Computation of Hydrodynamic Characteristics of Ships using CFD
- 1. Computation of Hydrodynamic Characteristics of Ships using CFD Authored by Md. Mashud Karim and Nabila Naz 2016 4th Asia Conference on Mechanical and Materials Engineering Presented by: Md. Mashud Karim Professor Department of Naval Architecture and Marine Engineering Bangladesh University of Engineering and Technology Dhaka-1000, Bangladesh
- 2. Objective To analyze the governing equation of fluid flow around ship hull To determine the flow around different ship hulls with free surface To determine the free- surface wave pattern and wave elevation around ship hulls at different speeds To compute wave making, viscous and total resistance components at different speeds To analyze the computed results for different mesh density To validate the obtained results with available experimental results The objective of present research are:
- 3. 3 Waves caused by the movement of Ship When ship moves through water creates pressure difference around it causes generation of waves at free surface to maintain constant atmospheric pressure.
- 4. 4 Lord Kelvin (1887) gave a characteristics wave for ship named as Kelvin wave pattern which consists of two types of waves: Transverse wave Divergent Wave Ship Wave Pattern
- 5. 5 Determination of Wave Characteristics Real Wave Pattern Wave Pattern simulated by CFD Wave Pattern by EFD at Towing Tank
- 6. 6 What is CFD? • Stands for Computational Fluid Dynamics •virtual towing tank • determines the flow characteristics around ship by mathematical modeling and numerical methods using commercially developed software •possible by the advent of digital computer and advancing with improvements of computer resources Substantial reduction of lead times and costs of new designs. Ability to study systems where controlled experiments are difficult or impossible to perform (e.g. very large systems). Ability to study systems under hazardous conditions at and beyond their normal performance limits (e.g. safety studies and accident scenarios). Practically unlimited level of detail of results. Advantages of CFD over EFD CFD software- SHIPFLOW developed by FLOWTECH International AB from the long term research done at the Hydrodynamics group of the Naval Architecture Department at Chalmers University of Technology, Gothenburg, Sweden.
- 7. 12/02/16 7 To compute the flow around a ship in an efficient way, zonal approach will be is used by Shipflow as shown in Fig. which divides the flow around a ship into three different zones with different solution methods. Fig . Zonal approach Zone 1: Region outside the boundary layer; the potential flow theory will be employed Zone 2: Thin boundary layer region near the forward part of the hull; ‘momentum boundary integral equation’ will be used. Zone 3: Stern/wake region where ‘Navier Stokes equation’ will be used
- 8. Governing Equation for Zonal Approach Zone 1 Potential flow region Fluid is incompressible and inviscid and the flow is irrotational Continuity equation becomes Water surface condition Ship hull surface condition Kinematic free-surface condition Dynamic free-surface condition
- 9. Zone 2 Thin boundary layer region near the hull Momentum integral equation Zone 3 Viscous flow at stern/ wake region Reynolds Averaged Navier Stokes (RANS) equations coupled with the time-averaged continuity equation:
- 10. Description of Hull Body plan 3D view of hull A mathematical hull with its geometric surface defined as: Wigley Hull LBP 1 m B/L 0.01 H/L 0.0631 WPA coefficient 0.667 CB 0.447 Model Dimensions:
- 11. Body plan 3D view Series 60 ship LBP 1 m B/L 0.134 H/L 0.05352 WPA coefficient 0.943 CB 0.6
- 12. Discrtetization of hull and free-surface for potential flow The surface of the ship and the water surface are divided into flat, ideally square panels commonly with constant source strength. (a) Wigley Hull (b) Series 60 ship
- 13. Boundary Conditions Boundary types employed are no slip, slip, inflow, and outflow. Both Dirichlet and Neumann boundary conditions are formulated in terms of pressure , velocity , turbulent kinetic energy , and turbulent frequency . Due to symmetry on the x-z plane, quarter of cylinder is used as computational domain with radius 3.0 L, downstream length 0.8L For zonal approach viscous computation starts from 0.5L behind the F.P of the ship as shown in Fig.Fig. Computational Domain
- 14. Grid Generation Fig. H-O type structured grid (a) computational domain (b) close-up view 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.
- 15. (b) Fr. 0.408 Wave Pattern around Wigley hull at Different Speed (a) Fr. 0.177 Wave pattern consists of two wave systems namely transverse and divergent waves Divergent waves which are the primary wave system at lower Fr., start at the bow and stern region at an angle of 19.47º
- 16. Wave Pattern around Series 60 Ship at Different Speed (a) Fr. 0.20 (b) Fr. 0.35 Transverse waves which are more important at higher Fr. are perpendicular to the ship's line of motion. Both wave patterns are contained within two straight lines making an angle of 19.47º on each side of line of motion show the characteristics of Kelvin wave pattern
- 17. Free Surface Wave Elevation Wigley hull at Fr. 0.267 The computed free-surface wave elevations around Wigley hull with different mesh configuration at Fr. 0.267 are compared with the experimental results as shown in Fig. It appears that with fine mesh wave elevation along hull shows good agreement with the experimental results except the stern region for Wigley hull
- 18. Free Surface Wave Elevation Series 60 ship at Fr. 0.316 The computed free-surface wave elevations around Series 60 ship with different mesh configuration at Fr. 0.316 are compared with the experimental results as shown in Fig. It appears that with fine mesh wave elevation along hull shows good agreement with the experimental results except the bow region for Series 60
- 19. This discrepancy between computed and experimental results 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.
- 20. Pressure Coefficient and Potential Flow Streamline (a) Wigley hull (b) Series 60 ship 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 perpendicular to the beginning of the after hull part as shown in Figs. at Fr. 25 for both hulls.
- 21. Resistance coefficient as a function of Fr. Total (Ct), wave making (Cw) and viscous (Cv) resistance coefficients as a function of Froude numbers (Fr) and Reynolds numbers (Re) are shown in Fig. Cv decreases with increasing Re for both hulls as it largely depends on it. 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. (a) Wigley hull (b) Series 60 ship
- 22. WIGLEY HULL: RELATIVE ERRORS OF TOTAL RESISTANCE COEFFICIENTS BETWEEN CFD AND EFD Fr. Re. Ct CFD Ct EFD Error (%) 0.177 2.2*106 0.00433 0.00419 -3.34129 0.25 3.1*106 0.00479 0.00455 -5.27473 0.27 3.3*106 0.00475 0.00447 -6.26398 0.316 3.9*106 0.00544 0.00516 -5.42636 0.35 4.3*106 0.00517 0.00489 -5.72597 0.408 5.0*106 0.00624 0.00573 -8.90052 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 I
- 23. SERIES 60 SHIP: RELATIVE ERRORS OF TOTAL RESISTANCE COEFFICIENTS BETWEEN CFD AND EFD Fr. Re. Ct CFD Ct EFD Error (%) 0.1 1.2*106 0.00474 0.00476 0.42017 0.15 1.7*106 0.00448 0.00456 1.75439 0.2 2.2*106 0.00455 0.00442 -2.94118 0.25 3.1*106 0.00457 0.00448 -2.00893 0.3 3.8*106 0.00619 0.00594 -4.20875 0.35 4.3*106 0.00653 0.00645 -1.24031 Table II Table II shows that for Series 60 ship at low Fr. CFD results of Ct is smaller than the EFD results but as Fr. increases, CFD starts to overestimate the Ct than EFD with maximum relative error of 4.21% at Fr. 0.3.
- 24. Conclusions In this research work, potential flow, boundary layer flow and viscous flow theories are used to determine flow characteristics at different regions. From the above mentioned results and discussions, following conclusions can be drawn: Zonal approach for computing flow characteristics takes less computational time than global approach as three solvers act successively to give results significant to that region . Wave pattern around ship hulls with different Fr. show characteristics of Kelvin wave pattern and computed wave elevations agree satisfactorily with experimental results i. With increasing Fr. wave making and total resistance is accompanied by a number of humps and hollows due to interaction of divergent waves and frictional resistance decreases as Re. increases for both hulls v. The computed results depend to a certain extent on the discretization of the body and the free-surface. The agreement between the computed and experimental results is quite satisfactory with increasing number of panels. However, it takes longer computation time.
- 25. ACKNOWLEDGMENT Authors are grateful to Bangladesh University of Engineering and Technology (BUET) and sub-project CP # 2084 of Department of Naval Architecture and Marine Engineering under Higher Education Quality Enhancement Project (HEQEP), UGC, Ministry of Education, Govt. of Bangladesh for providing necessary research facilities during the current research work.