Bulletin of the American Physical Society
2005 58th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 20–22, 2005; Chicago, IL
Session BS: Turbulent Shear Flows |
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Chair: John Foss, Michigan State University Room: Hilton Chicago Stevens 4 |
Sunday, November 20, 2005 10:56AM - 11:09AM |
BS.00001: From large scales energy injection to small scales heat dissipation in a modulated turbulent flow. Olivier Cadot, Frederic Plaza The turbulence is forced between two counter-rotating disks enclosed in a cylindrical cell. The injected power that fed the motion of the large scales is directly measured through the torques driving the disks. The cell is thermally isolated. Simultaneously to the injected power measurements, a temperature measurement is performed in the center of the flow. In order to study the time delay between injection and dissipation, the rotation frequencies of the forcing devices are modulated with a given modulation frequency. First, the synchronized averaging of the temperature measurements allows to detects temperature variation as small as 10$^{-4}$K. Second, during a cycle of modulation of energy injection, the temperature is found to globally increase but displays a modulation too. The temperature modulation encompasses 2 differents phases : a small temperature decrease followed by a large temperature increase. The first phase corresponds to the mixing of the temperature, the second to the energy dissipation due to the turbulence. A delay is found between the peaks of power injection and dissipation. The variation of this time delay is measured for different modulation frequencies of the energy injection in order to extract the turbulent cascade time. The cascade time corresponds to the duration of kinetic energy transfer from the large scales to small scales. [Preview Abstract] |
Sunday, November 20, 2005 11:09AM - 11:22AM |
BS.00002: Vorticity Based Properties of the Viscous Superlayer (VSL) for a Large Single Stream Shear Layer (SSSL) John Foss, Aren Hellum The MSU four-sensor transverse vorticity probe has been used to establish the intermittency function: I=1 if vorticity ($\omega _{z})$ is present, I=0 if it is not for a high R$_{e}$ SSSL, U$_{o}\theta $/$\nu $=9.6$\times $10$^{4}$. The rationale for the dwell time and amplitude threshold will be demonstrated. (The dwell time is set as k$\lambda _{t}$ for k=2 and $\lambda _{t}$= Taylor time microscale of the transverse velocity fluctuation. The amplitude threshold was selected as 40 sec$^{-1})$. Using these processing conditions, the present (averaged) $<$I($\eta )>$ values can be compared with the similar distribution (albeit derived from a surrogate signal) from Wyganski and Fiedler (1970). The two distributions are in close agreement if the mean velocity profiles are forced to agree. Pattern recognition software tools will also be used on the $\omega _{z}$(t) data to evaluate I(t). A planar array of 6 probes: x/$\theta $(o)=500, 0$\le \eta \le $3.05 for $\eta $=(y-y$_{c})$/$\theta $, will be used to track the I=0,1 transitions that signal the passage of the VSL. The inferred topological features of the VSL will be presented. Wyganski, I. and H.E. Fiedler (1970) ``The two-dimensional mixing region,'' \textit{J. Fluid Mech.}, \textbf{41}, 327-361. [Preview Abstract] |
Sunday, November 20, 2005 11:22AM - 11:35AM |
BS.00003: Scale by scale assessment of the approach to isotropy in a turbulent shear flow Peter E. Hamlington, Werner J.A. Dahm The local isotropy hypothesis of Kolmogorov's 1941 universal equilibrium theory states that all turbulent flows approach an increasingly isotropic state at small scales, regardless of the large scale behavior of the flow. Here we present an assessment of this scale-by-scale approach to isotropy for various anisotropy metrics using data from dual-plane stereo PIV measurements in a turbulent shear flow at several outer scale Reynolds numbers $Re_{\delta}$. Specifically we examine the existence of a power law scaling between any arbitrary anisotropy metric $Q$ and wavenumber scale $k$. Such a scaling implies residual anisotropies even at the smallest turbulent scales, perhaps accounting for the unexpectedly large departures from local isotropy noted in several previous studies. [Preview Abstract] |
Sunday, November 20, 2005 11:35AM - 11:48AM |
BS.00004: High Resolution Simulations of the Kelvin-Helmholtz Instability and Stratified Shear Turbulence: The Impact of Stratification on Shear-Layer Morphology, Transition to Turbulence, and the Restratified End State. Joseph Werne, Bjorn Anders Pettersson-Reif Results are reported for high-resolution direct numerical simulations of the Kelvin-Helmholtz instability and ensuing turbulence for three different values of the Richardson number: Ri=0.05, 0.15, and 0.20. Flow morphology and evolution are found to depend strongly on Ri. Whereas the lowest Ri (i.e., the least stable) case exhibits coherent KH ``billows,'' whose round cross-section and rapid solid-body rotation stabilizes them and delays the onset of turbulence in the billow cores, perhaps ironically the highest Ri (or most stable) case displays flatter billows which transition to turbulence immediately upon billow formation. In marked contrast, the lowest Ri, least stable case exhibits a much more complex transition to horizontally homogeneous turbulence, characterized by a sequence of distinct steps which include 1) layered billow formation, 2) secondary instability of billow edges, 3) vigorous turbulence in the braid region between billow cores, and 4) development of turbulence in the cores. Nevertheless, despite the dramatic differences in the flow evolution and dynamics, the final states exhibit nearly identical mid-layer stability profiles and shear/buoyancy timescale ratios. [Preview Abstract] |
Sunday, November 20, 2005 11:48AM - 12:01PM |
BS.00005: Computational study on the internal layer in a diffuser Xiaohua Wu, Jorg Schluter, Parviz Moin, Heinz Pitsch, Gianluca Iaccarino, Frank Ham We report an internal layer found in the turbulent flow through an asymmetric planar diffuser using large eddy simulation; we discuss five issues relevant to the internal layer: definition and identification, conditions for occurrence, connection with its outer flow, similarity with other equilibrium flows, and growth. The present internal layer exists in a region with stabilized positive skin friction downstream of a sharp reduction. The streamwise pressure gradient changes suddenly from slightly favorable to strongly adverse at the diffuser throat, and relaxes in a prolonged mildly adverse region corresponding to the skin friction plateau. Signatures of the internal layer include inflectional point in the wall-normal profiles of streamwise turbulence intensity, and a well-defined logarithmic slope in mean streamwise velocity underneath a linear distribution extending to the core region of the diffuser. It is interesting to note that some of these characteristics bear certain resemblance to those existing in the C-type of Couette-Poiseuille turbulent flows. Two point correlations with streamwise, wall-normal and temporal separations were used to examine connections between fluctuations inside the internal layer and those in the core region of the diffuser. The internal layer discovered from this study provides qualified support to a conjecture advanced by Azad and Kassab fifteen years ago. [Preview Abstract] |
Sunday, November 20, 2005 12:01PM - 12:14PM |
BS.00006: Control and Visualization of a Shear Layer Over a Weapons Bay Ryan Schmit, Ganesh Raman, Luis Lourenco, Valdis Kibens In July 2005, the AFRL program Flow Control Analysis Development (FlowCAD) tested the High Frequency Excitation Active Flow Control for Supersonic Weapons Release (HIFEX) generic weapons bay model in the Boeing's Polysonic windtunnel facility. The $10\%$ scaled weapons bay with an L/D of $5$ was tested at Mach $1.82$. Several flow control devices were tested, including: the goalpost, a wedge and pin configuration, and the splash jet, to determine their effectiveness at reducing the sound pressure levels inside the weapons bay. The results show the wedge and splash jet are equally effective at reducing the peak Rossiter tone by $20$ $dB$. The main objective of this test was to visualize the shear layer over the weapons bay cavity. By examining the cavity shear layer with a $10 kHz$ Focused Schlieren system the effects from the flow control devices can be understood to produce a more effective flow control device in the future. [Preview Abstract] |
Sunday, November 20, 2005 12:14PM - 12:27PM |
BS.00007: Flow field measurements along a row of confined cylinders Barton Smith, Eric Thorson, Donald McEligot The results of flow experiments performed in a cylinder array designed to mimic flow phenomena in a lower plenum of a ``Very High Temperature'' nuclear reactor are presented. Pressure drop and velocity field measurements were made. For initial velocity measurements, a domain centered on one cylinder extending upstream and downstream to the next rows of cylinders was measured, and 27 Reynolds number values in the range 237 $<$ Re $<$ 56,000 were examined. The pressure drop between each set of cylinders was measured simultaneously. Subsequently, high-resolution data sets consisting of three subdomains (one in front of, one below, and one behind a cylinder) revealed the mean and fluctuating separation location and the boundary-layer state for each case. Based on these measurements, five regimes of behavior are identified that are found to depend on Reynolds number. It is also found that the recirculation region behind the cylinders is shorter than those of half cylinders placed on the confining walls representing the symmetry plane. [Preview Abstract] |
Sunday, November 20, 2005 12:27PM - 12:40PM |
BS.00008: Effects of Streamwise Injection Angle on Skin Friction Distribution around a Row of Short Jets-in-Crossflow Sean Peterson, Michael Plesniak The velocity field and skin friction distributions around a row of five jets issuing into a crossflow via short (L/D = 1) holes inclined by 35 deg. in the streamwise direction are presented. The flow features are compared with previously published data from our laboratory for jets issuing through holes oriented normal to the crossflow. Streamwise injection eliminated the recirculation region and consequent downstream spiral separation node vortices observed in the 90 deg. case. The jetting processes caused by in-hole separations were also found to differ, with the bulk of the jet issuing from the leading half of the streamwise injected hole. The skin friction distribution around the injection holes was very similar to that of the normal holes, particularly in the wake of the hole, despite a much lower trajectory and increased spreading. This may have implications on the understanding of the degree to which the counter-rotating vortex pair impacts the wall shear stress. [Preview Abstract] |
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