Current: Yongxi Hou, Pablo Bueno, Noel T. Clemens and David S. Dolling
Alumni: Roderick G. Austin, Steven J. Beresh, Mark Comninos and Stephen C. Chan
PLS and PIV in Mach 5 flow
The data acquisition arrangement for Planar Laser Scattering (PLS) and Particle Image Velocimetry (PIV) in the Mach 5 wind tunnel is illustrated below.
For PLS imaging, the flow is seeded with an alcohol vapor which condenses into fine ice crystals as the flow expands through the nozzle, creating an efficient medium for Mie scattering of the laser light. The laser light source is the 532 nm output of an Nd:YAG laser, which is formed into a sheet and directed into the wind tunnel's test section. The scattered light is collected by a CCD camera. Pressure measurements are conducted simultaneously in the floor of the wind tunnel to monitor the position of the separation shock foot.
Laser light scatters effectively off of the fog in the free stream of the flow, but the temperature increase within the boundary layer evaporates much of the ice and thus provides a weaker signal. This produces a well-defined boundary layer interface, as seen in the PLS image above (click for an animated sequence of instantaneous images (495kB) , or an animated sequence of averaged images (365kB)) Additionally, if the alcohol seeding density is adjusted carefully, the separation shock becomes visible as residual ice crystals in the boundary layer are affected by the density increase across the shock. This PLS technique provides an excellent instantaneous picture of the gross structure of the boundary layer and separation shock, which can be used to examine the influence of the incoming boundary layer upon the shock.
For PIV, the flow is seeded with sub-micron alumina particles which are also illuminated through Mie scattering of the laser light. The laser is double-pulsed with a separation time much smaller than the characteristic flow velocity, effectively freezing the flow for the duration of the image. The resulting double exposure image is collected by a high-resolution CCD camera. Two traces are thus recorded for each particle. By measuring the particle displacements within certain interrogation windows, and using the known laser pulse separations, velocity vectors can be computed. This is repeated at regularly spaced intervals through the image to create a grid of velocity vectors.
The PIV vector field shown above, from which the mean velocity field has been subtracted, reveals the turbulent velocity fluctuations within the incoming boundary layer. The vector field is superposed on a countour plot of the local vorticity field, as calculated from the PIV measurements. Whereas PLS can show only an overall picture of the turbulent boundary layer, PIV is able to quantify the velocities throughout a portion of the flow. Here, the vector field examines the boundary layer just upstream of the interaction. Since the shock motion is measured by the pressure transducers located in the tunnel floor, the PIV measurements can be used to seek correlations between the incoming flow behavior and the shock motion.
The figure above shows the conditional ensemble average profiles of the streamwise fluctuations in the incoming boundary layer conditioned on the separation shock foot motion within a time period of 250 µs. The result was obtained by combining upstream PIV measurement with simultaneous fast response pressure measurement in the intermittent region. The positive velocity fluctuations in the lower part of the boundary layer were correlated with downstream motions of the separation shock, and negative velocity fluctuations were correlated with upstream shock motion. A cartoon based on it is shown below.
