Controlling the Flow Structures Within a Scramjet Isolator With Backpressure Manipulations

Abstract

Potentially, for hypersonic access to space vehicles, the scramjet engine is the propulsion system of choice and will be required to operate in a variety of flight conditions. In many cases, the freestream dynamic pressure may be held constant, however, the Mach numbers may range from 4 to 12. Operating in such a broad Mach range, will in turn require the combustor to accommodate varying conditions. Computational Fluid Dynamics as an engineering tool has been used in this paper to analyze be fluid field physics within a scramjet isolator. Currently, with proven capability to diagnose scramjet isolator design challenges, especially those tools that will predict and prevent unstarts, are lacking. To overcome these challenges, the Integro-Differential Scheme (IDS), which was developed and improved in Ref [1–2], is used in the computational analyses’ aspects of this effort. In addition, the numerical model is designed with back-pressure manipulation capability that seeks to influence the real-time flow behavior within the isolator based on experiment. The base-line scramjet isolator is model after a Mach 1.8 isolator with a length to height ratio of 8.40 has been simulated in this paper. The aerodynamic conditions used in the design of the numerical model was extracted from the experimental data presented in Ref. [3]. The flow physics within the isolator numerical model was studied under two sets of back pressure conditions; namely, (a) natural designed condition and (b) fixed adverse conditions. It is noteworthy to mention, backpressure studies were conducted through the use of ‘smooth’ and ‘discrete’ pressure jumps. In addition, the backpressure conditions were allowed to vary real-time as the flow structures within the isolator were observed. The engineering analysis conducted herein demonstrated results that are in excellent agreement with the available experimental data. It was observed that under design conditions, the isolator flow field consisted of an oblique shock train, which was strongest closest to the entrance of the isolator. Also, it was observed during each ‘discrete’ change in back pressure value, a wave, comprising of a coupled pair of oblique shocks and a normal shock, resembling the ‘lambda shock pattern’ emerges from the exit of the isolator. During each test, this ‘lambda shock’ travels to the front of the isolator, interacting with and dominating each set of reflected waves along its path. In each case, the lambda shock interacts with the front-most and strongest pair of oblique shocks, rocking back and forth before the entire isolator flow field settles down into a new configuration. This process intensifies as the back pressure discrete jump increases in strength, and the oblique shock train transformed into a form that closely mimics a normal shock train, with the strongest ‘lambda shock’ at the head of the isolator. In general, it appears as if the isolator flow patterned itself as a flexible spring within the constant area duct, constantly modifying its ‘net shock strength’ to accommodate the rising back pressures and while pushing the leading lambda shock small increments towards the entrance. The results showed that at a PBP with α = 2.1, the leading lambda shock moves rapidly towards the entrance, and with a PBP with α = 2.2 the isolator reach to ‘unstarts’ condition. In the end, different data sets have been provided with the relationship of backpressure variation and 1st ‘lambda’ location versus time by using IDS simulation.