In many fluid-structure interaction problems, we often needed to consider the fluid forces caused by arbitral accelerated motions of fluid and/or structures. Such forces have been understood using the concept of the "added mass" or the "virtual mass." In the present study, We propose a simple and efficient method to specify fluid-force coefficients of a three-dimensional oscillating object in viscous fluid. In this method, we consider incompressible and viscous fluid under the assumption of infinitesimal amplitude of a three-dimensional object, and properly modify the Navier-Stokes equations into linear equations, namely, the Brinkman equations. The solving method is based on a discrete singularity method (DSM). Then, we apply the DSM to many three-dimensional objects. As a result, we can specify the fluid forces of square-cross-section cylinders at aspect ratio b/a = 0.5-5.0. Moreover, we conduct the experiment, and we can confirm the amplitude effects.
