Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session M35: Turbulence: Shear Layers II - Experiments |
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Chair: Joseph Katz, Johns Hopkins University Room: 406 |
Tuesday, November 26, 2013 8:00AM - 8:13AM |
M35.00001: Vorticity Based Intermittency in the Single Stream Shear Layer (SSSL) John Foss, Kyle Bade, Richard Prevost, Douglas Neal The uniquely large scale and high Reynolds number, Re($\theta$)=65120, single stream shear layer (SSSL), extensively studied by Morris and Foss (2003), has been investigated using overlapped PIV images. The PIV data focus on the low-speed side: 0$<$u/U$<$0.8, of the SSSL. The PIV data complement the hot-wire data which were previously used to extensively examine the high-speed side of the SSSL. (The PIV results overcome significant uncertainty sources of hot-wire anemometry as the time-mean streamwise velocity approaches zero on the low-speed side of the SSSL.) Conventional stochastic values have been obtained using the PIV results. The substantial value of this study is to exploit the unique irrotational state of the entrainment stream, which allows the in-principle designation for intermittency as: I=1 if the point-wise vorticity is non-zero, and I=0 where vorticity equals zero with high spatial resolution. These measures can be used to identify the viscous super-layer (VSL) as the border between the two domains.\\[4pt] Morris, S.C. and Foss, J.F. (November 2003) ``Turbulent boundary layer to single-stream shear layer: the transition region,'' Jour. Fluid Mechanics, 494, pp. 187-221. [Preview Abstract] |
Tuesday, November 26, 2013 8:13AM - 8:26AM |
M35.00002: Coherent structures and momentum transport at various scales above an array of multiscale structures Kunlun Bai, Joseph Katz, Charles Meneveau Detailed PIV measurements are carried out to study the turbulence and coherent structures at various scales above a canopy composed of multiscale fractal tree-like objects. The fractal tree has five generations, each consisting of three branches. To study the turbulent structures and momentum transport at large scales, quadrant analysis of fluctuation velocity is carried out. It shows that close to the canopy, sweeping events have larger contribution to the Reynolds shear stress than ejections. Away from the canopy, on the other hand, sweeping contributes less to shear stress than ejection. When the ejection is at present, the flow is disturbed greatly, and the inclined angle of vortices packets can be more than 30 degree. Close to the canopy, the correlation spectrum , i.e. $-E_{uw}/(E_{uu}E_{ww})^{0.5}$ (where $E_{uw}$ is the co-spectrum and $E_{uu}$ and $E_{ww}$ are spectra of streamwise and vertical velocities, respectively), first decreases and then lifts up as wavenumber increases or scale decreases. It indicates the presence of small-scale coherent structures close to the canopy that contribute, at small scales, to momentum transport. A physical space filtering technique is applied to the velocity field to study such structures and the associated large-scale flow patterns. [Preview Abstract] |
Tuesday, November 26, 2013 8:26AM - 8:39AM |
M35.00003: Particle Image Velocimetry of a Supersonic Flow over a Finite-Width Rectangular Cavity Steven Beresh, Justin Wagner, John Henfling, Russell Spillers, Brian Pruett Stereoscopic particle image velocimetry measurements have been acquired in the streamwise plane for supersonic flow over a rectangular cavity of variable width, peering over the side wall lip to view the depths of the cavity. The complex camera angles were treated using two-axis scheimpflug focusing and perspective correction inherent in the camera calibration algorithm. The data reveal the turbulent shear layer over the cavity and the recirculation region within it. The mean position of the recirculation region was found to be a function of the length-to-width ratio of the cavity, as was the turbulence intensity within both the shear layer and the recirculation region. Compressibility effects were observed in which turbulence levels dropped and the shear layer thickness decreased as the Mach number was raised from 1.5 to 2.0 and 2.5. Supplemental measurements in the crossplane and the planform view suggest that zones of high turbulence were affixed to each side wall centered on the cavity lip, with a strip of turbulence stretched out across the cavity shear layer whose intensity was a function of the length-to-width ratio. [Preview Abstract] |
Tuesday, November 26, 2013 8:39AM - 8:52AM |
M35.00004: Nozzle Turbulent Boundary Layer Influence on Sound in a Mach 0.9 Jet Ryan Fontaine, Gregory Elliott, Joanna Austin, Jonathan Freund One of the principal challenges in the prediction and design of low-noise nozzles is the thinness of the near-nozzle mixing layers at the high-Reynolds-numbers of engineering conditions. The specific challenge will depend in part on the upstream conditions, but typically we expect that the jet leaving the nozzle will have $Re_D\sim5\times10^{6}$ at application scales. Including this in design approaches presents a significant challenge. It is well known that small-scale experiments will have relatively thicker near-nozzle shear layers, which can hamper their applicability to high-Reynolds-number design. Though they can nominally be run at sufficiently high Reynolds numbers, faithfully representing these regions within a large-eddy simulation is likewise a challenge because the locally largest scales are so small. A family of nozzles designed to change the exit thickness of the turbulent boundary layer with otherwise identical flow conditions is studied experimentally to quantify the sensitivity of the far-field sound to nozzle shear layer conditions, which are quantified with very-near-nozzle PIV measurements. The influence is pronounced, though less significant than the well-known sensitivity of far-field sound to laminar versus turbulent boundary near-nozzle shear layers. [Preview Abstract] |
Tuesday, November 26, 2013 8:52AM - 9:05AM |
M35.00005: An experimental investigation of the shear-layer and acoustic sources produced by a leading edge slat Stephen Wilkins, Patrick Richard, Joseph Hall Leading edge slats are a common addition to airfoils as part of a high lift configuration employed during take-off and landing; the unsteady flow caused by these slats is a major contributor to the overal airframe noise. As the next generation of aircraft seeks to reduce these noise concerns, a better understanding of the sources of aeroacoustic noise generation is sought. Particle Image Velocimetry (PIV) and simultaneous multipoint measurements of the unsteady surface pressure are used herein to investigate the unsteady flow around a leading edge slat coupled with an airfoil for several different configurations and a range of Reynolds numbers ($Re=156,000$ to $Re=1.2$ million based on the wing chord). Shear-layer development off the slat cusp and the related unsteady vortex structures are examined in detail to better establish and understand the mechanisms responsible for the generation of aeroacoustic slat noise. [Preview Abstract] |
Tuesday, November 26, 2013 9:05AM - 9:18AM |
M35.00006: Vortex Identification above the free-end of finite-height prisms and cylinders Rajat Chakravarty, Noorallah Rostamy, Donald Bergstrom, David Sumner The local flow field above the free end of a surface-mounted finite-height square prism of aspect ratio $=$ AR $=$ 5 was experimentally studied using 2D PIV measurements on the vertical symmetry plane. The velocity field data were post-processed using Proper Orthogonal Decomposition (POD) and swirling strength. POD was performed using 100 instantaneous snapshots to generate a reconstruction of the vorticity field. It was observed that flow separation from the leading edge of the free-end surface led to the formation of a separated shear layer and a mean recirculation zone below it. POD and swirling strength comparisons successfully isolated small-scale vortices at the shear layer interface not resolved in previous studies. These vortices became progressively larger and weaker, finally smearing out into the wake. Swirling strength, though inherently a 3D concept, was able to isolate the corresponding vortex structures even in regions where the 2D local velocity gradient tensor yielded complex eigenvalues. It was concluded that swirling strength can also be used in 2D flow fields on planes of symmetry to isolate vortices whose axes have no component in the mean flow direction. [Preview Abstract] |
Tuesday, November 26, 2013 9:18AM - 9:31AM |
M35.00007: Transition to turbulence in stratified shear flow: experiments in an inclined square duct Colin Meyer, Paul Linden We describe laboratory experiments of countercurrent stratified shear flow in an inclined square duct. To achieve this, a long water tank was partitioned into regions of higher and lower density saltwater that are connected by an inclined square duct. The flow regime was characterized to be turbulent, intermittent, Holmboe or laminar as a function of the duct inclination, $\theta$, and the density difference, $\Delta\rho$, between the two reservoirs. The density difference and duct angle were systematically varied and a phase plane of flow regime was developed. The transition between the interrmittent regime and turbulence was experimentally determined to occur at $\theta\Delta\rho \simeq 20$ [degrees kg m$^{-3}$]. This critical combination of parameters fits into the buoyancy-compensated Reynolds number scaling proposed by Brethouwer et al. (\emph{J. Fluid Mech.}, 2007). The turbulent interfacial thickness was found to be a function of the inclination angle, which can be predicted using the buoyancy lengthscale from Waite and Bartello (\emph{J. Fluid Mech.}, 2004) and others. Furthermore, we measured the density profiles at multiple points along the duct, and using these profiles, we modeled the entrainment at the interface. [Preview Abstract] |
Tuesday, November 26, 2013 9:31AM - 9:44AM |
M35.00008: 3D Evolution of Turbulent Flow Structures in Taylor-Couette Sedat Tokgoz, Gerrit E. Elsinga, Jerry Westerweel In this study, we use high-speed tomographic PIV to investigate the evolution of turbulent flow structures in Taylor-Couette flow. High-speed tomographic PIV enables fully volumetric time-resolved measurements and is well-suited for this purpose. Presently, the turbulent flow is created by exact counter-rotation of the cylinders ($Re_i = - Re_o$, $R_{\Omega} = 0.0$), where the wall velocities are the same with opposite sign. Under these circumstances, the mean flow is zero in the bulk. Results indicate that the structures still advance in one direction despite the zero mean. However, the observation time for the flow structures is still at least one order magnitude longer than in the boundary layer flows, which were considered before. Results also revealed the presence of azimuthal velocity streaks. It is found that the intense vortical structures are mostly located in the shear layer between these streaks. Different events regarding the evolution of the vortical structures, such as stretching and break-up of vortices, are observed. [Preview Abstract] |
Tuesday, November 26, 2013 9:44AM - 9:57AM |
M35.00009: The behavior of the wake behind a heated circular cylinder Morteza Khashehchi, Kamel Hooman The thermal effects on the characteristics of the wake behind a circular cylinder operating in the mixed convection regime are considered at relatively high Reynolds number using Particle Image Velocimetry. The experiments were conducted in a horizontal wind tunnel with the heated cylinder placed horizontally. With such assumptions, the direction of the thermally induced buoyancy force acting on the fluid surrounding the heated cylinder would be perpendicular to the flow direction. Experiments were conducted for three Reynolds numbers 1000, 2000 and 4000, where each of them were run at three different temperatures 25, 50 and 75$^{\circ}$C. By adjusting different temperatures in different Reynolds numbers, the corresponding Richardson number (\textit{Ri}$_{D}=$\textit{Gr/Re}$^{2})$ was varied between 0.0 (unheated) and 10, resulting in a change in the heat transfer process from forced convection to mixed convection. With increasing temperature of the heated cylinder, significant modifications of the wake flow pattern and wake vortex shedding process were clearly revealed. In low Richardson number, the size of the wake and the vortex shedding process in the wake was found to be quite similar to that of an unheated cylinder. As the Richardson number increased, the wake vortex shedding process was found to be altered and the relative position of the first detached vortices respect to the second one is changed. It was also found that the shedding frequency of the wake vortex structures and the wake closure length decreased with increasing Richardson number. [Preview Abstract] |
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