Turbulent flow characteristics over forward-facing obstacle

Abstract

The local flow field and wake region over a forward-facing obstacle were experimentally investigated, when the water surface above the obstacle was dipped compared to flow further upstream and further downstream. The obstacle was modelled resembling the shape of an airfoil with a flat top having stoss-side slope ${35}^{\circ }$ and downstream lee-side slope $6^{\circ }$ designed for minimal flow separation. The submerged obstacle showed an evolutionary change in the flow field with a dipped water surface shape over the crest; and consequently three distinct flow regions were observed: upstream region, crest region and the downstream. The velocity data were recorded using a three-dimensional micro-acoustic Doppler velocimeter along the flume centreline for three flow Reynolds numbers and different localised Froude numbers. The paper highlights the flow and turbulence prime parameters: second-order statistics, turbulence kinetic energy, eddy viscosity, turbulence production, probability-density functions and the contributions of stress fraction into four quadrants to the total shear stress along the flow. The spectral analysis was performed to examine the energy distributions, vortex-shedding frequencies with corresponding Strouhal numbers. Variations of turbulence integral length scale along the flow passing over the obstacle were investigated to examine the eddy turnover scales. The turbulent features were fairly inspiring since it led to a pronounced acceleration on the crest location like a jet with a potential core region, and elongated deflection in the downside slope. The results provide a basic turbulence in natural environment for various phases of ebb and tide over a submerged smooth obstacle or mountain hills in river/ocean.

KEYWORDS:

• Turbulence
• forward-facing obstacle
• Reynolds stress
• velocity spectra
• integral length scale

Acknowledgements

The authors are thankful to Kausik Sarkar, Sayahnya Roy and Krishnendu Barman for their helpful suggestions and discussions. They also acknowledge the Editor-in-Chief, and two anonymous Reviewers for their constructive comments and suggestions for improvement of the paper.

Disclosure statement

No potential conflict of interest was reported by the author(s).