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
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.
I’m pleased to see my student Dr. Jianhui Cheng defend her Ph.D. successfully at University of Florida.
Title: THE APPLICATION OF INSTABILITY WAVE THEORY ON ACOUSTIC RADIATION WITHIN OFF-DESIGN SUPERSONIC JETS AND ON TWO-POINT STATISTICS WITHIN HIGH-SPEED FLOW OVER SHARP AND BLUNT CONES WITH PLASMA ACTUATION
The intense noise generated by large-scale turbulent structures is the main component of acoustic radiation from jets, especially in the downstream direction, which is harmful to human health. Furthermore, the high-frequency waves created by boundary layer turbulence can cause intense vibrations on vehicle surface structures during rocket launch. It can be simplified as a cone flow. As instability waves are closely connected with the formation of large-scale turbulent structures, instability wave theory is applied to compute the radiated noise in the far-field for free jets and the pressure fluctuations on the cone surface. A phenomenological plasma actuator model is applied to alter the flow-fields and stability properties of the cone flows. We compute the radiated noise from instability waves within two off-design supersonic jet flows. The directivity and azimuthal properties of jet noise from large-scale turbulent structures in the downstream direction are captured. The spectral density of noise in the downstream directions agree well with the experimental measurements, whereas they are lower at the upstream direction. The auto-correlation shows similar behavior both in the downstream and upstream directions. The cross-correlation and coherence shows higher values than the prediction from the Kirchhoff surface method and experimental measurements. The results show that the noise from instability wave models in the upstream and downstream direction are highly correlated. The results could be used for future flow control on noise reduction from large-scale turbulent structures. We examine the effect of plasma actuation on the pressure fluctuations from the 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, 3.5, and 5 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. We find that plasma actuation stabilizes the flow-field and spatial coherence becomes smaller. The results are beneficial for future flow control to reduce the vibration from large-scale turbulent structures.
The information needed by design engineers of either aircraft or flow machinery is the pressure, the shearing stress, the temperature, and the heat flux vector imposed by the moving fluid over the surface of a specified solid body or bodies in a fluid stream of specified conditions. To supply this information is the main purpose of the discipline of gasdynamics.
The whole problem of aerodynamics, both subsonic and supersonic, may be summed up in one sentence: Aerodynamics is the science of slowing-down the air without loss, after it has once been accelerated by any device, such as a wing or a wind tunnel. It is thus good aerodynamic practice to avoid accelerating the air more than is necessary.
He was introduced to the sensitivity of stalling characteristics of wings to the boundary layer state near the suction peak by a change in paint scheme! The XBT-1 a forerunner to the famous SBD Dauntless alrcraft had good stall characteristics. The XBT-2 had exactly the same wing but had an unacceptable stall. The XBT-1 wing was completely painted with aluminum colored paint whereas the XBT-2 wing was painted yellow on the top and aluminum on the bottom. Where the colors joined at the leading edge, a slight ridge was formed by the masking tape during painting. John Wheatly, AMOs’ boss, took out his pocket knife and scraped away the ridge. When the XBT-2 was stalled again, the vicious stall had completely disappeared. This interaction of the boundary layer with stall characteristics obviously made an impression on AMO.
Original flap development was motivated by three desired benefits. 1) slower flying speeds, hence shorter takeoff and landing runs; 2) reduction of angle of attack near minimum flying speed; 3) increase of drag, or control of drag, in order to steepen glide angle in approach and reduce floating tendencies. Currently, because of large aircraft noise problems, the emphasis under the third item has changed. We are trying to reduce flap drag in order to reduce thrust requirements and hence the noise.
This week I received tenure and promotion to Associate Professor of Mechanical and Aerospace Engineering at the University of Florida. It is a privilege to research, teach, and perform service here. I’m looking forward to many more years of working with wonderful colleagues and friends. The best part of the work has been helping my students learn to think critically and independently.
My good friend Thomas D. Norum recently passed away. He worked as a researcher at NASA Langley over most of his career. I knew him starting in 2009 through 2016 while I was working there. He worked as an experimentalist in the jet noise lab of NASA Langley. As an experimentalist, he worked to understand the physics of jet turbulence and noise, while also reducing aircraft noise for the benefit of communities. I often see people referencing his publication on screech tones. Two major publications have been made on screech that are often cited (in my opinion). The first is by C. K. W. Tam, and the second is by T. D. Norum. Dr. Norum’s seems to be more predictive and based in measurement while Tam’s is based more purely on theory. Even today, there is no closed-form well accepted model for screech tones. I use to hang out with Dr. Norum at Afterburner’s at Langley. He always was positive (with a bit of snark) and am appreciative of our time together. God’s speed Dr. Norum.
Read more of his work through AIAA –
Screech suppression in supersonic jets by T. D. Norum – https://doi.org/10.2514/3.8059 Abstract – Screech from underexpanded supersonic jets has been investigated experimentally. Multiple screech modes, or stages, are found to be present at most jet operating conditions. The fundamental screech tone of each mode attains a maximum amplitude at about 20 deg from the inlet axis, with higher harmonics exhibiting multiple lobes. The directivity of each harmonic is predicted quite well from a stationary array of acoustic monopoles, with phasing between consecutive monopoles determined by the shock cell spacing and eddy convection velocity. Large reduction of screech amplitude can be obtained from modifications to the jet exit geometry, although the extent of this suppression is mode dependent.