The first part of this work addresses the importance of shear flows in astrophysical gas dynamics, where the evolution of the Kelvin-Helmholtz instability (KHI) is studied analytically and numerically. The dispersion relation for the two-dimensional KHI including viscous dissipation is derived. The resulting expression for the growth rate is then used to estimate the intrinsic viscosity of four numerical schemes depending on code-specific as well as on physical parameters. The second part of this work focuses on the importance of forthcoming CMB measurements and their corresponding implications for cosmology. The theoretical CMB-cross correlation bispectrum (between the Rees-Sciama and weak lensing effect) together with the corresponding signal-to-noise ratio are analyzed for dark energy models with a time varying equation of state - the quintessence. The main focus is on a special form of quintessence, called early dark energy. The theoretical predicted signals give insight if future experiments are able to distinguish between quintessence and the standard cosmological constant.