Rough wall turbulent boundary layers

Abstract

This paper describes a detailed experimental study of turbulent boundary-layer development over rough walls in both zero and adverse pressure gradients. In contrast to previous work on this problem the skin friction was determined by pressure tapping the roughness elements and measuring their form drag.Two wall roughness geometries were chosen each giving a different law of behaviour; they were selected on the basis of their reported behaviour in pipe flow experiments. One type gives a Clauser type roughness function which depends on a Reynolds number based on the shear velocity and on a length associated with the size of the roughness. The other type of roughness (typified by a smooth wall containing a pattern of narrow cavities) has been tested in pipes and it is shown here that these pipe results indicate that the corresponding roughness function does not depend on roughness scale but depends instead on the pipe diameter. In boundary-layer flow the first type of roughness gives a roughness function identical to pipe flow as given by Clauser and verified by Hama and Perry & Joubert. The emphasis of this work is on the second type of roughness in boundary-layer flow. No external length scale associated with the boundary layer that is analogous to pipe diameter has been found, except perhaps for the zero pressure gradient case. However, it has been found that results for both types of roughness correlate with a Reynolds number based on the wall shear velocity and on the distance below the crests of the elements from where the logarithmic distribution of velocity is measured. One important implication of this is that a zero pressure gradient boundary layer with a cavity type rough wall conforms to Rotta's condition of precise self preserving flow. Some other implications of this are also discussed.

Flow and skin friction over natural rough beds

“…Unfortunately, for most laboratory investigations of macrorough flow (including this one) this is necessary because of depth limitations, so the location of the zero plane (the "zero-plane displacement") is important in describing flow in the ILL. There is no universally accepted technique for finding the zero-plane displacement, although commonly it is chosen to optimize agreement between measured data and equation (1.6) (Perry et al, 1969). A theoretical study of the zero-plane displacement done by Jackson (1981) does not yield a straightforward method for its determination.…”

Free surface flow over square bars at intermediate relative submergence

“…Perry, Schofield, and Joubert (1969) were the first to identify two types of roughness, d-type and k-type, which give rise to fundamentally different roughness functions. Their pipe flow experiments showed that for k-type roughness the roughness function depended on the size of the roughness elements, whereas in d-type roughness it depended on the pipe diameter.…”

Channel flow over large cube roughness: a direct numerical simulation study

“…In any case, rough-wall flows of the type considered here require careful consideration of the effective origin of the wall-normal coordinate. It was perhaps Perry, Schofield & Joubert (1969) who first emphasized the complications posed by roughness because of the shift in the effective y = 0 location. For most of their roughnesses, as for all cases when the roughness height is very small compared with the boundary layer depth, there was no chance of making measurements within the roughness array itself, whereas for the larger roughness (comprising sharp-edged rectangular blocks) as in the present case, it was possible to determine the pressure drag forces by obtaining pressure fields around the obstacles.…”