Print Email Facebook Twitter A frequency dependent drag coefficient on the motion response of a hybrid STC wind-wave energy converter Title A frequency dependent drag coefficient on the motion response of a hybrid STC wind-wave energy converter Author Kohlmann, Lars (TU Delft Mechanical, Maritime and Materials Engineering) Contributor Wellens, Peter (mentor) Degree granting institution Delft University of Technology Programme Ship Hydrodynamics Date 2019-12-13 Abstract The vertical viscous drag force acting on a heaving cylinder with flat and hemispheric bases is investigated by means of an experiment, a linear and a nonlinear analytic model. Viscous effects create a significant uncertainty in estimating the motion response of oscillating cylinders in waves of different frequencies. Forced oscillation tests are compared with a linear analytic model and approximated with a nonlinear model to quantify the viscous drag force for a range of oscillating frequencies. The results asses the behaviour of the drag coefficient, CD, to be used for calculating the drag force. The main interest of this study is to improve the estimation of the motion response of floating hybrid wind-wave energy converters. The hydrodynamic parameters, added mass and radiation damping, used in the linear model are obtained with the boundary element method (BEM) solver NEMOH. The same parameters are obtained from the experiment for comparison. Near the natural frequency of the geometries the linear model underestimate the forces due to the absence of a nonlinear viscous drag term. The time traces of the force measurement around the natural frequencies show period doubling behaviour which can be described by the quadratic velocity term in the drag force calculation. By applying the least-squares optimization method on the obtained data, optimal variables in the fit-function including the quadratic drag term are found. The results show the drag coefficient, CD, is changing over the frequency of oscillation. The vertical drag force is frequency dependent and therefore important to consider when calculating the motion response of wave energy converters Subject Drag coefficientWECViscous dragDampingHeaveForced MotionFrequency To reference this document use: http://resolver.tudelft.nl/uuid:2f1dc98e-9544-4817-83f3-3c26d2964934 Part of collection Student theses Document type master thesis Rights © 2019 Lars Kohlmann Files PDF report_final.pdf 22.96 MB Close viewer /islandora/object/uuid:2f1dc98e-9544-4817-83f3-3c26d2964934/datastream/OBJ/view