Turbulent flow over the trough region formed by a pair of forward-facing bedform shapes

The present paper explores the turbulent flow characteristics and overall drag over the trough region of a pair of adjacent 2-D forward-facing dune-shaped artificial structures with two different stoss-side slopes and aims to make a comparative study. Structures considered here were of equal base length (λ) with a common gentle slope of 6°at the downstream face. The stoss-side angles were respectively 50°and 90°. Experiments were conducted at the Fluvial Mechanics Laboratory (FML) of Indian Statistical Institute (ISI), Kolkata. The velocity data were collected using a 3-D Micro-Acoustic Doppler Velocimeter (ADV) at the flume centerline to analyze the mean flows, Reynolds stresses and the overall drag at a Reynolds number, Reh≈1.44×105. One-dimensional profiles of turbulent flow parameters demonstrate that the flow separation bubble and a thin perturbed shear layer between the negative velocity region and outer layer with high velocity are the main sources of turbulence production. A change in turbulence characteristics between the two crest positions is identified. A greater amount of flow resistance is observed in the case of the structure having higher stoss-side slope. The spectral analysis reveals that peak power spectral density generally occurs at 0.25-1 Hz for the stream-wise velocity component and at 0.9-3.0 Hz for the cross-stream and vertical velocity components. Power spectra showed better defined peaks near the shear layer in the separation cell where it reaches to maximum which is two to three times greater than its values in the upstream and further downstream region. Strouhal numbers are calculated using the frequencies of eddies and vortices with different sets of characteristic length and velocity scales; and compared with the previous results of other researchers.