News

03/20/2017

"Turbulent spots in hypervelocity flow" published in Experiments in Fluids

This paper, based largely on Chapter 6 of my Ph.D. thesis, is currently online at Experiments in Fluids and slated to appear in print shortly. DOI: 10.1007/s00348-017-2317-y

The turbulent spot propagation process in boundary layer flows of air, nitrogen, carbon dioxide, and air/carbon dioxide mixtures in thermochemical nonequilibrium at high enthalpy is investigated. Experiments are performed in a hypervelocity reflected shock tunnel with a 5-degree half-angle axisymmetric cone instrumented with flush-mounted fast-response coaxial thermocouples. Timeresolved and spatially demarcated heat transfer traces are used to track the propagation of turbulent bursts within the mean flow, and convection rates at approximately 91, 74, and 63% of the boundary layer edge velocity, respectively, are observed for the leading edge, peak, and trailing edge of the spots. A simple model constructed with these spot propagation parameters is used to infer spot generation rates from observed transition onset to completion distance. Spot generation rates in air and nitrogen are estimated to be approximately twice the spot generation rates in air/carbon dioxide mixtures.

10/19/2016

"Boundary Layer Stability Analysis for Stetson's Mach 6 Blunt Cone Experiments" published in Journal of Spacecraft and Rockets

This paper is currently online at the Journal of Spacecraft and Rockets and slated to appear in print shortly. DOI: 10.2514/1.A33619

The results of a seminal experimental study of the effects of bluntness and swallowing length on transition on an 8 deg cone at zero angle of attack in Mach 6 high-Reynolds-number flow are analyzed with the STABL-2D computational fluid dynamics code package. Mean flow solutions and parabolized stability analyses for a total of 11 different nose-tip bluntnesses, ranging from sharp to a 15.24 mm radius, are obtained. For the sharpest cases, the N factor at transition is approximately seven; but, as bluntness increases and the calculated swallowing distance lengthens, the computed N factor at the experimentally observed transition location drops below the level at which Mack's second mode would be expected to lead to transition. These results indicate that the dominant instability mechanism for the bluntest cases is not the second mode, and oblique mode analysis also excludes these modes as the dominant instability mechanism.

09/16/2016

"Effects of Shock-Tube Cleanliness on Hypersonic Boundary Layer Transition at High Enthalpy" published in AIAA Journal

This paper is currently online at AIAA Journal and slated to appear in print shortly. DOI: 10.2514/1.J054897

An improved cleaning procedure in a hypervelocity shock tunnel improves the repeatability of transition measurements, demonstrating the need for researchers using impulse facilities for hypervelocity boundary-layer instability and transition research to operate the facility in a manner least likely to introduce particulate to the test flow. A statistical analysis of the correlation of tunnel parameters to transition location indicates that the coefficient of determination was significantly increased after the implementation of the cleaning regimen. This increase in the coefficient of determination is consistent with more repeatable transition locations and flow quality, and facilitates fundamental hypervelocity boundary-layer stability and transition research.

06/15/2016

Disturbance and Phase Speed Measurements for Shock Tubes and Hypersonic Boundary-Layer Instability

This paper was presented at AIAA Aviation 2016, which was held in June in Washington, DC..

Measurements and calculations of the phase speed of disturbances observed in a small shock tube are made, via cross-correlation of successive frames from focusing schlieren videography and double focused laser differential interferometry. The efficacy of the latter technique for measuring slender-body hypersonic boundary-layer instability wave-packets is also demonstrated. Shock tube experiments are performed to provide a known velocity input to both sensors for comparison, and measurements of phase velocity are made behind the reflected shock. Power spectral density curves from the FLDI and focusing schlieren from shock tube experiments at similar conditions are compared with each other and with pitot tube results, and reasonable agreement is found to the frequency limits of the sensors.

01/20/2016

Boundary Layer Stability Analysis for Stetson's Mach 6 Blunt Cone Experiments

This paper was presented at AIAA SciTech 2016, which was held in January in San Diego, California.

The results of a seminal experimental study of the effects of bluntness and swallowing length on transition on an 8-degree cone at zero angle of attack in Mach 6 high Reynolds number flow are analyzed with the STABL computational fluid dynamics code package. Mean flow solutions and PSE-Chem stability analyses for a total of 11 different nose tip bluntnesses, ranging from sharp to 15.24 mm radius, are obtained. For the sharpest cases, NTr ≈ 7, but as bluntness increases and the calculated swallowing distance lengthens, the computed N-factor at the experimentally-observed transition location drops below the level at which Mack’s second mode would be expected to lead to transition. These results indicate that the dominant instability mechanism for the bluntest cases is not the second mode.

06/15/2015

Correlation of HIFiRE-5 Flight Data With Computed Pressure and Heat Transfer

This paper was presented at AIAA Aviation 2015, which was held in June in Dallas, Texas.

The HIFiRE-5 test article was an elliptic cone with a 2.5-mm nose radius and 2:1 aspect ratio and a 7-degree minor-axis half-angle. The vehicle was flown in April 2012. The upper stage of the sounding rocket failed to ignite, resulting in a peak Mach number of about 3 instead of the target of 7. Flight heat flux and pressure data (reduced from almost 300 thermocouples and 50 pressure transducers) have been compared to α- and β-dependent CFD results for pressure distribution, as well as laminar and turbulent heat-transfer results. Computations were performed at three time points in the ascent trajectory. At each time point, five values each of angle of attack and yaw, ranging from -5.0° to 5.0°, were computed. CFD pressures, normalized with p, were interpolated to the flight Mach numbers at specified times throughout the ascent and descent trajectories. At each flight time, α and β were estimated from measured pressure by determining the α-β combination that minimized the RMS difference between the measured and computed pressures. The vehicle attitude, as determined from measured pressure, was compared to the vehicle attitude derived from Inertial Measurement Unit (IMU) results for α and β from the flight. The two methods showed excellent agreement for the entirety of the ascent and reentry portions of the trajectory. A similar normalization of the laminar and turbulent heat transfer CFD results with St was compared to flight heat transfer measurements, and transition locations were inferred. Finally, a computational heat conduction analysis was made to verify assumptions inherent in the calculation of heat flux from temperature.

02/01/2015

Fluid Analysis Indicates the Patriots May Not Have Cheated

Realistic modeling of the rapid filling process for a football, neglected in current media coverage, can easily account for a “greater than 2 psi” (ESPN, 21 January 2015) drop in internal pressure from the moment immediately after a ball is inflated and checked by the referees prior to an NFL game to the point, over two hours later, when it is used during competition. In general, this analysis supports the hypothesis that the New England Patriots did not present the referees with underinflated footballs prior to the AFC Championship Game in which they defeated the Indianapolis Colts on 18 January 2015.

10/10/2013

Fun New High-Speed Schlieren Videos

High speed Schlieren videos were produced highlighting the fluid mechanics found in everyday objects. This video (entry 102369) was submitted as part of the Gallery of Fluid Motion 2013, which is a showcase of fluid dynamics videos.

Several videos of common objects are produced to remind us that we are constantly surrounded by a variety of fluid mechanical processes in our everyday lives. The videos are taken using a Phantom v710 camera set at 1000 fps, with an exposure time of 330 ms. A conventional z-type Schlieren setup is used with an Osram Diamond Dragon LUW W5AP continuous LED white light source. The movies are played back at 25 fps, which corresponds to a reduction in speed by a factor of 40.

04/10/2013

Invited Lecture at the University of Notre Dame

In this talk, I present results from recent experimental investigations into a fundamental problem in compressible flow: measuring and characterizing the laminar to turbulent transition process in boundary layer flows in thermochemical nonequilibrium at high enthalpy.

In this flow regime, where the second or Mack mode instability dominates, nonequilibrium effects for certain species (in our work, carbon dioxide) can actually suppress transition through the absorption of energy from acoustic disturbances through vibrational relaxation. Higher thermal loads, by half an order of magnitude or more, result from the increased heat transfer due to turbulent flow, so laminar to turbulent transition is a critically important process in hypervelocity vehicle design. We measure the propagation and growth of turbulent spots within the boundary layer, characterize transition delay for flows in air with increasing mass fractions of carbon dioxide, and also investigate the efficacy of gas injection mechanisms into the hypervelocity boundary layer for inhibiting second mode transition, with the goal of gleaning useful data to be exploited in future aerospace vehicles.

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