Riblets is the wall configuration with longitudinal grooves proposed first by Walsh et al. and is extremely a practical drag reducing technique. Extensive experiments have been carried out to investigate the most effective geometry and dimensions of riblets; it was confirmed that the riblets can reduce the frictional drag about 8 percent at maximum in both external and internal flows.
The purpose of this review is to reveal the problems on practical applications of riblets. When a new technique is applied in practical fields, the effectiveness of such a technique must be discussed in the whole system, not in the part of the system. Therefore, it is important to evaluate the effects of riblets on various practical flow fields. On this view point, the effects of the riblets under various flow conditions are discussed based on the experimental data available.
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