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Introduction
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Computation of Hydrodynamic Characteristics of Ships Using CFD

Profile image of Mashud KarimMashud Karim

International Journal of Materials, Mechanics and Manufacturing

https://doi.org/10.18178/IJMMM.2017.5.4.322
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Abstract

This paper investigates various hydrodynamic characteristics of two conventional ships namely Wigley hull and Series 60 ship by commercial CFD software named Shipflow. Zonal approach is applied to incorporate 'potential flow solver' in the region outside the boundary layer and wake, 'boundary layer solver' in thin boundary layer region near the hull and 'Navier Stokes solver' in the wake region successively. Finally free-surface wave pattern, wave elevation, pressure coefficient on hull, boundary layer streamline and different resistance components at different speeds are computed and validated by comparing with experimental results.

Figures (11)
To compute the flow around a ship in an efficient way, zonal approach [8] is used as shown in Fig. 2. Which divides the flow around a ship into three different zones with different solution methods. Region outside the boundary layer and wake is considered to be incompressible, inviscid and irrotational. Therefore, in the outer flow (zone 1), the potential flow theory is employed. The inner flow is divided into the thin boundary layer (zone 2) and stern/wake region (zone 3). The coordinate system (x, y, z) for computation is defined as origin is located in the undisturbed free surface at fore perpendicular (F.P) of the hull so that the undisturbed incident flow with a constant speed U appears to be a streaming in the positive-x direction with y axis extends to the starboard side and z- axis upwards as shown in Fig. 1.
To compute the flow around a ship in an efficient way, zonal approach [8] is used as shown in Fig. 2. Which divides the flow around a ship into three different zones with different solution methods. Region outside the boundary layer and wake is considered to be incompressible, inviscid and irrotational. Therefore, in the outer flow (zone 1), the potential flow theory is employed. The inner flow is divided into the thin boundary layer (zone 2) and stern/wake region (zone 3). The coordinate system (x, y, z) for computation is defined as origin is located in the undisturbed free surface at fore perpendicular (F.P) of the hull so that the undisturbed incident flow with a constant speed U appears to be a streaming in the positive-x direction with y axis extends to the starboard side and z- axis upwards as shown in Fig. 1.
TABLE I: BOUNDARY CONDITIONS FOR COMPUTATIONAL DOMAIN ool In XCHAP solver the boundary conditions are implied on the computational domain to solve partial differential equations of RANS equation. Boundary types employed are no slip, slip, inflow, and outflow [11]. Both Dirichlet and Neumann boundary conditions are formulated in terms of pressure p, velocity u; , turbulent kinetic energy k , and turbulent frequency @. Descriptions of boundary types are given in Table I and in Fig. 4.
TABLE I: BOUNDARY CONDITIONS FOR COMPUTATIONAL DOMAIN ool In XCHAP solver the boundary conditions are implied on the computational domain to solve partial differential equations of RANS equation. Boundary types employed are no slip, slip, inflow, and outflow [11]. Both Dirichlet and Neumann boundary conditions are formulated in terms of pressure p, velocity u; , turbulent kinetic energy k , and turbulent frequency @. Descriptions of boundary types are given in Table I and in Fig. 4.
XPAN solver uses potential-flow panel method based on Rankine sources [9] with hull and nonlinear free-surface boundary conditions [10] by discretizing the hull and the free-surface by flat quadrilateral panels as shown in Fig. 3. Thin boundary layer near the hull is computed with XBOUND solver [8] using the momentum integral equation:
XPAN solver uses potential-flow panel method based on Rankine sources [9] with hull and nonlinear free-surface boundary conditions [10] by discretizing the hull and the free-surface by flat quadrilateral panels as shown in Fig. 3. Thin boundary layer near the hull is computed with XBOUND solver [8] using the momentum integral equation:
To compute the flow around ship hull three types of governing equations are used depending on the hypothesis and considerations assumed in different regions. Foz potential flow region XPAN solver [8] is used where continuity equation becomes:
To compute the flow around ship hull three types of governing equations are used depending on the hypothesis and considerations assumed in different regions. Foz potential flow region XPAN solver [8] is used where continuity equation becomes:
where n;,U;, op, denote normal to the surface, velocity at characteristics time and parameter direction crossing the boundary respectively. Finite volume method requires grid cells in order to discretize the partial differential equations and approximate algebraic equations. In XCHAP module only structured grids are used. Computational domain along with hull geometry is represented by a single block structured grid of H-O type with 0.45 M cells as shown in Fig. 5. where n;,U,, pg denote normal to the surface, velocity at
where n;,U;, op, denote normal to the surface, velocity at characteristics time and parameter direction crossing the boundary respectively. Finite volume method requires grid cells in order to discretize the partial differential equations and approximate algebraic equations. In XCHAP module only structured grids are used. Computational domain along with hull geometry is represented by a single block structured grid of H-O type with 0.45 M cells as shown in Fig. 5. where n;,U,, pg denote normal to the surface, velocity at
Nee eS eee eee ee ee ee Fig. 8. Free surface wave elevation along ship hulls. Fig. 7. Wave pattern around Series 60 ship at Fr. 0.20 and 0.35 respectively.
Nee eS eee eee ee ee ee Fig. 8. Free surface wave elevation along ship hulls. Fig. 7. Wave pattern around Series 60 ship at Fr. 0.20 and 0.35 respectively.
The computed free-surface wave elevations around Wigley and Series 60 hulls with different mesh configuration at Fr. 0.267 and at Fr. 0.316 respectively are compared with the experimental results [3], [7] as shown in Fig. 7. Wave patterns for Wigley hull at Fr. 0.177 and 0.408 and Series 60 hull at Fr. 0.20 and 0.35 are shown in Figs. 6 and 7 respectively, it is apparent that divergent waves which are the
The computed free-surface wave elevations around Wigley and Series 60 hulls with different mesh configuration at Fr. 0.267 and at Fr. 0.316 respectively are compared with the experimental results [3], [7] as shown in Fig. 7. Wave patterns for Wigley hull at Fr. 0.177 and 0.408 and Series 60 hull at Fr. 0.20 and 0.35 are shown in Figs. 6 and 7 respectively, it is apparent that divergent waves which are the
Pressure coefficient and potential flow streamlines are automatically traced from the potential flow solution. Boundary layer tracing is started from 0.05L behind the fore From Fig. 7, it appears that with fine mesh wave elevation along hull shows good agreement with the experimental results except the stern region for Wigley hull and bow region for Series 60. This discrepancy is likely to have been caused by the following reasons: (i) the wave profiles are taken from the free- surface elevations at the panels next to the body, not at the actual hull surface, which resulted in error especially near the bow and stern region. (ii) Potential flow methods assumes free surface as flat and rigid to avoid air/water interface of viscous flow which also results in variation between computed and experimental results.
Pressure coefficient and potential flow streamlines are automatically traced from the potential flow solution. Boundary layer tracing is started from 0.05L behind the fore From Fig. 7, it appears that with fine mesh wave elevation along hull shows good agreement with the experimental results except the stern region for Wigley hull and bow region for Series 60. This discrepancy is likely to have been caused by the following reasons: (i) the wave profiles are taken from the free- surface elevations at the panels next to the body, not at the actual hull surface, which resulted in error especially near the bow and stern region. (ii) Potential flow methods assumes free surface as flat and rigid to avoid air/water interface of viscous flow which also results in variation between computed and experimental results.
TABLE II: WIGLEY HULL: RELATIVE ERRORS OF TOTAL RESISTANCE COEFFICIENTS BETWEEN CFD AND EFD From Table I] it is seen that for Wigley hull CFD results of Ct is greater than the EFD results for all Fr. and maximum relative error is 8.9% at the highest Fr. of the range.
TABLE II: WIGLEY HULL: RELATIVE ERRORS OF TOTAL RESISTANCE COEFFICIENTS BETWEEN CFD AND EFD From Table I] it is seen that for Wigley hull CFD results of Ct is greater than the EFD results for all Fr. and maximum relative error is 8.9% at the highest Fr. of the range.
Total (Ct), wave making (Cw) and viscous (Cv) resistance coefficients have been computed for both Wigley and Series 60 hulls at different Froude numbers (Fr) and Reynolds numbers (Re) as shown in Fig. 10. Fig. 10 shows that Cv decreases with increasing Re for both hulls as it largely depends on Re. Curves of Cw and Ct consist of number of humps and hollows which occur when bow and stern waves are in and out of phase respectively which is validated with experimental results [4], [7].
Total (Ct), wave making (Cw) and viscous (Cv) resistance coefficients have been computed for both Wigley and Series 60 hulls at different Froude numbers (Fr) and Reynolds numbers (Re) as shown in Fig. 10. Fig. 10 shows that Cv decreases with increasing Re for both hulls as it largely depends on Re. Curves of Cw and Ct consist of number of humps and hollows which occur when bow and stern waves are in and out of phase respectively which is validated with experimental results [4], [7].

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  14. M. M. Karim was born in Dhaka, Bangladesh in 1967. He received his Ph.D. in naval architecture and

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