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Accepted Manuscripts

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research-article  
Shibdas Dholey
J. Fluids Eng   doi: 10.1115/1.4040572
This paper discusses an unsteady separated stagnation-point flow of a viscous fluid over a flat plate covering the complete ranges of the flow strength parameter a (> 0) and the unsteadiness parameter ß. Here ß varies from zero, Hiemenz's steady stagnation-point flow, to large ß-limit, for which the governing boundary layer equation reduces to an approximate one in which the convective inertial effects are negligible. An important finding of this study is that the governing boundary layer equation conceives an analytic solution for the specific relation ß = 2a. It is found that for a given value of ß(=0) the present flow problem always provides an unique attached flow solution (AFS), whereas for a negative value of ß the solution may or may not exist that depends on the values of a and ß. If the solution exists, it may either be unique or dual or multiple in nature. According to the physical structure of these solutions, they are of two kinds : one is AFS and other is reverse flow solution (RFS). Another interesting result of this analysis is the asymptotic solution which is more practical than the numerical solutions for large values of ß(> 0) depending on the values of a. The analysis reveals that for a positive value of ß the pressure is nonmonotonic along the stagnation-point streamline, there is a pressure minimum which moves towards the stagnation-point with the increase of ß > 0.
TOPICS: Flat plates, Stagnation flow, Flow (Dynamics), Pressure, Boundary layers, Fluids
research-article  
Hawwa/F Kadum, Devin Knowles and Raul Bayoan Cal
J. Fluids Eng   doi: 10.1115/1.4040568
Conditional statistics are employed in analyzing wake recovery and Reynolds shear stress (RSS) and flux directional out of plane component preference. Examination of vertical kinetic energy entrainment through describing and quantifying the aforementioned quantities has implications on wind farm spacing, design, and power production, and also on detecting loading variation due to turbulence. Stereographic particle image velocimetry measurements of incoming and wake flow fields are taken for a 3x4 model wind turbine array in a scaled wind tunnel experiment. RSS component is influenced by huvi component, whereas hvwi is more influenced by streamwise advection of the flow; u, v, and w being streamwise, vertical, and spanwise velocity fluctuations, respectively. Relative comparison between sweep and ejection events, Shuiuj, shows the role of streamwise advection of momentum on RSS values and direction. It also shows their tendency to an overall balanced distribution. huwi intensities are associated with ejection elevated regions in the in ow, yet in the wake, huwi is linked with sweep dominance regions. Downward momentum flux occupies the region between hub height and top tip. Sweep events contribution to downward momentum flux is marginally greater than ejection events'. When integrated over the swept area, sweeps contribute 55% of the net downward KEF and 45% is the ejection events contribution. Sweep dominance is related to momentum deficit as its value in near wake elevates 30% compared to inflow. Understanding these quantities can lead to improved closure models.
TOPICS: Wind turbines, Kinetic energy, Shear stress, Momentum, Wakes, Design, Energy generation, Wind farms, Wind tunnels, Flow (Dynamics), Particulate matter, Turbulence, Fluctuations (Physics), Inflow, Preferences, Statistics as topic
research-article  
Aydin Haci Dönmez, Zehra Yumurtaci and Levent A. Kavurmacioglu
J. Fluids Eng   doi: 10.1115/1.4040557
The aim of the current study is to investigate the effect of inlet blade angles on cavitation performance in a centrifugal pump. In order to reveal this relationship, both hub and shroud blade angles are considered and a two phase 3D Computational Fluid Dynamics (CFD) study is carried out. Shear Stress Transport (SST) turbulence and Rayleigh-Plesset cavitation models are used in simulations. Inlet blade angles for both hub and shroud are changed and pump performance (Head-Discharge) and cavitation (Head-Inlet Pressure) graphs are obtained for eight different designs. Afterwards, numerical cavitation tests are conducted, required net positive suction head values of the each design are calculated and variations are demonstrated. Results show that, hub and shroud blade angle variations have no significant effect on the pump characteristic curves excluding for shroud blade angle at high discharge values. However, cavitation performance of the pump is excessively affected for both hub and shroud blade angle alterations. Increasing hub blade angle has slightly negative effect on cavitation performance of the pump. On the other hand, while increasing shroud blade angle from 20° to 30° have positive effect on cavitation performance, it is negatively affected from 30° to 50°.
TOPICS: Cavitation, Blades, Centrifugal pumps, Pumps, Computational fluid dynamics, Design, Engineering simulation, Pressure, Turbulence, Suction, Simulation, Shear stress
research-article  
Promode Bandyopadhyay
J. Fluids Eng   doi: 10.1115/1.4040523
The propulsors of organisms from paramecia to dolphins have ball-and-socket jointed bases that allow large-amplitude, low-friction swings. Their olivo-cerebellar control also remains unchanged. Yet, the propulsive surfaces of small animals vary widely from flagellar filaments (0 < Re < 5) to flapping fins (Re > 20) with an intermediate range of Reynolds number (5 < Re < 20) where both types are present in the same swimming animal. Analysis suggests that these unsteady surfaces are mechanical oscillators coupled to their nonlinear wakes. A low-friction-driven oscillator that can interact with the oscillators of models or live swimming and flying animals could help us understand the hydro-structural events prompting evolution of such surfaces at specific Re values. A gearless underdamped hemispherical motor oscillator is described where efficiency increases by a factor of eight as the forces drop by a factor of ten from 10 N. The efficiencies at 0.8 N are comparable to the total thermal efficiencies of flies, and the quality factor is comparable. The fin oscillations of penguins, Clione antarctica and flies are reproduced. When flapping at 0.3 Hz, the oscillator would respond to all wake nonlinearities. Abrupt fin turning is modeled by switching the roll and pitch phase difference between -p/2 and p/2 in successive quadrants. Defining the fish-wake lock-in error as the difference between Triantafyllou's fish Strouhal number and the tangent of the vortex-shedding angle, an experiment is discussed for measuring the minimum drag of live fish.
TOPICS: Engines, Motors, Wakes, Friction, Locks (Waterways), Oscillations, Errors, Fins, Q-factor, Antarctic region, Vortex shedding, Thermal efficiency, Drag (Fluid dynamics), Reynolds number
research-article  
Bjørn W Solemslie and Ole G. Dahlhaug
J. Fluids Eng   doi: 10.1115/1.4040444
A post processing method has been developed to enable the extraction of quantifiable data from images captured from within the rotating frame of reference of a Pelton turbine. The turbine tested was the reference Pelton runner, designed at the Waterpower Laboratory (NTNU). The method relies on interpolation to 3D map the inner hydraulic surface of the bucket. Interpolation has been conducted with two different schemes, i.e., Barycentric triangular and biharmonic spline, where the latter showed significant increase in accuracy. The 3D mapping provides the world coordinates of the pixels within the bucket and enables the tracking of the water front as it propagates through the bucket. The method has been described and the uncertainties have been estimated in the order of 0.4mm for most of the hydraulic surface. The results follow expected, and previously observed, behaviour and show great promise with regard to validation of numerical simulations. Results obtained by the method will be of great interest for the CFD community as it can be used as direct validation data for flow propagation with numerical methods. The method relies heavily on manual input due to the high noise and low contrast of the available images, which causes an increase in both uncertainty and time consumption. Suggestion for reducing uncertainty and time consumption are presented and will be implemented in future publications.
TOPICS: Flow (Dynamics), Turbines, Uncertainty, Interpolation, Water, Computer simulation, Splines, Noise (Sound), Computational fluid dynamics, Numerical analysis
research-article  
Ming Liu, Lei Tan and Shuliang Cao
J. Fluids Eng   doi: 10.1115/1.4040502
The cavitating flow around the asymmetric leading edge (ALE) 15 hydrofoil is investigated through large eddy simulation with the modified Schnerr-Sauer cavitation model, which considers the effect of non-condensable gas. The statistical average velocity profiles obtained by simulation and experimentation show good agreement. The time evolution of cavity shape shows that cavity growth and separation start from the short side and spread toward the long side due to a side-entrant jet. The variation frequency of the cavity length of ALE15 hydrofoil at the long side is 163.93 Hz, and the cavitation shedding frequency at the short side is 306.67 Hz, which is about twice the value of the former. The filtered vorticity transport equation is employed to investigate the cavitation-vortex-turbulence interaction. Results indicate that vortex stretching is the major promoter of cavitation development, and vortex dilatation links vapor cavity and vortices. Baroclinic torque is noticeable at the liquid-vapor interface, and turbulent stress is related to cavitation inception. Moreover, a one-dimensional model for predicting pressure fluctuation is proposed, and results show that the model can effectively predict cavitation-induced pressure fluctuation on a hydrofoil, even on a three-dimensional ALE15 hydrofoil.
TOPICS: Pressure, Turbulence, Cavitation, Vortices, Hydrofoil, Large eddy simulation, Cavities, Vapors, Torque, Flow (Dynamics), Separation (Technology), Vorticity, Simulation, Stress, Shapes
research-article  
Kofi Freeman K. Adane and Martin Agelin-Chaab
J. Fluids Eng   doi: 10.1115/1.4040503
In this study, a qualitative assessment of transitional velocity engineering models for predicting non-Newtonian slurry flows in a horizontal pipe was performed using data from wide pipe diameters (25 - 268 mm). In addition, the Gamma Theta transition model was used to compute selected flow conditions. These models were used to predict transitional velocity in large pipe diameters (up to 420 mm) for selected slurries. In general, it was observed that most of the current engineering models predict transitional velocities conservatively. However, caution should be exercised in design situations where both pipe diameter and viscoplastic viscosity influence the value of the Hedström number. Based on the Gamma Theta transition model results and for large Hedström number, He (? 105), other methods should be used to predict transitional velocity if a change in pipe diameter (scale-up) results in an order of magnitude for the He. It was also found that the Gamma Theta transition model predicted a laminar flow condition in the fully developed region for flow conditions with a small plug region (low yield stress-to-wall shear stress ratio) which is contrary to what has been observed in some experiments due partly to the local fluid rheological parameters values which might be different from reported. However, the Gamma Theta transition model results are in good agreement with the experimental data for flow conditions with a large plug region (high yield stress-to-wall shear stress ratio).
TOPICS: Flow (Dynamics), Turbulence, Slurries, Pipes, Shear stress, Engineering models, Stress, Rheology, Design, Viscosity, Laminar flow, Fluids
research-article  
Meng Chen, Nay Zar Aung, Songjing Li and Changfang Zou
J. Fluids Eng   doi: 10.1115/1.4040500
The occurrence of self-excited noise felt as squealing noise is a critical issue for an electrohydraulic servo-valve that is an essential part of the hydraulic servo-control system. Aiming to highlight the root causes of the self-excited noise, the effect of oil viscosity on the noise production inside a two-stage servo-valve is investigated in this paper. The pressure pulsations characteristics and noise characteristics are studied at three different oil viscosities experimentally by focusing on the flapper-nozzle pilot stage of a two-stage servo-valve. The cavitation-induced and vortex-induced pressure pulsations characteristics at upstream and downstream of the turbulent jet flow path are extracted and analyzed numerically by comparing with the experimental measured pressure pulsations and noise characteristics. The numerical simulations of transient cavitation shedding phenomenon are also validated by the experimental cavitation observations at different oil viscosities. Both numerical simulations and experimental cavitation observations explain that cavitation shedding phenomenon is intensified with the decreasing of oil viscosity. The small-scale vortex propagation with the characteristic of generating, growing, moving and merging is numerically simulated. Thus, this study reveals that the oil viscosity affects the transient distribution of cavitation and small-scale vortex, which in turn enhances the pressure pulsation and noise. The noise characteristics achieve a good agreement with pressure pulsation characteristics showing that the squealing noise appears accompanied by the flow field resonance in the flapper-nozzle. The flow-acoustic resonance and resulting squealing noise possibly occurs when the amplitude of the pressure pulsations near the flapper is large enough inside a two-stage servo-valve.
TOPICS: Servomechanisms, Viscosity, Noise (Sound), Valves, Cavitation, Pressure, Vortices, Transients (Dynamics), Nozzles, Flow (Dynamics), Resonance, Computer simulation, Acoustics, Turbulence, Jets
Review Article  
Obinna Ehirim, Kevin Knowles and Alistair Saddington
J. Fluids Eng   doi: 10.1115/1.4040501
The ground-effect diffuser has become a major aerodynamic device on open-wheel racing and sports cars. Accordingly, it is widely considered to be indispensable to their aerodynamic performance, largely due to its significant downforce contribution. However, the physics and characteristics that determine how it generates downforce and its application in the auto racing industry require an in-depth analysis to develop an understanding. Furthermore, research that could generate further performance improvement of the diffuser has not been defined and presented. For these reasons, this review attempts to create a systematic understanding of the physics that influence the performance of the ground-effect diffuser. As a means of doing this, the review introduces research data and observations from various relevant studies on this subject. It then investigates advanced diffuser concepts mainly drawn from the race car industry and also proposes a further research direction that would advance the aerodynamic performance of the diffuser. It is concluded that although the diffuser will continue to be paramount in the aerodynamic performance of racing cars, research is needed to identify means to further enhance its performance.
TOPICS: Aerodynamics, Diffusers, Automobiles, Physics, Sports, Wheels
research-article  
Zhang Jianwen, Jiang Aiguo, Xin Yanan and He Jianyun
J. Fluids Eng   doi: 10.1115/1.4040445
The erosion-corrosion problem of gas well pipeline under gas-liquid two-phase fluid flow is crucial for the natural gas well production, where multiphase transport phenomena exposes great influences on the feature of erosion-corrosion. An Eulerian-Eulerian two-fluid flow model is applied to deal with the three dimensional gas-liquid two-phase erosion-corrosion problem and the effects of the liquid droplets dissolved with CO2 are taken into consideration. The erosion-corrosion feature at different parts including expansion, contraction, step, screw sections and bends along the well pipeline is numerically studied in detail. An erosion-corrosion model is setup to address the local corrosion and erosion induced by the droplets impinging on the pipe surfaces. Three typical cases are studied and the mechanism of erosion-corrosion for different positions is investigated. It is explored by the numerical simulation that the erosion-corrosion changes with the practical production conditions: Under lower production rate, corrosion is the main cause for erosion-corrosion; under higher production rate, erosion predominates greatly; and under very high production rate, erosion becomes the main cause. It is clarified that the parts including connection site of oil pipe, oil pipe set and valve are the places where erosion-corrosion origins and becomes serious. The failure mechanism is explored and good comparison with field measurement is achieved.
TOPICS: Steel, Corrosion, Erosion, Natural gas, Pipes, Pipelines, Natural gas wells, Drops, Failure mechanisms, Computer simulation, Screws, Fluid dynamics, Flow (Dynamics), Fluids, Valves, Transport phenomena, Carbon dioxide
research-article  
Kazuyuki Yada, Masaharu Uchiumi and Ken-ichi Funazaki
J. Fluids Eng   doi: 10.1115/1.4040466
This paper describes an imbalanced torque force, called the Thomas/Alford force, of a partial-admission turbine for the rocket engine turbopump. The Thomas/Alford force is a destabilizing force imposed on the rotor that could cause rotor dynamic instability. This, in turn, may impair stable operation of the rocket engine and trigger mission failure. Such destabilizing force should be avoided as its characteristics have not been discussed in detail. In this study, Thomas/Alford forces for typical symmetric partially open/closed nozzle patterns with an open/closed ratio are analyzed. For such an open/closed ratio, it was determined that the Thomas/Alford force varied with the whirling angle. And whether the open/closed ratio may impair stable operation and reliability of the rocket engine turbopump is contingent on avoiding such fluctuation. The reason for such fluctuation was investigated by mathematical methodology, which was then extended to determine a general rule of patterns for rocket engine designers. This rule would thus prove useful in the future development of a partial-admission turbine for a rocket engine turbopump.
TOPICS: Turbines, Rocket engines, Rotors, Whirls, Failure, Torque, Reliability, Nozzles
research-article  
Varun Chitta, Tausif Jamal and Dibbon K. Walters
J. Fluids Eng   doi: 10.1115/1.4040467
This paper investigates the ability of computational fluid dynamics (CFD) simulations to accurately predict the turbulent flow separating from a three-dimensional (3D) axisymmetric hill using a recently developed four-equation eddy-viscosity model (EVM). The four-equation model, denoted as k-kL-?-v2, was developed to demonstrate physically accurate responses to flow transition, streamline curvature, and system rotation effects. The model was previously tested on several two-dimensional cases with results showing improvement in predictions when compared to other popularly available EVMs. In this paper, we present a more complex 3D application of the model. The test case is turbulent boundary layer flow with thickness d over a hill of height 2d mounted in an enclosed channel. The flow Reynolds number based on the hill height (ReH) is 1.3 × 105. For validation purposes, CFD simulation results obtained using the k-kL-?-v2 model are compared with two other Reynolds-averaged Navier-Stokes (RANS) models (fully turbulent SST k-? and transition-sensitive k-kL-?) and with experimental data. Results obtained from the simulations in terms of mean flow statistics, pressure distribution, and turbulence characteristics are presented and discussed in detail. Results indicate that both the complex physics of flow transition and streamline curvature should be taken into account to significantly improve RANS-based CFD predictions for applications involving blunt or curved bodies in a low Re turbulent regime.
TOPICS: Flow (Dynamics), Computational fluid dynamics, Turbulence, Engineering simulation, Reynolds-averaged Navier–Stokes equations, Simulation, Simulation results, Statistics as topic, Boundary layer turbulence, Physics, Pressure, Rotation, Eddies (Fluid dynamics), Viscosity, Reynolds number
research-article  
Kazem Hejranfar and Saman Rahmani
J. Fluids Eng   doi: 10.1115/1.4040446
In this study, a theoretical analysis is performed to assess the interaction of free stream disturbances with a plane normal shock considering real gas effects. Such effects are important in a field with high velocities and high temperatures. To perform the theoretical analysis, the downstream disturbances field is expressed as a mathematical function of the upstream one by incorporating real gas effects in the formulation. Here, the linearized one-dimensional perturbed unsteady Euler equations are used for the classification of the downstream/upstream disturbances field and the linearized one-dimensional perturbed Rankine-Hugoniot equations are applied to provide a relationship between the disturbances field of two sides of the shock. To incorporate real gas effects in the formulation, real gas relations and equilibrium air curve-fits are used in the resulting system of equations. The general formulation presented here may be simplified to derive Morkovin's formulation by the perfect gas assumption. The magnitudes of downstream disturbances field resulting from different types of upstream disturbances field (entropy wave and fast/slow acoustic waves) with the shock are expressed by appropriate analytical relations. Results for different disturbance variables are presented for a wide range of upstream Mach number considering real gas effects and compared with those of the perfect gas and some conclusions are made. The effects of the presence of body are also studied theoretically and the analytical relations for the magnitude of the pressure disturbance at the body for different types of upstream disturbances field considering real gas effects are provided and their results are presented and discussed.
TOPICS: Shock (Mechanics), Theoretical analysis, Waves, Equilibrium (Physics), High temperature, Pressure, Mach number, Acoustics, Entropy
research-article  
Jie Wu, Haoxiang Desmond Lim, Xiaofeng Wei, T. H. New and Yongdong Cui
J. Fluids Eng   doi: 10.1115/1.4040447
Supersonic jets at design Mach number of 1.45 issuing from circular 30° and 60° double-bevelled nozzles have been investigated experimentally and numerically in the present study, with a view to potentially improve mixing behaviour. Reynolds-Averaged Navier-Stokes (RANS) simulations of the double-bevelled nozzles and a benchmark non-bevelled nozzle were performed at nozzle-pressure-ratios (NPR) between 2.8 to 5.0, and the results are observed to agree well with Schlieren visualizations obtained from a modified Z-type Schlieren system. Double-bevelled nozzles are observed to produce shorter potential core lengths, modifications to the first shock cell lengths that are sensitive towards the NPR and jet half-widths that are typically wider and narrower along the trough-to-trough and peak-to-peak planes respectively. Lastly, using double-bevelled nozzles lead to significant mass flux ratios at NPR of 5.0, with a larger bevel-angle demonstrating higher entrainment levels.
TOPICS: Jets, Flow (Dynamics), Nozzles, Visualization, Reynolds-averaged Navier–Stokes equations, Mach number, Simulation, Shock (Mechanics), Design, Engineering simulation, Pressure
research-article  
Jonathan Lai, Elia Merzari and Yassin A. Hassan
J. Fluids Eng   doi: 10.1115/1.4040464
Large eddy simulation (LES) is conducted for the flow over the shell side of a helical coil steam generator heat exchanger. Simulations are conducted on a simplified experimental test section that represents a one-column region of the helical coils using half-rods. Although the rods are wall-bounded, the flow still exhibits the turbulent characteristics and fluctuations from vortex shedding that one would expect from crossflow around a cylinder. The spectral element, computational fluid dynamics code Nek5000 is used to capture the physics, and the results are compared with particle image velocimetry (PIV) measurements. In order to ensure that the turbulence is resolved, analysis is conducted by using the Taylor length scales and normalized wall distance. Sensitivity to the inlet boundary conditions and the spatial discretization for different polynomial order solutions are also studied, finding only minor differences between each case. Pressure drop and velocity statistics show reasonable agreement with PIV. Proper orthogonal decomposition analysis reveals that the primary modes are similar between experiment and simulation, although the LES predicts higher turbulent kinetic energy than does PIV. Overall, the study establishes the resolution and resources required in order to conduct a high-fidelity simulation over 12 helical rods.
TOPICS: Boilers, Flow (Dynamics), Large eddy simulation, Turbulence, Simulation, Rods, Shells, Vortex shedding, Fluctuations (Physics), Resolution (Optics), Kinetic energy, Particulate matter, Computational fluid dynamics, Heat exchangers, Boundary-value problems, Cylinders, Polynomials, Pressure drop, Principal component analysis, Statistics as topic, Physics
research-article  
Ilias Gavrielatos, Ramin Dabirian, Ram S. Mohan and Ovadia Shoham
J. Fluids Eng   doi: 10.1115/1.4040465
A state-of-the-art, Portable Dispersion Characterization Rig (P-DCR) is used to investigate the effect of nanoparticles (NP) on oil-water emulsion formation and stabilization. Spherical silica nanoparticles of different wettabilities were used to investigate their effect on separation kinetics of solid stabilized emulsions in terms of solid particle concentration, wettability, initial dispersion phase, water-cut, and shearing time. The main findings of the study include the following: Nanoparticles, even at concentrations as low as 0.005% or 0.01% (by weight), can significantly increase separation time of oil/water emulsions from a few minutes to several hours or even days. The Portable Dispersion Characterization Rig (P-DCR) is recommended as an effective in-line tool to measure emulsion stability in the field.
TOPICS: Nanoparticles, Emulsions, Water, Separation (Technology), Particulate matter, Weight (Mass), Stability, Die cutting, Shearing (Deformation)
research-article  
Xiao Hu and G. Q. Zhang
J. Fluids Eng   doi: 10.1115/1.4040441
The Physical mechanism for the evolution and decay of Lamb-Oseen vortex pair in ground proximity with an obstacle has been investigated in detail by adopting the large eddy simulation. In the present research, we mainly focus on the vortex evolution and decay mechanism in ground proximity with obstacle, so we chose one fixed height of the obstacle case (h0=0.5b0) to investigate, and the obstacle is placed transversally to the axis of the primary wake to be analyzed. The trajectories of the primary wake vortex cores and the circulation profiles, as well as the distribution of the tangential velocity on different axial positions, have been specifically captured and analyzed. The "strake", "claw" and "ivory" vortices have been newly found and defined at the initial evolution stage, and they subsequently begin to harshly wind and rotate with the primary vortex. A flow structure with double helix conical shapes of the primary vortex has been found in the obstacle case. The pressure waves along the vortex axis have also been analyzed in detail. The wake vortex on each side would be pulled in opposite axial directions and eventually pinched off at the upper surface of obstacle. Moreover, it has also been newly found that the trajectories of the wake vortex in longitudinal directions at different axial distances away from the obstacle will experience two kinds of motion: only descending and rebounding after descending. Results obtained in this study provide a better understanding of mechanisms for the interaction of wake vortex and the obstacle.
TOPICS: Vortices, Wake turbulence, Shapes, Wind, Large eddy simulation, Pressure, Flow (Dynamics), Waves, Wakes
research-article  
Ayman Shaaban and Samir Ziada
J. Fluids Eng   doi: 10.1115/1.4040389
Flow over ducted shallow cavities can excite fluid resonant oscillations. A common industrial application is the flow in corrugated pipes that can be modelled as a series of consecutive shallow cavities. In the current study, the effect of the separation distance on the aeroacoustic source of multiple shallow cavities is investigated. The Standing Wave Method (SWM) is used to measure the source, where multiple microphones reconstruct the acoustic standing wave upstream and downstream of the cavities. The effect of the ratio between the separation distance to cavity length is investigated for a practical range from 0.5 to 1.375 for two and three-cavity configurations. At low and intermediate sound levels, constructive hydrodynamic interference, resulting in a strong source, is observed for the extremum spacing ratios of 0.5 and 1.375. However, at high excitation levels, 10% and higher, the source, slightly but consistently, decreases upon increasing the separation ratio. These trends persist for both the double and triple-cavity configurations. On the other hand, the separation distance of destructive interference is found to depend on the number of cavities of the tested configuration. Particle Image Velocimetry (PIV) measurements of the constructive interference cases show strong synchronized vorticity shedding in all cavities. Each cavity contribution to the total aeroacoustic source is then examined by means of Howe's analogy and the percentage contribution of each cavity is found to depend on the excitation level.
TOPICS: Separation (Technology), Cavities, Flow (Dynamics), Excitation, Standing waves, Vorticity, Pipes, Oscillations, Resonance, Fluids, Particulate matter, Acoustics, Microphones
research-article  
C.Y. Wang
J. Fluids Eng   doi: 10.1115/1.4040362
A modified Ritz method for solving non-uniform slip flow in a duct is applied to the semi-circular duct and the isosceles triangular duct. These ducts are important in micro-fluidics. Detailed flow fields and Poiseuille numbers show the large effects of non-uniform slip. A rare exact solution for the semi-circular duct with non-zero slip is also found.
TOPICS: Flow (Dynamics), Ducts, Poiseuille flow, Slip flow, Microfluidics

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