Measuring Vortex Breakdown Over* a Delta Wing with a Laser Doppler *Velocimeter
The flow over a delta wing is a vortex dominated flow field. The delta wing owes much of the lift that it is able to generate a subsonic speeds to the fact that the flow separates at the leading edge of the wing. This separated shear layer rolls into a large vortex over the leeward side of the wing. This vortex also has associated with it a large axial flow component. This high flow velocity in the core of the vortex is a region of low pressure and this low pressure generates a suction or lift. At high angles of attack, a phenomenon know as vortex breakdown occurs in the vortices above the wing. Vortex breakdown results in the destruction of the tight and coherent vortex. The core becomes highly turbulent, the diameter of the core increases, and the high axial velocity in the core ceases to exist. It is this last item that is expecially important, because with the loss of axial velocity the wing losses lift. This can be troubling to the pilot, especially if the breakdown is occurring over only one of the wings, as is sometimes the case. The force imbalance between the two wings can cause high roll movements. Even if the breakdown is symmetric, the loss of a large portion of the aircrafts lift can cause control problems. It is for these reasons that the vortex breakdown is being studied along with ways that the presence of breakdown can be controlled.
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This experiment was conducted in the water tunnel of the ESM Fluid Mechanics Research Laboratory at Virginia Tech. The water tunnel is capable of reaching speeds of up to 9 ft/sec with turbulence levels ranging from 0.6% to 1.5%. The top and sides of the test section is construcet of optical grade Plexiglass so that the Laser Doppler Velocimeter measurements can be taken from the top or sides of the tunnel.
Here are a set of images taken from data where the wing was set at a 35 degree angle of attack. The red color indicates areas of high axial velocity at 2.65 times the free stream velocity. The purple region is the wing itself. The slices were taken from the center of the wing, where the breakdown ocurred.
Side and Rear Views
Slices through the wing