The influence of y+ in wall functions applied in ship viscous flows

Pas, S. ten (2016) The influence of y+ in wall functions applied in ship viscous flows.

Abstract:One approach to describe the solution of the shear-stress at a wall is the application of wall functions which determine the shear-stress at the wall from semi-empirical equations. The y+ is the dimensionless quantity for the distance from the wall up to the centre of the first grid cell. In simulations without wall functions, this quantity is typically around 1, while wall functions allow us to take a y+ higher than 1. The boundary layer can be divide in two region: the inner layer and the outer layer. The former also consist of three regions, which are from the wall up to the outer layer: the viscous sublayer (y+ < 5), the buffer layer (5 < y+ < 30−50) and the log-law region (y+ > 30 − 50). Grid are made with Gridpro (structured grids) and Hexpress (unstructured grids), where several configurations are chosen. For the Gridpro grids, 14 different y+ values are chosen, with in every region of the inner layer some values. For most of the grids, also a grid refinement study is performed. For the Hexpress grids, 12 different y+ values are chosen. Due to the lack of time, no grid refinement study is performed. CFD simulations are done with MARIN’s in-house CFD solver ReFRESCO is used, which is an incompressible RANS solver. From the simulation the friction resistance coefficient CF and the pressure resistance coefficient CP are evaluated. From the results followed that an y+ between 50 and 100 is the most ideal case for both resistance coefficients. In the case only the friction resistance coefficient is considered, higher values in the log-law region can also be used to give satisfactory results.
Item Type:Internship Report (Master)
MARIN, the Netherlands
Faculty:ET: Engineering Technology
Subject:52 mechanical engineering
Programme:Mechanical Engineering MSc (60439)
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