A NOVEL INVESTIGATION OF THE OSCILLATORY FIELD OVER A TRANSPIRING SURFACE

Abstract

The flowfield character is investigated inside a long rectangular chamber in the presence of time-harmonic pressure waves. The chamber is designed with multiple interchangeable sections for the purpose of controlling the length and therefore the system's natural frequency. Pressure waves are induced externally at variable frequencies by means of a Scotch-yoke mechanism theoretically capable of imparting pure oscillatory motions. In characterizing the internal flowfield, velocity measurements are acquired inside a principal test section that can accommodate flat blocks of solid carbon dioxide (i.e., dry ice). As solid CO2sublimates, a flow of gaseous carbon dioxide is generated at the bottom of the principal section and enters the chamber in the transverse direction. The resulting generalized Stokes layer formed above the transpiring surface exhibits several features associated with oscillatory flows over impermeable surfaces, including an overshoot in the velocity amplitude in the vicinity of the transpiring wall known as Richardson's annular effect. Quantitative pressure and velocity measurements are in agreement with theoretical predictions obtained from recent models of the oscillatory field over transpiring surfaces. The acoustic Reynolds number based on the Stokes layer thickness increases linearly with increasing Scotch-yoke frequency except in the neighborhood of the system's natural frequency. Near resonance, a sharp non-linear increase in the acoustic Reynolds number is noted. Furthermore, both acoustic pressure and velocity amplitudes increase with the driving frequency in a manner that is consistent with current theories. Since the sublimation rate of dry ice can be expressed in a similar mathematical form to the regression rate at the burning surface of solid propellants, this experiment constitutes a cold flow simulation of the internal flowfield in solid rocket motors.