P. C. Palma

Fluorescence Velocimetry of the Hypersonic, Separated Flow over a Cone

Planar laser-induced fluorescence of nitric oxide is used to measure a component of the velocity field for the Mach 7 flow around a 30-deg half-angle, 50-mm-diam cone mounted to a long, 38-mm-diam shaft, or sting. Transverse velocities are measured in the freestream, the shock layer, and the separated region at the junction between the cone and the sting. For most of the flowfield, the uncertainty of the measurements is between ±50 and ±100 m/s for velocities ranging from -300 to 1300 m/s, corresponding to a minimum uncertainty of ±5%. The measurements are compared with the commercial computational fluid dynamics (CFD) code CFD-FASTRAN . The agreement between the theoretical model and the experiment is reasonably good. CFD accurately predicts the size and shape of the shock layer and separated region behind the cone as well as the magnitude of the gas velocity near the reattachment shock. However, the magnitude of the velocity in the shock layer and gas expansion differ somewhat from that predicted by CFD. The discrepancies are attributed to a small systematic error associated with laser-beam attenuation and also to inexact modeling of the flowfield by CFD

Fluorescence Imaging of Rotational and Vibrational Temperature in Shock-Tunnel Nozzle Flow

Two-dimensional rotational and vibrational temperature measurements were made at the nozzle exit of a freepiston shock tunnel using planar laser-induced e uorescence. The Mach 7 e ow consisted predominantly of nitrogen with a trace quantity of nitric oxide. Nitric oxide was employed as the probe species and was excited at 225 nm. Nonuniformities in the distribution of nitric oxide in the test gas were observed and were concluded to be due to contaminationofthetestgasbydrivergasorcoldtestgas.Thenozzle-exitrotationaltemperaturewasmeasuredand is in reasonable agreement with computational modeling. Nonlinearities in the detection system were responsible forsystematicerrorsin themeasurements. Thevibrational temperaturewasmeasured to beconstantwith distance from the nozzle exit, indicating it had frozen during the nozzle expansion.

Optical and Pressure Measurements in Shock Tunnel Testing of a Model Scramjet Combustor

Experimental measurements are presented of the flow in a model supersonic-combustion ramjet. A rectangular duct with a central streamwise-injected planar hydrogen jet has been tested at various enthalpies in a free-piston-driven shock tunnel. Several optical diagnostic techniques were employed to characterize the flow for a moderate enthalpy condition where pressure measurements indicated that significant combustion was occurring. Shadowgraph and emission images provided qualitative information on the density variations and temperature distributions, respectively. The planar laser-induced fluorescence technique has been used to examine the regions of ignition. Combustion was found to be occurring only in a thin mixing layer present between the air and hydrogen streams. The positions of shock waves within the duct compared well with pressure measurements performed along the floor of the duct.

Absolute intensity measurements of impurity emissions in a shock tunnel and their consequences for laser-induced fluorescence experiments

Absolute intensity measurements of impurity emissions in a shock tunnel nozzle flow are presented. The impurity emission intensities were measured with a photomultiplier and optical multichannel analyser and calibrated against an intensity standard. The various metallic contaminants were identified and their intensities measured in the spectral regions 290 to 330 nm and 375 to 385 nm. A comparison with calculated fluorescence intensities for predissociated laser-induced fluorescence signals is made. It is found that the emission background is negligible for most fluorescence experiments.

Numerical Simulation of Laser-Induced Fluorescence Imaging in Shock-Layer Flows

Planar laser-induced fluorescence (PLIF) images of nitric oxide in hypersonic flow over a wedge and a hemisphere are compared with a theoretical PLIF model. The theoretical PLIF images are based on computational fluid dynamics (CFD) models including a perfect-gas model and a nonequilibrium chemistry model. Two-dimensional maps of the flow parameters generated by the CFD are used to predict the theoretical PLIF images, including the effects of collisional quenching. We find good agreement between the model and the experimental measurements. We explain how this method of computational flow imaging can be useful for designing experiments.

Comparison of theoretical laser-induced fluorescence images with measurements performed in a hypersonic shock tunnel

Planar laser-induced fluorescence (PLIF) images of hypersonic flow over a wedge are compared with a theoretical PLIF model. The synthetic images, which include the effects of collisional quenching, are based on a perfect-gas treatment of the fluid mechanics, assuming frozen chemistry. We find very good agreement between the model and the experimental measurements.

Vibrational Temperature Measurements in a Shock Layer Using Laser Induced Predissociation Fluorescence

Single shot spatially and spectrally resolved laser induced predissociation fluorescence measurements in a shock layer around a cylinder in a pulsed supersonic free stream are presented. Fluoresence signals were produced using the tuned output of an argon fluoride excimer laser to excite a mixture of rovibrational transitions in molecular oxygen. The signals produced along a line inside the shock layer were focussed onto a two dimensional detector coupled to a spectrometer, thus allowing spectral and spatial resolution of the fluoresence. In this way, it was possible to detect two fluoresence signals from two different transitions simultaneously, allowing the determination of vibrational temperatures without the need for calibration. To minimize problems associated with low signal to noise ratios, background subtraction and spatial averaging was required.