This paper describes the development of a suction and oscillatory blowing (SAOB) active flow control (AFC) system, that was aimed towards achieving drag-reduction in a full-scale rotorcraft flight-test environment. The experimental work presented here includes benchtop characterization of the AFC actuators, development and testing of the AFC system using a full-scale two-dimensional airfoil at Tel Aviv University, and full scale testing of a UH-60 External Stores Support System (ESSS) in the CCDC AvMC Aviation Development Directorate 7- by 10-ft wind tunnel up to 140 knots. The airfoil tests composed of two stages, steady suction through holes on the trailing edge and the addition of pulsed blowing using SAOB actuators. The steady suction testing focused on identifying the optimal suction locations, while the SAOB experiments validated the suitability of the chosen actuator arrays to obtain the desired drag reduction at reasonable energetic cost. Tests for both steady suction and SAOB actuators resulted in 15-30% drag reduction compared to the clean baseline on the two-dimensional ESSS airfoil. Results from testing the full-scale ESSS identified the complexity of this configuration and the difficulty of transitioning directly from 2D component testing to three-dimensional testing of actual flight hardware. It became evident that by placing SAOB actuators only on the ESSS wing section, only 2-3% drag reduction was achievable. Further study is on-going to better understand aerodynamic interactions and develop a path towards more robust drag reduction.

An experimental effort was performed in order to identify and quantify the structural changes to a two-dimensional turbulent boundary layer under the influence of a high spanwise aspect ratio slot synthetic jet. A comprehensive test matrix of key synthetic jet variables including non-dimensional stroke length, jet Reynolds number, and slot angle, was conducted in quiescent air, zero pressure gradient, and a mild adverse pressure gradient turbulent boundary layer flows. Time averaged and phase averaged features of the modified boundary layer flowfield were examined using hot-wire anemometry. Velocity profiles and turbulence data measured downstream of the actuator were compared for the range of actuator parameters with the baseline turbulent boundary layers.

This work presents ongoing experiments toward developing fundamental understanding of suction and oscillatory blowing (SaOB) flow control mechanisms along with development of accurate and practical CFD simulation methodologies for complex unsteady active flow control systems. Experimental and computational studies were conducted on SaOB actuators’ internal flow and external interaction with two zero pressure gradient boundary layers. The experimental work incorporates detailed multi component hot-wire measurements of both laminar and turbulent boundary layers with steady suction and pulsed blowing for a single actuator and an array configuration. Large eddy simulation was employed for the computational model. Simulations were carried out on the actuator internal flow, and the resulting oscillatory blowing jet exit velocity profiles are characterized and fit with a functional form, in order to create simplified boundary conditions for future flow control simulations. Both experimental and computational results show that the suction hole geometry and configuration is an important factor in determining the structure and stability of the downstream laminar as well as turbulent boundary layer flow-fields. Measurements of oscillatory blowing jets interacting with a turbulent boundary layer demonstrate this flow control produces unsteady spanwise and streamwise vorticity components that can be interpreted as counter-rotating streamwise vortex patterns.

An active-flow-control study, using steady suction-and-oscillatory-blowing actuators, was conducted on an axisymmetric bluff-body model for a range of Reynolds numbers between 2 x 10(6) and 5 x 10(6). Previous work on the same model demonstrated the experimental implementation and efficient drag reduction of the suction-and-oscillatory-blowing actuator system, including comparisons to computational fluid dynamics results. The current study presents a detailed analysis of the experimental data, coupled with a refined computational model toward a flow-physics understanding of the drag-reduction mechanisms of the suction-and-oscillatory-blowing active-flow-control system. The boundary-layer response was examined using time-averaged and phase-averaged hot-wire measurements conducted on the aft portion of the model where active flow control was applied. The drag-reduction behavior was scaled using multiple active-flow-control parameters associated with the unique and complex features of the suction-and-oscillatory-blowing active-flow-control system. The results show that the drag-reduction mechanisms associated with the suction-and-oscillatory-blowing actuation system include boundary-layer suction, wall-jet momentum addition, unsteady shear-layer excitation, the generation of thrust, and streamwise vortices.

aft portion of the model. The drag reduction behavior was scaled using multiple AFC parameters associated with the unique features of the SaOB actuators. Results show that the drag reduction mechanisms associated with the SaOB actuation system include boundary layer suction, wall-jet momentum addition, unsteady shear layer excitation, thrust, and streamwise vortices.

A flow-control study using steady suction and pulsed blowing in close proximity was conducted on an axisymmetric bluff body at length-based Reynolds numbers between 1.0 and 4.0 x 10(6). The study included a coupled incremental computational-fluid-dynamics and experimental approach. It began with computations of various model setup designs. Subsequently, flow-control experiments and computations were used to optimize steady suction alone. Finally, flow control was provided by a synchronized array of 28 suction and oscillatory blowing actuators, positioned slightly upstream of the baseline separation. Results show suction alone has a limited ability to delay separation and reduce drag on this geometry. Suction located far from the baseline separation is shown to actually increase drag. Addition of pulsed blowing enables separation delay to the trailing edge and drag to be nullified. Increased overall system efficiency, including estimated total actuator power invested, was found at low momentum input for optimally located steady suction and pulsed blowing. This was partially attributed to the particular geometry used because the active flow-control system shows a robust ability to delay separation. Not all measured trends were predicted by computation due to the complex nature of this configuration and the active flow-control system characteristics.

A new scaling for strong adverse pressure gradient (APG) turbulent boundary layers (TBL) is presented. The new scaling is applied to both the author’s unsteady and steady APG TBL experiments as well as several previously published studies. The scaling is shown to provide a remarkable collapse of the mean velocity profiles in each case. The new scaling is motivated by the recognition that the physics of the strong APG TBL is dominated by the inflectional instability of an embedded shear layer within the boundary layer. The implications of the scaling on the physics of APG TBL flows in general is also discussed.

An experimental study was conducted to determine the effect of single dielectric barrier discharge plasma actuators on turbulent boundary-layer separation control. Two-component particle image velocimetry and laser Doppler velocimetry measurements showed the effect of plasma actuators on ambient air, a canonical zero-pressure gradient turbulent boundary layer, and a two-dimensional turbulent boundary-layer separation from a convex ramp section. Different actuator configurations and control strategies were implemented. Spanwise actuators were operated in both steady and unsteady modes. Plasma streamwise vortex generators were also used to enhance boundary-layer mixing. Flow visualization using a high-speed camera captured the temporal aspects of the separation control process and helped discern the physical mechanisms associated with each actuation strategy. The steady spanwise actuation produced a wall jet effect that augmented near-wall momentum and reattached the separated boundary layer. The streamwise oriented actuators also showed effective control authority by creating counter-rotating vortices within the boundary layer that promote mixing of high and low momentum fluid.

This paper presents the results of a parametric experimental investigation aimed at optimizing the body force produced by single dielectric barrier discharge plasma actuators used for aerodynamic flow control. A primary goal of the study is the improvement of actuator authority for flow control applications at higher Reynolds number than previously possible. The study examines the effects of dielectric material and thickness, applied voltage amplitude and frequency, voltage waveform, exposed electrode geometry, covered electrode width, and multiple actuator arrays. The metric used to evaluate the performance of the actuator in each case is the measured actuator-induced thrust which is proportional to the total body force. It is demonstrated that actuators constructed with thick dielectric material of low dielectric constant produce a body force that is an order of magnitude larger than that obtained by the Kapton-based actuators used in many previous plasma flow control studies. These actuators allow operation at much higher applied voltages without the formation of discrete streamers which lead to body force saturation.