Millimeter wave interferometry in optically thick particle-laden flows: an application to plume-surface interactions*

Millimeter wave radar interferometry provides a unique capability for measuring the volume fraction of particles in dispersed multiphase mixtures. It allows the measurement of particle concentrations in optically thick flows independently of the size distribution of the dispersed phase, in contrast with state-of-the-art methods based on optical attenuation and scattering. In this work, a proof-of-concept based on a COTS mmWave radar measures the concentration of particles in ejecta clouds generated by the impingement of a Mach 5 jet on a granular surface. This method is extended from a single path-integrated measurement to a multi-path tomographic measurement.

The instrument is calibrated under vacuum against a slot funnel producing gravity-driven particle sheets with known particle concentrations. The dielectric permittivity of the particle material is measured via the waveguide method in a separate experiment. Interferometric measurements are performed on a plume-surface impingement experiment using a 6.5 N cold gas thruster replicating the ambient pressures, Mach numbers and pressure ratios of lunar and martian landings.