High-Frequency Micro-Vortex Generators

With SBIR Phase I and II funding from Air Force Office of Scientific Research, FlexSys has developed two compact, high-frequency vortex generating systems for active flow control. Both devices can be deployed either statically or can oscillate.

Movies

An array of 16 vortex generators operating at low speeds. Each vortex generator consists of a blade mounted on an amplifier, which in turn is coupled with a piezo stack. (.wmv format) (308 KB)

Voice-Coil-Actuated System	Piezo-Actuated System
Actuator The voice-coil actuator is integrated with a 20X compliant displacement amplifier. The system produces 5 mm output displacement at 80Hz. It operates between 0-240 Hz.	Actuator The piezo-actuated system produces a maximum deflection of 2.97 mm and a maximum block force of 4.1 N across a frequency range from 0-300 Hz. The image illustrates an array of eight actuator-amplifiers with a total of 16 vortex generators blades (two blades per actuator).
In-Situ The system was installed in a wing with a rounded leading edge, variable angle trailing edge flap, and a formed pocket located just forward of the trailing edge flap to house the vortex generation hardware.	In-Situ The piezo-actuated system was installed in a 24 inch chord, 48 inch span PE 1423 test section, a modern trailing edge separation airfoil that had been tested previously. Sixteen active vortex generators were located on the upper leading edge surface of the model. The pitch of the model was electronically controlled by two brushless servo motors. The model was pitched from -5°- 18° (past stall) at a constant rate up to 25° per second. To resolve the flow effects, conventional pressure taps were used in conjunction with a high-speed pressure transducer and data acquisition system.
Test Conditions Aerodynamic testing was conducted in a 2' by 2' subsonic wind tunnel at the University of Michigan. The tunnel is an Eiffel-type open return facility capable of variable test section velocities up to 80 ft/sec.	Test Conditions Aerodynamic testing for the phase II program was conducted in the University of Michigan low speed wind tunnel. Primary flow control testing for this program was conducted in the University of Michigan 5' x 7' subsonic wind tunnel at 132 ft/sec. (Reynolds number = 1 million).

Phase I Test Results

Results of the Phase I and Phase II testing show that our vortex generators improve flow attachment on par with the best pneumatic flow control systems. Research also suggests that maintaining flow control in transonic environments is theoretically achievable with the proposed systems.

Low-speed wind tunnel testing proved that the high-frequency vortex generators increased the dynamic stall angle and increased maximum lift by 9%. Testing also verified that this effect diminishes as a function of pitch rate. Very high pitch rates (in excess of 25 degrees per second) produced approximately 3% increase in lift due to the large leading edge vortex that overwhelms the active flow devices during rapid pitching maneuvers.

This chart shows negative pressure vs. pressure tap number. The vortex generators are located 3 inches ahead of the flap. Operating the deployable vortex generators in high-frequency mode (deployment height 5mm) produced flow attachment on the forward portion of the flap upper surface, whereas none was present with the vortex generators statically deployed.

Phase II Test Results

The following figures detail a flow visualization study where tufts were placed in front of and behind the active VG system. The images show a close-up of the model leading edge and active flow system. The model begins at 11 degree angle of attack and is increase to 15 degrees. The VG system is switched on and off to demonstrate the effectiveness of the flow control capability.

Close-up of model surface with tufts positioned around the vortex generator system. The model is at 11 degrees AOA and the VG system is OFF (vortex blades retracted flush with the model surface). Note that the tufts are relatively static.

The model begins at 11 degrees AOA and the VG system is ON with the vortex blades pulsing at 75 Hz. Here, the tufts wiggle in a blurred envelope that clearly demonstrates how the VG system mixes high energy flow into the boundary layer as a result of the oscillatory vortex production. The model AOA is then increased to 15 degrees and the flow is still fully attached over the model.

The model is at 15 degrees AOA and the active VG system is switched OFF (flush to model surface). The flow immediately separates, showing flow reversal as indicated by the tufts on the model. Due to the large separation bubble that forms during stall, restoring vortex generator operation does not reattach the flow unless the angle of attack of the airfoil is lowered.

Quasi-static testing of active VG system. This plot compares the coefficient of lift from the model for three cases: VGs retracted flush with the model (VGL), VGs statically deployed (VGH) and VGs oscillating at 70 Hz (VG70). The active VG case demonstrates up to a 10% increase in the maximum coefficient of lift and stall angle of attack over no-VG and static-VG cases.

Dynamic pitch testing of active VG system at 7.5 degrees per second. This plot compares the coefficient of lift from the model for three cases: VGs retracted flush with the model (VGL), VGs statically deployed (VGH) and VGs oscillating at 90 Hz (VG90). The active VG case demonstrates up to a 10% increase in the maximum coefficient of lift and stall angle of attack over no-VG and static-VG cases.

Dynamic pitch testing of active VG system at 22.5 degrees per second. This plot compares the coefficient of lift from the model for three cases: VGs retracted flush with the model (VGL), VGs statically deployed (VGH) and VGs oscillating at 90 Hz (VG90). The active VG case demonstrates only a 3% increase in the maximum coefficient of lift over no-VG and static-VG cases as the active flow devices are overwhelmed by the leading edge vortex that forms during rapid pitch maneuvers.