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 BM: Separated Flows II |
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Chair: Israel Wygnanski, University of Arizona Room: Hilton Chicago PDR 2 |
Sunday, November 20, 2005 10:56AM - 11:09AM |
BM.00001: Amplitude-modulated Excitation of an Axi-symmetric Backward-facing-step Flow: Wall-pressure-array and Velocity Measurements Antonius Aditjandra, Barry Trosin, Ahmed Naguib Wall-pressure-array and velocity measurements were conducted to investigate the influence of amplitude-modulated excitation on the flow over an axi-symmetric backward-facing step. The forcing was implemented using an externally-driven Helmholtz resonator with an orifice located in the immediate vicinity of the separation point. At resonance, the fluidic disturbance produced by the resonator exhibited both unsteady and streaming components. By modulating the excitation waveform at substantially lower frequency, it was possible to couple the streaming disturbance to the flow effectively and shorten the separation bubble. The effect of both the modulation frequency and amplitude on the flow were examined, and the most effective forcing frequency was identified. Furthermore, the spatio-temporal characteristics of the wall-pressure and velocity field within and downstream of the separation bubble were captured under different forcing conditions. The results provide insight into the behavior of the flow structures and their wall-pressure signature under forced conditions. [Preview Abstract] |
Sunday, November 20, 2005 11:09AM - 11:22AM |
BM.00002: The effect of opening angle and Reynolds number in a planar asymmetric diffuser studied using LES Astrid Herbst, Dan Henningson The performance of diffuser-like flows occuring in many technical applications is strongly affected by separation and therefore flow control is needed. Here we study flow separation in a plane asymmetric diffuser by the means of LES. The numerical implementation follows the work by Kaltenbach et.al. using the code developed at CTR, Stanford. The incompressible Navier--Stokes equations are solved on a structured grid using a hybrid second order finite difference / spectral method with a dynamic subgrid model. Simulations have been conducted for two opening angles of $8.5^{\circ}$ and $10^{\circ}$ at the Reynolds number $Re_b=9000$ and at $Re_b=2000$ a simulation for an opening angle of $8.5^{\circ}$ matching the experimental configuration of Gullman-Strand et al. has been performed. We have found an increase in the separated region with increasing Reynolds number as well as for a wider opening angle. Future simulations of periodic forcing of the separated flow in the plane asymmetric diffuser will be presented at the conference. Gullman-Strand J. et al., Int. J. Heat and Fluid Flow, 25,pp. 451-460 (2004). Kaltenbach H. J. et. al, JFM, 390, pp. 151-185 (1999) [Preview Abstract] |
Sunday, November 20, 2005 11:22AM - 11:35AM |
BM.00003: Flow control in low pressure turbine blades using plasma actuators Karthik Ramakumar, Arvind Santhanakrishnan, Jamey Jacob An experimental study of plasma flow control actuators for flow separation control in low pressure turbine (LPT) blades is presented. The actuator arrangement consists of two copper strips separated by a dielectric medium with an input voltage of approximately 5kV and a frequency input varying from 3-5 kHz, creating a region of plasma used for boundary layer flow control. The effect of varying waveform on control efficacy is investigated using sine, square and saw tooth waveforms. The impact of duty cycle and forcing frequency on both displacement and momentum thickness are also examined. Boundary layer measurements are carried out using PIV while measurements of the wake downstream are performed using a 7-hole probe for Reynolds number ranging from 30,000 to 50,000. Separation is fully controlled in most configurations and boundary layer parameters reveal that the actuator entrains the free-stream flow at the actuator location and creates a region of high turbulence, essentially behaving similar to an active boundary layer trip. A small region of reversed flow near the surface indicates the presence of cross-stream vortical structures. The use of plasma synthetic jet actuators flow LPT flow control is also discussed. [Preview Abstract] |
Sunday, November 20, 2005 11:35AM - 11:48AM |
BM.00004: Fluidic Thrust Vectoring Modelling of a two-dimensional convergent-divergent nozzle Bruno Mangin, Laurent Jacquin, Amer Chpoun Injecting air into the divergent of the nozzle causes a shock in the primary flow, and turns the flow supersonically. The angle between thrust and nozzle axis is due to asymmetric wall pressure and secondary flow thrust. The main component of the normal force is due to the overpressure in the separated area upstream the injector. A theoretical modelling has been implemented to evaluate the flow structure and forces resulting from the injection in the divergent. Different penetration theories including Zukoski's blunt body theory and a new method based on characteristics method are coupled to separation and reattachment criteria to qualitatively evaluate the different parameters influences (total pressures and Mach numbers in the nozzle and in the injector). Applying this model on a NASA experiment (AIAA 2003-3802) enabled to evaluate the model accuracy. Thrust vector components are estimated within 20{\%}. RANS calculations have been realized thanks to the CFD code elsA. [Preview Abstract] |
Sunday, November 20, 2005 11:48AM - 12:01PM |
BM.00005: Fourier and POD Analyses of the Floor-Pressure Signature in a Low-Mach-Number Shallow Cavity Ahmed Naguib An experimental investigation of the pressure fluctuations on the bottom of a low-Mach-number ($M <$ 0.1) cavity flow was undertaken using a sixteen-microphone array to resolve the frequencies \textit{as well as} the wavenumbers associated with the wall-pressure signature. Both Fourier and POD analyses of the array data were utilized to identify the dominant cavity unsteadiness modes. In the present work, the results from the two different approaches are examined to provide insight into the strength and limitation of each of the techniques in capturing the proper cavity-flow physics. The effect of different cavity Reynolds numbers and aspect ratios on the results are also considered. [Preview Abstract] |
Sunday, November 20, 2005 12:01PM - 12:14PM |
BM.00006: Prediction capabilities in a turbulent flow field Julie Ausseur, Jeremy Pinier, Mark Glauser, Hiroshi Higuchi Part of our continuous efforts to implement an optimal closed-loop feedback control of the flow over a NACA 4412 airfoil is the development of an accurate model of the flow state. Combining several low-dimensional techniques, such as the Proper Orthogonal Decomposition (POD) and the modified Linear Stochastic Measurement (mLSM), we are able to access real-time information on the flow state just by sensing the pressure on the surface of the airfoil. In previous studies (Glauser et al. 2004), we have demonstrated using a simple proportional feedback control that the first POD coefficient estimated through the mLSM procedure alone contains this valuable information. In 2001 Ukeiley et al. derived a dynamical system able to predict the temporal dynamics of the POD modes and therefore of the flow. This presentation will mainly focus on the different methods that we are applying to solve this low-dimensional set of ODEs. We will present and discuss our prediction results as well as their incorporation in a more elaborate control system. [Preview Abstract] |
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