Investigation of the boundary layer characteristics for assessing the DBD plasma actuator control of the separated flow at low Reynolds numbers

Abstract

The present study intends to investigate the boundary layer characteristics to assess the potentiality of the single dielectric barrier discharge plasma actuators (SDBDPAs) to reattach the separated flow at low Reynolds numbers. The effect of the actuator geometrical parameters and of the Reynolds number on the device control authority was experimentally investigated. For this aim, a curved wall plate, which profile shape was designed to reproduce the suction surface of a low-pressure turbine (LPT) blade, was installed in the test section of a closed loop wind tunnel and a groove was made over it, at the front of the adverse pressure gradient region, for allocating a SDBDPA. Three actuators, characterized by different streamwise width, were manufactured by photolithography technique and they were tested. The velocity flow field, in both presence and absence of external flow, was investigated by particle image velocimetry (PIV) measurements. When the actuator was turned on, a sinusoidal voltage excitation with amplitude of 8kV and frequency of 2kHz was applied and the dissipated power (P‾el) was retrieved by electrical characterization.The effect of the active flow control was firstly estimated by analyzing the plasma induced velocity fields in absence of external flow. Subsequently the wind tunnel inlet free stream velocity (vx,∞in) was set to 1.54m/s. The velocity, turbulence intensity (Tu) and vorticity (ωz) fields together with the boundary layer shape factor (H12) and momentum coefficient (cμ) were evaluated in both presence and absence of actuation. All the aforementioned analyses together with the estimation of the device electrical-to-fluidic energy conversion efficiency (ηfm) allowed identifying the best actuator geometry. Then, that configuration was chosen to investigate the effects of the wind tunnel velocity on the device control authority. The tested vx,∞in values ranged from 1.54m/s up to 3.16m/s. In absence of actuation, a large reverse flow and high turbulence intensity was observed in the separation region. Considering the actuated cases, it was found that at P‾el≈7W, the SDBDPA operation always implied a reduction of the separated region, of the flow angle and of the turbulence intensity. Moreover, the plasma induced jet had a larger impact on the flow at lower velocities and a low flow control effect was noticed at the highest vx,∞in values. The H12 factor evaluation confirmed the flow regimes at the different tested velocities (i.e. cμ values). The whole data set allowed to evaluate the actuator success for separation control and to identify a threshold value of the cμ coefficient delimiting the still detached flow from the reattached one.