In the present paper, experiments are reported establishing a maximum drag reduction of five to seven percent in fully developed turbulent flow of water through 25.4mm and 50.8mm diameter pipes lined with a film of grooved equilateral triangles of base 0.11mm. The maximum reduction occurs when the height of the riblets is 11 to 16 wall units. This correlates well with the Taylor microscale of the fluctuating velocity gradient.
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Liu et al. (1990) studied this question.
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