Justin L. Wagner

Experimental Investigation of Unstart in an Inlet/Isolator Model in Mach 5 Flow

DOI: 10.2514/1.40966 The dynamics of unstart of a floor-mounted inlet/isolator model in a Mach 5 flow are investigated experimentally. The inlet section contains a 6-deg compression ramp, and the isolator is a rectangular straight duct that is 25.4-mm high by 50.8-mm wide by 242.3-mm long. Measurements made include 8-kHz schlieren imaging and simultaneous fast-response wall pressures along the length of the inlet/isolator. Unstart is initiated by deflecting a flap at the downstream end of the isolator. The shock system, induced by unstart, initially propagates upstream through the isolator at a velocity of about 35 m=s (in the laboratory frame of reference), then decelerates to about 20 m=s near the isolator entrance, and then accelerates to a velocity of about 74 m=s within the inlet. Throughout the isolator, unstart is seen to be strongly associated with boundary-layer separation. Once the inlet has unstarted, a highamplitudeoscillatory(periodic)unstarted flowensues,forwhichtheoscillationfrequencyisabout124Hz.However, under some conditions, an 84-Hz oscillatory unstarted flow mode, with lower pressure fluctuations, is observed. Under other conditions, a nonoscillatory unstarted flow, with much lower pressure fluctuations, is observed.

Interaction of a planar shock wave with a dense particle curtain: Modeling and experiments

The interaction of a planar shock wave with a dense particle curtain is investigated through modeling and experiments. The physics in the interaction between a shock wave with a dense gas-particle mixture is markedly differently from that with a dilute mixture. Following the passage of the shock wave, the dense particle curtain expands rapidly as it propagates downstream and pressures equilibrate throughout the flow field. In the simulations, the particles are viewed as point-particles and are traced in a Lagrangian framework. A physics-based model is then developed to account for interphase coupling. Compared to the standard drag law, four major improvements are made in the present interphase coupling model to take into account: (1) unsteady force contributions to particle force; (2) effect of compressibility on hydrodynamic forces; (3) effect of particle volume fraction on hydrodynamic forces; (4) effect of inter-particle collision. The complex behavior of the dense particle curtain is due to the interp...

Pulse-Burst PIV in a High-Speed Wind Tunnel.

Time-resolved particle image velocimetry (TR-PIV) has been achieved in a high-speed wind tunnel, providing velocity field movies of compressible turbulence events. The requirements of high-speed flows demand greater energy at faster pulse rates than possible with the TR-PIV systems developed for low-speed flows. This has been realized using a pulse-burst laser to obtain movies at up to 50 kHz, with higher speeds possible at the cost of spatial resolution. The constraints imposed by use of a pulse-burst laser are limited burst duration of 10.2 ms and a low duty cycle for data acquisition. Pulse-burst PIV has been demonstrated in a supersonic jet exhausting into a transonic crossflow and in transonic flow over a rectangular cavity. The velocity field sequences reveal the passage of turbulent structures and can be used to find velocity power spectra at every point in the field, providing spatial distributions of acoustic modes. The present work represents the first use of TR-PIV in a high-speed ground-test facility.

Shock Tube Investigation of Unsteady Drag in Shock- Particle Interactions

A reassessment of historical drag coefficient data for spherical particles accelerated in shock-induced flows has motivated new shock tube experiments of particle response to the passage of a normal shock wave. Particle drag coefficients were measured by tracking the trajectories of 1-mm spheres in the wake of incident shocks of Mach numbers 1.68, 1.93, and 2.05. Data clearly show that as the Mach number increases, the drag coefficient increases substantially, consistent with past experiments. This increase significantly exceeds the drag predicted by incompressible standard drag models, but recently developed compressible drag models return values quite close to the current measurements. Low values for the acceleration parameter indicate that unsteadiness should not be expected to contribute to the drag increase. These observations suggest that elevated particle drag coefficients can be attributed to increased compressibility rather than flow unsteadiness.

Fluid-structure interactions in compressible cavity flows

Experiments were performed to understand the complex fluid-structure interactions that occur during aircraft internal store carriage. A cylindrical store was installed in a rectangular cavity having a length-to-depth ratio of 3.33 and a length-to-width ratio of 1. The Mach number ranged from 0.6 to 2.5 and the incoming boundary layer was turbulent. Fast-response pressure measurements provided aeroacoustic loading in the cavity, while triaxial accelerometers provided simultaneous store response. Despite occupying only 6% of the cavity volume, the store significantly altered the cavity acoustics. The store responded to the cavity flow at its natural structural frequencies, and it exhibited a directionally dependent response to cavity resonance. Specifically, cavity tones excited the store in the streamwise and wall-normal directions consistently, whereas a spanwise response was observed only occasionally. The streamwise and wall-normal responses were attributed to the longitudinal pressure waves and shear layer vortices known to occur during cavity resonance. Although the spanwise response to cavity tones was limited, broadband pressure fluctuations resulted in significant spanwise accelerations at store natural frequencies. The largest vibrations occurred when a cavity tone matched a structural natural frequency, although energy was transferred more efficiently to natural frequencies having predominantly streamwise and wall-normal motions.

Width effects in transonic flow over a rectangular cavity

A previous experiment by the present authors studied the flow over a finite-width rectangular cavity at freestream Mach numbers 1.5–2.5. In addition, this investigation considered the influence of three-dimensional geometry that is not replicated by simplified cavities that extend across the entire wind-tunnel test section. The latter configurations have the attraction of easy optical access into the depths of the cavity, but they do not reproduce effects upon the turbulent structures and acoustic modes due to the length-to-width ratio, which is becoming recognized as an important parameter describing the nature of the flow within narrower cavities.

Joint Experimental/Computational Investigation Into the Effects of Finite Width on Transonic Cavity Flow.

Recently acquired experimental data on pressure fluctuations in cavities of equal length (L) to depth (D) ratio but varying length to width (L/W) ratio have shown substantial variations in the dominant modes in the cavity. These observations have been carried out at subsonic and transonic Mach numbers at cavity L/D=5, which puts the cavity flow in the “open” category. This paper presents results from a joint computational and experimental investigation undertaken at Sandia to explain these observations. To this end, simulations of L/D=5.0 cavity at L/W=1.0,1.67 and 5.0 have been carried out and analyzed. The results show strong differences in the mean flow structure between the three widths. The widest cavity shows significantly higher turbulence intensities across the cavity. The unsteady wall pressures reveal that in this case, significant tunnel wall interactions are present, intensifying the pressure fluctuations and the shear layer oscillations. The differences in the wall pressures and turbulent flow field are smaller for the L/W=1.0 and 1.67 cavities. The L/W=1.67 cavity is strongly influenced by the streamwise vortices at the spanwise edges of the cavity, resulting in strong three dimensional variations in mean flow across the width of the cavity. In the case of the narrowest cavity, this effect is minimal, with the resultant flow field showing predominantly two-dimensional character.

Complex Geometry Effects on Cavity Resonance

The flow over an aircraft bay is often represented using a rectangular cavity; however, this simplification neglects many features of actual flight geometry that could affect the unsteady pressure field and resulting loading in the bay. To address this shortcoming, a complex cavity geometry was developed to incorporate more realistic aircraft-bay features including shaped inlets, internal cavity structure, and doors. A parametric study of these features was conducted based on fluctuating pressure measurements at subsonic and supersonic Mach numbers. Resonance frequencies and amplitudes increased in the complex geometry compared to a simple rectangular cavity that could produce severe loading conditions for store carriage. High-frequency content and dominant frequencies were generated by features that constricted the flow such as leading-edge overhangs, internal cavity variations, and the presence of closed doors. Broadband frequency components measured at the aft wall of the complex cavities were also sig...

Experiments in Unsteady Forcing of Mach 2 Shock Wave/Boundary Layer Interactions

An experimental study was conducted to study the effects of unsteady upstream forcing on Mach 2 shock wave/boundary layer interactions. Two separate experiments were perfor med. In the first experiment, phase -locked wide -field PIV was used to study how pulsed vortex generator jets affected the interaction generated by a circular cylinder. The jets were pitched at an angle of 60o and skewed at an angle of 90o, and operated eit her continuously or at a frequency of 100 Hz. The jets were shown to increase the velocity in the lower boundary layer, making it more resistant to separation. Additionally, the scale of the interaction was changed as the primary horseshoe vortex became sm aller while the secondary cylinder -root vortex became larger. In the second experiment wide -field PIV and planar flow visualization were used to study the effects of counter -rotating -vane vortex generators at a frequency of 50 Hz on a 20 o unswept ramp int eraction. The vortex generators were shown to energize the upstream boundary layer and had a significant effect on the location and strength of the separation shock. The vortex generators also showed a significant decrease in size of the separation flow scale .

Turbulent eddies in a compressible jet in crossflow measured using pulse-burst particle image velocimetry

Pulse-burst Particle Image Velocimetry (PIV) has been employed to acquire time-resolved data at 25 kHz of a supersonic jet exhausting into a subsonic compressible crossflow. Data were acquired along the windward boundary of the jet shear layer and used to identify turbulent eddies as they convect downstream in the far-field of the interaction. Eddies were found to have a tendency to occur in closely spaced counter-rotating pairs and are routinely observed in the PIV movies, but the variable orientation of these pairs makes them difficult to detect statistically. Correlated counter-rotating vortices are more strongly observed to pass by at a larger spacing, both leading and trailing the reference eddy. This indicates the paired nature of the turbulent eddies and the tendency for these pairs to recur at repeatable spacing. Velocity spectra reveal a peak at a frequency consistent with this larger spacing between shear-layer vortices rotating with identical sign. The spatial scale of these vortices appears si...