On Transition

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.

On AMO Smith, Life of a Gentle Genius

Development of Flaps

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.

AMO Smith

Associate Professor and Tenure

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.

Thomas D. Norum

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.

Laws of Thermodynamics

A long time ago, the running joke of the three laws of thermodynamics was introduced. They are the following:

  1. You cannot win.
  2. You cannot break even.
  3. You cannot get out of the game.

Sonic Boom for Lilliputians

Absurdly tiny models -0.25 to 1 inch in size-were tested in the Ames and Langley supersonic wind tunnels. With such miniaturization, the tunnel walls were up to 150 body lengths away from the models. The Lilliputian models generated shock waves all right, but they were so weak that new pressure sensors had to be conceived. Further, tunnel conditions had to be held more nearly uniform because slight changes in humidity or compressor speed would create transient flow conditions that confused the shock wave data. By taking great care, Whitham’s theory of sonic booms was verified in the idealized environment of the wind tunnel.

Numerical prediction of loudness metrics for N-waves and shaped sonic booms in kinematic turbulence

Abstract, “The effects of a kinematic field of velocity fluctuations on the loudness metrics of two waveforms are examined with a three-dimensional one-way propagation solver. The waveforms consist of an N-wave and a simulated low-boom from NASA’s X-59 QueSST aircraft. The kinematic turbulence is generated using a von Karman composite spectrum, which is dependent on a root mean square (rms) velocity and outer scale of the turbulence. A length scale is proposed to account for the effect of the rms velocity and integral scale on the focusing and defocusing of the sonic boom waveform. The probability density function of the location of the first caustic attains a maximum value when the propagation distance is equal to the proposed length scale. Simulation results indicate that for small values of the nondimensional propagation distance, the standard deviation of the loudness metrics increases linearly. The loudness metrics follow a normal distribution within a given range of the nondimensional propagation distance. Results indicate the potential to parameterize the loudness metric distributions by the rms velocity and integral length scale.”

Carr, A. N., Lonzaga, J. B., Miller, S. A. E., “Numerical Prediction of Loudness Metrics for N-Waves and Shaped Sonic Booms in Kinematic Turbulence,” The Journal of the Acoustical Society of America, Vol. 151, No. 3580, 2022. DOI: ​​10.1121/10.0011514 [Link via DOI][PDF][PDF Preprint]