Bulletin of the American Physical Society
2006 59th Annual Meeting of the APS Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2006; Tampa Bay, Florida
Session BO: Separated Flows I |
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Chair: Haecheon Choi, Seoul National University Room: Tampa Marriott Waterside Hotel and Marina Meeting Room 11 |
Sunday, November 19, 2006 11:00AM - 11:13AM |
BO.00001: On the effect of large amplitude excitations on the boundary layer separation Rouslan Krechetnikov It is known that periodic excitations of a boundary layer may lead to the delay of separation or to the reattachment of initially separated flow. The ``transition'' from separated states to reattached ones is described by a curve in the perturbation frequency-amplitude space. This phenomenon of reattachment takes place for periodic excitations of small amplitudes. However, the flow, which is reattached due to periodic excitations, becomes separated again if the amplitude of the perturbation increases up to a considerable level. This, in turn, defines a new curve for the transition from reattached to separated flow in the same frequency-amplitude space. This talk is devoted to a minimal model, which explains the existence of this latter transition curve. The model is based on the unsteady Prandtl boundary layer equations, which are analyzed as a problem of receptivity to high amplitude periodic perturbations. The flow separation is identified using Wang's separation criteria (Wang, 1982), i.e. formation of limiting streamlines in Lagrangian space. [Preview Abstract] |
Sunday, November 19, 2006 11:13AM - 11:26AM |
BO.00002: Boundary Layer Separation Control via Spanwise Lorentz Forces Srinivasan Dattarajan, Hamid Johari The ability of a Lorentz force actuator to suppress boundary layer separation in a weakly conductive fluid was examined experimentally. The actuator comprising of an array of alternately arranged electrodes and magnets was flush mounted on the surface of a two dimensional hydrofoil model with a trailing flap inclined at 20\r{ } to the freestream flow in a water tunnel. The actuator was powered with a unipolar square wave at various frequencies and amplitudes in order to generate time-periodic, spanwise Lorentz forces. Velocity fields measured using PIV indicated complete flow reattachment on the inclined flap as a result of the application of Lorentz forces above a certain forcing amplitude. Performance of the actuator was weakly dependent on the forcing frequency, with improved performance found at a non-dimensional frequency of unity. The effectiveness of forcing varied with the Reynolds number, with better control achieved at higher Reynolds number for the same forcing amplitude and frequency. The ability of the actuator in suppressing separation appeared to be related to the appearance of distinct streamwise vortices as a result of Lorentz forcing. [Preview Abstract] |
Sunday, November 19, 2006 11:26AM - 11:39AM |
BO.00003: Manipulation of separation by transverse blowing Leon van Dommelen, Han Zhao Recent experiments have shown the potential to manipulate boundary-layer separation by means of transverse blowing using supersonic microjets. Such a mechanism is not explainable by means of a conventional boundary layer description; it requires the resolution of G\"ortler scale spanwise vorticity in a parabolized Navier-Stokes approach. It is shown that the G\"ortler mechanics of blowing with discrete jets is characterized by an viscous-inviscid interaction between opposing convection effects in the viscous flow near the wall and the immediately overlying irrotational flow that leads to two layers of counter-rotating vortices. Initially, the corresponding vortex motion is dominated by spanwise convection, but when the boundary layer thickens while approaching separation, nonlinear interactions between the vortices become possible to increase mixing. Effects of the mechanics of those vortices on the forming separation process will be discussed during the meeting. Since blowing can be much more reliable than suction under real-life conditions, the results can have significant potential practical application. [Preview Abstract] |
Sunday, November 19, 2006 11:39AM - 11:52AM |
BO.00004: Evaluation of Passive Control of Cavitation in Separated Flows Hailing An, Michael Plesniak Vortex generators have been used for many years to successfully control separated flows. The goal of this study is to evaluate the effectiveness of such passive devices for controlling cavitation in separated flows. Cavitation has been associated with the low pressure cores of turbulent coherent structures. The hypothesis is that weakening these structures or disrupting their spanwise coherence will impact cavitation. However, the devices studied generate relatively strong streamwise vortices which could enhance cavitation. The net result will be a balance of competing effects. Three vortex-generating separation control devices (surfaced-mounted fence, spanwise cylinder and vane-type vortex generators) are compared. Their performance in reducing the separation bubble length downstream of a backward facing step is evaluated, as well as their effect on cavitation phenomena. Measurements are made using LDV, PIV and high speed cinematography to facilitate comparisons between the baseline and the controlled cases. These measurements also provide benchmarks for companion CFD studies. The effects of separation control on both the flow physics and cavitation phenomena are discussed. [Preview Abstract] |
Sunday, November 19, 2006 11:52AM - 12:05PM |
BO.00005: Critical mass and vortex dynamics for rising and falling spheres Matthew Horowitz, Charles H.K. Williamson We study the dynamics of spheres rising or falling freely through a fluid at two different Reynolds numbers, Re = 450 and 10,000. At both values of Re, falling spheres, which have a mass ratio, or relative density greater than 1, descend rectilinearly. In the case of a rising sphere, we find that there exists a critical value of the mass ratio, below which the sphere will undergo large-amplitude oscillations. Despite the difference in the modes of vortex formation at these two Reynolds numbers (due to the instability of the separated shear layer at higher Re), a critical mass exists for both cases. For the higher Reynolds number, we find a critical mass ratio of 0.61, in good agreement with the result for tethered and elastically mounted spheres at similar Re (Govardhan and Williamson, 2004, JFM). At Re = 450, performing experiments in glycerin-water mixtures to allow for precise control of the Reynolds number, we find that the critical mass ratio takes a distinctly lower value of 0.36. Using laser-induced fluorescence to visualize the wake of a vibrating sphere at this Reynolds number reveals another interesting phenomenon; rather than two alternately signed vortex loops being shed in a cycle, as might be expected, four vortex structures are shed in each cycle of oscillation. [Preview Abstract] |
Sunday, November 19, 2006 12:05PM - 12:18PM |
BO.00006: Effect of free-stream turbulence on flow over a sphere. Kwang Min Son, Jin Choi, Woo-Pyung Jeon, Haecheon Choi In this study, the effect of free-stream turbulence on flow over a sphere is experimentally investigated by installing various types of grids upstream of the sphere. We measure the drag, surface pressure and velocity fields in the wake and boundary layer, and conduct surface visualization at $Re = 0.5~ {\times}~10^5 \sim 2.8~ {\times}~10^5$. The free-stream turbulence generates small separation bubble above the sphere surface and decreases the critical Reynolds number at which the drag coefficient rapidly decreases. With further increasing the Reynolds number, the laminar separation point is delayed downstream but the reattachment point closing the separation bubble is fixed at $115^{\circ}$. The main separation point is also fixed at $130^{\circ}$, resulting in constant drag coefficient after the critical Reynolds number. As the Reynolds number is further increased, the small separation bubble finally disappears but the main separation point is still fixed at $130^{\circ}$. Therefore the formation, regression and disappearance of the separation bubble are the key to the drag change due to the free-stream turbulence. More results of detailed measurement around the separation bubble and the boundary layer will be presented. [Preview Abstract] |
Sunday, November 19, 2006 12:18PM - 12:31PM |
BO.00007: Characteristics of flow around a sphere with a surface trip. Jin Choi, Kwang Min Son, Woo-Pyung Jeon, Haecheon Choi The drag on a sphere in a freestream is significantly changed by a surface trip wire. Although this behavior is well known, the flow characteristics have not been clearly presented except that the surface trip wire promotes transition to turbulence. In our study, we vary the diameter and location of the trip wire, measure the drag, surface pressure and velocity profiles inside the boundary layer at $Re = 0.5 {\times} 10^5 \sim 3 {\times} 10^5$, and conduct flow visualization. With a thick trip wire ($k/d=1.3 {\times} 10^{-2}$), a separation bubble is formed right after the trip wire and transition to turbulence occurs there, resulting in main separation delay and drag reduction. On the other hand, with a thin trip wire ($k/d=0.3 {\times} 10^{-2}$), transition to turbulence does not occur at the trip wire but a separation bubble is newly formed at $100^{\circ} \sim 115^\circ$, which significantly delays the main separation. At high Reynolds number, this separation bubble disappears and transition to turbulence occurs at the trip wire. When the trip wire is located downstream, transition to turbulence and drag crisis occur at lower Reynolds number. [Preview Abstract] |
Sunday, November 19, 2006 12:31PM - 12:44PM |
BO.00008: Prediction of the flow around a golf ball using different turbulence models. Clinton Smith, Kyle Squires Turbulence models, including Reynolds-Averaged Navier-Stokes (RANS) and Detached Eddy Simulation (DES), are applied to prediction of turbulent flow around a golf ball. Both non-rotating and rotating cases are considered. One of the primary goals of the work is to assess the capability of the turbulence treatments in predicting the flow around the ball. Predictions are presented from computations performed at a Reynolds number based on the diameter of the ball and a freestream velocity of around 160,000. The time history of the drag force for the rotating and non-rotating cases are similar, with the values of the average drag for the rotating case nominally higher for all turbulence treatments used. Predictions of the lift and drag coefficients using RANS and DES are in good agreement with measurements, in spite of the simplifications made to treatment of the boundary layer. [Preview Abstract] |
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