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 HC: Turbulent Shear Layers |
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Chair: Joe Hall, University of New Brunswick Room: 002A |
Monday, November 24, 2008 10:30AM - 10:43AM |
HC.00001: A Detailed Analysis of Nonlinear Interaction of Cavity Tones in Shallow Subsonic Resonant Cavity Flows M. Samimy, J. Malone, M. Debiasi, J. Little The Rossiter equation and its derivatives with two empirical parameters have been used over several decades to understand and interpret shallow cavity tones. A recently developed alternative approach based on signal processing theory interprets the Rossiter equation as the result of an amplitude modulation process between a fundamental aeroacoustic loop frequency and a modulating lower frequency. We present the results of this approach applied to detailed, high quality spectral data of shallow-cavity flows over Mach number range of 0.2 to 0.65. The results show that the spectra can be grouped into several ranges of Mach number with similar behavior, separated by transitional regions. Clear identification of all the tones observed in these spectra validates the ability of the new approach in explaining the complex spectral features of this type of flow. The asymptotic growth with Mach number of the fundamental aeroacoustic loop frequency and the relatively constant modulating frequency provide insight into the variation of the Rossiter mode parameters. The results also indicate that the empirical parameters in the Rossiter equation vary slightly with Mach number for fixed cavity geometry. [Preview Abstract] |
Monday, November 24, 2008 10:43AM - 10:56AM |
HC.00002: POD and Fourier analysis of composite-field-of-view PIV applied to the far turbulent axisymmetric jet: Part I Knud Erik Meyer, Maja W\"{a}nstr\"{o}m, William K. George PIV measurements were made in a streamwise plane parallel to and intersecting the centerline of a 1 cm turbulent round air jet at exit Re = 20,000. Two HiSense 4 Mpxl cameras were positioned with overlap to produce a total field-of-view which covered the extent of the jet from x/D=31 to x/D=100. Radial coverage of the jet was $\pm 0.2*x$ at all downstream positions. The goal was to be able to effect a logarithmic coordinate transformation of the velocity field into a homogeneous field in similarity coordinates (i.e., $\xi = \ln (x/D)$, see $[1]$) for which Fourier transformation in $\xi$ is the theoretically optimal representation. The extended streamwise field was necessary to minimize finite window effects. The magnification of each camera was adjusted to attempt to maintain constant relative spatial resolution. \\ 1.\ Ewing, D. et al.\ {\bf J. Fluid Mech., 577}, 309-330, 2007. \\ [Preview Abstract] |
Monday, November 24, 2008 10:56AM - 11:09AM |
HC.00003: POD and Fourier analysis of composite-field-of-view PIV applied to the far turbulent axisymmetric jet: Part II Maja W\"{a}nstr\"{o}m, William K. George, Knud Erik Meyer This paper concerns the processing and results of data acquired from 30 to 100 diameters in a far turbulent air jet using a composite-field-of-view PIV. The Reynolds number of the flow was 20,000 based on the nozzle exit diameter which was 1 cm. Following $[1]$, the snapshots for the entire field were first mapped into similarity coordinates, $\eta =r/\delta_{1/2}(x)$ and $\xi = \ln[(x-x_o)/D]$. Then the mapped field was decomposed using the classical POD in $\eta$ and Fourier analysis in the new homogeneous coordinate $\xi$. The streamwise extent of the field was chosen to minimize window effects on the Fourier analysis and the lateral extent to insure capture of the energy by the POD. The fields were then reconstructed using partial decompositions in the manner first proposed by $[2]$. \\ 1.\ Ewing, D. et al.\ {\bf J. Fluid Mech., 577}, 309-330, 2007. \\ 2.\ Elteyeb, et al.\ {\bf Bull. Am. Phys. Soc., 48}, 10, p. 53, 2003. [Preview Abstract] |
Monday, November 24, 2008 11:09AM - 11:22AM |
HC.00004: Measurement of Pressure-Rate-of-Strain, Pressure Diffusion and Velocity-Pressure-Gradient Tensors around an Open Cavity Trailing Corner Xiaofeng Liu, Joseph Katz A non-intrusive measurement technique that is capable of simultaneously measuring the instantaneous velocity, material acceleration and pressure distribution over a sample area has been applied to measure the pressure related turbulence statistics of a shear layer flow around the trailing corner of a 2D open cavity. This technique utilizes four-exposure PIV to measure the distribution of material acceleration, and integrating it by means of circular virtual boundary omni-directional algorithm to obtain the instantaneous pressure distribution. Results based on 13600 instantaneous flow field measurement samples enable direct estimates of components of the pressure-rate-of-strain, pressure diffusion and velocity-pressure-gradient tensors. Analysis of error propagation and comparisons between these terms indicate that the results are reliable. In the cavity shear layer, the pressure-rate-of-strain term R11 is mostly negative while R22 is positive but with smaller magnitude, implying that the streamwise turbulent energy is redistributed to the lateral direction. Conventional pressure-strain turbulence models are also compared with the measurement data. [Preview Abstract] |
Monday, November 24, 2008 11:22AM - 11:35AM |
HC.00005: The Triple Layer Structure of a Transitional Wall-Jet Sam Raben, Wing Ng, Pavlos Vlachos Wall-jets are commonly found in heating and cooling applications such as gas turbine blade cooling as well as flow control [i]. The structure of wall-jets can be described by three distinct regimes: near wall boundary layer, mixing layer, and outer shear layer [ii]. Previously, wall-jet studies have focused on the self-similar fully developed length scale regime. This study examines the transitional regime using Time Resolved Digital Particle Image Velocimetry over a range of Reynolds numbers between 200-10,000. Analyses of the data, using previously developed scaling laws [ii], support the notion of incomplete similarity, and demonstrate that a transition point exists past which this similarity can be applied. However, although before this transition point the incomplete similarity can be used to describe the outer shear layer, yet it fails to scale the inner near wall layer and mixing layer. The present work provides the first investigation of the triple layer structure of a wall-jet within the regime of transitional lengths and across a wide range of Reynolds numbers with high spatio-temporal resolution. i. Launder BE, and Rodi W (1983) \textit{The Turbulent Wall Jet -- Measurements and Modeling}, Ann Rev Fluid Mech \textbf{15}:429-459 ii. Barenblatt GI, et al, (2005) \textit{The Turbulent Wall Jet: A Triple-Layered Structure and Incomplete Similarity.} Proc. of the Nat. Academy of Sciences, \textbf{102 }(25) [Preview Abstract] |
Monday, November 24, 2008 11:35AM - 11:48AM |
HC.00006: Turbulence interaction with fractal trees in a turbulent boundary layer over a rough surface Hyung-Suk Kang, Charles Meneveau An experimental study is performed of turbulence interacting with multi-scale fractal objects placed in turbulent boundary layer flows. The main objectives of the study are to quantify drag forces generated by such interactions and to compare with the simulation results of Chester et al. (Journal of Computational Physics, volume 225, 2007) who used renormalized numerical simulation (RNS) to predict the forces. In the present study, fractal trees with branches in a single cross-plane are considered. Each branch has three subbranches and the scale ratio is 0.5, so the similarity fractal dimension is about 1.58. The models are placed in a wind-tunnel model of the atmospheric-like turbulent boundary layer over a rough surface. The inflow is generated by a combination of an active grid and a row of strakes. Six trees corresponding to increasing generations of branchings are considered. The fractal trees are mounted on a load cell unit, which enables us to measure the drag forces depending on the included generation of branches. The averaged total drag coefficients agree well with a branch-resolved simulation (Chester et al. 2007). Also, turbulence data are sampled at 4 different downstream locations of the fractal trees by using an X-type hot-wire probe. The effects of generation number on the turbulence spectra and PDFs are quantified. [Preview Abstract] |
Monday, November 24, 2008 11:48AM - 12:01PM |
HC.00007: Wall-Shear Stress Distribution in Turbulent Duct Flow Sebastian Gro{\ss}e, Wolfgang Schr{\"o}der The wall-shear stress sensor MPS$^{3}$ based on flexible micro-pillars has been used to experimentally assess the two-dimensional wall-shear stress distribution in turbulent duct flow at moderate Reynolds number. A sensor array covering an area of 90x125 viscous length-scales along the streamwise and spanwise direction, respectively and 1-D sensor-lines spanning 125 viscous length-scales have been applied. The results evidence the co-existence of low- and high-shear regions representing ``foot-prints'' of near-wall coherent structures. Applying Taylor's hypothesis allows to crudely assess the streamwise length-scales of the near-wall flow field. Especially the low-shear regions attain streamwise dimensions of approximately 1,000 viscous length-scales. These low-shear regions resemble long meandering bands locally interrupted and deflected by regions of high-shear stress. A qualitative comparison evidences the structures detected in the present study to be similar to wall-shear stress distributions reported in the literature and to structures found in higher regions of the boundary layer. The results allow assessing statistics of the streamwise and spanwise wall-shear stress distribution, such as convection velocities of wall-shear stress fluctuations, two-point-, and auto-correlations. Geometric properties of the streak-like wall-shear stress distributions will also be discussed. [Preview Abstract] |
Monday, November 24, 2008 12:01PM - 12:14PM |
HC.00008: On A Turbulent Mixing Layer Created Downstream Of A $\Lambda $-Shaped Trailing Edge Boris Zakharin, Israel Wygnanski Measurements were carried out in a turbulent mixing layer formed by two parallel streams differing in velocity downstream of a splitter plate that had a $\Lambda $-shaped trailing edge. It appears that the transverse rate of spread of the mixing layer is defined by the local distance downstream of the trailing edge. However the mixing layer shifted toward the high-speed stream at its center span that is directly downstream of the $\Lambda $-notch in the splitter plate, while its sides bent toward the low speed stream. This bending persisted over long distances from the splitter plate. The existence of a pair of counter rotating streamwise eddies that dominate the flow and substantially increase its level of turbulence relative to the classical plane mixing layer could explain these observations. The idea was tested by adding vortex generators and was shown to be valid. [Preview Abstract] |
Monday, November 24, 2008 12:14PM - 12:27PM |
HC.00009: PIV ---Galilean Identification of Large Scale Coherent Motions in a Shear Layer John Foss, Kyle Bade 1,000 PIV realizations were processed in a Galilean reference frame image of the shear layer's velocity field. Images with at least one node were processed as a collapsed sphere with four holes (Foss (2004) and (2007)). A square domain of side L, surrounding the node-center, was evaluated for its circulation: Gamma(L). The size of the ``large Scale Coherent Motion (LSCM)'' was defined as the maximum value of $<$omega$>$ where $<$omega$>$ = Gamma(L)/L**2. Lm designates the domain size for the maximum $<\omega >$ value. Statistical properties of the LSCMs include their sizes [Lm/theta(x)], strengths [$<$omega m$>$Lm/Uo], their position and their probability of occurrence within the observation window. These LSCMs are ``rare events'' as shown by their $<$omega m$>$/(uniformly distributed vorticity) = $\sim $ 100. The LSCMs may be related to the anomalous omega z (t) traces seen in direct vorticity measurements. J.F. Foss (2004) ``Surface Selections and Topological Constraint Evaluations for Flow Field Analyses,'' \textit{Experiments in Fluids}, Springer-Verlag, \textbf{37}, pp. 883-898. J. Foss, \textit{Springer Handbook of Experimental Fluid Mechanics}, Chapter C.13, Springer-Verlag, Berlin, 2007. [Preview Abstract] |
Monday, November 24, 2008 12:27PM - 12:40PM |
HC.00010: Large-scale intermittency of liquid-metal channel flow in a magnetic field Oleg Zikanov, Thomas Boeck, Dmitry Krasnov, Andre Thess We predict a novel flow regime in liquid metals and other electrically conducting fluids under the influence of a magnetic field. It is characterized by long periods of nearly steady, two-dimensional flow interrupted by violent three-dimensional turbulent bursts. Our prediction has been obtained from direct numerical simulations in a channel geometry at low magnetic Reynolds number and translates into physical parameters which are amenable to experimental verification under laboratory conditions. The new regime occurs in a wide range of parameters and may have implications for metallurgical applications and the dynamo problem. [Preview Abstract] |
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