The effects of roughness and suction on the boundary-layer flows under an influence of varying factors

This thesis investigates the effects of surface roughness and wall suction on the convective stability of boundary-layer flow over three body types. It compares the impact of these effects with other factors in delaying the onset of laminar-turbulent transition. Three distinct flow types over three geometric surfaces are considered: a still fluid over a rotating broad rough cone in a fixed frame of reference; an enforced axial flow over a rotating rough disk in a rotating frame of reference; and flow of a fluid with temperature-dependent viscosity over a permeable heated plate under the influence of parietal suction and injection. For each model, effects of both anisotropic and isotropic surface roughness on distinct instability properties of the boundary-layer flow over a rotating body are examined. This approach requires modification of the no-slip boundary condition to allow partial slip. Similarly, wall suction or injection is applied to the flow by simple modification of the no-penetration condition and existing boundary conditions on the flat plate. The Navier-Stokes equations are used to obtain the steady mean flow system, and linear stability equations are then formulated to obtain neutral stability curves. Stability analysis results are validated using linear growth rates and energy analysis. Results from these studies indicate that streamwise-aligned radial grooves have a strong destabilising effect on type II (viscous) instabilities, whereas concentric grooves and isotropic surface roughness stabilise the boundary-layer flow for type I (inviscid or crossflow) instabilities. A pronounced destabilising effect is also associated with increasing injection or increasingly temperature-dependent viscosity over a heated plate. In contrast, increasing either wall suction or temperature dependence results in significantly greater flow stability. Practical applications and extensions of these findings are considered in the context of chemical vapour deposition (CVD) reactors. Note that one from these studies is submitted by Al-Malki et al.∗, and two papers have been published by Al-Malki et al.†