ABSTRACT:

This paper presents a method of verification of the diffraction force simulated in a NWT. A method called ULSM/L+Q was recently developed and verified against the measured force due to the laboratory storm seas in MARINTEK wave tank. It can predict the force given the free incident wave elevation time series at the center of the structure and the geometry of the structure with the water depth. The ULSM prediction uses the incident wave time series simulated in a NWT and the result is compared with the NWT force. Its prediction agrees with the force due to the Stokes 5th-order-like laboratory wave of MARINTEK. ULSM agrees with the force of a column tested at TAMU wave tank, which has the same column dimensions as that in the above NWT. The ULSM prediction found that NWT force agreed well overall.

INTRODUCTION

The time domain verification of numerical wave tank (NWT) diffraction force against experimental wave tank (EWT) data is a crucial step necessary for the advancement of NWT technique. However, it is difficult to acquire the precisely needed EWT data, because of the high cost of experiment and other technical difficulties. We propose a quasi-experimental technique called ULSM/L+Q that can circumvent the difficulty mentioned above. The universal linear system model (ULSM) that employs the linear and quadratic transfer function (L+Q) for the diffraction force is employed in the paper. Given the time history of the free incident wave measured at the center of the structure and geometry of the structure with water depth, ULSM/L+Q can predict the force acting on the body in the time domain. Comparing extensively with the test data obtained in the wave tank of MARINTEK (Stansberg et al, 1995), Kim and Wang (1999) already validated the model.

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