Bulletin of the American Physical Society
2005 58th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 20–22, 2005; Chicago, IL
Session BG: Vortex Dynamics I |
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Chair: Mark Stremler, Vanderbilt University Room: Hilton Chicago Williford A |
Sunday, November 20, 2005 10:56AM - 11:09AM |
BG.00001: Turbine Blade Tip-Gap Flow Physics and Control Travis Douville, Julia Stephens, Thomas Corke, Scott Morris A linear cascade that is designed to simulate the flow around blades in the low-pressure turbine stage of turbo-jet engines is used to study the physics of the tip-gap flow and vortex. The cascade consists of three Pratt \& Whitney ``PakB'' blades. The experiment investigated gap sizes of 0.5 to 5.0 percent of the blade axial chord, and Reynolds numbers from 100K to 500K that correspond to tip relative Mach numbers of 0.04 to 0.21. Static pressure ports at mid and tip spanwise locations recorded blade pressure distributions. Static end wall taps recorded pressures in the gap region. A five-hole Pitot probe that was traversed in the blade wakes was used to determine total pressure loss coefficients and local velocity vectors. Baseline measurements were analyzed across the range of Reynolds numbers and gap sizes to categorize their effects. These were then compared to flow changes produced by a passive flow control device placed at the end of a blade to locally reduce the gap height. Its effect on the tip-gap flow is presented. [Preview Abstract] |
Sunday, November 20, 2005 11:09AM - 11:22AM |
BG.00002: Tip Gap Flow Control of A Pak-B Turbine Blade Julia Stephens, Travis Douville, Thomas Corke, Scott Morris A high-speed linear cascade is used to investigate passive and active approaches for controlling the over-tip leakage flow associated with the turbine blades in the low-pressure stage of a gas turbine engine. The cascade consists of Pratt \& Whitney ``PakB'' blades with varying gap sizes ranging from 0.5 to 5.0 percent of the blade axial chord. Reynolds numbers between 100K and 500K that correspond to tip relative Mach numbers of 0.04 to 0.21 were investigated. Pressure ports on the endwall as well as at the midspan and tip of the blade are used to evaluate the flow. Additionally, a five-hole probe that was traversed in the blade wakes was used to determine total pressure loss coefficients and local velocity vectors. Two types of flow control devices were investigated. One consisted of a passive partial ``squeeler'' tip that locally reduced the gap size. The other consisted of a plasma actuator located on the blade tip was designed to produce unsteady disturbances which were receptive to the over-tip flow jet and shear layers. The effects of both these approaches are contrasted. [Preview Abstract] |
Sunday, November 20, 2005 11:22AM - 11:35AM |
BG.00003: The Unsteady Wake Generated by a Rotor in Ground Effect Devi Pulla, A.T. Conlisk Helicopters flying close to the ground encounter many handling qualities problems. Large unsteady forces near the main and tail rotors are believed to be a major cause of these problems. We investigate the structure of the unsteady wake generated by multibladed rotors in ground effect. A vortex lattice method with a free wake model is used to simulate the tip-vortex and the method of images is used to model the ground and the blades are modeled using the lifting surface method. The rotor wake is advanced in time until periodicity is attained. Unsteadiness in the form of instantaneous velocities is studied at different planes on the advancing and retreating sides of the tail rotor to identify the plane with the maximum unsteadiness. The root-mean-square of the deviation of the instantaneous velocity from the time averaged velocity over a given period is used to quantify the unsteadiness [Preview Abstract] |
Sunday, November 20, 2005 11:35AM - 11:48AM |
BG.00004: Vortex merging in stably stratified fluid Laura Brandt, Keiko Nomura This study investigates the effects of stable stratification on the two-dimensional vortex merging process. Numerical simulations of two horizontal co-rotating vortices in a vertically stably stratified fluid are performed. The merging process in both unstratified and stratified flows occurs in three stages: adaptation (initial diffusion and deformation), convective merger, final diffusion. The convective merger is associated with the structure of the mutually induced flow field. As in previous studies, analysis in the rotating frame reveals the existence of two recirculation regions with rotation in the opposite sense to that of the primary vortices which lead to the formation of filaments. In stably stratified fluid, horizontal density gradients are established as the flow stirs the fluid, generating vorticity of opposite sign through baroclinic torque. In the case of weak to moderate stratification levels, the baroclinic vorticity appears outside the recirculation regions and assists in the formation of the filaments. An earlier onset and a more rapid convective merger are observed; thus, merging is accelerated. In the case of strong stratification, the baroclinic vorticity significantly alters the ghost vortices and causes the primary vortices to shed their filaments. Results provide further insight into the physics of vortex merging. [Preview Abstract] |
Sunday, November 20, 2005 11:48AM - 12:01PM |
BG.00005: Turbine Tip Clearance Active Flow Control using Plasma Actuators Daniel VanNess, Thomas Corke, Scott Morris A low-speed linear cascade was used to examine the tip gap leakage flow and leakage vortex that exists within the low pressure turbine stage of a gas-turbine engine. The cascade array is composed of nine Pratt \& Whitney ``PakB" blades, with the center blade having a variable tip gap up to five percent chord. Reynolds numbers based on axial chord varied from $10^4$ to $10^5$. Static pressure taps located at the midspan and near the tip of the blade were used to characterize the blade pressure distribution. A five-hole probe was also traversed in the downstream blade wake to ascertain velocity vectors and total pressure loss. Flow control in the form of a single-dielectric-barrier plasma actuator mounted on the blade tip was used to alter the leakage vortex by acting on the blade tip separation bubble, the blade tip shear layer instability, or the gap flow jet instability through the production of high frequency unsteady disturbances. The flow was documented through measurements with and without flow control for varying tip gaps and Reynolds numbers. The effect of the actuation on the tip leakage vortex and efficiency are investigated. [Preview Abstract] |
Sunday, November 20, 2005 12:01PM - 12:14PM |
BG.00006: Effects of stable stratification on the elliptic instability in vortex pairs Keiko Nomura, Laura Brandt, James Rottman The effects of stable stratification on the dynamics of both counter-rotating and co-rotating vortex pairs are studied using direct numerical simulations. Here, the vortices are oriented horizontally in a vertically stratified fluid. In particular, we are interested in the three-dimensional elliptic instability that may occur in these flows and the two-dimensional dynamics that influence its development. The instability is associated with the ellipticity of the streamlines due to the strain induced by one vortex on the other and results in an antisymmetric sinusoidal deformation of the vortex cores. In the case of the counter-rotating vortex pair, for weak to moderate stratification, the primary effect of stratification is to reduce the vortex separation distance which enhances the mutually induced strain. Consequently, the instability has an earlier onset and higher growth rate but otherwise develops the same as in unstratified flow. For strong stratification, the baroclinic vorticity detrains fluid from the primary vortices and significantly alters the instability. In the case of the co-rotating vortex pair, the two-dimensional merging process is a dominant aspect of the flow evolution. For weak to moderate stratification, baroclinic torque assists merging thereby impeding the development of the instability. [Preview Abstract] |
Sunday, November 20, 2005 12:14PM - 12:27PM |
BG.00007: 3D Vortices in Rotating Stratified Flows Xylar Asay-Davis, Sushil Shetty, Philip Marcus We have carried out high resolution numerical simulations of 3D vortices in rotating flows in which stratification is strong in the sense that the Brunt-Vaisala frequency $N$, the Coriolis parameter $2 \Omega$ and the inverse of the turn-around time of the vortex $1/\tau$ are all the same order. Although these vortices are of importance in protoplanetary disks around forming stars, the vortices can also be created in the laboratory. Most previous analyses of laboratory vortices have been for weak stratification, ie, $N \ll 1/\tau$. The focus of this talk is on the theoretical predictions of scaling laws for laboratory vortices and the numerical validation of the scalings. Of most importance is how the prediction for the aspect ratio (height to average horizontal diameter) of the allowable equilibria depends on the non-dimensional ratio of the 3 times scales (ie, Rossby number, Froude number, etc.). We also discuss the stability of the allowed equilibria as functions of these same dimensionless numbers. For example as stratification becomes weak, the aspect ratio becomes large (and the vortices look more like Taylor columns), but instabilities, rather than the phase space of allowable equilibria, set the maximum aspect ratio. [Preview Abstract] |
Sunday, November 20, 2005 12:27PM - 12:40PM |
BG.00008: Dynamic of a tilted vortex in a stratified fluid Nicolas Boulanger, Patrice Meunier, Stephane Le Dizes The dynamic of a vortex tilted with respect to the stratification is investigated theoreticaly and experimentally for small inclination angle. Expanding the velocity with respect to the small angle we have been able to find the base flow. The main result of this analysis is the appearance of a critical layer where the angular velocity equal the Brunt-Vaisala frequency. Smoothed by viscosity this critical layer is responsible for the appearance of a vertical velocity with a strong shear and a radial variation of the density both with a strong azimuthal dependancy. This result is in good agreement with PIV measurements. Moreover shadowgraph visualisations and PIV measurements reveals the developpement of an instability which could be of K-H type due to the strong shear in the critical layer. This instability is responsible for a strong mixing and internal wave generation. Quantitative measurements remains to be done but this instability could be of great interest specifically concerning stratified turbulence. [Preview Abstract] |
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