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The noise analysis of a large-scale aquaculture vessel reveals that during its navigation, the primary equipment noise, particularly from the propeller, exerts a notable influence on the aquaculture environment for large yellow croaker. The free surface greatly impacts the noise performance of propellers, which is a significant factor affecting the fish's habitat. This study adopts the numerical simulation method to analyze the hydrodynamic and acoustic characteristics of the E1619 propeller operating near the free surface. The open-water performance and noise calculations of the propeller are verified through experiments, and the effects of different immersion depths and advance coefficients on the propeller are explored. The results demonstrate that the free surface significantly affects the thrust, torque, and noise of the propeller, especially at shallow immersion depths and low advance coefficients. Surface wave pattern causes the instability and breakup of tip vortices, causing increased thrust and torque fluctuations, reduced efficiency, and significant overall sound pressure levels in the entire flow field. As immersion depth and advance coefficients increase, the interaction between tip vortices and the free surface weakens, wake vortex instability decreases, and noise levels gradually reduce. These analyses and conclusions can guide the design of next-generation propellers for aquaculture vessels to optimize performance near the free surface. [ABSTRACT FROM AUTHOR]

Toroidal propellers hold significant potential as underwater propulsion systems compared to traditional propellers, primarily due to their unique shape, which effectively reduces and minimizes hydrodynamic noise and enhances structural stability and overall strength. To investigate hydrodynamic loads, flow fields, and vortex characteristics of toroidal propellers, numerical simulations were conducted on both toroidal and conventional propellers using the detached eddy simulation (DES) method in Star CCM+ computational fluid dynamics software. Results show that at low advance coefficients, the primary thrust generated by toroidal blades comes from pressure difference in the front section, whereas at high advance coefficients, it originates in the back section. A high-velocity region exists between the front and back sections of the toroidal propeller, with the range and intensity of this region gradually increasing from front to back. The wake vortex of the toroidal propeller comprises two parts: the tip vortex, where the front section tip vortex, back section tip vortex, and transition section leakage vortex merge, and the trailing edge vortex, which forms from the fusion of the front and back section leakage vortices. The fusion of these vortices is influenced by the advance coefficient. Compared to conventional propellers, the toroidal propellers exhibit a more extensive and intense trailing edge vortex in the wake flow field. These findings provide guidance for the optimization design research of toroidal propellers. [ABSTRACT FROM AUTHOR]

Propeller cavitation is one of the main causes of pressure fluctuation and noise around marine propellers, and tip vortex cavitation is one of the main causes of high-frequency underwater radiated noise. To predict tip vortex cavitation, it is necessary to correctly describe the radius of the vortex core and the vortex circulation of the tip vortex. Therefore, in this study, non-cavitating flow field measurements around the tip of model propellers were made for the 0.75 m diameter working section of the large cavitation tunnel at the National Maritime Research Institute, Japan (NMRI) using 2D-PIV. Identification procedures were investigated to develop a tip vortex model. The vortex properties (radius of vortex core and vortex circulation) of the tip vortex were obtained by applying by a Rankine vortex model and a Burgers vortex model. The measured velocity and vorticity values around the tip vortex as identified by the Burgers vortex model were in better agreement than those given by the Rankine vortex model. The Burgers vortex model was suitable for obtaining vortex properties from the measured flow field around the tip vortex. The vortex properties obtained from the identification using the Burgers vortex model showed the Reynolds number demonstrated a greater effect on the radius of the vortex core and vortex circulation. The higher the Reynolds numbers, the smaller the radius of the vortex core and the smaller vortex circulation tends to be. It is also shown that this Reynolds number effect differs depending on the blade shape of the propellers. [ABSTRACT FROM AUTHOR]

The circulation distribution of the wing-tip vortex core is analyzed in the near-field region of a rectangular wing at the downstream distance of several wing chord lengths. Experimental results, obtained earlier using particle image velocimetry, are compared to the analytical Burnham–Hallock vortex model. A modification of Burnham–Hallock model is proposed, that allows obtaining the circulation distribution in the wing-tip vortex core region based on the flowfield conditions and the downstream distance of the wing. [ABSTRACT FROM AUTHOR]

In this article, a modified immersed boundary method (IBM) with a ghost-cells strategy is introduced to study a kind of vertical-axis wind turbine (VAWT). In order to improve the accuracy of the numerical calculation and enhance its adaptability, some treatments on the IBM for the VAWT especially are carried out. The method is testified by the numerical simulation of an Eppler387 airfoil. With the help of the method, some flowing characteristics of the wind turbine have been thoroughly observed and understood. Furthermore, the effects of blade shape, tip vortex and secondary flowing on the turbine performance are discussed in detail based on the numerical results. [ABSTRACT FROM AUTHOR]

In this work, the internal flow behaviour and characteristic pressure fluctuations of an axial pump with varying water conditions are analysed. The impact of tip vortex flow on the pattern of turbulent flow is simulated numerically by the application of the CFD technique and experimentally using an acoustics analysis method. The numerical CFD data are verified with an experimental test model for accuracy and reliability. Based on the results, the difference in pressure in the internal flow and at the surfaces of the blade can be impacted through tip leakage vortex regions, which leads to changes in internal flow. Subsequently, the flow in the clearance and tip leakage vortex regions is changed. Moreover, the results reveal that the suction wall upstream is more unsteady near the surface due to more mixing, secondary flow, and tip leakage vortices. Pressure fluctuation occurs near the tip of the blade, caused by the increasing vortex flow velocity and hence raising the turbulent kinetic energy (TKE). Using different monitoring points at the blade impeller reveals high values of the pulsation amplitude. Owing to the region of clearance backflow under low-water conditions, the axial pump displays larger fluctuations in pressure near the tip blade area. Because the leakage flow leaves the gap at a high flow rate, shear layers are formed quickly between the main flow and the leakage flow. Near the end wall, there is a negative-vorticity-induced vortex. Moreover, as the flow rate increases, the pump's amplitude decreases along with its main frequency. For the low-water flow, the results reveal that there is an important clearance backflow because the axial pump has large clearance. [ABSTRACT FROM AUTHOR]

The numerical modeling of the flow around a rotor in the in-situ operating mode at different rotation speeds is considered. Based on the numerical modeling, an analysis of changes in the circulation of the rotor tip vortex in the near field was carried out. [ABSTRACT FROM AUTHOR]

When examining the literature for flow effects on circular cylinders one can find many studies on infinite cylinders and cantilevered cylinders but minimal data related to cylinders with two free ends (Shepard, T., Law, D., Dahl, J., Reichstadt, R., and Selvamani, A. S., 2022, "Impact of Aspect Ratio on Drag and Flow Structure for Cylinders With Two Free Ends," ASME Paper No. V001T03A031.). The limited data available shows that the cylinder aspect ratio affects the drag and frequency content of flow within the wake however these studies were done at discreet Reynolds numbers. In order to better understand the combined impact of aspect ratio and Reynolds number a series of wind tunnel tests and numerical simulations has been conducted for cylinders with two free ends having aspect ratios of 2-15. Tests were carried out in the subcritical regime with Reynolds numbers ranging 13000-105,000. Tip vortex effects, which vary with aspect ratio, are shown to impact the cylinder surface pressure, drag coefficient, and wake Strouhal numbers though Reynolds number effects are minor for the conditions studied. The results are compared against existing historical data and show the trend of drag coefficient increasing with cylinder aspect ratio (Shepard, T., Law, D., Dahl, J., Reichstadt, R., and Selvamani, A. S., 2022, "Impact of Aspect Ratio on Drag and Flow Structure for Cylinders With Two Free Ends," ASME Paper No. V001T03A031). [ABSTRACT FROM AUTHOR]

The effect of a porous wingtip on tip vortex formation and properties in a supersonic flow is studied. It is established that a porous wingtip has a considerable effect on the vortex structure and parameters reducing its intensity. [ABSTRACT FROM AUTHOR]

Yuan, H.; Li, F.; Yan, Q.; Zhang, W., and Hu, J., 2023. Investigating pressure fluctuations on marine vessel rudders: Numerical results from propeller–rudder–hull interactions. Journal of Coastal Research, 39(2), 284–295. Charlotte (North Carolina), ISSN 0749-0208. Severe pressure fluctuations induced by propeller wake enhance rudder vibrations. In the present study, the pressure fluctuations on the rudder are numerically investigated by using k-ω Menter's shear stress transport turbulence model. The pressure fluctuations on the port and starboard sides of the rudder are compared in time and frequency domains. Furthermore, the impact of the advance coefficient on the displacement of propeller wake and pressure fluctuations is also discussed. The numerical results indicate that the propeller wake displacement is oriented upward and downward at the port and starboard sides, respectively. The most intense pressure fluctuation occurs at the propeller blade passage frequency. As a result of vortex displacement, the suction-side fluctuations are weaker than those on the pressure side at frequencies of 100–300 Hz over an advance coefficient range of J = 0.75–0.925. The displacement of the propeller wake weakens when the advance coefficient decreases. [ABSTRACT FROM AUTHOR]

Tip loading is a common strategy to increase the propulsive efficiency of propellers. Solutions such as contracted and tip-loaded (CLT) and "New generation" CLT propellers exploit the presence of an endplate ("true" or as the result of a dedicated modification of the rake distribution) to sustain the increased load at the tip of the blade, at the cost of more complex vortical structures. Their evolution, and the mutual interaction of secondary vortices originated by the endplate itself, however, has not been completely and deeply investigated. The current paper addresses this topic by improved delayed detached eddy simulations (IDDES) of the flow field around two propellers of this type at different loading conditions. The presence of secondary vortices from the endplate root (or from the bended blade at tip), partially observed in recent experiments, is evidenced by high-fidelity CFD calculations. The interaction mechanism with the primary vortices (those from the endplate tip), and the resulting strengthening of the vortical structures, also through the interaction with the blade trailing vortical wake that promote the leapfrogging phenomenon, is discussed as well, comparing the phenomena in the case of two optimally designed geometries (a CLT and a New Generation CLT propeller) exploiting the same pressure side tip-loading concept in a slightly different way. Results show a rather different instability mechanism depending on the endplate configuration and open the discussion on the effectiveness of splitting a single tip vortex into pairs of vortical structures that may induce similar (or even worse) side effects in terms of pressure minima in the wake and earlier wake destabilization. [ABSTRACT FROM AUTHOR]

A new white-light volumetric flow measurement technique is presented that can be used in large-scale facilities. The technique enables large volumes to be measured with high temporal and spatial resolution and without the need for a class-4 laser. This LED-based Lagrangian particle tracking velocimetry is demonstrated by measuring the tip vortex formation and the near wake of a 1.2 m diameter tidal turbine in a 25 m diameter, 2 m deep tank. Seven streamwise-distributed volumes of interest are combined, each 334 mm long, 244 mm wide and 140 mm deep, reaching up to one diameter downstream of the turbine. The system does not require re-calibration when moved. By assuming a periodic flow field, a phase-averaged flow field was reconstructed with a temporal resolution of 3.9 ms and a spatial resolution of 5.4 mm. The large volume and high time and spatial resolution could enable key research questions to be addressed on high-Reynolds-number flows and could provide valuable benchmark data for numerical model development and code validation. [ABSTRACT FROM AUTHOR]

The efficiency and accuracy of numerical simulation on power unit is the key to study the relevant aerodynamic layout with multiple rotating power units. However, the numerical simulation of the power unit using real geometry all faces the problem of low solution efficiency. Taking the rotor hovering state as an example, the real blade was firstly simplified and replaced by a thin mesh disk to establish the effective momentum source method. Then, using fan-shaped mesh region that changes with time to replace real blade and simulate the rotation, the unsteady momentum source method which could get the revolution of tip vortex was proposed. The results show that the momentum source method with the input of accurate blade force distribution can simulate rotor wake better, and the influence that blade geometry acts on wake mainly reflects in the blade force distribution. In addition, the unsteady momentum source method can simulate the revolution of tip vortex, and its consumptions of computing resources and calculation time are only about 1/8 of the unsteady numerical simulation based on the real geometry.

Propeller cavitation is a main source of fluctuating pressure and noise induced by propellers, and the tip vortex cavitation is the principal source. The present study measures the flow fields near the blade tip using the 2D-PIV technique. The experimental setup and scheme are introduced. We monitor the process of generation and shedding of the propeller tip vortex in real time and analyse the dynamic structure of the tip vortex by testing the propeller wake field under different phases of the axial plane. The distribution characteristics of radial and axial velocity are also analysed. The influence range and the vorticity of the tip vortex and trailing vortex are obtained. All of the measured quantitative data are useful for future propeller design.

The study investigates helical vortices, which are fundamental structures in fluid dynamics, and a basic model of tip vortices behind wind turbines. In connection with the intensive development of wind energy, interest in modeling helical vortex wakes behind the rotors has increased. Therefore, the purpose of this mini-review is to compare the existing methods for calculating the induced velocities of screw vortices. The three methods for calculating the motion of helical vortices are compared. Two typical forms of vorticity with uniform (Rankine-type) and Gaussian distributions in the core of helical vortices are compared, and the minimum distance between the vortex filaments or their turns is identified with sufficient accuracy in both cases. The results presented in this mini-review can be used to model the helical vortices in the rotor wakes, central helical vortices in vortex devices, or natural phenomena such as tornadoes, dust tornadoes, and waterspouts.

Wingtip vortices of a finite span wing are analyzed numerically with the Reynolds-averaged Navier–Stokes equations. Vortex core parameters were identified using the Winding Angle method, the Q-criterion, λ2-criterion, and a modified Q-criterion defined as the difference of vorticity and strain rate tensors. [ABSTRACT FROM AUTHOR]

Using delayed detached-eddy simulations, the objective of this paper is to show the effects of some design parameters on the wake recovery of vertical-axis turbines. To that end, the wake dynamics of a single-blade turbine is analyzed and compared for two different positions of blade attachment point and for two designs of blade support structures (struts). The results show that the chordwise position of the blade attachment point affects the temporal evolution of the blade circulation during the turbine revolution. More precisely, we demonstrate that an attachment point positioned near the trailing edge of the blade leads to a temporal vorticity distribution that is detrimental to the wake recovery. Moreover, we show that the use of tip struts, that are highly beneficial to the turbine efficiency, affects the spatial distribution of the vortex structures shed by the turbine as well as the wake recovery. The results presented in this paper suggest that the velocity recovery in the wake is highly dependent on the very near-wake vortex dynamics. Therefore, we can infer that any geometric parameter affecting the temporal and spatial distributions of the vorticity shed by the blades can significantly alter the wake recovery of vertical-axis turbines. [ABSTRACT FROM AUTHOR]

Three-dimensional numerical simulations of hydrokinetic flapping-foil turbines with fully-prescribed motions are carried out using a Delayed Detached-Eddy Simulation (DDES) approach. The objective is to assess the efficiency and the wake recovery rate of flapping-foil turbines equipped with either end-plates or detached end-plates. The power coefficient and the mean streamwise velocity recovery in the wake of a turbine with different geometries of end-plates and detached end-plates are analyzed and the results are compared to a high-efficiency reference turbine without end-plates. We show that the turbine efficiency is significantly increased when detached end-plates are used. Indeed, the best configuration simulated in this work has an efficiency value that is 21% larger than that of the reference turbine. Moreover, the main physical mechanisms affecting the wake recovery rate are the same whether or not detached end-plates are used. Interestingly, for a turbine with small detached end-plates, the mean streamwise velocity recovery in the wake is similar to that of the reference turbine, while the efficiency is increased by 15%. This suggests that such detached end-plates could be quite beneficial for flapping-foil turbines operating in turbine farms. • End-plates and detached end-plates improve the efficiency of flapping-foil turbines. • Detached end-plates can importantly affect the vortex dynamics in the near-wake. • The use of small detached end-plates does not negatively impact the wake recovery. [ABSTRACT FROM AUTHOR]

This paper presents an experimental investigation of tip vortex formation noise produced by wall-mounted finite airfoils. Acoustic measurements were taken in the open jet anechoic wind tunnel at Brandenburg University of Technology with a planar 47-microphone array. A parameter space of eight airfoil models with variations in airfoil profile shape (thickness and camber) and tip geometry were tested at a range of angles of attack and Reynolds numbers. The acoustic survey shows that tip noise is a strong contributor to the overall airfoil noise signature at mid-to-high frequencies above 2–3 kHz. A critical frequency was defined to distinguish the effect of the thickness of the airfoil on the noise spectra, which increases with Reynolds number and appears constant with angle of attack. Above the critical frequency, a thicker airfoil is shown to produce a lower amplitude tip noise peak, especially at mid-to-high frequencies above 5 kHz. An increase in camber does not affect the frequency of the tip noise peak but increases the noise level, especially for f≥3 kHz and high angles of attack. Further, the presence of a rounded tip is found to be effective in decreasing the noise levels by up to 5.6 dB over the frequency range of 5–15 kHz. To explore the tip flowfield, velocity data were also obtained using planar particle image velocimetry in the anechoic wind tunnel at the University of New South Wales for a few targeted cases. The results show that the size of the vortex and its turbulence intensity are influenced by the tip geometry. Specifically, the vortex size for the cambered airfoils decreases along the streamwise direction, while for symmetric airfoils it increases from x/c=1.2 to 1.8. Collectively, these databases can be used to aid tip noise prediction models and validate computational fluid dynamic and computational aeroacoustic simulations. [ABSTRACT FROM AUTHOR]

Tip loading is a common strategy to increase the propulsive efficiency of propellers. Solutions such as contracted and tip-loaded (CLT) and “New generation” CLT propellers exploit the presence of an endplate (“true” or as the result of a dedicated modification of the rake distribution) to sustain the increased load at the tip of the blade, at the cost of more complex vortical structures. Their evolution, and the mutual interaction of secondary vortices originated by the endplate itself, however, has not been completely and deeply investigated. The current paper addresses this topic by improved delayed detached eddy simulations (IDDES) of the flow field around two propellers of this type at different loading conditions. The presence of secondary vortices from the endplate root (or from the bended blade at tip), partially observed in recent experiments, is evidenced by high-fidelity CFD calculations. The interaction mechanism with the primary vortices (those from the endplate tip), and the resulting strengthening of the vortical structures, also through the interaction with the blade trailing vortical wake that promote the leapfrogging phenomenon, is discussed as well, comparing the phenomena in the case of two optimally designed geometries (a CLT and a New Generation CLT propeller) exploiting the same pressure side tip-loading concept in a slightly different way. Results show a rather different instability mechanism depending on the endplate configuration and open the discussion on the effectiveness of splitting a single tip vortex into pairs of vortical structures that may induce similar (or even worse) side effects in terms of pressure minima in the wake and earlier wake destabilization.

With the continuous increase in the total quantity and quality of wind energy used by society, the aerodynamic complexity of wind turbine impellers has also gradually increased. This requires a more accurate analysis and understanding of the aerodynamic performance of important parts of the wind turbine impeller. The blade tip vortex is undoubtedly one of the most important issues. This article used the state-of-the-art lattice Boltzmann method (LBM), combining large eddy simulation (LES) and wall-adapting local-eddy (WALE) model, to investigate the unsteady flow characteristics of the blade tip region due to impeller wake for a wind turbine. The trend of this calculation result is consistent with the experimental data, both for the axial force and the torque on the impeller. Therefore, it indicates the calculation model of LBM is reasonable and effective. The relevant in-depth results clearly showed the dynamic stability of a spiral structure will increase as operating conditions continue to advance and it is also synchronously affected by the tower wake flow structures. The fluctuation amplitude of the static pressure value on both sides of the impeller gradually decreases with time. The distribution of static pressure near the leading edge of the tip is very different for each blade due to the influences of turbulence intensity and the tower shadow effect. The random minor fluctuations of the axial force and torque both exhibit similar characteristics, and the overall torque is more sensitive to random changes in the tip vortex on the impeller surface. [ABSTRACT FROM AUTHOR]

It is well known that a shroud placed around a wind turbine can increase its power coefficient, but it brings complex mechanisms by which the shroud alters the flow passing through the rotor. Such mechanisms impose numerical challenges, as the shrouded turbines present nonlinear behavior in the wake. This paper deals with a comprehensive analysis of tip vortex trajectory in shrouded wind turbines using Reynolds Averaged Navier–Stokes numerical solutions. The analysis includes aerodynamic performance and vortex characteristics of the whole wind turbine. The Multiple Reference Frame is used on a high-order unstructured compressible solver to study both, isolated and shrouded rotor. The NREL Phase VI Unsteady Aerodynamic Experiment rotor is used as a test case. The accuracy of results for wind speeds between 7 and 25 m s − 1 is discussed. Overall, good agreement is achieved between the computed pressure distributions and the experimental reference values. At stalled blade, more efforts are needed to improve numerical solutions, especially for integrated load quantities. The vortex structure is examined, showing that shroud impacts tip vortex trajectory by the increase of the axial induced velocity at the rotor plane. This result, demonstrates that the classical Prandtl tip loss is not accurate for shrouded turbine analysis, and modern finite blade functions are needed. The influence of the flow conditions on the tip vortex trajectory, flow separation and shroud interaction are also discussed. • Tip vortex trajectory of the NREL Phase Vi. • Use of a simplified Multiple Reference Frame model with open-source high-order CFD solver, avoiding conformal interface complexities and enabling efficient shrouded turbine simulation. • Accurate and efficient computation of the integrated loads on the wind turbine. • Unprecedented perspective into profoundly altered tip vortex trajectory behavior within shrouded turbines using validated higher-order framework. • Study of the interaction between the tip vortex and shroud's boundary layer. [ABSTRACT FROM AUTHOR]

The application of roughness on the blade surfaces is an effective and appealing tip vortex cavitation (TVC) suppression technique. However, there are few studies on the suppression of TVC by dimpled roughness, and its physical mechanism remains to be elucidated. In this work, the suppression of TVC on an elliptical foil using dimpled roughness is numerically investigated at a Reynolds number of 4.7 × 1 0 5 . Efforts are made to analyse the difference in vortex structures and vorticity distributions between several roughness configurations, and the suppression mechanism of the dimpled roughness is uncovered. It is found that the dimpled roughness can disrupt the tip vortex during its formation stage, thus preventing the accumulation of vorticity and reducing the tip vortex strength. Meanwhile, a thicker boundary layer on the dimpled surface increases the vortex core radius. Subsequently, the vortex footprint is analysed to identify the effective area of dimpled roughness. The minimum pressure criterion is selected to determine the TVC inception index. The results show that the optimum dimpled roughness application areas are at the leading edge, tip, and trailing edge of the suction side. Under the optimum configuration with fewer dimples, the TVC inception index decreases by 15.1% compared to the smooth foil. • A method of suppressing tip vortex cavitation (TVC) by dimpled roughness is numerically investigated using the LES turbulence model. • Suppression mechanisms on TVC of dimpled roughness are demonstrated in detail with the vortex structures. • Vortex footprints are proposed to find effective areas for roughness applications and are expected to serve as a reference for different operating conditions. • Optimal roughness configuration reduces the TVC inception by 15.1% relative to the smooth foil and minimally affects foil performance. [ABSTRACT FROM AUTHOR]

In a large wind farm, the wakes of upstream and downstream wind turbines can interfere with each other, affecting the overall power output of the wind farm. To further improve the numerical accuracy of the turbine wake dynamics under atmosphere turbulence, this work proposes some improvements to the actuator line-large-eddy simulation (AL-LES) method. Based on the dynamic k-equation large-eddy simulation (LES), this method uses a precursor method to generate atmospheric inflow turbulence, models the tower and nacelle wakes, and improves the body force projection method based on an anisotropic Gaussian distribution function. For these three improvements, three wind tunnel experiments are used to validate the numerical accuracy of this method. The results show that the numerical results calculated in the far-wake region can reflect the characteristics of typical onshore and offshore wind conditions compared with the experimental results. After modeling the tower and nacelle wakes, the wake velocity distribution is consistent with the experimental result. The radial migration velocity of the tip vortex calculated by the improved blade body force distribution model is 0.32 m/s, which is about 6% different from the experimental value and improves the prediction accuracy of the tip vortex radial movement. The method proposed in this paper is very helpful for wind turbine wake dynamic analysis and wind farm power prediction. [ABSTRACT FROM AUTHOR]

The vorticity confinement (VC) method was applied to tip vortices shed by edges of wings in order to predict induced drag using far-field integration. The optimal VC parameter was determined by its application to tip vortices shed by 3-D stationary and rotating wings. The VC was used with a total variation diminishing (TVD) approach to reduce the needed confinement. The TVD minmod and differentiable flux limiters were tested to evaluate their effect on the VC parameter. The 3-D inviscid simulations were post-processed using the wake-integral technique to determine lift-induced drag force. Dependence of the VC parameter on the flight Mach number and the angle of attack was evaluated. Grid convergence studies were conducted for induced drag generated by 3-D wing. The VC approach was combined with the Reynolds stress equation turbulence model, and the results were compared to experimental data of tip vortex evolution. [ABSTRACT FROM AUTHOR]

This study presents results of experiments where roughness applications are evaluated in delaying the tip vortex cavitation inception of an elliptical foil. High-speed video recordings and laser doppler velocimetry (LDV) measurements are employed to provide further details on the cavitation behavior and tip vortex flow properties in different roughness pattern configurations. The angular momentum measurements of the vortex core region at one chord length downstream of the tip indicate that roughness leads to a lower angular momentum compared with the smooth foil condition while the vortex core radius remains similar in the smooth and roughened conditions. The observations show that the cavitation number for tip vortex cavitation inception is reduced by 33% in the optimized roughness pattern compared with the smooth foil condition where the drag force increase is observed to be around 2%. During the tests, no obvious differences in the cavitation inception properties of uniform and non-uniform roughness distributions are observed. However, the drag force is found to be higher with a non-uniform roughness distribution. [ABSTRACT FROM AUTHOR]

In this paper, due to its advantages of high accuracy and moderate computational cost, detached eddy simulation (DES) is adopted to observe detail flow characteristics of a kind of vertical axial wind turbine equipped with four helical blades. The numerical results obtained from the method have been validated by experiments as well as been compared with those from unsteady Reynolds-averaged Navier-Stokes equations (U-RANS). Several critical parameters, such as the torque, power coefficient, and tangential force, are studied in detail. It is found that the results from DES are closer to the measurement data than those from U-RANS. And, the characteristics of the flow field in the turbine can be clearly revealed by DES. Furthermore, the effects of tip vortex, Reynolds number, and pitch angle on turbine performance have also been analyzed and discussed. [ABSTRACT FROM AUTHOR]

In the present work, Delayed Detached-Eddy Simulations (DDES) are conducted to evaluate the performance of an H-Darrieus vertical-axis turbine with detached end-plates, namely stationary end-plates that are not in contact with the turbine blades. More precisely, the force and the power coefficients of the turbine are analyzed for two different geometries of detached end-plates. The turbine wake dynamics is also presented and the main mechanisms responsible for the wake recovery rate are assessed. The results show that the presence of detached end-plates significantly increases the efficiency of the vertical-axis turbine considered. In addition to this improved efficiency, a semi-annular shape of detached end-plate leads to an improved recovery of the mean streamwise velocity in the turbine wake. The important wake recovery rate in the near-wake of the turbine with semi-annular detached end-plates is mainly related to the transport of momentum by the mean spanwise velocity field, which is also known to be the dominant contribution to the wake recovery rate of a turbine without detached end-plates. The results presented in this paper confirm that detached end-plates have a great potential to improve the performance of H-Darrieus vertical-axis turbines used both individually and within turbine farms. • The use of detached end-plates improves the efficiency of vertical-axis turbines. • Detached end-plates affect importantly the vorticity dynamics in the turbine wake. • The use of semi-annular detached end-plates improves the turbine wake recovery rate. • Transport of momentum by the mean spanwise velocity governs the wake recovery rate. [ABSTRACT FROM AUTHOR]

An experimental study of the near wake up to four rotor diameters behind a model wind turbine rotor with two different wing tip configurations is performed. A straight‐cut wing tip and a downstream‐facing winglet shape are compared on the same two‐bladed rotor operated at its design tip speed ratio. Phase‐averaged measurements of the velocity vector are synchronized with the rotor position, visualizing the downstream location of tip vortex interaction for the two blade tip configurations. The mean streamwise velocity is found not to be strongly affected by the presence of winglet tip extensions, suggesting an insignificant effect of winglets on the time‐averaged inflow conditions of a possible downstream wind turbine. An analysis of the phase‐averaged vorticity, however, reveals a significantly earlier tip vortex interaction and breakup for the wingletted rotor. In contradistinction, the tip vortices formed behind the reference configuration are assessed to be more stable and start merging into larger turbulent structures significantly further downstream. These results indicate that an optimized winglet design can not only contribute to a higher energy extraction in a rotor's tip region but also can positively affect the wake's mean kinetic energy recovery by stimulating a faster tip vortex interaction. [ABSTRACT FROM AUTHOR]

The blade of horizontal axial tidal turbine (HATT) will experience unsteady characteristics, such as dynamic stall, hysteresis loop, and stall delay, leading to unsteady loads that challenge its performance and survivability in the complex ocean environment with turbulent flow, waves, and shear flow. Installing vortex generators (VGs) on the blade surface effectively controls flow separation and improves the performance of the HATT blades. However, most existing VGs studies focus on two-dimensional models that do not consider the tip vortex, requiring further research to investigate the influence of VGs on the blade tip and their potential benefits. This study investigates the static and dynamic effects of different VGs parameters on the NACA63820 hydrofoil installed at the tip through hydrofoil water tunnel experiments and numerical simulations to understand the advantages of using VGs in this configuration. The results show that VGs alter the pressure distribution on the hydrofoil's surface, leading to the formation of pressure humps in the pressure curve of the hydrofoil section. The number and orientation of these humps depend on the openings of the VGs. This study comprehensively analyzes the factors causing these humps and their effects. In conclusion, installing appropriate VGs at the tip significantly improve the hydrofoil's performance and reduce the size of the hysteresis loop opening. Comparing different VGs identifies optimal parameters for HATT blades and evaluate their performance through numerical simulations. The results show that installing VGs at the 30% chord position of the airfoil section can improves the power coefficient by 1.6% at the optimal blade tip speed and broadens the operating range of the HATT. [ABSTRACT FROM AUTHOR]

There is currently a significant focus on using boundary layer control (BLC) approach for controlling the flow around bodies, especially the foil sections. In marine engineering this is done with the hope of increasing the lift - to - drag ratio and efficiency of the hydrofoils. In this paper, effects of the method on hydrodynamic characteristics and tip vortex formation of a hydrofoil are studied. Steady water injection at the tip of the hydrofoil is simulated in different conditions by using ANSYS-CFX commercial software. Validity of the proposed simulations is verified by comparing the obtained results against available experimental data. Effects of the injection on the lift, drag, and lift - to - drag ratio are studied and the ranges within which the injection has the most positive or negative effects, are determined. Furthermore, flow pattern and pressure variation are studied upon the water injection to determine the most positive and negative case and to ascertain the main reasons triggering these phenomena.

The wake flow around the tip of a surface piercing flat plate at an angle of incidence was studied using two-dimensional particle image velocimetry as part of benchmarking the particle image velocimetry technique on the moving carriage in the Australian Maritime College towing tank. Particle image velocimetry results were found to be in close agreement with those of the benchmarking work presented by the Hydro Testing Alliance, and a method of tracking the tip-vortex core near a free surface throughout numerical simulation has been demonstrated. Issues affecting signal to noise ratio, such as specula reflections from the free surface and model geometry were overcome through the use of fluorescing particles and a high-pass optical filter. Numerical simulations using the ANSYS CFX Solver with the volume of fluid method were validated against the experimental results, and a methodology was developed for tracking the location of the wandering vortex core experimentally and through simulation. The ability of the scale-adaptive simulation shear stress transport turbulence model and the shear stress transport model to simulate three-dimensional flow with high streamline curvature was compared. The scale-adaptive simulation shear stress transport turbulence model was found to provide a computationally less resource-intensive method of simulating a complex flow topology with large eddies, providing an insight into a possible cause of tip-vortex aperiodic wandering motion. At high angles of attack, vortex shedding from the leading edge separation of the test geometry is identified as a possible cause of the wandering phenomena. In this study, the vortex centre and point of extreme core velocity were found not to be co-located. The point of extreme stream wise velocity within the vortex core was found to be located within half the vortex radius of the vortex centre. [ABSTRACT FROM AUTHOR]

The acoustic noise and flow condition of tandem propellers in a horizontal axis-type tidal stream power unit, which is composed of counter-rotating propellers and double-rotational armature-type generator without traditional stators, were experimentally investigated. The front and rear propellers rotate in opposite directions and drive the inner and outer armatures, respectively; the rotational torque of the front propeller coincides with the torque of the rear propeller. The front blade profile exerts a great effect on noise generation from the tandem propellers, but the effect of the rear blade profile is comparatively small. The sound pressure level is largely concentrated at low frequencies, and most predominant frequencies can be predicted by Hanson's method. These predominant frequencies are mainly caused by the tip vortex shedding from the front and rear blades in the counter-rotating propellers. The vortex shedding from the front blade does not affect the rear blade. As a result, the acoustic noise is weakened as long as the diameter of the rear propeller is smaller than the diameter of the front propeller. [ABSTRACT FROM AUTHOR]

Highlights • Different curvature correction models are described, and incorporated into the SST k − ω model • By considering the classical Rankine vortex, the response of the curvature correction models in the viscous vortex core, and in the inviscid outside region are investigated. • It is shown that some of the curvature correction models are so sensitive to their clipping values, or their calibration parameters. • It is noted that the curvature correction models that can accurately predict the tip vortex flow fields, alter the boundary layer behaviour on the foil, and form a leading edge vortex. Abstract This paper presents and studies effects of curvature correction (CC) methods to improve two equation RANS simulations of tip vortex flows, exemplified using the SST k − ω turbulence model. Performance of the CC models is first evaluated in the classical Rankine vortex flow field, and then extended into the study of tip vortex flows over an elliptical foil. The results have been compared with experimental measurements in terms of the vortex strength and velocity field, and the importance of the turbulence closure in tip vortex simulations is highlighted. Contribution of the CC models in different terms of the turbulent kinetic energy and specific dissipation transport equations are described, and it is discussed why a CC model may have mesh resolution dependent results. By considering the distribution of the CC function, it is shown that although some of the models can predict the location of the tip vortex core accurately, they still do not significantly improve the vortex prediction as the impact on the turbulent viscosity is wrong or not enough. It is further noted that as some of these models have been calibrated on specific vortex flows, they may not be completely applicable for other cases without recalibration. It is shown that some CC models provide accurate tip vortex predictions, primarily the ones based on the sensitization of the turbulent viscosity. Further, it is noteworthy that the successful models are active not only around the vortex, but also change the boundary layer characteristics on the foil, and the boundary layer separation lines, which consequently can provide the required momentum for the vortex core accelerated axial velocity. [ABSTRACT FROM AUTHOR]

In this paper, a theoretical derivation from the perspective of the force on the slender bodies arranged along the hydrofoil span is performed to explain why the vortex circulation Γvortex should theoretically be equal to the maximum bound circulation Γ0 around the hydrofoil. Then, a more accurate and operable prediction formulation based on hydrofoil load, Γ v o r t e x = k Γ 0 ‾ , is proposed, where k = 0.9–1. Γ 0 ‾ is the averaged hydrofoil circulation along spanwise direction and its value can be obtained easily from hydrofoil load. To validate our new formulation, two typical vortex flows, i.e., tip leakage vortex (TLV) flow and tip vortex (TV) flow, are simulated with large eddy method. Our numerical results confirm that the variations of hydrofoil load along span would lead to streamwise vorticity, and then the rolling-up of latter leads to the vortex formation (Green, 1995). However, since the influential region of vortex is limited that not all streamwise vorticity could be entrained into the vortex and there are viscous and dissipation effects, the actual Γvortex cannot reach Γ0 but it is quite close to Γ 0 ‾. The prediction formulation agrees well with the numerical results and available measured data. • Connection between hydrofoil load distribution and vortex circulation is theoretically derived. • A mathematical derivation illustrates that the variation of hydrofoil load along span leads to streamwise vorticity. • A prediction formulation for vortex circulation based on hydrofoil load is proposed. [ABSTRACT FROM AUTHOR]

The aerodynamic optimization of horizontal axis wind turbine has became one of the most important challenge in the renewable energy field. Over the past few years, many researchers have drawn more attention to the physical processes of the wind energy conversion and precisely the identification of the main causes of energy losses. This paper presents an experimental investigation of near wake dynamics for a model horizontal axis wind turbine in a wind tunnel. The coherent structures downstream of the rotor were studied for different tip speed ratios using the Particle Image Velocimetry (PIV) technique. The influence of the tip vortex meandering was discussed and analyzed using the Proper Orthogonal Decomposition (POD) method. The high-energy modes show that radial meandering is the most energetic source of perturbation in each tip vortex sub-region. The energy fraction of these modes increase gradually during the development of the helical tip vortex filament, which confirm the growth of vortex wandering amplitude in the near wake.

Abstract Three-dimensional steady-state RANS simulations of the flow around a stationary finite-span blade have been carried out. The analysis is directed toward the vorticity dynamics, the forces acting on the blade and their spanwise distributions when using end-plates or detached end-plates, namely end-plates that are not in contact with the blade tips. The objective is to understand the physics at play in the vicinity of the blade tips with an emphasis on the detached end-plates, as the use of such devices could be beneficial for optimizing the performance of vertical-axis turbines. This paper also aims at providing simple guidelines regarding the use of end plates and detached end-plates. The results show that the presence of detached end-plates leads to an important increase of the blade lift coefficient in addition to a significant reduction of its drag coefficient. This is explained by the fact that the detached end-plates modify the path of the vorticity lines shed by the blade, thus affecting the vorticity distribution and the resulting circulation along the blade. This work also shows that the forces on the blade are more sensitive to the dimensions of the detached end-plates in the transverse direction than to any other geometric parameter. Highlights • Both end-plates and detached end-plates strongly affect the forces on a blade. • Detached end-plates alter the distribution of the vorticity shed in the wake. • Detached end-plates do not affect the total circulation of the trailing vortices. • The forces are mainly sensitive to the transverse dimensions of the end-plates. [ABSTRACT FROM AUTHOR]

Appropriate changes to the blade tip pattern can effectively improve fan performance. In this research, the effect of two blade tip patterns and speed variation on aerodynamic performance of a ducted axial-flow fan was numerically investigated. In order to ensure the accuracy of the solving method, numerical results were compared with the experimental data from wind tunnel of the NACA Propeller-Research Center. Numerical results show that both the coefficients of pressure and torque increases with the appropriate groove at the tip of the blade. But due to the higher rate of increase in the coefficient of pressure than that of the torque, aerodynamic efficiency has also increased significantly. This increase has been observed in both patterns and in all operational speed of the fan. But, the increase in aerodynamic coefficients had rising trend up to 3000 rpm and, then, declined. The results determine the best pattern for the tip of the blade. In fact, the structure of the groove is such that it traps a rotating airflow with high kinetic energy at the tip, and this vortex, like a barrier, prevents air leakage. This causes reduction in losses due to mixing of the leakage flow and passage flow. With increasing fan rpm, the generated vortexes in tip groov are amplified, which, in addition to a further decrease in the leakage flow rate from the tip region, increases the viscosity and turbulence losses in the area. [ABSTRACT FROM AUTHOR]

Highlights • Flow-field across the span of a low aspect ratio wall-mounted airfoil was studied. • Laminar separation and transitional laminar separation were observed. • Mean and fluctuating velocity field varied across the span at lifting conditions. • Near-wake vortex shedding varies with spanwise location and angle of attack. • A region of spanwise shear with weak dependency on angle of attack was observed. Abstract Measurements on a wall-mounted NACA 0012 airfoil with an aspect ratio (A R) of 0.5 operating in the low Reynolds number (274,000, based on chord) regime were performed. Measurements included oil flow visualizations and velocity data obtained with a combination of pitot and hotwire probes. Three different geometric angles of attack ( α g ) equal to 0°, 5°, and 10° were considered and the effective angles of attack corresponding to these were quantified through measurements of the velocity vector in the potential flow around the airfoil. The flow around such a low A R airfoil is complex and can be three-dimensional across the span due to strong interaction between the junction, airfoil, and tip flows. In the mid-span region of the airfoil, laminar separation with and without reattachment was present on the suction-side and pressure-side respectively. The pressure-side separation, which is located near the trailing-edge, leads to vortex shedding in the near-wake of the airfoil. The character of this shedding, however, is different between α g = 5° and 10°. Towards the airfoil root (at 25% span), this shedding is supressed by the turbulent junction flow. Towards the free-end of the airfoil (at 75% span), shedding is observed even in the absence of pressure-side laminar separation and may be attributed to the interaction between the spanwise tip flow and the suction-side separation bubble. The airfoil boundary layer in this region is also thinner compared to that at the mid-span point. Further towards the airfoil tip (at 90% span), the shedding still exists; however, its intensity and character are modified by the dominant vortex dynamics. In proximity to the airfoil tip, the velocity and turbulence are affected by both the primary and secondary vorticity for the highest angle of attack studied. [ABSTRACT FROM AUTHOR]

In present study, aerodynamics of a NACA4412 wings with aspect ratio of 1 and 3 was considered experimentally at Reynolds numbers of 2.5 × 104, 5 × 104 and 7.5 × 104. Studies for AR = 1 wing showed that stall was delayed and extra (vortex) lift was obtained, because separation bubble got smaller in both chordwise and spanwise axes with effect of wing-tip vortices. Oil-flow experiments at higher angles of attack clarified the reason for vortex lift obtained from AR = 1 wing. However, there was an increase in drag coefficient as well as vortex lift, and stall delayed due to tip vortex. Turbulence intensity distributions pointed out location of the transition to turbulence; Reynolds stress and turbulence kinetic energy distributions indicated shear layer. Furthermore, in experiments of AR = 3 wing, the viscous forces and leading edge vortices were effective at Re = 2.5 × 104 and Re = 5 × 104, but flow over the wing at Re = 7.5 × 104 acted as a 2D flow. After α = 12°, bubble burst and stall consisted abruptly because effectiveness of 3D flow decreased over wing. Strouhal (St) numbers of vortex shedding frequencies in wake of AR = 3 wing had a certain difference from St = 0.17/sinα curve at lower angle of attack (α = 0° − 10°) due to separation bubble, but AR = 1 wings showed that St numbers were near St = 0.17/sinα curve. [ABSTRACT FROM AUTHOR]

Abstract The current study investigates different cavitation inception prediction methods to characterize tip vortex flows around an elliptical foil, and a high skewed low-noise propeller. Adapted inception models cover different levels of complexity including wetted flow, Eulerian cavitation simulations, and Rayleigh-Plesset bubble dynamics models. The tip vortex flows are simulated by Implicit Large Eddy Simulation on appropriate grid resolutions for tip vortex propagation, at least 32 cells per vortex diameter according to previous studies guidelines. The results indicate that the cavitation inception predictions by the minimum pressure criterion of the wetted flow analysis are similar to weak water inception measurements. In the wetted flow analysis, the proposed energy criterion is noted to provide reasonably accurate inception predictions, similar to the predictions by Eulerian cavitation simulations with much lower computational costs. Comparison between high speed videos and numerical results of the propeller shows the capability of the numerical methodology in predicting tip vortex structures in different conditions. The interaction between vortices and their impact on the pressure field and the cavitation inception are also highlighted. The strong dependency of the inception on the initial nuclei sizes are demonstrated, and it is shown that for weaker tip vortices this dependency becomes more significant. Highlights • Cavitation inception is evaluated numerically by considering different models including wetted and cavitation simulations. • Inception characteristics of tip vortex flows around an elliptical foil, and a high skewed low-noise propeller are evaluated. • Detailed numerical simulations of tip vortex flow are presented, and compared with high speed videos. • Strong dependency of the inception on the initial nuclei sizes is demonstrated. • The terms of weak and strong water used in experimental measurements are addressed by the numerical results. [ABSTRACT FROM AUTHOR]