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

4pPAa6. A study of the role of diffraction in the behavior of focusing step shocks using NPE-type equations and a finite difference program.

Andrew A. Piacsek

Graduate Prog. in Acoust., Penn State Univ., P.O. Box 30, State College, PA 16804

A finite difference solution to an NPE-like equation is used to study the linear and nonlinear behavior of focusing step shocks in regions where caustics are formed. The initial condition is a nominally plane step pulse with a hyperbolic tangent rise profile of thickness l[sub sh]; the wavefront has a single ripple of length scale L[sub wf], concave towards the direction of propagation. When (epsilon)=l[sub sh]/L[sub wf] is much less than unity, the linear numerical simulation showed that the shock behaves nearly according to geometric theory. Several different values of (epsilon) were investigated, verifying the linear theory predictions of normalized amplitude ~(epsilon)[sup -1/4] at the cusp [J. Hunter and J. Keller, Wave Motion 9, 429--443 (1987)]. Numerical solutions are also obtained for three cases of nonlinear propagation with dissipation. In addition to (epsilon), the distinguishing parameters are shock amplitude and dissipation. It is observed that for cases in which nonlinearity is stronger relative to diffraction and focusing, the wavefront forms a cusp at a later time, caustics are less pronounced, and the (normalized) rise time is shorter. Implications of these results on the effect of caustics on sonic boom rise times is discussed.