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

1pPA5. Experimental study of reflection of airborne, spark-produced N waves by a paraboloidal dish.

Lawrence J. Gelin

Jamie M. Shorey

Daniel M. Hester

David T. Blackstock

Appl. Res. Labs. and Mech. Eng. Dept., Univ. of Texas at Austin, P.O. Box 8029, Austin, TX 78713-8029

An experimental investigation of the transient response of a paraboloidal reflector is reported. An inhomogeneous plane N wave was produced by locating an electrical spark at the focus of a machined aluminum paraboloidal reflector (focal length z[sub F]=5.08 cm, radius a=10.80 cm). A second reflector (z[sub F]=5.17 cm, a=10.05 cm) was constructed by spinning a container of epoxy at constant speed and allowing it to cure. Peak pressure P and arrival time were measured across the beam (fixed axial distance z measured from the reflector surface, variable radial distance r) and along the axis. The range of measurements was r(less than or equal to)80 mm and 20.74 mm(less than or equal to)z(less than or equal to)95.74 mm. Small-signal N waves (P=400 Pa, duration T=9 (mu)s) were measured as well as stronger ones (P=1000 Pa, T=12 (mu)s). For small-signal N waves the axial measurements generally confirm Hamilton's theoretical prediction (previous paper, 1pPA4) although the edge waves are weaker than forecast. Transverse measurements agree with ray-theory predictions off axis but are up to 10% low in the axial region. For stronger N waves, transverse measurements of arrival time and peak pressure show evidence of self-refraction (ray bending due solely to finite-amplitude effects). [Work supported by ONR, ARL:UT IR&D program, and NASA.]