Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session D26: Rotating Flow I |
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Chair: John Cimbala, Pennsylvania State University Room: 329 |
Sunday, November 20, 2011 2:10PM - 2:23PM |
D26.00001: Spin-up of a stratified fluid in a square cylinder with a sloping bottom M.R. Foster, R.J. Munro We consider the spin-up of a linearly stratified fluid, contained in a closed rectangular cylinder whose bottom is sloped at a small angle $\alpha$. The full three-dimensionality of this flow leads to effects not evident in axisymmetric containers. We discuss asymptotic results for small Ekman number, $E$, and Rossby number $\epsilon \ll E^{1/2}$, and a series of experiments conducted at the University of Nottingham. Provided that $\alpha \gg E^{1/2}$, motion occurs on four timescales. The adjustment begins with impulsive motion, caused by the fact that the vertical walls are not streamlines of rigid rotation, and then in times of order $\alpha^{-1}$, internal waves driven by the sloped bottom wall arise. Ekman-layer eruption on the $E^ {-1/2}$ time scale damps this periodic motion, after which the fluid near the lower wall is fully spun up, but the whole of the fluid interior is not. On the $E^{-1}$ time scale, final adjustments occur, to bring the fluid up to the new rotation rate. In the four experiments performed, the rectangular cylinder was $38\,\textrm{cm}\times38\,\textrm{cm}$ in cross-section, with an average depth of $52\,\textrm{cm}$, and a base slope $\alpha=10^\circ$. In all cases, the Rossby and Burger numbers were between 0.019-0.022 and 3.1-6.8, respectively, with Ekman number $O\left( 10^{-5}\right)$. Throughout each experiment, horizontal-plane velocity components (and corresponding vertical vorticity) were obtained using 2D PIV. [Preview Abstract] |
Sunday, November 20, 2011 2:23PM - 2:36PM |
D26.00002: Large-scale barotropic circulation in rotating convection Antonio Rubio, Keith Julien, Ian Grooms High resolution DNS for rotating Rayleigh-Benard convection using the non-hydrostatic balanced geostrophic equations (NHBGE) (Julien et al 1998, 2006) was carried out in the geostrophic turbulence regime, revealing the existence of a slow-growing large scale barotropic mode. Such large scale modes have been previously observed as an inverse cascade in stable layer quasi-geostophic dynamics or via instability mechanisms of thermal Rossby waves occuring in presence of sloping endwalls (i.e quasi-geostrophic beta-convection). In this talk we report on a natural large scale organization resulting from smaller scale geostrophic thermal turbulence in an unstably stratified layer. This large scale circulation is discussed in detail in terms of spectra of the term-by-term balances of the baroclinic and barotropic vorticity equations of the NHBGE. Specifically, simulations studies reveal a tendency for the kinetic energy spectrum to follow a -3 scaling exponent at large horizontal scales and a -5/3 exponent in an inertial range of (horizontally) homogenous turbulence occuring at scales smaller than the convective forcing. [Preview Abstract] |
Sunday, November 20, 2011 2:36PM - 2:49PM |
D26.00003: PIV measurement of flow around an irregularly rotating disk with bluff bodies Sunghyuk Im, Young Jin Jeon, Hyung Jin Sung Flow structure interaction between a rotating disk with bluff bodies and surrounding flow has been evaluated by using a tomographic PIV. To examine the dynamics of disks and the fluid flow simultaneously, fluorescence tracer particles and long pass filters were used. Particles and a marked surface pattern were separated by an image processing. The geometries of the bluff bodies and the disk were then obtained by analyzing the marked pattern. Subsequently, a particle volume was reconstructed by MLOS-SMART. The particle displacement was then calculated by the PIV algorithm from the reconstructed particle volume. Rotating disk dynamics and fluid flow were discussed with variety of bluff body shapes and arrangements. Furthermore, the influence of bluff bodies on the flow field was also considered. [Preview Abstract] |
Sunday, November 20, 2011 2:49PM - 3:02PM |
D26.00004: Time-Resolved PIV in the Flow Around Cylinders Under the Effect of Coriolis Forces Filippo Coletti, Ignacio Mayo Yague, Tony Arts The effect of Coriolis forces on the dynamics of bluff body wakes is relevant to fields as diverse as turbomachinery and meteorology. Nevertheless such flows are largely unexplored, due to practical difficulties in measuring velocity in the rotating frame of reference. We present both ensemble-averaged and time-resolved PIV measurements around square and circular cylinders at Re=1000, in the presence of system rotation around an axis parallel to the cylinder. The PIV system, including a continuous laser diode and a high speed CMOS camera, is put in rotation on the same turntable as the test section at an angular velocity of 130 rpm, for a rotation number of Ro=0.13. The unique arrangement allows the same level of accuracy and spatio-temporal resolution as in a non-rotating rig. Coriolis forces break the wake symmetry, resulting in one cyclonic and one anti-cyclonic shear layer shed from the opposite sides of the cylinder. For the square cylinder the fluid entrainment from the cyclonic to the anti-cyclonic side deforms the flow topology to the point that only one focus is present in the mean wake. Time series and space-time two-point correlations suggest an overall loss of periodicity and coherence in the presence of system rotation. [Preview Abstract] |
Sunday, November 20, 2011 3:02PM - 3:15PM |
D26.00005: Bubble deformability is crucial for strong drag reduction in turbulent Taylor-Couette flow Chao Sun, Daniela Narezo Guzman, Dennis P.M. van Gils, Detlef Lohse Bubbly Taylor-Couette flow in the turbulent regime is studied both globally and locally at Reynolds numbers of $5.1 \times 10^5 - 2.0 \times 10^6$ for pure inner cylinder rotation. We measure the drag reduction (DR) based on the global torque for global gas volume fractions ($\alpha_{global}$) up to 4$\%$, and observe a moderate DR for Re $= 5.1 \times 10^5$, and a strong DR for Re $= 1.0 \times 10^6$ and $2.0 \times 10^6$. Remarkably, more than 40$\%$ of DR is achieved for $\alpha_{global} = 4\%$ at Re $= 2.0 \times 10^6$. We investigate the statistics of the liquid flow velocity, and directly measure the local bubble concentration and Weber number for two Reynolds numbers in different drag reduction regimes, i.e. Re $ = 1.0 \times 10^6$ (strong DR) and $5.1 \times 10^5$ (moderate DR). By combining global and local measurements we reveal that bubble deformability is crucial for strong drag reduction in bubbly turbulent Taylor-Couette flow. [Preview Abstract] |
Sunday, November 20, 2011 3:15PM - 3:28PM |
D26.00006: PIV measurements of a jet impinging on an opened rotor-stator system at low gap spacing Thien Nguyen, Julien Pell\'{e}, Souad Harmand The current work experimentally investigates the flow characteristics of an air jet impinging to an opened rotor-stator configuration at a low nondimensional spacing $G=0.02$ and very low aspect ratio $e/D=0.25$. The rotational Reynolds numbers varied from $0.33\times10^5$ to $5.32\times10^5$ while the jet Reynolds numbers ranged from $17.2\times10^3$ to $43\times10^3$. PIV measurements were performed at three axial planes for the entire disk diameter. The obtained PIV results agreed with those obtained by LDA measurements and numerical simulation reported in Poncet et al. 2005 (Physics of Fluids 17, 075110). A recirculation flow region, which centered at the impinging point and possessed high turbulent intensities, was observed. The mean flow and turbulent intensities were evaluated with the local heat transfer coefficients measured by Pell\'{e} and Harmand 2009 (Applied Thermal Engineering 29: 1532-1543). It is shown that the local peaks and the gradually rising of the radial heat transfer coefficients $Nu$ are due to the secondary peaks and the increases near the outer radius of the turbulent intensity distributions respectively. POD analysis was applied to the cases of the impinging jet with and without rotation. It is shown that the first POD mode captured nearly $60\%$ total kinetic energy and the low-order POD modes revealed a spiral structure in the jet-dominated region. [Preview Abstract] |
Sunday, November 20, 2011 3:28PM - 3:41PM |
D26.00007: Blade Vortex Interaction of Bi-directional flow in a Uni-directional Impulse Turbine Carlos Velez Uni-directional impulse turbines are used for the extraction of wave energy by converting oscillating air flow generated by waves into uni-directional rotational energy. The symmetric airfoil design requires a large camber, in order to function in bi-directional flow, which creates a large boundary layer separation region towards the trailing edge of the blade. A three-dimensional, viscous, transient turbulent CFD model with rotating reference frame is created to model the blade vortex interaction (BVI) which occurs during transient bi-directional air flow. Various LES models are compared to determine an adequate turbulence model to accurately resolve the vortices created on the blade trailing edge. A study of the adverse effects of this BVI is conducted and a novel blade jet technique is introduced to prevent the separation of air flow from the trailing edge of the blade. Results show strong stresses arise from BVI during bi-directional transitional flow and the~effectiveness of the blade jet technique in diminishing flow separation is successfully demonstrated. Results indicate that the increase in blade lift is linearly proportional to the blade jet mass flow rate once the jet velocity reaches approximately 120{\%} of the turbine inlet velocity and that the increase in efficiency created by the blade jets are greater than the loss in efficiency in reducing the mass flow rate extracted from the inlet to the blade jet. [Preview Abstract] |
Sunday, November 20, 2011 3:41PM - 3:54PM |
D26.00008: Validation of a Transient Rotating Reference Frame CFD Model Othmane Khoungui, Justin Ladd, Carlos Velez Uni-directional impulse turbines are used for the extraction of wave energy by converting oscillating air flow generated by waves into uni-directional rotational energy. Due to the inconsistent nature of ocean waves, airflow within an OWC is bi-directional and inherently transient. Such complex fluid dynamics require a varying rotor RPM incorporated in the CFD simulation to adequately resolve the flow field during turbine startups and changing air flow direction. The software Numeca is used to introduce a user defined function which defines a varying rotor rpm in a three dimensional transient viscous simulation of air flow through a uni-directional turbine. A scaled turbine prototype is used in a wind tunnel to measure the rotors RPM and Torque. Additionally, a radial pressure profile is developed in front and behind of the rotor blades. The experimental data is used to validate the accuracy of this varying rotating reference frame CFD model. [Preview Abstract] |
Sunday, November 20, 2011 3:54PM - 4:07PM |
D26.00009: A Parametric Study of Unsteady Rotor-Stator Interaction in a Simplified Francis Turbine Alex Wouden, John Cimbala, Bryan Lewis CFD analysis is becoming a critical stage in the design of hydroturbines. However, its capability to represent unsteady flow interactions between the rotor and stator (which requires a 360-degree, mesh-refined model of the turbine passage) is hindered. For CFD to become a more effective tool in predicting the performance of a hydroturbine, the key interactions between the rotor and stator need to be understood using current numerical methods. As a first step towards evaluating this unsteady behavior without the burden of a computationally expensive domain, the stator and Francis-type rotor blades are reduced to flat plates. Local and global variables are compared using periodic, semi-periodic, and 360-degree geometric models and various turbulence models (k-omega, k-epsilon, and Spalart-Allmaras). The computations take place within the OpenFOAM$^{\textregistered}$ environment and utilize a general grid interface (GGI) between the rotor and stator computational domains. The rotor computational domain is capable of dynamic rotation. The results demonstrate some of the strengths and limitations of utilizing CFD for hydroturbine analysis. These case studies will also serve as tutorials to help others learn how to use CFD for turbomachinery. [Preview Abstract] |
Sunday, November 20, 2011 4:07PM - 4:20PM |
D26.00010: Stirring in a tilted-rotating tank Thomas Ward, David Swan, Andrew White Inhomogeneous fluid mixing in a tilted-rotating tank is discussed at O(1-10) Reynolds and small $<1$ capillary numbers. At low Reynolds numbers the flow exhibits two large vortices. As the Reynolds number increases to the laminar regime, the two vortices exhibit interactions with the bottom wall and begin a cascading effect that is similar to the well known Moffatt (J. Fluid Mech., 1964) vortices in Stokes flow in cavities. The additional vortices aid in transporting material from the the walls to the bulk of the region between them. But the vortices also intensify in magnitude with increasing rotation rate leading to the appearance of KAM surfaces, which are barriers to efficient mixing. Experiments are performed to study dispersing water in vegetable oil (5\% water by volume) and using laser fluorescence to illuminate the vortices via experimental Poincar\'e mapping. The resulting images are analyzed to determine the mixed cross sectional area versus elapsed time as a function of the system parameters which are the tilt angle and Reynolds number. [Preview Abstract] |
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