Accepted Manuscripts

Saddam Hossain Joy, Saeedur Rahman and A. B. M. Toufique Hasan
J. Fluids Eng   doi: 10.1115/1.4038214
Present investigation deals with the interaction of an incident oblique shock wave on a turbulent boundary layer over a wavy surface. The oblique shock wave was generated by a 8° wedge in a free stream Mach number of 2.0. Three dimensional Reynolds averaged Navier-Stokes (RANS) equations with k-? SST turbulence model were used for numerical computation. The computed results are in good agreement to the experimental measurement and DNS data in case of the interaction of an oblique shock with plain flat plate. To identify the effect of surface waviness on shock-wave/turbulent boundary-layer interaction (SWBLI), a section of the flat plate was replaced by a wavy surface. Computations have been conducted for different magnitude of wavy amplitude. Further, the wave length of the wavy surface has been varied. Results showed that the presence of wavy surface induces supplementary shock and expansion waves in the flow field which are referred as topographic waves. This supplementary system of waves interacts with the counterpart of intrinsic SWBLI in a complex manner. Flow structure, separation behaviour and aerodynamic characteristics are studied. It is revealed that the amplitude is dominant than the wavelength of waviness in case of SWBLI on a wavy surface.
TOPICS: Shock waves, Boundary layer turbulence, Waves, Shock (Mechanics), Flow (Dynamics), Computation, Flat plates, Turbulence, Reynolds-averaged Navier–Stokes equations, Wedges, Mach number, Wavelength, Separation (Technology), Boundary layers
Francisco J Huera-Huarte
J. Fluids Eng   doi: 10.1115/1.4038168
A parametric study is presented in this paper, showing the impulsive performance of foils with different flexural stiffness, pitching in a quiescent flow. A wide range of Reynolds numbers (different imposed kinematics) and foil rigidities is covered, depicting how flexibility effects on impulse, are more important at the largest Reynolds numbers. The impulsive performance of the system is derived from direct thrust force measurements. Passive flexibility alters vortex strength and formation in the wake of the pitching foil. These changes in the wake formation can be used to explain the differences in the measured impulses. The wake dynamics is studied after quantitative analysis of particle image velocimetry data, and it is linked to the momentum transfer generated by the foil.
TOPICS: Fluids, Chords (Trusses), Impulse (Physics), Wakes, Reynolds number, Particulate matter, Thrust, Kinematics, Dynamics (Mechanics), Momentum, Flow (Dynamics), Vortices, Force measurement, Stiffness
Technical Brief  
Aaron S. Alexander and Arvind Santhanakrishnan
J. Fluids Eng   doi: 10.1115/1.4038166
Savonius VAWTs typically suffer from low efficiency due to detrimental drag production during one half of the rotational cycle. The present study examines a stator assembly created with the objective of trapping cylindrical flow for application in a Savonius vertical axis wind turbine (VAWT). While stator assemblies have been studied in situ around Savonius rotors in the past, they have never been isolated from the rotor to determine the physics of the flow field, raising the likelihood that a moving rotor could cover up deficiencies attributable to the stator design. The flow field created by a stator assembly, sans rotor, is studied computationally using 3D numerical simulations in the commercial computational fluid dynamics (CFD) package Star-CCM+. Examination of the velocity and pressure contours at the central stator plane show that the maximum induced velocity exceeded the free stream velocity by 65%. However, flow is not sufficiently trapped in the stator assembly, with excess leakage occurring between the stator blades due to adverse pressure gradients and momentum loss from induced vorticity. A parametric study was conducted on the effect of the number of stator blades with simulations conducted with 6, 12, and 24 blades. Reducing the blade number resulted in a reduction in the cohesiveness of the internal swirling flow structure and increased the leakage of flow through the stator. Two unique energy loss mechanisms have been identified with both caused by adverse pressure gradients induced by the stator.
TOPICS: Flow (Dynamics), Vertical axis wind turbines, Savonius wind turbines, Stators, Rotors, Blades, Manufacturing, Computational fluid dynamics, Leakage, Pressure gradient, Swirling flow, Design, Engineering simulation, Cycles, Drag (Fluid dynamics), Simulation, Energy dissipation, Vorticity, Computer simulation, Physics, Pressure, Momentum
John R. Murdock, Aamir Ibrahim and Song-Lin Yang
J. Fluids Eng   doi: 10.1115/1.4038167
To provide porous media substrates that are quick to generate and characterize for lattice Boltzmann analysis, we propose a straightforward algorithm. The method leverages the benefits of the lattice Boltzmann method, and is extensible to multiphysics flows. Several parameters allow for simple customization. The generation algorithm and lattice Boltzmann method are reviewed, and suggested implementation covered. Additionally, results are discussed and interpreted to evaluate the approach. Several verification tools are employed such as Darcy’s law, the Ergun equation, the Koponen correlation, a newly proposed correlation, and experimental data. Agreement and repeatability are found to be excellent, suggesting this relatively simple method is a good option for engineering studies.
TOPICS: Filters, Lattice Boltzmann methods, Algorithms, Flow (Dynamics), Porous materials, Darcy's law
Cheng Liu, Wei Wei, Qingdong Yan, Brian Weaver and Houston G. Wood
J. Fluids Eng   doi: 10.1115/1.4038115
Cavitation in torque converters may cause degradation in hydrodynamic performance, severe noise, or even blade damage. Researches have highlighted that the stator is most susceptible to the occurrence of cavitation due to the combination of high flow velocities and high incidence angles. The objective of this study is to therefore investigate the effects of cavitation on hydrodynamic performance as well as the influence of stator blade geometry on cavitation. A steady-state homogeneous computational fluid dynamics (CFD) model was developed and validated against test data. It was found that cavitation brought severe capacity constant degradation under low speed ratio operating conditions and vanished in high speed ratio operating conditions. A design of experiments (DOE) study was performed to investigate the influence of stator design variables on cavitation over various operating conditions and it was found that stator blade geometry had a significant effect on cavitation behavior. The results show that stator blade count and leaning angle are important variables in terms of capacity constant loss, torque ratio variance, and duration of cavitation. Large leaning angles are recommended due to their ability to increase the cavitation number in torque converters over a wide range of speed ratios, leading to less stall capacity loss as well as a shorter duration of cavitation. A reduced stator blade count is also suggested due to a reduced torque ratio loss and capacity loss at stall.
TOPICS: Torque converters, Cavitation, Blades, Geometry, Stators, Computational fluid dynamics, Torque, Flow (Dynamics), Noise (Sound), Design, Experimental design, Steady state, Damage
Xiao Yexiang, Zhu Wei, Wang Zhengwei, Zhang Jin, Ahn Soo-Hwang, Zeng Chongji and Luo Yongyao
J. Fluids Eng   doi: 10.1115/1.4038077
The S-shaped characteristic curves in pump-turbines, complicate synchronization with the electrical grid and affect system safety. Misaligned Guide Vanes (MGVs) are one of the most effective solutions to avoid S-shaped characteristics. The internal flow mechanism with the MGV for improving S-shaped characteristics was studied by numerical analysis. Six operating conditions were modeled in the S-shaped region. Four guide vanes were arranged as the MGVs to qualitatively and quantitatively analyze the flow behavior. The internal flow was quite complex at the four operating points without the MGV, here the attack angle and the flow behavior had no obvious difference at each vane. For the similar conditions with MGVs, attack angles and internal flow fields varied clearly at each vane, especially in the vaneless region and in the runner blade passages. For the same discharge rates, total openings and rotating speeds, the internal flows were quite different between with and without the MGVs. The MGVs disrupt the high-speed circumferential water ring (appreciably faster compared to the main flow) in the vaneless region and maintain operation with higher unit speeds. Consequently, the unit speed is larger at the same unit discharge in the S-shaped region. Therefore, the MGV method can reduce S-shaped characteristics.
TOPICS: Pump turbines, Numerical analysis, Guide vanes, Internal flow, Flow (Dynamics), Safety, Blades, Power grids, Synchronization, Water
Foo Kok, Roy Myose and Klaus A. Hoffmann
J. Fluids Eng   doi: 10.1115/1.4038087
The onset condition of flow separation in diverging tee junctions was investigated numerically. Flow separation and recirculation at the proximal region of a bypass graft can contribute to early-phase graft failure in aortocoronary bypass surgery. Rounding the inlet edge of the branch reduces the likelihood of flow separation and recirculation. The recirculating zone at the upstream end of the branch is fully eliminated when a threshold value of mass flow rate ratio is reached. The corresponding flow characteristics obtained from diverging tees with a diameter ratio ≤ 0.2 and a radius of curvature ≤ 0.25 for Reynolds number ≤ 1817 indicate that an increasing flow rate ratio induces an exponential decrease in the recirculation length. An increase in the diameter ratio and Reynolds number increases both the onset condition of the flow separation and the recirculation length at the upstream end of the branch. However, a decrease in the diameter ratio reduces the onset condition of separation more effectively than the reduction of the radius of curvature at the junction.
TOPICS: Flow separation, Flow (Dynamics), Reynolds number, Junctions, Surgery, Failure, Separation (Technology)
P. Liu, A. Patil and G. Morrison
J. Fluids Eng   doi: 10.1115/1.4038080
This study is focused on the development and validation of the analytical model to predict the performance characteristics of multiphase flow twin-screw pump for wide range of operating conditions. A 200 HP, 635 GPM capacity multiphase flow twin-screw pump was tested with inlet pressure varying from 15 psi to 100 psi at Gas Void Fraction (GVF) varying from 0 to 100% to validate the model. A new model is proposed to study the leakage flow in the twin screw pump. Adiabatic compressible flow is assumed in the circumferential clearance. The acceleration of the two-phase flow is taken into account in the new model. The change of Mach number of the leakage flow in the clearance and the possibility of choked flow at the outlet of the clearance was studied. Model provided important information about pressure distribution across the screw length, volumetric efficiency of the pump and chocked flow condition. Model verification using experimental data concluded the paper.
TOPICS: Screws, Multiphase flow, Pumps, Clearances (Engineering), Pressure, Flow (Dynamics), Leakage flows, Performance characterization, Mach number, Two-phase flow, Compressible flow, Porosity
Masoud Darbandi and Moslem Sabouri
J. Fluids Eng   doi: 10.1115/1.4038085
This work utilizes the direct simulation Monte Carlo (DSMC) calculations and examines the influence of rarefication on the mixing length and effective diffusion coefficient in a two-species mixing problem. There have been efforts in past rarefied mixing flow studies to bridge between the mixing evolution rate and Knudsen number. A careful review of those efforts shows that the past derived relations did not determine the weights of Reynolds (or Peclet) number in the rarefaction influences. Although they indicated that an increase in Knudsen would decrease the mixing length, such reductions were primarily due to the Reynolds (or Peclet) reduction. Therefore, those studies could not explicitly appraise the contribution of rarefaction in the total mass diffusion magnitude. This work focuses specifically on the role of rarefaction in the total diffusive mass transfer magnitude in rarefied gas mixing problems. It excludes the contributions of momentum and heat to the mass diffusion via imposing suitable velocity, pressure, and temperature fields in the mixer domain. The results show that there will be some decreases in the diffusive mass fluxes and some increases in the mixing length as Knudsen increases. Using the Fick's law, the effective diffusion coefficient is then calculated in the mixer zone. The results show that this coefficient may vary considerably throughout the mixer zone due to the local rarefaction level variation. The results of all investigated cases indicate that the trends of their effective diffusion coefficient variations approach to a limiting value as the rarefaction level decreases.
TOPICS: Mass transfer, Simulation, Diffusion (Physics), Knudsen number, Bridges (Structures), Flux (Metallurgy), Pressure, Momentum, Flow (Dynamics), Heat, Temperature
Seung Joong Kim and Hyung Jin Sung
J. Fluids Eng   doi: 10.1115/1.4038088
Large eddy simulations (LESs) are carried out to predict the flow noise produced in the solenoid valve of an antilock braking system (ABS) using Lighthill's acoustic analogy and the Ffowcs Williams and Hawkings (FW-H) surface integral method. The fluid inside the valve is assumed to be incompressible at a fixed temperature. The solenoid valve operation is realized by applying an overset grid methodology to the moving plunger, and the plunger has a linear motion in the axial direction. Several types of solenoid valves are numerically designed to maximally reduce the flow noise. The upstream flow is detached through a small opening between the plunger and the seat, which generate pressure fluctuation around the narrow gap, which is subject to high wall pressure fluctuations and shear stresses. LESs are performed by varying the position of the flow separation. An optimal design of the valve is obtained, featuring a small radius of surface curvature, a smooth surface, and a large plunger tip area angle. Measurements are obtained from the optimal design to validate the design in a real vehicle performance test, and the predicted pressure frequency in the solenoid valve agreed well with the experimental results.
TOPICS: Flow (Dynamics), Noise (Sound), Design, Valves, Solenoids, Antilock braking systems, Pressure, Fluctuations (Physics), Testing performance, Large eddy simulation, Shear stress, Vehicles, Flow separation, Temperature, Fluids, Acoustics
Rodolfo Vaghetto, Nolan Goth, Philip Jones, Mason Childs, Saye Lee, Duy Thien Nguyen and Yassin A. Hassan
J. Fluids Eng   doi: 10.1115/1.4038086
To achieve longer-life liquid-metal fast reactor cores, designers are considering to increase the wall gap of the wire-wrapped hexagonal fuel bundles to account for volumetric void swelling and radiation creep. A new wire-wrapped hexagonal test bundle has been constructed, with a wall gap larger than prior experiments, and experimental pressure drop data has been generated under laminar, transition, and turbulent flow regimes (corresponding to Re of 250 – 19,000), to complement the existing database of small wall gap experimental bundles. The comparison of the experimental data set with the predictions of four existing correlations (Baxi and Dalle Donne, Cheng and Todreas Detailed, Kirillov, and Rehme), showed general agreement between data and the selected correlations. However, the Cheng and Todreas Detailed correlation most accurately predicted the experimental trend, and the transition between flow regimes. The analysis of the experimental data also revealed that the larger wall gap size caused a lower bundle pressure drop due to the increased bypass flow area.
TOPICS: Pressure measurement, Fuels, Wire, Flow (Dynamics), Pressure drop, Fast neutron reactors, Creep, Radiation (Physics), Liquid metals, Databases, Turbulence
Zhi-jiang Jin, Zhi-xin Gao, Jin-yuan Qian, Zan Wu and Bengt Sunden
J. Fluids Eng   doi: 10.1115/1.4038090
Hydrodynamic cavitation occurs inside globe valves increases the energy consumption burden of the whole piping system, and leads to severe damages to the valve body and piping system with a large economic loss. In this paper, in order to reduce the hydrodynamic cavitation inside globe valves, effects of valve body geometry parameters including bending radius, deviation distance and arc curvature linked to in/export parts on hydrodynamic cavitation are investigated by using a cavitation model. To begin with, the numerical model is compared with similar works to check its accuracy. Then, the vapor volume fraction in each computational cell and the total vapor volume are predicted. The results show that vapor primarily appears around the valve seat and connecting downstream pipes. The hydrodynamic cavitation does not occur under a small inlet velocity, a large bending radius, and a large deviation distance. Cavitation intensity decreases with the increase of the bending radius, the deviation distance and the arc curvature linked to in/export parts. This indicates that valve geometry parameters should be chosen as large as possible, while the maximal fluid velocity should be limited. This work is of significance for hydrodynamic cavitation or globe valve design.
TOPICS: Cavitation, Valves, Vapors, Geometry, Piping systems, Damage, Energy consumption, Computer simulation, Design, Pipes, Fluids
Patrick R. Richard, Stephen John Wilkins and Joseph W. Hall
J. Fluids Eng   doi: 10.1115/1.4038091
Air traffic volume is expected to triple in the U.S. and Europe by 2025, and as a result, the aerospace industry is facing stricter noise regulations. Apart from the engines, one of the significant contributors of aircraft noise is the deployment of high-lift devices, like leading-edge slats. In particular, Particle Image Velocimetry (PIV) measurements were performed on a scale-model wing equipped with a leading-edge slat in a wind tunnel. Two Reynolds numbers based on wing chord were studied: Re=6x10^5 and 1.3x10^6. A snapshot Proper Orthogonal Decomposition (POD) analysis indicated that differences in the time-averaged statistics between the two were tied to differences in the coherent structures formed in the slat cove shear layer. In particular, the lower Re flow seemed to be dominated by a large-scale vortex formed in the slat cove that was related to the unsteady flapping and subsequent impingement of the shear-layer onto the underside of the slat. A train of smaller, more regular vortices was detected for the larger Re case which seemed to cause the shear-layer to be less curved and impinge closer to the tail of the slat than for the lower Re case. The smaller structures are consistent with Rossiter modes being excited within the slat cove. The impingement of the shear-layers on, and the proximity of the vortices to the slat and the main wing are expected to be strong acoustic dipoles in both cases.
TOPICS: Flow (Dynamics), Particulate matter, Shear (Mechanics), Vortices, Wings, Noise (Sound), Chords (Trusses), Aerospace industry, Acoustics, Engines, Reynolds number, Dipoles (Electromagnetism), Air traffic control, Statistics as topic, Aircraft, Principal component analysis, Regulations, Trains, Wind tunnels
Mirae Kim, Hyun Dong Kim, Eunseop Yeom and Kyung Chun Kim
J. Fluids Eng   doi: 10.1115/1.4038089
Three-dimensional (3D) curved wall jets are a significant topic in various applications related to local heat and mass transfer. This study investigates the effects of the impinging angle and Reynolds number with a fixed distance from the nozzle to the surface of a cylinder. The particle image velocimetry (PIV) method was used to measure the mean streamwise velocity profiles, which were normalized by the maximum velocity along the centerline of the impinging jet onto the cylinder. After the impingement of the circular jet, a 3D curved wall jet develops on the cylinder surface due to the Coanda effect. At a given Reynolds number, the initial momentum of the wall jet increases, and flow separation occurs further downstream than in normal impingement as the impinging angle increases. At a given impinging angle, flow separation is delayed with increasing Reynolds number. A self-preserving wall jet profile was not attained in the 3D curved wall jet. The turbulence intensity and the Reynolds shear stress were obtained to analyze the turbulence characteristics. The radial turbulence intensity showed similar tendencies to a two-dimenisional (2D) curved wall jet, but the streamwise turbulence intensity was dissimilar. The Reynolds shear stress decreases downstream of the cylinder wall due to the decreased velocity and centrifugal force.
TOPICS: Flow (Dynamics), Jets, Curved walls, Cylinders, Turbulence, Reynolds number, Flow separation, Shear stress, Nozzles, Momentum, Centrifugal force, Heat, Mass transfer, Particulate matter
Yuehua Fan, Zhenxun Gao, Chongwen Jiang and Chun-Hian Lee
J. Fluids Eng   doi: 10.1115/1.4038092
A naval aircraft has the potential to experience inlet performance decline when taking off from the carrier deck with the steam-driven catapult assistance. The steam ingested into inlet may cause to time-dependent rise and spatial distortion of the total temperature on the inlet-exit, which would decrease the compressor stall-margin and then lower the performance of the turbine engine. In this paper, these temporal and spatial temperature non-uniformities are numerically studied using the dual-time-step transient method with a real aircraft/inlet model taken into account. The flowfield characteristics of a designed baseline case are first analyzed, indicating that the engine's suction effect and the wind velocity relative to the aircraft are two key factors affecting the steam ingestion. The former is dominant at the beginning of take-off since the aircraft's velocity is low, while the latter is increasingly significant as the aircraft accelerates. Next, parametric studies show that the greater the wind speed is, the less significantly the flowfield of the inlet-exit would be influenced by the steam. The effects are also studied among various steam leakage profiles - two are constant in time-histories of the steam leakage rate, whereas the other two are nonlinear with the maximum value at different instants. It is found that the temperature rise rate of the inlet-exit would increase apparently if the steam leakage rate reaches the maximum earlier.
TOPICS: Steam, Aircraft, Leakage, Temperature, Wind velocity, Transients (Dynamics), Gas turbines, Suction, Compressors
Zhao Xiaoran, Xiao Yexiang, Wang Zhengwei, Luo Yongyao and Cao Lei
J. Fluids Eng   doi: 10.1115/1.4037973
Unsteady flow phenomena like rotating stall frequently occur in centrifugal pumps under off design conditions. Rotating stall could lead to flow instabilities and pressure pulsation, which affect the normal operation of pumps. The mechanism of rotating stall has not been sufficiently understood in previous researches. In this study, the impact of rotating stall in the impeller on centrifugal pump stability and pressure pulsation is numerically investigated. This paper aims to detect the unsteady flow characteristics inside the centrifugal pump by CFD technology, analyze pressure pulsations caused by rotating stall and explore the propagation mechanism of rotating stall. Unsteady numerical simulations were performed by ANSYS 16.0 to model the unsteady flow within the entire flow passage of a centrifugal pump under 0.4QBEP and 0.6QBEP working conditions. Through flow characteristics research, the generation and propagation of rotating stall are discovered. Flow separation appears near the leading edge of the pressure side and transforms into vortexes, which move along the passage. Meanwhile, the stall cells rotate circumferentially in the impeller. Additionally, frequencies and amplitudes of pressure pulsations related to rotating stall are investigated by spectrum analysis. The results detect a possible characteristic frequency of rotating stall and show that the interaction between stall cells and the volute tongue could have an influence on RSI.
TOPICS: Pressure, Design, Centrifugal pumps, Unsteady flow, Impellers, Flow (Dynamics), Spectroscopy, Computer simulation, Stability, Flow instability, Pumps, Vortices, Emission spectroscopy, Computational fluid dynamics, Flow separation
Matthieu Boudreau and Guy Dumas
J. Fluids Eng   doi: 10.1115/1.4037974
An analysis of the vortex dynamics in the wake of three different free-stream turbine concepts is conducted to gain a better understanding of the main processes affecting the energy recovery in their wakes. The turbine technologies considered are the axial-flow turbine (AFT), the cross-flow turbine (CFT), also known as the H-Darrieus turbine, and the oscillating-foil turbine (OFT). The analysis is performed on single turbines facing a uniform oncoming flow and operating near their optimal efficiency conditions at a Reynolds number of $10^7$. Three-dimensional Delayed Detached-Eddy Simulations (DDES) are carried out using a commercial finite-volume Navier-Stokes solver. It is found that the wake dynamics of the AFT is significantly affected by the triggering of an instability while that of the CFT and the OFT are mainly governed by the mean flow field stemming from the tip vortices' induction.
TOPICS: Dynamics (Mechanics), Wakes, Turbines, Vortices, Flow (Dynamics), Electromagnetic induction, Eddies (Fluid dynamics), Reynolds number, Simulation, Wake turbulence, Energy recovery, Engineering simulation, Axial flow, Cross-flow, Delay differential equations
David Marten, George Pechlivanoglou, Christian Navid Nayeri and Christian Oliver Paschereit
J. Fluids Eng   doi: 10.1115/1.4037978
Recently a new interest in vertical axis wind turbine (VAWT) technology is fueled by research on floating support structures for large scale offshore wind energy application. For the application on floating structures at multi megawatt size, the VAWT concept may offer distinct advantages over the conventional horizontal axis wind turbine (HAWT) design. As an example VAWT turbines are better suited for upscaling and, at multi megawatt size, the problem of periodic fatigue cycles reduces significantly due to a very low rotational speed. Additionally, the possibility to store the transmission and electricity generation system at the bottom, compared to the tower top as in a HAWT, can lead to a considerable reduction of material logistics costs. However, as most VAWT research stalled in the mid 90's, no sophisticated and established tools to investigate this concept further exist today. Due to the complex interaction between unsteady aerodynamics and movement of the floating structure fully coupled simulation tools, modelling both aero- and structural dynamics are needed. A nonlinear Lifting Line Free Vortex Wake code was recently integrated into the open source wind turbine simulation suite QBlade. This paper describes some of the necessary adaptions of the algorithm, which differentiates it from the usual application in HAWT simulations. A focus is set on achieving a high robustness and computational efficiency. A short validation study compares LLFVW results with those of a 2D URANS simulation.
TOPICS: Design, Vertical axis wind turbines, Horizontal axis wind turbines, Simulation, Floating structures, Robustness, Wind turbines, Aerodynamics, Fatigue, Logistics, Wakes, Ocean engineering, Structural dynamics, Algorithms, Modeling, Turbines, Vortices, Wind energy, Cycles, Electric power generation
Chang Cai, Zhigang Zuo, Shuhong Liu and Takao Maeda
J. Fluids Eng   doi: 10.1115/1.4037980
Leading-edge protuberances on airfoils or wings have been considered as a viable passive control method for flow separation. In this paper, the aerodynamic performance of a modified airfoil with a single leading-edge protuberance was investigated and compared with the baseline NACA 634-021 airfoil. Spalart-Allmaras turbulence model was applied for the numerical simulation. Compared to the sharp decline of baseline lift coefficient, the stall angle of the modified foil decreased and the decline of the lift coefficient became mild. The post-stall performance of the modified airfoil was improved, while the pre-stall performance was declined. Asymmetric flows along the spanwise direction were observed on the modified airfoil, and the local region around one shoulder of the protuberance suffered from leading edge separation at pre-stall angles of attack, which may be responsible for the performance decline. At post-stall angles of attack, the attached flows along the peak of the protuberance with a sideward velocity component, would help improving the total performance of the airfoil. Experimental visualization methods, including surface tuft and smoke flow, were performed, and the asymmetric flow pattern past the protuberance was successfully captured. This specific phenomenon may be largely related to the formation of the bi-periodic condition and other complicated flow patterns induced by multiple leading-edge protuberances. The formation mechanism and suppression method of the symmetry breaking phenomenon should be investigated more deeply in the future to guide the practical application of this passive control method.
TOPICS: Airfoils, Flow (Dynamics), Passive control, Separation (Technology), Turbulence, Computer simulation, Visualization, Flow separation, Smoke, Wings
Robert Jaron, Antoine Moreau, Sébastien Guérin and Rainer Schnell
J. Fluids Eng   doi: 10.1115/1.4037981
A major source of contra-rotating open rotor tonal noise is caused by the interaction of the front-rotor wakes with the aft-rotor blades. Inspired by chevron nozzles, which increase the mixing process in jet shear layers, serrations are implemented at the front rotor trailing-edge in order to increase the wake mixing and thus reduce the tones. The depth and width of the serrations are optimized with a multi-objective, metamodel-assisted evolutionary algorithm. For each member a steady-state Reynolds-Averaged Navier-Stokes (RANS) simulation is performed which is coupled with an analytical noise prediction method to evaluate the noise reduction due to the serrations. The results confirm that tonal interaction noise can be reduced by means of trailing-edge serrations. It is found that the major noise reduction mechanism for wake interaction is attributed to increased destructive interferences occurring in spanwise direction. The tonal noise generated through the interaction of the rear rotor potential field with the front rotor trailing edge is also slightly reduced because of the circumferential and axial shift of the serrated trailing edge. Furthermore the present study demonstrates the feasibility of performing an acoustic optimization with a hybrid approach that predicts the noise analytically and extracts the aerodynamic input data from a steady-state Reynolds -Averaged Navier-Stokes flow solution.
TOPICS: Noise (Sound), Optimization, Rotors, Wakes, Noise control, Steady state, Blades, Evolutionary algorithms, Reynolds-averaged Navier–Stokes equations, Acoustics, Simulation, Shear (Mechanics), Flow (Dynamics), Nozzles

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