Bulletin of the American Physical Society
2006 59th Annual Meeting of the APS Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2006; Tampa Bay, Florida
Session EL: Turbulent Shear Layers II |
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Chair: John Foss, Michigan State University Room: Tampa Marriott Waterside Hotel and Marina Meeting Room 8 |
Sunday, November 19, 2006 4:15PM - 4:28PM |
EL.00001: Measurement of Pressure Fluctuation and Pressure-Velocity Correlations in a Cavity Shear Layer by Integrating the Material Acceleration 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 (Liu and Katz, Exp Fluids 41:227-240) has been applied to measure the pressure related turbulence statistics of a 2D cavity flow. This technique utilizes four-exposure PIV to measure the distribution of material acceleration, and integrating it by means of omni-directional virtual boundary integration algorithm to obtain the pressure distribution. Statistics characterizing the mean and turbulent flow properties within the shear layer and the cavity includes rms values of velocity and pressure fluctuations as well as triple velocity correlation terms. In most of the shear layer the fluctuating pressure is negatively correlated with the streamwise velocity fluctuation. However, as the shear layer impinging on the trailing wall of the cavity, the pressure-streamwise velocity correlation reverses its sign and becomes positive. Pressure-velocity correlations are significantly higher than the triple velocity correlation terms. PDF of pressure fluctuations show that the highest fluctuation occurs near the cavity trailing edge, and the pressure peaks agree with measured cavitation inception indices. [Preview Abstract] |
Sunday, November 19, 2006 4:28PM - 4:41PM |
EL.00002: Simultaneous Cinematographic PIV and PLIF measurements at kilohertz frame rate for turbulent cavity flows S. Bian, S.L. Ceccio, J.F. Driscoll A temporally-resolved flow visualization method has been developed for simultaneous measurement of velocity and concentration quantities in time-varying flows through the combination of cinematographic Particle Imaging Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF). PIV images are acquired using two high-speed CMOS cameras that were optically combined and registered by using a non-polarizing beam splitter and a calibration target with known dimensions. The alignment uncertainty contributes up to 5{\%} to the overall velocity measurement uncertainty. A third camera was added to simultaneously record PLIF images. The illumination of both particles and fluorescent dye is provided by two high repetition rate frequency-doubled Nd:YAG lasers. A long-pass color filter in front of PLIF camera eliminates the laser light reflected by PIV particles. A sequence length of 1083 image pairs with 1600 x 800 pixels was obtained at 1500 frames per second while simultaneous PLIF images with a full resolution of 800 x 600 pixels are taken through synchronizing signal generator. Both mean and instantaneous velocity and concentration fields are presented along with dynamic pressure data to illustrate the effect of the large-scale structures on the self-sustained oscillation inside the cavity. Correlations between the fluctuating velocities and concentration in the impinging shear layer reveal the dynamics of the transient mixing process. [Preview Abstract] |
Sunday, November 19, 2006 4:41PM - 4:54PM |
EL.00003: Measurements of energy spectra in two-dimensional turbulence with sheared mean flow. P.W. Fontana, M. Kearney-Fischer, S. Rogers, S. Windell Measurements of forced turbulence in the presence of mean flow shear in quasi-two-dimensional flows in a circular Couette cell are underway. Initial observations indicate suppression of the turbulence by the shear, as suggested by observations of transport barriers in geostrophic flows and laboratory fusion plasmas. The apparatus generates flows in a liquid film of dilute soap solution suspended freely in a horizontal annular channel. The channel is 7 cm wide with an average radius of 46.5 cm. Turbulence is forced electromagnetically, while mean flow shear is generated independently by rotating the outer boundary. The mean sheared flow profiles provide a new method of estimating the coefficient of drag between the films and the air; the result, $\sim 9 s^{-1}$, is compared with estimates from other soap film experiments using different techniques. Two-dimensional turbulence spectra are measured using particle imaging velocimetry, and data showing the effect of shear on the spectrum are presented. [Preview Abstract] |
Sunday, November 19, 2006 4:54PM - 5:07PM |
EL.00004: Vorticity Based (External) Intermittency measurements in a High R$_{\theta}$ Single-Stream Shear Layer John F. Foss, Aren Hellum The MSU four-sensor (transverse) vorticity probe has been used to determine the intermittency function (I(t)=0,1) in a single-stream shear layer. The measurements were taken in the self-preserving region (x/$\theta $(0)=484) with R[$\theta $(x)=9$\times $10$^{4}$]. The I(t) signal permits conditionally sampled measures of the intermittent turbulence field. The mean intermittency $<$I$>$ distribution is compared with that previously reported (1) and (2). It is also compared with the $<$I$>$ distribution created from the use of an advanced surrogate method (3). Distinctive conditionally sampled results include: i) a constant [mean(u$'$v$'$)]/[$\sigma $(u)$\sigma $(v)] distribution for the I=1 condition, ii) clear evidence of fluid from both high- and low-speed irrotational streams to the peak $<$I$>$ location ($\bar{\mbox{u}}$/U$_{\circ}$=0.5), and iii) robust dissipation even as $<$I$> \to $ for large values of [(y-y/2)/$\theta $(x)]. 1. Wygnanski, I. and Fiedler, H. E., (1970). ``The two-dimensional mixing region''. \textit{JFM}, Vol. 41, pp. 327-361. 2. R.C. Haw, J.K. Foss and J.F. Foss, ``Vorticity Based Intermittency Measurements in a Single Stream Shear Layer'' \textit{proc. Second European Turb. Conf. Advance in Turbulence 2,} Ed. H.H. Fernholz and H.E. Fiedler Spring Verlag, Berlin (1989). 3. Hedley, T.B., and Keffer, J.F., (1974). ``Turbulent/non-turbulent decisions in an intermittent flow''. \textit{JFM, }Vol. 64, pp. 625-644 [Preview Abstract] |
Sunday, November 19, 2006 5:07PM - 5:20PM |
EL.00005: Mixing and Entrainment Characteristics of Countercurrent Reacting Shear Layers Phani Rajavolu, David Forliti This investigation is concerned with entrainment issues associated with countercurrent shear layers in the presence of heat release. Heat release has a detrimental effect on the shear layer growth which in turn affects the mixing and entrainment into a shear layer. Previous studies have shown that non-reacting countercurrent shear layers have higher normalized turbulence levels, shear layer growth rates and enhanced entrainment characteristics compared to co-flowing shear layers. The objective of the current project is to explore the effect of heat release on the enhanced entrainment characteristics of the countercurrent shear layer. The mixing characteristics of countercurrent shear layers with heat release is experimentally studied. A primary stream consisting of a rich mixture of methane and air interacts with a counterflowing air stream such that a flame stabilizes within the shear layer upon ignition. Particle image velocimetry is employed as the diagnostic to obtain flow field measurements. The enhanced flame stabilization and entrainment obtained using countercurrent shear may facilitate the design of compact combustion systems with increased efficiency and reduced environmental impact. [Preview Abstract] |
Sunday, November 19, 2006 5:20PM - 5:33PM |
EL.00006: POD Analysis of Subsonic Cavity Flow Fields Lawrence Ukeiley, Nathan Murray, George Shumway Subsonic flow over an open cavity with a length to depth ratio of 6 is studied experimentally using the Proper Orthogonal Decomposition of two-component PIV measurements. Over 1000 velocity field snapshots were acquired at 5 different free stream Mach numbers ranging from 0.2 through nearly 0.75 for the analysis. These Mach numbers had different features of the surface pressure which evolved from completely broad band to being dominated by Rossiter mode 2 and 3. Additionally there were significantly differences in mean flow recirculation patterns which evolved from a single bubble in aft part of the cavity to one approaching the center of the cavity. Although there were drastic differences in those properties the dominant POD modes have many similarities. In the presentation quantitative analysis of the differences in the POD modes will be discussed along with a discussion of the development of a set of modes which are valid over the full range of Mach numbers studied. [Preview Abstract] |
Sunday, November 19, 2006 5:33PM - 5:46PM |
EL.00007: Small scale anisotropy in shear turbulence Carlo Massimo Casciola, Renzo Piva, Paolo Gualtieri, Boris Jacob Recent developments in turbulence are focused on the effect of large scale anisotropy on the small scale statistics of velocity increments. According to Kolmogorov, isotropy is recovered in the large Reynolds number limit as the scale is reduced and, in the so-called inertial range, universal features - namely the scaling exponents of structure functions - emerge clearly. However this picture is violated in a number of cases, typically in the high shear region of wall bounded flows. The common opinion ascribes this effect to the contamination of the inertial range by the larger anisotropic scales, i.e. the residual anisotropy is assumed as a weak perturbation of an otherwise isotropic dynamics. This kind of reasoning fails when the anisotropic effects are strong as in the production range of shear dominated flows. This regime is analyzed here by means of both numerical and experimental data for a homogeneous shear flow. A well defined scaling behavior is found to exist, with exponents which differ substantially from those of classical isotropic turbulence. A systematic use of the SO(3) decomposition on the correlation tensors of velocity fluctuations enables to extract the different anisotropic contributions. They vanish at small scale at a relatively fast rate under weak shear. Under strong shear instead they keep a significant amplitude up to viscous scales, thus leaving a signature on the gradients. [Preview Abstract] |
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