It would be difficult to create the right mood under the present circumstances, but perhaps by accident another beautiful differential equation will come along again, as it once did when I worked with you.
Letter from Meyer to Prandtl, May 5, 1918. Ref. no. GOAR:2647, DLR-Gottingen Archives
The prediction of cross-spectra from first mode instability waves within high-speed flow over sharp and blunt cones with plasma actuation
Abstract: Leading edge geometries, such as cones, moving at high-speed undergo intense loading due to the growth of instability waves and turbulent transition. These instability waves are highly spatially coherent. Aerodynamic loading related to instability waves and transition cause large-amplitude vibrations within the underlying structure, which may lead to flight-vehicle failure. We examine the effect of plasma actuation on the pressure fluctuations from first mode instability waves on the cone surface via theory with flow-fields predicted by computational fluid dynamics. We present predictions for a seven-degree half-angle cone at free-stream Mach 2.0, 3.5, and 5.0 with varying nose radii. Nose radii range from 0.038 to 38.1 mm and represent both sharp and large leading edge bluntness. For non-actuated flows, we observe that very small radii leading edges do not alter the maximum growth rates. Large radii cones have lower growth rates due to a thicker boundary layer. Spatial coherence of the instability waves decreases with increasing frequency. The growth rates are smaller at higher freestream Mach number. The effect of the simulated plasma actuator adds local heating to the flow-field. Increased nose radii lowers the relative temperature difference between the actuated and base flow-fields. The relative temperature differences are higher at higher freestream Mach number. We find that plasma actuation stabilizes the flow-field and spatial coherence becomes smaller.
- Cheng, J. and Miller, S. A. E., “Cheng, J. and Miller, S. A. E., “The Prediction of Cross-Spectra from First Mode Instability Waves within High-Speed Flow Over Sharp and Blunt Cones with Plasma Actuation,” Aerospace Science and Technology, Vol. 130, No. 107870, 2022. pp. 1-21. DOI: 10.1016/j.ast.2022.107870 [Link via DOI][PDF Preprint]
On Challenges in Turbulent Flow Theory and Experiment
Research in macroscopic classical physics, such as fluid dynamics or aspects of condensed matter physics, continues to confront baffling challenges that are by no means less demanding than those at the post-Newtonian frontiers of physics that have been explored since the beginning of this century. This is so even though the basic equations of macroscopic classical physics are knownindeed, have been known for centuries in many cases. Chaos and nonlinear dynamics are examples of the topics that pose new challenges to our understanding of macroscopic classical systems. Turbulence, a phenomenon related to but distinct from chaos, and having strong roots in engineering, has been increasingly in the focus of physics research in recent years. Turbulence occurs in a very wide variety of flows, ranging from the mixing of cream in a coffee cup to the dispersal of pollutants in the atmosphere, from the formation of galaxies in the early universe to thermal convection in stars, from flows around automobiles, ships and aircraft to combusting flows in turbomachinery.
Uriel Frisch and Steven A. Orszag, DOI: 10.1063/1.881235
On Lift
It’s easy to explain how a rocket works, but explaining how a wing works takes a rocket scientist
Philippe Spalart, Boeing Technical Fellow
Mach Experiment
Syzygy
Would you like to know the best possible method for solving a Syzygy problem? It is this. Having ascertained from your doctor that you are in a state of health to bear, without risk, severe brainwork and keen intellectual excitement, from the most candid of your intimate friends that you are in a pleasant temper, and free from all morbid irritability; and from yourself that you are in the humour for the task seat yourself in an easy-chair, taking care that you have writing materials within reach, and that there are no dictionaries in the room; close your eyes, and paint the two given words on your mental retina.
Lewis Carroll
On the On-Set of Hypersonics
Almost everyone has their own definition of the term hypersonic. If we were to conduct something like a public opinion poll among those present, and asked everyone to name a Mach number above which the flow of a gas should properly be described as hypersonic there would be a majority of answers round about 5 or 6, but it would be quite possible for someone to advocate, and defend, numbers as small as 3, or as high as 12.
Philip Roe, verbal comment at von Karman Institute, 1970.
Fourty One Years of the NASA STS
Associate Professor
It is my first day as an Associate Professor of Mechanical and Aerospace Engineering at the University of Florida. Just about six years ago I left NASA and joined as an Assistant Professor. My goals remain the same. First is to perform excellent research of fundamental nature. The second is to inspire others and myself. Time seems to be speading up.
Congratulations to Dr. Alex Carr on Defending his Ph.D.
My graduate student, Alex Carr, defended his dissertation on sonic boom through the turbulent atmosphere.
Title: SONIC BOOM PROPAGATION IN THE TURBULENT ATMOSPHERIC BOUNDARY LAYER
Abstract:
Sonic boom waveforms measured at ground level exhibit variability due to scattering and diffraction caused by the presence of turbulence in the atmospheric boundary layer (ABL). Sonic boom propagation in the ABL is considered in the context of a partially one-way equation for a finite amplitude pressure perturbation that incorporates turbulence effects. This equation is shown to simplify to several well known equations in nonlinear acoustics, if certain assumptions are made. A wide-angle parabolic approximation is applied to the heterogeneous terms of the governing equation. The forward solution along the propagation direction is computed with a split-step method. The solution at each propagation plane is the composition of solutions to subproblems that account for each physical effect: nonlinear distortion, diffraction, atmospheric absorption, and the effects of mean flow and turbulence. Simulation results for benchmark acoustic problems are compared to the corresponding analytical solutions to validate the code. Turbulence is synthesized in the computational domain with Fourier synthesis techniques that have been used previously for sound propagation simulations. The turbulent kinetic energy spectrum is approximated by a von Ka ́rma ́n model. Simulations of traditional N-wave and shaped booms are performed through homogeneous turbulence, as well as inhomogeneous turbulence that is representative of daytime conditions in the atmospheric boundary layer. A length scale is proposed to non-dimensionalize the propagation distance and collapse the probability density functions of the caustic locations obtained from the N-wave simulations. For the simulations performed through homogeneous turbulence, the average loudness levels for both waveforms are shown to decrease along the propagation direction due to turbulence effects. The standard deviation of the loudness metrics increases linearly for small non-dimensional distances. The maximum standard deviation of each loudness metric considered for both waveforms fell between 2 to 4 dB. For the simulations performed through inhomogeneous turbulence, the effects of ABL height and convection level on a traditional N-wave and shaped waveform are considered. An empirical modification to the scaling length is proposed to account for the varying ABL height altering the turbulence integral length scales in the mixed layer region. Results for the loudness metrics are shown to follow a normal distribution for both waveforms when the non-dimensional distance is less than 2, beyond which the observations become increasingly skewed to the right of a normal distribution. Results indicate that the loudness metric distributions of both waveforms are likely to be normally distributed in the region undertrack of the flight path for weak to strong convection levels in the ABL. Additional simulations are conducted of sonic boom decay into the shadow zone region. Results indicate that loudness levels of both waveforms in the shadow zone region are sensitive to the turbulence levels in the ABL. As the turbulence level increases, the average of each loudness metric increases.
Carr, Defense, 2022.