Modeling for Control Design of an Axisymmetric Scramjet Engine Isolator.

摘要

Renewed interest in scramjet propulsion has motivated efforts to construct models of the scramjet engine flow path that capture transient flow dynamics to an extent that allows model-based control design. In particular, a model of the propagation of the shock wave due to a change in backpressure is needed for use in designing a controller for unstart prevention. A simplified modeling process considers the engine as the cascade of an inlet/isolator subsystem and a combustor subsystem, with emphasis placed on constructing a model of the (axisymmetric) isolator. Two types of models were considered in this effort: physics-based models constructed through spatial discretization of the compressible Euler equations and data-based models constructed through system identification using computational fluid dynamics (CFD) data. While physics-based models constructed from the two-dimensional Euler equations were found to be too expensive, a reduced-order model based on the one-dimensional Euler equations was found appropriate for the task at hand. Concurrently, a set of linear isolator models were constructed through application of subspace state-space identification technique to CFD data. An unstructured representation of uncertainty in this model was constructed from covariance data. Combining these results with error and residual analysis, a single linear model was chosen for consideration in control design. Based on a mixed-sensitivity approach, an unstable and a stable controller were designed for this model with the goal of anchoring the shock wave at a specified location in the presence of an input disturbance. While both controllers were successful in maintaining the shock location within 3 cm of the expected location for the nominal model, it was found that the controller with stable dynamics resulted in enhanced robustness for the closed-loop system. The model was then validated in closed-loop with the physics-based model, where the controller successfully meets the design goal despite limitations in the nonlinear model. This shows the promise of applying reduced-order model-based control design to the problem of active unstart prevention in a scramjet engine.