10:30
Fluid structural inter-actions IV
Chair: Yousheng Wu
10:30
30 mins
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NUMERICAL STUDY OF A COUPLED WELL BOAT- FISH FARM SYSTEM IN WAVES AND CURRENT DURING LOADING OPERATIONS
Yugao Shen, Marilena Greco, Odd Faltinsen
Abstract: Dynamic response of a coupled well boat-fish farm system in waves and current is investigated numerically. The main purpose of the study is to determine how the presence and motions of the well boat will affect the behaviour of the different components of the fish farm. In particular, we focus on the influence on the maximum forces in the mooring lines during loading operations in extreme sea conditions.
Before simulating the coupled system, some aspects related to the modelling of the well boat as well as the individual components of the fish farm system are presented. For the well boat, the equations of motions are solved in a coordinate system that coincide with the mean position of the well boat. Connection line forces as well as possible contact forces between the well boat and the fish farm system are duly considered. To avoid unnecessary complexity, we model a generic yet realistic fish farm system. The idealized fish farm system consists of an elastic floater, a flexible-circular-bottomless net cage, sink weights and a realistic mooring system. The various components are modelled according to state-of-the-art theoretical and numerical formulations [1],[2].The effect of the flow around the net cage and the transient responses of the well boat and the elastic floater are also accounted for.
Then, the strategy to handle the coupling between different system components and the method used to estimate the contact force between the elastic floater and the well boat are explained. In particular, an intrinsic coupling strategy has been preferred to handle better strong coupling between different fish farm components while an iterative strategy is adopted for weaker coupling between the well boat and the fish farm. Intrinsic coupling means that the equation system is solved simultaneously. Numerical simulations for the coupled system in waves and current are performed to assess the robustness of the solution algorithm and to analyze the validity of the numerical predictions.
Finally, a sensitivity analysis is performed to identify the most important parameters influencing the maximum mooring-line forces. The considered parameters include the distance between the well boat and the floater, the elasticity of the floater, the contact stiffness acting between the floater and the well boat, the pretension in the mooring lines and the mooring lines stiffness.
REFERENCES
[1] Kristiansen, T. and O. M. Faltinsen (2015). "Experimental and numerical study of an aquaculture net cage with floater in waves and current." Journal of Fluids and Structures 54: 1-26.
[2] Endresen, P. C., et al. (2014). Simulation and Validation of a Numerical Model of a Full Aquaculture Net-Cage System. ASME 2014 33rd International Conference on Ocean, Offshore and Arctic Engineering, American Society of Mechanical Engineers.
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11:00
30 mins
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EXTREME WAVE INTERACTION ON TRUSS STRUCTURE
Sruthi Chandrasekar, Sriram Venkatachalam
Abstract: Owing to increased support for renewable resources, offshore wind turbines have got lot of attention these days. One important installation in offshore wind turbine is the substructure which is subjected to wave loads. The truss tower has several key advantages like less weight and a more flexible design. Extreme load may result from breaking waves. The magnitude of loading will depend on the substructure type which may also be a controlling factor in design. In this study experiments were conducted on truss structure. Forces so far calculated on the whole structure is based on formulas proposed for single cylinder. Hence to have clarity, the whole truss structure was tested. Focusing waves were generated and waves are made to break at specified location. Focusing waves were chosen for breaking of waves without slope. The focusing point was changed so that the waves break in-front of the structure, on the front leg, between the braces, on the rear leg which will form different loading cases. In order to understand the braced effects, the structure was dismantled and tested. Two legs with braces in the direction of wave and another setup with braces transverse to wave direction were installed. The same loading cases was executed for the above mentioned experimental setup. Load transducers were used to measure the response of the structure. Further the results were analyzed using Duhamel integral and the impact force estimated. The main aim of study is to understand the time variation of load and the time for which load acts. Comparison with existing theoretical formulas proposed for slamming load on cylinder proposed by Goda, et al. (1966), Wienke and Oumeraci (2005) were also done.
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11:30
30 mins
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EFFECT OF COMPRESSION ON THE HEAD-ON COLLISION BETWEEN TWO HYDROELASTIC SOLITARY WAVES
M. M. Bhatti, D.Q. Lu
Abstract: Interaction and collision of different fluids and elastic boundaries originates in various mechanical environments. These types of problems are very much difficult mathematically, due to its coupling between deformable bodies and moving fluids. Another interesting thing we observed that when the solitary waves collide each other, they transfer their position and energies with each other and then regain their original shape and position after separating off. It is noteworthy here that, during the whole process of collision, solitary waves are very much stable in preserving their identities. To obtain the solution of head-on collision between solitary waves, one must employ the asymptotic technique to solve the highly nonlinear partial differential equations. Inverse scattering transform method (IST) is also use to get the solution of KdV equation, but this method is only applicable for overtaking interaction between solitary waves. For this purpose we will employ Poincar$\acute{\textrm{e}}$--Lighthill--Kuo (PLK) method. It is originally the method of strained coordinates which was introduced by Poincare $1982$ for ordinary differential equations, Later Lighthill $(1949)$ and Lin $(1954)$ applied this scheme to hyperbolic differential equations.
We investigate the head-on collision between two hydroelastic solitary waves traveling in a fluid covered by an elastic plate under the influence of compression. The fluid having constant density is incompressible and inviscid and the motion is irrotational. The horizontal plane bottom is situated at $z=0$ where the normal velocity of the fluid is considered to be zero. The deflection of the plate is presented at $z=H(x,t)$. With the help of potential flow theory, the governing equation is the Laplace equation. The Euler beam theory is taken for the elastic part in the dynamical boundary conditions. Both the solitary waves are small in amplitude ($a$) and are long in wavelengths $\lambda$, namely $ a/H \ll 1 $ and $\lambda / H \gg 1$. The physical parameters are related to Ursell's ordinary theory of shallow water, i.e $H^3 \approx a \lambda^2 $. The solutions of the nonlinear equation has been obtained with the help of the method of strained coordinates (the PLK method) upto fourth-order approximation. The behavior of all the physical parameters of interest are discussed and demonstrated graphically.
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