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 H22: Turbulent Mixing II |
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Chair: Mark Kimber, University of Pittsburgh Room: 317 |
Monday, November 25, 2013 10:30AM - 10:43AM |
H22.00001: Measurements of scalar probability density functions and conditional expectations Amir Behnamian, Stavros Tavoularis High resolution, multi-sensor, hot/cold-wire measurements were made in passively heated, uniformly sheared turbulence in a wind-tunnel, with focus on terms in the scalar PDF equation that require closure models. For the homogeneous scalar field that was produced by a uniform mean temperature gradient, results conformed with the literature: the scalar PDF was essentially Gaussian; the conditional expectations of velocities upon the scalar value were approximately linear; and the conditional expectation of the scalar dissipation rate upon the scalar value was mildly anisotropic and had a shape that was similar to those of any of its three parts, which justifies the use of the streamwise part as a surrogate for the total. All these properties behaved very differently in two inhomogeneous scalar fields, namely a thermal mixing layer and the plume of a heated line source: the scalar PDF were distinctly sub-Gaussian; the conditional velocity expectations were non-linear functions of the scalar value; and the conditional scalar dissipation rates were very strongly anisotropic, as well as depending on the scalar value in fashions that differed strongly from those of any of their three parts. [Preview Abstract] |
Monday, November 25, 2013 10:43AM - 10:56AM |
H22.00002: Revisiting measurements of small scale temperature fluctuations Christian Gebauer, Carla Bahri, Gilad Arwatz, Yuyang Fan, Marcus Hultmark It is well known that high frequency temperature measurements are attenuated due to a non-flat frequency response. Based on the temperature correction model proposed by Arwatz et. al (under review), new experimental data is compared with existing measurements. Focus is laid on structure functions, probability density functions, and the behavior of small scale temperature fluctuations. Additionally, a new temperature sensor developed at Princeton University is utilized for further improvement of temperature measurements. The effect of temporal resolution on the temperature spectrum is investigated by comparing uncorrected data to corrected data and data acquired with the new fast response temperature sensor. [Preview Abstract] |
Monday, November 25, 2013 10:56AM - 11:09AM |
H22.00003: Inverse cascades sustained by the transfer rate of angular momentum in a 3D turbulent flow Javier Burguete, Miguel Lopez-Caballero The existence of energy cascades as signatures of conserved magnitudes is one of the universal characteristics of turbulent flows. In this work we present the evidence of an inverse cascade in a fully developed 3D experimental turbulent flow where the conserved magnitude is the angular momentum. We analyze the behavior of a fluid in a closed cavity where two inhomogeneous and strongly turbulent flows collide in a thin region. The experimental volume is a closed cylinder (diameter of 20cm) where two impellers rotate in opposite directions. A key characteristic of this setup the high stability of the propellers (the instantaneous fluctuations are below $0.1\%$). We have performed PIV and LDA measurements of the velocity fields. Typical characteristics of the turbulent flow in this setup are: turbulence intensity $50\%$, the $Re_\lambda = 900$, the Taylor microscale $\lambda_T=1.8$mm and the integral scale $L_I=15$mm. The analysis of the data series reveal that below the injection scales an inverse cascade can be identified (-1/3 in time, -7/3 in space) that can be explained as the transfer of angular momentum between the diferent fluid layers. A. de la Torre, J. Burguete, Phys Rev Lett 99 (2007) 054101. M. Lopez-Caballero, J. Burguete, Phys Rev Lett 110 (2013) 124501. [Preview Abstract] |
Monday, November 25, 2013 11:09AM - 11:22AM |
H22.00004: The horizontal planar structure of kinetic energy in a model vertical-axis wind turbine array Anna Craig, Robert Zeller, Francisco Zarama, Joel Weitzman, John Dabiri, Jeffrey Koseff Recent studies have indicated that arrays of vertical axis wind turbines (VAWTs) could potentially harvest significantly more power per unit land area than arrays composed of conventional horizontal axis wind turbines. However, to design VAWT arrays for optimal power conversion, a more comprehensive understanding of inter-turbine energy transfer is needed. In the presented study, a geometrically scaled array of rotating circular cylinders is used to model a VAWT array. The horizontal inter-cylinder mean fluid velocities and Reynolds stresses are measured on several cross-sections using 2D particle image velocimetry in a flume. Two orientations of the array relative to the incoming flow are tested. The results indicate that cylinder rotation drives asymmetric mean flow patterns within and above the array, resulting in non-uniform distributions of turbulent kinetic energy. The variability is observed to be directly related to the ratio of the cylinder rotation speed to the streamwise water velocity. Emphasis is placed on the implications of the asymmetries for power production. [Preview Abstract] |
Monday, November 25, 2013 11:22AM - 11:35AM |
H22.00005: Anisotropy tensor invariant assessment for counter-rotating wind turbine wakes Nicholas Hamilton, Ra\'{u}l Bayo\'{a}n Cal Model wind turbine arrays were tested in a suite of wind tunnel experiments to determine the wake-to-wake interaction and mixing for different counter-rotation schemes of turbine rotors. All configurations were comprised of a standard Cartesian arrangement ($4\times3$) of turbines. A uniform rotation scheme formed the control against which were tested row-by-row, column-by-column, and checkerboard counter-rotation configurations. Stereo PIV measurements were made immediately upstream and downstream of both entrance and exit row turbines in the center of the wind tunnel. The full Reynolds stress anisotropy tensor, $a_{ij}$, was calculated for all measurement locations showing effects of sense of rotation of rotor blades on turbulent stresses. The invariants of the anisotropy tensor were calculated and compared further demonstrating the effects of rotation and further characterizing the turbulence within a wind turbine canopy layer. Results have implications on return-to-isotropy models used in wind turbine array simulations. [Preview Abstract] |
Monday, November 25, 2013 11:35AM - 11:48AM |
H22.00006: Variation of the slope of the velocity power spectrum and intermittency factor corresponding to 160 \textless\ Re$_{\lambda}$ \textless\ 490 Alejandro Puga It has been observed in many studies, at sufficiently high Reynolds number and for an intermediate range of wavelengths that $E_{11} (\kappa )\sim \kappa^{-n}$. Kolmogorov's 1$^{\mathrm{st}}$ and 2$^{\mathrm{nd}}$ hypotheses state that $n$ should approach 5/3 as the Reynolds number tends to infinity and that the variation of $n$ from 5/3 is due to intermittency in the dissipation. Wind tunnel experiments are conducted where high intensity turbulence is generated by means of an active turbulence grid modeled after Makita's 1991 design (Makita, 1991) as implemented by Mydlarski and Warhaft (M{\&}W, 1998). The goal of this study is to document the variation of $n$ over a range of Re$_{\lambda}$ from 160 to 490. The corresponding values of $n$ are 1.46 and 1.55 where $n=5/3-3.23Re_{\lambda }^{-0.56} $. This is in disagreement with Mydlarski and Warhaft who found that $n=5/3-5.25Re_{\lambda }^{-2/3} $. The intermittency factor, $\mu $, is obtained from the slope of the dissipation spectrum where $E_{11}^{\varepsilon } (\kappa )\sim \kappa^{\mu -1}$ and its variation is determined. The intermittency factor is calculated using the spatial derivative of the downstream velocity as determined from the temporal derivative and Taylor's hypothesis. As turbulence intensity increases, it had been hypothesized that $\mu $ would become zero. However, Sreenivasan and Kailasnath (S{\&}K, 1992), in agreement with Praskovsky and Oncley (P{\&}O, 1994), have found that $\mu $ appears to be nearly a constant of $0.25\pm 0.05$. In the current study it is found that the intermittency exponent is nearly a constant, in agreement with Sreenivasan and Kailasnath, but has a value of 0.7. [Preview Abstract] |
Monday, November 25, 2013 11:48AM - 12:01PM |
H22.00007: Turbulent velocity and concentration measurements in a macro-scale multi-inlet vortex nanoprecipitation reactor Zhenping Liu, Rodney Fox, James Hill, Michael Olsen Flash Nanoprecipitation (FNP) is a technique to produce monodisperse functional nanoparticles. Microscale multi-inlet vortex reactors (MIVR) have been effectively applied to FNP due to their ability to provide rapid mixing and flexibility of inlet flow conditions. A scaled-up MIVR could potentially generate large quantities of functional nanoparticles, giving FNP wider applicability in industry. In the presented research, the turbulent velocity field inside a scaled-up, macroscale MIVR is measured by particle image velocimetry (PIV). Within the reactor, velocity is measured using both two-dimensional and stereoscopic PIV at two Reynolds numbers (3500 and 8750) based on the flow at each inlet. Data have been collected at numerous locations in the inlet channels, the reaction chamber, and the reactor outlet. Mean velocity and Reynolds stresses have been obtained based on 5000 instantaneous velocity realizations at each measurement location. The turbulent mixing process has also been investigated with passive scalar planar laser-induced fluorescence and simultaneous PIV/PLIF. Velocity and concentration results are compared to results from previous experiments in a microscale MIVR. Scaled profiles of turbulent quantities are similar to those previously found in the microscale MIVR. [Preview Abstract] |
Monday, November 25, 2013 12:01PM - 12:14PM |
H22.00008: Characterization of the Flow Field Over an Ablative Surface Michael Allard, Christopher White, Yves Dubief Experiments are performed in a small-scale wind tunnel to investigate the complex coupling between an erodible surface and an eroding agent. The flow configuration is a spatially developing heated boundary layer flow over an ablative surface. Several variations of the inlet conditions, both for flow and temperature, are used to study the temporal and spatial development of ablation driven by coherent structures, such as vortices, and the response of turbulence to wall recession and emergence of roughness (ablation patterns). Characterization and comparison of velocity and thermal fields over ablative and non-ablative surfaces are reported in addition to qualitative observations of ablation patterns for vortex driven, laminar, and turbulent flow over an ablative surface. [Preview Abstract] |
Monday, November 25, 2013 12:14PM - 12:27PM |
H22.00009: Turbulent dispersivity under conditions relevant to airborne disease transmission between laboratory animals Siobhan Halloran, William Ristenpart Virologists and other researchers who test pathogens for airborne disease transmissibility often place a test animal downstream from an inoculated animal and later determine whether the test animal became infected. Despite the crucial role of the airflow in pathogen transmission between the animals, to date the infectious disease community has paid little attention to the effect of airspeed or turbulent intensity on the probability of transmission. Here we present measurements of the turbulent dispersivity under conditions relevant to experimental tests of airborne disease transmissibility between laboratory animals. We used time lapse photography to visualize the downstream transport and turbulent dispersion of smoke particulates released from a point source downstream of an axial fan, thus mimicking the release and transport of expiratory aerosols exhaled by an inoculated animal. We show that for fan-generated turbulence the plume width is invariant with the mean airspeed and, close to the point source, increases linearly with downstream position. Importantly, the turbulent dispersivity is insensitive to the presence of meshes placed downstream from the point source, indicating that the fan length scale dictates the turbulent intensity and corresponding dispersivity. [Preview Abstract] |
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