### Session GA: Turbulent Boundary Layers: Experiments II

Chair: Beverley McKeon, California Institute of Technology
Room: 101A

 Monday, November 23, 2009 8:00AM - 8:13AM GA.00001: Measurements of spatio-temporal spectra in a zero-pressure gradient turbulent boundary layer J. LeHew , M. Guala , B.J. McKeon The structure of the velocity field in wall-parallel planes in a zero pressure gradient turbulent boundary layer is interrogated using time-resolved digital particle image velocimetry at moderate Reynolds number ($Re_{\tau}=500$). In order to investigate turbulent boundary layer structure in light of the emerging understanding of the nature of very large scale motions (VLSMs), two high speed cameras are placed side-by-side to recover streamwise structures over ten times the boundary layer thickness in length, while still resolving the dissipative scales. The simultaneous spatial (streamwise and spanwise) and temporal joint spectra and correlations are used to investigate the validity of Taylor's hypothesis as the wall is approached, in light of the known significant wall normal extent of the VLSMs and the associated range of convection velocities. Monday, November 23, 2009 8:13AM - 8:26AM GA.00002: Instantaneous shear stress distribution in a turbulent wall-bounded flow Omid Amili , Julio Soria Knowledge of wall shear stress is crucial for understanding of all wall-bounded turbulent flows and also for many technical applications. The aim of the present work is to develop a novel stress sensor which is capable of measuring surface shear stress over an extended region of the flow. This sensor as a direct method for measuring surface stresses consists of mounting a thin film made of a elastic polymer on the surface of the solid model. The geometry and mechanical properties of the elastomer are measured, particles acting as markers are applied on the film surface, and an optical technique is used to measure the film deformation caused by the flow. While the technique can be used in air or water, its sensitivity can be tuned for different flow conditions. The static and dynamic calibration of the sensor, and its application to a fully developed turbulent channel flow at moderately high Reynolds numbers will be addressed, and results will be compared with indirectly measured wall shear stress from PIV experiment. Monday, November 23, 2009 8:26AM - 8:39AM GA.00003: Time-resolved PIV in fully developed turbulent pipe flow Leo Hellstroem , Alexander Smits Stereoscopic particle image velocimetry was used to study the three component flowfield in both fully developed turbulent pipe flow and at several locations downstream of a horizontal $90^{\circ}$ bend. The data was acquired with a high speed camera, making it possible to resolve the flow field in time for Reynolds numbers up to $35'000$. The secondary motions downstream the bend appears to be governed by either a Dean type motion were two swirls with opposite signs coexists, one in the upper and one in the lower half of the pipe respectively. Or a motion where two swirls, as large as the pipe, with opposite sings are alternating between each other. Both motions are present at the same Reynolds numbers, but the unsteady behavior appears to be more common for higher Reynolds numbers. Monday, November 23, 2009 8:39AM - 8:52AM GA.00004: 3-D Flow Visualization of a Turbulent Boundary Layer Brian Thurow , Steven Williams , Kyle Lynch A recently developed 3-D flow visualization technique is used to visualize large-scale structures in a turbulent boundary layer. The technique is based on the scanning of a laser light sheet through the flow field similar to that of Delo and Smits (1997). High-speeds are possible using a recently developed MHz rate pulse burst laser system, an ultra-high-speed camera capable of 500,000 fps and a galvanometric scanning mirror yielding a total acquisition time of 136 microseconds for a 220 x 220 x 68 voxel image. In these experiments, smoke is seeded into the boundary layer formed on the wall of a low-speed wind tunnel. The boundary layer is approximately 1.5'' thick at the imaging location with a free stream velocity of 24 ft/s yielding a Reynolds number of 18,000 based on boundary layer thickness. The 3-D image volume is approximately 4'' x 4'' x 4''. Preliminary results using 3-D iso-surface visualizations show a collection of elongated large-scale structures inclined in the streamwise direction. The spanwise width of the structures, which are located in the outer region, is on the order of 25 -- 50{\%} of the boundary layer thickness. Monday, November 23, 2009 8:52AM - 9:05AM GA.00005: Harvesting energy from turbulence in boundary layers by using piezoelectric generators Yiannis Andreopoulos , Dogus H. Akaydin , Niell Elvin The availability of significant kinetic energy in fluid flows distributed over a number of temporal and spatial scales creates a unique opportunity to convert this energy into electrical output by using piezoelectric generators. The unsteadiness due to turbulence can produce mechanical strain energy in the piezoelectric material which in turn can generate a build up of charge that can be used to power electronic devices. In the present work, short length piezoelectric beams were placed in a zero pressure gradient two dimensional turbulent boundary layer at Reynolds numbers based on momentum thickness up to 6500 to evaluate their performance as energy generators. The piezoelectric beam was traversed across the boundary layer to determine the location where the output power is maximized. It was found that the location of maximum power is not close to the wall where most of the turbulent activities are high but further away from the wall. The work has shown that there is a three-way coupled interaction~between the fluid flow, the piezoelectric structure and its electromechanical field. Monday, November 23, 2009 9:05AM - 9:18AM GA.00006: Turbulent Coherent Structures in a Thermally Stable Boundary Layer Owen Williams , Sean Bailey , Alexander Smits An experiment was conducted to examine the effect of thermal stability on turbulent coherent structures occurring in a flat plate boundary layer. The objective is to further characterize the turbulence in thermally stable atmospheric boundary layers, commonly found in the arctic regions, focusing on Reynolds number independent effects. This experiment was conducted in a 16 foot long, 4'x2' cross-section, open-return wind tunnel by replacing the upper surface with a heated half inch aluminum plate. The plate was maintained at an isothermal condition, the boundary layer along this surface was tripped and the tunnel run at the lowest speed possible, in order to maintain both a fully turbulent boundary layer and a large Richardson number. A wide range of stabilities were investigated, with Richardson numbers ranging from 0 to 0.5, covering both the weakly and strongly stable regimes. Using thermocouple temperature measurements and time resolved particle image velocimetry; an attempt was made to identify changes in coherent turbulent motions corresponding to changing flow stability. Additionally, an attempt was made to identify significant features of the turbulence that could be used to identify clearly delineating features of the weakly stable and strongly stable flow regimes. Monday, November 23, 2009 9:18AM - 9:31AM GA.00007: Buoyancy effects on large- and small-scale turbulent motions in the atmospheric surface layer during the transition through neutral stability Meredith Metzger The present study examines how the transitory nature of the atmosphere, both on diurnal and meso timescales, affects the evolution of Very Large-Scale Motions in the Atmospheric Surface Layer (ASL) during the transition through neutral thermal stability. It is hypothesized that the finite time duration of the near-neutral period arrests the development of VLSMs in the ASL, compared to those expected in a canonical turbulent boundary layer having equivalent Reynolds number; and that, this, in turn, affects the structure of the small-scale turbulence by impeding inner-outer interactions. These scientific questions are addressed using simultaneously sampled hot-wire and sonic anemometry time series obtained during a field campaign in Utah's western desert. The pointwise data span a wall-normal distance between 1 mm and 30 m above the surface over a time period of several hours centered around neutral transition. Velocity spectra as a function of time (i.e., thermal stability) are shown as well as statistics associated with the turbulent bursting process. Results are compared against those obtained both at lower Reynolds number in the laboratory and in the neutral ASL. Monday, November 23, 2009 9:31AM - 9:44AM GA.00008: Turbulent structures in smooth and rough open channel flows Vesselina Roussinova , Ram Balachandar Turbulent flow in open channels is unique because it is bounded by the free surface and the flow is entirely dominated by the bed turbulence. Both experiments and simulations agree that the large-scale near-wall structures interact with the free surface without significant reduction of their strength. Still the link between bed and free-surface turbulence is not well understood and it is of a particular importance for the processes occurring at the surface in shallow geophysical flows. In this paper, high resolution particle image velocimetry (PIV) measurements in an open channel flow are presented. Velocity measurements were obtained in the streamwise - wall normal (x-y) plane and streamwise-spanwise (x-z) plane. Streamwise-spanwise (x-z) planes were acquired at various vertical locations. The focus of this study is to investigate the streamwise oriented vortices along the flow depth as documented through the analysis of swirling strength, conditional averaging and vortex statistics. Turbulence measurements in rough open channel flow are also presented and compared with those on the smooth wall. Monday, November 23, 2009 9:44AM - 9:57AM GA.00009: A turbulent boundary layer on a rough wall at hypersonic speeds Dipankar Sahoo , Parthav Desai , Alexander Smits Previous experiments on hypersonic turbulent boundary layers have documented the general features of the mean flow behavior on a smooth plate, but virtually no data exist describing the boundary layer behavior on a rough wall for Mach numbers greater than about 5. Here, we report PIV measurements of the mean flow and two components of velocity fluctuations on a flat plate with three different roughness geometries: a square bar roughness, and two diamond roughness elements of different height. The boundary layer develops at Mach 7.2 in a perfect gas, at a Reynolds number based on momentum thickness of about 3600. The results are compared with DNS under identical flow conditions. Supported under NASA Grant NNX08AB46A, Program Manager Catherine McGinley.