ON THE PREDICTION OF RIBLET PERFORMANCE WITH ENGINEERING TURBULENCE MODELS

The paper reports the outcome of a numerical study of fully developed flow through a plane channel composed of ribleted surfaces adopting a two-equation turbulence model to describe turbulent mixing. Three fami lies of riblets have been examined: idealized blade-type, V-groove and a novel U-form that, according to computations, achieves a superior p erformance to that of the commercial V-groove configuration. The maxim um drag reduction attained for any particular geometry is broadly in a ccord with experiment though this optimum occurs for considerably larg er riblet heights than measurements indicate. Further explorations bri ng out a substantial sensitivity in the level of drag reduction to the channel Reynolds number below values of 15 000 as well as to the thic kness of the blade riblet. The latter is in accord with the trends of very recent, independent experimental studies. Possible shortcomings i n the model of turbulence are discussed particularly with reference to the absence of any turbulence-driven secondary motions when an isotro pic turbulent viscosity is adopted. For illustration, results are obta ined for the case where a stress transport turbulence model is adopted above the riblet crests, an elaboration that leads to the formation o f a plausible secondary motion sweeping high momentum fluid towards th e wall close to the riblet and thereby raising momentum transport.