Christopher Ward

Hypersonic Boundary-Layer Transition Experiments in a Mach-6 Quiet Tunnel

The Boeing/AFOSR Mach-6 Quiet Tunnel has achieved quiet flow to stagnation pressures of 146 psia, and intermittently quiet flow between 146 and 169 psia. In an attempt to measure natural transition under quiet flow, a 3-m-circular-arc compression cone was tested with a nearly sharp nosetip. Using temperature-sensitive paint, hot streaks were observed to develop near the rear of the cone at high pressures under quiet flow. The streaks do not appear under noisy flow. The cause of the hot streaks remains unknown, though they may be instabilities or artifacts of nonlinear breakdown. Under quiet flow, the cone boundary layer remained laminar up to N factors of at least 15 and possibly as high as 19. Transition occurred at N = 9 under noisy flow. It is unknown why laminar flow persisted to such high N factors. As part of an investigation of crossflow vortices, a 7 half-angle cone was tested at 6 angle of attack with temperature-sensitive paint finishes of varying roughness. The roughness of the paint finish was observed to have an effect on crossflow vortices, in some cases inducing transition under noisy flow. Heat-transfer measurements were made at the stagnation point of a hemisphere to observe the effect of freestream noise; no effect was evident.

Hypersonic Boundary-Layer Transition Experiments in the Boeing/AFOSR Mach-6 Quiet Tunnel

This paper reports progress from six different projects studying hypersonic boundarylayer transition. Efforts to measure instabilities in the wake of an isolated roughness element in the laminar nozzle-wall boundary layer of the BAM6QT are on-going. A new set of instabilities have been measured when the roughness is at a height that causes incipient transition within the measurement range. The second project tested a method of calibrating temperature-sensitive paints using Schmidt-Boelter heat transfer gauges. A 7-deg half-angle cone was tested at 0-deg angle of attack and the heat transfer compared well with theory. A forward-facing cavity was used to identify the critical depth where self-sustaining oscillations begin. This was found to be about 1.2 cavity diameters. A shock tube is being constructed at Purdue to calibrate PCB-132 sensors. The tube will create clean, weak shocks of a magnitude similar to a second-mode wave in a wind tunnel, allowing accurate calibrations for instability measurements. A flared cone was tested in the BAM6QT, and showed natural transition under fully quiet flow. The streamwise vortices visible on the surface of the cone were found to be body fixed. A blunt 3-deg half-angle cone was tested with various distributed and isolated roughness elements.

Transition Research with Temperature-Sensitive Paints in the Boeing/AFOSR Mach-6 Quiet Tunnel

Abstract : The Boeing/AFOSR Mach-6 Quiet Tunnel (BAM6QT) is used for the study of noise effects on transition. A 12-inch ball valve was installed in the BAM6QT in April 2011, replacing a slow gate valve. This ball valve allows the tunnel to run without the use of diaphragms, though experiments are still typically run with the double burst diaphragm system. Four projects in the BAM6QT are also described in this paper. The first project tested a method of calibrating temperature-sensitive paints using Schmidt-Boelter heat transfer gauges. A 7-deg half-angle cone was tested at 0-deg angle of attack and compared to theory. The second project tested two von Karman ogive models. On the 7.62-cm- diameter ogive model at 0-deg angle of attack, the flow remained laminar for a smooth nosetip, a nosetip with a two-dimensional roughness strip, and a nosetip with distributed roughness. Isolated roughness elements larger than 51 m cause transition on this model at higher Reynolds numbers. A smaller 5.08-cm-diameter ogive model was constructed to allow the model to start at 2-deg angle of attack and to prevent a reflected bow shock from impinging on the model. Transition occurred on the lee ray on the smaller model. Forward-facing and aft-facing steps on the model nosetip did not appear to affect transition. Third, a 3-m circular-arc flared cone was run in different axial positions in the tunnel to determine if there was an effect. Sensors were also installed aft of the model to try to measure noise levels with an installed model in an attempt to show that transition occurs on the cone in fully quiet flow. For the last project, roughness dots were added to the same flared cone in an attempt to change vortex spacing. The flared cone remains a subject for future research.

Heat-Flux Measurements with Temperature-Sensitive Paint in a Mach-6 Quiet Tunnel

A procedure based on an analytical inverse method is developed to calculate heat-flux distributions on a model using a time sequence of temperature-sensitive-paint images acquired in the Purdue qui...

Instability and Transition Research in a Mach-6 Quiet Tunnel

Pitot-probe measurements were made to determine the spatial extent of the uniform quiet flow core for the laminar nozzle-wall boundary layer. The uniformity of the flow for a turbulent nozzle-wall boundary layer was determined as well. The nozzle was re-polished in August, and quiet flow is currently being achieved at higher stagnation pressures, around 162 psia. The laser perturber system for the BAM6QT is being re-established. The laser used for this system performs as in previous experiments and a new optical system has been designed for the BAM6QT. Tests were conducted to characterize the effect of suction downstream of the nozzle exit on starting larger blunt models in hypersonic flow. Pressure measurements were made in the upstream pipe-insert section and found that suction levels were highest with a 0.375-in. gap. Models of the Crew Exploration Vehicle (CEV) with diameters of 1.000 and 1.250 in. started regardless of model positioning and gap suction. A 1.500-in. model started in quiet flow, but did not start in noisy flow with pipe-insert suction alone. The 1.500-in. CEV model did, however, start in noisy flow with bleeds open above the maximum quiet pressure. Another project looked at the effect of nosetip roughness on the crossflow instability with a 7-deg half-angle cone at 6-deg angle of attack. When the average roughness of the nosetip was varied by an order of magnitude, the temperature-sensitive paint data showed no effect on the stationary vortices. It appears that the paint step at the nosetip-frustum junction was dominating the generation of the stationary vortices. Lastly, measurements upstream of a cylindrical roughness have confirmed the existence of an absolute instability in the separation region, as suggested by computations. A second absolute instability was also detected at lower Reynolds numbers.

Secondary Instability of Stationary Crossflow Vortices on an Inclined Cone at Mach 6

Using temperature-sensitive paint (TSP) to measure global surface heat transfer on a cone at angle of attack in Mach-6 flow, stationary crossflow waves were observed as hot streaks. When the stationary waves grew large and appeared to begin to break down to turbulence in the vicinity of a pressure sensor, a high-frequency disturbance was detected. This disturbance may be evidence of the secondary instability required to break down the stationary crossflow instability to turbulence. The frequency of the instability appeared to be sensitive to the local boundary layer thickness and small changes in azimuthal position. The azimuthally concentrated signature and the apparent dependence on a threshold stationary wave amplitude indicate that this instability may be caused by the secondary instability of the primary stationary crossflow wave.

Boundary-Layer Transition Experiments in a Hypersonic Quiet Wind Tunnel

This paper reports the progress of three projects in the Boeing/AFOSR Mach-6 quiet tunnel at Purdue University. The first project used a 7-deg half-angle cone at 6-deg angleof-attack with temperature-sensitive paint applied to the frustum and small roughness dots added near the nosetip. Depending on the spacing of the roughness, the spacing and breakdown of the stationary vortices was altered. The second project looked at modifications to a pulsed jet perturber to reduce the perturbation duration. This was accomplished through both electronic and physical modifications to the perturber system. Shorter duration perturbations were achieved, but further progress is required. The third project measured low-frequency disturbances in the first-mode instability frequency range with Kulite pressure transducers on the surface of a cone-ogive-cylinder model. Initial measurements show that the magnitude of the low-frequency disturbance on the cone-ogive-cylinder is greatest outside the boundary layer, which may indicate an entropy-layer instability.