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.
07/14/2013
Recent Conference Presentation: 29th International Symposium on Shock Waves
The 29th International Symposium on Shock Waves will be held July 14-19 in Madison, WI. Home to the University of Wisconsin-Madison. All conference events will take place at the Memorial Union located on the University of Wisconsin - Madison campus and on the shores of Lake Mendota.
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.