Abstract
AbstractThe streamwise vortices generated by dielectric-barrier-discharge plasma actuators in the laminar boundary layer were investigated using particle image velocimetry to understand the vortex-formation mechanisms. The plasma vortex generator was oriented along the primary flow direction to produce a body force in the spanwise direction. This created a spanwise-directed wall jet which interacted with the oncoming boundary layer to form a coherent streamwise vortex. It was found that the streamwise vortices were formed by the twisting and folding of the spanwise vorticity in the oncoming boundary layer into the outer shear layer of the spanwise wall jet, which added its own vorticity to increase the circulation along the actuator length. This is similar to the delta-shaped, vane-type vortex generator, except that the circulation was enhanced by the addition of the vorticity in the plasma jet. It was also observed that the plasma vortex was formed close to the wall with an enhanced wall-ward entrainment, which created strong downwash above the actuator.
Highlights
The dark grey isosurface visualizes the lower shear layer of the wall jet, whilst the white isosurface shows the upper shear layer. It is the negative vorticity at the outer edge of the upper shear layer that rolls up to form the streamwise vortex
It implies that the streamwise vortex growth depends upon the wall jet length and velocity scales, not those of the external flow. These were demonstrated by Jukes & Choi (2012), who studied the vorticity with free-stream speed and the length of the exposed electrode, where the vortex circulation at a range of flow conditions collapsed when scaled with the spanwise wall jet thickness and the wall jet velocity, Γ /Wpδp. This idea can been expanded to conclude that the discharge plasma actuatorsDielectric-barrier-discharge (DBD)-Vortex generators (VGs) streamwise vortex location scales with x2/3, the circulation scales with x1/3 and the peak vorticity scales with x−1
The streamwise vortex produced by a DBD-VG has been studied to identify the vortex-formation mechanisms with a comparison with vane-type VGs
Summary
Dielectric-barrier-discharge (DBD) plasma actuators are purely electric devices which can directly couple momentum into gas flows. The concept can be traced back to Roth, Sherman & Wilkinson (1998), where symmetric, streamwise-oriented plasma actuators produced EHD forcing in both positive and negative spanwise directions to produced counter-rotating vortex pairs with common-flow down arrangement This increased the skin friction in a laminar boundary layer through promoting transition. For boundary-layer control, He, Corke & Patel (2007) studied turbulent separation over a ramp model, which is possibly the earliest reference to plasma VGs. Flow visualization revealed that symmetric streamwise actuators generate counter-rotating vortices similar to vane-type VGs, which could improve the pressure recovery over the ramp.
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