Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session M25: Flow Control: External flows |
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Chair: Thomas Corke, University of Notre Dame Room: 31A |
Tuesday, November 20, 2012 8:00AM - 8:13AM |
M25.00001: Distributed forcing of the flow past a blunt-based axisymmetric bluff body Thierry Jardin, Yannick Bury The topology of bluff body wakes may be highly sensitive to forcing at frequencies close to intrinsic flow instabilities. In a similar way, a steady but spatially varying forcing at wavelengths close to specific flow instabilities can lead to analogous outcomes. Such forcing is commonly referred to as distributed forcing. However, although distributed forcing has proven to be a relevant control strategy for three-dimensional flows past nominally two-dimensional geometries (e.g. extruded circular cylinder at Re $>$ 180), its impact on the flow past nominally three-dimensional geometries is still unknown. Here we assess the receptivity of the flow past a blunt-based axisymmetric bluff body to an azimuthally distributed forcing applied at the periphery of the bluff-body base. We show that the impact of RSPa, RSPb and RSPc instabilities on the drag fluctuations experienced by the bluff body can be suppressed, depending on the forcing wavelengths. [Preview Abstract] |
Tuesday, November 20, 2012 8:13AM - 8:26AM |
M25.00002: Bio-Inspired Pressure Sensing for Active Yaw Control of Underwater Vehicles Amy Gao, Michael Triantafyllou A towed underwater vehicle equipped with a bio-inspired artificial lateral line was constructed and tested with the goal of active detection and correction of the vehicle's angle of attack. Preliminary experiments demonstrate that a low number of sensors are sufficient to enable the discrimination between different orientations, and that a basic proportional controller is capable of keeping the vehicle aligned with the direction of flow. We propose that a model based controller could be developed to improve system response. Toward this, we derive a vehicle model based on a first-order 3D Rankine Source Panel Method, which is shown to be competent in estimating the pressure field in the region of interest during motion at constant angles of attack, and during execution of dynamic maneuvers. To solve the inverse problem of estimating the vehicle orientation given specific pressure measurements, an Unscented Kalman Filter is developed around the model. It is shown to provide a close estimation of the vehicle state using experimentally collected pressure measurements. This demonstrates that an artificial lateral line is a promising technology for dynamically mediating the angle of a body relative to the oncoming flow. [Preview Abstract] |
Tuesday, November 20, 2012 8:26AM - 8:39AM |
M25.00003: Large eddy simulation of the flow past a twisted cylinder Jae Hwan Jung, Hyun Sik Yoon, Chang young Choi Large eddy simulation of flow past a twisted cylinder has been carried out at a Reynolds number of 300 based on the cylinder diameter and the free stream velocity using finite volume method. The twisted cylinder has been formed by rotating the elliptic cross sectional area along the spanwise direction. For an ellipse, different eccentricities are considered to observe the effect of eccentricity on the flow fields. The excellent comparisons with previous studies for the cases of a smooth cylinder and a wavy cylinder having sinusoidal variation in cross sectional area along the spanwise direction guarantee the accuracy of present numerical methods. The instantaneous vortical structures in wake of the twisted cylinder are compared with those of the circular and wavy cylinders. In general, the shear layer of the twisted cylinder covering the recirculation region is more elongated than those of the circular and the wavy cylinders. Successively, vortex shedding of the twisted cylinder was considerably suppressed, compared with those of the circular and wavy cylinders. Consequently, the twisted cylinder achieved a large amount of the drag reduction and especially the significant suppression of the fluctuating lift coefficient. [Preview Abstract] |
Tuesday, November 20, 2012 8:39AM - 8:52AM |
M25.00004: Flow Separation Control with Rotating Cylinders James Schulmeister, Jason Dahl, Gabriel Weymouth, Michael Triantafyllou The use of small counter-rotating rotating cylinders to control flow separation and reduce the drag of a fixed circular cylinder in cross-flow is investigated experimentally at Reynolds number (Re) 52,000 and computationally at Re 100 and 10,000. The moving surface of the control cylinders imparts momentum to the flow near the location of flow separation. The transfer of momentum delays separation further downstream and thereby reduces drag. The relationship between drag and rotation rate is found to be Reynolds number regime dependent; at Re = 100 the drag decreases linearly with rotation rate and at Re = 10,000, the relationship is non-linear. This non-linearity appears to be due to the interaction between vortex shedding from the small control cylinders (which does not occur at Re 100) and the main cylinder wake. Finally, the power consumed by the active control mechanism is considered and estimated to be significantly smaller than the power savings in reduced drag. [Preview Abstract] |
Tuesday, November 20, 2012 8:52AM - 9:05AM |
M25.00005: Optimization of Feedback Control of Flow over a Circular Cylinder Donggun Son, Euiyoung Kim, Haecheon Choi We perform a feedback gain optimization of the proportional-integral-differential (PID) control for flow over a circular cylinder at $Re = 60$ and $100$. We measure the transverse velocity at a centerline location in the wake as a sensing variable and provide blowing and suction at the upper and lower slots on the cylinder surface as an actuation. The cost function to minimize is defined as the mean square of the sensing variable, and the PID control gains are optimized by iterative feedback tuning method which is a typical model free gain optimization method. In this method, the control gains are iteratively updated by the gradient of cost function until the control system satisfies a certain stopping criteria. The PID control with optimal control gains successfully reduces the velocity fluctuations at the sensing location and attenuates (or annihilates) vortex shedding in the wake, resulting in the reduction in the mean drag and lift fluctuations. [Preview Abstract] |
Tuesday, November 20, 2012 9:05AM - 9:18AM |
M25.00006: Control of the Shock-Induced Flow Separation over Convex Surfaces Abraham N. Gissen, Bojan Vukasinovic, Ari Glezer The present experimental investigation focuses on shock-induced flow separation off a convex surface geometry under subsonic upstream channel flow. In particular, the emphasis is placed on the narrow pre-chocked regime (M $\approx $ 0.6) with a signature localized shock that induces the flow separation off the surface. Both uncontrolled and controlled flows are studied by characterization of both the shock dynamics and the resulting separated flow over the convex surfaces of varying curvatures. The diagnostics tools include static and dynamic surface pressure measurements, assisted by qualitative and quantitative characterizations of the separated flow. Alteration of the shock wave dynamics and its coupling to the boundary layer separation and shear layer dynamics is achieved by active generation of streamwise vorticity. The active control is aimed at direct manipulation of either the shock dynamics or the flow separation. The former demonstrates control authority on the shock dynamics and its coupling to the shear layer, while the latter assesses both upstream and downstream coupling of the flow separation delay with the shock and shear layer, respectively. [Preview Abstract] |
Tuesday, November 20, 2012 9:18AM - 9:31AM |
M25.00007: Aero-thermal optimization of film cooling flow parameters on the suction surface of a high pressure turbine blade Carole El Ayoubi, Ibrahim Hassan, Wahid Ghaly This paper aims to optimize film coolant flow parameters on the suction surface of a high-pressure gas turbine blade in order to obtain an optimum compromise between a superior cooling performance and a minimum aerodynamic penalty. An optimization algorithm coupled with three-dimensional Reynolds-averaged Navier Stokes analysis is used to determine the optimum film cooling configuration. The VKI blade with two staggered rows of axially oriented, conically flared, film cooling holes on its suction surface is considered. Two design variables are selected; the coolant to mainstream temperature ratio and total pressure ratio. The optimization objective consists of maximizing the spatially averaged film cooling effectiveness and minimizing the aerodynamic penalty produced by film cooling. The effect of varying the coolant flow parameters on the film cooling effectiveness and the aerodynamic loss is analyzed using an optimization method and three dimensional steady CFD simulations. The optimization process consists of a genetic algorithm and a response surface approximation of the artificial neural network type to provide low-fidelity predictions of the objective function. The CFD simulations are performed using the commercial software CFX. The numerical predictions of the aero-thermal performance is validated against a well-established experimental database. [Preview Abstract] |
Tuesday, November 20, 2012 9:31AM - 9:44AM |
M25.00008: Numerical Investigation of Active Flow Control on Wind Turbines under Yaw Misalignment Steven Tran, David Corson, Onkar Sahni Yaw misalignment dramatically increases unsteady aerodynamic loading on wind turbine blades over each revolution. The resulting fluctuating loads on each blade cause fatigue in the system and subsequently, failure leading to increased maintenance costs and unnecessary downtime. In this study we numerically analyze the effects of yaw misalignment on complete rotating wind turbines with blades of O(5m) in length. We consider two wind speeds at rated and above-rated regimes, where the effect of yaw misalignment is more pronounced. For the baseline configuration comparisons are made with the existing experimental data. To mitigate the resulting unsteady aerodynamic loading, we apply synthetic-jet based fluidic actuation in order to achieve fast-time response (in contrast to traditional yaw control strategies). O(5-10) jets are placed along the outer half of blade span. Along the chord two jet locations (x/c = 0.05 and 0.40) are considered. Actuation strategies for jets are based on partial loop control with pulse modulation. All simulations are based on unsteady Reynolds-averaged Navier-Stokes (URANS) equations. [Preview Abstract] |
Tuesday, November 20, 2012 9:44AM - 9:57AM |
M25.00009: Feasibility Study of Using Gurney Flaps for Flow Control of Wind Turbine Blades Pourya Nikoueeyan, Andrew Magstadt, John Strike, Jonathan Naughton Unsteady wind turbine aerodynamics due to atmospheric unsteadiness and rotation of the blade through a shear layer are phenomena that exceed the rate at which conventional blade pitch control mechanisms operate. Depending on the location on the blade, these rapidly varying effects can cause reduced aerodynamic efficiency, stall, unwanted oscillatory loads, and accompanying deflections. In this study, actively controlled Gurney flaps are investigated as a practical solution for alleviating these effects. Because of recent growth of the use of flatback airfoils in the root section of wind turbine blades, a DU97-W-300 derived flatback airfoil has been used in this study. The effect of flap height on lift and moment in static and dynamic conditions has been investigated by means of time-resolved pressure measurements. Static results show $\pm$30\% changes in the section lift coefficient $C_l$ with the Gurney flap, indicating sufficient authority for active control. An aeroelastic model based on the flatback airfoil geometry and correlations derived from the experimental Gurney flap results has been developed. Using a feedback control algorithm, the model indicates that a step disturbance can be quickly damped using closed-loop control of the Gurney flap. [Preview Abstract] |
Tuesday, November 20, 2012 9:57AM - 10:10AM |
M25.00010: Creation and Optimization of Compliant Flow for an Existing Wind Turbine Rotor Geometry Theodore Williams, Thomas Corke, John Cooney A compliant flow is created on a wind turbine rotor through geometric optimization in order to make it more susceptible to active flow control. Feasible designs were limited to ones that can be implemented without permanent modification to the existing geometry. Computational fluid dynamics and quantitative optimization methods are employed to evaluate different design families that incorporate plasma flow control. Designs that resulted in the largest left control authority are presented. The application of these active lift designs on a horizontal wind turbine is discussed. [Preview Abstract] |
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