Naturally occurring cracks have rough surfaces which mate in such a fashion as to close the crack completely when the surfaces are pressed together. Experimental work shows that friction factors are given by a Nikuradse type of equation when the crack surfaces are widely spaced. The equation remains applicable as the crack closes until roughness elements from opposing surfaces start to overlap and then an upper limit is achieved. Further reduction in the crack wall separation causes a reduction in the friction factor, which may fall to the level applicable to a smooth-walled tortuous channel. These observations are in accord with theoretical concepts.
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