Experimental studies of aero-optical properties of subsonic turbulent boundary layers

Abstract

This paper gives the most-complete characterization to date of the optical aberrations imposed on a laser beam propagated through a subsonic, compressible, turbulent boundary layer in a zero-pressure gradient environment, over a range of boundarylayer thicknesses, oblique propagation angles and Mach numbers. This characterization is based on optical measurements using optical-wavefront-sensing instruments that have only become available in the last decade. The optical characterization includes and discusses in detail: optical-wavefront spectra, convective velocities of optically active large-scale structures and correlation functions in both streamwise and cross-stream directions, as well as root-mean-square optical path difference levels for different apertures. The scaling law based on the extended strong Reynolds analogy is derived and is shown to successfully collapse optical data collected in a number of facilities. Anisotropy of aero-optical distortions for different oblique viewing angles was experimentally quantified and is discussed.

Subsonic Boundary-Layer Wavefront Spectra for a Range of Reynolds Numbers

“…For both the Caltech wavefront data obtained at Reθ = 1,700 the convective velocity was found to be 0.83U ∞ using the correlation of the two beams, a value which is consistent with wavefront measurements obtained previously at higher Reynolds numbers [9,14]. For wavefronts obtained in the Notre Dame facility, the convective velocity was found to be 0.82U ∞, which is also consistent with previous measurements in this facility at higher Reynolds numbers.…”

“…The peaks for all measured spectra are at St δ = 1.0, independent of Reynolds number. In [14] it was shown that using Kolmogorov-like arguments for the inertial range, the deflection angle amplitude spectrum at large frequencies should behave as ~f (−2/3) . This spectra behavior is Smith et al…”

“…To remove the aero-optic contribution of the, un-heated TBL along the lower wall in the TWT facility in the Malley probe wavefront measurements, boundary layers on opposite walls of the test section are assumed to be statistically independent. Then the contribution of the heated wall single boundary layer (SBL), may be isolated from the contribution of the un-modified boundary layer using an extension of the statistical relationship shown in [11,14], where is the value of OPD rms measured by a wavefront sensor in the double boundary layer (DBL) configuration, in which the beams pass through both the wall-heating modified boundary layer and the un-modified boundary layer. Likewise is the DBL measured value of OPD rms for two un-modified boundary layers in the control case.…”

“…Using a bulk-flow analysis they derived the more-general scaling law for OPD rms including supersonic regime as [ ] [13,14] derived an alternative general scaling law for OPD rms ,…”

“…To model the observed changes in the (high frequency slope) size of the inertial range of deflection angle spectra for different Reynolds numbers, the spectral model from [14] was modified in order to account for changes in the spectra roll-off as a function of Re θ . The fall-off of the spectra was assumed to take a form inspired by Tatarski's modification of Kolmogorov's atmospheric wavefront spectrum to account for the presence of inner scale dissipative structures [17].…”