Abstract
Turbulence in open channel flows is ubiquitous to hydro-environmental applications and has recently increased in importance with the deployment of tidal stream turbines, as turbulence impacts both the turbine performance and the blade fatigue life. Tidal turbine analysis requires fully developed turbulence characteristics at the inlet of numerical simulations where generally the length scale information is limited. In this study, fully resolved large eddy simulations (LES) with flat beds were undertaken using an open source code at friction Reynolds numbers () of 150, 400 and 1020. It was found that the effects of the free surface on turbulence length scales were felt in approximately the uppermost 10% of the channel only, although the influence on Reynolds stresses extended further downwards. Furthermore, the cross-correlation length scales of both streamwise and spanwise velocities were found to be significantly affected by the free surface where turbulent eddies were flattened to the two-component limit.
Highlights
Turbulence near shear-free, approximately-flat surfaces occurs in a wide variety of industrial and environmental flows, and studying the structure of turbulence near the free surface is important for understanding the complex interaction of the gas– liquid interface
3.1 Validation and comparison with earlier studies Grid sensitivity studies for all the cases were performed with grids of different sizes and aspect ratios (half the grid size in Figure 2 u+vs y+ plots for different Reτ values: (a) Reτ = 150, (b) Reτ = 400, (c) Reτ = 1020 all directions referred to as low resolution and double the grid size in all directions referred to as high resolution); it was found that the mean velocity and Reynolds stresses were not affected by these changes
Large-eddy simulations were performed for fully-developed open-channel flow with a stress-free, impermeable lid
Summary
Turbulence near shear-free, approximately-flat surfaces occurs in a wide variety of industrial and environmental flows, and studying the structure of turbulence near the free surface is important for understanding the complex interaction of the gas– liquid interface. Previous experimental studies by Komori, Ueda, Ogino, and Tokuro (1982), Komori, Murakami, and Ueda (1989), Nezu and Rodi (1986), Rashidi and Banerjee (1988) and Kumar, Gupta, and Banerjee (1998) have established the behaviour of root-mean-squared (rms) velocity fluctuations in the vertical direction for open-channel flows These experiments reported that the fluctuations in the vertical direction were damped, whereas fluctuations in the tangential direction were enhanced near the free surface. A wealth of information regarding open-channel flows has already been provided by experiments; there are some difficulties related to measurements very close to the deformable interface at the free surface This results in a lack of accuracy of measurements for the rms velocities near the free surface and derived quantities such as Reynolds stresses, vorticity and turbulent dissipation, all of which cannot be obtained with accuracy
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