Flow Visualization of Density in a Cryogenic Wind Tunnel Using Planar Rayleigh and Raman Scattering

Abstract Using a pulsed Nd:YAG laser (532 nm) and a gated, intensifiedcharge-coupled device, planar Rayleigh and Raman scattering tech-niques have been used to visualize the unseeded Mach 0.2flow density ina 0.3-meter transonic cryogenic wind tunnel Detection limits are de-termined Jor density measurements by using both unseeded Rayleigh andRaman (N2 vibrational) methods. Seeding with C02 improved theRayleigh flow visualization at temperatures below 150 K. The seededRayleigh version was used to demonstrate the observation of transientflow Jeatures in a separated boundary layer region, which was excitedwith an oscillatory jet. Finally, a significant degradation of the laserlight sheet, in this cryogenic J_tcility, is discussed. Introduction NASA Langley Research Center (LaRC) has an ongoing effort (see citations in ref. 1) for the devel-opment of Rayleigh scattering (ref. 2) in a variety of wind tunnels. Many wind tunnels, including LaRC's0.3-Meter Transonic Cryogenic Tunnel (TCT), operate near the condensation point of the working fluid;hence, clusters of molecules may form in the unseeded flow. With clusters, the quantitative aspect of thedensity measurement from the Rayleigh signal is compromised. Since the Rayleigh scatter is elastic andproportional to the sixth power of the cluster size, the scatter from the clusters can easily be larger thanthe molecular scatter. Since the cluster-size distribution is not typically known, the density of molecularscatters is hard to deconvolve from the cluster signal. In this case, Rayleigh scattering cannot easilymeasure flow density and is limited to qualitative flow visualization. Recently, (ref. 3) it was demon-strated that clustering does not occur in the free-stream flow for typical run conditions in the TCT, mak-ing Rayleigh scattering one promising method for quantitative density measurements in the TCT.In this report, planar imaging of density in the unseeded flow is demonstrated in the TCT withRayleigh scattering from a laser beam formed into a sheet. Seeding CO2 into the flow is shown to induceclustering in the flow medium, increasing the magnitude of the elastic light scattering and enhancing thequality of the images. This seeded version of planar Rayleigh scattering is used to detect transient flowstructure downstream of the blowing slot in a near-zero-mass-flow oscillatory blowing experiment.Planar Raman scattering from the 0-1 vibrational transition in N2 is also demonstrated as a second poten-tial method to visualize the unseeded flow. Finally, beam steering effects, by the cryogenic fluid in TCT,are described.The elastic light scatter is in the Rayleigh regime for the unseeded flow; however, adding CO2 toinduce clusters raises questions about the size of the clusters and whether the seeded elastic light scatter isin the Rayleigh or Mie regime. For the purpose of this work, it is not important if the elastic scatter is inthe Rayleigh or Mie regime, and we will hereafter refer to the seeded scattering as Rayleigh scattering.