Arvind Santhanakrishnan

Flow control with plasma synthetic jet actuators

This paper presents an experimental investigation of the characteristics of a plasma actuator design for flow control consisting of an annular electrode in quiescent and flat plate boundary layer flows. In quiescent flow, the circular plasma region produced on actuation was observed to generate a vertical zero-net mass flux (or synthetic) jet, hence the name plasma synthetic jet actuator, the characteristics of which were found to be affected by the actuator operation mode (steady or unsteady). Pulsed operation of the actuator results in the formation of a starting vortex ring that advects ahead of the jet and secondary vortex rings near the actuator surface due to the additional plasma-induced fluid entrainment in the boundary layer. By varying the actuator pulsing frequency, multiple vortex rings were created in the flowfield and the resulting vortex ring interactions were found to increase both the peak velocity and streamwise extent of the jet. The interaction of the actuator with a crossflow was observed to be similar to that seen in conventional or non zero-net mass flux jets with the plasma synthetic jet penetrating into the mean flow. As expected, the influence of the jet on the freestream was found to decrease with increasing mean velocity and the impact on displacement and momentum thickness values diminishes as well.

Flow Control Using Plasma Actuators and Linear/Annular Plasma Synthetic Jet Actuators

This paper investigates the use of dielectric barrier discharge plasma actuators in low Re flow control applications. Three different actuator geometries have been tested: a conventional design using two rectangular strip electrodes (the linear actuator) that produces a nearly two-dimensional horizontal wall jet upon actuation, and two new designs that render the plasma induced flow in the form of a vertical jet that can be either three-dimensional using an annular electrode array actuator construction the plasma synthetic jet actuator, PSJA or nearly two dimensional using a modified linear actuator construction consisting of two exposed electrodes and one embedded electrode, the L-PSJA. The modification in actuator design can be used to broaden its applicability and enhance the flow control effects. 2-D PIV measurements are used to characterize the operation of these actuators in quiescent flow, a flat plate boundary layer, and flow over a circular cylinder. In quiescent flow, these actuators add momentum to the residual fluid with significant velocity fluctuations. The interaction of the plasma induced flow with a mean flow is shown to vary with the actuator geometry. The PSJA and L-PSJA geometries enhance the penetration of the plasma induced jet as compared to the linear actuator. The actuators act as an active boundary layer trip, the effectiveness of which is seen to decrease with increasing freestream velocity. While the PSJA affects the global flowfield, the L-PSJA and linear actuator affect primarily the near wall region. The linear actuator is observed to be a better configuration for flow control on a circular cylinder as opposed to the L-PSJA.

On Plasma Synthetic Jet Actuators

The term plasma actuator refers to an asymmetric arrangement of two electrodes (typically rectangular strips) separated by dielectric material that can be used as active flow control devices. A plasma actuator design consisting of an annular electrode array, the plasma synthetic jet actuator (PSJA), is experimentally investigated in this paper. This particular geometry creates a zero-net mass flux (or “synthetic”) jet upon actuation, and can be operated in a pulsed or steady manner for flow control or thrust generation. Unlike synthetic jets, the actuator configuration can be reversed to act as a suction device. 2-D PIV measurements are used to characterize the actuator mounted on a flat plate in quiescent flow. Pulsing the actuator results in formation of three counter-rotating vortex rings: a starting vortex ring that advects downstream ahead of the jet, a secondary vortex ring that is found to be “trapped” during the actuation phase, and a weak strength tertiary vortex ring created as a result of fluid entrainment in the boundary layer. Examination over a range of frequencies reveals varying values of peak jet velocity and momentum distribution based upon interactions of the starting vortices. The effects of changing pulsing frequency on the jet characteristics are discussed. Preliminary observations on a PSJA used for suction are also presented.

Effect of Regular Surface Perturbations on Flow Over an Airfoil

This paper presents an investigation on the effect of introducing large-scale roughness through static curvature modifications on the low speed flow over an airfoil. The surfaces of a standard Eppler 398 airfoil have been modified with regular perturbations or “bumps” of the order of 2%c for this purpose. While the actual E398 airfoil is not a suitable candidate for low Re cases due to extensive prevalence of boundary layer separation, it is expected that the bumps would exercise passive flow control by promoting early transition to turbulence, thereby reducing the extent of separation and improving the performance. The fluid dynamic mechanism behind this separation control methodology is not clearly understood, however. Smoke-wire flow visualization is performed for qualitative observation of the separation region in both the perturbed and unmodified airfoil geometry cases. At higher Re values, pressure probe measurements are made to quantify the wake momentum deficits. Unsteady 2D PIV measurements are employed to understand the near-wall flowfield behavior . The size and strength of vortical structures formed in the separating shear layer are examined, along with measurements on the laminar separation bubble. All the experiments are conducted for chord based Re values ranging from 25,000 to 500,000.

Enabling Flow Control Technology for Low Speed UAVs

This paper discusses several technologies currently being investigated for use on low speed UAVs. This includes adaptive wing technology that controls separation via active changes in the camber, use of large scale surface roughness to control separation, plasma actuators to control stall and enhance lift, and inflatable wings that can be tailored using wing warping. Results from wind tunnel and flight tests from each of the systems is presented.

Effect of Plasma Morphology on Flow Control Using Plasma Synthetic Jet Actuators

The plasma synthetic jet actuator (PSJA) is a geometric variant of a plasma actuator, consisting of an annular electrode array that results in a circular region of dielectric barrier discharge plasma. In quiescent conditions, it is observed that this plasma region drives the residual fluid in the form of a vertical zero net mass flux (synthetic) jet that is useful for flow control applications, particularly separation reduction. Pulsatile operation of the PSJA results in formation of multiple counter-rotating vortical structures in the flowfield, resembling conventional synthetic jet actuators. The velocity and momentum of the resulting jet are found to be aected by the input power, embedded electrode diameter, pulsing frequency and structure of the plasma region. This paper specifically investigates the effect of variation in plasma morphology on the isolated jet characteristics. Three distinct morphological states are examined for both steady and pulsed operation of the actuator. PIV based measurements of the flowfield are used in conjunction with time exposed images captured by a CCD camera to relate non-dimensional relative plasma intensities to the jet characteristics. It was found that for the particular actuator investigated herein, increasing the intensity and extent of the plasma resulted in improving the plasma induced jet velocity and momentum substantially. The eciency of the actuator in injecting momentum to the fluid was found to be aected by both the plasma intensity and pulsing frequency, with the former being the most critical factor.

Formation and Scaling of Plasma Synthetic Jet Actuators

The plasma synthetic jet actuator is a plasma actuator consisting of an annular electrode array separated by dielectric material, with a larger electrode that is exposed to the atmosphere and a smaller electrode embedded on a surface. Under input of high voltage, high frequency AC or pulsed DC, a region of plasma is formed starting from the toroidal edge of the outer electrode. This plasma renders the surrounding air in the form of a round synthetic jet and pulsatile operation of the actuator results in the production of multiple three-dimensional vortical structures. In quiescent conditions, the flowfield consists of a primary vortex ring that advects ahead of the jet and spatially fixed secondary vortical structures formed on account of plasma induced boundary layer entrainment near the inner edge of the outer electrode. The peak velocity and momentum of the plasma synthetic jet are found to be strongly aected by a combination of factors, including the input power, pulsing frequency, embedded electrode diameter and plasma morphology. The formation of the plasma synthetic jet and scaling of the jet characteristics with respect to the above variables is investigated in this paper. A simple analytical scaling model is derived starting from fundamental fluid dynamics principles. The relations obtained from the above model are compared with results from experiments for validation.

Optimization and Validation of a Variable Camber Airfoil

This paper presents an investigation on the development of an adaptive wing configuration using piezoelectric actuators to vary the suction surface camber of a modified NACA 4415. Three profiles from a NACA 4415 baseline profile differing in their upper surface camber and maximum thickness have been constructed and analyzed using XFOIL. The optimum operating points have been determined with consideration to cruise angle of attack at maximum lift over drag ratio. Wind tunnel diagnostic measurements show that at low Reynolds numbers, the adaptive wing controls separation by exerting either passive flow control due to static surface curvature modification or active flow control due to dynamic oscillation of the camber. It is observed from the results of experiments and computations that at chord based Reynolds numbers less than 100,000, the stall angle of attack is increased in the case of static camber variation, while the oscillating camber case shows promising results in controlling stall. An inverse design based shape optimization routine is currently under development to determine ideal airfoil geometry for the previously determined optimum operating points. Preliminary results from the optimization routine are presented.

Unstructured Numerical Simulation of Experimental Linear Plasma Actuator Synthetic Jet Flows

This paper presents a comparison between experimental data and computational results for the linear plasma synthetic jet actuator (L-PSJA). The L-PSJA conguration consists of two exposed electrode strips separated by an embedded electrode, similar to two linear dielectric barrier discharge actuators being placed back-to-back. Experiments have shown that the resulting oweld is similar to a surface jet, and by pulsing the power to the electrodes an oscillatory oweld similar to a synthetic jet can be produced. The computational model used for the plasma forcing is the Suzen{Huang plasma actuator model which has been placed in an imcompressible, unstructured grid code. Both quiescent and crossow conditions are simulated and compared to the available experimental measurements. The objective is to assess the capabilities of the plasma actuator model for the L-PSJA conguration.