Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session L23: Turbulence: Theory V - Measurements |
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Chair: Greg Voth, Wesleyan University Room: 318 |
Monday, November 25, 2013 3:35PM - 3:48PM |
L23.00001: Extracting Turbulent Spectral Transfer from Under-Resolved Velocity Fields Nicholas Ouellette, Rui Ni, Greg Voth The strong nonlinearities in turbulent flows drive the transfer of energy and other quantities among different scales of motion. In 3D turbulence, this transfer organizes into the classic Richardson-Kolmogorov cascade of energy to small scales; in 2D turbulence, it leads to an inverse cascade of energy to large scales and a forward cascade of enstrophy to small scales. Directly measuring this spectral transfer is difficult, particularly in experiments. Recent developments of filtering techniques allow spectral fluxes to be measured locally, but have been assumed to require finely resolved velocity fields that are typically not available in 3D experiments. Here we show, using experimental data in 2D and DNS data in 3D, that poorly resolved velocity fields can still be used to extract information about spectral transfer processes. Our results also have implications for locality in the cascades. [Preview Abstract] |
Monday, November 25, 2013 3:48PM - 4:01PM |
L23.00002: Decaying turbulence in the presence of a shearless uniform kinetic energy gradient Adrien Thormann, Charles Meneveau The study of decaying isotropic turbulent flow is an important point of reference for turbulence theories and numerical simulations. For the past several decades, most experimental results have focussed on possible power-law decays and found exponents between -1 and -1.4, approximately. Another class of experiments have been shear less mixing layers in which there are two regions with different kinetic energy levels that slowly diffuse into each other downstream. In this study we consider flow without shear-production of turbulence with a cross-stream uniform spatial gradient of kinetic energy k(z) = C z. Such gradient is generated with the use of an active grid and screens mounted upstream of the wind-tunnel's test section iteratively designed to produce a linear gradient of kinetic energy without mean shear. In such a flow, deviations from constant lateral flux of kinetic energy are due only to spatial variations in turbulent diffusivity of k (turbophoresis). Data are acquired using X-wire thermal anemometry at different spanwise and downstream locations. Tests of homogeneity, as well as spectral characteristics of the flow, decay and diffusion rates of the kinetic energy will be presented. [Preview Abstract] |
Monday, November 25, 2013 4:01PM - 4:14PM |
L23.00003: Multi-scale grid generated turbulence in an internal flow application Piyush Ranade, Scott Morris Turbulence generation using multi-scale, or fractal grids, is a method of creating high turbulence intensity flows passively by utilizing the intrinsic scales associated with the grid. This has become the topic of research in many external flow applications. In turbomachinery, the flow at the exit of the combustor and into the first nozzle stage is highly turbulent. In order to create high turbulence intensities in a lab setting passively, multi-scale turbulence generation grids are proposed. The presence of multiple length scales in the grid geometry innately gives rise to turbulent motions of a wide spectrum being shed immediately downstream of the grid, leading to high turbulence intensity flow. The biggest challenge with using such a grid in an internal flow, however, is to achieve spatial uniformity. In this research, three grid geometries commonly found in literature were tested in an experimental set-up consisting of flow between two flat plates. In addition, several other fractal grid geometries were created and tested in an attempt to maximize turbulence intensity while maintaining spatial homogeneity. This research hopes to begin giving insight into the development of turbulence downstream of a multi-scale grid in an internal flow setting. [Preview Abstract] |
Monday, November 25, 2013 4:14PM - 4:27PM |
L23.00004: Alignment of two-point statistics with respect to mean deformation field in anisotropic turbulent flows Khandakar Morshed, Lakshmi Dasi We study the variations in two-point correlation functions and second-order structure functions in the strongly anisotropic turbulent flow past a backward facing step. Time-resolved particle image velocimetry measurements were performed in a stationary turbulent flow past a backward facing step at Reynolds numbers 13,600, 9,000, and 5,500 based on the maximum velocity and step size. Measurements revealed a strongly anisotropic large-scale flow with an intense turbulent free-shear layer downstream of the step. Comparison among local two-point correlation functions and second-order structure functions at varying locations within the measurement domain reveals a mechanistic relationship between the magnitude of mean flow deformation field and the spatial organization of the two-point statistics in 360 degrees. It is shown that the local spatial variation in rms velocity significantly induces local anisotropy at arbitrarily small length scales. [Preview Abstract] |
Monday, November 25, 2013 4:27PM - 4:40PM |
L23.00005: Reconstruction of Coherent Structures from Time Shifted Data Andrew Newman, Luciano Castillo The possibility of extracting coherent structure information from experimental data which has a time shift is considered. The presented techniques are especially applicable to particle image velocimetry (PIV) data collected from large experiments where all PIV planes cannot be collected simultaneously. The method relies on the Proper Orthogonal Decomposition and considers ways to construct POD modes for two adjacent data domains (but with a time shift between them) out of POD modes computed from the individual domains where all data is collected simultaneously. It is first shown that such composite modes exist. Further, it is shown through example how this procedure is done. Lastly, it is shown that the technique is applicable to experimental data though the problem becomes increasing complex. [Preview Abstract] |
Monday, November 25, 2013 4:40PM - 4:53PM |
L23.00006: Scale-by-scale energy transfer in the production region of a fractal grid Rafael Gomes Fernandes, Bharathram Ganapathisubramani, Christos Vassilicos An experimental study of turbulence generated by low-blockage space-filling fractal square grids was performed using time-resolved Particle Image Velocimetry in a water tunnel. Scale-by-scale energy transfer is computed using the transport equation of the second order structure function for inhomogeneous flows. The balance of each of the equation terms is presented. In some particular locations in the production region, the map of the radial divergence of the energy flux shows an upward energy transfer in the direction of the mean flow and downwards in the perpendicular direction. In these locations, the energy spectra already exhibits a well-defined -5/3 power law over more than one decade; and the third order structure function of the velocity component parallel to the mean flow, evaluated in that direction at the centerline, has a positive sign throughout the range of scales dominated by the -5/3 scaling. Any velocity derivative checks of small-scale isotropy available to us indicate that the small-scales are indeed isotropic in these locations. [Preview Abstract] |
Monday, November 25, 2013 4:53PM - 5:06PM |
L23.00007: Fluctuations in the energy input determine Kolmogorov constants in turbulence Gregory Bewley, Florent Lachauss\'ee, Johannes Kassel, Greg Voth, Eberhard Bodenschatz Attention to turbulence is often focused, for good reason, on flows that either maintain a steady state or decay freely. But these conditions are not typical in natural or industrial flows. We ask what effect deviations from these conditions have on the turbulence itself. To answer the question, we employ a new active grid with many independently controllable degrees of freedom to generate turbulence in a wind tunnel. We find the following: The anisotropy in the flow can be set to various states, including an isotropic one, by adjusting the correlations between motions on the grid. Some part of the fluctuations in the flow can be attributed to the instantaneous configuration of the grid, in the sense that it is reproduced when the grid returns to the same configuration. The value of the Kolmogorov constants for the structure functions of different order can be adjusted by changing over time the degree to which the active grid agitates the flow. We interpret these variations in agitation as variations of the energy input rate. We then find that the Kolmogorov constants, in particular those of order higher than two, can be made to have universal values when the variation of the energy input rate is accounted for by a model based on the refined similarity theory. [Preview Abstract] |
Monday, November 25, 2013 5:06PM - 5:19PM |
L23.00008: Inertial range ESS scaling deteriorates with increasing Reynolds number Eberhard Bodenschatz, Michael Sinhuber, Gregory Bewley, Margit Vallikivi, Marcus Hultmark, Alexander Smits We examined the scaling of velocity structure functions in turbulence generated by a classical biplanar grid of crossed bars in the Variable Density Turbulence Tunnel in G\"{o}ttingen. The flow had neither a mean shear nor strong anisotropy. Despite this, the structure functions did not exhibit power-law scaling unless Extended Self-Similarity (ESS) was employed. The ESS exponents were remarkably stable at Taylor Reynolds numbers between 100 and 1600. That is, at higher Reynolds numbers than in any other comparable flow. However, the extent to which ESS applied at small scales deteriorated as the Reynolds number increased. The experiments were performed in pressurized sulfur hexafluoride gas at pressures between 1 and 15 bar. The data were acquired with both classical hot wires, and with the NSTAP anemometers developed at Princeton. [Preview Abstract] |
Monday, November 25, 2013 5:19PM - 5:32PM |
L23.00009: Detrended analysis of Reynolds stress in a decaying turbulent flow in a wind tunnel with active grids Zhiming Lu, Imtiaz Ahmad, Yongxiang Huang Multi-scale properties of Reynolds stress in decaying turbulence at a wind tunnel with high Reynolds number are investigated. Two filtering technique i.e., zeroth-order and first-order detrending method are applied to the two velocity components, where the local mean value (resp. local linear trend) is removed in the former (latter) technique. Some basic statistics for thirty measurements show that the variation is very large at first two locations and relatively small at last two locations. Moderately good power law is found for the mean value of local Reynolds stress at last three measurement locations with scaling exponents approximately being 1.0 and a dual power law exists for the mean value of standard deviation of local Reynolds stress at all four measurement locations with scaling exponents being 0.53 and 0.58 for zeroth- and first-order filtering respectively. [Preview Abstract] |
Monday, November 25, 2013 5:32PM - 5:45PM |
L23.00010: Anomalous scaling of passive scalar fluctuations in a spatially developing turbulent mixing layer Antonio Attili, Fabrizio Bisetti A detailed statistical analysis of fluctuations of a passive scalar in the fully developed region of a spatially developing turbulent mixing layer from a direct numerical simulation is presented. Passive scalar spectra show inertial ranges characterized by scaling exponents $-4/3$ and $-3/2$ in the streamwise and spanwise directions, in agreement with recent theoretical analysis of passive scalar scaling in shear flows [Celani {\it et al.} J. Fluid Mech. 523, 99 (2005)]. Scaling exponents of high-order structure functions in the streamwise direction show saturation of intermittency with an asymptotic exponent $\zeta_{\infty}=0.4$ at large orders. Saturation of intermittency is confirmed by the self-similarity of the tails of the probability density functions of the scalar increments at different scales $r$ with the scaling factor $r^{-\zeta_{\infty}}$ and by the analysis of the cumulative probability of large fluctuations. Conversely, intermittency saturation is not observed for the spanwise increments, and the relative scaling exponents agree with recent results for homogeneous isotropic turbulence with mean scalar gradient. Probability density functions of the scalar increments in the three directions are compared to assess anisotropy. [Preview Abstract] |
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