Aero-Optical Analysis of a Film-Cooled Optical Window Based on Linear Stability Analysis

Abstract

Film cooling is a common practice to protect the optical window of high-speed flying vehicles from overheating. However, the optical distortions caused by density fluctuations in the cooling film may deteriorate the quality of the window. The fluctuations are mainly due to the instability of the shear layer formed between the outer flow and the injected cooling-gas film. Therefore, a stability analysis of the shear layer, as well as the corresponding aero-optical analysis, is highly desired before the design of the window. In this paper, a method of assessing the aero-optical effect based on a stability analysis is proposed. Two simplified models are used to highlight the essential mechanisms through which the optical performance is affected by stability characteristics of the shear layer. Three cases with different cooling gases are investigated, namely, the commonly used air, a light gas (that is, helium), and a slightly heavier gas (that is, carbon dioxide). Among the three candidates, helium performs the best under the criterion of optical path difference because the low density of helium is highly beneficial in reducing density fluctuations in the flow, resulting in much better optical performance as compared with the other two gases.