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

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 zF=5.08 cm, radius a=10.80 cm). A second reflector (zF=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≤80 mm and 20.74 mm≤z≤95.74 mm. Small-signal N waves (P=400 Pa, duration T=9 μs) were measured as well as stronger ones (P=1000 Pa, T=12 μs). 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.]