Characterisation of suction effects on a submarine body operating near the free surface

Submarines operating near the free surface will experience a depth and speed dependent heave force. This force can have a significant impact on the submarine causing it to either broach the surface or descend if it is insufficiently controlled. In this condition, a submarine generally has limited control due to typically low operating speeds and small control surfaces. Therefore, in the design process, it is critical to be able to accurately predict how the heave force changes with varying speeds and depths to ensure the trim tanks and control surfaces that are typically utilised in this condition are effective. Moreover, when developing simulation models of a submarine, correct characterisation of the heave force is essential in order to guarantee representative behaviours close to the free surface. Using Reynolds Averaging Navier Stokes (RANS) computational fluid dynamics (CFD) simulations, this paper presents data at varying depths and speeds typical for a submarine operating near the free surface for the evolved DST Group/MARIN generic BB2 submarine at model scale [9]. The non-dimensional depth (H*) tested ranged from H* = 1.4 up to H* = 3.0 with Froude number between 0.16 and 0.31. At these depths, the heave force is directed upward towards the free-surface, increasing the risk of a submarine broaching. However, it is shown that small changes to either the depth or the speed will result in a significant change in heave force, which could lead to instabilities in the control of the submarine. The hydrodynamic heave force coefficient is presented independently of buoyancy and mass. © 2018 Australasian Fluid Mechanics Society