Equivalent sand roughness

The equivalent sand roughness depends on the arrangement (pattern), distance (density) and shape, of the roughness elements such as groove, sand grains, waves or cuboids, therefore it is possible that these elements differentiate in roughness although they have the identical geometrical roughness height k. Plate-shaped roughness elements for example, are about the same height as sand grains, but its roughness may/can be eight times bigger, depending on the distance between the plates (density). (Zanke)

Nikuradse made very extensive and systematic measurements in rough pipes. He used circular pipes, which were covered as densely as possible with sand of a certain grain size.

The sand roughness, which was examined by Nikuradse can be characterised by a maximum roughness density. In many technical applications the roughness density is substantially smaller. Such a roughness can no longer be indicated by a roughness height k. For this purpose it has proved to classify the roughness by comparing it to a scale of a normal roughness and then choose the Nikuradse sand roughness.

Any roughness can be generally assigned to a so-called equivalent sand roughness ks, which is understood as the sand grain roughness, with the same friction coefficient ?.


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