Experimental measurements of the mean velocity profiles produced by axially symmetric turbulent boundary layers on cylinders of various diameters are described. The profile measurements were made with very small hot wires developed for this investigation. Measurements of the wall shear stress on cylinders ranging from 0.02 to 2.0 in. in diameter are also reported. In the boundary layer on cylinders, well-defined regions exist in which the two-dimensional law of the wall and a three-dimensional wake law are valid. There was no evidence that the boundary layer was not fully turbulent even on the cylinders of smallest diameter. Measurements of wall pressure fluctuations beneath the boundary layer on a 1 in. diameter cylinder are also described. The results were much the same as those previously reported by Willmarth & Yang (1970) for a 3 in. diameter cylinder. The only difference was the discovery that the wall pressure was correlated in the transverse direction approximately half-way around the cylinder. This was not true on the 3 in. diameter cylinder.
An experimental study aimed at the characterization of energy deposition of nanosecond Dielectric Barrier Discharge (ns-DBD) plasma actuators was carried out. Special attention was given on the effect of the thickness and material used for dielectric barrier. The selected materials for this study were polyimide film (Kapton), polyamide based nylon (PA2200), and silicone rubber. Schlieren measurements were carried out in quiescent air conditions in order to observe density gradients induced by energy deposited. Size of heated area was used to qualify the energy deposition coupled with electrical power measurements performed using the backcurrent shunt technique. Additionally, light intensity measurements showed a different nature of discharge based upon the material used for barrier, for a fixed thickness and frequency of discharge. Finally, a characterisation study was performed for the three tested materials. Dielectric constant, volume resistivity, and thermal conductivity were measured. Strong trends between the control parameters and the energy deposited into the fluid during the discharge were observed. Results indicate that efficiency of energy deposition mechanism relative to the thickness of the barrier strongly depends upon the material used for the dielectric barrier itself. In general, a high dielectric strength and a low volumetric resistivity are preferred for a barrier, together with a high heat capacitance and a low thermal conductivity coefficient in order to maximize the efficiency of the thermal energy deposition induced by an ns-DBD plasma actuator.
An experimental investigation was conducted about the induced velocity and density gradients due to the effect of a nanosecond pulsed dielectric barrier discharge (ns-DBD) plasma actuator on a flat plate boundary layer. A particle image velocimetry (PIV), with a magnification about 1.26, was set-up in order to carry out this study. The tested plasma actuator was made out of a 2 layers Kapton tape barrier, with non-overlapping copper tape electrodes. It was mounted in a dedicated groove which allowed the tested actuator to be flush to the wall of a flat plate. Moreover, the flat plate was furnished with a super-elliptical leading edge so to produce a small positive pressure gradient meant to damp down instabilities due to surface aberrations and flow imperfections. Investigated parameters were the position of the covered electrode with respect to the flow direction and the energy input. Back-current shunt technique was applied in order to monitor the pulse discharged and to calculate the energy input. Results of discharge bursts of 50 pulses at 10kHz indicated that effects induced by a ns-DBD plasma actuator had a strong dependence on the stream wise location of the covered electrode with respect to the direction of the flow. When the covered high voltage electrode was in upstream position a region of about 0.5m/s decelerated flow was observed. On the contrary, when it was in downstream position a much thinner region about 0.5m/s accelerated flow was observed, highlighting the presence of a small directional body force. Moreover, density gradients were calculated resolving the compressible continuity equation implemented on a backward-time and backward-space finite difference discretization scheme. Results suggest that the area where density field is affected is larger for a covered electrode placed in upstream position. Moreover, a superposition of body force and density gradient could play a significant rule in optimization process of flow control strategies based on ns-DBD plasma actuators.
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