Reliable damping of free-surface waves in numerical simulations

Abstract

This paper generalizes existing approaches for free-surface wave damping via momentum sinks for flow simulations based on the Navier-Stokes equations. It is shown in 2D flow simulations that, to obtain reliable wave damping, the coefficients in the damping functions must be adjusted to the wave parameters. A scaling law for selecting these damping coefficients is presented, which enables similarity of the damping in model-and full-scale. The influence of the thickness of the damping layer, the wave steepness, the mesh fineness and the choice of the damping coefficients are examined. An efficient approach for estimating the optimal damping setup is presented. Results of 3D ship resistance computations show that the scaling laws apply to such simulations as well, so the damping coefficients should be adjusted for every simulation to ensure convergence of the solution in both model and full scale. Finally, practical recommendations for the setup of reliable damping in flow simulations with regular and irregular free surface waves are given.

Analytical prediction of reflection coefficients for wave absorbing layers in flow simulations of regular free-surface waves

“…For practical purposes, it is desired to be able to estimate the amount of undesired wave reflection before running the simulations. Of the various techniques for reducing undesired reflections (see Perić and Abdel-Maksoud, 2016), this paper is concerned with the ones that apply source terms to the governing equations in a zone adjacent to the corresponding domain boundaries. Such approaches have been presented under many different names, such as absorbing layers (e.g.…”

“…The main goal with forcing-zone-type approaches is to achieve reliable minimization of undesired wave reflections. However, the source term func-tion contains tunable parameters which must be adjusted for every simulation (Mani, 2012;Perić and Abdel-Maksoud, 2016). It was argued that these "parameters and profiles [...] can only be determined by trail and error" (Colonius, 2004, p. 337), which at the time of writing is still common practice in flow simulations with ocean waves (Perić and Abdel-Maksoud, 2016).…”

“…However, the source term func-tion contains tunable parameters which must be adjusted for every simulation (Mani, 2012;Perić and Abdel-Maksoud, 2016). It was argued that these "parameters and profiles [...] can only be determined by trail and error" (Colonius, 2004, p. 337), which at the time of writing is still common practice in flow simulations with ocean waves (Perić and Abdel-Maksoud, 2016). So far, no theoretical foundation has been presented which guides how to choose these parameters and predicts reflection coefficients (Colonius, 2004;Hu, 2008; Modave et al, 2010;Mani, 2012).…”

“…So far, no theoretical foundation has been presented which guides how to choose these parameters and predicts reflection coefficients (Colonius, 2004;Hu, 2008; Modave et al, 2010;Mani, 2012). This is especially problematic since in industrial practice reflection coefficients are seldom evaluated; instead, mostly the default coefficients are used, which can lead to significant errors (Perić and Abdel-Maksoud, 2016). Thus prediction and minimization of undesired wave reflections is a key problem of great practical importance.…”

“…Apart from acoustics, absorbing layers are applied e.g. to reduce reflections of free surface waves, Kármán vortex streets and fully turbulent flows at domain boundaries 11,14,15,16,17,18 . Although widely used accross various disciplines, absorbing layers are currently not well understood.…”