Visualization of small density fluctuations of the atmosphere with spatial frequency filters

The recently improved ultra-fast aircraft resistance thermometer measures with a time constant of the order 0.1 ms. For an aircraft speed of 100 m/s this time constant corresponds to a spatial resolution of a few centimeters. Measurements made both in the atmosphere and in the low-turbulence wind tunnel at air speed 80 m/s are corrupted with noise of a few kHz frequency. Authors of the thermometer suggest that this noise results from turbulence introduced by vortex shedding from the protective shield. To achieve further improvement of the instrument we have to understand the nature of these aerodynamic disturbances. The present study is carried in two complementary directions. In the first, flow modeling is made with the FEATFLOW 1.2d - a finite element software for the incompressible Navier-Stokes equations. The results of flow simulation are in qualitative agreement with the experiment. In the second, we simulate visualization of the flow using two optical spatial filters: the Foucault filter that gives output intensity signal where bright bias is modulated with 1-D Hilbert transform of an object phase function and modified Zernike phase filter that shifts phase of the spectrum dc term by 0.2π.