A. F. P. Houwing

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

Laminar boundary layer separation at a fin-body junction in a hypersonic flow

Abstract. Planar laser-induced fluorescence (PLIF) imaging was performed to visualize the fin bow shock, separation shock, viscous shear layer and recirculation region of the flowfield at the junction of a blunt fin and a flat plate. Making use of the temperature dependence of the PLIF technique, images were made sensitive to temperature to provide qualitative information on the flowfield. The PLIF technique was also used as the basis for a flow-tagging technique, making it possible to measure a velocity component and to demonstrate the reverse flow of the separated region. Flow visualisation of the plane of symmetry allowed determination of the point of boundary layer separation, the angle of the separation shock and the bow shock standoff distance. These parameters were compared with predictions made by computational fluid dynamic simulations of the flowfield. Good agreement between theory and experiment was achieved. Comparisons between theoretical and experimental velocity measurements showed good agreement.

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.

Instabilities of shock and detonation waves

It is shown that the compatibility conditions for spontaneous splitting of a single shock into two shocks plus a contact discontinuity are satisfied whenever the instability limits derived by D’yakov, Kontorovich, and Erpenbeck are violated. They can be stated as −1<j2 (dV/dP)H <(1−M2−M2V0/V)(1−M2+M2V0/V)−1 <1+2M, where j2=( P−P0)/(v0−v), M=‖(D−u)/c‖, and the derivative is along the Hugoniot curve. One‐dimensional splitting can occur when the outer limits are exceeded. Violation of the intermediate upper limit (Kontorovich condition) admits spontaneous generation of steady transverse waves similar to those often observed in detonations.

Rotational and vibrational temperature measurements using CARS in a hypervelocity shock layer flow and comparisons with CFD calculations

Broadband single pulse coherent anti-Stokes Raman scattering (CARS) experiments employing a folded-box phase-matching geometry in a pulsed hypervelocity blunt body flow are presented. Rovibrational spectra of molecular nitrogen, produced in the freestream and within the shock layer at moderately high enthalpy (8.4 MJ/kg), are examined. Difficulties peculiar to the application of a single pulse optical technique to a high enthalpy pulsed flow facility are discussed and measurements of flow temperatures are presented. Theoretically calculated values for temperatures based upon algorithms used to determine freestream and shock layer conditions agree well with experimental measurements using the CARS technique. The measurements indicate that thermal non-equilibrium conditions exist within the freestream, and that near thermal equilibrium exists at the point of measurement within the shock layer. The comparison between the experiment and theory in the shock layer is improved by using the measured freestream temperatures as input to the shock layer computations.

Computational fluid dynamics validation using multiple interferometric views of a hypersonic flowfield

The validity of the computed three-dimensional perfect-gas inviscid density field of the shock layer about a blunt body in a hypersonic argon freestream has been investigated. Mach-Zehnder interferometry was used to generate interferograms of such a flowfield produced in the T3 free-piston shock tunnel. Two-dimensional phase maps (representing line-of-sight integrated density) were produced from the interferograms using a two-dimensional Fourier transform fringe analysis method. Theoretical maps, computed from the computational fluid dynamics solution, compare extremely well with experimental maps for each of seven different viewing angles used to generate interferograms. The multiple angles remove the ambiguity associated with comparing theoretical and experimental integrated quantities. Thus, confidence can be placed in the validity of the three-dimensional density computations.

Comparison of planar fluorescence measurements and computational modeling of shock-layer flow

Planar laser-induced fluorescence is used to image transient and quasisteady flow phenomena associated with an impulsively started supersonic jet incident on a circular cylinder. The transient phenomena observed are consistent with established theoretical work on the development of nozzle flows in a shock tunnel and with well-understood shock reflection processes. The technique is also used to measure the rotational temperature in the jet and in the shock layer on the cylinder after the establishment of quasisteady flow conditions. The inviscid flow between the bow shock and the edge of the boundary layer and the viscous flow within the boundary layer are modeled numerically using an iterative scheme. Good agreement is achieved between the computational and experimental results for most of the imaged field, except for the region near the shock vertex, where flow nonuniformities near the centerline perturb the shock from the shape expected for a uniform incident flow.