### ASA 128th Meeting - Austin, Texas - 1994 Nov 28 .. Dec 02

## 3aPAa2. Sonic boom rise time.

**Robin O. Cleveland
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David T. Blackstock
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*Appl. Res. Labs. and Mech. Eng. Dept., Univ. of Texas at Austin, P.O. Box
8029, Austin, TX 78713-8029
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The rise time of a sonic boom shock in air depends strongly on the
relaxation processes of nitrogen and oxygen. Stratification of the atmosphere
leads to significant variation of the relaxation processes with altitude. J.
Kang [Ph.D. thesis, Penn State (1991)] argues that the shock profile can adjust
quickly enough to changes in attenuation that it always appears to be in steady
state. If correct, then rise time at the ground can be calculated from local
conditions only. A time domain computer algorithm, based on work by Lee and
Hamilton (``Time domain modeling of pulsed finite-amplitude sound beams,''
submitted to J. Acoust. Soc. Am. in March 1994), is presented for a
Burgers-type equation. The algorithm includes the effects of nonlinear
distortion, thermoviscous absorption, molecular relaxation, and ray tube
spreading. A parametric study of the effect of change in relative humidity
shows that the steady-state assumption is not justified for sonic boom shocks
in the atmosphere. A sonic boom propagated through a real atmosphere is shown
to have a rise time that, in all cases run so far, is shorter than that of a
steady-state shock calculated using atmospheric properties at the ground only.
[Work supported by NASA.]