  Shock wave surfing is investigated experimentally in GALCIT's Mach 4.0
Ludwieg Tube. Shock wave surfing occurs when a secondary free-body follows the
bow shock formed by a primary free-body; an example of shock wave surfing
occurs during meteorite breakup. The free-bodies in the current investigation
are nylon spheres. During each run in the Ludwieg tube a high speed camera is
used to capture a series of schlieren images; edge tracking software is used to
measure the position of each sphere. Velocity and acceleration are had from
processing the position data. The radius ratio and initial orientation of the
two spheres are varied in the test matrix. The variation of sphere radius ratio
and initial angle between the centers of gravity are shown to have a
significant effect on the dynamics of the system.
  The air flow in each fluid dynamics video is from left to right. The Mach
number is 4.0 with total pressure in each run of roughly 200kPa. Each run
begins with the spheres tethered and no flow. Flow arrives and the tethers are
quickly broken. The spheres are then free-bodies with the dominant force being
the pressure distribution around the surface of the sphere, which, in the case
of the secondary sphere, is strongly manipulated by the shock wave interaction.
Gravity is not a considerable body force because of the short test times in
these experiments. The secondary spheres are visibly seen to be affected by the
bow shock wave of the primary bodies in the form of quite beautiful
trajectories. Computational images were obtained from a simulation using the
AMROC code. A fully three-dimensional simulation of two free-flying bodies was
performed, assuming inviscid flow, whereby the trajectory of each sphere was
calculated according to the instantaneous aerodynamic forces experienced.
