Evaluation of the Viscous Drag for a Domed Cylindrical Moored Wave Energy Converter

Abstract

Viscous drag, nonlinear in nature, is an important aspect of the fluid–structure interaction modelling and is usually not taken into account when the fluid is assumed to be inviscid. Potential flow solvers can competently compute radiation damping, which is related to the radiated wave field. However, the drag damping primarily related to the viscous effects is usually neglected in the radiation/diffraction problems solved by the boundary element method (BEM), also known as the boundary integral element method (BIEM). This drag force can have a significant impact in the case of structures extending much deeper below the free surface, or for those that are completely submerged. In this paper, the drag coefficient C d was quantified for the heave and surge response of a structure which consists of a moored horizontally oriented domed cylinder with two surface piercing square columns located at the top surface. The domed cylinder is the primary part and is submerged. The drag coefficient is estimated using the experimental measurements related to harmonic monochromatic wave–structure interaction. Finally, this estimated drag coefficient was used in the modified time domain model, which includes the nonlinear viscous correction term, and the resulting device response in heave and surge directions is presented for an irregular incoming wave field. The comparison of the numerical model and the experiments validates the estimated C d values obtained earlier. Prior to the time domain model, frequency-dependent parameters such as added mass, radiation damping, and excitation force were computed using three mainstream potential flow packages (that is, ANSYS AQWA, WAMIT, and NEMOH), and a comparison is presented. The effect of free surface on the drag coefficient is investigated through differences in C d values between heave and surge modes.

VD-PQ; A Velocity-Dependent Viscous Damping Model for Wave-Structure Interaction Analysis

“…The WEC consists of two rigid bodies connected with two vertical tendons. The first body is a fully submerged horizontal cylinder with domed ends and rectangular-shaped surface piercing columns, while the second one is a fully submerged mass [36,37]. A system of mooring lines is used to keep the WEC in place by providing horizontal and heave stiffness.…”

Efficient Nonlinear Hydrodynamic Models for Wave Energy Converter Design—A Scoping Study

“…An effective approach to overcome the scaling issue is to perform full-scale experiments using RANS simulations, as demonstrated in [17,[210][211][212][213][214][215][216]. The discrepancies in drag coefficient values and methods to calculate them for a spherical HPA is investigated in Giorgi and Ringwood [217], where it is concluded that estimating the drag coefficient is challenging, resulting in uncertainties and inconsistencies in reported values for the same geometry in different publications.…”

Mooring Analysis of a Floating OWC Wave Energy Converter

“…Numerical codes based on the potential flow theory are also frequently used to evaluate the performance of point-absorber-type WECs [9][10][11], and discrepancies in motions are often noted between the numerical and experimental results for motion. This issue is attributed to the absence of viscous effects in such numerical models [12,13], in which the assumption of zero viscosity in the fluid can lead to over-estimation of the motions for bodies when the pressure and skin friction drag from viscous effects is significant.…”

“…Typically, the region surrounding the resonant frequency is most influenced by the viscous effects [14]. Accurate modelling of the motions about the natural periods is important, and several investigations have sought to mitigate the effects of viscous damping by tuning the numerical models [9,10,15,16]. A common approach taken in potential flow investigations is to incorporate an external, frequency-independent damping force that is linearly proportional to the structure's displacement or rotational velocity; some examples include the simulation of side-by-side vessel motions [15] and multiple point absorber WECs integrated into a floating platform [16].…”