Amanda Chou

Instability and transition measurements in the Mach-6 quiet tunnel

The Boeing/AFOSR Mach-6 Quiet Tunnel achieved quiet flow to a stagnation pressure of 163 psia in Dec. 2008, the highest value observed so far. It remains quiet at pressures above 160 psia. Under noisy conditions, nozzle-wall boundary-layer separation and the associated tunnel shutdown appear to propagate slowly upstream, whereas under quiet conditions, the propagation is very rapid. A new diffuser insert has been designed, fabricated, and installed in the tunnel in order to start larger blunt models and increase run time. A flared cone with a circular-arc geometry was designed to generate large second-mode N factors under quiet flow conditions. When the computed N factor was 13, large instability waves were measured under quiet flow conditions using fast pressure sensors, but the flow remained laminar. Transition was observed only under noisy conditions. A laminar instability was detected in the wake of an isolated roughness element in the boundary layer on the nozzle wall; this appears to be the first such measurement at hypersonic speeds.

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.

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.

Transition Prediction in Hypersonic Boundary Layers Using Receptivity and Freestream Spectra

Boundary-layer transition in hypersonic flows over a straight cone can be predicted using measured freestream spectra, receptivity, and threshold values for the wall-pressure fluctuations at the tr...

Measurements of Resonance in a Forward-Facing Cavity at Mach Six

An experimental study of the resonance of a cylindrical forward-facing cavity was conducted in a Mach 6 quiet-flow wind tunnel. The diameter of this cavity was fixed and the depth was varied in order to find the critical depth at which the cavity resonance became self-sustained. At Mach 6, this critical depth was 1.2 diameters deep, regardless of the freestream noise levels. For cavities deeper than 1.2 diameters, measurements of root-mean-square pressure fluctuations were orders of magnitude larger than those in shallower cavities. In quiet flow, this increase was about 2.5 orders of magnitude. In noisy flow, this increase was only about one order of magnitude. However, the magnitude of the pressure fluctuations within deep cavities was about the same in quiet flow as it was in noisy flow. The damping characteristics of a shallow cavity (depth less than 1.2 diameters) were also studied by observing the cavity response to a freestream laser-generated perturbation. The perturbation convects with the flow a...

Characterization of a Laser-Generated Perturbation in High-Speed Flow for Receptivity Studies

Abstract : A better understanding of receptivity can contribute to the development of an amplitude-based method of transition prediction. This type of prediction model would incorporate more physics than the semi-empirical methods, which are widely used. The experimental study of receptivity requires a characterization of the external disturbances and a study of their effect on the boundary layer instabilities. Characterization measurements for a laser-generated perturbation were made in two different wind tunnels. These measurements were made with hot-wire probes, optical techniques, and pressure transducer probes. Existing methods all have their limitations, so better measurements will require the development of new instrumentation. Nevertheless, the freestream laser-generated perturbation has been shown to be about 6 mm in diameter at a static density of about 0.045 kg/m3. The amplitude of the perturbation is large, which may be unsuitable for the study of linear growth.

Measurements of the Interaction of an Upstream Laser Perturbation with a Forward-Facing Cavity

The laser perturber is a tool to be used for receptivity studies in the Boeing/AFOSR Mach-6 Quiet Tunnel. To demonstrate that the laser perturber is functioning correctly, it is tested with a forward-facing cavity. Results are similar to previous studies by Ladoon and Segura in a Mach-4 quiet tunnel. The cavity base pressure uctuations are nearly quiescent at shallow cavity depths. When a freestream perturbation interacts with the forward facing cavity, the cavity base pressure experiences oscillations that decay exponentially. The measured cavity resonance frequency matches the theoretical cavity resonance frequency to within 6%. The measured cavity response appears to be initiated by the acoustic disturbance that results from the laser-generated perturbation. The damping characteristics of the forward-facing cavity are also examined. For cavities with similar resonant frequencies, the damping constants can di er by almost an order of magnitude. However, cavities with similar non-dimensional depths have similar damping constants.

Receptivity and Forced Response to Acoustic Disturbances in High-Speed Boundary Layers

Supersonic boundary-layer receptivity to freestream acoustic disturbances is investigated by solving the Navier-Stokes equations for Mach 3.5 flow over a sharp flat plate and a 7-deg half-angle cone. The freestream disturbances are generated from a wavy wall placed at the nozzle wall. The freestream acoustic disturbances radiated by the wavy wall are obtained by solving the linearized Euler equations. The results for the flat plate show that instability modes are generated at all the incident angles ranging from zero to highly oblique. However, the receptivity coefficient decreases by about 20 times when the incident angle increases from zero to a highly oblique angle of 68 degrees. The results for the cone show that no instability modes are generated when the acoustic disturbances impinge the cone obliquely. The results show that the perturbations generated inside the boundary layer by the acoustic disturbances are the response of the boundary layer to the external forcing. The amplitude of the forced disturbances inside the boundary layer are about 2.5 times larger than the incoming field for zero azimuthal wavenumber and they are about 1.5 times for large azimuthal wavenumbers.