Abstract
Reducing fluid drag and the energy required for fluid transport is an important issue. Riblet is used as a method to reduce frictional drag. Direct numerical simulation (DNS) was used to research the drag reduction effect of the riblet and the torque generated at the base of the riblet. The riblet is a three-dimensional riblet whose height and span vary with respect to the flow direction. The riblet is represented by a superposition of two sine waves, with amplitude and phase determined randomly. The torque and drag reduction effects were investigated for 1108 cases of riblets with different geometries. The friction Reynolds number, defined by the friction velocity uτ [m/s] and the kinematic viscosity ν [m2/s], is 110. The results show that torque is a shape parameter and is most dependent on pressure of the riblet surface. A Paretooptimal solutions that minimizes both the torque and the drag increase ratio was investigated. As a result, four Pareto solutions were found. The drag force generated on each face of the riblet was also analyzed and compared to the Pareto solution in all cases. The results show that the Pareto optimal solution group has a smaller fraction of drag due to pressure than all cases.
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