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Abstract:
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The motion instability is an important issue that occurs during the operation of towed
underwater vehicles (TUV), which considerably affects the accuracy of high precision
acoustic instrumentations housed inside the same. Out of the various parameters
responsible for this, the disturbances from the tow-ship are the most significant one. The
present study focus on the motion dynamics of an underwater towing system with ship
induced disturbances as the input. The study focus on an innovative system called two-part
towing. The methodology involves numerical modeling of the tow system, which consists
of modeling of the tow-cables and vehicles formulation. Previous study in this direction
used a segmental approach for the modeling of the cable. Even though, the model was
successful in predicting the heave response of the tow-body, instabilities were observed in
the numerical solution. The present study devises a simple approach called lumped mass
spring model (LMSM) for the cable formulation. In this work, the traditional LMSM has
been modified in two ways. First, by implementing advanced time integration procedures
and secondly, use of a modified beam model which uses only translational degrees of
freedoms for solving beam equation. A number of time integration procedures, such as
Euler, Houbolt, Newmark and HHT-α were implemented in the traditional LMSM and the
strength and weakness of each scheme were numerically estimated.
In most of the previous studies, hydrodynamic forces acting on the tow-system such as drag
and lift etc. are approximated as analytical expression of velocities. This approach restricts
these models to use simple cylindrical shaped towed bodies and may not be applicable
modern tow systems which are diversed in shape and complexity. Hence, this particular
study, hydrodynamic parameters such as drag and lift of the tow-system are estimated using
CFD techniques. To achieve this, a RANS based CFD code has been developed. Further, a
new convection interpolation scheme for CFD simulation, called BNCUS, which is blend of
cell based and node based formulation, was proposed in the study and numerically tested.
To account for the fact that simulation takes considerable time in solving fluid dynamic
equations, a dedicated parallel computing setup has been developed. Two types of computational parallelisms are explored in the current study, viz; the model for shared
memory processors and distributed memory processors. In the present study, shared
memory model was used for structural dynamic analysis of towing system, distributed
memory one was devised in solving fluid dynamic equations. |