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When an oncoming boundary layer flow encounters an appendage fixed on the plate, the flow separates in front of the body due to the blocking effect. The separated flow forms several vortices around the body, which are called horseshoe vortices because their top view looks like a horseshoe. The flow is completely 3D and becomes complicated as the horseshoe vortex interacts with the boundary layer developed on the body surface. This convoluted three-dimensional flow has been called a juncture flow. Juncture flow can affect the lift and stability/control characteristics of appendages through the generation of horseshoe vortices from these appendages. In spite of its engineering importance, there is no established method for estimating the appendage resistance or wake characteristics since the detailed mechanism of the juncture flow is not fully understood. Although some systematic experimental studies have been performed through conventional tests, the estimation of the resistance is hampered by the uncertainty associated with the scale effect. The flow complexity is due to the number of vortices originating upstream of the strut. These vortices result from the separation of the boundary layer due to the adverse pressure gradient in front of the strut. Creation of the vortex system in front of the circular cylinder is clearly seen in the famous photograph taken by Sutton where an array of vortices is visualized with the injection of smoke. Sutton's photograph has helped clarify the horseshoe vortex system. The number of vortices increases as the Reynolds number is increased or the thickness of the oncoming boundary layer decreases. Although many experimental studies have been conducted to understand the structure of the horseshoe vortex system, the precise flow topology is still controversial due to the lack of resolution in experimental data. However, rapid advances in computing technology have cleared the road to simulate the flow by solving the RANS equations.
The master thesis focuses on the numerical simulation of 3D flow around junctures, in which the unsteady nature of the flow represents the main point of interest. Various geometries of the juncture are investigated to grasp a better understanding of the phenomena and develop proper modeling techniques. Besides, the study also focuses on the free-surface influence on the overall hydrodynamic field around the juncture.
In the first part of the thesis, the theoretical aspects of the numerical approach are briefly described. The governing equations and the initial and boundary conditions are presented. In addition, some quantities which are used to estimate and evaluate the solutions of the governing equations are also mentioned. Emphasis is put on the turbulence treatment. A comparison between different turbulence models is performed to motivate the choice for the Spalart-Allmaras model, which proved to be the most suitable one for this work. The validation of the computed solution is achieved through comparisons with the experimental data provided by the literature.
The second part of the work discusses the 3D flow around a circular cylinder mounted on a plate. The characteristics of the flow around a circular cylinder such as Reynolds and Strouhal numbers, vortex shedding, drag and lift coefficients are also pointed out specifically. In these cases, the free surface effect is not taken into account. The shape of plate is changed ranging from the flat one to a concave or a convex one. For each geometric configuration of the plate, the circular cylinder will be inclined with various aspect angles (10°, 20° and 30°) longitudinally and laterally. All simulation cases are done for two different Reynolds numbers of 3900, and 1 million, respectively.
In the third part, the unsteady simulation is performed at Reynolds number of 3,900. Only the circular cylinder mounted on the convex plate and inclined laterally is studied and the numerical solution is compared with the cases of the steady flow corresponding case. Besides, once again, these simulations do not take into account the effect of free surface. The mechanism of vortex shedding will be unveiled by the results of this simulation.
Finally, several conclusions outline the achievements and findings of the work, drawing out the
potential directions for further studies.

Ingénierie, informatique & technologie > Ingénierie civile

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The challenge of international market and the search of increasing performances lead the process of ship design to a continuous enhancement in merchant field where the dedicated time for a thorough design is ever decreasing. While improving the performance of hull form, seakeeping is a crucial part for a cruise liner in order to serve the customers with maximum comfort onboard and considering the wave added resistance in calculating the total resistance of hull form for selecting the appropriate engine power. Therefore, this work is motivated by the challenge to understand and clarify which approach is particularly suited for the performance of seakeeping behavior of the hull form design and select the optimum hull form design considering a couple of scenarios of different sea-states of operating routes and different speeds. In this thesis, parametric modeling of the hull form is modified from the original geometry model of the ship hull which is provided from Meyer Shipyard by using Friendship Framework/CAESES. After getting parametric model, it is simulated by GL Rankine and validated with experimental results from HSVA. GL Rankine is coupled with CAESES and then, with different optimization approaches, optimal ship hull form is obtained in calm water condition. The optimized model is checked for wave added resistance and analyzed for seakeeping behavior in moderate sea-state under different scenarios. In the another approach, the hull form is directly optimized for the total resistance of the ship, comprising the calm water resistance and the added resistance in seaway, followed by the analysis of seakeeping behaviors. Optimization is to be done especially on the fore body of the hull (e.g. bulbous bow). Finally, the results are analyzed to compare the optimal hull form with original design.

Ingénierie, informatique & technologie > Ingénierie civile

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In the preliminary stage of ship design, resistance prediction is a primary challenge for
designers. The objective of the thesis is to predict the total resistance and its components of
JBC bulker with the RANS method. As a mature approach, CFD is an effective tool to
estimate resistance. The well investigated hull form KVLCC2 is used to study the accuracy of
the CFD calculation and to validate the resistances and the flow field around the hull by
comparing it with existing experimental data.
Simulations are performed using the open source software package OpenFOAM. A steady
state RANS solver is used to solve the viscous flow around the hull and two-equation KOmega
SST model is selected for the turbulence modeling. The vessels are implemented in a
fixed towing condition where the effect of trim and sinkage is neglected. The commercial
mesher HEXPRESS is used to generate unstructured hexahedral meshes of the fluid domain.
Local grid refinements are carried out by setting the multi-block meshes and the free surface
is captured via VOF method. Additional refinement mesh is given in the boundary areas of
the hull and the wall function is implemented to simulate the turbulence characteristics.
A suitable value of turbulence intensity is obtained after a set of tests in order to get the right
turbulence parameters. In addition, the effect of grids with similar configuration is
investigated and 5-level gradual refinement meshes are studied. The resistance components
and the wave elevations in three different wave cuts are compared with the experimental
results, which show a good agreement. The most proper mesh configuration with good
accuracy is selected to convert to the JBC bulker.
At the end, the calculated resistances and three different transverse wave cuts of the bare hull
JBC bulker in calm water condition are presented.

Ingénierie, informatique & technologie > Ingénierie civile

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Despite the early discovery of hydrofoils, first conceived in 1906 [1], there has been renwened interest in recent years, particularly for racing yachts, most likely thanks to the progress in material properties. As a consequince, the concept is begining to filter through to the cruising yacht sector despite the limited technical literature. The main benefits of including hydrofoils on a sailing yacht are: performance, velocity along with stability, and seakeeping. There does however remain challenges to the designs and hydrodynamics when adopting foils. The aim of this master thesis is to implement three existing concepts of hydrofoil into a standard design of cruising yacht, and to establish and resolve the design integration problems and tackle the hydrodynamic advantages. Firstly, the design is going to be explored in order to find the best way to include the hydrofoils into the boat. Subsequently, the selected designs will be tested by experimental fluid dynamic approach in a towing tank testing and their hydrodynamic results outlined.

Ingénierie, informatique & technologie > Ingénierie civile

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A numerical simulation code using Discrete Element Method has been developed by HSVA,
which can generate brash ice and ice ridges as well as analyse ship navigation through ice
channels. The current version is simulating the problem in model scale for ease of validation.
This thesis aims to enhance the software's capabilities, reduce the computational time, and to
enhance performance and capabilities by modifying internal source code.
To improve the performance GPU programming has been introduced. GPU programming
extension CUDA, developed by NVIDIA, has led to numerous advances in computing over the
last few years. The CUDA API makes it relatively easy for users to access the graphics card
hardware, which allows users to perform parallel computations with thousands of CUDA cores.
The following report investigates the methodology and advantages of using the CUDA API for
DEM computations. In order to achieve this, existing CPU code had to be rewritten for the
GPU. Both implementations show significant improvements with regard to iteration time, and
performance depending on of GPU architecture.
Additionally, it has been demonstrated that the GPU can be sped up by simply varying certain
parameters, which boosts the code's performance overall. Another investigation dives into the
overhead associated with programming memory intensive scripts to the GPU and shows what
effect this has on the total calculation times for the application. Further a more complex IceStructure interaction algorithm can improve the quality of results.
In this case a different number of simulations is done varying the element number to find out
the dependency of the elements to the computational time. Eventually, several tests were
conducted for different types of brash ice and Ice ridge channels to see how the software would
react.

DEM, GPU, CUDA, Time consumption --- Ingénierie, informatique & technologie > Ingénierie mécanique