EVALUATION OF LINEAR AND NONLINEAR HYDRODYNAMIC COEFFICIENTS OF UNDERWATER VEHICLES USING CFD

Hydrodynamic coefficients (HDCs) in the equations of motion for trajectory simulation of any underwater vehicle are inherent characteristics of the body geometry. At the concept design stage of underwater vehicles, there is a requirement for affordable and efficient use of general-purpose CFD tools to get a first estimate of the HDCs, with greater accuracy than empirical methods, but at lesser cost and time than model testing. This paper reports the prediction of values of straight-line and rotary HDCs for a submerged axisymmetric body of revolution using a general-purpose RANSE solver and with modest, readily available computational facilities. Various alternatives for grid generation and grid size have been compared and a solution strategy evolved to optimize requirements of accuracy, time and computational resources. Linear and nonlinear HDCs for straight-line and rotary motions were obtained by curve-fitting of the forces and moments plotted against angle of attack and angular velocity. To compute rotary HDCs, an approach was evolved to define a circular domain and effectively simulate the motion of a model in Rotating Arm captive model tests. Results were compared with experimental results from Planar Motion Mechanism tests of the same body. The trends in variation of forces and moments are captured well by computations. Accuracy obtained for yaw moment HDCs is comparable to the uncertainty levels from model tests. It is shown that CFD techniques offer flexibility for computing nonlinear coefficients as well, by simulating combinations of linear and angular velocities.