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In previous work at the U.S. Air Force Academy, the phenomenology and behavior of the aerodynamic plasma actuator, a dielectric barrier discharge plasma, was investigated. To provide insight into the phenomenology associated with the transfer of momentum to air by a plasma actuator, the velocity distributions upstream and downstream of a plasma actuator with an induced boundary layer were measured using freestream velocities of approximately 4.6 and 6:8 m/s for a range of frequencies (5-20 kHz) and voltages (5-10-kV amplitude). The body forces on the air were calculated using a control volume momentum balance. In a second experiment, time-averaged results were also obtained by measuring the reaction force using a pendulum. A third experiment uses an accelerometer to gain insight into the time-dependent forces or, more specifically, the direction of the forces. The results show that the body force acts within the first 4 mm above the surface of the actuator (within the boundary layer). For a constant peak-to-peak voltage, the body force is proportional to frequency, producing a constant impulse per cycle, and the energy dissipation per cycle and efficiency are independent of frequency. The time-dependent measurements support the theory that the body force of the actuator consists of one large push followed by one small pull during each cycle. [ABSTRACT FROM AUTHOR]

A thermal tuft method for the measurement of low wall shear stresses is described. Previous work described how the thermal tuft method can be used for flow visualization, but this work extends the technique to quantitative measurements of low values of wall shear stress. The laser thermal tuft involves heating a spot on a surface with a laser, which produces a teardrop-shaped surface temperature distribution pointing downstream of the heated spot. The temperature profile can be determined with liquid crystals, infrared thermography, or other methods. In the present study, it is demonstrated by theory and experiment that the lengths of these teardrop-shaped tufts are determined by the wall shear stress. Theoretical results evaluate the effects of laser power, laser spot size, and liquid crystal cutoff temperature on the tuft length. Effects of the thermal boundary-layer thickness are evaluated and found to be negligible for heights up to half of the hydrodynamic boundary-layer thickness. Experimental results agree with theoretical predictions, indicating shorter tuft lengths as shear stress increases. Thermal tufts can be used to measure the wall shear stress at multiple locations, thereby mapping out the wall shear stress distribution. [ABSTRACT FROM AUTHOR]