Turbulence between two inline hemispherical obstacles under wave–current interactions

• Turbulent flow over hemispheres in the presence of surface waves.
• Mean flows and Reynolds stresses are analyzed.
• Effect of deep water and intermediate water waves at the near-bed boundary.
• Coherent structures over hemispheres are analyzed.
• Power Spectral density distrubutions for different frequencies of wave.

This paper reports an experimental investigation of open channel turbulent flow between two inline surface mounted hemispherical obstacles in tandem arrangement. A series of experiments are performed under combined wave–current interaction with seven relative spacing L/h, where L is center to center spacing distance and h is the obstacle height for Reynolds number Re = 5.88 × 104. The observations are particularly focused on the changes induced in the mean velocity components, turbulence intensities and Reynolds shear stress due to superposition of surface waves on the ambient flow, and are compared to that of flat-surface and a single hemisphere. The paper also investigates the dominant turbulent bursting events that contribute to the Reynolds shear stress for different relative depth influenced by hemispheres. It is observed that the contributions to the total shear stress due to ejection and sweep are dominant at the wake region for single and double hemisphere near the bed, while towards the surface outward and inward interactions show significant effect for wave–current interactions which is largely different from that over the flat-surface case. Spectral analysis of the observed velocity fluctuations reveals the existence of two distinct power law scaling regime near the bed. At high frequency, an inertial sub-range of turbulence with −5/3 Kolmogorov scaling is observed for the flat-surface. The spectral slope is calculated to show the shifting of standard Kolmogorov scale for both only current and wave-induced tests.