An investigation of two body forces in a vortex wake

Summary form only given. Computer models and tow-tank trials are being used to study the roll behavior of a sonar twobody in two-part underwater twoing. Two-part two configurations, consisting of a depressor generating downforce on the main two cable to hold down a neutrally-buoyant connecting cable and twobody, are used to isolate towed devices from ship motion. However, during sea trials of one such arrangement with a hydrodynamic depressor, the twobody would periodically roll violently. This unacceptable motion was believed to be caused by the two trailing vortices which dominate the delta-shaped depressor's wake. An investigation has commenced into this problem and how it might be avoided. First, a series of computer programs for estimating the wake's influence on the twobody is being written. The initial code uses potential-flow calculations in the body's crossflow plane to determine the instantaneous lift, sideforce, and roll moment induced on the tail section by a pair of ideal vortices. A later version will consider viscous boundary layer and vortex-core effects. The first simulation has been run for a range of twobody positions relative to the wake. Its results show that large moments are generated when the body is very close to the vortices, and that the lift and sideforce can move the body into such hazardous proximity over time. To verify the simulation output, depressor-generated forces on the actual body are to be measured in a two tank. Trials will be run for several twobody positions to validate the predictions of the computer models. This project represents a novel approach to twoing simulation. The complete investigation should confirm that the depressor's wake induces twobody roll, and reveal twoing situations which must be avoided. The results can be used as a guide to adjusting two-part two configurations to maintain stability.<
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