For the purpose of investigating the time series variation of the flow driven by plasma actuator (PA), flow visualization has been conducted and the flow separation height from airfoil was evaluated using image processing. The experiments were conducted on NACA0012 airfoil at angle of attack 1.5^º and a chord Reynolds number of 50000. The PA was mounted on the airfoil and input two types of burst modulation. Firstly, the step input of burst modulation was applied in order to investigate the transient phenomena of the flow driven by PA. The flow separation height was investigated in the time range immediately after starting and stopping the burst modulation. The flow separation height was smoothed by simple moving average, and the smoothed data of the height demonstrated similar configuration to the step response. Secondly, the PA was driven by alternatively repeating a fixed number of burst wave and a fixed period of stopping. Under certain conditions the smoothed data of flow separation height becomes similar to square wave response of first order system.

Acoustic characteristics of stationary hybrid rockets were investigated experimentally. The test grain was set and operated horizontally, and the acoustic radiation was acquired using free field microphones and flush-dish microphones set along an arc at 8m from the nozzle exit. In the sound spectrum, low frequency noise that considered to be caused by the combustion oscillation, the air-column oscillation and so on were superimposed onto the turbulent mixing noise. The combustion noise was presumably due to insufficient differential pressure between the combustion chamber and the injector. Periodic modulation was also observed in the sound spectrum, that caused by the ground reflection of sound ray, which shown to be canceled using the flush-dish microphone. By using spectral fitting, the combustion noise is eliminated from the spectra. While the exhaust velocity of the hybrid rocket is on the order of 1,500 m/s which is even larger when compared with supersonic aircraft engines, the spectrum and radiation angle characteristics for the turbulent mixing noise exhibited a reasonable continuity with the jet noise data for subsonic/supersonic aircrafts. And the overall sound pressure level is intermediate between the aircrafts and rockets.

Self-oscillation of a shock wave, which is called a transonic shock buffet, occurs on a wing surface in a transonic flow. Lee and Crouch explained the self-sustained mechanism by different pressure wave propagations but there is no experimental evidence of which feedback loop is dominant. The principal aim of the present study is to identify the dominant feedback path of pressure waves related to the shock buffet using high-speed focusing schlieren movies. The experimental data showed that the flow fields include complicated phenomena such as a lot of pressure waves with a broad range of frequency. Therefore, we performed dynamic mode decomposition (DMD) analysis to extract dominant phenomena for the buffet. DMD modes whose frequencies corresponded to the shock oscillation frequencies were extracted and investigated. DMD modes revealed that pressure perturbations generated at the shock foot and the perturbation propagated to the height-wise direction along with the shock wave. This feedback pass is similar to that observed by Crouch using the global stability analysis for NACA 0012 airfoil.

This paper considers the effect of wind on stability derivatives of longitudinal equations of motion of an aircraft and estimating steady wind by estimating the stability derivatives using an algorithm such as the least squares method (LSM). The wind defined in the inertial frame appears in the derivatives concerning the pitch angle in the linear form and is computed from the estimated derivatives. The numerical results computed for NASA's Generic Transport Model (GTM) verify the linear model in the presence of steady wind. The simulation results obtained for GTM using the LSM and an extended Kalman filter indicate that although the vertical wind has a significant estimation error, the horizontal wind is estimated well despite considerable estimation error of the stability derivatives. We also examined the reason analytically and numerically.