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This study numerically investigates the mechanisms for generating and suppressing the leading edge vortex (LEV) in a vaned diffuser of a centrifugal compressor, aiming to enhance the compressor's stable operating range. Detailed analyses focus on the mechanisms of the rotating stall and the initiation process of the LEV. Notably, the end wall contouring method is applied to vaned diffuser to suppress the LEV and improve diffuser stability. The suppression mechanisms of the LEV are examined to deepen the understanding of stability enhancement mechanisms. Results demonstrate that, under stall conditions, the vaned diffuser experiences two-cell rotating stalls moving opposite to the impeller rotation direction, each rotating at approximately 12% of impeller rotation speed. The formation of diffuser stall is attributed to a longitudinal vortex developing from the LEV and causing the passage blockage. The increasing spanwise distortion of leading edge (LE) incidence angles and circumferential distortion of static pressures at the diffuser inlet promote the evolution of the longitudinal vortex from LEV. The end wall contouring of vaned diffuser effectively enhances the compressor's stable operating range by 15.10% and 8.90% toward lower flow rate conditions for machine Mach numbers 1.3 and 1.2, respectively. At near-stall points, the end wall contoured diffuser reduces spanwise distortion of LE incidence angles and circumferential distortion of static pressures at the diffuser inlet, mitigates the LE flow separation and corner separation, suppresses the generation and development of the LEV, delays the onset of rotating stall in vaned diffuser, and thereby improves the stable operating range of centrifugal compressor stage. [ABSTRACT FROM AUTHOR]

The impeller of the multi-blade centrifugal fan significantly impacts the fan's internal flow field and aerodynamic performance. To improve the axial flow distribution along the impeller and suppress the blade passage backflow at the impeller outlet of multi-blade centrifugal fans, a method of variable chord length blade design is proposed. Simultaneously, optimization models are established with static pressure and efficiency as objectives. By altering the control points of the Bezier curve, the leading-edge profile of the impeller is optimized. The results indicate that using variable chord length blades can expand the operating range of the multi-blade centrifugal fan. The maximum flow rate improvement reaches 10.6%. Significant enhancements in the aerodynamic efficiency of the multi-blade centrifugal fan are observed at low flow rates, with a maximum improvement of 4.9%. The variable chord length blade design method enables adjustment of the axial flow distribution at the impeller outlet to better match the impeller's flow requirements. This leads to increased outlet flow on both the shroud and hub sides of the impeller, consequently reducing the backflow area on both sides and improving the blade passage backflow phenomenon. Moreover, variable chord length blades facilitate a more uniform circumferential flow distribution at the impeller outlet, reducing the diversion pressure of the volute tongue, mitigating flow separation within the blade passage, weakening the interaction between the impeller and the volute tongue, and lowering energy losses in the impeller–volute tongue regions. Consequently, this enhances the aerodynamic performance of multi-blade centrifugal fans. [ABSTRACT FROM AUTHOR]

The permeation diffusion behavior of chloride ions with seawater is one of the key factors causing damage to reinforced concrete structures due to seawater erosion of steel bars. This study establishes a new mathematical model for chloride ion transport in reinforced concrete structures exposed to seawater. The model captures the effect of chloride ions on underwater tunnel structures effectively and provides a valuable theoretical basis for the maintenance and safety of these structures. The proposed model uniquely incorporates the microscopic pore structure characteristics of concrete and the effects of static water pressure and porosity on chloride ion diffusion. The results show that: (1) the diffusion coefficient of chloride ions first increases rapidly, then remains basically unchanged and gradually decreases; (2) there is a non-linear positive correlation between porosity, static water pressure, and maximum corrosion thickness; and (3) the fractal dimension of pore curvature is opposite to the fractal dimension of pore size and crack length. [ABSTRACT FROM AUTHOR]

As an advanced flow-drive technology, micro-electrohydrodynamic (EHD) conduction pumping has become a new prospect in many micro-scale industrial applications, including lab-on-chip devices and microfluidic cooling systems. Under micro-scale conditions, the effect of the electric double layer (EDL) has to be considered. Zeta potential is an adjustable and measurable experimental value and has been proposed to estimate the strength of EDL in simulations. In this work, the effect of zeta potential on the performance of micro-EHD conduction pumping has been numerically investigated. A method to estimate the surface charge density without the Debye–Hückel approximation was introduced. A two-dimensional flush electrode configuration with a typical size of 50 μm was considered. The coupled series of governing equations was implemented in the finite-volume framework of OpenFOAM® and solved based on the PIMPLE algorithm. The results show that zeta potential can enhance the asymmetry of the electric field and change the distribution of the Coulomb force. For the construction considered in this work, negative zeta potential can reduce the size and strength of the vortex in the flow field and improve the pump's net flow rate and static pressure. In contrast, positive zeta potential has the opposite effect. Maximum performance enhancement up to 94.8%–115.1% has been observed for different electrode length ratios within the parameters studied in this paper. The results guide the zeta potential optimization of micro-EHD conduction pumping. By matching the pairs of solid and liquid materials, researchers can adjust zeta potential to an optimal value, thereby improving the pump performance. [ABSTRACT FROM AUTHOR]

The non-cavitating tip vortex in the near field of an elliptical hydrofoil is studied utilizing tomographic particle image velocimetry. Both the instantaneous and time-averaged flow fields are analyzed to elucidate the flow characteristics of the near-field tip vortex. The tip vortex is mainly formed on the suction side of hydrofoil and exhibits a tube-like shape. The turbulence intensity is at a relatively high level around the hydrofoil tip due to the roll-up process of the separated shear layers from the pressure side. With increasing angle of attack, the initiating position of the tip vortex moves upstream along the hydrofoil outline. In the near field, the axial flow within the tip vortex manifests a jet-like profile at higher angles of attack (α ≥ 10 °), and the majority of the vorticity is contained within the vortex core. A special position is identified during the streamwise evolution of the tip vortex, where the vortex circulation reaches its local maximum for the first time and the tip vortex cavitation is more prone to incept. In the vicinity of this crucial position, the pressure–velocity relation is derived along the vortex centerline by combining the three-dimensional measured velocity fields with the governing equations. It is revealed that the mean static pressure is directly related to the local mean axial velocity, adhering to the form of Bernoulli's equation. Conversely, corresponding pressure fluctuation depends on both the mean and fluctuating parts of the local axial velocity. [ABSTRACT FROM AUTHOR]

Research into the flow mechanisms and heat transfer characteristics of two-dimensional supersonic cooling films has been extensive, yet studies on supersonic conical cooling films remain scarce. This study examined the flow characteristics and mechanisms of supersonic conical cooling film under varying ratios of static pressure (RSPs) using nanoparticle planar laser scattering and particle image velocimetry techniques. The mainstream Mach number was M a ∞ = 3.8 , and the supersonic conical cooling film was tangentially injected through a precisely calibrated annular nozzle of M a j = 2.8. Analysis of the instantaneous flow structure characteristics along and normal to the flow direction segmented the flow downstream of the slot into three distinct zones: the potential core region, the large-scale coherent structure region, and the turbulent boundary layer region. The reattachment location was assessed both qualitatively and quantitatively based on time-averaged and statistical velocity field data. The results indicated that the reattachment location extended with higher RSP. Moreover, the reattachment point of the supersonic conical cooling film was further forward than that of a two-dimensional supersonic cooling film. It was found that the supersonic conical cooling film was more significantly affected by the compression effect of the conical stream tube compared to the impact of RSP. [ABSTRACT FROM AUTHOR]

An unsteady simulation with a full-annular computed domain was conducted to explore the flow unsteadiness near the tip region and the mechanism of rotating instability (RI) in the transonic rotor. The numerical static pressure signals were monitored at the tip region. RI was identified by a frequency hump in the spectrum of the static pressure signal in a small mass flow coefficient with a narrow range of 0.9510–0.9174. The cross-power spectrum of two static pressure signals obtained in adjacent passages showed that the circumferential propagation speed of RI decreased with the decrease in the mass flow coefficient. The relative circumferential propagation speed of RI of the prominent mode order was −0.65 to −0.61 times the rotor speed. Details of the flow field near the tip showed that a new vortex structure was induced by the interaction of the tip leakage vortex and the shock wave, which was called as tip secondary vortex (TSV). The TSV was the key parameter to the formation of RI. The impact of the TSV on the pressure side surface generated a low-pressure area and changed the tip load of the adjacent blade. RI is result of the propagation of the interaction between the TSV and the tip load. [ABSTRACT FROM AUTHOR]

The effects of back pressure on the transonic cascade operating state are crucial and can determine the structure of internal shock waves and secondary flows. In this paper, numerical methods validated by experiments were employed to investigate the evolution mechanisms of the inlet flow field, shock structure, secondary flow structure, and cascade performance under different back pressures. Analysis revealed that transonic cascade exhibited unique incidence characteristics in the inlet flow field under both subsonic and supersonic regimes, although these two regimes involved different physical mechanisms. The results revealed that the operating state of the transonic compressor cascade under the unique incidence condition was influenced by the outlet back pressure, and there existed a critical static pressure ratio. The critical static pressure ratio shifted from 1.61 for two-dimensional flow to 1.37 for three-dimensional (3D) flow at M1 = 1.1, due to the corner separation and the characteristics of 3D shocks. The 3D shock structure exhibited a non-uniform distribution along the spanwise direction due to the influence of back pressure and the separated boundary layer. The vortex structures analysis revealed that the secondary flow structure on the sidewalls of the transonic compressor cascade was primarily dominated by corner vortices, whose formation mechanism was related to the interaction between the shock wave and the sidewall boundary layer. Additionally, this interaction also led to the formation of detached shock and lip shock vortex structures. Finally, loss analysis indicated that the wake region of the transonic cascade primarily includes six types of loss, and the total loss of the cascade decreased with the rise in back pressure. [ABSTRACT FROM AUTHOR]

Rotor tip clearance has significant influences on both the performances and internal flow fields of compressors. To explore the relationship between these influences and compressor loading levels, four single-stage compressors, with loading levels ranging from 0.36 to 0.59, have been designed for experiments. Compressor characteristics and flow fields at blade row inlet and outlet planes were measured under two rotor tip clearance conditions: 1% and 2.2% blade height. Based on these experimental results, an analysis was conducted to examine the influence of loading levels on the sensitivity to rotor tip clearance. The experimental results indicate that increase in the loading level enhances the sensitivity of compressor stage characteristics to rotor tip clearance. At the design point, an increase of 1% in the rotor tip clearance causes reductions of 1.3% in the stage efficiency and 2.3% in the stage static pressure rise when the loading level is 0.36. However, when the loading level is 0.59, the reductions are 2.9% and 3.4% in stage efficiency and stage static pressure rise, respectively. Increasing the rotor tip clearance brings the static pressure rise at the rotor tip region closer to its limit, resulting in a rapid growth of local loss. This trend becomes increasingly pronounced with higher compressor loading level. [ABSTRACT FROM AUTHOR]

The storm sewer geyser is a process where an air–water mixture violently erupts from a manhole. Despite the low hydrostatic pressure, violent eruptions can achieve a height of tens of meters above the ground. This current study experimentally investigates large-scale violent geysers using a large air pocket inserted from a pressurized air tank. The total length of the pipe system is approximately 88 m with a 0.1572 m diameter pipe. This large-scale experiment facilitates the investigation of spontaneous geyser eruptions. This study identifies the role of air–water volume ratio and coefficient of pressure (ratio of absolute initial static pressure to initial dynamic pressure) on the geyser intensity using eruption images and pressure plots. A total of 116 cases are tested, in which the volume ratio is parametrically increased from 0 to 1.1 under various operating conditions. A geyser score is defined to quantify the geyser eruption nature based on visual observations. The key findings are as follows: first, a sharp transition in geyser intensity is observed at the critical volume ratio of 0.5, and pre-transition and post-transition intensity exhibit a linear relationship with the volume ratio; and second, the critical volume ratio linearly varies with the coefficient of pressure. [ABSTRACT FROM AUTHOR]

A directly connected transient scramjet simulator was developed for supersonic combustion experiments to gain a deeper understanding of the process and mechanism of mode transitions in dual-mode scramjet engines. This study presents experimental simulations of flame development under constant combustor inlet conditions with increasing equivalence ratios, utilizing advanced high-speed measurement techniques such as high-frequency static pressure, schlieren imaging, and methylene radical chemiluminescence. The results demonstrated that abrupt changes occurred during the mode transition as the equivalence ratio increased. Specifically, lower equivalence ratios correspond to the shear layer mode, whereas higher equivalence ratios correspond to the jet-wake and jet-front modes. The stable position of the precombustion shock wave is determined by the heat released during combustion, whereas both the precombustion shock wave and the fuel-injection depth determine the combustion mode and flame-anchoring position. Finally, the dynamic variation of shock waves inside the combustor is significant in contributing to abrupt mode transitions. [ABSTRACT FROM AUTHOR]

The aerodynamic performance of the variable cycle engine fan changes sharply during mode transition. Investigating the variations of flow structure and understanding the loss mechanism are helpful in providing guidance for the fan design. Three-dimensional models of single bypass and double bypass compression systems are established, and static pressure is applied at the bypass stream outlet to simulate the opening of the mode selection valve. The characteristic band of variable cycle engine fan is obtained by gradually increasing the bypass stream pressure while maintaining specific values for the core stream pressure. Results show that the overall performance of the double bypass configuration, without bypass recirculation, is almost identical to that of the conventional single bypass configuration during the throttling process. With the increase in bypass pressure, the shock wave and the trajectory of tip leakage vortex gradually move forward, thereby increasing the blockage region induced by the interaction between the shock and tip leakage vortex. In addition, the performance of fan with reverse flow is also calculated. The recirculation causes the operating point to move closer to the stability limit, reducing the isentropic efficiency. Additionally, the recirculation changes the radial distribution of axial velocity and total pressure, leading to inlet distortion in the core driven fan stage. Furthermore, the loss mechanism is clarified by modeling the splitter and conducting entropy generation analysis. The sharp expansion of bypass stream could cause severe flow separation, and reducing the curvature of casing can effectively suppress the viscous shear loss. [ABSTRACT FROM AUTHOR]

A novel method to characterize a supersonic plasma jet using a double-jacketed enthalpy probe is presented and applied to Ar supersonic plasma jets generated by a plasma torch operated at an input power level of 5.7 kW and a chamber pressure of 2.3 kPa. The basis of this method is to measure total stagnation pressure, total enthalpy, and static pressure at the backside of a shock wave formed in front of the probe with a single insertion of the probe. Once these three variables are known, normal shock relations before and after the shock wave can disclose information on static pressures, Mach numbers, temperatures, and velocities of the supersonic plasma jet under a calorically perfect gas assumption. For example, measurement experiments carried out with the proposed probe revealed that static pressure of Ar supersonic plasma jet oscillated around the chamber pressure of 2.3 kPa in a range of 1–5 kPa along the jet axis, clearly showing an aerodynamic non-equilibrium. Corresponding to the behaviors of static pressures, Mach numbers also oscillated in the range of 1.1–3.5 along the jet axis. In addition, oscillation patterns of static pressures and Mach numbers agreed well with those of compression and expansion wave zones observed in the photograph of an over-expanded Ar supersonic plasma jet. Although relatively large errors were accompanied due to a low input power level, plasma temperatures and velocities were measured to be decreasing and increasing, respectively, in the expansion wave zone while opposite behaviors were observed in the compression wave zone. [ABSTRACT FROM AUTHOR]

This study investigates the fluid dynamics and mixing characteristics of an oscillating sonic jet injected into a supersonic cross flow of Mach 2.1 using experimental and computational techniques. The oscillating jet is produced by a novel fluidic oscillator, which consists of a primary rectangular duct that expands into an outer duct with sudden expansion. Control jets are injected in the lateral direction from the side walls of the sudden expansion in an out-of-phase manner to oscillate the injected jet in the spanwise direction of the crossflow. Experimental and numerical investigations based on wall static pressure and mass fraction fluctuations, respectively, revealed that the injected jet oscillation frequency matches the control jet frequency. The iso-surface of lambda-2 criterion showed the presence of various dominant vortex structures, such as counter-rotating vortex pairs, horseshoe vortex, sidewall vortices, and trailing vortices. Helicity contour plots showed that the streamwise vortices oscillate in the spanwise direction with the control strategy and promote the spread of the injected jet in the spanwise direction. The spatiotemporal reconstruction (z–t plot) of the density gradients at a particular streamwise location revealed that the bow shock produced by the interaction of the injected jet and the crossflow oscillates with the actuation of the control strategy. The power spectral density of the z–t plot revealed that the shock wave oscillation frequency matches the control jet frequency. The oscillating jet produced by the control strategy showed significant mixing enhancement in supersonic crossflow compared to a simple rectangular injection. [ABSTRACT FROM AUTHOR]

Tip leakage flow is one of the significant factors influencing the internal flow stability of mixed-flow pumps, and in severe cases, it can lead to channel blockage and energy loss. In order to gain a deeper understanding of the energy dissipation mechanism induced by tip leakage vortex cavitation, this study is based on the Wray–Agarwal (WA) turbulence model and the homogeneous flow model, investigating the cavitation flow characteristics of mixed-flow pumps. Additionally, the entropy production theory is employed to evaluate the energy losses within the mixed-flow pump and analyze the components of energy loss in the impeller and guide vanes. The research results reveal that with increasing cavitation intensity, the low-pressure region at the leading edge of the blade extends toward the trailing edge, influencing the static pressure distribution on the blade's pressure side. Leakage flow and the spatial distribution of leakage vortices move closer to the suction side of the blade with increasing cavitation intensity. Cavitation primarily affects the energy losses in the impeller region, with turbulent dissipation being the main source of energy loss. High turbulent dissipation zones are concentrated at the trailing edge of the blade, correlating with recirculation vortices and trailing-edge vortices. This study provides theoretical insights with practical implications for enhancing the cavitation performance of mixed-flow pumps, offering valuable guidance for design and operation. [ABSTRACT FROM AUTHOR]

This research was inspired by the long-eared owl's ability to fly silently. For the first time in this study, a wind turbine blade is designed to mimic the wing surface and the leading edge of the long-eared owl. The commonly used two-dimensional blade profile in previous studies is replaced by a more effective three-dimensional profile. This change leads to improved aerodynamic performance of the multi-blade centrifugal fan and reduced noise levels. The airfoil and leading edge profile parameters of the long-eared owl were extracted and utilized. These parameters were used to develop a fitting formula based on their correlation. This formula facilitated the design and optimization of a bionic blade (B-Blade). The results indicate a 4.1% enhancement in the maximum flow rate compared to the original blade fan, alongside a noise reduction of 1.3 dB(A) under identical static pressure conditions. An examination of the internal flow, noise, and sound source characteristics of both fan types was conducted, elucidating the aerodynamic noise mechanism. Fan noise propagation showed pronounced dipole sound source traits. The sound source area at the B-Blade fan's inlet and the volute tongue was more compact, leading to a decrease in mid-low frequency discrete noise. The sound source intensity was also diminished. The B-Blade fan also ameliorated the flow distribution at the impeller outlet, reducing the unstable interaction between the impeller and volute tongue, thereby effectively diminishing noise. [ABSTRACT FROM AUTHOR]

Due to the fact that it serves as both a pump and a compressor, the helico-axial multiphase pump is better suited for mixed-transport oil-gas two-phase flow and is frequently used for deep-sea oil extraction. The gas–liquid separation caused by the asynchrony of the gas–liquid two-phase medium is the primary cause of the damage or failure of the helico-axial multiphase pump in the project, resulting in the pump's damage or inability to operate. To investigate the variation of phase separation in the helico-axial multiphase pump under diverse operating conditions, a test system was designed and the theory of fluid flow mechanics was applied. The effects of different flow fields on the energy conversion characteristics of the pump were investigated. The results indicate that the gas–liquid separation position in the impeller occurs near the 2/3 of the airfoil bone line. After the gas–liquid separation, gas mass will be formed, pocket flow will be readily induced, and the gas block-up phenomenon will result in energy loss in the compression unit. Simulations indicate that the static pressure recovery and total pressure loss in a diffuser fluctuate over time. The static pressure recovery efficacy of the diffuser is highest when the imported gas volume fraction is 10% and lowest when the imported gas volume fraction is 60%. On the surface of the impeller blade, perpendicular to the flow direction, the separation phenomenon near the hub side is more severe than that near the rim. Along the flow direction, the pattern of phase separation is comparable. [ABSTRACT FROM AUTHOR]

This study aims to investigate the effect of front and aft wall modifications and their combinations on the sound pressure level response of the deep configuration of the M219 cavity at transonic speed. Experimental results for the baseline geometry of the cavity were validated with the high-fidelity lattice Boltzmann method computational fluid dynamics solver, SIMULIA PowerFLOW®. In this study, inclined and parallel cuts were applied to the front and aft walls of the M219 cavity to determine their efficiency in reducing the overall sound pressure level (OASPL). The most effective configurations on reducing OASPL at the front and aft walls were combined to form mixed configurations. All front wall and aft wall configurations as well as mixed configurations were validated through grid independence studies. Simulations showed that modifications to the aft wall were more effective in reducing noise than modifications to the front wall. The spectral analysis of pressure oscillations provided information about the broadband and tonal pressure responses of the mixed configurations. The findings showed that the mixed configurations were effective in reducing the frequency and amplitude of the Rossiter modes. The most successful configurations on noise reduction also showed a decrease in the drag coefficient and the average static pressure distribution along the cavity. [ABSTRACT FROM AUTHOR]

The flow field characteristics downstream of the evaporative flameholder in the subsonic–supersonic mixing flow were experimentally investigated. The study focused on examining the effects of different inlet static pressure ratios characterized by supersonic and subsonic flow parameters. The results indicated that the increase in the static pressure ratio enhanced the fragmentation of the multiple vortices downstream of the flameholder located in the subsonic mainstream. It also exacerbated the asymmetry in the recirculation zone downstream of the flameholder and strengthened the tendency of the fluid to flow from one vortex to another. The regions with higher vorticity were mainly concentrated in the subsonic–supersonic shear layer between the subsonic and supersonic mainstream and the subsonic–subsonic shear layer region downstream of the flameholder. Furthermore, an increase in the static pressure ratio widened the range of peak distribution while reducing the magnitude of the peaks. The recirculation zone downstream of the flameholder exhibited four distinct changes in the vortex structure as the static pressure ratio increased from 1.07 to 1.96. These typical changes in the vortex structure observed are as follows: asymmetric dual-vortex structure, single vortex structure (away from the supersonic mainstream region), asymmetric dual-vortex structure, and single vortex structure (near the supersonic mainstream region). [ABSTRACT FROM AUTHOR]

The probability-based robust optimization methods require a large amount of sample data to build probability distribution models of uncertain parameters. However, it is a common situation that only scarce sampled data are available in practice due to expensive tests. This study proposes a data-driven robust optimization framework by embedding a novel uncertainty quantification (UQ) method, which can quantify the uncertainty based on the statistical moments of scarce input data. The computational robustness and accuracy of the developed UQ methods are validated. Then, the data-driven multi-objective optimization framework is applied to improve the mean performance and aerodynamic robustness of a two-dimensional compressor blade with real stagger angle errors. Uncertainty analysis shows that there is a probability of 47.55% to deviate from the nominal total pressure loss coefficient by more than 1% for the actual performance values at high positive incidence i = 7 ° condition. In the optimization process, the total pressure loss coefficient is selected as the objective function, while the static pressure ratio is used as a constraint. The Gaussian process regression model is trained to improve the robust optimization efficiency. The robust optimization is conducted under the most sensitive conditions. Optimized results indicate that compared with the nominal blade, the mean performance of the selected robust blades is increased by 10.9%, 8.56%, and 0.83%; the performance dispersion is decreased by 19.0%, 24.8%, and 35.3%, respectively. The optimized results can provide useful references for the robust design of compressor blades. [ABSTRACT FROM AUTHOR]

The secondary flow in the impeller of an axial flow pump is an important factor affecting the safe and stable operation of the unit. However, there is still a lack of systematic research on the generation mechanism of secondary flow and corresponding control strategies in axial flow pumps. To better understand the secondary flow characteristics in the axial flow pump, based on the momentum equation of relative motion, the basic distribution characteristics of the potential rothalpy gradient (PRG, or the reduced static pressure gradient) in the impeller of an axial flow pump were systematically analyzed. Two typical secondary flows were found, namely, trailing-edge hub-shroud type secondary flow at the blade outlet hub side and leading-edge hub-shroud type secondary flow at the blade inlet shroud side. The generation of these secondary flows is directly related to the effect of natural adverse PRG. A new blade design method is proposed. The essential idea of this method is to give the blade loading strategy based on grasping the macro-flow characteristics and control PRG characteristics by adjusting the real blade loading δp (i.e., the static pressure difference between the blade pressure and suction surfaces) and, thereby, control the above-mentioned secondary flows. The application of an axial flow pump showed that the blades designed based on this method can effectively control these secondary flows and reduce pressure fluctuations. The average decrease in pressure fluctuation on the blade inlet shroud side and the outlet hub side is 17.79% and 20.03%, respectively. [ABSTRACT FROM AUTHOR]

Effects of porous carbon-fiber-reinforced carbon-matrix ceramics (C/C) on the stability of second-mode waves on a 7°-half-angle cone were investigated for Reynolds numbers R e m = 2.43 × 10 6 – 6.40 × 10 6 m − 1 at the freestream Mach number of M ∞ = 7.4 , for both sharp and 2.5-mm-round nose tips. A broadband time-domain impedance boundary condition was used to model the effects of the C/C porosity on the flow dynamics leveraging direct ultrasonic benchtop experiments and homogenous absorber theory. A spectral linear stability solver based on orthogonal Laguerre functions, naturally vanishing in the free stream, was used to predict linear spatial growth rates, which are in agreement with independent pulsed axisymmetric direct-numerical simulations. The latter were carried out with the quasi-spectral viscosity closure—a dynamic quasi-spectral procedure capable of deactivating the sub-filter scale stresses in the absence of turbulent break down—verifying its suitability to carry out transitional calculations without affecting ultrasonic wave dynamics. The effectiveness of a porous C/C surface is shown to decrease drastically with static pressure and its presence is shown to decrease the second-mode growth rates in regions where it is unstable as well as increasing the attenuation rates in regions where it is stable. [ABSTRACT FROM AUTHOR]

Boundary layer ingesting (BLI) propulsion represents a promising concept that can achieve a great fuel burn reduction compared to conventional aircraft. This study numerically investigates the BLI inflow distortion and its impact on propulsion fan performance using commercial software CFX. First, Reynolds-averaged Navier–Stokes (RANS) simulations were performed to obtain the inflow distortion of a propulsive fuselage aircraft with various geometries and flight conditions. Then, full-annulus URANS simulations were carried out to assess the effect of various inflow distortions on the performance of NASA rotor 67. The results show that the fuselage sweep dominates the BLI inflow distortion, while the effects of the wing and vertical tail are local. As the angle of attack increases, the BLI inflow distortion is alleviated due to the freestream being ingested from the bottom of the propulsor. With the increase in the angle of sideslip, the redistributing and impinging flow significantly aggravates the total pressure and swirl distortion. With regard to the fan performance, various BLI inflow distortions share a similar static pressure distribution upstream of the rotor, as a result of the comparable total pressure distribution, so the cases without swirl distortion show a similar distribution of migration-induced swirl angle. At the peak efficiency condition, a minimum reduction in efficiency of approximately 4.5% and a total pressure ratio reduction of over 2.9% are observed for the investigated inflow distortions, in comparison with the Clean case. Moreover, the findings reveal that the effect of swirl distortion on fan performance is minor for the cruise case, but substantial for the two sideslip cases. These results offer useful insights into BLI propulsion. [ABSTRACT FROM AUTHOR]

Ratio of static pressure (RSP) is a critical factor affecting the development of supersonic cooling film flow field. To give a good account of flow characteristics and physical mechanisms on the influences of RSPs, experiments are conducted employing the nanoparticle planar laser scattering (NPLS) and particle-image velocimetry (PIV) techniques. A supersonic cooling film is tangentially injected at a Mach number of M a j = 2 into a laminar boundary layer at a mainstream Mach number of M a ∞ = 3 with different RSP cases. The flow field is divided into four sections: the wake region, the potential core region, the wall-jet region, and the fully developed turbulent boundary layer region. The velocity profile extracted at the fully developed turbulent boundary layer region is proved to be consistent with the theoretical composite profile. Moreover, a new criterion that distinguishes the specific position of the impingement point where the mixing layer approaches the bottom boundary layer is proposed based on the distribution of principal strain rate of the mean velocity field. The analysis of turbulent structures finds that the K–H vortex structures are sensitive to RSPs, and more broken vortices generated when the two fluids mix. The intensity of wave structures is depicted in the synthetic schlieren images when subjected to different RSPs. In addition, the turbulent characteristics show a weaker mixing process with increased RSP, indicating a prolonging in the effective cooling length. Furthermore, the disturbance at the slot and variations of velocity gradient downstream are addressed and analyzed to reveal the physical mechanisms for the mixing process on RSP differences. [ABSTRACT FROM AUTHOR]

Ensuring the high efficiency and stable operation of a supersonic through-flow fan (STFF) in a wide range of inlet Mach numbers is of vital importance. The influence of inlet Mach number (M) ranging from 0.3 to 2.36 on the aerodynamic performance and flow structure in the STFF cascade is studied. The results indicate that at design incidence, the transonic inflow condition has a greater loss, a lower static pressure ratio, and a larger flow turning than the non-transonic inflow condition. The evolution of shock structure with the increasing inlet Mach number is as follows: shock-free → passage quasi-normal shock → leading edge (LE) local normal shock + λ shock → LE normal shock + trailing edge (TE) fishtail shock → LE bow shock + TE fishtail shock + multiple reflected oblique shocks → multiple oblique shocks. Under subsonic inflow conditions, the blade loss is dominated by boundary layer friction loss and wake loss. Under transonic inflow conditions, when the inlet Mach number rises from 0.66 to 0.86, the rear branch of passage λ shock impinges on the thickened separation region of the adjacent blade. This increases the loss. When the inlet Mach number rises to 1.06, the λ shock evolves into fishtail shocks, and the LE shock becomes stronger, reducing shock loss and separation loss significantly. As the inlet Mach number goes up to 1.26, the LE normal shock evolves into a bow shock, inducing a sharp reduction in LE shock loss. Under the supersonic inflow conditions, when the inlet Mach number rises from 1.96 to 2.36, the loss variation is mainly stemmed from LE bluntness loss, accounting for 66% of overall loss change. [ABSTRACT FROM AUTHOR]

An Euler two-fluid model and a Shear Strain Transports (SST) turbulence model were used to study the gas–liquid flow law in a diffuser of a Helico-Axial Multiphase Pump by building a bedstand in this paper and studied energy recovery and gas–liquid separation characteristics of the diffuser in the Helico-Axial Multiphase Pump at different stages. The results show that the pressurization capacity of an impeller decreases gradually when the increase in IGVF (inlet gas volume fraction), and static pressure drops slightly in a dynamtic–static interface. Energy recovery characteristics are more complex at medium and high IGVF in the diffuser. The pressure gradient change perpendicular to the flow direction is the main factor causing two-phase separation on the back surface of the diffuser. The gas–liquid separation is mainly caused by the pressure gradient difference perpendicular to the flow direction on the working surface of the diffuser under the low IGVF, while the pressure gradient difference in the two directions is jointly influenced by the pressure gradient difference at medium and high IGVF. [ABSTRACT FROM AUTHOR]

The jet-wedge combinatorial initiation (JWCI) is a novel functional form for initiating an oblique detonation wave (ODW) with reduced total drag within the oblique detonation combustor under the conditions of a low Mach number and low static pressure inflow. It can suppress the instability of the detached ODW. The evolution of the combustion wave during combinatorial initiation is dominated by the intersection between the bow shock wave and the oblique shock wave and consists of four stages: the shock-induced combustion stage, the shock–deflagration coupling stage, the hybrid combustion stage, and the oblique detonation stage. Three combustion regimes can be formed by using the JWCI: the shock-induced combustion regime, the hybrid combustion regime, and the oblique detonation combustion regime. These regimes have distinct characteristics of combustion and flow structure that can be controlled by changing two non-dimensional variables: the ratio of momentum flux (J) and the penetration ratio (PR). This is significant as it can facilitate the application of different combustion regimes under a variety of realistic flight conditions. In this study, the criterion for the transformation of the combustion regime is quantitatively investigated, and it is shown that the structure of the combustion wave does not transform until both non-dimensional variables have reached their respective thresholds. J is crucial for determining the combustion regime and facilitating its transformation, but the PR accounts only for the height of the combustion wave structure. The work is beneficial for research on the initiation of the ODW in applications of oblique detonation engines. [ABSTRACT FROM AUTHOR]

In order to explore the influence of different crude viscosities on the energy conversion characteristics of an impeller of a Helical-Axial Multiphase Pump, a numerical simulation based on the standard k–ε equation and Euler multiphase flow model under different viscosities was carried out. At the same time, the internal flow structure of the Helical-Axial Multiphase Pump was analyzed by experiments when water was transported. The changes in pressure, GVF (gas volume fraction), flow field, and pressurization of the impeller are obtained with different viscosities and inlet gas volume fractions. The results show that the liquid viscosity is greater, the inlet pressure of the impeller is increasing, the velocity vector is greater near the rim and hub side, and the turbulent kinetic energy is increasing near the hub side and rim side, resulting in a greater flow loss. The pressurization capacity of the impeller is weakened, making its head lower. At the same time, the fluctuation of GVF from inlet to outlet is smaller, and the low GVF area is mainly concentrated in high pressure area. The viscosity has a greater impact on static pressure energy of the first half of the impeller and a smaller impact on the second half of the impeller, and the power of the impeller is mainly concentrated in the first half of the impeller. [ABSTRACT FROM AUTHOR]

The unsteady three-dimensional separated flow on a wall induced by a square protrusion (approximately twice the local boundary layer thickness in width and height), and its control by means of passive suction through holes, is investigated using wind tunnel experiments at Mach 2.87. The baseline flow without any control was characterized and compared against the cases with bleed. A bow-shaped separation line on the wall with a mid-span separation length of 5.57 δ from protrusion face was traced from the oil-flow visualization. The averaged pressure distribution surveyed using static pressure ports placed on the wall has mapped plateau, high-pressure, and low-pressure regions in the separated flow, distinctive to three-dimensional interactions. Ten control configurations were tested with suction holes placed along mid-span in the different pressure regions. Significant spanwise "Mean Reduction in Separation Length" of up to 0.93 δ was observed from oil-flow visualization. A comparison of observations from various control configurations suggested that bleeding the flow from the high-pressure region could delay the separation and reduce the bubble size in general. Furthermore, time-resolved Schlieren visualizations have confirmed reduction in both "mid-span separation length" and "shock-intermittent-region" with the introduction of suction in high-pressure region. Fourier and Proper Orthogonal Decomposition analysis done on the Schlieren data has confirmed the presence of low-frequency separation-shock oscillations at Strouhal Numbers of order 10 − 2 , both with and without control. Furthermore, the amplitudes of separation-shock oscillations in the spectrum were reduced with the introduction of suction simultaneously from two holes placed in high- and low-pressure regions. [ABSTRACT FROM AUTHOR]

Bonneted slide gates are widely used in dam bottom outlets for flow regulation. High kinetic energy flow generates turbulence, which induces vibrations in bonneted slide gates under partially open conditions. Significant vibrations can indicate problems and cause damage or expedited deterioration of the gate if left unchecked. This experimental study focuses on two aspects: turbulent flow formation due to rectangular bonneted slide gates and elimination of random fluctuating loads on the gate using a turbulence inhibitor. Five reservoir heads were combined with nine gate openings (10%–90%) to produce 45 different discharges of 12–182 l/s. The different types of the gate flow create different turbulence and random loads based on the water–air mixture and the kinetic energy of the flow. Flow analysis indicates considerable random static pressure fluctuations under the slide gate. The results show the formation of swirling secondary flows in the gate's side guide slots, and their movement and collision with the gate are the leading cause of the turbulent multiphase flow and random fluctuations in static pressure. Also, the use of turbulence inhibitors on the gate can prevent the formation of secondary flows, which results in a 98.92% reduction in the amplitude of random static pressure fluctuations and the elimination of the random fluctuating loads on the slide gate. After removing the swirling secondary flows, a stable air boundary layer was formed under the gate. Finally, the gate flow changed from a turbulent two-phase flow to a steady single-phase jet of water. [ABSTRACT FROM AUTHOR]

The flow around a cylinder is a classical aerodynamic problem involving the effect of Reynolds number (Re). Thus far, the impact of turbulence has not been fully clarified despite its important practical value in engineering applications. This study mainly investigates the influence of turbulence in the critical Re regime on the smooth and turbulent flows around a cylinder. The foregoing is accomplished by conducting static pressure measurement model experiments in the Re range of 1.6 × 105–6.1 × 105 and turbulence intensity range (Iu) of 5%–13%. Consequently, a series of useful results for engineering wind resistance design is obtained. The effect of turbulence on each sub-flow regime varies. The structural response to turbulence is more dangerous under conditions such as when the wind pressure coefficients fluctuate strongly and non-Gaussianity is strong. The foregoing effects do not necessarily increase linearly with the turbulence intensity. In addition, the influence of turbulence evidently depends on the Re and corresponding flow regime. Therefore, turbulence must be cautiously managed in scaled model experiments and actual wind resistance design. [ABSTRACT FROM AUTHOR]

The aerodynamics of finite-span inclined flat plates in ground effect is experimentally investigated at a chord-based Reynolds number of 50 000 for aspect ratios of 1 and 2. The minimum ground height is varied between 0.1 and 1.0 chord lengths, and lift and drag forces are measured using a force balance for angles of attack between −90° and 90°. Planar, two- and three-component particle image velocimetry is used to perform streamwise and cross-plane measurements at the midspan and one chord length downstream of the trailing edge, respectively. Ground effect is significant at ground clearances below 0.5 chord lengths, most notably near the stall angle, where it leads to significant changes to flow development. At sufficiently low free flight pre-stall angles, the increase in edge velocity at low gap ratios caused greater suction, generating higher lift with a minimal increase in drag for both orientations. Closer to the free flight stall angle, a decrease in aerodynamic loading is observed for negative orientations due to earlier onset of stall with a decreasing gap ratio. The exception was the higher aspect ratio plate at negative orientations, where the loading was largely invariant to changes in gap ratio for all angles tested. At positive orientations, the increase in average static pressure along the pressure surface in ground proximity led to an overall increase in loading prior to deep-stall conditions for both aspect ratios. The ground effect was minimal at post-stall angles of attack. The results may be used to guide the design of photovoltaic supports at relevant latitudes. [ABSTRACT FROM AUTHOR]

Due to the demand for higher aerodynamic performance of compressors, thoroughly investigating the high-loss flow in the corner region and effectively controlling it are important. In this paper, a novel parameterization method based on the extended free form deformation (EFFD) technique and the constraints for EFFD's control points is proposed. Then, considering the features of typical control techniques and the degrees of freedom of both the blade and hub geometries, the combined control approach is implemented in the corner region of a linear cascade. Furthermore, large eddy simulation is used to simulate the flow, verify the effects of the combined control approach, and explore the underlying physical mechanisms of corner separation. The numerical results show that the combined control can significantly decrease the mean total pressure loss. The loss reduction at the design point reaches 6.05%, while it decreases by almost 2.5% near the stall/blockage operating conditions. The combined control increases the radial pressure gradient at the rear of the blade by depressing the hub and stretching the suction surface. Consequently, although the radial flow slightly increases the mixing loss in the mainstream at large incidences, the accumulation of low-energy flow in the boundary layer and the corresponding development of the corner vortex are significantly restrained. Moreover, by redistributing the static pressure on the hub, the combined control weakens the migration of crossing flow and obstructs the low-velocity flow from the pressure side involved in the separation. Overall, the combined control contributes to reducing the corner separation and improving the aerodynamic performance. [ABSTRACT FROM AUTHOR]

Thrust abruption due to mode transition could be catastrophic for hypersonic vehicles. To understand the underlying physics, a direct-connected transient Flight Trajectory Simulator 1 (FTS-1) has been developed at the Institute of Mechanics, Chinese Academy of Sciences. This facility uses advanced high-speed measuring techniques, including thrust and static wall pressure measurement, and Schlieren and hydrocarbon radical chemiluminescence (CH* chemiluminescence) imaging. Kerosene-fueled dual-mode combustor experiments are designed in an acceleration trajectory. The basic operation parameters and the flame and flow dynamics of the acceleration induced mode transition are evaluated. Discussions are given on the triggering mechanisms responsible for the ram to scram mode transition in a simulated flight acceleration. [ABSTRACT FROM AUTHOR]

The behavior of submerged granular flow is strongly dependent on the solid volume fraction and the viscosity discontinuity over a wide range of flow regimes. To obtain a general description of this type of flow, this study proposes a new model to compute solid effective stresses of submerged granular materials across multiple flow regimes. Here, based on the critical state soil mechanics framework, a new equation is proposed to describe the evolution of elastic reference of materials caused by elastoplastic deformation. The evolution of elastic reference subsequently informs the development of static pressure, and together with the dynamic pressure computed using a well-established blended model, resulting in a new approach to compute the solid pressure induced by both dynamic and static effects. The proposed model is then implemented in the Eulerian–Eulerian approach using the finite volume method to simulate the collapses of submerged granular columns, covering different flow regimes from quasi-static to viscous depositions. Simulation results agreeing well with experimental and numerical data in the literature are a testament to the performance of a well-developed constitutive law. In addition, the simulation results comprehensibly demonstrate the important role of interstitial fluid flow as well as the initial solid volume fraction in the collapsing process across different flow regimes with different packing densities. Furthermore, the effects of initial volume fraction, fluid pressure, and phase interaction forces on the flow responses are also discussed. [ABSTRACT FROM AUTHOR]

A new method to characterize air test conditions in hypersonic impulse facilities is introduced. It is a hybrid experimental–computational rebuilding method that uses the Fay–Riddell correlation with corrections based on thermochemical nonequilibrium computational fluid dynamic results. Its benefits include simplicity and time-resolution, and using this method, a unique characterization can be made for each individual experimental run. Simplicity is achieved by avoiding the use of any optical techniques and overly expensive numerical computations while still maintaining accuracy. Without making any assumptions to relate the reservoir conditions to the nozzle exit conditions, the work done characterizing four test conditions in a reflected shock tunnel is presented. In this type of facility, shock compression is used to produce an appropriate reservoir, which is then expanded through a nozzle to produce hypersonic flow. Particular focus is given to the nozzle exit total enthalpy where a comparison is made with the reservoir enthalpy obtained using the measured shock speed and pressure in the shock tube. Good agreement is observed in all cases providing validation of the new approach. Additionally, static pressure measurements showed clearly that conditions III and IV have a thermochemical state which likely froze shortly after the nozzle throat. Also, the nozzle flow is shown to be almost isentropic. Due to the simplicity of the current method, it can be easily implemented in existing facilities to provide an additional independent estimate alongside existing results. [ABSTRACT FROM AUTHOR]

During the transient processes in load rejection in a pumped-storage system, water column separation (WCS) can occur in the draft tube when the local pressure is less than the vapor pressure. The reverse water hammer due to water column bridging affects the safety of the unit and the tailrace tunnel. However, what are the conditions that trigger WCS? What is the physical mechanism? These questions have not been elucidated experimentally. Therefore, it is necessary to investigate the conditions that lead to WCS and the impact of the reverse water hammer generated by WCS bridging on the stability of the unit. In this study, a semi-open test rig was modified by installing a variable-frequency pump to reduce the static pressure, resulting in a cavitation vortex in the draft tube. The static pressure and the water hammer pressure derived from load rejection cause the pressure at the draft tube wall to fall below the vapor pressure for some time, interrupting the flow in the draft tube. This is a typical case of WCS in a pump turbine. If the pressure in the draft tube does not remain at the minimum pressure for long enough, imperfect water column separation may occur with bubble groups. The amplitude of the pressure pulsations during the initial and development stages was (−0.2 m, 0.2 m). It was 4.96% of the initial static pressure. Thus, the pressure fluctuations have a negligible effect on WCS. [ABSTRACT FROM AUTHOR]

Scramjet engines are one of the most economical and reliable high-speed airbreathing propulsion technologies that can be used for low-cost satellite launchers and hypersonic atmospheric transportation. For access to space, the flight envelope practically comprises varying freestream conditions and altitude during ascent flight, constituting a complex optimization problem subject to various constraints from design and operation perspectives. This paper presents the results and insights obtained from a multi-objective optimization study of three classes of axisymmetric scramjet intakes, i.e., three-ramp, Busemann-based, and generic intakes represented by smooth Bézier curves, with the same intake mass flow rate for all classes. Optimization is conducted by means of surrogate-assisted evolutionary algorithms coupled with computational fluid dynamics simulations using a Reynolds-averaged Navier–Stokes flow solver for steady-state flowfields. It aims to minimize the intake drag and maximize the compression efficiency at Mach 7.7 at an altitude of 30 km simultaneously. It has been found that generic intakes, which offer greater local shape control, can achieve the highest compression efficiency of 94.8%, the highest total pressure recovery, and the highest flow uniformity at the intake exit. The Busemann-based intakes, on the other hand, can produce the highest static pressure ratio while incurring the lowest drag force. The utility of principal component analysis has effectively reduced the dimensions of the solutions, and has identified clusters of non-dominated solutions according to their characteristics such as geometric attributes, exit flow profiles, and wall property distributions, thereby conducing to obtain further physical insights into the optimal configurations. [ABSTRACT FROM AUTHOR]

Numerical simulations of the particle-laden gas–solid flow in horizontal circular pipes have been used to identify the role of particle collision coefficients in flow regimes within it. A four-way coupling Euler–Lagrangian approach was employed, using direct numerical simulations of the gas phase and Lagrangian particle tracking to account for the drag, gravitational and lift forces, together with particle–wall and inter-particle interactions. The influences on the flow of the mass loading ratio ( Φ m ) and of the coefficients of restitution for collisions both between particles and the wall ( e p − w ) and between particles ( e p − p ) are assessed by examining the fluid and particle velocities, particle concentration distribution, turbulence kinetic energy, static pressure, inter-phase transferred momentum, and the secondary flow motions of both the fluid and particle phases. Three dominant flow regimes that include three sub-regimes based on their secondary flow patterns are identified, the transition between which depends on the combination of Φ m , e p − w , and e p − p . Additionally, the quantitative dependence of these transitions on these three parameters is also reported for a series of Stokes and Froude numbers. [ABSTRACT FROM AUTHOR]

Mathematical modeling is applied as an effective tool for prediction of cavitation in hydraulic components and systems. A multiphase mathematical model based on the change in phase between water and vapor is typically used to investigate the cavitation flow. However, dissolved air can significantly affect the cavitation. This study proposes a new approach based on a multiphase turbulent mathematical model by adding the air into the mixture to solve the dynamics of cavitation. To clearly assess the significance of air in the multiphase model, four variants of the mixture are investigated (water; water and vapor; water and air; and water, vapor, and air together). The software of the computational fluid dynamics ANSYS Fluent was applied to numerically solve the proposed mathematical models. The influence of gaseous components is analyzed through evaluation of hydraulic parameters and spectral characteristics of the cavitation bubble. To verify the proposed mathematical models, a hydraulic water circuit was built to generate cavitation in a transparent Venturi nozzle. Cavitation in the experiment was identified by measuring the flow rate, static pressure, and noise and visualized with a camera. The numerical results of the extended multiphase flow confirmed very good agreement with experimentally obtained basic hydraulic parameters and frequency-related characteristics. Knowledge obtained from the multiphase mathematical model of cavitation can be applied to cavitation in the oil flow (pump suction and flow through the valve) in future research, where the effect of the air on cavitation is more important than the effect of vapor. [ABSTRACT FROM AUTHOR]

The flow field of an impulsively started round, confined nitrogen jet was investigated using combined high speed schlieren imaging and particle velocimetry (PIV) measurements. PIV measurements were carried out at five different, normalized times (55 ≤ t * ≤ 392) relative to jet intrusion into a constant volume chamber. Between 100 < t * < 250, the NPR linearly increased to that for a moderately underexpanded jet (NPR ≈ 3.5). Distributions of the mean flow and Reynolds normal and shear stresses revealed two different stages in jet development. In stage I ( t * = 55–103), prior to clear shock cell appearance, the jet was characterized by a leading, toroidal vortex whose induced recirculatory motion inhibited the growth of the trailing jet's shear layer instabilities and radial spreading. In stage II ( t * = 196 and 392), the jet became moderately underexpanded (NPR ≥ 2) and close to the nozzle exit, flow characteristics resembled those of a "co-annular" jet. The co-annular region did not extend beyond 15 D. An analysis of instantaneous vortex numbers and strengths further supported the two identified stages in jet development and their connection to shear layer instability growth. Based on the distributions of mean flow and Reynolds stresses, it was shown that the static pressure gradient along the jet's centerline is mainly governed by the dynamic pressure gradient. Gradients of the Reynolds normal and shear stresses play a minor role. Important for gaseous fuel injection at high injection pressures, results point at limited mixing during stage I and enhanced mixing during stage II. [ABSTRACT FROM AUTHOR]

For a better understanding of the shock train structure and its dynamic oscillation features, wind tunnel experiments with linear and stepwise increase backpressure were conducted with supersonic isolator flows at Mach 1.85 and 2.70. High-frequency wall static pressure measurements were performed along the primary and corner regions to capture the behavior of the primary and corner shock train. The fine structures of the shock train were recorded using schlieren visualization with circular, horizontal, vertical, and color knife edges. The pressure results show that the shock train leading shock at Mach 2.70 is more three-dimensional. The flow field exhibits the following features near the corner: the pressure fluctuation amplitude is smaller, the shock train leading shock is closer to the upstream regions, and the oscillation of the shock train leading shock can propagate a longer distance downstream. Schlieren snapshots obtained using horizontal and vertical knife edges show shock train structures with alternating distributions of the vertical and horizontal density gradients. Further application of color knife edges clearly distinguishes these regions. The power spectra analysis of a series of schlieren snapshots was performed to characterize the dominant oscillation structures in the flow field with different visualization variables, and the motion relationship between the structures in the shock train was clarified based on the coherence and phase analyses of the schlieren images. The disturbance downstream first causes the movement of the shock train leading shock, and the closer the downstream shock is to the shock train leading shock, the earlier it moves. [ABSTRACT FROM AUTHOR]

In aeroengines, purge flow directly fed from the compressor (which bypasses the combustor) is introduced through the disk space between blade rows to prevent the hot ingress. Higher quantity of purge gas fed through the wheel space can provide additional thermal protection to the passage endwall and blade surfaces. However, the interaction of purge flow with the mainstream flow leads to higher secondary losses. Secondary losses inside a turbine blade passage can be reduced effectively by endwall contouring. This paper presents computational investigation on the influence of non-axisymmetric endwall contouring over endwall secondary flow modification in the presence of purge flow with the pressure side bubble (PSB). The experimental analysis was conducted for the base case without purge and base case with purge (BCP) configurations having flat endwalls. The total pressure loss coefficient and exit yaw angle deviation were measured with the help of a five-hole pressure probe. Static pressure distribution over the blade midspan was obtained by 16 channel Scanivalve. Aerodynamic performances of three different profiled endwalls are numerically analyzed and are compared against the BCP configuration. The effects of different contoured endwall geometries on endwall static pressure distribution and secondary kinetic energy were also discussed. Analysis shows that in the first contoured endwall configuration (EC1), the formation of stagnation zones at a contour valley close to the suction surface causes the exit total pressure loss coefficient to increase. The shifting of the contour valley near to the pressure surface (EC2 configuration) has resulted in local acceleration of the diverted pressure side leg of the horseshoe vortex over the hump toward the end of the passage. In the third configuration (EC3 configuration), reduced valley depth and optimum hump height have effectively redistributed the endwall pitchwise pressure gradient. The increased static pressure coefficient at the endwall near to the pressure surface has eliminated the PSB formation. In addition, computational results of unsteady Reynolds averaged Navier–Stokes simulations are obtained for analyzing transient behavior of PSB, with more emphasis on its migration on the pressure surface and transport across the blade passage. The additional work done by the mainstream fluid to transport the low momentum PSB fluid has caused higher aerodynamic penalty at the blade exit region. In this viewpoint, the implementation of contoured endwalls has shown beneficial effects by eliminating the PSB and related secondary vortices. At 27% of axial chord downstream of the blade trailing edge, a 4.1% reduction in the total pressure loss coefficient was achieved with endwall contouring. [ABSTRACT FROM AUTHOR]

The effect of thermodynamic and transport properties on the convective-to-absolute transition in heated round jets is investigated with the spatio-temporal linear stability theory, by considering three sets of properties with increasing complexity. Present models include (i) a constant property model often used in the literature, (ii) a simplified model with variable properties, and (iii) a more accurate equilibrium air mixture model, accounting for dissociation reactions in the flow. A family of arbitrary single-stream and dual-stream jet profiles, representative of typical configurations studied in the literature, is investigated and adapted to each model. Our results show that considering a variable viscosity and thermal conductivity has a destabilizing effect on absolute instabilities in the viscous regime. Furthermore, this destabilization is stronger for the outer mode in dual-stream jets than for the inner mode or the jet-column mode in single-stream jets. With sufficient heating (S < 0.3), results obtained with the equilibrium model strongly depart from those of calorically perfect gas models and display absolute domains deformed by the chemical activity. For absolute instabilities triggered by the baroclinic torque such as jet-column and inner modes, the convective-to-absolute transition is shifted toward thinner and hotter configurations, while the opposite is observed for the outer mode. Finally, we observe a dependence of the equilibrium model stability properties on the static pressure. [ABSTRACT FROM AUTHOR]

The influence of subsonic adiabatic choking on frictional resistance inside three-dimensional (3D) microchannels has not been studied for rarefied gas flows. In the present work, the variation of the Poiseuille number with respect to the Mach number has been documented for a 3D microchannel of aspect ratio (width/height) 0.49. Measurements of mass flow rate, static pressure, and temperature have been conducted with nitrogen in highly compressible and slightly rarefied (slip flow) regime: outlet Mach number (0.43–0.99), outlet Knudsen number (4.04 × 10−3 –7.04 × 10−3 ), and pressure ratio (8.17–8.72). The present 3D measurements are compared with available analytical solutions for isothermal and adiabatic flows. A maximum deviation of only 4.8% from the adiabatic slip flow solution points toward the adiabatic nature of the exit choked state, which is being experimentally demonstrated for the first time in the highly compressible slip flow regime. Furthermore, the influence of losses of microchannel end manifolds on the overall pressure drop is calculated to be negligible. We further propose the ranges of the area ratio, Reynolds number, and Knudsen number for which these losses continue to be unimportant for gaseous slip flow. This study gives insights into the influence of subsonic choking on the frictional resistance at various mass flow rates and is relevant for future space expeditions and in certain biological applications. [ABSTRACT FROM AUTHOR]

Passive noise control for a tandem NACA 65-710 airfoil configuration is experimentally investigated by applying leading-edge serrations on the rear airfoil. With a sliding side-plate mechanism that allows the rear airfoil to move in the vertical direction relative to the front airfoil, the position of maximum turbulence interaction noise is first identified from the far-field noise measurements. Subsequently, detailed static surface pressure distribution and unsteady surface pressure fluctuations are acquired to shed more light on the physical phenomenon and underlying noise-reduction mechanism of the leading-edge serrations. The far-field noise measurements confirm that a notable turbulence interaction noise reduction can be achieved from 600 Hz < f < 3000 Hz, agreeing well with the previous literature on the effectiveness of the leading-edge serrations. The near-field hydrodynamic analyses obtained using remote-sensing techniques of the fluctuating pressure fields over the airfoil show that a significant reduction in the surface pressure fluctuation levels up to 20 dB/Hz can be observed at the serrated-tip plane of the rear serrated airfoil close to the leading-edge regions, over the range of frequencies investigated. Although reduction can also be observed on the serrated-root plane, the magnitude is much less significant. The present results suggest that the modification of the unsteady loading on the rear airfoil by the leading-edge serrations plays a crucial role in the reduction of turbulence interaction noise in the tandem airfoil configuration, which may find practical application for noise reduction in aerodynamic systems involving rows of airfoils, such as contra-rotating open rotors and outlet guide vanes. [ABSTRACT FROM AUTHOR]

A predictive model is developed for the pressure loss coefficient for a viscous flow through a rectangular orifice on a pipe-installed thick plate. The model is developed based on the 1-dimensional Navier-Stokes equation and an asymptotic increase in velocity modeled to have a direct relation with the flow convergence in the near-inlet region. Here, the flow velocity increases asymptotically from the steady mean upstream value to the orifice velocity. This phenomenon is represented by a convergence parameter, ϕ , used in the velocity transition model to quantify the length of the convergence zone. The static pressure drop is measured experimentally for varying orifice aspect ratio, AR, at creeping Reynolds numbers (0.01 ≤ Re ≤ 0.1). A significantly wider range of AR is covered (1 ≤ AR ≤ 250), compared to related works in the literature. Results show that the relative dominance of the convergence phenomenon is affected by AR. The maximum length of convergence is for the square orifice (AR = 1), as the flow experiences comparable convergence from all directions, whereas for higher AR, convergence becomes less dominant in one of the two midplanes of investigation. The loss coefficient thus decreases as AR increases. At constant Re, higher AR generally leads to higher pressure drop but lower values of the loss coefficient. The velocity gradient in the convergence zone is also determined as a function of AR and Re which verifies that lower AR takes a longer distance for the velocity transition due to increased convergence. [ABSTRACT FROM AUTHOR]

In this paper, Navier-Stokes equations were solved with high-order accurate schemes to investigate the basic structure and regularity of the flow field during the interaction of a supersonic jet and a codirectional supersonic incoming flow. A double backward-facing step model was proposed to investigate the interaction between the jet/supersonic incoming flow shear layers. The two shear layers interact to produce a secondary jet. The secondary jet produced by the action has a unique periodicity that is related to the overall oscillation of the shear layer. The secondary jet is generated when the horizontal angle of the jet shear layer reaches a certain value. This paper focused on the analysis and discussion of the periodicity of the secondary jet. When the aspect ratio is different, the period of the secondary jet changes significantly. However, when the static pressure ratio is different, the period of the secondary jet does not change much. [ABSTRACT FROM AUTHOR]

A major aim of the present work is to understand and thoroughly document the generation, the three-dimensional distribution, and the evolution of the secondary motion as the fluid progresses downstream through a branched network. Six generations (G0-G5) of branches (involving 63 straight portions and 31 bifurcation modules) are computed in one go; such computational challenges are rarely taken in the literature. More than 30 × 106 computational elements are employed for high precision of computed results and fine quality of the flow visualization diagrams. The study of co-planar vis-á-vis non-planar space-filling configurations establishes a quantitative evaluation of the dependence of the fluid dynamics on the three-dimensional arrangement of the same individual branches. As compared to the secondary motion in a simple curved pipe, three distinctive features, viz., the change of shape and size of the flow-cross-section, the division of non-uniform primary flow in a bifurcation module, and repeated switchover from clockwise to anticlockwise curvature and vice versa in the flow path, make the present situation more complex. It is shown that the straight portions in the network, in general, attenuate the secondary motion, while the three-dimensionally complex bifurcation modules generate secondary motion and may alter the number, arrangement, and structure of vortices. A comprehensive picture of the evolution of quantitative flow visualizations of the secondary motion is achieved by constructing contours of secondary velocity ∣→vS∣ , streamwise vorticity ωS, and λ2 isosurfaces. It is demonstrated, for example, that for in-plane configuration, the vortices on any plane appear in pair (i.e., for each clockwise rotating vortex, there is an otherwise identical anticlockwise vortex), whereas the vortices on a plane for the out-of-plane configuration may be dissimilar, and there may even be an odd number of vortices. We have formulated three new parameters (ES/P, δSF, and δGn) for a quantitative description of the overall features of the secondary flow field. δSF represents a non-uniformity index of the secondary flow in an individual branch, ES/P represents the mass-flow-averaged relative kinetic energy of the secondary motion in an individual branch, and δGn provides a measure of the non-uniformity of the secondary flow between various branches of the same generation Gn. The repeated enhancement of the secondary kinetic energy in the bifurcation modules is responsible for the occurrence of significant values of ES/P even in generation G5. For both configurations, it is found that for any bifurcation module, the value of ES/P is greater in that daughter branch in which the mass-flow rate is greater. Even though the various contour plots of the complex secondary flow structure appear visually very different from one another, the values of δSF are found to lie within a small range (0.37 ≤ δSF ≤ 0.66) for the six-generation networks studied. It is shown that δGn grows as the generation number Gn increases. It is established that the out-of-plane configuration, in general, creates more secondary kinetic energy (higher ES/P), a similar level of nonuniformity in the secondary flowin an individual branch (similar δSF), and a significantly lower level of non-uniformity in the distribution of secondary motion among various branches of the same generation (much lower δGn), as compared to the in-plane arrangement of the same branches. [ABSTRACT FROM AUTHOR]