Large Eddy Simulation of Micro-Riblet Performance Under Upstream Flow Separation

Micro-riblets are effective for reducing drag in turbulent layers.Flow separation is a prevalent flow phenomenon in transport.However, the precise drag reduction performance of riblets downstream of a flow separation remains unclear.An inclined forward step model is proposed to investigate the interaction between riblet and upstream flow separation.The large eddy simulation method is applied to simulate the flow over geometries with different step angles and riblet positions.The results show that the riblet downstream of a flow separation still reduce drag and have two characteristics, with two distinct behaviours.Firstly, drag reduction rises with separation intensity; a step angle increase from 0 to 30 degree boosts drag reduction from 9.5% to 12.6%.This is due to intensified Q2 motions enhancing riblet effects.Secondly, the position of the riblets has a significant impact on the pressure drag.Riblets close to the point of separation enhance flow separation, altering the surface pressure distribution and thus increasing the resistance.The precise performance of riblets under complex flow conditions is are important for their practical engineering application on the train.