Interaction of oscillations of channel flow and flow separation at duct discontinuities

The subject of vortex flow and coupling between vortices and acoustic wave motions in rocket motor chambers has become a topic of high interest to rocket motor stability specialists in the past several years. Although the phenomenon was first noted to be a significant factor in the stability of very large booster motors, recent experiences in tactical-sized motors indicate that vortex-acoustic wave interactions can become an important factor in determining acoustic wave amplitudes in all solid-propellant rocket motors, regardless of size. A two-dimensional experiment was used to study the flow fluctuations in the separated region resulting from a wall-slot-step (convergent step). This geometry corresponds to the geometry present in a segmented solid rocket motor in the region of the ends of adjoining segments. A modulated tunnel flow of controlled oscillation frequency was used, and the effect on flow fluctuations in the separated region was examined by hot-wire velocity measurements. The transfer function between free stream oscillations and separated flow fluctuations at the driven frequency was determined over a range of frequencies and main stream flow velocities. The transfer function exhibited a frequency dependence with maximum in the 10-25 Hz range, with the maximum occurring at higher frequency when the mean flow velocity was higher.