Numerical Investigation of the Influences of the Features of Transonic Flow over a Hemispherical Turret on Beam Wavefront Distortions

Abstract

Complex features of transonic flow over a turret make it challenging to use passive flow control to reduce aero-optical effects. In this study, the influence of different flow features on wavefront distortions is numerically investigated through improved delayed detached eddy simulation coupled with a modified sub-grid scale. The proper orthogonal decomposition (POD) method is used to study the spatiotemporal characteristics of the flow features. The flow field changes in the wake along with the motion of the shock. Two features, namely, lateral shift (dominant in modes 1 and 2) and wall-normal fluctuation (dominant in modes 3 and 4) of the wake, are the most dominant in the flow field. All beams share the common feature of transmitting the flow field, in which a large component of optical path difference (OPD) appears at St=0.35, indicating the high impact of wall-normal fluctuation on the distortion of the wavefront. After the different POD modes, which contain 30% of the mode energy, are removed, all beams transmitted through different reconstructed fields show very different features for OPD. The flow features that do not exhibit higher-order modes from modes 21 to 92 affect the OPD slightly, as the OPD components in the low-St region are almost unchanged. With the removal of modes from 3 to 32, wavefront distortions are considerably reduced, particularly at St=0.35. The wavefront distortions are most reduced after the lower order modes from 1 to 20 are removed, as the components of OPD in the low-St region are dramatically reduced. The significant relations between OPD and the flow features reveal that controlling the dominant flow features has significant potential for reducing aero-optical effects.