Wavefront sensing of supersonic aero-optical effects on side-mounted windows

摘要

In order to gain insight into the different mechanisms involved in the optical degradation transmitted by infrared (IR) windows at supersonic flight speeds, wave front sensing has been used to quantify aero-optical effects on side-mounted windows of a supersonic fore-body in a series of wind-tunnel experiments. Flow-induced aberrations have been measured at partially duplicated flight conditions at Mach 3.7 and for different mass-density values corresponding to altitudes ranging from 14 up to 22 km. For each altitude condition different lines of sight and window slopes have been considered. Apart from steady shockwave effects, laminar-to-turbulent transition took place on the upstream window and generated intermittency phenomena in the boundary layer whereas on the downstream window, shear layer was completely detached so that full turbulent occurred. To capture these unsteady and turbulent effects the flow-induced optical distortions have been "frozen" by using a 15 ns, 40 Hz pulsed laser at 532 nm. The ability of the Shack-Hartmann wave front sensing technique employed to substract a reference permitted a simple calibration procedure to ensure accurate measurements. Sets of curves of optical phase power spectra and FTMs obtained relating to the various aerodynamic parameters involved will be presented. Their distribution shows a correct behavior according to aerodynamic parameter variations. Isotropic turbulent characteristics could be inferred from the results on the downstream window whereas complete anisotropy prevailed for the upstream window. First comparisons of the results with turbulent aero-optical model will be further discussed at the conference. Further results induced by steady flow components are compared in a companion paper~4 in the Conference with CFD steady-state Reynolds-averaged Navier-Stokes simulations.