Abstract

This article discusses the role of conventions in defining the concept of inertial reference frame, and it specifies key historical evidence, up to now widely ignored, connecting Poincaré, Einstein, and Reichenbach’s analyses of simultaneity.

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Experimental measurements of the displacements of shock accelerated microparticles from shortly after shock interaction to the particle relaxation time show time-dependent drag coefficients ($C_{D}$) that are much higher than those predicted by quasi-steady and unsteady drag models. Nylon particles with mean diameter of $4~\unicode[STIX]{x03BC}\text{m}$, accelerated by one-dimensional normal shocks (Mach number $M_{s}=1.2$, 1.3 and 1.4), have measured $C_{D}$ values that follow a power-law behaviour. The drag is a function of the time-dependent Knudsen number, $Kn^{\ast }=M_{s}/Re_{p}$, where the particle Reynolds number ($Re_{p}$) is calculated using the time-dependent slip velocity. Some portion of the drag can be attributed to quasi-steady forces, but the total drag cannot be predicted by current unsteady force models that are based on the Basset–Boussinesq–Oseen equation and pressure drag. The largest contribution to the total drag is the unsteady component ($C_{D,us}$) until the particle attains $Kn^{\ast }\approx 0.5{-}1.0$, then the unsteady contribution decays. The quasi-steady component ($C_{D,qs}$) increases almost linearly with $Kn^{\ast }$, intersects the $C_{D,us}$ at $Kn^{\ast }\approx 2$ and becomes the primary contributor to the drag towards the end of the relaxation zone as $Re_{p}\rightarrow 0$. There are currently no analytical models that are able to predict the nonlinear behaviour of the shock accelerated particles during the relaxation phase of the flow.

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