Abstract: Detailed instantaneous velocity fields of a jet in crossflow have been measured with stereoscopic particle image velocimetry (PIV). The jet originated from a fully developed turbulent pipe flow and entered a crossflow with a turbulent boundary layer. The Reynolds number based on crossflow velocity and pipe diameter was 2400 and the jet to crossflow velocity ratios were R=3.3 and R=1.3. The experimental data have been analysed by proper orthogonal decomposition (POD). For R=3.3, the results in several different planes indicate that the wake vortices are the dominant dynamic flow structures and that they interact strongly with the jet core. The analysis identifies jet shear-layer vortices and finds that these vortical structures are more local and thus less dominant. For R=1.3, on the other hand, jet shear-layer vortices are the most dominant, while the wake vortices are much less important. For both cases, the analysis finds that the shear-layer vortices are not coupled to the dynamics of the wake vortices. Finally, the hanging vortices are identified and their contribution to the counter-rotating vortex pair (CVP) and interaction with the newly created wake vortices are described.

Abstract: A CFD code has been developed based on a Large-Eddy Simulation (LES) approach. The turbine is simulated by concentrated drag forces, and is placed in an environment with turbulence anisotropy properties similar to the ones of the real atmosphere. Comparisons with experimental data and with analytical correlations have been performed, and the results are found to be in good agreement with both, suggesting that LES is a potentially useful tool in the investigation of detailed wake flow.

Abstract: The wake of a wind turbine operating in an atmospheric turbulent inflow without mean shear is simulated using a numerical method, which combines large eddy simulations with an actuator line technique. A turbulent inflow with the same spectral characteristics as the atmosphere is produced by introducing time varying body forces in a plane upstream the rotor. The results of the simulation are compared to those obtained on a wind turbine in uniform inflow at the same mean wind speed and from this comparison a number of features of the influence of inflow turbulence on wake dynamics are deduced. Furthermore, the results are used to verify the validity of some of the basic assumptions employed in simpler engineering models and to study their bounds of application. The large amount of data from the wake simulation can easily be used in simple engineering methods to model a wind turbine operating in the wake of an upstream turbine

Abstract: The tip-leakage flow in a turbomachinery cascade is studied using large-eddy simulation with particular emphasis on understanding the underlying mechanisms for viscous losses in the vicinity of the tip gap. Systematic and detailed analysis of the mean flow field and turbulence statistics has been made in a linear cascade with a moving endwall. Gross features of the tip-leakage vortex, tip-separation vortices, and blade wake have been revealed by investigating their revolutionary trajectories and mean velocity fields. The tip-leakage vortex is identified by regions of significant streamwise velocity deficit and high streamwise and pitchwise vorticity magnitudes. The tip-leakage vortex and the tip-leakage jet which is generated by the pressure difference between the pressure and suction sides of the blade tip are found to produce significant mean velocity gradients along the spanwise direction, leading to the production of vorticity and turbulent kinetic energy. The velocity gradients are the major causes for viscous losses in the cascade endwall region. The present analysis suggests that the endwall viscous losses can be alleviated by changing the direction of the tip-leakage flow such that the associated spanwise derivatives of the mean streamwise and pitchwise velocity components are reduced.

Abstract: Flow over traveling wavy foils in a side-by-side arrangement has been numerically investigated using the space-time finite element method to solve the two-dimensional incompressible Navier-Stokes equations The midline of each foil undergoes lateral motion in the form of a streamwise traveling wave, which is similar to the backbone undulation of swimming fish Based on the phase difference between the adjacent undulating foils, two typical cases, ie, in-phase and anti-phase traveling wavy movements, are considered in the present study The effects of lateral interference among the foils on the forces, power consumption, propeller efficiency, and flow structures are analyzed It is revealed that the lateral interference is of benefit to saving the swimming power in the in-phase case and enhancing the forces in the anti-phase case Some typical vortex structures, eg, vortex-pair row, single vortex row, and in-phase and anti-phase synchronized vortex-street, are observed in the wake of the traveling wavy

Abstract: A flexibly mounted circular cylinder in cross-flow, with natural frequencies in the inline and transverse directions having a ratio close to 2:1, exhibits drastic changes in the vortex structures in its wake, the frequency content of the fluid forces, and the orbital shape of its resulting motions. Stable multivortex patterns form in the cylinder wake, associated with large high-frequency force components.

Abstract: We investigate the two-dimensional flow of a liquid foam around a circular obstacle by measuring all the local fields necessary to describe this flow: velocity, pressure, and bubble deformations and rearrangements. We show how our experimental set-up, a quasi-two-dimensional ‘liquid pool’ system, is adapted to the determination of these fields: the velocity and bubble deformations are easy to measure from two-dimensional movies, and the pressure can be measured by exploiting a specific feature of this system, a two-dimensional effective compressibility. To describe accurately neighbour swapping (so-called ‘T1’ processes), we propose a new, tensorial descriptor. All these quantities are evaluated via an averaging procedure that we justify by showing that the fluctuations of the fields are essentially Gaussian. The flow is extensively studied in a reference experimental case; the velocity presents an overshoot in the wake of the obstacle, and the pressure is maximum at the leading side and minimal at the trailing side. The study of the elastic deformations and of the velocity gradients shows that the transition between plug flow and yielded regions is smooth. Our tensorial description of T1s highlights their correlation both with the bubble deformations and the velocity gradients. A salient feature of the flow, notably for the velocity and T1 distribution, is a marked fore–aft asymmetry, the signature of the elastic behaviour of the foam. We show that the results do not change qualitatively when various control parameters (flow rate, bubble area, fluid fraction, bulk viscosity, obstacle size and boundary conditions) vary, identifying a robust quasi-static regime. These results are discussed in the framework of the foam rheology literature. A movie is available with the online version of the paper.

Abstract: In an effort to better understand landing-gear noise sources, we have been examining a simplified configuration that still maintains some of the salient features of landing-gear flow fields. In particular, tandem cylinders have been studied because they model a variety of component level interactions. The present effort is directed at the case of two identical cylinders spatially separated in the streamwise direction by 3.7 diameters. Experimental measurements from the Basic Aerodynamic Research Tunnel (BART) and Quiet Flow Facility (QFF) at NASA Langley Research Center (LaRC) have provided steady surface pressures, detailed off-surface measurements of the flow field using Particle Image Velocimetry (PIV), hot-wire measurements in the wake of the rear cylinder, unsteady surface pressure data, and the radiated noise. The experiments were conducted at a Reynolds number of 166 105 based on the cylinder diameter. A trip was used on the upstream cylinder to insure a fully turbulent shedding process and simulate the effects of a high Reynolds number flow. The parallel computational effort uses the three-dimensional Navier-Stokes solver CFL3D with a hybrid, zonal turbulence model that turns off the turbulence production term everywhere except in a narrow ring surrounding solid surfaces. The current calculations further explore the influence of the grid resolution and spanwise extent on the flow and associated radiated noise. Extensive comparisons with the experimental data are used to assess the ability of the computations to simulate the details of the flow. The results show that the pressure fluctuations on the upstream cylinder, caused by vortex shedding, are smaller than those generated on the downstream cylinder by wake interaction. Consequently, the downstream cylinder dominates the noise radiation, producing an overall directivity pattern that is similar to that of an isolated cylinder. Only calculations based on the full length of the model span were able to capture the complete decay in the spanwise correlation, thereby producing reasonable noise radiation levels.

Abstract: The present paper describes a wind tunnel study of flow downstream a small horizontal axis wind turbine (HAWT). The experimental investigations were carried out with the use of particle image velocimetry (PIV). To obtain the flow field in the rotating frame of reference, the phase-locked technique was applied. Explorations were carried out in azimuth planes with different angles. The 3D velocity field was reconstituted by processing the images resulting from the explored azimuth planes. In addition to PIV investigations, hot-wire measurements were also carried out immediately behind the wind turbine rotor at different radial and axial distances. The obtained results are very useful to analyze wind turbine wake and to constitute a reference for CFD computation.

Abstract: Plasma-based active flow control was simulated numerically for the subsonic flow through a highly loaded low-pressure turbine. The configuration corresponded to previous experiments and computations which considered flow at a Reynolds number of 25,000 based upon axial chord and inlet conditions. In this situation, massive separation occurs on the suction surface of each blade due to uncovered turning. The present exploratory numerical study was performed to investigate the use of asymmetric dielectric-barrier-discharge actuators for mitigating separation, thereby decreasing turbine wake losses and increasing efficiency. Solutions were obtained for the Navier-Stokes equations, which were augmented by a phenomenological model that was used to represent plasma-induced body forces imparted by the actuator on the fluid. The numerical method used a high-fidelity time-implicit scheme, employing domain decomposition to carry out calculations on a parallel computing platform. A high-order overset grid approach preserved spatial accuracy in a locally refined embedded region. The magnitude of the plasma-induced body force required for control is examined, and both continuous and pulse-modulated actuations are considered. Novel use of counterflow actuation is also investigated, and the effects of pulsing frequency and duty cycle are considered. Features of the flowfields are described, and resultant solutions are compared with each other, with previous mass-injection control cases, and with the baseline situation where no control was enforced.

Abstract: A finite element code based on the level-set method is used to perform direct numerical simulations (DNS) of the transient and steady-state motion of bubbles rising in a viscoelastic liquid modelled by the Oldroyd-B constitutive equation. The role of the governing dimensionless parameters, the capillary number (Ca), the Deborah number (De) and the polymer concentration parameter c, in both the rising speed and the deformation of the bubbles is studied. Simulations show that there exists a critical bubble volume at which there is a sharp increase in the terminal velocity with increasing bubble volume, similar to the behaviour observed in experiments, and that the shape of both the bubble and its wake structure changes fundamentally at that critical volume value. The bubbles with volumes smaller than the critical volume are prolate shaped while those with volumes larger than the critical volume have cusp-like trailing ends. In the latter situation, we show that there is a net force in the upward direction because the surface tension no longer integrates to zero. In addition, the structure of the wake of a bubble with a volume smaller than the critical volume is similar to that of a bubble rising in a Newtonian fluid, whereas the wake structure of a bubble with a volume larger than the critical value is strikingly different. Specifically, in addition to the vortex ring located at the equator of the bubble similar to the one present for a Newtonian fluid, a vortex ring is also present in the wake of a larger bubble, with a circulation of opposite sign, thus corresponding to the formation of a negative wake. This not only coincides with the appearance of a cusp-like trailing end of the rising bubble but also propels the bubble, the direction of the fluid velocity behind the bubble being in the opposite direction to that of the bubble. These DNS results are in agreement with experiments.

Abstract: The dual-jet flow generated by a plane wall jet and a parallel offset jet at an offset ratio of d/w = 1.0 has been investigated using Particle Image Velocimetry (PIV). The particle images are captured, processed, and subsequently used to characterize the flow in terms of the 2D velocity and vorticity distributions. Statistical characteristics of the flow are obtained through ensemble averaging of 360 instantaneous velocity fields. Also presented is a time series of instantaneous flow fields to illustrate the dynamic interaction between the two jets. Results reveal that the near field of the flow is characterized by a periodic large-scale Karman-like vortex shedding similar to what would be expected in the wake of a bluff body. The existence of the Karman-like vortices results in periodic interactions between the two jets; in addition, these vortices produce noticeable impact on the jet outer layers, i.e., the free shear layer of the offset jet and the wall boundary layer of the wall jet. A schematic of vortex/shear layer interaction is proposed to illustrate the flow pattern.

Abstract: A numerical method is developed for modelling the interactions between incompressible viscous fluid and moving boundaries. The principle of this method is introducing the immersed-boundary concept in the framework of the lattice Boltzmann method, and improving the accuracy and efficiency of the simulation by refining the mesh near moving boundaries. Besides elastic boundary with a constitutive law, the method can also efficiently simulate solid moving-boundary interacting with fluid by employing the direct forcing technique. The method is validated by the simulations of flow past a circular cylinder, two cylinders moving with respect to each other and flow around a hovering wing. The versatility of the method is demonstrated by the numerical studies including elastic filament flapping in the wake of a cylinder and fish-like bodies swimming in quiescent fluid. Copyright © 2006 John Wiley & Sons, Ltd.

Abstract: An experimental and theoretical investigation of the flow field around small-scale mesh disk rotor simulators is presented. Wake characteristics of the rotor simulators have been measured in the 21m tilting flume at the Chilworth hydraulics laboratory, University of Southampton. A three-dimensional Eddy-viscosity numerical model based on an established wind turbine wake model has been modified to account for the change in fluid and the presence of a bounding free surface. This model shows good agreement with the measured experimental data and further work will be conducted to refine the model. This work has been conducted as part of a DTI-funded project to develop a numerical modelling tool which can predict the flow onto a marine current turbine within an array. The work presented in this paper feeds into this project and will eventually assist the layout design of arrays which are optimally spaced and arranged to achieve the maximum possible energy yield at a given tidal energy site.

Abstract: This paper is prompted by a recent numerical study that shows that for a two-dimensional (2-D) elliptic airfoil undergoing prescribed heaving motion in a viscous fluid, both leading-edge vortices and trailing-edge vortices contributed to the formation of the wake structures. However, an earlier dye-visualization study on a heaving NACA 0012 airfoil appears to show that the wake structures were derived from trailing-edge vortices only. The dissimilarity in the two studies remains unclear because there is no corresponding experimental data on a 2-D heaving elliptic airfoil. In this study, digital particle image velocimetry technique was used to investigate the wake-structure formation of a 2-D elliptic airfoil undergoing simple harmonic heaving motion. For the range of flow conditions investigated here, our results show that the type of wake structures produced is controlled by when and how the leading-edge vortices interact with the trailing-edge vortices

Abstract: The tip-clearance flow in axial turbomachines is studied using large-eddy simulation with particular emphasis on understanding the unsteady characteristics of the tip-leakage vortical structures and the underlying mechanisms for cavitation-induring low-pressure fluctuations. A systematic and detailed analysis of the velocity and pressure fields has been made in a linear cascade with a moving end-wall. The generation and evolution of the tip-leakage vortical structures have been investigated throughout the cascade using mean streamlines and λ 2 contours. An analysis of the energy spectra and space-time correlations of the velocity fluctuations suggests that the tip-leakage vortex is subject to a pitchwise low frequency wandering motion. Detailed statistics of the pressure fields has been analyzed to draw inferences on cavitation

Abstract: Among the fundamental problems in canopy turbulence, particularly near the forest floor, remain the local diabatic effects and linkages between turbulent length scales and the canopy morphology. To progress on these problems, mean and higher order turbulence statistics are collected in a uniform pine forest across a wide range of atmospheric stability conditions using five 3-D anemometers in the subcanopy. The main novelties from this experiment are: (1) the agreement between second-order closure model results and measurements suggest that diabatic states in the layer above the canopy explain much of the modulations of the key velocity statistics inside the canopy except in the immediate vicinity of the trunk space and for very stable conditions. (2) The dimensionless turbulent kinetic energy in the trunk space is large due to a large longitudinal velocity variance but it is inactive and contributes little to momentum fluxes. (3) Near the floor layer, a logarithmic mean velocity profile is formed and vertical eddies are strongly suppressed modifying all power spectra. (4) A spectral peak in the vertical velocity near the ground commensurate with the trunk diameter emerged at a moderate element Reynolds number consistent with Strouhal instabilities describing wake production.

Abstract: Large-eddy simulation of compressible transitional flows in a low-pressure turbine cascade is performed by using sixth-order compact difference and a 10th-order filtering method. Numerical results without freestream turbulence and those with about 5 % of freestream turbulence are compared. In these simulations, separated flows in the turbine cascade accompanied by laminar-turbulent transition are realized, and the present results closely agree with past experimental measurements in terms of the static pressure distribution around the blade. In the case where no freestream turbulence is taken into account, the unsteady pressure field essentially differs from that with strong freestream turbulence. In the no freestream turbulence case, pressure waves that propagate from the blade's wake region have noticeable effects on the separated-boundary layer near the trailing edge and on the neighboring blade. Also, based on the snapshot proper orthogonal decomposition analysis, dominant behaviors of the transitional boundary layers are investigated.

Abstract: A hybrid model has been developed in order to represent the flow downstream a wind turbine rotor. The blades are replaced by their mean surfaces and a pressure jump boundary condition is applied here. The hybrid model combines CFD solver with a blade element method (BEM). The solving method is iterative: at the beginning of iteration a BEM determines the pressure discontinuities along the blade span, using the rotor inflow and the aerodynamic properties of blade sections. Then the CFD solver applies this pressure discontinuity in order to model the blade forces and the wake downstream the rotor. At the end of iteration the obtained rotor inflow, the wake development and the residuals are compared with those obtained from previous iteration. If the required precision is attained, the calculation stops. The solution is obtained after thousands of iterations, depending on the number of nodes, residuals etc. The approach of replacing blades by pressure discontinuity surfaces is validated by comparison the wakes in the cases of flat plate and the S809 airfoil. Finally, the proposed hybrid model is used to calculate the performance of NREL Phase VI wind turbine and the results obtained are satisfactory.

Abstract: Phase-averaged organized oscillation velocities (U,V,W) and random fluctuation Reynolds stresses (uu,uu,ww,uv ,uw) are presented for the nominal wake of a surface ship advancing in regular head (incident) waves, but restrained from body motions, ie, the forward-speed diffraction problem A 3048 X 3048 X 100 m towing tank, plunger wave maker, and towed, 2D particle-image velocimetry (PIV) and servo mechanism wave-probe measurement systems are used The geometry is DTMB model 5415 (L= 3048 m, 1/466 scale), which is an international benchmark for ship hydrodynamics The conditions are Froude number Fr=028, wave steepness Ak=0025, wavelength λ/L=15, wave frequency f=0584 Hz, and encounter frequency f e =0922 Hz Innovative data acquisition, reduction, and uncertainty analysis procedures are developed for the phase-averaged PIV The unsteady nominal wake is explained by interactions between the hull boundary layer and axial vortices and incident wave There are three primary wave-induced effects: pressure gradients 4%Uc, orbital velocity transport 15%U c , and unsteady sonar dome lifting wake In the outer region, the uniform flow, incident wave velocities are recovered within the experimental uncertainties In the inner, viscous-flow region, the boundary layer undergoes significant time-varying upward contraction and downward expansion in phase with the incident wave crests and troughs, respectively The zeroth harmonic exceeds the steady-flow amplitudes by 5-20% and 70% for the velocities and Reynolds stresses, respectively The first-harmonic amplitudes are large and in phase with the incident wave in the bulge region (axial velocity), damped by the hull and boundary layer and mostly in phase with the incident wave (vertical velocity), and small except near the free surface-hull shoulder (transverse velocity) Reynolds stress amplitudes are an order-of-magnitude smaller than for the velocity components showing large values in the thin boundary layer and bulge regions and mostly in phase with the incident wave

Abstract: Swimming and flying animals generate unsteady locomotive forces by delivering net momentum into the fluid wake. Hence, swimming and flying forces can be quantified by measuring the momentum of animal wakes. A recently developed model provides an approach to empirically deduce swimming and flying forces based on the measurement of velocity and vortex added-mass in the animal wake. The model is contingent on the identification of the vortex boundary in the wake. This paper demonstrates the application of that method to a case study quantifying the instantaneous locomotive forces generated by the pectoral fins of the bluegill sunfish (Lepomis macrochirus Rafinesque), measured using digital particle image velocimetry (DPIV). The finite-time Lyapunov exponent (FTLE) field calculated from the DPIV data was used to determine the wake vortex boundary, according to recently developed fluid dynamics theory. Momentum of the vortex wake and its added-mass were determined and the corresponding instantaneous locomotive forces were quantified at discrete time points during the fin stroke. The instantaneous forces estimated in this study agree in magnitude with the time-averaged forces quantified for the pectoral fin of the same species swimming in similar conditions and are consistent with the observed global motion of the animals. A key result of this study is its suggestion that the dynamical effect of the vortex wake on locomotion is to replace the real animal fin with an `effective appendage', whose geometry is dictated by the FTLE field and whose interaction with the surrounding fluid is wholly dictated by inviscid concepts from potential flow theory. Benefits and limitations of this new framework for non-invasive instantaneous force measurement are discussed, and its application to comparative biomechanics and engineering studies is suggested.