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 R32: General Fluid Dynamics: Rotating Flows and Multi-Physics Phenomena |
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Chair: Miron Kaufman, Cleveland State University Room: 313 |
Tuesday, November 24, 2015 12:50PM - 1:03PM |
R32.00001: Thermodynamic Phase Transitions and Creeping Flows in Cavities Miron Kaufman, Petru S. Fodor We discuss the analogy between the stream line function of creeping flows in rectangular cavities and the thermodynamic potential at critical points and at phase transitions. Assuming no-slip boundary conditions, the corners of the rectangular cavity are stationary (fixed) points. We analyze two such points: 1. Corner where one wall is moving and the other is stationary; 2. Corner where both walls are stationary. The first one is analogous to a to a first-order transition (discontinuity) point while the second one is analogous to a thermodynamic critical point (second-order transition). Moffatt eddies, which impede mixing [P. S. Fodor, M. Kaufman, Proceedings of PPS-30, AIP Conf. Proc. 1664 (2015)], are present in the neighborhood of the second stationary point. The results discussed here are based on numerical solutions of the Navier-Stokes equations combined with analytical work valid in the vicinity of the stationary points. [Preview Abstract] |
Tuesday, November 24, 2015 1:03PM - 1:16PM |
R32.00002: Heat transfer analysis in rotating sphericall shells Ares Cabello, Ruben Avila The study of flow patterns within rotating spherical annular geometries with natural convection, is essential to understand the internal dynamics of the planets. We investigate the convective flows and the heat transfer rate in an spherical gap in which a temperature difference between the inner sphere and the outer sphere is present. A self gravity field which varies as a function of $1/r^n$ (where $r$ is the radial position and the integer exponent $n$ has the values 2,3,4,5) is assumed. The Boussinesq fluid equations are solved by using a spectral element method (SEM). To avoid the singularity at the poles, the cubed-sphere algorithm is used to generate the spherical mesh. Heat fluxes at the surface of both spheres are analyzed. We find, for several Ekman and Rayleigh numbers, that there exists a high correlation between the azimuthal motion of both the Busse cells and the zones where the maximum surface heat fluxes occur. The azimuthal position, as a function of time, of the maximum heat flux zones (which are located symmetrically with respect to the equator), allows to speculate on the nature of the phenomena occurring (in geological times) on the surface of the terrestrial planets. [Preview Abstract] |
Tuesday, November 24, 2015 1:16PM - 1:29PM |
R32.00003: Convective flow patterns in inclined rectangular cavities with rotation Ruben Avila, Diana Perez-Espejel The natural convection in inclined three dimensional rectangular cavities with rotation is numerically investigated by using a spectral element method. When the rate of rotation ($Ta$ number) is equal to zero, the critical Rayleigh number $Ra_c$ for the onset of transverse or longitudinal rolls is obtained by solving (using the Tau-Chebyshev spectral method) the equations of the linear stability theory. In the numerical approach, the rotation is imposed once the steady state of the longitudinal or transverse rolls is attained. The cavity rotates around an axis that is orthogonal to its cold and hot surfaces, and passes through the center of these surfaces. In all the analyzed cases, the tilted angle $\delta$, from the horizontal, varies in the interval 0$^{\circ}\le \delta < 90^{\circ}$ (the cavity is heated from its lower surface, then an unstable condition prevails) and 90$^{\circ} < \delta \le$ 180$^{\circ}$ (the cavity is heated from its upper surface, then a stable condition prevails). We report the influence of the $Ta$ number on the critical $Ra$ number, the average Nusselt number (evaluated at the hot surface), and the flow patterns in the tilted cavity. [Preview Abstract] |
Tuesday, November 24, 2015 1:29PM - 1:42PM |
R32.00004: Interpreting global behavior of quasi-Keplerian flows as a response to boundary forcing E. M. Edlund, H. Ji A series of experiments conducted in the Hydrodynamic Turbulence Experiment (HTX), a modified Taylor-Couette device, have explored the response of the azimuthal flows to the forcing imposed by the boundaries. The HTX device has rings on the axial end-caps that can take speeds different from that of the inner and outer cylinders. This extra degree of freedom allows us to tune the mean flow profiles, with the possibility of achieving flows remarkably close to the ideal Couette profile that are expected in the absence of axial boundaries. These ''optimized'' cases have the interesting property that the azimuthal velocity profiles are effectively independent of Reynolds number. In contrast, non-optimized cases show progressive departure from ideal Couette as the Reynolds number is increased. We present a model that captures this Reynolds number dependence and interpret this from the perspective of angular momentum flux across the boundaries. By varying the boundary components, we also show that optimized flows can only be achieved when there exists pressure balance between the boundary and the bulk. These observations have important implications for the design of Taylor-Couette experiments that attempt to make connections to astrophysics at large Reynolds numbers. [Preview Abstract] |
Tuesday, November 24, 2015 1:42PM - 1:55PM |
R32.00005: Transient growth and its consequences in rotating channel flow Sharath Jose, Vishnu Prasad, Beno\^it Pier, Rama Govindarajan We know that pressure-driven flow through a channel, which is being rotated about its spanwise coordinate, is unstable for a range of rotation numbers Ro. The critical Reynolds number is very sensitive to the rotation rate. We present here nonmodal stability characteristics of the system in the parameter regime where exponential instabilities do not exist. We show that transient growth is markedly different at low and high Ro, with high growth rates and asymmetric streamwise structures at low Ro and spanwise structures created by the far weaker Orr mechanism in operation at high Ro. The latter is a demonstration of the Taylor-Proudman theorem. In the nonlinear regime, except when the rotation rate is very high, chaotic flows of varying strength are obtained. At high Ro weak structures confined to one side of the channel give way to a completely laminar profile. [Preview Abstract] |
Tuesday, November 24, 2015 1:55PM - 2:08PM |
R32.00006: Numerical and Experimental study of secondary flows in a rotating two-phase flow: the tea leaf paradox Antoni Calderer, Douglas Neal, Richard Prevost, Arno Mayrhofer, Alan Lawrenz, John Foss, Fotis Sotiropoulos Secondary flows in a rotating flow in a cylinder, resulting in the so called “tea leaf paradox”, are fundamental for understanding atmospheric pressure systems, developing techniques for separating red blood cells from the plasma, and even separating coagulated trub in the beer brewing process. We seek to gain deeper insights in this phenomenon by integrating numerical simulations and experiments. We employ the Curvilinear Immersed boundary method (CURVIB) of Calderer et al. (J. Comp. Physics 2014), which is a two-phase flow solver based on the level set method, to simulate rotating free-surface flow in a cylinder partially filled with water as in the tea leave paradox flow. We first demonstrate the validity of the numerical model by simulating a cylinder with a rotating base filled with a single fluid, obtaining results in excellent agreement with available experimental data. Then, we present results for the cylinder case with free surface, investigate the complex formation of secondary flow patterns, and show comparisons with new experimental data for this flow obtained by Lavision. [Preview Abstract] |
Tuesday, November 24, 2015 2:08PM - 2:21PM |
R32.00007: Design optimization of a vaneless ''fish-friendly'' swirl injector for small water turbines Ajith Airody, Sean D. Peterson Small-scale hydro-electric plants are attractive options for powering remote sites, as they draw energy from local bodies of water. However, the environmental impact on the aquatic life drawn into the water turbine is a concern. To mitigate adverse consequences on the local fauna, small-scale water turbine design efforts have focused on developing ``fish-friendly'' facilities. The components of these turbines tend to have wider passages between the blades when compared to traditional turbines, and the rotors are designed to spin at much lower angular velocities, thus allowing fish to pass through safely. Galt Green Energy has proposed a vaneless casing that provides the swirl component to the flow approaching the rotor, eliminating the need for inlet guide vanes. We numerically model the flow through the casing using ANSYS CFX to assess the evolution of the axial and circumferential velocity symmetry and uniformity in various cross-sections within and downstream of the injector. The velocity distributions, as well as the pressure loss through the injector, are functions of the pitch angle and number of revolutions of the casing. Optimization of the casing design is discussed via an objective function consisting of the velocity and pressure performance measures. [Preview Abstract] |
Tuesday, November 24, 2015 2:21PM - 2:34PM |
R32.00008: Numerical investigation of power consumption and mixing time in a stirred vessel with regular and multiscale impellers Salur Basbug, George Papadakis, Christos Vassilicos The flow field inside a stirred tank is obtained by means of direct numerical simulation based on finite volume method at Re$=$500. Two different types of four-bladed radial impellers are considered: the first one is a regular type with rectangular blades and the second one is a modified version of the former with irregular blade edges, having the same thickness and the surface area. The shaft power is averaged over more than sixty revolutions and the comparison between the two cases shows that the impeller with irregular blades has lower energy consumption. Moreover, a passive scalar is injected into the vessel for a quarter period of revolution and the scalar transport equation is solved to investigate the mixing times. The coefficient of variation of the passive scalar is averaged over the whole volume in order to obtain a quantitative indicator of the mixing progress. The homogenization curves depend on the instantaneous flow conditions due to the transient nature of the mixing process, therefore multiple curves are averaged to obtain a representative result. There are indications that irregular blades can decrease mixing time with respect to regular ones. [Preview Abstract] |
Tuesday, November 24, 2015 2:34PM - 2:47PM |
R32.00009: Experimental investigation of the flow field and power consumption characteristics of regular and fractal blade impellers in a dynamic mixer K. Steiros, P.J.K. Bruce, O.R.H. Buxton, J.C. Vassilicos Experiments have been performed in an octagonal un-baffled water tank, stirred by three radial turbines with different geometry impellers: (1) regular rectangular blades; (2) single-iteration fractal blades; (3) two-iteration fractal blades. Shaft torque was monitored and the power number calculated for each case. Both impellers with fractal geometry blades exhibited a decrease of turbine power number compared to the regular one (15{\%} decrease for single-iteration and 19{\%} for two iterations). Phase locked PIV in the discharge region of the blades revealed that the vortices emanating from the regular blades are more coherent, have higher kinetic energy, and advect faster towards the tank's walls where they are dissipated, compared to their fractal counterparts. This suggests a strong link between vortex production and behaviour and the energy input for the different impellers. Planar PIV measurements in the bulk of the tank showed an increase of turbulence intensity of over 20{\%} for the fractal geometry blades, suggesting higher mixing efficiency. Experiments with pressure measurements on the different geometry blade surfaces are ongoing to investigate the distribution of forces, and calculate hydrodynamic centres of pressure. [Preview Abstract] |
Tuesday, November 24, 2015 2:47PM - 3:00PM |
R32.00010: Leading-edge vortex trajectories under the influence of Coriolis acceleration Eric Limacher, Chris Morton, David Wood Leading-edge vortices (LEVs) can form and remain attached to a rotating wing indefinitely, but the mechanisms of stable attachment are not well understood. Taking for granted that such stable structures do form, a practical question arisesof where an LEV core persists in the body-fixed frame of reference. Noting that span-wise flow exists within the LEV core, it is apparent that a mean streamline aligned with the axis of the LEV must exist. The present work usesthe Navier-Stokes equations alongthis steady, axial streamline in order to consider the accelerations that act in the steamline-normal direction to affect its local curvature. With some simplifying assumptions, an ordinary differential equation is derivedthat describes the trajectory of the axial streamline through the vortex core. Usingempirical values of axial velocity in the vortex core from previous studies, it can be shown that Coriolis and centrifugal forces alone can account for the tilting of the stable LEV into the wake within several chord lengths from the root of rotationin the span-wise direction. This result supports an hypothesisthat LEVs are observed at inboard locations because Coriolis force must actover a finite distance to tilt the stable LEV away from the leading edge. [Preview Abstract] |
Tuesday, November 24, 2015 3:00PM - 3:13PM |
R32.00011: A Multiscale simulation method for ice crystallization and frost growth Miad Yazdani Formation of ice crystals and frost is associated with physical mechanisms at immensely separated scales. The primary focus of this work is on crystallization and frost growth on a cold plate exposed to the humid air. The nucleation is addressed through Gibbs energy barrier method based on the interfacial energy of crystal and condensate as well as the ambient and surface conditions. The supercooled crystallization of ice crystals is simulated through a phase-field based method where the variation of degree of surface tension anisotropy and its mode in the fluid medium is represented statistically. In addition, the mesoscale width of the interface is quantified asymptotically which serves as a length-scale criterion into a so-called “Adaptive” AMR (AAMR) algorithm to tie the grid resolution at the interface to local physical properties. Moreover, due to the exposure of crystal to humid air, a secondary non-equilibrium growth process contributes to the formation of frost at the tip of the crystal. A Monte-Carlo implementation of Diffusion Limited Aggregation method addresses the formation of frost during the crystallization. Finally, a virtual boundary based Immersed Boundary Method (IBM) is adapted to address the interaction of ice crystal with convective air during its growth. [Preview Abstract] |
Tuesday, November 24, 2015 3:13PM - 3:26PM |
R32.00012: Geometrical Scaling of an Ablative Bluff Body under Different Outer Flow Velocity and Temperature Configurations Michael Allard, Christopher M. White, Yves Dubief Experimental results investigating the geometrical scaling and local properties of an eroding low temperature ablator (para-dichlorobenzene) are presented. The bluff body is placed in a heated open-circuit wind tunnel and the effects of incoming outer flow velocity (uniform and spatially varying) and temperature on the ablation process are investigated. Image sequencing of the projected area in the streamwise-spanwise and streamwise-wall normal flow direction are used to quantify the time evolution of the geometrical shape and compute local recession rates and curvature. The geometrical self-similarity and local recession rates are evaluated and compared to Moore \textit{et al.} (Phys. Fluids (2013) 25:116602) and Huang \textit{et al.} (J. Fluid Mech. (2015) 765:R3) who investigated erosion under the action of fluid shear force and dissolution, respectively. [Preview Abstract] |
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