Resistance and flow

Abstract

The flow over a submarine will influence its top speed, its acoustic signature, and the effectiveness of its own sensors. In particular, flow separation should be avoided. The various components of resistance include: surface friction; form drag; induced drag; and wave making resistance. The latter only becomes important when the submarine is operating on, or close to, the water surface. The effect of the hull form on the various components of resistance is discussed, and it is shown that a teardrop hull may not always be the best solution. A submarine hull is usually considered in three parts: fore body; parallel middle body; and aft body. The main driver for the hydrodynamic design of the fore body is to control the flow such that there is laminar flow over the sonar array. A fuller fore body may be beneficial for this. The length of the parallel middle body influences the length to diameter ratio, and it is shown that there is an optimum value of the L/D to minimise resistance, depending on the hull form. The aft body shape can be characterised by the half tail cone angle, which defines its fullness. The primary aim of the design of the aft body is to avoid flow separation, and ensure good flow into the propulsor. Model testing and Computational Fluid Dynamics techniques are discussed. In addition, an empirical method of predicting the resistance of a submarine, suitable for use in the early stage of the design, is presented.