Flow over smooth surfaces has been studied ever since the start of aerodynamic research. Non-smooth irregularities in rough walled surfaces will impact on the flow and induce changes in friction and heat transfer. The term 'rough walled' encompasses a range of shapes, ranging from material roughness to wall profiles with heights up to the boundary layer thickness. The effect of material roughness has been studied extensively, but data on large roughness elements is scarce and case-specific, depending on flow conditions and roughness element shape.