The persistence of regular reflection during strong shock diffraction overrigid ramps

We report on calculations and experiments with strong shocks diffracting ov er rigid ramps in argon. The numerical results were obtained by integrating the conservation equations that included the Navier-Stokes equations. The results predict that if the ramp angle 8 is less than the angle theta, that corresponds to the detachment of a shock, theta < <theta>(e), then the ons et of Mach reflection (MR) will be delayed by the initial appearance of a p recursor regular reflection (PRR). The PRR is subsequently swept away by an overtaking corner signal (cs) that forces the eruption of the MR which the n rapidly evolves into a self-similar state. An objective was to make an ex perimental test of the predictions. These were confirmed by twice photograp hing the diffracting shock as it travelled along the ramp. We could get a P RR with the first exposure and an MR with the second. According to the von Neumann perfect gas theory, a PRR does not exist when theta < <theta>(e). A viscous length scale x(int) is a measure of the position on the ramp where the dynamic transition PRR --> MR takes place. It is significantly larger in the experiments than in the calculations. This is attributed to the fact that fluctuations from turbulence and surface roughness were not modelled in the calculations. It was found that x(int) --> infinity as theta --> the ta (e) Experiments were done to find out how x(int) depended on the initial shock tube pressure p(0). The dependence was strong but could be greatly r educed by forming a Reynolds number based on x(int). Finally by definition, regular reflection (R) never interacts with a boundary layer, while PRR al ways interacts; so they are different phenomena.