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 H20: Turbulent Taylor-Couette and Mixing |
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Chair: Bruno Eckhardt, Philipps-Universitaet Marburg Room: 208 |
Monday, November 23, 2015 10:35AM - 10:48AM |
H20.00001: Momentum transport in Taylor-Couette flow with vanishing curvature Bruno Eckhardt, Hannes Brauckmann, Matthew Salewski We study the influence of system rotation on torque and on mean angular momentum profiles in turbulent Taylor-Couette flow for large cylinder radii in direct numerical simulations. In this limit, curvature effects that can cause a stabilization of the outer flow region become negligible. We find that the torque as a function of the system rotation shows two maxima at a shear Reynolds number of $2 \times 10^4$. The broad torque maximum for a moderate system rotation is related to strong turbulent Taylor vortices. A model based on marginal stability of boundary layers reproduces this torque maximum. The comparison between our simulations and the model suggests that the second torque maximum at weak system rotation is caused by the transition to turbulent boundary layers. [Preview Abstract] |
Monday, November 23, 2015 10:48AM - 11:01AM |
H20.00002: Exploring the phase space of multiple states in highly turbulent Taylor-Couette flow Roeland van der Veen, Sander Huisman, On Yu Dung, Ho Lun Tang, Chao Sun, Detlef Lohse It was recently found that multiple turbulent states exist for large Reynolds number $(\mathrm{Re}=10^6)$ Taylor-Couette flow in the regime of ultimate turbulence. Here we investigate how the transitions between the multiple states depend on the Reynolds number in the range of $\mathrm{Re}=10^5$ to $2\cdot10^6$, by measuring global torque and local velocity while probing the phase space spanned by the rotation rates of the inner and outer cylinder. This sheds light on the question whether multiple states persist for Reynolds numbers beyond those currently reached. By mapping the flow structures for various rotation ratios in two Taylor-Couette setups with equal radius ratio but different aspect ratio, we furthermore investigate the influence of aspect ratio on the characteristics of the multiple states. [Preview Abstract] |
Monday, November 23, 2015 11:01AM - 11:14AM |
H20.00003: Turbulence-driven mean flow generation within laboratory Taylor-Couette flow M.J. Burin, H. Ji, G. Tynan, E. Edlund, E. Gilson, K. Caspary, R. Ezeta Aparicio, P. Dang, M. McNulty We report on a new experimental effort to study mean (or zonal) flow generation within a turbulent laboratory fluid with a wide-gap Taylor-Couette apparatus. Mean flows from externally-forced turbulence are observed, both with and without a linearly sloped end-cap, which when present enforces a radial variation of potential vorticity. We characterize the dependence of the mean flow on various experimental parameters, such as forcing strength and azimuthal mode number, and offer accounts of their dynamical origins. [Preview Abstract] |
Monday, November 23, 2015 11:14AM - 11:27AM |
H20.00004: Enhanced transport by grooved walls in turbulent Taylor-Couette flow Xiaojue Zhu, Rodolfo Ostilla-Monico, Roberto Verzicco, Detlef Lohse We present direct numerical simulations of Taylor-Couette flow with grooved walls at a fixed radius ratio $\eta=r_i/r_o=0.714$ with inner cylinder Reynolds number up to $Re_i=3.76\times10^4$, corresponding to Taylor number up to $Ta=2.15\times10^9$. The grooves are axisymmetric V-shaped obstacles attached to the wall with a tip angle of $90^\circ$. Results are compared with the smooth wall case in order to investigate the effects of grooves on Taylor-Couette flow. We focus on the effective scaling laws for the torque, flow structures, and boundary layers. It is found that, when the groove height is smaller than the boundary layer thickness, the torque is the same as that of the smooth wall cases. With increasing $Ta$, the boundary layer thickness becomes smaller than the groove height. Plumes are ejected from tips of the grooves and a secondary circulation between the latter is formed. This is associated to a sharp increase of the torque and thus the effective scaling law for the torque vs. $Ta$ becomes much steeper. Further increasing $Ta$ does not result in an additional slope increase. Instead, the effective scaling law saturates to the ``ultimate'' regime effective exponents seen for smooth walls. [Preview Abstract] |
Monday, November 23, 2015 11:27AM - 11:40AM |
H20.00005: The near-wall region of highly turbulent Taylor-Couette flow Rodolfo Ostilla Monico, Roberto Verzicco, Detlef Lohse Direct numerical simulations of the Taylor-Couette (TC) problem, the flow between two coaxial and independently rotating cylinders, have been performed. The study focuses on TC flow with mild curvature (small gap) with a radius ratio of $\eta=r_i/r_o=0.909$, an aspect ratio of $\Gamma=L/d=2\pi/3$, and a stationary outer cylinder. Three inner cylinder Reynolds of $1\cdot10^5$, $2\cdot10^5$ and $3\cdot 10^5$ were simulated, corresponding to frictional Reynolds numbers between $Re_\tau\approx 1400$ and $Re_\tau \approx 4000$. An additional case with a large gap, $\eta=0.5$ and driving of $Re=2\cdot10^5$ was also performed. Small-gap TC was found to be dominated by spatially-fixed large-scale structures, known as Taylor rolls (TRs). TRs are attached to the boundary layer, and are active, i.e. they transport angular velocity through Reynolds stresses. For small-gap TC, evidence for the existence of logarithmic velocity fluctuations, and of an overlap layer, in which the velocity fluctuations collapse in outer units, was found. Profiles consistent with a logarithmic dependence were also found for the angular velocity in large-gap TC, albeit in a very reduced range of scales. [Preview Abstract] |
Monday, November 23, 2015 11:40AM - 11:53AM |
H20.00006: Angular statistics of fluid particle trajectories in confined two-dimensional turbulence Benjamin Kadoch, Wouter Bos, Kai Schneider The directional change of fluid particles can be characterized by the angle between subsequent particle displacement increments evaluated as a function of the time lag [1]. At small values of the time-increment the so-defined angle is proportional to the curvature of the trajectory. At large values this coarse-grained curvature should be affected by the presence of solid no-slip walls around the flow domain. In [2] we applied these statistics to three-dimensional isotropic turbulence, here we compare homogeneous and confined two-dimensional turbulent flows. We show that at long times the probability density function of the angles carries the signature of the confining domain if finite size effects are present. At short times, the PDF of the cosine of the angle is given by a power law with a well defined exponent, reminiscent of the close to Gaussian character of the velocity field. \\[4pt] [1] S. Burov, S.A. Tabei, T. Huynh, M.P. Murrell, L.H. Philipson, S.A. Rice and A.R. Dinner. Distribution of directional change as a signature of complex dynamics. Proc. Natl. Acad. Sci., 110(49), 19689, 2013. \\[0pt] [2] W.J.T. Bos, B. Kadoch and K. Schneider. Angular Statistics of Lagrangian Trajectories in Turbulence. Physical Review Letters 114 (21), 214502, 2015. [Preview Abstract] |
Monday, November 23, 2015 11:53AM - 12:06PM |
H20.00007: Impact of Scalar Forcing Simulation Techniques on High Schmidt Number Turbulent Mixing K. Jeff Rah, Guillaume Blanquart Numerous Direct Numerical Simulations of turbulent scalar mixing have been performed with a forcing technique to prevent the turbulent mixing from decaying. In this work, two scalar forcing techniques, namely the mean gradient forcing and the linear scalar forcing, are compared to assess the validity of Batchelor's theory. For high Schmidt number scalars, it predicts a $\kappa^{-1}$ scaling of energy spectrum in the viscous-convective region. When using the mean gradient forcing technique, the energy spectrum agrees well with Batchelor's theory. When using the linear scalar forcing, on the other hand, the energy spectrum does not follow the $\kappa^{-1}$ scaling. This difference can be explained by considering Yaglom's equation for the scalar structure functions and are due to the form of the forcing source term. These results give a hint to the disagreement between theoretical predictions and experimental data in turbulent mixing literature. [Preview Abstract] |
Monday, November 23, 2015 12:06PM - 12:19PM |
H20.00008: Turbulent velocity and concentration measurements in a multi-inlet vortex nanoprecipitation reactor Michael G. Olsen, Zhenping Liu, Emmanuel Hitimana, James C. Hill, Rodney O. Fox Turbulent flow characteristics in a multi-inlet vortex reactor (MIVR) are of interest due to this reactor's importance in nanoprecipitation applications. In the presented work, velocity and passive scalar concentration fields in a macroscale MIVR have been investigated by using stereoscopic particle image velocimetry (SPIV) and planar laser induced fluorescence (PLIF). The measurements are focused near the reactor center where the turbulent mixing occurs. The investigated Reynolds numbers based on the bulk velocity and diameter at the reactor outlet range from 16800 to 42000, resulting in a complex turbulent swirling flow within the reactor. The mean velocity field can be divided into free vortex region and forced vortex region from the wall to the reactor center. Back flow appears due to the low pressure in the forced vortex region. Most of turbulent fluctuation and mixing also occur in the forced vortex region while two point spatial correlations show turbulent eddies undergo shear stretching in the free vortex region. The flow is found to be unsteady with a wandering vortex center. As expected, a mixing performance is found to improve with increasing Reynolds number. [Preview Abstract] |
Monday, November 23, 2015 12:19PM - 12:32PM |
H20.00009: Statistics of High Atwood Number Turbulent Mixing Layers Jon Baltzer, Daniel Livescu The statistical properties of incompressible shear-driven planar mixing layers between two miscible streams of fluids with different densities are investigated by means of Direct Numerical Simulations. The simulations begin from a thin interface perturbed by a thin broadband random disturbance, and the mixing layers are allowed to develop to self-similar states. The temporal simulations are performed in unprecedented domain sizes, with grid sizes up to 6144 x 2048 x 1536, which allows turbulent structures to grow and merge naturally. This allows the flow to reach states far-removed from the initial disturbances, thereby enabling high-quality statistics to be obtained for higher moments, pdfs, and other quantities critical to developing closure models. A wide range of Atwood numbers are explored, ranging from nearly constant density to At=0.87. The consequences of increasing the density contrast are investigated for global quantities, such as growth rates, and asymmetries that form in statistical profiles. Additional simulations in smaller domains are performed to study the effects of domain size. [Preview Abstract] |
Monday, November 23, 2015 12:32PM - 12:45PM |
H20.00010: Effects of Mass and Volume Fraction Skewness in Variable Density Mixing Processes Adam J. Wachtor, Jozsef Bakosi, Raymond Ristorcelli Among the parameters characterizing mixing by variable density turbulence of fluids involving density variations of a factor of 5 to 10 are the Atwood, Froude, Schmidt, and Reynolds numbers. There is evidence that the amount of each fluid present when the two pure fluids mix, as described by the probability density function of the mass or molar (volume) fraction, also strongly affects the mixing process. To investigate this phenomena, implicit large-eddy simulations (ILES) are performed for binary fluid mixtures in statistically homogenous environments under constant acceleration. These coarse grained simulations are used as data for theory validation and mix model development. ILES has been demonstrated to accurately capture the mixing behavior of a passive scalar field through stirring and advection by a turbulent velocity field. The present work advances that research and studies the extent to which an under-resolved active scalar drives the subsequent fluid motion and determines the nature of the mixing process. Effects of initial distributions of the mass and molar (volume) fraction probability density function on the resulting variable density turbulence and mixing are investigated and compared to direct numerical simulations from the Johns Hopkins Turbulence Database. [Preview Abstract] |
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