Publications

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Simultaneous stereo PIV measurements of a round free jet were obtained from narrow and wide camera angles while a fifth camera viewed the laser sheet from 90 degrees to determine the two-component velocity field free of errors resulting from stereo calibration. Errors in mean velocities were small, but artificially reduced turbulent stresses were generated when self-calibration was not used, owing to a smearing effect that occurs when the two cameras are inadequately registered to each other. This difficulty worsened with increased laser sheet thickness. Spatial error in the stereo calibration process can artificially displace vector fields from the expected origin, which was detected through comparison to the simultaneous two-component measurement. Although this spatial offset typically is small with respect to statistical properties of a data set, it can be prominent when instantaneous snapshots of the velocity field are examined, particularly where the velocity gradient is momentarily large.

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Experiments were conducted at freestream Mach numbers of 0.55, 0.80, and 0.90 in open cavity flows having a length-to-depth ratio L/D of 5 and an incoming turbulent boundary having a thickness of about 0.5D. To ascertain aspect ratio effects, the length-to-width ratio L/W was varied between 1.00, 1.67, and 5.00. Two stereoscopic PIV systems were used simultaneously to characterize the flow in the plane at the spanwise center of the cavity. For each aspect ratio, trends in the mean and turbulence fields were identified, regardless of Mach number. The recirculation region had the weakest reverse velocities in the L/W = 1.67 cavity, a trend previously observed at supersonic Mach numbers. Also, like the previous supersonic experiments, the L/W = 1.00 and L/W = 5.00 mean streamwise velocities were similar. The L/W = 1.00 cavity flows had the highest turbulence intensities, whereas the two narrower cavities exhibited lower turbulence intensities of a comparable level. This is in contrast to previous supersonic experiments, which showed the lowest turbulence levels in the L/W = 1.67 cavity.

High-frequency pressure sensors were used in conjunction with a high-speed schlieren system to study the growth and breakdown of boundary-layer disturbances into turbulent spots on a 7° cone in the Sandia Hypersonic Wind Tunnel at Mach 5 and 8. To relate the intermittent disturbances to the average characteristics of transition on the cone, the statistical distribution of these disturbances must be known. These include the boundarylayer intermittency, burst rate, and average disturbance length. Traditional low-speed methods to characterize intermittency identify only turbulent/nonturbulent regions. However at high M, instability waves become an important part of the transitional region. Algorithms to distinguish instability waves from turbulence in both the pressure and schlieren measurements are being developed and the corresponding intermittency, burst rate, and average burst length of both regions have been provisionally computed for several cases at Mach 5 and 8. Distinguishing instability waves from turbulence gives a better description of the intermittent boundary layer at high M and will allow the fluctuations associated with boundary-layer instabilities to be incorporated into transitional models.

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