Scott Stanfield

Stability Analysis for HIFiRE Experiments

The HIFiRE-1 flight experiment provided a valuable database pertaining to boundary layer transition over a 7-degree half-angle, circular cone model from supersonic to hypersonic Mach numbers, and a range of Reynolds numbers and angles of attack. This paper reports selected findings from the ongoing computational analysis of the measured in-flight transition behavior. Transition during the ascent phase at nearly zero degree angle of attack is dominated by second mode instabilities except in the vicinity of the cone meridian where a roughness element was placed midway along the length of the cone. The growth of first mode instabilities is found to be weak at all trajectory points analyzed from the ascent phase. For times less than approximately 18.5 seconds into the flight, the peak amplification ratio for second mode disturbances is sufficiently small because of the lower Mach numbers at earlier times, so that the transition behavior inferred from the measurements is attributed to an unknown physical mechanism, potentially related to step discontinuities in surface height near the locations of a change in the surface material. Based on the time histories of temperature and/or heat flux at transducer locations within the aft portion of the cone, the onset of transition correlated with a linear N-factor, based on parabolized stability equations, of approximately 13.5. Due to the large angles of attack during the re-entry phase, crossflow instability may play a significant role in transition. Computations also indicate the presence of pronounced crossflow separation over a significant portion of the trajectory segment that is relevant to transition analysis. The transition behavior during this re-entry segment of HIFiRE-1 flight shares some common features with the predicted transition front along the elliptic cone shaped HIFiRE-5 flight article, which was designed to provide hypersonic transition data for a fully 3D geometric configuration. To compare and contrast the crossflow dominated transition over the HIFiRE-1 and HIFiRE-5 configurations, this paper also analyzes boundary layer instabilities over a subscale model of the HIFiRE-5 flight configuration that was tested in the Mach 6 quiet tunnel facility at Purdue University.

Spectral Characteristics of Separation Shock Unsteadiness

Spectra of wall-pressure fluctuations caused by separation shock unsteadiness were compared for data obtained from wind-tunnel experiments, the Hypersonic International Flight Research Experimentation flight test 1, and large-eddy simulations. The results were found to be in generally good agreement, despite differences in Mach number and two orders of magnitude difference in Reynolds number. Relatively good agreement was obtained between these spectra and the predictions of a theory developed by Plotkin. The predictions of this theory are also qualitatively consistent with the results of experiments in which the shock motion was synchronized to controlled perturbations. The results presented here support the idea that separation unsteadiness has common features across a broad range of compressible flows and that it behaves as a selective amplifier of large-scale disturbances in the incoming flow.

Traveling Crossflow Instability for the HIFiRE-5 Elliptic Cone

A 38.1%-scale model of the Hypersonic International Flight Research Experimentation Program’s Flight Five 2:1 elliptic cone flight vehicle was used to investigate the traveling crossflow instability in a Mach 6 quiet wind tunnel. Traveling crossflow waves were detected with pressure sensors mounted flush with the model surface. The crossflow instability phase speed and wave angle were calculated from the cross spectra of the three pressure sensors. Both quantities showed good agreement with linear stability theory. Duplicate runs at the same initial conditions showed excellent repeatability in traveling crossflow wave properties. Traveling crossflow waves in quiet flow showed very low levels of nonlinear interactions. No traveling crossflow waves were observed for any Reynolds number for elevated freestream noise levels, but transition occurred for a much lower Reynolds number than in quiet flow. Due to the lack of nonlinear growth in quiet flow and the absence of traveling crossflow waves in noisy flow, ...

Rotational and Vibrational Temperature Distributions for a Dielectric Barrier Discharge in Air

Spatially resolved rotational and vibrational temperatures for N 2 and rotational temperatures for N + 2 , as a function of voltage, have been obtained for an asymmetric surface mode dielectric barrier discharge using emission spectroscopy. The rotational temperatures were obtained from a nonlinear least-squares fit of a two-temperature theoretical spectrum with the measured spectra of the N 2 (C 3 Π u ― B 3 Π g ) and N + 2 (B 2 Σ + u ― X 2 Σ + g ) electronic band systems. The vibrational temperatures were obtained by applying the Boltzmann plot method to the Δv = ―2 sequence of the N 2 (C 3 Π u ― B 3 Π g ) electronic band system. It was observed that the rotational temperatures for N 2 ? and N + 2 decreased in the induced flow direction and increased with increasing voltage. Values started at 390 ± 10 K and decreased to 340 ± 10 K for N 2 and started at 500 ± 15 K and decreased to 450 ± 15 K for N + 2 . The vibrational temperatures also decreased in the induced flow direction from 3250 to 2850 ± 300 K. A difference in rotational temperatures between N 2 and N + 2 was observed for all voltages studied, and these differences increased with increasing voltage. The rotational temperatures of both species fluctuated in the spanwise direction. These fluctuations damped out in the streamwise direction and were weakly correlated with the attachment points of the microdischarges on the edge of the exposed electrode.

Study of Plasma Electrode Arrangements for Optimum Lift in a Mach 5 Flow

Abstract : This work is an experimental effort to study the power efficiency of using a plasma discharge to alter the lift on a body or surface. In this paper several electrode geometries are considered in an effort to reduce the plasma power required for a given change in lift. The cathode electrode position and electrode size are studied. For all cases studied the anode electrode is kept the same. Results are presented for four different size cathodes and four different cathode positions. The primary result presented is the lift change produced by the discharge per unit power input. The lift is determined by measuring the deflection of the model under the applied plasma. This type of a measurement system has some advantages and disadvantages compared to a load cell lift measurement system used by the authors in past work. Results from each of these lift measurement tools compare well. Results for 9 and 24 mA DC discharges are shown in this paper. For the conditions utilized in this work the results indicate that both cathode position and cathode size affect the lift change caused by a plasma discharge per unit of power input.

Crossflow Instability for HIFiRE-5 in a Quiet Hypersonic Wind Tunnel

Both the stationary crossflow instability and traveling disturbances were experimentally investigated for the HIFiRE-5 2:1 elliptic cone geometry in Purdue University’s quiet, Mach 6 wind tunnel using surfacemounted pressure transducers, temperature-sensitive paint, and oil-flow visualization. With quiet conditions, stationary crossflow vortices and co-located traveling disturbances were observed. With noisy flow, no traveling disturbances were detected, even at the lowest Reynolds number tested. However, near the model’s leading edges, streamwise streaks were seen in the oil flow, suggesting the presence of stationary crossflow vortices. Discrete roughness elements with various spanwise roughness spacings were utilized in an attempt to change the unperturbed stationary crossflow wavelength. For quiet flow, the unperturbed stationary wavelength was found to be damped out with some roughness spacings, which also excited wavelengths that had been stable in the unperturbed boundary layer. These same roughness elements were also found to damp the measured traveling disturbance with greater effectiveness for decreasing roughness spacing. In noisy flow, some roughness spacings led to detectable streamwise vortices that persisted downstream through what was, in the smooth-wall case, a turbulent boundary layer.

Traveling Crossflow Instability for HIFiRE-5 in a Quiet Hypersonic Wind Tunnel

A scale model of the 2:1 elliptic cone HIFiRE-5 flight vehicle was used to investigate the traveling crossflow instability at Mach 6 in Purdue University’s Mach-6 quiet wind tunnel. Traveling crossflow waves were measured with surface-mounted pressure sensors. The crossflow instability phase speed and wave angle were calculated from the cross spectra of three surface-mounted pressure sensors. Both quantities show good agreement with computational values from about 30-50 kHz. Repeated runs at the same initial condition show excellent repeatability in traveling crossflow wave properties, and give an estimate of the experimental uncertainty associated with this technique. Additionally, autobispectral analysis showed the onset and development of moderate nonlinear quadratic phase-locking prior to transition, but not for the peak traveling crossflow wave. The bicoherence achieved only moderate values. No traveling crossflow waves were observed when freestream noise levels were intentionally elevated, but transition occurred for a much lower Reynolds number. It appears that the traveling crossflow instability is not the primary transition mechanism in the noisy flow of Purdue’s Mach 6 wind tunnel.

HIFiRE-1 Data Analysis: Boundary Layer Transition Experiment During Reentry

Analyses of data collected during reentry of the HIFiRE-1 flight for the boundary layer transition experiment are reported. The data collected was complicated by the dynamic motion of the flight vehicle at large angles-of-attack, and by missing data, which resulted in an unevenly spaced time domain. The unevenly spaced time data increased the complexity of spectral analysis. Normalized root-mean-square (RMS) surface pressures were computed, and correlated well with periodic surface pressure fluctuations, boundary layer separation, and the boundary layer transition front. The periodic surface pressure fluctuations were first discernable underneath a laminar boundary layer on the leeside surface, at Re = 1.9x10 6 , and persisted until the boundary layer transitioned to turbulent, at Re = 5.1x10 6 . At Re values of 1.9x10 6 and 5.1x10 6 , the extent of the fluctuations in the azimuthal direction ranged between 15and 75-degrees, respectively. Corresponding wavelengths, computed using the Lomb-Scargle periodogram, ranged between 4.0 and 9.5 mm. The wavelengths of the periodic surface pressure fluctuations decreased as the vehicle descended and Re increased.

HIFiRE-5 Attachment-Line and Crossflow Instability in a Quiet Hypersonic Wind Tunnel

Experiments were conducted on the HIFiRE-5 geometry in a quiet Mach-6 wind tunnel with both low and conventional freestream noise levels. The effect of freestream noise on natural transition on the windward surface and roughness-induced transition on the attachment line was investigated. Elevated freestream noise seemed to change the primary transition mechanism on the windward surface. For quiet flow, there appears to be significant stationary crossflow waves. Traveling crossflow waves were also possibly observed coincident with the stationary waves. With noisy flow, neither stationary nor traveling crossflow instabilities were observed. The primary windward transition mechanism with noisy flow remains unknown. Both two-and three-dimensional roughness geometries were tested on the attachment line. As expected, elevated levels of freestream noise served to decrease the critical roughness height for all roughness geometries tested. Three-dimensional roughness elements proved to be the most destabilizing for both noisy and quiet flow. Based on these results, the specified flight roughness tolerance appears to be quite conservative and should prevent early roughness-induced transition.

Rotational and Vibrational Termperatures for a Dielectric Barrier Discharge in Air Using Emission Epectroscopy

Spatially resolved rotational and vibrational temperatures have been obtained for a dielectric barrier discharge (DBD) using emission spectroscopy. The temperatures were obtained by ma tching the measured nitrogen second positive 0 -2, 1 -3 and 2 -4 transitions with a calculated one. The temperature had a periodic profile in the spanwise direction where the peaks correlated with the strong attachment points within the discharge. These resul ts, as well as visual observations, indicate that there are variations in the structure of the DBD and it is not a fully uniform glow discharge. It may be that these temperature high points indicate localized flow structure in the form of small plasma jets . The maximum rotational temperature in the DBD was 410 K located at the interface between the exposed and buried electrode. The minimum value of temperature measured was 310 K located at the edge of the discharge region. The vibrational tem peratures had a maximum of 6200 K at the i nterface, and a minimum of 5700 K at the edge of the discharge region.