N. J. Parziale

Differential Interferometric Measurement of Instability in a Hypervelocity Boundary Layer

The prediction of laminar–turbulent transition location in high-speed boundary layers is critical to hypersonic vehicle design because of the weight implications of increased skin friction and surface heating rate after transition. Current work in T5 (the California Institute of Technology’s free piston reflected shock tunnel) includes the study of problems relevant to hypervelocity boundary layer transition on cold-wall slender bodies. With the ability to ground-test hypervelocity flows, the study of energy exchange between the boundary layer instability and the internal energy of the fluid is emphasized. The most unstable mode on a cold-wall slender body at zero angle of incidence is not the viscous instability (as in low-speed boundary layers) but the acoustic instability. Quantitative characterization of this disturbance is paramount to the development of transition location-prediction tools.

Effect of Gas Injection on Transition in Hypervelocity Boundary Layers

A novel method to delay transition in hypervelocity flows in air over slender bodies by injecting CO2 into the boundary layer is presented. The dominant transition mechanism in hypersonic flow is the inviscid second (Mack) mode, which is associated with acoustic disturbanceswhich are trapped and amplified inside the boundary layer [8]. In dissociated CO2-rich flows, nonequilibrium molecular vibration damps the acoustic instability, and for the high-temperature, high-pressure conditions associated with hypervelocity flows, the effect is most pronounced in the frequency bands amplified by the second mode [3].

Shock Tunnel Noise Measurement with Resonantly Enhanced Focused Schlieren Deflectometry

The character of the boundary layer noise and ambient tunnel noise are of interest in the experimental study of laminar to turbulent transition. The instability mechanism in hypersonic flow over slender bodies is the acoustic mode. A number of investigations of flow over a slender cone in high-enthalpy facilities have been performed; however, measurements of the boundary layer noise and ambient tunnel noise have not been made. In cold hypersonic facilities the frequency range of the acoustic mode typically lies below 500 kHz; in high-enthalpy facilities, 5-20 MJ/kg, the most strongly amplified acoustic mode frequency is approximately 1-3 MHz. These high frequencies are well beyond the reach of the piezo-electric pressure transducers typically used in cold hypersonic facilities. A logical approach is to investigate the use of optical methods. Measurements of the boundary layer noise and ambient tunnel noise on a five degree half angle cone in the Caltech T5 hypervelocity shock tunnel are made with a single point focused schlieren system and a resonantly enhanced focused schlieren system.

Krypton Tagging Velocimetry (KTV) in Supersonic Turbulent Boundary Layers

The krypton tagging velocimetry (KTV) technique is applied to the turbulent boundary layer on the wall of the Mach 3 Calibration Tunnel at Arnold Engineering Development Complex (AEDC) White Oak. Profiles of velocity were measured with KTV and Pitotpressure probes in the Mach 2.75 turbulent boundary layer comprised of 99% N2/1% Kr at momentum-thickness Reynolds numbers of ReΘ = 800, 1400, and 2400. Agreement between the KTVand Pitot-derived velocity profiles is excellent. The KTV and Pitot velocity data follow the law of the wall in the logarithmic region with application of the Van Driest I transformation. Also, the velocity data in the wake region are consistent with data from the literature for a turbulent boundary layer with a favorable pressure gradient history. A modification of the Mach 3 AEDC Calibration Tunnel is described which facilitates operation at several discrete unit Reynolds numbers consistent with AEDC Hypervelocity Tunnel 9 run conditions of interest. Moreover, to enable near-wall measurement with KTV, an 800 nm longpass filter was used to block the reflection and scatter from the 760.2 nm read-laser pulse. With the longpass filter, the 819.0 nm emission from the re-excited Kr can be imaged to track the displacement of the metastable tracer without imaging the reflection and scatter from the read laser off of solid surfaces.

Krypton Tagging Velocimetry for Use in High-Speed Ground-Test Facilities

In this work, we present the excitation strategy, experimental setup, and results of an implementation of krypton tagging velocimetry (KTV) as applied to an underexpanded jet of three mixtures. We demonstrate that the KTV technique can be employed with gas mixtures of relatively low krypton concentration (0.5% Kr/99.5% N2) and conclude that the KTV technique shows promise as a velocimetry diagnostic with krypton as an inert, dilute, long-lifetime tracer in gas-phase flows.

Differential Interferometric Measurement of Instability at Two Points in a Hypervelocity Boundary Layer

The focused laser differential interferometer (FLDI) was used to investigate disturbances in a hypervelocity boundary layer on a sharp five degree half-angle cone. The T5 hypervelocity free-piston driven reflected-shock tunnel was used as the test facility; in such a facility the study of thermo-chemical/fluid-dynamic energy exchange is emphasized. Two sensitive FLDI probe volumes were aligned along a generator of the cone that recorded time-traces of density fluctuation at sufficient time resolution, spatial resolution, and signal to noise ratio, so that the boundary layer instability could be resolved. This arrangement of the FLDI allows for the interpretation of disturbances at two points and the correlation between them. The acoustic instability is detected with narrow-band peaks in the spectral response at a number of frequencies (500 kHz to 1.29 MHz). The data indicate that the instability driving the boundary layer to turbulence is acoustic in nature. Preliminary analysis indicates that there is not a significant difference between N2 and air acoustic boundary layer disturbance amplification factors for the representative cases presented. Computation of acoustic damping by thermo-chemical relaxation processes is presented for the same representative cases, and indicates that there is a negligible amount of absorption for both air and N_2 at the observed disturbance frequencies.

Pulsed Laser Diode for Use as a Light Source for Short-Exposure, High-Frame-Rate Flow Visualization

A pulsed laser diode (PLD) is demonstrated as a practical light source for high-speed digital schlieren and shearing-interferometric cinematogrpahy. Frame rates of greater than 300k fps with exposure times on the order of 10 ns have been achieved with an inexpensive and user-friendly setup. The light source has primarily been used in our laboratory to study nonsteady phenomena in high-speed gas flows. Examples are presented to illustrate the usefulness of the PLD as a light source with characteristics of a narrow band of wavelength, short exposure time, high frame-rate, and long pulse train duration.

Nonintrusive Freestream Velocity Measurement in a Large-Scale Hypersonic Wind Tunnel

H IGH-SPEED wind tunnels typically rely on pressure and/or temperature measurement and nozzle-flow calculations to determine the freestream conditions. This practice can require a complex treatment of the thermochemical state of the gas. The calorically perfect gas assumption begins to break down when producing air or N2 flows from a stagnated reservoir to freestream Mach number M∞ > 6. Rapid expansion in the nozzle can require modeling thermodynamic nonequilibrium processes, and if the gas is stagnated to high enthalpy, nonequilibrium chemistry must also be considered [1].Moreover, an excluded-volume equation of state may need to be used for high reservoir densities [2,3]. Although the modeling framework of these flows is tractable, some of the fundamentals pertaining to the thermochemical rate processes continue to be an ongoing topic of research [1]. One means of validating these run condition and nozzle-flow calculations is direct measurement in the freestream. Particle-based methods of velocimetry, such as particle image velocimetry, can produce high-quality multicomponent velocity data [4]. However, the engineering challenges associated with implementing particlebased techniques in large-scale high-speed facilities include timing, particle-seeding density and uniformity, and minimizing flow disturbances when injecting particles [5]. More importantly, there is the fundamental limitation of reduced particle response at Knudsen and Reynolds numbers [6] typical of high-speed wind tunnels, which can compromise the resolution of fine time and length scales. In contrast to the limitations of particle-based techniques, implementation of tagging velocimetry is not constrained by the aforementioned issues in large-scale high-speed facilities. Noted methods and tracers of tagging velocimetry include VENOM [7], APART[8],RELIEF [9], FLEET [10], STARFLEET [11], PLEET [12], nitrogen oxides [13–15], iodine [16], acetone [17], and the hydroxyl group techniques [18–21]. Continually advancing laser and imaging technology has enabled tagging velocimetry to be used in large-scale facilities where other approaches are difficult to implement. This technical note reports the direct measurement of freestreamvelocity profiles in AEDC Hypervelocity Tunnel 9 (Tunnel 9) with krypton tagging velocimetry (KTV). The KTV experimental setup is described, followed by an explanation of Tunnel 9 and the conventional method of run-condition calculation. KTV exposures are presented for four different Tunnel 9 conditions. Then, for two conditions, instantaneous velocity profiles and a comparison of the freestream velocity as calculated by conventional methods and KTV are presented.

Krypton Tagging Velocimetry (KTV) Investigation of Shock-Wave/Turbulent Boundary-Layer Interaction

Seven profiles of streamwise velocity and velocity fluctuations were measured in the incoming boundary layer and immediately upstream of a 24-degree compression corner in a M∞ = 2.8, ReΘ = 1750 shock-wave/turbulent boundary-layer interaction. The measurements were made with Krypton Tagging Velocimetry (KTV) in 99% N2/1% Kr flow. Globally seeding 1% Kr into the flow (premixed N2/Kr K-bottles from distributor) alters the major non-dimensional transport properties by 0.1-0.3%. The mean-velocity and velocityfluctuation profiles in the incoming supersonic turbulent boundary layer were found to agree with datasets in the literature, thus, the baseline flow was established. The meanvelocity profiles in the region immediately upstream of the 24-degree compression corner were found to agree with Direct Numerical Simulation (DNS) results available in the literature. In addition, the presence of a shear layer was detected, and its location and orientation are compared to that in the literature.

Krypton Tagging Velocimetry in the Stevens Shock Tube

Krypton Tagging Velocimetry (KTV) is implemented in the flow immediately following the incident shock wave in the Stevens Shock Tube. This is motivated by the long-term goal of using KTV to measure velocity in large-scale impulse facilities. Two example cases are presented in 99% N2/1% Kr at incident shock Mach numbers of 2.86 and 2.94. The velocities as measured by KTV are, in general, higher than those calculated from the shock-speed measurements. The discrepancy is most likely due to the misalignment of splitter plate installed in the shock tube (an expansion fan likely accelerated the flow). A new excitation scheme for KTV is used that results in a higher SNR as compared to previous work. A justification of the alternate scheme is presented via a three energy level model. An overview of the shock tube is given, and a model predicting the non-equilibrium thermodynamic state of the gas immediately following the incident shock is presented.