Abstract
Pressure losses and flow dynamics are studied experimentally in a cavity formed by two ramps on the channel wall and filled with triangular riblets. The length and amount of riblets are changed from one large to three smaller, occupying the entire cavity length and dividing it into smaller subcavities between riblets. Up to four subcavities, characterised by length-to-depth ratio λ = 8.8–2.8, are formed. Pressure loss regularities and flow structure above such cavity are investigated in a wide range of Re numbers (430–18000) covering laminar, transitional and turbulent flow regimes. The pressure loss regularities were found depended on the ramps forming the initial plane cavity, while riblets contribution is minor. However, their influence on the distribution of flow velocity and turbulence parameters is significant. Decreasing subcavities size between riblets, pressure losses increase until the critical size of subcavities is reached. Further reducing subcavities size, the interaction of the main flow and flow in subcavities diminishes, leading to decreased pressure losses. Measured velocity and shear rate profiles reveal dynamics of instabilities above vertices of riblets and their dependence on flow regime and subcavities size.
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