M.Sc Thesis

M.Sc StudentKadri Usama
SubjectThe Flowfield and Forces on Two Slender Bodies Moving in
Close Proximity
DepartmentDepartment of Aerospace Engineering
Supervisor RESEARCH PROFESSOR E Daniel Weihs


When two bodies move in close proximity in a fluid, or when a moving body passes a moored body, interactions involving significant hydrodynamic forces and moments will occur. These forces and moments may lead to collision, breakage of mooring lines, or grounding. In this thesis, the flow field and forces acting on two slender bodies moving in close proximity are studied for two primary cases; (1) the interaction forces acting on two bodies moving with equal speeds and (2) the interaction forces acting on a moored body due to a presence of a passing body. The fluid is assumed to be unbounded, inviscid and incompressible, and steady, time independent conditions assumed. Thus, when the speed of the bodies is not equal, this serves as a quasi-steady, instantaneous approximation only. Various cases of the separation distances, lengths ratios and speed ratios are presented in the first part of the work. Then the work focuses on some of the effects neglected in the first approximation solution in order to obtain a solution that includes smaller lateral separation distances. The nonlinear response due to the body movement, and the neglected terms of the potential function were considered. First, we considered the exact value of the separation distance; as a result we found small additional forces that become more significant at smaller separation distances. Then we use the method of matched asymptotic expansions and Taylor’s series expansions of the potential and its derivatives and found the second and the third approximation terms. As expected, for large separation distance the contribution of the higher order approximation terms is very small.  They become significant for small separation distances, and thus they should be taken into consideration in this case.