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 GS: Turbulence: Mixing II |
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Chair: Yves Dubief, University of Vermont Room: Salt Palace Convention Center Ballroom EG |
Monday, November 19, 2007 10:30AM - 10:43AM |
GS.00001: Large eddy simulations and experiments on mixing in a confined rectangular turbulent jet Bo Kong, Anup Gokarn, Francine Battaglia, Michael Olsen, Rodney Fox, James Hill Large eddy simulations were performed for a confined rectangular co-flowing liquid jet at Reynolds number 20,000 based on the average velocity and hydraulic diameter of the channel. An incompressible finite-difference formulation of the filtered Navier-Stokes and mass conservation equations on a partially-staggered grid was used. The effects of grid resolution, numerical schemes, and subgrid models on the LES solutions were studied. Validation was performed by comparing LES statistics with those obtained from low- and high-speed particle image velocimetry and laser-induced fluorescence measurements. These statistics include mean, velocity and scalar variances, Reynolds stress, one- and two-point correlation coefficients, skewness, and kurtosis; all compare well with experimental data. The good agreement with two-point spatial correlations suggests that structures in the LES field are similar to those in the actual flow. [Preview Abstract] |
Monday, November 19, 2007 10:43AM - 10:56AM |
GS.00002: Investigation of large fluctuations of scalar dissipation related to coherent vortices and flow topology Bertrand Rollin, Yves Dubief For Schmidt numbers greater than one, turbulent mixing of passive scalars produces large fluctuations of scalar dissipation contained in thin and elongated volumes. Recent research has shown that those sheet-like structures form in biaxial extensional flows. The first objective of this study is to relate this particular topology of turbulent flows to coherent vortices. Next, the dynamics of the regions of strong scalar dissipation is investigated as a function of the surrounding vortices and biaxial extensional flows. The study is based on direct numerical simulation of a turbulent Kolmogorov flow at low Reynolds number, with the adequate resolution for Schmidt number larger than unity. The local topology of the flow is characterized by the invariants and eigenvalues of the velocity gradient tensors. We use algorithms of vortex skeleton identification to examine the spatio- temporal correlation between the topology of the regions of strong scalar gradients and the neighboring vortices. [Preview Abstract] |
Monday, November 19, 2007 10:56AM - 11:09AM |
GS.00003: Turbulence Measurements of a High Reynolds Number Inclined Jet in Crossflow using PIV and FRAP Vipluv Aga, Claudio Feliciani, Ndaona Chokani, Reza Abhari The TKE, turbulence intensity and Reynolds shear stress for a jet inclined at $30^{\circ}$ to the freestream with a blowing ratio of 2 and Reynolds numbers, based on hole diameter, ($Re_{d}$) of $30000$ are measured using 3-D Stereoscopic PIV and a miniature Fast Response Aerodynamic Probe ($FRAP$). The Maximum Entropy Method is used to compose a spectrum for variance calculations from PIV data, thereby ameliorating the low sampling rate and systematic noise. The PIV measurements are compared with those from the ${FRAP}$ and found to be within error estimates. TKE contours indicate that the two main sites of turbulence production are the counter rotating vortex pair and the shearing surface of the jet. It is observed that the turbulence within the vortex pair is higher and dissipates slower than that in the jet shear surface. The eddy diffusivities of momentum in different cardinal directions are also compared and found to have an anistropic distribution. The mechanisms of turbulent mixing in this complex flow and their relevance to turbulence modeling are commented upon. [Preview Abstract] |
Monday, November 19, 2007 11:09AM - 11:22AM |
GS.00004: Experimental validation of a new closure scheme for scalar diffusion. Partha Sarathi, Roi Gurka, Paul Sullivan, Gregory Kopp The study of contaminant diffusion in environmental flows is important to the assessment of the hazards that result from pollutants released into atmosphere. The reduction of scalar concentration is only due to molecular diffusion and this is usually described through a probability density function. Approximations that are made for terms in the differential equations that govern the evolution of the moments of the probability density function of scalar concentration are shown to have solutions that provide qualitative agreement with observed distributions of the first four moments. It is expected that a reasonable approximation of the probability density function is derived through inversion of the first four moments. The new closure scheme for both the convective and diffusive terms has produced some promising qualitative results.~Simultaneous measurements of velocity and scalar concentrations, using particle image velocimetry (PIV) and planer laser-induced fluorescence (PLIF) respectively, on~a plume in a~grid-turbulence~water tunnel experiment are used to quantitatively explore this closure scheme.~The velocity and concentration fields are measured and analyzed in order to characterize the flow statistics such as turbulent fluxes and distributed moments. [Preview Abstract] |
Monday, November 19, 2007 11:22AM - 11:35AM |
GS.00005: Fine-scale modeling of entrainment and mixing of cloudy and clear air Steven Krueger, Helena Schlueter, Pamela Lehr The EMPM (Explicit Mixing Parcel Model) predicts the evolving in-cloud variability due to entrainment and finite-rate turbulent mixing using a 1D representation of a rising cloudy parcel. The 1D formulation allows the model to resolve fine-scale variability down to the smallest turbulent scales (about 1 mm). The EMPM calculates the growth of thousands of individual cloud droplets based on each droplet's local environment. Our analyses of EMPM results address two fundamental difficulties that the large-eddy simulation (LES) approach faces when attempting to represent the effects of entrainment and mixing on droplet microphysics. One is representing the subgrid-scale (SGS) variability of subsaturation and its impact on droplet size distribution (DSD) evolution. Another is accounting for the finite rate of SGS mixing and therefore of droplet evaporation. We have used EMPM results to characterize the evolution of the DSD due to a single isobaric entrainment and mixing event for a range of conditions. We have also used the EMPM to quantify the dependence of the relative humidity time scale on droplet evaporation, turbulent mixing, and droplet sedimentation time scales. [Preview Abstract] |
Monday, November 19, 2007 11:35AM - 11:48AM |
GS.00006: Dispersion of inertial particles in stably stratified turbulence M. van Aartrijk, H.J.H. Clercx The dispersion of heavy particles in statistically stationary stably stratified turbulence is studied by means of direct numerical simulations. The distribution of the particles over the domain clearly shows the effect of preferential concentration. This particle distribution reflects the anisotropy of the flow. Large-scale horizontal structures can be seen, whereas in vertical direction thin, sheared layers are observed. It is found that with increasing stratification the effect of preferential concentration decreases. Also single-particle dispersion displays different behavior in horizontal and vertical directions. In horizontal direction its behavior for inertial particles is very similar to that for fluid particles. An increased long-time behavior (O($t^{2}$)) is found compared to the classical diffusive regime ($\propto t$) in isotropic turbulence. In vertical direction, however, with increasing inertia the long-time dispersion is clearly enhanced compared to that of fluid particles. The typical plateau found for vertical fluid particle dispersion becomes less pronounced and the transition to a final linear diffusion limit sets in at earlier times. [Preview Abstract] |
Monday, November 19, 2007 11:48AM - 12:01PM |
GS.00007: Lagranian measurements of inertial particle trajectories in a turbulent boundary layer Sergiy Gerashchenko, Nicole Sharp, Stephanie Neuscamman, Sathyanarayana Ayyalasomayajula, Zellman Warhaft We describe Lagrangian measurements of water droplets in a boundary layer with high Reynolds number free stream turbulence. The high Reynolds number turbulence is produced using a active grid in a wind tunnel. Water droplets are introduced into the flow using an array of nozzles attached down stream of the active grid. A boundary layer is formed above a glass plate and the particle trajectories are tracked by a high speed camera moving along side of the tunnel [Ayyalasomayajula et al Phys. Rev. Lett.,~97,~2006). We show that the probability density function of the~acceleration of the particles in the boundary layer has stretched exponential tails that have pronounced asymmetry, reflecting the deceleration events as the particles approach the wall.The pdf's are measured as a function of distance from the wall and the free stream turbulence level is is also varied in order to change the turbulence characteristics in the boundary layer. The Stokes number of the particles is varied up to a value of approximately 0.2. [Preview Abstract] |
Monday, November 19, 2007 12:01PM - 12:14PM |
GS.00008: Infotaxis or how to search without gradient Emmanuel Villermaux, Massimo Vergassola, Boris Shraiman Chemotactic bacteria rely on local concentration gradients to guide them towards the source of a nutrient. Such local cues pointing towards the location of the source are not always available at macroscopic scales because mixing in a flowing medium breaks up regions of high concentration into random and disconnected patches. Thus animals sensing odors in air or water detect them only intermittently as patches of odor sweep by, carried by winds and currents. A macroscopic searcher must devise a strategy of movement based upon sporadic cues and partial information. We propose a search algorithm, which we call ``infotaxis'' (Vergassola et al. \textit{Nature} \textbf{445}, 2007), designed to work under such conditions. Any search process can be thought of as acquisition of information on source location and in infotaxis the latter plays a role similar to concentration in chemotaxis. The infotaxis strategy locally maximizes the expected rate of information gain. Its efficiency is demonstrated using a model of odor plume propagation as well as experimental data on real mixing flows. Infotactic trajectories feature zigzagging and casting paths similar to those observed in flights of moths. The proposed search algorithm is also relevant to the design of olfactory robots, but the general idea of infotaxis can be applied more broadly in the context of searching with sparse information. [Preview Abstract] |
Monday, November 19, 2007 12:14PM - 12:27PM |
GS.00009: Transition from Vortex Shedding to Turbulence Stanley Steers, Walter Goldburg Two-Dimensional turbulence can be produced in soap films by inserting a one-dimensional grid (a comb); a single rod produces a von Karman vortex street. But what of the transition between these two flow phenomena? An investigation is made using two interacting vortex streets in a soap film driven by gravity. Since the size of the shed vortices is dependent upon the diameter of the generating object, for two rods a fixed distance apart, increasing their diameter increases the size of the vortices and thereby increases the interaction of the two streets. Two cones 1 cm apart are penetrated to various depths in the film to allow for the selection of different diameters, typically in the 2 to 5 cm range for flow speeds on the order of 1 $m/s$. Using a laser Doppler velocimeter, time series are taken of the vortex street velocity components both parallel and perpendicular to the mean flow at various points on an axis transverse to the mean flow. Power spectra of these velocity records contain notable features. Although the spectra still contain peaks at the shedding frequency of the vortex street, power law scaling is observed over a limited range. [Preview Abstract] |
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