|
Current Projects
Flow Separation Control
At the Fluid Mechanics Research Laboratory there are two concurrent efforts underway to study the control
of flow separation. The first effort, being conducted in conjunction with NASA Ames Research Laboratory in Moffett Field,
California, studies the effects of microjet control on a dynamically pitching airfoil. Microjets, 400 micrometers in diameter,
are placed in a field (similar to an air hockey table) on the upper, forward surface of a NACA 0015 airfoil.
|
The microjets are pressurized while the airfoil is being sinusoidally pitched from 0° to 20° of attack in a
compressible, subsonic flow (Mach 0.3-0.4). This control scheme allows for manipulation of the occurrence of dynamic stall by
preventing or delaying the formation of the dynamic stall vortex. This has the effect of controlling the time and strength of
the occurrence of dynamic stall, which thus controls the strength of the nose-down pitching moment that is so detrimental to the
lifespan of helicopter rotor blades. Using a “Z”-type Schlieren Optical System modified for Point Diffraction Interferometry (PDI)
this control scheme and its effects are well documented. Using the interferograms numerous quantities can be ascertained about
the flowfield as well. Later efforts have used high frequency pressure transducers on the airfoil surface to directly measure
surface pressures and to verify quantities calculated from the interferograms.
|
Flow over a NACA 0015 airfoil
|
The second project, with supports from NASA and AFRL, aims at adapting the microjet for real-time separation control.
The study employs microjet on a backward facing Stratford ramp, geometrically similar to the serpentine inlets of diffusers, for
controlling flow separation. The study is being conducted in the closed subsonic wind-tunnel facility of AAPL, which boasts speed
up to 70m/s with excellent optical accessibility from all the sides.
|
Experimental data obtained using this wind-tunnel testing provides a comprehensive database for modeling
flow patterns based on different set of initial and boundary conditions and subsequently develop flow control schemes. The
present control scheme which employs microjets (400μm) has already been tested to attach the separated flow in its entirety
with very low mass flux. Using these microjets, which are very simple and easy to implement because of their small sizes,
the undesirable effects of separation including aerodynamic stall and unsteady loading on the compressor blades can be eliminated,
thus extending the operating range of aircraft navigation and preventing severe deteriorations on compressor and fan blades.
A sample result of the stream-wise component of the flow field is shown below.
|
Wind Tunnel Test Section
|
 
Streamwise Velocity Flowfield:
a) No control b) Microjet controlled
In addition to developing an active control device, we are also investigating the physics behind it using latest
techniques like Particle Image Velocimetry (PIV). The pronounced effect of microjets as observed is primarily due to generation of
stream-wise vortices when it interacts with the cross-flow, a phenomenon that was widely been exploited in separation control. Recent
efforts to explore and reconstruct this flow and its control using surface pressure is being accomplished using synchronized velocity
field and high frequency unsteady surface pressure measurements.
 
Evolution of a simple jet - Streamwise Vorticity Field:
a)Baseline b)Microjet controlled @ (0,0,0)
|
