Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session KQ: Turbulence: Shear Layers I |
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Chair: John Foss, Michigan State University Room: Salt Palace Convention Center 251 E |
Tuesday, November 20, 2007 8:00AM - 8:13AM |
KQ.00001: A Comparison of Entrainment Characteristics Between Reacting and Nonreacting Countercurrent Shear Layers Andrew Stuntz, David Forliti Countercurrent shear layers are known to have higher turbulence levels and entrainment rates compared to single stream and coflowing shear layers. The current work is motivated to understand the affect of heat release on the countercurrent shear layer. Heat release in the shear layer is created through igniting the shear layer formed between a rich methane/air primary stream and a counterflowing air stream. Countercurrent shear applied to a reacting shear layer suppresses flame liftoff and enhances combustion rates. The current focus is to document the entrainment characteristics of the reacting and nonreacting countercurrent shear layer, in particular the shear layer spreading rate and entrainment ratio as a function of velocity ratio will be described. Heat release is known to suppress shear layer growth for coflowing and single stream shear layers. The results will provide important insight into the potential benefit of using countercurrent shear for enhancing shear flows containing heat release. [Preview Abstract] |
Tuesday, November 20, 2007 8:13AM - 8:26AM |
KQ.00002: Experimental study of an active grid-generated shearless mixing layer Hyung-Suk Kang, Charles Meneveau The interaction between two dominant turbulence scales is investigated in a shearless mixing layer. The shearless mixing layer is composed of three regions, i.e., two nearly homogeneous decaying high- and low-energy regions, and a mixing region. There is no mean velocity gradient across the layer and so there is no production in the turbulent kinetic energy budget. Reynolds numbers higher than those of prior studies of this flow are achieved by combining an active grid with different winglets and stationary fine meshes to avoid mean velocity gradients. Measurements are performed at 5 different downstream locations in the Corrsin wind tunnel by using an X-type hot-wire probe and a stereoscopic PIV system. The Reynolds numbers based on the Taylor's microscale at the high- and low-energy regions are 250 and 110, respectively. The integral length scale ratio, defined as the integral length in the high-energy region to that in the low-energy region, is 2.65. Comparisons with earlier studies (Veeravalli \& Warhaft 1989, Knaepen et al 2004, Tordella \& Iovieno 2006) will be presented. The results are compiled into a database format to facilitate comparisons with Large-Eddy Simulations, and sample comparisons are presented. [Preview Abstract] |
Tuesday, November 20, 2007 8:26AM - 8:39AM |
KQ.00003: Statistical Features of the Large Scale Coherent Motions in a Single-Stream Shear Layer (SSSL) John Foss, Kyle Bade A dedicated PIV flow facility provides uniform seeding in the primary \textit{and} the entrainment flows for the subject SSSL. PIV images within the nominal range: 500$\le $x/$\theta $(0)$\le $680 have been examined by subtracting U$_{o}$/2 from each streamwise component of the measured velocity (in-plane) vectors. The resulting vector fields exhibited a sparse distribution of clearly defined nodes and saddles in contrast with the dense population of singular points in the original images. A total of 917 isolated coherent motions, identified by the topological considerations utilized in Foss (2004), have been identified in 1000 PIV images. Of these, 538 involved one node (coherent motion) in the observation window. The population of the areas that produced the largest spatially averaged vorticity: $<\omega _{z}>$A=$\Gamma $, and the $<\omega >$ values will be presented. 28.6{\%} of the realizations did not exhibit coherent motions, 9.3{\%} showed two or more nodes. Other statistical properties of the coherent motions will be reported. 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. [Preview Abstract] |
Tuesday, November 20, 2007 8:39AM - 8:52AM |
KQ.00004: Small scale isotropy and universality of axisymmetric jets Carlo Massimo Casciola, Francesco Picano Self-similarity is a fundamental phenomenology in turbulent flows, see e.g. the law of the wall near solid boundaries. In certain cases it can be extended to universality, e.g. each zero pressure gradient turbulent boundary layer is identical, despite differences in the transitional phases. In these conditions, the longstanding debate about jets and their far-field universality comes as no surprise. The initial ansatz of a universal spreading rate is inconsistent with more accurate and recent measurements where the scatter in the opening angle is found to exceed the experimental accuracy, implying a true lack of universality in these basic quantity. A universal scaling theory may still be proposed by including the spreading rate in the similarity transformations. We will show that this approach leads to a conflict with the presumed recovery of isotropy in the dissipative scales of the flow. Finally we will show why we expect isotropy to be recovered to conclude that axisymmetric jets cannot be universal, unless the spreading rate is universal itself. The issue is supported by means of two direct numerical simulations of free axisymmetric jets with different inlets and by experimental data available in the literature. [Preview Abstract] |
Tuesday, November 20, 2007 8:52AM - 9:05AM |
KQ.00005: Characterizing the Edge of Turbulence for Shear Flows Lina Kim, Jeff Moehlis We characterize the edge of turbulence, the boundary which separates the basins of attraction of the laminar and turbulent states, for a nine-dimensional model for sinusoidal shear flow in order to gain a greater understanding of the nature of and transition to turbulence. The model has three qualitatively distinct trajectories which either decay to the laminar state, or become transiently chaotic before decaying to the laminar state, or become transiently chaotic before moving towards a non-trivial attractor. The boundary which separates the laminar and chaotic behavior is the eight-dimensional stable manifold of an unstable periodic orbit, at least for moderately small Reynolds numbers. Furthermore, a probabilistic analysis of the transition to turbulence is performed by computing the probability, for a large range of Reynolds numbers, that perturbations of a given energy will lead to turbulence and/or linear transient energy growth, a mechanism which may trigger nonlinear effects that lead to sustained turbulence in shear flows. [Preview Abstract] |
Tuesday, November 20, 2007 9:05AM - 9:18AM |
KQ.00006: The spatial relationships between dissipation and production rates and vortical structures in turbulent boundary and mixing layers. J.M. Wallace, J. Diorio, D.H. Kelley Data bases of all three velocity components as well as six components of the velocity gradient tensor measured with multi-sensor hot-wire probes in a turbulent boundary layer and a two-stream turbulent mixing layer were analysed. The remaining three velocity gradients were determined using Taylor's hypothesis. With these data, the ``instantaneous'' production and dissipation rates, defined by $P = - {\partial \overline{U}_i}/{\partial x_j}(u_i u_j)$ and $D = -\nu \left( \left({\partial u_i}/{\partial x_j} \right)^2 + ({\partial u_i}/ {\partial x_j})({\partial u_j}/{\partial x_i}) \right)$, respectively, were determined. Cross-correlating the fluctuations of these two signals reveals significant levels of correlation and an asymmetric pattern that persists at several cross-stream locations for both flows. Furthermore, correlating both the dissipation and production rates with a vortex identifier, $\omega_{x-y} = [(\omega_x)^2 + (\omega_y)^2]^{\frac {1}{2}}$, also reveals consistent cross-stream patterns. The magnitude of these correlations and their persistent shapes across the flows suggest that regions of concentrated rates of dissipation are primarily located in the cores of quasi- streamwise vortices for both these types of turbulent shear flow, whereas regions of rates of production are more concentrated on the peripheries of the vortices . [Preview Abstract] |
Tuesday, November 20, 2007 9:18AM - 9:31AM |
KQ.00007: The Kelvin-Helmholtz Instability in the Atmosphere: Comparisons of High Resolution Numerical Simulations, Cloud observations, and Aircraft Measurements. Joseph Werne, Donald Wroblewski, Bj{\O}rn Anders Pettersson-Reif Results are reported for high-resolution direct numerical simulations (DNS) of the Kelvin-Helmholtz instability (KHI) and ensuing turbulence for four different values of the Richardson number (Ri=0.05, 0.10, 0.15, and 0.20) in relatively large domains: (4$\lambda $, 2$\lambda $, 2$\lambda )$ in the (streamwise, spanwise, vertical) directions. The resulting flow morphology and evolution depend strongly on Ri, and this can be used to determine the relevant values of Ri for the observed atmospheric motions using cloud observations and aircraft data. A conundrum results, with divergent values for Ri suggested by the different data sources. We will discuss resolution of this apparent paradox and describe our efforts to evaluate a census of atmospheric Ri values in the regions of the upper troposphere and lower stratosphere for which we have data. Other universal aspects of KHI-induced turbulence (independent of Ri) have also been discovered from the DNS results, and these will also be discussed. [Preview Abstract] |
Tuesday, November 20, 2007 9:31AM - 9:44AM |
KQ.00008: Experimental Study of Airborne Contaminant Migration in an Aircraft Cabin Model Stephane Poussou, Paul Sojka, Michael Plesniak The cabin air ventilation system in wide body jetliners is designed to provide a comfortable and controlled environment for passengers. Inside the cabin, the air flows continuously from overhead vents into sidewall exhausts, forming a circular pattern designed to minimize cross flow between adjacent seat rows. However, spreading of gaseous or particulate contaminants is possible when flight attendants or passengers walk along an aisle, perturbing the ventilation flow. Such unsteady flow perturbations have been found to alter the cabin air distribution and quality. A better fundamental understanding of the turbulent transport phenomena is needed to improve air quality monitoring and control systems and to validate numerical simulations. The velocity field in a 15:1 model of a simplified aircraft cabin is probed to investigate the wake of a rectangular body moving through a steady two-dimensional flow at a Reynolds number (based on body height) of the order of 50,000. Planar Laser Induced Fluorescence is used to visualize wake structure and scalar contaminant transport. The interaction between the wake and the ventilation flow is measured with PIV. The data are compared to numerical studies of cabin airflows in the literature. [Preview Abstract] |
Tuesday, November 20, 2007 9:44AM - 9:57AM |
KQ.00009: The influence of SPIV calibration misalignment on the modal decomposition of axisymmetric jet turbulence Maja W\"{a}nstr\"{o}m, William K. George, Knud Erik Meyer As shown in W\"{a}nstr\"{o}m et al\footnote{M. W\"{a}nstr\"{o}m, W.K. George, K E. Meyer and C. Westergaard ``Identifying sources of stereoscopic PIV measurement errors on turbulent round jets'' \textit{2007 FEDSM2007-3725} Proc. FEDS 2007 San Diego, USA.\par }$, $the single point second moment statistics of Cartesian coordinate system velocity components are subject to artificial surplus when the camera-to-camera disparity is large relative to the Taylor microscale of the turbulence. The size and magnitude of the disparity field depends on the misalignment between the calibration plane and the measurement plane. The geometry of the SPIV camera setup distributes the noise unequally over the Cartesian components, particularly over the in-plane ones. As a consequence, the jet flow inherent axisymmetry in a polar coordinate system is distorted and the azimuthal invariance lost, so that the azimuthal Fourier series expansion is questionable. This work aims to investigate the sensitivity of a classical Fourier series expansion/POD decomposition of cross-plane SPIV data to the errors described above. Data sets with varying degrees of calibration misalignment at different downstream positions are analyzed and compared to the result obtained with an attempted correction method. [Preview Abstract] |
Tuesday, November 20, 2007 9:57AM - 10:10AM |
KQ.00010: Numerical Simulations of Beam Propagation through a Free Shear Layer Jurgen Seidel, Stefan Siegel, Tom McLaughlin On airborne platforms, the propagation of light or laser beams is severely hampered by the flow field around the turret typically mounted on the air vehicle. Flow separation from the turret results in a free shear layer, which, due to its natural Kelvin-Helmholtz instability, develops large, coherent structures. These structures and the associated density variations result in large optical distortions because of the dependence of the index of refraction on the density. While current experimental techniques can measure the integrated aberrations in terms of the optical path length, no direct measurements of the density field are possible. Therefore, correlating the observed aberrations with the flow structures is exceedingly difficult. Using high resolution computational fluid dynamics, the structure of the flow as well as the optical aberrations are studied in detail, which provides insight into the correlation between flow features and these optical aberrations. The notion that the vortex cores are responsible for the largest aberrations is critically examined. [Preview Abstract] |
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