Computational study of optical distortions by separated shear layers and turbulent wakes

Abstract

The flow over a circular cylinder at ReD = 3900 and 10000 and M = 0.4 is considered a platform to study the aero-optical distortions by separated shear layers and turbulent wakes. The flow solution is obtained by large eddy simulation (LES) and validated against previous experimental and numerical results. The fluctuating refractive index obtained from LES is used in a ray-tracing calculation to determine wavefront distortions after the beam passes through the turbulent region. Free-space propagation to the far field is computed using Fourier optics. The optical statistics are analysed for different conditions in terms of optical wavelength, aperture size and the beam position. It is found that there exists an optimal wavelength which maximizes the far-field peak intensity. Optical results at both Reynolds numbers are compared. The optical distortion by the downstream turbulent wake is found to be Reynolds number insensitive. However, due to their different transition mechanisms, distortions by the near wake regions are different in the two flows. The aero-optical effects of different flow scales are examined using filtering and grid refinement. Through a grid convergence study it is confirmed that an adequately resolved LES can capture the aero-optics of highly aberrating flows without requiring additional subgrid scale model for the optics.

An adaptive implicit–explicit scheme for the DNS and LES of compressible flows on unstructured grids

“…Note that for Case 3 all the grid points except the ones on the cylinder wall have their fluxes advanced Comparison of the energy spectra of the vertical velocity as a function of non-dimensional frequency at x/D = 2 (top) and x/D = 5 (bottom). -: current results; : Mani et al[46] …”

“…17, the energy spectra at 2D and 5D downstream of the cylinder are compared with simulation results of Mani et al [46] in which they used a staggered sixth-order compact Pade scheme of [43]. Due to sufficient resolution near the cylinder, the spectra are in good agreement even at high frequencies.…”

“…The objective of this section is to validate our method and study its performance for LES. We perform LES of flow over a circular cylinder at subcritical Reynolds number Re D ¼ U1D m1 ¼ 3900, which has been extensively studied for both incompressible [45] and compressible [46] flows. The boundary layer around the cylinder requires a very fine grid, whereas the LES away from the cylinder is less resolution demanding.…”

Large-eddy simulation of the compressible flow past a wavy cylinder

“…Furthermore, the separated shear layer and its evolution are usually associated with transition mechanisms (e.g. Zdravkovich 1997;Mani, Moin & Wang 2009). As the growth rate of the shear layers shed from the wavy cylinder is relatively small, the effects of pre-separation transition on post-separation turbulence are weak.…”

Suitability of artificial bulk viscosity for large-eddy simulation of turbulent flows with shocks

“…[8], and validation test cases for the flow over a cylinder at different conditions can be found in Ref. [9].…”