Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session EZ: Couette and Swirling Flows |
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Chair: Richard Lueptow, Northwestern University Room: Hyatt Regency Long Beach Regency F |
Sunday, November 21, 2010 4:10PM - 4:23PM |
EZ.00001: Subcritical Transition and Spiral Turbulence in Taylor-Couette Flow M.J. Burin, C.J. Czarnocki, T. DaPron, K.R. McDonald We present measurements characterizing the transition to turbulence in Taylor-Couette flow for~a fully cyclonic regime, i.e. with only the outer cylinder rotating. Under this arrangement the flow is linearly-stable and the shear-driven transition to turbulence is understood to be~both `catastrophic' and spatiotemporally intermittent. En route to a fully turbulent state, we observe a regime featuring co-extant laminar/turbulent domains known as spiral turbulence. To better understand this regime, and the transition in general, we have obtained~velocimetry data (via LDV) and~angular momentum transport estimates (via torque), in~addition to flow visualization. These observations are discussed with respect to~similar transition phenomena~in planar and counter-rotating Couette flows. By utilizing three different inner cylinder radii within the apparatus, we also demonstrate the sensitivity of the subcritical transition scenario to annular~gap width. [Preview Abstract] |
Sunday, November 21, 2010 4:23PM - 4:36PM |
EZ.00002: Torque scaling in turbulent Taylor-Couette flow with independently rotating cylinders Matthew S. Paoletti, Daniel P. Lathrop We present experimental studies of the turbulent flow of water between independently rotating cylinders. The Taylor-Couette system is capable of both strong turbulence ($Re > 2 \times 10^6$) and rapid rotation. The torque required to drive the inner cylinder and the wall shear stress at the outer boundary are precisely measured as a function of the two angular velocities $\Omega_1$ and $\Omega_2$. We find that the dynamics, which are fully determined by the Reynolds number $Re$ and Rossby number $Ro = \Omega_1 - \Omega_2/\Omega_2$, are different in four different regions of the ($\Omega_1, \Omega_2$) parameter space. Our measurements allow us to estimate the skin friction coefficient $c_f$. We compare our measurements of $c_f$ with those of previous experiments and discuss the potential relevance for angular momentum transport in astrophysical flows. [Preview Abstract] |
Sunday, November 21, 2010 4:36PM - 4:49PM |
EZ.00003: Turbulent scaling in rotating spherical Couette flow Daniel S. Zimmerman, Santiago Andr\'{e}s Triana, Daniel P. Lathrop We study the parameter dependence of torque and other flow
quantities in rapidly rotating spherical Couette flow with radius
ratio $\eta = r_{\mathrm{i}}/r_{\mathrm{o}} = 0.35$ using the
University of Maryland 3m system. We examine the dependence of the
dimensionless torque, $G = T/\rho\nu^2 L_{gap}$, on the Reynolds
number, $Re = \Delta\Omega L_{gap}^2/\nu$, and Rossby number, $Ro =
\Delta\Omega/\Omega_{\mathrm{o}}$, for $-5 |
Sunday, November 21, 2010 4:49PM - 5:02PM |
EZ.00004: Taylor--Couette--Poiseuille flow with a permeable inner cylinder Nils Tilton, Denis Martinand, Eric Serre, Richard Lueptow We consider laminar Taylor--Couette--Poiseuille flow between an outer, fixed, impermeable cylinder and a concentric, inner, rotating, permeable cylinder with radial suction. Due to centrifugal instabilities the steady flow transitions to Taylor vortex flow. This system is used in filtration because the vortices wash contaminants away from the permeable cylinder. The coupling between the axial pressure drop driving the annular Poiseuille flow, and the transmembrane pressure driving the suction induces axial variations of the velocity field of the subcritical flow, which can evolve from suction to injection (cross flow reversal) or consume the whole axial flow (axial flow exhaustion). Moreover, the stability of this flow departs from that of Taylor--Couette flow. We propose an asymptotic solution to the subcritical flow assuming slow axial variations of the velocity and pressure fields. The transmembrane suction and pressure are coupled through Darcy's law. This solution is then used as a base flow to study the appearance of instabilities in the form of global modes. The analytical results for the subcritical and supercritical flows are then compared with dedicated 3-D spectral direct numerical simulations implementing Darcy's law on the inner cylinder. [Preview Abstract] |
Sunday, November 21, 2010 5:02PM - 5:15PM |
EZ.00005: Precessional states in a laboratory model of the Earth's core Santiago Triana, Daniel Zimmerman, Daniel Lathrop A water-filled three-meter diameter spherical shell built as a model of the Earth's core shows evidence of precessionally induced flows. We identified the flow to be primarily the spin-over inertial wave mode, i.e., a uniform vorticity flow whose rotation axis is not aligned with the container's rotation axis. The mode's amplitude dependence on the Poincar\'{e} number is in qualitative agreement with Busse's laminar theory (\textit{JFM} \textbf{33}:739-751, 1968) while its phase differs significantly, perhaps due to topographic effects. At high rotation rates free shear layers concentrating most of the kinetic energy of the mode have been observed. Comparison with previous computational studies and implications for the Earth's core are discussed. [Preview Abstract] |
Sunday, November 21, 2010 5:15PM - 5:28PM |
EZ.00006: Enhanced Angular Momentum Transport in Magnetized Spherical Couette Flow Matthew Adams, Daniel Lathrop We present experimental studies of the turbulent flow of a conducting fluid in a spherical shear flow in the presence of a magnetic field. Our experimental apparatus uses sodium as the working fluid, and both the inner and outer spheres can be rotated independently. An axial magnetic field of varying strength can be applied to the experiment, and magnetic field measurements are used to extract information about the global flow within the device. In addition, we measure the torque required to drive the inner and outer spheres at their respective rotation rates. The geometry of the experiment makes these studies applicable to geophysical and astrophysical bodies. With the inner sphere rotating faster than the outer sphere, we observe enhanced angular momentum transport from the inner to the outer sphere as the applied magnetic field is increased. In a previous experiment of the same geometry, enhanced angular momentum transport was observed with a stationary outer sphere [1]. In this case the source of enhanced transport was identified as the magnetorotational instability. Results for the case of rotating outer sphere also indicate the possible presence of the magnetorotational instability with independently rotating spheres. \\[4pt] [1] Sisan, \textit{et. al.}, \textit{PRL}, 2004. [Preview Abstract] |
Sunday, November 21, 2010 5:28PM - 5:41PM |
EZ.00007: Drag Measurements over Embedded Cavities in a Low Reynolds Number Couette Flow Caleb Gilmer, Amy Lang, Robert Jones Recent research has revealed that thin-walled, embedded cavities in low Reynolds number flow have the potential to reduce the net viscous drag force acting on the surface. This reduction is due to the formation of embedded vortices allowing the outer flow to pass over the surface via a roller bearing effect. It is also hypothesized that the scales found on butterfly wings may act in a similar manner to cause a net increase in flying efficiency. In this experimental study, rectangular embedded cavities were designed as a means of successfully reducing the net drag across surfaces in a low Reynolds number flow. A Couette flow was generated via a rotating conveyor belt immersed in a tank of high viscosity mineral oil above which the plates with embedded cavities were placed. Drag induced on the plate models was measured using a force gauge and compared directly to measurements acquired over a flat plate. Various cavity aspect ratios and gap heights were tested in order to determine the conditions under which the greatest drag reductions occurred. [Preview Abstract] |
Sunday, November 21, 2010 5:41PM - 5:54PM |
EZ.00008: Computational Analysis of Low Reynolds Number Couette Flow Over Embedded Cavities Chase Leibenguth, Amy Lang, Will Schreiber Bio-inspired surface patterning research has shown the potential drag reduction qualities of micro-geometric embedded cavities placed on the surface of an object, analogous to the spaces formed between successive rows of scales on a butterfly wing. Vortices form inside the cavities and contribute to a net partial slip condition that interacts with the boundary layer over the surface. The interaction potentially affects the global flow field over an object to delay separation and reduce drag. In the present study, embedded cavity geometry in a Couette flow was modeled in GAMBIT and analyzed with FLUENT to qualitatively determine the cavity's drag reduction capabilities and the presence of a partial slip condition. The GAMBIT models consisted of a top plate moving transversally over a single cavity with periodic boundary conditions, differing rectangular geometry configurations, and varied gap heights. FLUENT was used to analyze the flow over a range of Reynolds Numbers from 0.01 to 100. Data was obtained to analyze cavity vortex formation, pressure and shear distributions inside the cavity, and the velocity distribution near the cavity. [Preview Abstract] |
Sunday, November 21, 2010 5:54PM - 6:07PM |
EZ.00009: Slow dynamics in a highly turbulent von K\'arm\'an swirling flow Miguel Lopez, Javier Burguete In this work we present an experimental analysis of the dynamics of the coherent structures that appear in a von K\'arm\'an swirling flow, in a fully developped turbulent regime. The objective is to determine the effect of the fluctuations in the dynamics of these vortices. To achieve this goal, we have measured the flow in a water experiment. The fluid has been stirred in a cylindrical cavity up to a Reynolds number of $10^6$. We show that the average velocity field of the turbulent flow bifurcates subcritically breaking some symmetries of the problem and becomes time-dependent because of equatorial vortex moving with a precession movement. This subcriticality produces a bistable regime, with a hysteresis region for an extremely small range of parameters. Three different time-scales are relevant to the dynamics, two of them very slow compared to the impeller frequency. We have studied the different time scales of the system, changing a enclosure volume (neutrally buoyant spheres) assuming that the density of the sphere is homogeneous. Also we change the frequency of the impellers ($10 Hz$ - $50 Hz$) to explore another parameter of the system. We follow this volume in a period of time and we compare the results in different spatial scales. [Preview Abstract] |
Sunday, November 21, 2010 6:07PM - 6:20PM |
EZ.00010: Turbulent counterflow driven by swirl decay Anatoli Borissov, Vladimir Shtern Swirling counterflows occur in vortex combustors, hydrocyclones, and vortex tubes where the Reynolds number can exceed million. It is explained here why the elongated counterflows survive wild turbulent mixing in these devices. To this end, an analytical solution to the Reynolds averaged Navier-Stokes equations is obtained that describes the turbulent flow in a cylindrical container. The Reynolds stresses are modeled using the Prandtl mixing length approach modified here for swirling flows. A fluid enters the container through a tangential inlet and leaves through a central exhaust both located at the same end wall. Despite the inlet and exhaust are close, there is no short-cut flow. The fluid goes from the inlet near the sidewall to the dead end, turns around, and goes back near the axis to the exhaust. This global counterflow occurs due to swirl decay caused by friction at the sidewall. The combined effect of swirl and friction causes that pressure drops from the inlet to the dead end near the sidewall and from the dead end to the exhaust near the axis. Such a pressure distribution drives the counterflow and provides its survival against turbulent mixing. A simple experiment is performed confirming the counterflow geometry. [Preview Abstract] |
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