Simplified CFD model to understand turbulent flow fields around a Wave Energy Converter
Point absorbers are one of the various designs of Wave Energy Converters (WEC) analyzed at West Coast Wave Initiative (WCWI), University of Victoria. The point absorbers are relatively small floating devices when compared to typical wave length of the ocean waves. The vertical component of velocity of the waves is utilized and is responsible for the heaving motion of the WEC. The relative motion of one component (Float) with respect to another (Spar) is the source of energy which is harnessed by a Power Take Off (PTO) mechanism.
During design optimization, when different designs are tested and when the difference in solution time between a simplified model and a comprehensive model is significant; the intuition resorts to the simplified model. Similarly when evaluating the performance of a WEC purely in heaving motion and involving surface gravity waves, the dynamics of air above the free surface can be neglected as the air pressure is assumed to be constant without any significant effect on the final outcome.
In this study the free surface is being modelled as specified boundary condition, with an incompressible Navier-Stokes solver in a static grid that approximates waves through a transient wave height field. The effect of free surface is otherwise simulated through mapping of different fluid characteristics to the physical domain and solving for the interactions between them.
On the free surface the solver computes the wave height and the pressure change due the free surface profile change. The change in the free surface profile is calculated by integrating the fluid velocity at the free surface patch over time.
The waves at the free surface are about 10 % of the domain height which is 1m, yet the grid shows no deformation. The free surface profile is represented by the zeta field (see Fig.1).
To visualize the free surface zeta has been magnified by (x 100) and animated with respect to time as below. Fig.3 shows the variation of area averaged value of zeta over the simulation time.
Kinematic pressure is always the value obtained by dividing pressure by the fluid density. Dynamic pressure, however may refer to either the pressure NOT divided the fluid density or to the part of the total pressure that is caused by dynamic effects. In this study the kinematic pressure is referred to the pressure due to dynamic effects. The pressure change due to the variation of the free surface position relative to the still water level, is shown in the animation below.
Though the waves at the free surface being approximated by a wave height potential; in the interior domain, the solver is capable of handling different types of turbulence models. Thus it is possible to see the effect of complex geometrical shapes on the flow field, without simulating the effect of second fluid which anyways has no significant effect for small gravity waves.
