TRANSIENT PHENOMENA ASSOCIATED WITH MODE TRANSITION IN STRUT-BASED DUAL-MODE SCRAMJET ENGINES*

A strut-based, parallel-flow, supersonic combustor, based on the DLR geometry, was numerically simulated for mode transition, both from supersonic to subsonic and from subsonic to supersonic modes by throttling the fuel flow rate while holding the air flow rate constant. Both RANS and URANS simulations were performed using ANSYS-Fluent software to capture steady behaviour at fixed fuel-flow rate and unsteady behaviour for step changes in fuel-flow rate respectively.

Temporally resolved wall pressure data were used for identifying eigenfrequencies associated with different fuel-flow rates and the variation in eigenfrequencies during shock readjustments and mode transition. Pressure data from supersonic, choked and subsonic regions of the combustor flowpath were compared both in time domain and frequency domain to assess modal behavior, coherence, and signal attenuation. Spatio-temporally resolved shock structure oscillations, both during fixed fuel-flow rate operations and during sudden change in fuel flow rates were tested for assessing oscillation frequency, phase, and their correlation with wall pressure oscillations. Hysteresis behaviour associated with mode transition was investigated using temporally resolved numerical schlieren images and wall pressure data.