Rene Woszidlo

Parameters Governing Separation Control with Sweeping Jet Actuators

Parameters governing separation control by sweeping jet actuators are investigated experimentally on a generic “Multiple Flap Airfoil” (MFA). Neither the flow rate nor the momentum input is found to be a sole parameter governing the lift for varying distance between adjacent actuators. However, the product of the mass flow coefficient and a square root of the momentum coefficient collapses the lift onto a single curve regardless of the actuator spacing. Surface flow visualization on the flap suggests the formation of counterrotating pairs of streamwise vortices caused by the interaction of neighboring jets. The actuation intensity required to attach the flow increases with increasing distance from the flap shoulder and increasing flap deflection. No obvious dependence of the ideal actuation location on flap deflection, angle of attack, or actuation intensity is found within the tested range. Comparisons between experimental and numerical results reveal a strong dependence on the thickness of the last flap segment at its hinge. In absence of this geometrical effect potential flow solution appears to be a suitable predictor for the obtainable lift. The flap size affects the achievable lift, the accompanying drag, the required flap deflection, and actuation intensity. By controlling separation the range of achievable lift coefficients is doubled without significant penalty in drag even when considering a safety margin for the maximum applicable incidence.

Parametric study of sweeping jet actuators for separation control

In this paper, studies of separation control over a generic Multiple Flap Airfoil (MFA) using sweeping jet actuator arrays are presented. These jets, exiting from millimeter-scale nozzles, oscillate from side-to-side in a sweeping manner similar to windshield wipers and are henceforth referred to as sweeping jet actuators. Different flap sizes and flap deflections up to 45 o were investigated in the experiments. The MFA, with integrated rows of sweeping jet actuators at several chordwise locations on the flaps, enabled an extensive variation of geometrical and fluid dynamical parameters to study separation control. The effect of flap size, actuator location and actuation parameters on the lift and drag coefficients of the airfoil are discussed.

Single dielectric barrier discharge plasma actuators for improved airfoil performance

The applicability of single dialectic barrier discharge plasma actuators for use as active flow control devices, capable of enhancing the performance of airfoils, was assessed in this investigation. Measurements were carried out on two thick airfoils with simple flaps: a NACA0021 and an airfoil that is similar to those commonly used on tiltrotor aircraft. The chord length of the airfoils was approximately 0.3 and 0.25 m, respectively, and the span was approximately 0.6 m. They were both tested in the same wind tunnel with a test section of 0.6 x 1.1 m. Freestream velocities varying from 5 to 15 m/s were tested, corresponding to chord Reynolds numbers ranging between 0.8 × 10 5 and 3 × 10 5 . The lift, moment, and form drag were obtained from the pressure distributions over the airfoils surface, and the total drag was calculated from a wake survey. The range of incidence angles α varied from ―4deg <α < +20 deg and flap deflections δ f of 0 and 15 deg were tested. The location of the actuation was also altered. Two data sets are presented: one in which the actuator was placed at approximately 5 % of the chord and the other in which it was located just upstream of the flap shoulder at a chord location corresponding to about 75 %. The momentum input of the single dialectic barrier discharge plasma actuators was measured with a hot wire and was in good agreement with previously published results. The input momentum is very weak and is not sufficient to prevent separation at Reynolds numbers greater than 100,000. The single dialectic barrier discharge plasma actuators used in this study may only provide sufficient momentum to be effective at very low Reynolds numbers, such as those appropriate to micro air vehicles. Under special circumstances, their passive presence on the surface may trip the boundary layer, making it more resistant to separation, but in those cases, a proper roughness strip or vortex generators may delay separation more effectively.

Experimental Study on Bluff Body Drag Reduction with Fluidic Oscillators

In this study fluidic oscillators are examined for separation control purposes over base flaps attached to a three-dimensional bluff body which resembles the rectangular shape of a tractor-trailer model. Fluidic oscillators continuously emit a high velocity jet which is spatially oscillating along the flaps’ span at high frequency. These flow control actuators successfully prevent flow separation over the base flaps, thereby significantly reducing the model’s drag. Even when accounting for an overestimated momentum input, the total drag is reduced by 16% without any system or setup optimization. It was found that the drag reduction improves when increasing the flaps’ length at a constant actuation level. Furthermore, a larger spacing between adjacent actuators reduces the minimal momentum input required to attach the flow. Instead of the momentum coefficient, the velocity ratio governs the actuation intensity for changing actuator spacing. A velocity ratio of five yields the most efficient results. Some qualitative flow field observations demonstrate the actuators’ effectiveness in preventing flow separation over the entire flap. Additionally, vortical structures are observed in the vicinity of the jets’ exit, which are suggested to be the reason for the oscillators’ superior performance.

Discrete Sweeping Jets as Tools for Separation Control

Experiments aimed at delaying flow separation through discrete jets pointing in the direction of streaming and sweeping side-to-side along the span were conducted on two airfoils, a NACA0015 and a V-22 airfoil with and without deflected trailing edge flaps. The results indicated substantial drag reduction and lift increase at moderately low inputs of mass and momentum. Additional experiments were carried out on a semi-span V-22 wingnacelle combination and they too provided an increase in L/D of approximately 60%, even with the presence of nacelle drag and induced drag. The effectiveness of the sweeping jets on reducing the download force acting on a V-22 powered model in hover was also examined. A 29% reduction in download was realized using the embedded sweeping jets.

Phase-Averaging Methods for the Natural Flowfield of a Fluidic Oscillator

The presented study examines various methods for phase averaging the naturally oscillating flowfield of a scaled-up fluidic oscillator. No external trigger is employed to control the oscillation of the flow. Mathematical and signal conditioning approaches for phase averaging the data are categorized and described. The results of these methods are evaluated for their accuracy in capturing the natural flowfield. The respective criteria are based on the minimum fluctuation in oscillation period length, the conservation of velocity amplitudes, and the number of snapshots per phase-averaging window. Although all methods produce reasonable qualitative results, only two methods are identified to provide the desired quantitative accuracy and suitability for the investigated flowfield. The first method is based on conditioning a time-resolved pressure signal from the feedback channels in the oscillator. An autocorrelation applied to the reference signal improves the period identification. The second method employs...

Manipulating the Flow over Spherical Protuberances in a Turbulent Boundary Layer

Means of controlling the flow over a large spherical protuberance were examined. The role of suction around the base of the protuberance in reducing or even eliminating the necklace vortex created by the protuberance was considered. In the absence of suction, this vortex was lifted by the low base pressure existing behind the protuberance into the wake, thus affecting the turbulence level along its path. Large vortex generators placed upstream of the protuberance were able to delay local separation of the flow over the protuberance, thus affecting the symmetry of the wake and the level of turbulence on one side or the other. Observations made using flow visualization were supplemented by hot-wire measurements. The experiments were carried out at low speed at Reynolds numbers that did not exceed 3 x 105.

Phase-Averaging Methods for a Naturally Oscillating Flow Field

The presented study examines various methods for phase-averaging the naturally oscillating flow field of an enlarged fluidic oscillator acquired by a high-speed PIV system. Because of the absence of an external trigger, phase-averaging the acquired data is challenging. Mathematical and physical methods are categorized and described. The results of these methods are evaluated for their accuracy in capturing the natural flow field. It is found that the mathematical methods, especially the method of proper orthogonal decomposition, produce reasonable qualitative results. However, compared to the physical methods, shortcomings in quantitative accuracy are revealed. The physical methods require a time-resolved reference signal. Two possibilities to identify the oscillation periods in the reference signal are described and compared. It is found that applying an autocorrelation on the reference signal improves the period identification due to consideration of a locally fluctuating mean value. This period identification method and according phase-averaging yields the best results regarding the minimum fluctuation of the oscillation period lengths. The according procedure is described in detail and applied to the internal and external flow field of the fluidic oscillator.

Numerical Modeling and Validation of the Flow in a Fluidic Oscillator

A fluidic actuator is a device, which only needs one fluid supply to generate a self-induced and self-sustaining oscillating jet at its outlets. The present study investigates numerically the flow dynamics of a fluidic oscillator operated with water. Simulation results are validated with experimental data obtained with PIV and time-resolved pressure measurements. The numerical simulations are based on unsteady Reynolds-averaged Navier-Stokes equations (URANS) considering a turbulent, incompressible, and isothermal flow. Beforehand, a sensitivity analysis regarding the turbulence closure, the spatial grid solution, and the outlet geometry was conducted. In addition, to gain a deeper understanding of the flow dynamics a modal analysis is provided. It was found that the two-dimensional simulation employing the SST was sufficient to describe the flow field and dynamics qualitatively as well as quantitatively. However, nonlinear effects could only be observed in the threedimensional computations.

Fluidic Oscillators for Bluff Body Drag Reduction in Water

The presented work examines the application of fluidic oscillators for active flow control in water. The oscillators are installed into a bluff body to reduce the pressure induced drag force. Experiments in water are accompanied by challenges such as cavitation and scaling effects. However, the properties of the investigated oscillator design agree with other publications with air as working fluid. Therefore, the transferability between different working fluids can be used for parametrization and scaling. Cavitation is detected at the external nozzle by means of hydrophone measurements. No significant influence on the natural oscillation of the fluidic oscillators and on the efficacy of the actuation is detected. The measured momentum agrees well with theoretical values. Based on these bench tests, the feasibility of the actuation is proven by towing tank measurements. The net drag of the bluff body is considerably reduced by the actuation. The drag improvements agree well with previous wind tunnel experiments on the same model.