Doppler-free Velocimetry as a High-Speed Flow Diagnostic

Abstract: This paper presents the potential application of a new Doppler-free velocimetry technique based upon saturated absorption of rubidium atoms to make precise, spatially-resolved velocity measurements in the freestream of a low-density high-enthalpy shock tunnel facility. This technique aims to overcome the line-of-sight disadvantage encountered in tuneable diode laser absorption spectroscopy (TDLAS) velocity measurements. The technique is non-intrusive to the flow, provides a good signal-to-noise ratio upon seeding the tunnel with rubidium, therefore allowing it to be used in low-density flows and most importantly, unlike TDLAS provides spatially resolved measurements. The counter-propagating pump-probe laser beam arrangement results in the formation of narrow resonances (? 6 MHz spectral half width) on the Doppler-broadened absorption spectrum with FWHM of 600 MHz. These narrow dips make for a much more precise zero-velocity reference point for absorption measurements because their wavelength is independent of the Doppler shift induced by the bulk velocity of the flow and the broadening of spectral features induced by the random motion of the rubidium atoms. This paper will present modelling of the potential use of saturated absorption spectroscopy as a means of determining spatially resolved velocity within a small volume in a hypersonic nozzle-exit flow. The changes in sensitivity of the measurement with respect to various important flow and laser parameters will also be presented. Keywords: Hypersonic flow, velocity diagnostics, diode laser spectroscopy