MHz-rate pulse-burst laser imaging system: Development and application in the high-speed flow diagnostics

This dissertation presents the development of a state-of-art MHz-repetition-rate high-power pulse-burst laser imaging system and its application in high-speed flow diagnostics. The Nd:YAG based laser combines high power (100 mJ/pulse @ 1.064 μm and 25 mJ/pulse @0.532 μm) and near-Fourier-transform-limit linewidth with 1 MHz pulse repetition rate. The frequency of the laser can be tuned within 1 cm−1 at 1.064 μm. Coupled with a MHz framing rate CCD camera, this pulse-burst imaging system has been successfully demonstrated to be a powerful tool in capturing unsteady supersonic flow phenomena.; The primary diagnostic technique for flow visualization is CO₂-enhanced filtered Rayleigh scattering. The CO₂ condenses into clusters in the expansion to supersonic flow and the clusters sublimate in the hot boundary layer, resulting in high contrast in revealing the turbulent structure. The condensed cluster scattering represents closely the flow distribution across the boundary outer layer. The “average” CO₂ cluster radius has been estimated around 10 nm. Theoretical models predict that the cluster can accurately follow the flow fluctuations and shock motions.; The first application of the MHz-rate imaging system is the visualization of shock-wave/turbulent-boundary-layer interaction in a Mach 2.5 wind tunnel. The shock waves were produced by two-dimensional compression corners having angles of 14° and 24°, and the flows were, respectively, attached and separated. The interaction has been visualized in both planform and streamwise views. The MHz-rate sequential images clearly indicate the correlation between the shock motion and incoming turbulent boundary layer.; The second application of the imaging system is the transition study of hypersonic flow over a three-dimensional body. A three-dimensional imaging system was set up to record planform and spanwise views simultaneously with one pulse-burst laser and one camera. Under the “frozen flow” assumption, the sequential spanwise view images were used for three-dimensional volumetric reconstruction. The volumetric images clearly reveal the off-centerline action. It appears that the off-centerline waves may be the dominant instability in causing early transition over this particular three-dimensional body.