Joseph H. Harris
School of Engineering, University of Warwick
The rotating disk boundary-layer is the paradigm for three-dimensional transitional flows, and has been studied extensively in the past to resolve issues of instability mechanisms, transition methods and more complicated rotating geometries. However the considered disk geometry usually involves perfectly smooth surfaces. We consider the effects of distributed roughness on the boundary-layer transition over a rotating disk using both theoretical and experimental approaches. The theoretical approach looks to obtain the steady-flow solutions for a sinusoidal surface distribution function. Full three-dimensional stability analyses are then conducted on these for a range of roughness parameters. The experimental approach seeks to verify the steady-flow solutions and investigate the transition mechanisms at play with increasing surface roughness. Experiments are carried out using the rotating disk facility here at Warwick, on specially manufactured rough disks. Both approaches confirm a reduction in the number of spiral vortices present on the disk surface, a behaviour that could lead to a bypass of the traditional transition mechanism.