Research On Active Absorption Techniques Of Wavemaker

Physical model test is an important tool and method to study various problems in coastal engineering or ocean engineering field. But in the process of model test implementation, there is a main difference when comparing wave flume or wave basin with real ocean state-re-reflection. When the test devices have highly reflective structures, the progressive wave generated by wavemaker becomes a reflected wave after acting on the device, reflection occurs again when the reflected wave impinging on the paddle, and producing a re-reflected wave leaving the paddle and being added to the progressive wave. After a while, reflection occurs many times between test structures and wavemaker, experimental data cannot accurate describe the properties of wave structures interaction. To solve the problem, active absorption technique has been developed. With this technique, wavemaker can generate waves and absorb reflected waves at the same time. Such wavemaker, according to hydrodynamics feedback signal, drives the paddle motion by the control system to absorb reflections. Not only that, active absorption technique can prevent resonant oscillations in wave flume or wave basin; reduce the water stilling time between tests; avoid spurious waves. However, active absorption technique research need improvement, especially active absorption technique of 3D wavemaker is still at an early stage, no common framework has been established yet. Therefore, carrying out the research on active absorption techniques has an important significance in the field of model experiments.Based on linear wavemaker theory, and considering the single frequency feature of regular wave, the discrete solution of paddle position for 2D regular wave absorption is proposed. An absorbing wavemaker system is built based on original control system. Efficiency of the discrete solution for 2D regular wave absorption is verified by physical experiments, the error of the uniformity is less than 6%. A first order high pass filter method which can filter low frequency wave is used to prevent paddle drifting.Considering the multi-frequency feature of irregular wave, frequency domain solution of paddle position is given for 2D irregular wave absorption. Properties of the transfer function are discussed. When the magnitude of the transfer function is large at zero frequency, the paddle drifting problem will be caused. Therefore, a modification on the transfer function is applied to avoid this problem. Because absorption transfer function is not an analytic expression, we use a 1D digital filter to approximate the modified transfer function, and be translated into a time domain difference equation. Based on IIR filter model, we propose an iterative reweighted least-squares (IRLS) algorithm to improve the fitting performance. Experimental results show that the filter designed with the IRLS algorithm is improved nearly three times more than that designed by normal method.To improve the absorption rate and the accuracy of wave simulation, considering the presence of time delays in the control loop of wavemaker system, we propose a more accurate active absorption governing equation and derived the transfer function with delay compensation. Results of physical experiments demonstrate that the system is more stable than that without time delay compensation for small period waves. And the available frequency range of absorbing wavemaker system is expanded.In the aspect of 3D wavemaker active absorption, with delay compensation, frequency domain solution of paddle position for active absorption is given. Because of complexities of magnitude and phase response of 3D active absorption transfer function, model approximation precision is difficult to be guaranteed. We propose a more stable model based on 2D transfer function and a 3D modification function. A wave direction estimate method is also given. The main direction of reflections can be estimated on line according to surface elevation signal on the paddle. In this way full 3D active absorption is realized. Considering incident waves with different period and direction, the effectiveness and accuracy of the present method has been experimentally verified by establishing 3D physical test. Results show that the present method is applicable for 3D physical model test, and can improve the experiment efficiency.