Study of the hydrodynamic derivatives of vertical-axis tidal current turbines in surge motion

Highlights

A 2D numerical model is built to simulate the influence of induced speed on the hydrodynamic loads of vertical axis turbines (VATs).

The hydrodynamic load is expressed as an explicit function of the flux relative income speed.

The least squares method is adopted to separate the coefficient from the numerical result of the CFD simulation.

Numerical results show that the hydrodynamic load of the turbine can be separated as loads in the uniform, damping force and added mass forces.

In contrast to the damping of general marine structures, the damping of VATs is a periodical function of the azimuth of the blades.

Abstract

Both the particle velocity of waves and the response of floating platforms influence hydrodynamic loads of floating tidal current turbines. In this paper, the influence of surge motion on vertical-axis turbines was studied; numerical simulation results were validated by experimental results. Based on numerical simulation results, a double trigonometric function was developed to fit the time history curves of hydrodynamic derivatives because of the dual frequency characteristics of vertical-axis turbines. Then least squares method was used to solve hydrodynamic derivatives of force coefficient. The results showed that in the working condition, surge motion results in the periodic variation of peak value of instantaneous hydrodynamic loads and that maximum loads on the turbine increased, which is bad for structural strength of the turbine. Under small surge motion, hydrodynamic loads on the vertical-axis turbines are linearly related to surge motion velocity and acceleration. Under stable conditions, damping coefficient in surge motion is not dependent on the amplitude, phase and frequency of surge motion but is related to the tip speed ratio, phase angle of the blade. The research results are beneficial to the design of mooring systems and are significant for forecasting the motion response characteristics of floating tidal current power stations.

Keywords

  • Tidal current energy;
  • Vertical-axis turbine;
  • Hydrodynamic derivative;
  • Surge motion

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