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Nanoparticles (NPs) can be highly beneficial as additives to lubricating fluids, and the tribotronic response of charged NPs tuned by external fields represents an area of great technological potential. Tribotronic response, however, is expected to be highly size dependent, which represents a significant design challenge. To explore this issue, quartz crystal microbalance and cyclic voltammetry were employed to characterize nanotribological and electrochemical behavior of platinum–nanofluid interfaces formed by aqueous suspensions of different-sized negatively charged titanium dioxide (TiO2) NPs. Suspensions of 5, 40, and 100 nm NPs were all observed to reduced interfacial frictional drag forces upon introduction into pure water in zero field conditions, with reductions for the 40 nm NPs about twice those of 5 nm particles at comparable concentrations. Suspensions of 100 nm NPs produced even greater reductions, but rapidly precipitated from the suspension when left unstirred. NPs were also driven to and from Pt electrode surfaces by applying external electric fields with varying amplitudes and modulation frequencies. For electric fields of sufficient amplitude and duration, the 40 nm TiO2 nanosuspension exhibited tribological properties consistent with a reversible electrophoretic deposition of the NPs, accompanied by changes in the electrochemical attributes and increasing interfacial drag. The 5 nm NP properties were consistent with progressive reductions in interfacial drag forces at the NP–suspension interface linked to field-induced increases in concentration. [ABSTRACT FROM AUTHOR]

Flow around a near-wall circular cylinder with the splitter plate is numerically performed at Reynolds number of 500, with the objective of investigating the wake characteristics and hydrodynamic forces. Five gap ratios G ∕ D = 0. 1 , 0. 3 , 0. 5 , 0. 7 and 0. 9 (G is the gap between the lower surface of the cylinder and the wall, D is the diameter of the cylinder) are selected, and the splitter plate length L ∕ D ranges from 0 to 4. 5. The flow characteristics of an isolated cylinder with the splitter plate are investigated first for comparison, and four wake flow modes are observed, which include 2S mode (L ∕ D ≤ 0. 1 5), P+S mode (0. 3 ≤ L ∕ D ≤ 1. 0), 2S+S mode (1. 2 5 ≤ L ∕ D ≤ 2. 0) and 2P mode (L ∕ D ≥ 3. 0). As L ∕ D increases from 0 to 0. 7 5 , the mean drag coefficient ( C D ¯) is decreased, and there is a slight increase of C D ¯ for 0. 7 5 < L ∕ D ≤ 1. 2 5. In addition, the cases of L ∕ D = 0. 7 5 and L ∕ D = 1. 0 produce a very significant reduction of drag, and the C D ¯ is reduced by as much as 3 9 % and 3 8 % , respectively. The wake characteristics and hydrodynamic forces of a near-wall cylinder with the splitter plate are investigated in detail, and five wake regimes are observed, which include the wake vortex merging regime I, merged vortex attaching regime II, steady flow regime III, wall shear layer elongation regime IV and upper shear layer attaching regime V. For G ∕ D = 0. 1 , the wake vortex shedding is suppressed. For G ∕ D ≥ 0. 3 , the Strouhal number (St) is decreased as L ∕ D increases from 0 to 1.0, and there is an increase of St at L ∕ D = 1. 2 5. At L ∕ D ≥ 1. 2 5 , the St of the near-wall cylinder is larger than that of the isolated cylinder, and the increase in St is affected by the deflected gap flow. What is more, the hydrodynamic characteristics are affected by the wall. For G ∕ D > 0. 1 , the variations of C D ¯ with L ∕ D are similar to that of the isolated cylinder. It is found that the cases of L ∕ D = 0. 7 5 and L ∕ D = 1. 0 still produce a significant reduction of drag for G ∕ D > 0. 3 , and the C D ¯ is increased for all cases of L ∕ D as G ∕ D increases. [ABSTRACT FROM AUTHOR]

Flow-like events usually occur during heavy rainfall and pose significant threats to ecosystems and human life and property because of their suddenness, high speed, and long distances. To study the solid–fluid transition and subsequent high fluidity and hypermobility mechanism of redeposited loess, we conducted a series of flume tests and rheological tests, and the results showed that loess exhibits different degrees of fluidized movement characteristics under different rainfall intensities, and a rainfall intensity of 90 mm/h was the most likely to trigger loess flowslides. Additional rheological analyses indicated that viscosity (shear rate) bifurcation characterizes the rheological response of loess solid–fluid transition, and the decrease in viscosity caused by shear thinning can explain the drag reduction effect and its high fluidity in the process of solid–fluid transition, which corresponds well with the results of flume tests. We proposed a two-step yielding characteristic and introduced structural dynamics to establish a unified solid–fluid transition model incorporating a hydro-mechanical coupling and rheological property. The model can be used to describe both the solid-like behavior of soil before phase transition using an elastoplastic model and fluid-like behavior after phase transition using a viscoplastic model. The research results provide a new understanding of solid–fluid phase transition characteristics of loess from the perspective of rheology, which can also provide a new idea for studying the fluidization movement of rock avalanches and pyroclastic flows and their geomorphic evolution. [ABSTRACT FROM AUTHOR]

Response surface methodology predicts the best condition of the parameters that are critical in achieving desired goal and is the statistical analysis carried out to obtain the optimized conditions of parameters. The model for the examination is developed for the flow of Casson-Carreau nanofluids over stretched curved sheet swayed by the magnetic dipole. The sheet is liable to radiation and second-order slip and melting heat conditions is contemplated. The current study reveals that the velocity regime decreases with decreasing slip effects, and the ferrohydrodynamic interaction parameter increases with increasing slip effects. Additionally, the heat dissipation parameter influences the thermal profile. By response surface methodology analyzed on the skin friction coefficient reveals that histogram for the experimental runs is normally distributed and the R 2 is 100% promoting the accuracy of the model designed. Pareto chart has given the picture of 2.2 to be the critical point for the three parameters under consideration. The magnetic and porous parameters exhibit negative sensitivity at low and medium L s 1 values, while at L s 1 = 0.4 , they demonstrate no sensitivity or negligible sensitivity. The first-order slip parameter exhibits a positive sensitivity at low and medium concentrations, but a negative sensitivity at high concentrations of C . [ABSTRACT FROM AUTHOR]

Because of its superior thermal-hydraulic qualities, liquid sodium has been applied to a variety of industries, including energy storage, solar energy, sodium-cooled fast reactors, and aerospace. However, fires brought on by sodium leaks at high pressure can have major thermodynamic repercussions and put employees and equipment in use at risk directly or indirectly. As a result, a realistic and accurate forecast of the combustion behavior of sodium droplet swarm can offer technical backing for the use of liquid sodium in engineering as well as a way of sodium fire prevention and control. Spray dynamics (droplet settling, droplet particle size distribution, etc.), combustion kinetics (premixed combustion, gas phase combustion, etc.), sodium aerosol diffusion, and other specialized phenomena all contribute to the complex process of sodium droplet swarm combustion. The NACOM code created by Brookhaven National Laboratory for sodium droplet swarm combustion is utilized in this paper as a framework. The code is first validated using the benchmark of the sodium droplet swarm combustion tests carried out by prestigious institutions. The validation results demonstrate that the code's drag model, droplet combustion model, and heat transfer model are to blame for the significantly overestimated thermodynamic effects of sodium droplet swarm combustion. NACOM is subsequently developed twice for the authors' previously developed vapor-liquid two-layer-structure drag model, chemical kinetic combustion model, and suitable heat transfer coefficient. It is then thoroughly assessed for the separate-effects tests and integral-effects test. The evaluation results demonstrate that the optimized drag model accelerates the settling of sodium droplets due to the consideration of the sodium-vapor drag reduction effect, reducing the thermodynamic effects of liquid sodium combustion; the optimized premixed combustion model can accurately predict the low-temperature sodium droplet swarm combustion conditions, resolving the issue of serious misvaluation of the original version of NACOM. The associated research findings can serve as valuable resources and tools for deeper comprehension of the combustion effects and mechanisms of sodium droplet swarm under various operating settings (such as leakage rate and oxygen concentration). [ABSTRACT FROM AUTHOR]

Multi-blade centrifugal fans find wide application across various fields, with their internal airflow exhibiting complex turbulent behavior in three dimensions. Historically, blade optimization relied on constant thickness airfoils, limiting the effectiveness of optimization efforts. However, marine organisms have developed airfoil structures with highly efficient drag reduction, offering a novel approach to optimizing multi-blade centrifugal fans. This study proposes an airfoil optimization design method utilizing variable-thickness airfoils to maintain consistent pressure surface leading-edge parameters. By integrating the Non-dominated Sorting Genetic Algorithm II with biomimetic optimization design, performance improvements are achieved. The study constructs an asymmetric bionic blade using a Bezier curve to fit the mean camber line of the blade. Experimental testing validates the optimized fan's performance, demonstrating the effectiveness of the proposed design approach in reducing the unsteady interaction between the impeller and the volute tongue. This reduction significantly diminishes sound pressure fluctuations on the blade surface. Notably, at the maximum volume flow rate, the optimized fan featuring the asymmetric bionic blade exhibits a remarkable enhancement, with a 10.5% increase in volume flow rate and a notable 1.7 dB reduction in noise compared to the original fan configuration. [ABSTRACT FROM AUTHOR]

On a flat road, at race speeds, aerodynamic drag is the main resistive force a cyclist must overcome. Computational fluid dynamics (CFD) can be a useful tool to predict and understand the complex flow and, therefore, drive developments to reduce drag. However, cycling aerodynamics is complex. The effects of Reynolds number, surface roughness, boundary layer transition, flow separation, and turbulent wakes are challenging to accurately predict. High fidelity time-resolved computations, such as Large eddy simulations (LES), require high-performance computing and lengthy simulation times. This paper examines whether lower fidelity CFD, such as Reynolds averaged approaches, can predict the drag of a cyclist with sufficient accuracy and within practical timescales on a desktop PC. Wind tunnel tests of a rider model (without bicycle) were conducted at Reynolds numbers equivalent to speeds of ~ 20–70 km/h. Measured drag showed a notable Reynolds number dependency with the drag coefficient reducing almost linearly by ~ 20% from 0.88 to 0.71. The computational accurately replicated this relationship but only when employing a boundary layer transition model. The steady computations underpredicted the magnitude of the measured drag coefficient by ~ 3% but the unsteady computations were within ~ 2%. Examination of the predicted flow field revealed variations in boundary layer transition, separation, and wake formation from each body part which combine in a complex wake system. Overall, the data confirm validity and suitable accuracy of the CFD, and therefore this provides a practical time and cost-effective tool for further examination of drag reduction within cycling. [ABSTRACT FROM AUTHOR]

Riblet surface is a biomimetic surface that not only has the ability to reduce surface drag resistance of mechanical products in fluids, but also improves surface hydrophobicity. Therefore, studying its manufacturing technology has practical significance for solving its engineering applications. In order to grind the riblet surface on the cylindrical workpiece, an innovative hob-grinding strategy for reducing drag on cylindrical riblet surfaces using superhard grinding wheels with spiral abrasive pattern has been developed. To achieve this strategy, firstly, based on the analysis of the geometric topology features of the riblet surface, an engineered CBN grinding wheel with spiral arrangement of abrasive particles was designed based on the principle of gear hobbing machining. Then, the relationship between hob-grinding parameters and abrasive particle arrangement parameters on the parameters of the ground riblets was analyzed, and the process measures to increase the number of riblets on the cylindrical surface, the ratio of the height to spacing of riblets, and the impact on the surface geometry of the riblets were explored. Finally, the surface of the cylindrical riblet surface was ground through experiments, and under the conditions of this experiment, a riblet surface was obtained that its spacing, height, and ratio of the height-spacing were 93.85 µm, 14.65 µm, and 0.156, respectively. The research results indicate that using this strategy can grind the cylindrical riblet surface, and the proposed process strategy of increasing the number of riblets and the ratio of the height to spacing is feasible. [ABSTRACT FROM AUTHOR]

In recent years, hydrodynamic invisibility cloaks have attracted significant attention from the scientific and engineering communities due to their potential in applications such as fluid flow manipulation, drag reduction, submarine stealth, and biomedical engineering. However, cloaks based on transformation mapping theory typically require porous medium flow, limiting practical implementation. To address this, we draw inspiration from Hele-Shaw flow and develop a homogeneous hydrodynamic cloak composed of multiple layers in free fluid flow at low Reynolds numbers (R e ≪ 1). Through structural optimization, we construct a cloaking shell with ten concentric layers of alternating heights, achieving near-perfect cloaking as demonstrated by simulations and experiments. This non-porous cloak design offers valuable insights into the rapidly advancing field of hydrodynamic metamaterials and enables methods for controlling fluid flow at the microscale, with promising applications in microfluidics. [ABSTRACT FROM AUTHOR]

A turbulent pipe flow experiment was conducted where the surface of the pipe was oscillated azimuthally over a wide range of frequencies, amplitudes, and Reynolds numbers. The drag was reduced by as much as 35%. Past work has suggested that the drag reduction scales with the velocity amplitude of the motion, its period, and/or the Reynolds number. Here, we find that the key parameter is the acceleration, which greatly simplifies the complexity of the phenomenon. This result is shown to apply to channel flows with spanwise surface oscillation as well. This insight opens potential avenues for reducing fuel consumption by large vehicles and for reducing energy costs in large piping systems. [ABSTRACT FROM AUTHOR]

Deep-pine operation is often required for the high-power tractor for traction at present, leading to low operating efficiency. More importantly, the total power consumption of vibratory deep-pine operation is ever-increasing with the increase of vibration frequency. Therefore, there is an urgent need to carry out research on the high-efficiency vibratory deep-pine machine for high operational efficiency and low energy loss. In this study, a deep loosening machine was designed to cooperate with the cutting and local vibration. The loosening mode was adopted for the cutting first and vibration second, i.e. the front deep loosening shovel was fixed to break the soil. The angle of entry was always in the position of the lowest soil resistance. The rear shovel was vibrated around the shaft to loosen the soil. The soil resistance was reduced to increase the range of loosening for less crushing of soil, in order to effectively reduce the consumption of vibration energy. The discrete element method (DEM) was used to analyze the effect of cut-and-vibrate deep loosening on soil disturbance. Hertz Mindlin with Bonding model in EDEM was used to establish the contact model among soil particles. The cut-and-vibrate and traditional deep loosening were simulated to compare at the same time. The experimental model of the deep-pine shovel was established using SolidWorks2022 software and then saved in a format suitable for EDEM. The model was also imported into EDEM for constraining, driving, and simulation. The motion state of the simulation model was divided into two strokes of breaking and vibratory deep loosening, according to the actual vibratory during deep loosening. The cut-and-vibrate co-operative mode effectively reduced the tillage resistance of the deep-pine machine; The one-factor test showed that the optimal intervals operating speed and vibration frequency were 0.9-1.1 m/s, and 9-15 Hz, respectively; The orthogonal test demonstrated that the optimal parameters were obtained for the deep-pine operation: operating speed 0.96 m/s, tillage depth 300.24 mm, and vibration frequency 11.43 Hz. The actual performance of drag reduction was tested on the cut-and-vibrate deep-threader. The simulation was also combined with the best operating parameters of deep-threading. The field tests were conducted, where the forward speed of the tractor was set at 1.0 m/s, the operating depth was 300 mm, and the vibration frequency was set at 9, 12, and 15 Hz. The field test showed that the cut-and-vibrate deep pine machine shared the outstanding drag reduction under the same speed and plowing depth, compared with the non-vibrating deep pine machine. The plowing resistance was reduced by 15.43%-37.56%; The power consumption was reduced by 8.87%-32.32%. The drag reduction was the most significant when the vibration frequency was 12 Hz. The plowing resistance was reduced by 37.56%, the power consumption was reduced by 32.32%, the solidity of the soil was reduced by 74.11%, and the soil disturbance coefficient was 56.32%, and the deep pine depth of 309 mm, deep pine was reduced by 15 Hz. The stability coefficient for the deep pine depth of 309 mm was 98.16%. The deep pine depth and its stability were in line with the industry standard. The cutting and vibration co-production deep pine machine shared outstanding performance to reduce the resistance and consumption. The finding can provide a new reference to optimize the vibration deep pine machine. [ABSTRACT FROM AUTHOR]

A fundamental understanding of wet clutches' drag loss behavior is essential for designing efficient clutch systems. It has been widely recognized that the separation behavior immediately after disengaging the clutch and the resulting clearance distribution influence the drag loss behavior. However, these influencing factors have yet to be systematically investigated. Therefore, this study aimed to experimentally investigate the effects of plate separation and clearance distribution on drag loss behavior under different operating conditions and modes. For this purpose, image series of an operating clutch system were recorded and subsequently analyzed using image processing algorithms to evaluate the movements of the plates. Based on this, the effects on drag loss behavior were analyzed. The investigations were carried out on a clutch system used in an industrial application. The measurements show that the axial movements of the plates comprise main and superimposed non-periodic movements of much a smaller amplitude. The separation of the plates is primarily driven by the applied differential speed so that the set total clearance is only utilized mainly in the higher differential speed range. The separation behavior, therefore, decisively influences the drag loss behavior. The plates can even remain in contact in the low differential speed range. The investigations also showed that the separation behavior and, thus, the drag loss behavior can be improved by using waved plates, especially in the low differential speed range. It was also found that a high plate number and a large set total clearance support a non-uniform clearance distribution. Based on the investigations conducted, it is possible to expand our fundamental knowledge of separation behavior and clearance distribution, allowing for a reduction in the drag losses of wet clutches. The findings can thus contribute to the development of low-loss and compact clutch systems. [ABSTRACT FROM AUTHOR]

The article delves into the use of patterned heating to minimize pressure losses in sloping channels by creating separation bubbles that reduce friction and induce fluid rotation. Factors like Reynolds number, heating wavelength, and channel inclination impact the effectiveness of heating in reducing pressure losses. While judicious heating can significantly decrease pressure losses, excessive heating may lead to increased energy costs. The study suggests that heating can be a viable method for drag reduction in specific channel orientations, emphasizing the potential for energy efficiency and flow rate optimization through patterned heating. [Extracted from the article]

The article delves into the computational analysis of deformable fluid particle migration in viscoelastic fluid within a square channel under pressure-driven flow. Factors such as particle deformability, fluid elasticity, shear thinning, and fluid inertia impact the migration dynamics. Varying shear thinning alters the direction of particle migration, with deformable particles moving towards the wall in shear-thinning fluids. The study also uncovers elastic flow instability, path instability, and secondary flow features induced by particle migration, shedding light on the intricate interplay between fluid properties and particle behavior in microfluidic systems. [Extracted from the article]

The article in the Journal of Fluid Mechanics examines the use of a wall-attached ferrofluid film to reduce drag in turbulent channel flow. Experiments with water as the working fluid revealed unstable waves at the ferrofluid-water interface, showing potential for up to 95% drag reduction, particularly with smaller wave amplitudes. The study emphasizes the effectiveness of the ferrofluid coating in reducing drag by analyzing interface stability and its impact on flow characteristics, shedding light on the complex relationship between slip velocity, interface waviness, and drag reduction efficiency. [Extracted from the article]

The article "Direct numerical simulation of drag reduction in rotating pipe flow up to" delves into the impact of axial rotation on drag reduction in pipe flow up to Reynolds number Reτ ≈ 3000. The research demonstrates that drag reduction can reach up to 70% with higher Reynolds and rotation numbers, without causing flow relaminarization. The study emphasizes how rotation alters turbulent structures, widens streaks, and suppresses large-scale turbulent motions, potentially reducing energy consumption in high-Re internal flows like fluid transportation in large pipelines. The findings shed light on the role of rotation in enhancing drag reduction in axially rotating pipe flow, offering valuable insights into practical applications and challenges. [Extracted from the article]

Direct numerical simulations are performed to investigate the characteristics of a turbulent core–annular flow with water-lubricated high viscosity oil in a horizontal pipe. Six different superficial velocity ratios ($\,j_w/j_o = 0.057\unicode{x2013}0.41$) are examined by changing the water superficial velocity $j_w$ for a fixed oil superficial velocity $j_o$. The pressure drops in the pipe and the shapes of the phase interface agree well with those from previous experiments. The oil core flow is almost a plug flow, and the gaps between the phase interface and pipe wall are narrow and wide near the upper and lower surfaces of the pipe, respectively, due to the buoyancy. Within a narrow gap, water is confined mostly in a valley region of the wavy phase interface and hardly goes through its crest. On the other hand, water near the phase interface at a wide gap convects downstream almost at the core speed and the flow near the wall is similar to that of single-phase wall-bounded turbulent flow. The annular flow is characterized by three different regimes depending on the clearance Reynolds number ($Re_c$) based on the core velocity and local gap size: laminar Couette flow driven by the core for $Re_c \le 600$ , transitional flow for $600 and turbulent flow for $Re_c \ge 2500$. The minimum pressure drop occurs at $j_w/j_o = 0.11$ in the early transition regime. For all $j_w/j_o$ considered, the negative lift force acting on the core comes from the pressure force which balances the buoyancy. [ABSTRACT FROM AUTHOR]

Harmonic oscillations of the walls of a turbulent plane channel flow laden with inertial particles are studied by point-particle direct numerical simulation to improve our understanding of the physical mechanism for friction drag reduction. We specify a high wall oscillation amplitude and choose particle parameters that feature a considerable drag-reduction effect. The particle effect on the drag modulation is investigated by varying the wall oscillation period ( T + ) across a wide range. We find that particles enhance drag reduction for T + ≤ 30 while attenuating it for T + > 30. Specifically, we observe drag increase near the optimal oscillation period, i.e., T + = 50 and 75. To explore the coupling mechanism of drag modulation by particles and spanwise wall oscillations, we examine the modifications of turbulence and particle statistics. Moreover, the self-sustaining cycle of near-wall turbulence is modulated by wall oscillations and/or particles. We find that the quasi-streamwise vortices are tilted and weakened by wall oscillations while significantly depopulated by particles. The modulated turbulence also affects the near-wall particle accumulation and clustering patterns, which results in different fluid–particle interactions compared with the non-actuated particle-laden channel flow. The mechanism of drag modulation is governed by the competition between reduced fluid Reynolds shear stress and particle stress. To give a quantitative view of the drag modulation, we compare the contributions of different stress components to the friction drag coefficient. Although the fluid Reynolds shear stress is reduced by particles, which corresponds to reduced turbulent kinetic energy production, the particle stress contribution remains significant, especially for the drag-increase case. Furthermore, the anisotropy invariant maps are provided, which show a striking resemblance of increased near-wall turbulence anisotropy as observed in other drag-reduced flows. This suggests that the constraint of increased turbulence anisotropy might be only a necessary but not sufficient condition for achieving drag reduction since this constraint is satisfied in our drag-increase case. [ABSTRACT FROM AUTHOR]

Riblets can be potentially employed to passively reduce the turbulent friction drag. However, the drag reduction performance of riblets does not currently meet expectations, which could assist in emission reduction and energy conservation in green transportation. This study proposes and validates a topological form of hierarchical nested riblets (HNR) that significantly enhances the drag reduction performance. To explore the drag reduction enhancement mechanism, direct numerical simulations are performed for flow simulation on the riblet surface under different Reynolds numbers. The results show that under the riblet dimensionless spacing of the riblet s + ≈ 21 , the drag reduction performance of the HNR surface improves by about 70% compared to that of the uniform riblet surface, which is inspired by shark skin. From the perspective of turbulence statistics, the HNR surface reduces the turbulent mixing near the wall, weakening the momentum transfer. Furthermore, the transient flow field shows that the secondary riblet in HNR prevents some turbulent flow and streamwise vortices from entering the groove, considerably weakening the dispersive stress induced by the secondary flow. Moreover, owing to the influence of the secondary riblet, small-scale turbulence develops and strengthens into large-scale turbulent motion, which is advantageous to the boundary layer flow and results in drag reduction improvement. [ABSTRACT FROM AUTHOR]

In this study, the wall shear stress in the channel flow of polyacrylamide polymers was investigated through electrochemical experiments, and the effects of the polymer concentration were evaluated at different Reynolds numbers. The objective was to explore the relationship between the changes in the drag reduction and near-wall turbulence structure induced by the polymer. The experiments were conducted using polymer concentrations of 0, 20, 50, 100, and 150 ppm, and a drag reduction of approximately 23% was achieved at a bulk Reynolds number of R e b = 18 750. In the electrochemical method, the working electrode was arranged spanwise, and simultaneous measurements were performed for eight electrodes to discuss the scale of the near-wall low-speed streaks and burst events. A comprehensive analysis of the correlation of the wall shear stress in the streamwise direction and the cross-spectrum of two points in the spanwise direction revealed that large streamwise and spanwise scales of near-wall low-speed streaks were generated at high polymer concentrations. Furthermore, the results obtained using the variable interval time averaging technique indicated that polymer incorporation suppressed the wall shear stress fluctuations and weakened both the intensity and frequency of the bursting events. [ABSTRACT FROM AUTHOR]

Using riblet structures to modulate the turbulent boundary layer to reduce frictional drag is important for vehicle stability and range. The flow on the airfoil surface at different angles of attack is different, and the effect of the riblets structure on the turbulent boundary layer flow will be changed, which in turn affects the drag reduction effect. In this paper, the drag reduction characteristics of V-shaped riblets are investigated experimentally by the particle image velocimetry technique. The distributions of mean velocity, friction coefficient, drag reduction rate, and coherent structure in the boundary layer on the airfoil surface at angles of attack from 0° to 12° are analyzed, and the effects of smooth and riblets surfaces on the turbulent flow characteristics near the wall are compared. Based on these results, the airfoil surface was zonally covered with riblets to investigate further the effects of the riblets' covering positions on the drag reduction of the airfoil at different angles of attack. The results show that the riblets reduce the intensity, distribution, and probability of the eject and sweep events in the turbulence bursts by suppressing the velocity fluctuations at the near-wall surface, thus realizing the reduction of turbulence drag. The zoned arrangement of riblets can effectively regulate the airfoil surface flow and reduce the unfavorable effects of the riblets on the laminar flow region and flow separation region on the surface of the airfoil with different angles of attack, and the results of the study can provide a reference for the influence of the coverage range of the riblets on the drag reduction effect. [ABSTRACT FROM AUTHOR]

This study numerically investigates the impact of micro-blowing/suction parameters on the flow and heat transfer characteristics. Different models and parameters are evaluated by energy saving rate and comprehensive performance coefficients. The numerical results indicate that micro-blowing reduces the wall friction drag, while micro-suction increases the wall friction, and the magnitude of drag reduction (or drag increase) increases with the augment of the micro-blowing/suction coefficient. The maximum drag reduction rates of the three models are 2.60%, 3.59%, and 3.83%, respectively. When micro-blowing and micro-suction are arranged at the bottom and top of the channel, respectively, or both arranged at the bottom of the channel, the Nusselt number increases by at least 69.75% and 89.45%, respectively. The arrangement of micro-blowing and micro-suction both arranged at the bottom of the channel is optimal for heat transfer and drag reduction. In addition, with the present three arrangements, energy saving in the channel flow can be achieved. [ABSTRACT FROM AUTHOR]

Existing research on reflux self-excited oscillating nozzles (RSONs) has primarily focused on flow drag reduction and combustion mixing, with relatively little investigation of their impact on cavitation. This study employs the large-eddy simulation framework to conduct numerical simulations of the three-dimensional cavitating jet generated by an RSON. We analyze the impact of vortex dissipation and the nozzle throat structure on the cavitation phenomena and the evolution of vortex structures. Further analysis examines the impact pressure, pulse frequency, cavitation phenomena, and distribution patterns of vortex structures in the flow field for RSONs and an organ pipe nozzle under inlet pressures of 7, 14, and 21 MPa. The results show that the dissipation of spanwise vortices is jointly determined by the shape of the nozzle outlet and the intensity of vortex structures, with nozzles featuring a reflux structure producing faster dissipation. The main frequency of jet pulsation initially increases and then decreases with the development of the jet. The impact pressure of the jet is closely related to the intensity of the cavitation cloud and the location of its collapse. The RSON with a throat structure produces the maximum impact pressure near the nozzle outlet. This study deepens our understanding of the RSON flow field characteristics and provides a scientific basis for RSON applications in a broad range of fields. [ABSTRACT FROM AUTHOR]

Deviation from the traditional no-slip boundary condition due to factors like surface roughness and wettability is of paramount importance in microfluidics and nanofluidics, as it is attributable to its significance in drag reduction, flow control and enhancement and improved mixing. Augmentation in mixing, in turn, is known to strongly correlate with potential instabilities in the flow structure. Reported research studies indicate that slip is an inherent flow stabilizer in microfluidics, to the extent that with sufficient slip, the flow becomes linearly stable against all wavelike disturbances for all wavelengths and Reynolds numbers ["The linear stability of slip channel flows," Phys. Fluids 34,074103(2022)]. Contrary to such intuitive proposition, here we show that slip effects can destabilize microchannel flows under spanwise rotation, delving on the interplay of rotational forces and slippery hydrodynamics. Our results reveal that increasing the slip length decreases the critical rotation speed, indicating lower rotational effort required to destabilize the flow, whereas the critical Reynolds number for the flow remains effectively unaltered for different slip lengths in a spanwise rotating system. As the slip length increases progressively, the critical rotation number (dimensionless rotational speed) for the onset of instability decreases further, then remains constant up to a certain limit, and subsequently declines with additional enhancement in the slip length. This indicates the potential for deploying customized hydrophobic (slippery) substrates to facilitate transitions from stable to unstable modes by simple tuning of the rotational speed—a paradigm that offers great promise in various applications ranging from materials synthesis to biomedical technology. [ABSTRACT FROM AUTHOR]

This experimental study investigates the fluid dynamic function of butterfly scales to reduce friction drag below that experienced in a laminar boundary layer. A previous study on flight-testing live monarch butterflies showed that the removal of the scales from the wings had a significant effect on reducing flying efficiency. Monarch butterfly scales have a typical size of 0.1 mm in length, where the tips of the scales extend slightly upward to create microscopic grooves within a roof-shingle patterned surface. The hypothesis was tested that flow passing transverse to the grooves, for very low Reynolds number flow (Red < 10, based on cavity depth) as would occur on a butterfly wing, would result in the formation of cavity vortices that result in a partial slip condition to the outer boundary layer flow, resulting in reduced surface or skin friction drag. This flow phenomenon is designated the "roller-bearing effect." Tow tank studies, using both water and high viscosity mineral oil to vary the Reynolds number, measured the flow over various butterfly scale inspired grooves, or transverse cavity, models. A flow visualization study first proved the presence of an entrapped vortex inside each cavity that can act as a fluidic bearing. Drag measurements, using a momentum analysis of the velocity field obtained by digital particle image velocimetry, confirmed the presence of sub-laminar drag at low Reynolds numbers for multiple groove geometries. Results also confirmed that drag reduction is lost, as the Reynolds number increases an order of magnitude, due to instabilities developing for the embedded cavity flow. [ABSTRACT FROM AUTHOR]

This paper investigates the effects of introducing wavy cylinders of appropriate wavelength into tube bundles for the passive control of flow-induced vibration. The Reynolds-averaged Navier–Stokes approach RANS was used to study the turbulent flow characteristics within staggered tube bundles having transverse and longitudinal pitch-to-diameter ratios of 3.8 and 2.1, respectively. Two wavy models are tested with different wavelength-to-mean diameter ratios, namely: Model-A with λ / D m = 2 and Model-B with λ / D m = 1 , and their performances are compared with those of a smooth circular tube bundle Model-C (λ / D = ∞), in a range of subcritical Reynolds numbers 0. 5 × 1 0 4 ≤ Re ≤ 4. 1 × 1 0 4 . Mean velocity and pressure fields as well as aerodynamic coefficients are presented and compared. According to the obtained results, the wavy tube bundle with λ / D m = 2 exhibits the best performance, with slightly smaller C D values than the smooth tube bundle. However, results show that the wavy model with λ / D m = 1 is clearly the worst with an increase of C D up to 40%. Numerical outcomes also reveal that the interaction of neighboring rows and columns affects the fluctuation of C D values relative to each tube position. Moreover, C D evolutions depend significantly on the variation of Re as well, where drag reduction is more substantial by increasing Re. Thus, wavy-shaped tubes with optimum parameters could be more suitable for flow control by enhancing the secondary flow, turbulence and fluid mixing. [ABSTRACT FROM AUTHOR]

Summary Biological materials are often characterized by the fact that their structuring is at least equally, sometimes even more important for generating certain properties or property combinations than their chemical composition. This is not only the case for volume properties, but also for the skin of animals or the surface of leaves or blossoms. Biological structures for hydrophobization or antireflection of surfaces, adaptive coloring, drag reduction and increase of adhesion have been selected in this article as examples how such structures can be transferred (by means of bio‐inspiration) to the development of new materials. [ABSTRACT FROM AUTHOR]

The Tarim area, characterized by deep reservoirs, high temperatures, and limited fresh water resources, necessitates a fracturing fluid system that exhibits excellent temperature shear resistance, low friction, and salinity tolerance. This study presents the development of a zwitterionic hydrophobic polymer, HPC-5, as an effective thickener using five types of polymeric monomers, including AM, AA, DMC, AMPS, and a non-ionic hydrophobic monomer. The method employed for synthesis was free-radical polymerization in solution. A series of experiments including viscosity measurement with variation of salinity, solubility and drag reduction test, crosslinking test, thermal and shear resistance, sand-carrying test, gel breaking evaluation, and core damage test were conduct under the simulated reservoir conditions. The zwitterionic design imparts great salt tolerance to HPC-5, and the apparent viscosities of HPC-5 solutions can maintain comparably high values with 10×104 ppm NaCl and CaCl2 concentration. Meanwhile, the molecules of HPC-5 associate with each other to form tight net structures, resulting in an excellent viscoelasticity of the solution. To achieve high pump rate during hydraulic fracturing operation in ultra-deep reservoirs, the delayed crosslinking agent ZDC-L was prepared for forming a delayed crosslinking gel fracturing fluid system using reservoir brine, and the drag reduction rate can reach over 70% before crossing link within 4 min. Under pH = 4 conditions, the crosslinking time can be significantly delayed to over 4 min while maintaining exceptional temperature resistance up to 160 ℃ for the gel. These properties make it highly suitable for hydraulic fracturing operations in ultra-deep wells with temperatures reaching up to 7000 m depth at pump rates of 4~5m3/min. [ABSTRACT FROM AUTHOR]

As for blunt-body hypersonic vehicles, the extreme drag and aero-heating pose great challenges to the conceptual design. Different from the conventional configurations, a blunt body slotted with a channel is proposed in this paper. The channel is further combined with a jet on the shoulder to reduce drag and aero-heating. Simultaneously, based on the position of the separation point and vortex edge, a modified effective body approach is developed to accurately investigate the drag reduction mechanism. Specifically, the more slender the modified effective body is, the more obvious the drag reduction is. The simulation results show that the spike-channel-jet can push the shear layer on the spike away from the wall. In this way, the position of shear reattachment point moves downstream, which can enlarge the recirculation zone and weaken the shock-shock interaction. Additionally, the reattachment shock is pushed far away from the blunt body, which is also weakened. For drag, net power is introduced to evaluate the practical value of spike-channel-jet for drag reduction. Compared with a simple spike (spike length L / D = 1) of the same size, the spike-channel-jet can reduce the total drag by 24.68 %, the peak pressure coefficient of the blunt body by 38.54 %, the peak temperature of the blunt body by 65.75 %, and the peak temperature of the spike by 10.63 %. The maximum net power can reach 13.30 kW. Considering the fluid-thermal interaction, the findings reveal the influence of the spike length and jet mass flow rate on aerodynamic performance, by modified effective body further analyze the drag reduction mechanism with the jet, and validate the different reasons for the peak temperature and pressure of the blunt body. • Drag and aeroheating reduction mechanism of the spike-channel-jet concept under hypersonic conditions is investigated by the numerical simulation. • The influence factors are analyzed and compared comprehensively. • Combined spike-channel-jet concept is a promising drag and aeroheating reduction technique. [ABSTRACT FROM AUTHOR]

In this paper, analyze the influence of the stepped planning structure on the drag performance by observing waveform diagrams at the stern of the double M-ship and water–air and pressure distribution diagrams at the bottom of the ship. This study uses the combined stepped planning-air drag reduction technology to improve the sailing characteristics of the double M-ship. Research findings: The stepped planning contributes to a reduction in bottom pressure, enhances water–air distribution, and augments the amplitude of hull movement. Within the design speed range, the maximum drag reduction rate achieved by the stepped planning is 7.574%. However, this enhancement comes at the expense of increased viscous pressure resistance, which becomes the predominant resistance when sailing at full speed; Injecting air at the stepped planning can effectively reduce the viscous pressure resistance increased by the stepped planning. The combined drag reduction technology of stepped planning and air successfully realizes the total drag reduction at the double-M ship's high speed. The total resistance experienced when air is injected at the stepped planning is reduced by up to 20.981% compared to the original hull. [ABSTRACT FROM AUTHOR]

To enhance the service life of shipping equipment and minimize surface wear, this study employs biomimetic principles, integrating fitted structures with jet dynamics to model three configurations: non-smooth structures, single jet structures, and non-smooth jet-coupled structures. We utilized the SST k-ω turbulence model for numerical simulations to investigate the drag reduction characteristics of these structural models. By varying the jet angle and speed, we analyzed the changes in viscous resistance, pressure differential resistance, and drag reduction rates at the wall surface. Furthermore, the mechanisms of compressive stress, velocity fields, vortex structures, and shear stress on drag-reducing surfaces were elucidated, revealing how these factors contribute to drag reduction in non-smooth jet-coupled structures. The results indicate that the non-smooth jet-coupled structure exhibits superior drag reduction performance at a main flow field velocity of 20 m/s. As the jet velocity increases, the viscous drag on the surface of the non-smooth jet-coupled structure decreases, while the pressure differential drag increases. Conversely, variations in the jet angle have a minimal effect on viscous drag but lead to a reduction in pressure differential drag. Specifically, when the jet velocity is set at 1 m/s, and the jet angle is 60°, the drag reduction achieved by the non-smooth jet-coupled structure peaks at 7.48%. Additionally, the non-smooth jet-coupled structure features a larger area characterized by low shear stress, along with an increased boundary layer thickness at the bottom; this configuration effectively reduces surface velocity and consequent viscous drag. [ABSTRACT FROM AUTHOR]

This study investigates the aerodynamic performance of various trains with different nose shapes, using as the design variables two angles α , β for the head shape and the bluntness angle γ , without crosswind. The effects on aerodynamic performance, such as the train drag coefficient, pressure distribution along the train surface, flow structures around the train and the wake, and head pressure pulse, are analyzed. The results indicate that the increase in the train nose length for flat shapes decreases the C D values by 21.47 % and 19.11 % , decreasing the high-pressure region in the leading head. The duck nose configuration emerges as a compromise between drag reduction and nose length. Increasing the angle γ , a further drag reduction of 8.5 % is featured. Drag formation along the train is also analyzed. The steeper the variation in the geometry, the higher the peak intensity and the slope of the curve. Regarding the flow features around the train, two main counter-rotating vortices are captured in the wake. Moreover, the higher the nose length and the higher the bluntness angle γ , the weaker and narrower the wake. Again, a longer nose shape yields a softer jump in terms of pressure difference, crucial for train homologation and safety. [ABSTRACT FROM AUTHOR]

Copyright of China Petroleum Machinery is the property of China Petroleum Machinery Editorial Department and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Autonomous underwater vehicles (AUVs) have various applications in both military and civilian fields. A wider operation area and more complex tasks require better overall range performance of AUVs. However, until recently, there have been few unified criteria for evaluating the range performance of AUVs. In the present work, a unified range index, i.e., L*, considering the cruising speed, the sailing distance, and the volume of an AUV, is proposed for the first time, which can overcome the shortcomings of previous criteria using merely one single parameter, and provide a uniform criterion for the overall range performance of various AUVs. After constructing the expression of the L* index, the relevant data of 49 AUVs from 12 countries worldwide have been collected, and the characteristics of the L* range index in different countries and different categories were compared and discussed. Furthermore, by analyzing the complex factors affecting the range index, methods to enhance the L* range index value, such as efficiency enhancement and drag reduction, have been introduced and discussed. Under this condition, the work proposes a unified and scientific criterion for evaluating the range performance of AUVs for the first time, provides valuable theoretical insight for the development of AUVs with higher performance, and then arouses more attention to the application of the cutting-edge superlubricity technology to the field of underwater vehicles, which might greatly help to accelerate the coming of the era of the superlubricitive engineering. [ABSTRACT FROM AUTHOR]

Inspired by the "Lotus Leaf Effect" in nature, the phenomenon of superhydrophobia has attracted tremendous attention from researchers. Due to their special surface wettability, the superhydrophobic surfaces have been found to have broad potential applications in the fields of marine engineering, medical equipment, and aerospace. Based on the introduction of the principles of wettability, the advantages and disadvantages of various preparation methods for superhydrophobic surfaces were studied and summarized in this paper. The research progress on superhydrophobic surfaces in marine engineering applications was analyzed according to their self-cleaning, anti-corrosion, heat transfer, drag reduction, anti-fouling, ant-icing, and oil/water separation properties. Finally, to advance practical applications, the current challenges associated with superhydrophobic surfaces are highlighted, and potential future development directions are proposed. [ABSTRACT FROM AUTHOR]

In order to explore the drag reduction characteristics of different jet parameters in the bulb through pump, the bionic design was carried out for the shark gills, and the model of the bulb through the pump with a jet port at the front end of the bulb body was established, and the influence mechanism of the placement position, jet velocity and number of jet ports on the flow field of the bulb through the pump was studied by CFX. The results show that the near-wall main stream collides with the jet, and the axial momentum transmitted by the main stream to the jet weakens the axial velocity of the near-wall main stream. The radial momentum of the jet to the main stream increases the radial velocity of the near-wall mainstream, and lifts the near-wall high-velocity zone away from the bulb body, so that the near-wall medium and low velocity bands are widened, the high-velocity zone is contracted, and the wall shear stress is reduced. The calculation predicts that the drag reduction effect is up to 23.069%, the head is increased by 0.083 m, and the efficiency is increased by 1.729%, which can provide guidance for the optimal design and efficient operation of the bulb through-flow pump. [ABSTRACT FROM AUTHOR]

Copyright of Oilfield Chemistry is the property of Sichuan University, Oilfield Chemistry Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

To solve problems such as inadequate proppant carrying capacity, a complex viscosity changing process, and limited environmental friendliness associated with traditional slick water fracturing fluids, a variable viscosity friction reducer called Jianghan friction reducer (JHFR) with low damage, high drag reduction, and strong proppant‐carrying capacity is prepared by inverse emulsion polymerization. The properties of JHFR are investigated by viscometer, JHFR‐II high‐temperature and high‐pressure slick water drag reduction rate tester, surface tension meter, and JHCQ Core dynamic damage evaluation device. The result shows that the dissolution time of JHFR is less than 120 s. The drag reduction of 0.08 wt% JHFR (4.83 mPa s) is 80.3%. When the concentration of JHFR increases from 0.08 to 0.8 wt %, the viscosity of the JHFR increases from 4.83to 97.41 mPa s. After sheared at 90°C and 170 s−1 for 60 min, the viscosity of 0.8 wt% JHFR still maintains 51 mPa s. The damage rate of JHFR to core matrix permeability is lower than 30%. JHFR also shows the advantages of easy breakage and low residue. It provides guidance for the design and selection of fracturing fluid for unconventional oil and gas reservoir development. [ABSTRACT FROM AUTHOR]

Adding polymers to turbulent Newtonian fluid flows can have dramatic effects. A well-known example is a significant drag reduction by flexible polymers in turbulent wall-bounded flows. In numerical studies of polymer drag reduction, polymers are often modelled as dumbbells of two beads connected by a finitely extensible nonlinear elastic (FENE) spring. There are natural queries whether this highly simplified coarse-grained model is adequate for describing a polymer macromolecule in turbulent flows. By carrying out Eulerian–Lagrangian simulations of polymers, described by different models, in a turbulent pipe flow, Serafini et al. (J. Fluid Mech. , vol. 987, 2024, R1) have demonstrated that the FENE dumbbell model can accurately capture polymer extension statistics as compared with the realistic Kuhn chain model. Their work further reveals the surprising result that increasing the number of beads in a FENE chain worsens its accuracy in characterizing polymer spatial conformations at large Weissenberg numbers. [ABSTRACT FROM AUTHOR]

ABSTRACT Considering the rapid liquid transport characteristics of bionic dragonfly wings, an experimental investigation into the stability and drag reduction properties of an air film on various textured surfaces is being conducted. The study examines the impact of different wetting gradient textures on the stability and drag reduction properties of air film. Experimental results demonstrate an enhanced fluid transport efficiency, resulting in a maximum drag reduction of 9.1%, attributed to the size effect of the multi‐scale structure of bionic dragonfly wings. Surfaces featuring wetting gradients exhibit increased stability of the air film within the texture and the ability to trap air bubbles. Based on a near‐wall flow two‐phase flow theory model, the simulation considers the morphological changes of the air film at structured interfaces and their influence on near‐wall flow characteristics. The results indicate that the drag reduction effect arise from the slippage effect between the internal vortex in the air film inside the texture and the flow field near the wall surface. The synergistic effect of near‐wall flow fields among multiple texture layers is evident. This interplay across different regions contributes to the sustained drag reduction within the near wall area. [ABSTRACT FROM AUTHOR]

A series of recent studies has indicated that the component of the bottom drag caused by irregular small-scale topography in the ocean varies non-monotonically with the flow speed. The roughness-induced forcing increases with the speed of relatively slow abyssal currents but, somewhat counterintuitively, starts to decrease when flows are sufficiently swift. This reduction in drag at high speeds leads to the instability of laterally uniform currents, and the resulting evolutionary patterns are explored using numerical and analytical methods. The drag-law instability manifests in the spontaneous emergence of parallel jets, aligned in the direction of the basic flow and separated by relatively quiescent regions. We hypothesize that the mechanisms identified in this investigation could play a role in the dynamics of zonal striations commonly observed in the ocean. [ABSTRACT FROM AUTHOR]

The practical implementation of machine learning in flow control is limited due to its significant training expenses. In the present study the convolutional neural network (CNN) trained with the data of the restricted nonlinear (RNL) model is used to predict the normal velocity on a detection plane at $y^+=10$ in a turbulent channel flow, and the predicted velocity is used as wall blowing and suction for drag reduction. An active control test is carried out by using the well-trained CNN in direct numerical simulation (DNS). Substantial drag reduction rates up to 19 % and 16 % are obtained based on the spanwise and streamwise wall shear stresses, respectively. Furthermore, we explore the online control of wall turbulence by combining the RNL model with reinforcement learning (RL). The RL is constructed to determine the optimal wall blowing and suction based on its observation of the wall shear stresses without using the label data on the detection plane for training. The controlling and training processes are conducted synchronously in a RNL flow field. The control strategy discovered by RL has similar drag reduction rates with those obtained previously by the established method. Also, the training cost decreases by over thirty times at $Re_{\tau }=950$ compared with the DNS-RL model. The present results provide a perspective that combining the RNL model with machine learning control for drag reduction in wall turbulence can be effective and computationally economical. Also, this approach can be easily extended to flows at higher Reynolds numbers. [ABSTRACT FROM AUTHOR]

Armour has been present in vertebrates for millions of years and has evolved independently several times in the ray finned fishes. While armour is typically considered defensive, it is a multifunctional trait with many plausible alternative functions. We explore the hydrodynamic function of armour in nine species of poachers (Agonidae) by visualizing flow to assess drag reduction. We used microcomputed tomography scans to categorize armour morphology into morphotypes, and compared the rugosity (surface roughness) across these morphotypes. We then used digital particle image velocimetry to visualize boundary layer separation along 3D-printed whole-body models made using the scans. Poacher armour comprises eight rows that, some distance behind the second dorsal fin, merge into six. We found four morphotypes, with higher rugosity observed for the prominent spine morphotype and the lowest rugosity in the no-spine morphotype. Principal component analysis revealed that much of the variation in armour morphology is driven by plate shape, spine size, and protrusion. The boundary layer was retained longer in species with larger spines. Overall, the presence of a spine increased boundary layer retention, decreasing the drag, which is advantageous for these benthic fish. [ABSTRACT FROM AUTHOR]

The drag reduction efficacy of a large-scale flow control over a rough surface is studied via direct numerical simulations of turbulent channels (at friction Reynolds numbers R e τ = 180) by combining together wall riblets and streamwise counter-rotating swirls. In particular, the height of triangular riblets is h + ≈ 10 (+indicating wall units), while the number of riblets ( N Rib in the range 1–56) along the periodic spanwise direction is varied to find the optimum. The swirls are generated by the spanwise opposed wall-jet forcing (SOJF) in the Navier–Stokes equation, whose controlling parameters follow the optimal ones as for the smooth wall. In total, 12 cases of combined SOJF and riblets are performed to investigate the coupling effects between the two methods. We find a range of N Rib = 7 –14 (with the spanwise width z + ≈ 140 − 280) yields the largest drag reduction (up to 20%) for R e τ = 180 , much higher than riblets control only (about 3%). Compared to SOJF control only, riblets suppress the secondary swirls of SOJF hence decreasing drag, while the lateral and down washing motions of SOJF impinging on riblets would increase drag—the opposite two effects thus giving rise to an optimal. Through examinations on coherent structures, we elucidate that the attenuation of both large-scale coherent motions and small-scale random fluctuations leads to the net drag reduction. We conclude that large-scale control is a robust approach in the cases of rough surfaces, and the parameters can be selected for maximum drag reduction in each particular situation. [ABSTRACT FROM AUTHOR]

Airflow separation at the rear area of the automobile is the main source of automobile aerodynamic drag. To suppress the airflow separation, minimize the aerodynamic drag of the automobile, realize energy saving and emission reduction, a bionic drag reduction device was designed based on the dorsal fin of the orca with low drag characteristics. A numerical computation method was established to maximize the drag reduction performance of the bionic device, and the parametric modeling of the bionic drag reduction device was carried out. The design of experiments, the Kriging surrogate model, and an optimization algorithm were used to optimize the bionic drag reduction device. The validity of the optimization design was validated by the wind tunnel test. Finally, the mechanism and effectiveness of the bionic device in reducing aerodynamic drag were investigated through the comparison of flow field. The results show that the optimized bionic drag reduction device can delay the airflow separation and effectively reduce turbulence intensity of the automobile. According to the wind tunnel test, the aerodynamic drag coefficient of the optimized model is reduced by 6.16% compared with the original model. [ABSTRACT FROM AUTHOR]

In this work, an improved anisotropic k-ε-v2-f model based on the finite extensible nonlinear elastic model with the Peterlin approximation for viscoelastic channel flows is proposed. This model is tested using direct numerical simulation (DNS) data for friction Reynolds numbers (Reτ) in the range of 120–1000, friction Wiesenberg numbers (Wiτ) in the range of 25–116, viscosity ratios (β) in the range of 0.6–0.9, and maximum polymer extensibility values (L2) in the range of 900–14 400. The flow characteristics of viscoelastic fluids with various parameters obtained from the new model agree well with existing DNS results. By adding closures for the flow, shear, and transverse components, the incomplete prediction of nonlinear terms from interactions between the fluctuating components of the conformation and velocity gradient tensors is improved. Compared with DNS results, these closures can fully obtain each component and significantly improve the accuracy of the flow direction component in the intermediate and high drag reduction regimes. Furthermore, the model in this paper retains the advantages of an anisotropic model, does not require a damping function, is simple to construct, and is easily extended to a variety of bounded flows. [ABSTRACT FROM AUTHOR]

We move forward the important topic of water entry by documenting splash dynamics arising from the impact of hydrophilic spheres with buoyant millimetric microplastics, mimicked in our study by polystyrene beads. Collision with small, buoyant beads is yet another means to manipulate splash dynamics. In this experimental study, we investigate the fluid–structure interactions between beads and hydrophilic spheres for Froude numbers in the range of 20 − 100. Generally, hydrophilic spheres entering a liquid bath below the critical velocity of 8 m/s produce minimal fluid displacement and no cavity formation. The presence of proximally adjacent beads atop the fluid with respect to impacting spheres promote flow separation and compound cavities for sufficiently large Froude numbers, while suppressing the growth of splash crowns. Compound cavities consist of a shallow, quasi-static first cavity that seals near the water line, and a second, deeper cavity produced in the wake of descending spheres. A vertically protruding Worthington jet follows cavity collapse. The resulting splash metrics differ from those of hydrophobic spheres with respect to the properties of impacted beads. We find impactors traversing a deep liquid pool layered with beads experience drag reduction when compared to entry into a clean pool due to the drag-reducing benefits of flow separation while not offering a high inertial penalty. Our study unravels the physics behind the widely encountered interaction of solid projectiles impacting passively floating particles, and our results translate to the entry dynamics of water-diving creatures and projectiles into water bodies polluted by floating millimetric microplastics. [ABSTRACT FROM AUTHOR]

This document is a special issue that summarizes the latest research on carbon neutrality technology and symmetry principles in power engineering and engineering thermophysics. The research papers cover various topics related to aviation technologies, including experimental equipment, control system design, emission control technology, and vehicle drag reduction measures. The goal of this special issue is to provide new ideas and methods for solving current challenges and promoting sustainable development in science, technology, and industry. The document also discusses the exploration of alternative fuel engine optimization and adaptive design in the aviation industry to mitigate environmental impact while maintaining operational efficiency. It highlights the potential of sustainable aviation fuel, hydrogen, and ammonia in reducing aviation emissions. The optimization of thermal cycles and components of engines is also crucial in enhancing engine efficiency and reducing emissions. The document further emphasizes the importance of optimizing the design of aircraft to achieve emission reduction while considering the profitability of the aviation network. The integration of renewable energy systems in power systems is also discussed, along with the optimization of aeropropulsion propulsion system test facilities and the design of hydraulic drive systems. The document concludes by highlighting the advancements made in peak load regulation, alternative fuel applications, engine performance optimization, and sustainable aviation design, and the potential for future innovation and development in the field. This document also summarizes several research contributions related to various aspects of aerospace engineering, including the optimization of gas foil bearings, analytical modeling for flanged joints, sliding friction models for gear systems, detection efficiency of condensation particle counters, calibration methods for gas turbine thermodynamic models [Extracted from the article]

Shark skin is composed of denticles, or complex scale-like features, which have been shown to reduce drag in turbulent and laminar flow. The denticle crown features undulating structures called riblets that interact with the turbulent boundary layer to reduce drag. Two mechanisms thought to contribute to the drag-reducing properties of riblets include the lifting of streamwise vortices and the hampering of spanwise vortex interactions to reduce crossflow, which could translate to similar flow mechanisms for denticles. Because of the varied morphologies of dermal denticles on different shark species, which also depend on body location, the impact of these denticle geometries on flow is of interest to the biology community, including related fields such as fluid mechanics and oceanography. This review highlights the past 15 years of manufacturing techniques and experimental measurements of drag over denticle-inspired surface structures, including real shark skin samples and engineered denticles and riblets. State-of-the-art additive manufacturing and other techniques are primarily limited to mm-length denticle scales, which have demonstrated drag reduction in lower flow speeds, under 1 m s−1. New manufacturing approaches can create sub-mm length denticles and nanotextured surface structures, which have achieved reported drag reductions of up to 31%. We synthesize results from the literature to illustrate the drag reduction properties of bioinspired denticles and riblets according to their geometry and flow conditions. Using these trends, we suggest design features and focus areas for future research, such as increasing studies of different denticle morphologies, hydrophobicity, antifouling properties, and acoustic noise reduction. Continued work on bioinspired denticles for drag reduction has wider implications in comparative biology and applications to design more energy-efficient, persistent vehicles for environmental monitoring. [ABSTRACT FROM AUTHOR]