Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Fluid Dynamics
Volume 53, Number 15
Sunday–Tuesday, November 23–25, 2008; San Antonio, Texas
Session LA: Turbulent Boundary Layers: Experimental Studies III |
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Chair: Ronald J. Adrian, Arizona State University Room: 001A |
Monday, November 24, 2008 3:35PM - 3:48PM |
LA.00001: Observations of Large-Scale Meandering Motions in Rough-Wall Turbulence R. Mejia-Alvarez, Y. Wu, K.T. Christensen Recent experimental evidence suggests the existence of meandering low-speed motions in the logarithmic region of smooth-wall turbulence with streamwise extents well exceeding the boundary- layer thickness. The present contribution explores the possible existence of these large-scale motions in turbulent flow over a rough surface. Time-resolved particle-image velocimetry experiments are performed in a streamwise--spanwise plane in the vicinity of a rough wall ($y\cong 0.065\delta$) replicated from a turbine blade damaged by deposition of foreign materials. This surface is highly irregular and contains a broad range of topographical scales. Taylor's hypothesis is utilized to reconstruct velocity fields over $8\delta$-long in the streamwise direction from the time- resolved PIV fields. Similar to previously-reported smooth-wall observations, these reconstructed velocity fields are marked by connected regions of low-speed fluid [$O(\delta)$ wide in the spanwise direction] that extend well beyond $\delta$ in the streamwise direction and meander significantly in the spanwise direction. In addition, wall- normal vortex cores of opposing rotation are found to populate the boundaries of these meandering, low-speed regions. The average characteristics of these motions in rough-wall turbulence are explored and compared to observations from smooth-wall flow. [Preview Abstract] |
Monday, November 24, 2008 3:48PM - 4:01PM |
LA.00002: Addition of Isotropic Free-stream Turbulence Promotes Anisotropy in a Turbulent Boundary Layer Sheilla Torres-Nieves, Brian Brzek, Jose Lebron, Luciano Castillo, Raul Cal, Charles Meneveau The effects of nearly isotropic free-stream turbulence in transitionally rough zero pressure gradient turbulent boundary layers are studied using data obtained from Laser Doppler Anemometry measurements. Measurements are carried out at Re$_{\theta }\le $ 11,300 with up to 6.2{\%} free-stream turbulence generated with an active grid. Remarkably, there is a large portion of the boundary layer in which the addition of nearly isotropic turbulence in the free-stream results in significant increases in anisotropy of the turbulence. In order to quantify which turbulence length-scales contribute mostly to creating this trend, second order structure functions for velocity components u or v are examined at various distances to the wall. Results show that the anisotropy created by adding nearly isotropic turbulence in the free-stream resides mostly in the larger scales of the flow. [Preview Abstract] |
Monday, November 24, 2008 4:01PM - 4:14PM |
LA.00003: ABSTRACT WITHDRAWN |
Monday, November 24, 2008 4:14PM - 4:27PM |
LA.00004: Inertial particle accelerations in a turbulent boundary layer Zellman Warhaft, Sergiy Gerashchenko, Nicole Sharp, Stephanie Neuscamman Two dimensional Lagrangian acceleration statistics of inertial particles in a turbulent boundary layer with free stream turbulence are determined by means of a high speed particle tracking technique (Ayyalasomayajula et al. PRL, 95, 144507, 2006). The boundary layer is formed above a flat plate, and water droplets are fed into the flow from sprays placed down-stream from an active grid, and from tubes fed into the boundary layer from humidifiers. The free stream Stokes number is varied from 0.035 to 0.47. As the boundary layer plate is approached, the tails of the pdfs narrow, become negatively skewed, and their peak occurs at negative accelerations (decelerations in the stream-wise direction). The mean deceleration and its r.m.s. increase to large values close to the plate and are more pronounced with increasing Stokes number, in marked contrast to what is found in isotropic turbulence where the acceleration r.m.s. decreases with increasing Stokes number. A model shows the significance of the combined effects of shear and gravity on the acceleration statistics. The work is funded by the US NSF. [Preview Abstract] |
Monday, November 24, 2008 4:27PM - 4:40PM |
LA.00005: Complex dynamics of a boundary layer with free stream turbulence Nicole Sharp, Stephanie Neuscamman, Zellman Warhaft Boundary layers in nature and in engineering applications often occur with turbulent free streams above them. Previous work by Hancock and Bradshaw (JFM, \textbf{205}, 1989), Thole and Bogard (J. Fluids Eng., \textbf{118}, 1996), and others has shown that free stream turbulence affects the statistics of a boundary layer significantly. In the present wind tunnel study using hot wire anemometry, a flat plate generates a boundary layer that is subjected to a variety of free stream turbulence conditions using active and passive grids. The free stream varies in turbulence intensity from 0.25{\%} to 11{\%} and in free stream turbulent Taylor- scale Reynolds number from 20 to 550. The ratio of the free stream length scale to the boundary layer thickness is also varied. Spectral data reveal a double-peaked energy spectrum, indicating the interaction of two different, major length scales. The double peak develops as the plate is approached from the free stream, and, though the feature is most pronounced at higher free stream Reynolds numbers, it is also evident at very low free stream turbulence intensities. This work was supported by the US NSF. [Preview Abstract] |
Monday, November 24, 2008 4:40PM - 4:53PM |
LA.00006: High Aspect Ratio Cylindrical Boundary Flow and Wall Pressure Spectra Kimberly Cipolla, William Keith, Damien Bretall, Deborah Furey, Paisan Atsavapranee High resolution stereo-PIV measurements were made on a long ($>$ 1300 m), 38 mm diameter cylinder towed from a vertical strut at speeds of 7 to 30 kts. Wall pressure measurements were collected simultaneously at select axial locations. The experiments were performed in the high speed tow tank at NSWCCD. The cylinder was ballasted to be approximately neutrally buoyant and towed through a stationary laser sheet oriented perpendicular to the tow direction. The objective of the study was to quantify the boundary flow along the cylinder for correlation with the surface pressure data. The average velocity data was analyzed to compute boundary flow parameters used for nondimensionalizing the boundary layer pressure spectra. Mean and fluctuating streamwise and cross-stream velocities will be presented along with the corresponding pressure spectra. [Preview Abstract] |
Monday, November 24, 2008 4:53PM - 5:06PM |
LA.00007: The Effect of Upstream Vane Wakes on Annular Diffuser Flows Erica Cherry, Angelina Padilla, Christopher Elkins, John Eaton Experiments were performed to determine the sensitivity to inlet conditions of the flow in two annular diffusers.~ One of the diffusers was a conservative design typical of a diffuser directly upstream of the combustor in a jet engine.~ The other had the same length and inlet shape as the first diffuser but a larger area ratio and was meant to operate on the verge of separation.~ Each diffuser was connected to two different inlets, one containing a fully-developed channel flow, the other containing wakes from a row of airfoils. Three-component velocity measurements were taken on the flow in each inlet/diffuser combination using Magnetic Resonance Velocimetry.~ Results will be presented on the 3D velocity fields in the two diffusers and the effect of the airfoil wakes on separation and secondary flows. [Preview Abstract] |
Monday, November 24, 2008 5:06PM - 5:19PM |
LA.00008: Local flow topology dynamics in a turbulent boundary layer Gerrit Elsinga, Ivan Marusic Experimental data from time-resolved 3D Tomographic Particle Image Velocimetry is used to study the dynamics of the local flow topology in the logarithmic region of a turbulent boundary layer. Specifically, we determine the invariants of the velocity gradient tensor defining the local topology and compute their mean material derivatives as a function of the invariants themselves. Subsequent time integration yields trajectories, which reveal spiralling, periodic orbits representative of the flow evolution in the mean sense. The period is nearly constant and can be thought of as a characteristic life-time for the eddies. It has an associated wavelength of approximately 10~boundary layer thicknesses (using the local average velocity as the convective velocity). This corresponds well with the location where a peak appears in the pre-multiplied power spectra of the streamwise component of velocity in wall-bounded turbulence. Previous studies have linked that peak to the very large-scale motions or superstructures observed in wall turbulence. Hence, these results may provide a link between the local topology dynamics and the coherent structures commonly observed in boundary layers. [Preview Abstract] |
Monday, November 24, 2008 5:19PM - 5:32PM |
LA.00009: Investigation of large-scale features in turbulent duct flows Henry Ng, Jason Monty, Nicholas Hutchins, Min Chong, Ivan Marusic, Bharathram Ganapathisubramani Recent studies reveal that long low-speed meandering structures (referred to as ``superstructures'' or Very Large Scale Motions) exist in the log region of fully developed turbulent pipe and channel flows as well as the turbulent boundary layer. These studies have been carried out using hot-wire arrays which are physically limited in terms of wall proximity. Here we use an array of multiple wall skin friction sensors to study the ``footprint,'' that is, the influence of these large scale features at the wall. Hot-wire velocity profiles measured in conjunction with the multiple skin-friction sensor array are used to study the three dimensional coherence of the large scale structure and the ensemble averaged statistics. Experiments are carried out in fully developed turbulent pipe and channel flow facilities with a similar outer length scale (pipe radius, $R = 49.4{\rm mm}$ and channel half height, $h = 50{\rm mm}$) enabling direct comparison of the flows at a matched Reynolds number. [Preview Abstract] |
Monday, November 24, 2008 5:32PM - 5:45PM |
LA.00010: Two-Point Cross-Spectral and POD Analysis of High Reynolds Number Zero Pressure Gradient Turbulent Boundary Layer M. Tutkun, W.K. George, J.-M. Foucaut, S. Coudert, M. Stanislas, J. Delville This study reports some of the two-point cross-spectral and proper orthogonal decomposition analyzes performed on the zero pressure gradient flat plate turbulent boundary layer experiments at R$_{\theta}=$9800 and 19,100. They are based on hot-wire measurements using a rake of 143 single wire probes placed in the LML wind tunnel, which has a 20 m long test section with approximately 30 cm of boundary layer thickness. Elongated correlations and their variation across the boundary layer are shown using two-point space-time correlations computed over very long records. The possible interaction between the inner and outer layer is discussed using the cross-correlations analysis and also a POD-based reconstruction of the velocity fields on a plane normal to the streamwise direction. Organization of the large scale structures on the same plane is also demonstrated using a combination of different spanwise Fourier and POD modes. [Preview Abstract] |
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