The minimal channel: a fast and direct method for characterising roughness. (April 2016)

• Record Type: Journal Article
• Authors: MacDonald, Michael; Chung, Daniel; Hutchins, Nicholas; Chan, Leon; Ooi, Andrew; García-Mayoral, Ricardo
• Is Part Of: Journal of physics. Volume 708(2016)
• Publication Date: 2016-04
• Subjects: Physics -- Congresses; 530.5
• DOI: 10.1088/1742-6596/708/1/012010
• Languages: English
• ISSNs: 1742-6588

Abstract

Abstract: Roughness only alters the near-wall region of turbulent flow and leaves the outer-layer unaffected, making it a prime candidate for the minimal-span channel framework which only captures the near-wall flow. Recently, Chung et al. ( J. Fluid Mech., vol. 773, 2015, pp. 418-431) showed that the minimal-span channel can accurately characterise the hydraulic behaviour of roughness. Following on from this, we aim to further optimise the minimal-span channel framework by primarily noting that the outer layer it produces is inherently incorrect, and as such modifications to this region can be made to improve performance. Firstly, a half-height channel with slip wall is shown to reproduce the near-wall behaviour seen in a standard channel, but with half the number of grid points. Next, a forcing model is introduced into the outer layer of a half-height channel. This reduces the high streamwise velocity associated with the minimal channel and allows for a larger computational time step. The streamwise length of the channel is also investigated independent of the previous improvements, and suggests the minimum length should be at least 3 times the spanwise width and also 1000 viscous-units long, whichever is longer. Finally, an investigation is conducted to see if varying the roughness Reynolds number with time is a feasible method for obtaining the full hydraulic behaviour of a rough surface, instead of running multiple simulations at fixed roughness Reynolds numbers.