Session: 10-03-02: Fundamental Issues and Perspectives in Fluid Mechanics - II

Paper Number: 69420

Start Time: Monday, 04:00 PM

69420 - Unstart Phenomena Induced by Forced Excitations in a Dual-Mode Scramjet 

The recompression of supersonic gas flow by shock train occurs in a scramjet inlet-isolator. The isolator which isolates the inlet from the pressure build-up inside the combustion chamber is a critical component of the scramjet engine. Seldom pseudo-shock wave formed in the scramjet isolator shows self-excited oscillations when subjected to a certain range of steady back-pressures. In an actual scramjet, the combustor undergoes unsteady combustion giving rise to fluctuating backpressure, which causes forced shock train oscillations. Hence, the actual mechanism of recompression can be very different with fluctuating backpressure. The occurrence of such forced, unsteady fluctuations in an isolator can seriously limit its performance.
The pressure build-up from combustion heat release causes thermal flow choking and may lead to inlet unstart. One of the popular ways to mimic thermal flow choking, experimentally, is by using flow blockages in the form of wedges, in which pressure build-ups due to flow deceleration. The problem with a fixed geometry wedge is that it can simulate only a fixed backed pressure, whereas, the actual combustion chamber pressure variation is highly unsteady. Until now, no experimental study on the forced shock train oscillation in hypersonic flows is reported, mainly due to the limited available ground facilities that can reproduce such conditions. However, the effects of fluctuating backpressure on shock train are numerically addressed by several authors employing a movable flap at the exit of the isolator.  The movement of the flap is achieved by the dynamic mesh formulation. Such studies are limited by the inability of capturing a complete period of shock oscillation inside the isolator due to the limited range (angle) of flap movement.
The present study numerically addresses a complete period of shock oscillation inside the isolator of a dual-mode scramjet engine subjected to downstream periodic excitations, which were generated by a throttling elliptical-shaped disc, which rotates at low (~ 20 Hz) and high (~100 Hz) frequencies. For the unsteady simulations, the elliptical disc was rotated using a sliding mesh formulation at frequencies 10 and 50 Hz to produce a periodic variation in backpressure at a frequency double that of shaft rotation. In the sliding mesh formulation, all cells retain their original shape and volume, unlike in dynamic mesh motion. To the best of our knowledge, this is the first study employing sliding mesh formulation for the simulation of forced excitation of shock train in a dual-mode scramjet.
The present work provides a better understanding of forced, unsteady fluctuations of shock-train, its interaction with the asymmetric boundary layers on top and bottom walls and flow choking driven unstart. 

Presenting Author: V. Lijo Government Engineering College Thrissur

Authors:

Alvin Alex Government Engineering College Thrissur
V. Lijo Government Engineering College Thrissur