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 M20: Turbulence: Wakes and Flows Behind Grids |
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Chair: Gregory Bewley, Max Planck Institute for Dynamics and Self-Organization, Germany Room: 208 |
Tuesday, November 24, 2015 8:00AM - 8:13AM |
M20.00001: Turbulence decay downstream of an active grid Gregory Bewley, Eberhard Bodenschatz A grid in a wind tunnel stirs up turbulence that has a certain large-scale structure. The moving parts in a so-called ``active grid'' can be programmed to produce different structures. We use a special active grid in which each of 129 paddles on the grid has its own position-controlled servomotor that can move independently of the others. We observe among other things that the anisotropy in the amplitude of the velocity fluctuations and in the correlation lengths can be set and varied with an algorithm that oscillates the paddles in a specified way. The variation in the anisotropies that we observe can be explained by our earlier analysis of anisotropic ``soccer ball'' turbulence (Bewley, Chang and Bodenschatz 2012, Phys. Fluids). We define the influence of this variation in structure on the downstream evolution of the turbulence. [Preview Abstract] |
Tuesday, November 24, 2015 8:13AM - 8:26AM |
M20.00002: Some effects of vortex shedding in grid-generated turbulence Gianfrancesco Melina, Paul J.K. Bruce, John Christos Vassilicos We perform hot-wire measurements in a wind tunnel downstream of different types of turbulence-generating grids: a regular grid (RG60), a fractal square grid (FSG17) and a single square grid (SSG). We characterize the flow highlighting similarities and differences between the grids and between the production and the decay regions of turbulence. We focus on the effects of vortex shedding from the bars of the grids. For this purpose we design a novel 3D configuration formed by the SSG and a set of four splitter plates detached from the grid. We show that, by placing the splitter plates, the peak of turbulence intensity on the centerline is reduced and its location is moved downstream. We compare data from the different turbulence generators and find that a reduction of vortex shedding energy correlates with an increase in the magnitudes of the skewness and flatness of the turbulent velocity fluctuations in the production region. [Preview Abstract] |
Tuesday, November 24, 2015 8:26AM - 8:39AM |
M20.00003: Alignments and small scale statistics in the production region of grid turbulence Immanuvel Paul, George Papadakis, John Christos Vassilicos Direct Numerical Simulation (DNS) of turbulent flow generated by a single square grid is investigated using an unstructured finite volume method. The maximum value of the Taylor length-based Reynolds number throughout the computed flow field is about 40. The main focus of this study is on the production region which lies in the lee of the grid where turbulence builds up. Statistics of vorticity and of eigenvalues ($\lambda_{i}$, where i$=$1,2,3) and eigenvectors (e$_{i}$, where i$=$1,2,3) of the fluctuating strain rate tensor (S$_{ij})$ are analyzed. It is observed that the PDFs of all the eigenvalues in the production region are highly non-gaussian. The PDFs of the compressive ($\lambda_{3})$ and intermediate ($\lambda _{2})$ eigenvalues are strongly skewed to negative and positive values respectively. The energy spectrum of the streamwise fluctuating velocity has a well-defined power law with an exponent around -2 or -5/3 over more than one decade depending on the position in the production region. It is also observed that the most extensive eigenvector (e$_{1})$ and the intermediate eigenvector (e$_{2})$ align significantly with vorticity vector in the production region, which in turn increases average enstrophy production. [Preview Abstract] |
Tuesday, November 24, 2015 8:39AM - 8:52AM |
M20.00004: Decay of grid turbulence in a closed box Stéphane Perrard, William Irvine We investigate the decay of a turbulent flow in the absence of mean flow. By accelerating a square grid in a water tank, we generate an array of wakes that induces a 3 dimensional turbulent flow with a Reynolds number of about Re $ \approx 5\times 10^4$. After the impulse excitation (about 100ms), a decay in time of this turbulent flow is observed. The entire decay process lasts for hours while the dissipative length rises up through scales over time. We follow and characterize both in space and time this turbulent decay process through several decades. [Preview Abstract] |
Tuesday, November 24, 2015 8:52AM - 9:05AM |
M20.00005: The turbulent flow generated by inhomogeneous multiscale grids Shaokai Zheng, Paul J K Bruce, J Michael R Graham, John Christos Vassilicos A group of inhomogeneous multiscale grids have been designed and tested in a low speed wind tunnel in an attempt to generate bespoke turbulent shear flows. Cross-wire anemometry measurements were performed in different planes parallel to the grid and at various streamwise locations to study turbulence development behind each of the different geometry grids. Two spatially separated single hot wires were also used to measure transverse integral length scale at selected locations. Results are compared to previous studies of shearless mixing layer grids and fractal grids, including mean flow profiles and turbulence statistics. [Preview Abstract] |
Tuesday, November 24, 2015 9:05AM - 9:18AM |
M20.00006: Power Law Decay in High Intensity Turbulence Timothy Koster, Alejandro Puga, Baolong Nguyen, John LaRue In the study reported herein, the region where the power decay law is applicable for active grid generated turbulence is found by an iterative approach which determines the largest range where the ratio of the dissipation from the power law and the dissipation from the temporal velocity derivative are unity. The square of the Taylor microscale, as noted by Batchelor (1953), is linearly related to downstream distance relative to the virtual origin and can be used in a straightforward manner to find the virtual origin. The fact that the decay of downstream velocity variance is described by a power law is shown to imply power law behavior for various other parameters such as the dissipation, the integral length scale, the Taylor microscale, the Kolmogorov microscale and the Taylor Reynolds number and that there is an algebraic relationship between the various power law exponents. Results are presented for various mean velocities to show the decay exponent as a function of the Taylor Reynolds number. [Preview Abstract] |
Tuesday, November 24, 2015 9:18AM - 9:31AM |
M20.00007: Dissipative Effects on Inertial-Range Statistics at High Reynolds Numbers Michael Sinhuber, Gregory Bewley, Eberhard Bodenschatz Using the unique capabilities of the Variable Density Turbulence Tunnel at the Max Planck Institute for Dynamics and Self-Organization, we were able to measure extremely long time series of up to $10^{10}$ samples of the turbulent fluctuating velocity in a well-controlled environment at a wide range of high Reynolds numbers up to $R_\lambda = 1600$. These classical grid measurements were conducted using both classical hot-wire probes as well as NSTAP probes developed at Princeton University. With these long datasets, we were able to uncover fine details of the structure functions and their scaling behavior. We find that deviations from ideal scaling is anchored to the small scales and that dissipation influences the inertial-range statistics even up to $r/\eta =1000$. [Preview Abstract] |
Tuesday, November 24, 2015 9:31AM - 9:44AM |
M20.00008: Interaction of two high Reynolds number axisymmetric turbulent wakes M. Obligado, S. Klein, J.C. Vassilicos With the recent discovery of non-equilibrium high Reynolds number scalings in the wake of axisymmetric plates (Nedic et al., PRL, 2013), it has become of importance to develop an experimental technique that permits to easily discriminate between different wake scalings. We propose an experimental setup that tests the presence of non-equilibrium turbulence using the streamwise variation of velocity fluctuations between two bluff bodies facing a flow. We have studied two different sets of plates (one with regular and another with irregular peripheries) with Hot-Wire Anemometry in a wind tunnel. By acquiring streamwise profiles for different plate separations and identifying the wake interaction length for each separation it is possible to estimate the streamwise evolution of the single wake width. From this evolution it is also possible to deduce the turbulence dissipation scalings. This work generalizes previous studies on the interaction of plane wakes (see Gomes-Fernandes et al., JFM, 2012) to include axisymmetric wakes. We find that the wake interaction length proposed in this cited work and a constant anisotropy assumption can be used to collapse the streamwise developments of the first three moments. [Preview Abstract] |
Tuesday, November 24, 2015 9:44AM - 9:57AM |
M20.00009: Scale-by-scale energy fluxes in anisotropic non-homogeneous turbulence behind a square cylinder Felipe Alves Portela, George Papadakis, John Christos Vassilicos The turbulent wake behind a square section cylinder is studied by means of high resolution direct numerical simulations using an in-house finite volume code. The Reynolds number based on the cylinder side is 3900. Single- and two-point statistics are collected in the lee of the cylinder for over 30 shedding periods, allowing for an extensive description of the development of the turbulence. The power spectrum in the frequency domain of velocity fluctuations displays a near -5/3 power law in the near wake, where the turbulence is neither isotropic nor homogeneous. In the same region of the flow, two-point statistics reveal a direct cascade of fluctuating kinetic energy down the scales as a result of the combined effect of linear and non-linear interactions. For scales aligned with the mean flow the non-linear interactions dominate the cascade. Conversely, for scales normal to the mean flow the cascade is dominated by the linear interactions while the non-linear term is mostly responsible for redistributing energy to different orientations. [Preview Abstract] |
Tuesday, November 24, 2015 9:57AM - 10:10AM |
M20.00010: On the Large Scale Dynamics in the Wake of a Fractal Obstacle Jonathan Higham, Wernher Brevis In a water flume three-dimensional Particle Tracking Velocimetry is used to capture the turbulent wake of two full-width and wall-mounted obstacles: The first obstacle is a uniformly spaced array of square cylinders of same length-scale; the second is a three-iteration pre-fractal based on a the deterministic Sierpinski Carpet. Both obstacles emerge from the water surface and had the same porosity. For the description of the instantaneous vortical structures the velocity gradient tensor is analysed. It is found that whilst the largest length scales of the fractal dominated the vorticity field in the wake, the smaller length-scale within the obstacle caused intense vortical structures within the near field of the wake. To further investigate the spatio-temporal behaviour of the wake a simple and integrated use of the Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) is introduced. POD is used to rank the spatial structures relatable to the total variance (i.e. vorticity) while DMD is used to identify their dominant oscillation frequencies and spatial characteristics. From the POD it is clear that the largest length-scale creates spatially dominant structures, whilst the DMD extracts a set of oscillatory frequencies relatable to each fractal length-scale. [Preview Abstract] |
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