Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session M19: Turbulence: Measurements |
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Chair: Blair Perot, University of Massachusetts, Amherst Room: 207 |
Tuesday, November 24, 2015 8:00AM - 8:13AM |
M19.00001: The Decay of Turbulence After it Stops Rotating. J. Blair Perot, Chris Zusi It is well known that the value of the power-law decay rate is reduced when turbulence is rotated. Less well known is how rotating turbulence behaves when system rotation stops. 512$^{3}$ DNS simulations of properly initialized isotropic turbulence at a variety of Reynolds numbers and rotation rates are used to show that immediately after rotation stops decaying turbulence has an exponential and not a classical power-law decay. Exponential decay is equivalent to an infinite power-law decay exponent and is a result of a constant physical turbulent timescale. In contrast, classical power-law decaying turbulence has a turbulent timescale that is proportional to the time itself. The implications for the modeling of the dissipation rate, and the physics of the turbulent decay process, are discussed. [Preview Abstract] |
Tuesday, November 24, 2015 8:13AM - 8:26AM |
M19.00002: On the Relation between Spatio-Temporal Forcing and Structure of Turbulence Douglas Carter, Filippo Coletti The different methods to force turbulence in physical or numerical experiments can have significant effects on the fluid dynamics. Understanding how a given forcing scheme maps on the flow statistics is important both to reproduce desired flow features, and to gain insight in the transfer of energy across the scales. Here we consider the case of the turbulent flow generated by the interaction of pulsating jets. We present a novel installation where pressurized air is issued through 256 independently actuated valves, arranged in symmetric rectangular arrays over two facing planes. The small net mass flux and the randomization of the actuation produces a large region of approximately homogeneous flow at the center of the apparatus, with fluctuations much larger than the mean velocity. The turbulence structure under different firing sequences is statistically analyzed using Particle Image Velocimetry. The forcing parameters include: jet Reynolds number, spacing of the active jets, actuation frequency, and spatio-temporal correlation of the firing patterns. The observables include: integral length scales, turbulent dissipation, energy spectra, and isotropy. We discuss the relation between forcing schemes and flow features, and implications for modeling and flow control. [Preview Abstract] |
Tuesday, November 24, 2015 8:26AM - 8:39AM |
M19.00003: Correlational signatures of time-reversal symmetry breaking in two-dimensional flow Charlie Hogg, Nicholas Ouellette Classical turbulence theories posit that broken spatial symmetries should be (statistically) restored at small scales. But since turbulent flows are inherently dissipative, time reversal symmetry is expected to remain broken throughout the cascade. However, the precise dynamical signature of this broken symmetry is not well understood. Recent work has shed new light on this fundamental question by considering the Lagrangian structure functions of power. Here, we take a somewhat different approach by studying the Lagrangian correlation functions of velocity and acceleration. We measured these correlations using particle tracking velocimetry in a quasi-two-dimensional electromagnetically driven flow that displayed net inverse energy transfer. We show that the correlation functions of the velocity and acceleration magnitudes are not symmetric in time, and that the degree of asymmetry can be related to the flux of energy between scales, suggesting that the asymmetry has a dynamical origin. [Preview Abstract] |
Tuesday, November 24, 2015 8:39AM - 8:52AM |
M19.00004: Determining the direction of a turbulent cascade Walter Goldburg, Rory Cerbus In two-dimensional (2D) turbulence, one expects a cascade of energy to larger spatial scales, while the enstrophy cascade is to smaller ones. Here we present a new tool to study cascades using simple ideas borrowed from information theory. It is entirely unrelated to the Navier-Stoke's equations or any scaling arguments. We use the conditional entropy (conditioned uncertainty) of velocity fluctuations on one scale conditioned on another larger or smaller scale. If the entropy is larger after conditioning on larger scales rather than smaller ones, then the cascade is to smaller scales. By varying the scale of the velocity fluctuations used in the conditioning, we can test both direction and locality. We use these tools on experimental data taken from a flowing soap film, an approximately 2D turbulent flow. The Reynolds number is varied over a wide range to determine the entropy's scaling with Reynolds number [Preview Abstract] |
Tuesday, November 24, 2015 8:52AM - 9:05AM |
M19.00005: On velocity gradient dynamics and fine-scale structure: experiments support DNS and models John Lawson, James Dawson The fine scales of turbulence are embodied by statistics of velocity gradients. In solving exact equations for their evolution, the challenge is to specify how the pressure Hessian acts. This is determined by the footprints that ``structures'' of enstrophy and strain leave in conditional average pressure fields. We use direct and approximate conditional averaging methods to extract this structure from different turbulence datasets: a direct numerical simulation and a unique scanning tomography experiment in a ``French washing machine''. Direct comparisons between simulation and experiment show the structure and resulting dynamics are in excellent, quantitative agreement. This evidence supports existing modelling approaches and provides insights towards their refinement. Moreover, it demonstrates the dynamical significance and the reproducibility of fine-scale structure. [Preview Abstract] |
Tuesday, November 24, 2015 9:05AM - 9:18AM |
M19.00006: Multi-level segment analysis: definition and applications in turbulence Lipo Wang The interaction of different scales is among the most interesting and challenging features in turbulence research. Existing approaches used for scaling analysis such as structure-function and Fourier spectrum method have their respective limitations, for instance scale mixing, i.e. the so-called infrared and ultraviolet effects. For a given function, by specifying different window sizes, the local extremal point set will be different. Such window size dependent feature indicates multi-scale statistics. A new method, multi-level segment analysis (MSA) based on the local extrema statistics, has been developed. The part of the function between two adjacent extremal points is defined as a segment, which is characterized by the functional difference and scale difference. The structure function can be differently derived from these characteristic parameters. Data test results show that MSA can successfully reveal different scaling regimes in turbulence systems such as Lagrangian and two-dimensional turbulence, which have been remaining controversial in turbulence research. In principle MSA can generally be extended for various analyses. [Preview Abstract] |
Tuesday, November 24, 2015 9:18AM - 9:31AM |
M19.00007: Estimation of Turbulent Wall Jet Velocity Fields for Noise Prediction Adam Nickels, Lawrence Ukeiley, Robert Reger, Louis Cattafesta Estimation of the time-dependent turbulent velocity field of a planar wall jet based on discrete surface pressure measurements is performed using stochastic estimation in both the time and frequency domain. Temporally-resolved surface pressure measurements are measured simultaneously with planar Particle Image Velocimetry (PIV) snapshots, obtained at a relatively reduced rate. Proper Orthogonal Decomposition (POD) is then applied to both the surface pressure probes and the PIV snapshots, allowing for the isolation of portions of the wall pressure and velocity field signals that are well correlated. Using the time-varying pressure expansion coefficients as unconditional variables, velocity expansion coefficients are estimated and used to produce reconstructed estimates of the velocity field. Optimization in terms of number of unconditional probes employed, location of probes, and effects of PIV discretization are investigated with regards to the resulting estimates. Coupled with this analysis, Poisson's equation for fluctuating pressure is solved such that the necessary source terms of an acoustic analogy can be calculated for estimates of the far-field acoustics. Specifically in this work, the effects of using estimated velocity fields to solve for the hydrodynamic pressure and acoustic pressure will be studied. [Preview Abstract] |
Tuesday, November 24, 2015 9:31AM - 9:44AM |
M19.00008: An experimental Lagrangian study of inhomgeneous turbulence Nickolas Stelzenmuller, Nicolas Mordant We investigate experimentally the Lagrangian properties of inhomogeneous turbulence in the general scope of dispersion studies in natural and industrial flows. Lagrangian studies of homogeneous turbulence are becoming common, but very little Lagrangian experimental data exists for inhomogeneous turbulence despite the vast range of applications. Particle tracking velocimetry using a very high speed camera in a fully developed turbulent channel flow in water is achieved at $Re_H=33,000$. This technique provides Lagrangian velocity and acceleration statistics fully resolved at the smallest turbulent scales near the wall. These statistics, conditioned by the distance to the wall, allow the the investigation of the inhomogeneity of the statistical properties of this flow. Autocorrelations of velocity and acceleration show increasing Lagrangian turbulent scales as distance from the wall increases, as well as decreasing anisotropy. PDF's and moments of Lagrangian quantities are presented by showing the evolution of structure functions across the boundary layer. These results are compared to direct numerical simulation results from a similar flow, and their implications for stochastic models of inhomogeneous flows are discussed. [Preview Abstract] |
Tuesday, November 24, 2015 9:44AM - 9:57AM |
M19.00009: Skin-Friction Measurements on Mathematically Generated Roughness in a Turbulent Channel Flow Julio Barros, Michael Schultz, Karen Flack Engineering systems are affected by surface roughness, however, predicting frictional drag has proven to be challenging. One open question is how roughness topography, whether it is idealized 2D and 3D or irregular with multi-scale features, impacts the frictional drag. A previous study from Flack and Schultz (2010) presented a new model to estimate frictional drag based on surfaces statistics. The present work takes a systematic approach by generating and manufacturing surfaces roughness where surface statistics, such as \textit{rms}, skewness and power-spectral density can be controlled. Skin-friction measurements are conducted in a high Reynolds number turbulent channel flow facility, where the experiments cover all roughness regimes, from hydraulic-smooth to fully-rough. The surface roughness studied herein is produced using the random Fourier modes method with a varying power-law spectral slope, whereas the \textit{rms} and surface amplitude are kept constant ($k_{rms}$ $\sim$ 45$\mu $m and $k_{t}$ $\sim$ 200$\mu $m) while still possessing a Gaussian probability-density-function. These surfaces are then 3D-printed and replicated using a mold/cast technique to generate the top and bottom walls of the channel flow facility. [Preview Abstract] |
Tuesday, November 24, 2015 9:57AM - 10:10AM |
M19.00010: Inner--outer interactions in a turbulent boundary layer overlying complex roughness Gokul Pathikonda, Kenneth T. Christensen Stereo PIV and hot-wire measurements were performed in a rough-wall turbulent boundary layer to investigate the inner-outer interactions across the roughness sublayer. The sPIV was performed in a spanwise--wall-normal plane and hot-wire measurements were conducted with single- and two-probe methods. The complex roughness with a wide distribution of roughness scales has been shown previously to induce alternating high- and low- momentum pathways (HMPs and LMPs)--- imprints of roughness-induced secondary flows. Differences in the streamwise velocity and turbulent kinetic energy between a HMP-LMP pair are established in the current study. The respective inner-outer interactions are quantified by the amplitude modulation correlation coefficient, the time-delayed velocity correlation coefficient maps and by a 2-point calibration method proposed by Mathis et al. (J. Fluid Mech. 681 (2011): 537-566). It is observed that the strength of such interactions, as measured in this calibration-framework, is generally stronger in the rough-wall than smooth-wall flow, and relatively stronger at an LMP than at a corresponding HMP. [Preview Abstract] |
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