潜艇的水动力设计开发

1 STATE OF THE ART CFD ANALYSIS FOR HYDRODYNAMIC DESIGN IN SUBMARINE DEVELOPMENT Henrik Gustafsson, Anna Eriksson, P-O Hedin, Johan Jensen Conceptual Design/ Hydromechanics TKMS Submarine Division / Kockums AB Sweden henrik.gustafsson@kockums.se SYNOPSIS Computational Fluid Dynamics (CFD) is widely used for analysis in submarine development within TKMS Submarine Division. This technique is primarily used to study different areas regarding hydrodynamic performance of the submarines. One important area of interest is hydrodynamic resistance but several other subjects are studied as well. Much effort is spent on determining the characteristics of the wake in order to optimise the flow conditions for the propulsor. Detailed studies are performed for e.g. fin (sail) and control surfaces to point out the most favourable position on the hull. Different locations of sensors are also studied in order to ensure optimal performance of each device. Simulations of surface piercing masts are performed in order to analyse the resulting wave pattern and water spray. By using a more advanced turbulence modelling technique called Large Eddy Simulation (LES), analysis of flow induced noise is performed. Additionally, CFD is also applied within other, i.e. non hydrodynamic areas of submarine design, such as on-board environment analyses with regard to climate control. In order to increase the efficiency even more for the well proven Stirling AIP-system, CFD analyses are conducted to optimise the design of the combustion chamber. Introduction The aim of CFD is to numerically solve the governing equations for fluid flow. Due to the complexity of turbulent engineering flows, a numerical solution of the original equations is computationally too demanding. Two approaches are used for solving the problem, averaging (RANS, Reynolds Averaging Navier-Stokes) and filtering (LES, Large Eddy Simulation) of the equations. With RANS all turbulence is modelled. Using RANS is the classical way of solving turbulent engineering flows. However, if time accurate pressure fluctuations and velocities are of importance, as in flow noise studies, LES has to be used instead. LES is a more advanced turbulence modelling technique in which the large scale turbulence is resolved and the small scale turbulence is filtered out and modelled. This will, however, increase the computational time from a few days to several weeks. There are two major reasons for the increased computational time. First, the computational mesh has to be a lot denser in order to properly resolve the turbulent scales. Second, the simulation has to be time dependant and due to the small cells in the mesh the time step has to be very small. History 2 CFD has been used within TKMS Submarine Division for about 20 years. The first ambition was to increase the accuracy in resistance prediction by enabling analytical results of pressure as well as viscous resistance to be obtained. Earlier analytical methods only took the pressure resistance into account, using potential flows, leaving the amount of viscous resistance to an estimation based on towing tank tests. As different projects went by and questions of many kinds were discussed, a wider potential for the CFD technology has emerged. However, the main topics for CFD analyses lie within the area of hydrodynamics. Hardware and Software TKMS Submarine Division uses up-to-date pc-cluster technology with 28 computational nodes for CFD analyses. At the moment two different commercial CFD softwares are available within the division, ANSYS CFX and Fluent. Since all softwares have their special advantage the market is constantly observed to ensure that the most suitable one is used for each purpose. The CFD Process The CFD process normally starts with creation of the actual geometry. This is usually done with a CAD tool such as ProEngineer. Thereafter the geometry is imported to a mesh tool, ANSYS ICEM CFD, where the mesh is created. Now, the mesh can be read in to the CFD solver where the case is set up and solved. The results are finally analysed using dedicated post processing softwares, CFX Post or Fieldview. This process is tuned in to be as reliable as possible to provide sufficient results within a limited time frame. Applications Below follows a selection of applications where CFD has been used to study different aspects in submarine design. The first application is described more in detail to get a general feeling of the analysis work. Flow Field Analysis for Different Configurations of Mast Openings at the top of the Fin The characteristics of the flow field around the submarine are to some extent affected by the configuration of mast openings at the top of the fin. By simulating both the flow around the submarine as well as the flow in the free flooded spaces, analyses of the flow field characteristics can be carried out for different combinations of mast openings at the top of the fin to determine the effect of changes in the mast opening configuration. The reason for this study was to get an understanding of the characteristics of the flow in order to decide if the openings should be covered or not. 3 Results presented in this paper come from an initial study within the area. The different configurations were generated from the openings shown in Figure 1. Due to the academic level of the study, the submarine geometry used is generic. This implies difficulties in the validation process due to lack of measured data. The study however pointed at some interesting phenomena which makes the subject interesting for further analysis. From the results variations in the flow field, both inside and around the submarine, can be seen when the configuration of the openings is altered. Generally, there is a flow entering through the slots in the casing and passing, in a very complex way, through the free flooded spaces to exit both through the fin top openings and the slots. The exit and egress point at the slots as well as the structure of the flow in the free flood spaces does, however, differ from case to case. Figure 1. Openings in the casing and at the top of the fin for generating different cases Figure 2. Streamlines and contours of vorticity Vortices are generated in the vicinity of both the slots and the mast openings. The characteristics of the vortices are, however, case dependent as well. The results also indicate that the configuration of a single opening can produce effects on other openings. This is seen in Figure 3 which shows a comparison of two of the evaluated cases. To visualise one of the differences, velocity vectors coloured by x-component of vorticity are plotted in a plane that cuts through the aft starboard opening. 4 For the case with two openings at the fin top (left) only separation vortices are generated whereas for the case with three openings at the fin top (right), a horse shoe vortex is produced at the opening as a result of an increased flow through that opening. Differences in the flow field can give rise to differences in signatures and resistance, parameters that are important in submarine development. The results of the study showed that this is an area that has to be analysed further to fully understand the different aspects of the flow. The generic submarine, designed by Kockums, is open and available for anyone who wants to use it for CFD projects. The study is also open and available. Please, contact any of the authors to get a copy. Figure 3. Velocity vectors in a vertical plane that cuts through the aft starboard opening 5 Design Optimisation Using CFD, different designs are analysed with respect to resistance and flow field. Fin designs with different shapes of the top has been analysed, i.e. flat and rounded top respectively. By improving the design, the resistance of the fin was reduced with f 50%. In Figure 4 and 5 a dramatic difference in the flow field between the two fins can be seen. Figure 4. Rounded fin top Figure 5. Flat fin top Certain objects such as outboard mounted sonars with fairings have been parametrically analysed to find the most favourable geometry and position. These analyses are of great importance in order to provide an optimum environment for e.g. sensors. The same analysis will also make sure that existing apparatus is not disturbed by the new details and that effects leading to increased signatures or disturbing the propeller wake are not present. Figure 6. Streamlines coloured by turbulent intensity 6 Wake and Propeller Analyses In cooperation with the Swedish Defence Material Administration, FMV, and other research institutes, extensive research has been carried out regarding propulsor design and determination of wake characteristics. CFD analyses show very good agreement with unique full scale measurements on a Swedish submarine. In this case the CFD results exceed results from the model test of the same submarine. Figure 7. CFD vs. experimental data Figure 8. Wake fraction CFD has also been used for analysing the flow conditions downstream of a propeller. The analysis method used was validated using a four bladed propeller from which measurements using Laser Doppler Velocimetry were available (INSEAN, Italy), see Figure 9 and Figure 10. Figure 9. Axial velocity, normalised Figure 10. CFD vs. experimental data 7 Free Surface Flow It is very important for a submarine in snorting condition to show as small Radar Cross Section (RCS) as possible in order to avoid detection. Therefore it is of certain interest to perform analysis of surface piercing masts. These simulations are made to analyse resulting wave pattern and the amount of water spray. The mast resistance and frequencies of vortex shedding at different speeds are also studied. By doing this, speed limitations are set to guarantee safe conditions for the use of masts. Mast analyses are performed for different mast geometries and for different configurations of several masts. Other characteristics such as the periscope’s line of sight are also determined. The mast positions can be optimised with regard to RCS and resistance. In order to calculate the total RCS on a submarine using masts at periscope depth, the wave pattern is then exported to software where the RCS of surface and masts are calculated. Figure 11. Single mast, cylindrical cross section Figur 12. In line masts, profiled cross section The method for calculating wave pattern in free surface flows has been validated using a NACA0024 profile. Wave pattern from a CFD analysis was compared with experimental data and showed good agreement; see Figure 13 and Figure 14. Due to mesh resolution, the CFD-results are smoother than the real surface. Figure 13. Wave pattern Figure 14. CFD vs. experimental data -0,035 -0,03 -0,025 -0,02 -0,015 -0,01 -0,005 0 0,005 0,01 -2 -1 0 1 2 3 4 x [ m ] w a ve h ei g h t [m ] CFD results vf 05 Experimental Data 8 Flow Induced Noise Prediction By using Large Eddy Simulation (LES) analyses of flow induced noise are performed. This is an advanced turbulence modelling technique for resolving the turbulent quantities and pressure oscillations in more detail. This method is currently being used and further developed. Making this technique applicable in submarine development will open up a new branch of research, leading to even further capacity of an already successful progress in signature management. Non Hydrodynamic Analyses Additionally, CFD is also applied with other, i.e. non hydrodynamic areas in submarine design such as on-board environment with regard to heat and ventilation. Studies are made of the battery compartment to ensure that the hydrogen concentration remains below the safety limit in the whole space with no local high gas concentrations during charging of the batteries. Environmental studies have been performed of the living quarters to investigate the heat distribution. In order to increase the efficiency even more for the well proven Stirling AIP-system, CFD analyses are conducted to optimise the design of the combustion chamber. Figure 15. Battery compartment Figure 16. Living quarter Summary CFD has been used for many years in submarine development and the number of applications is still increasing. By using CFD in submarine hydrodynamic design the operational performance has been increased due to optimisation of the design and positioning of equipment such as sonars. Unexpected phenomena are avoided by performing detailed studies of outboard arrangements. Compared to tank tests, CFD analyses are more efficient and has also shown better agreement with full scale measurements. CFD is also used in other, non hydrodynamic, areas such as ventilation analysis of the battery compartment, heat distribution studies and combustion chamber analysis. KockumsKockums 9 Author’s Biography Henrik Gustafsson graduated from Lund Institute of Technology, Sweden, in 2001 with a Master of Science in Mechanical Engineering. After graduation he started his career working for Volvo Aero Corporation as a development engineer with main interest in fluid mechanics. He is now working for Kockums at the Conceptual Design Department, as a specialist in hydrodynamics. His work involves a wide range of applications regarding Computational Fluid Dynamics. Anna Eriksson graduated as Master of Science in Naval Architecture in 2005. The studies were conducted at the Royal Institute of Technology and at the Department of Shipping and Marine Technology at Chalmers University of Technology, Sweden. Since 2005 she is employed at Kockums as specialist in hydrodynamics at the Conceptual Design Department. She is primarily working with Computational Fluid Dynamics within a wide range of applications in particularly submarine but also surface vessel development. Per-Ola Hedin started his career in the RSwN serving as a technical officer on the Sea Serpent class submarines between 1989 and 1995. During these years he was also involved in submarine rescue and diving operations. In 1999 he graduated as Master of Science in Naval Architecture from the Department of Vehicle Engineering at The Royal Institute of Technology, Sweden, and started to work for Kockums as a Hydrodynamic specialist. Since 2005 he is deputy manager of the Conceptual Design Department, responsible for submarine development. Johan Jensen started his career in the RSwN serving as an operational officer (sonar, navigation, steering) on the Sea Serpent and the Näcken class submarines between 1990 and 1995. In 2000 he graduated as Master of Science in Naval Architecture from Department of Shipping and Marine Technology at Chalmers University of Technology, Sweden. In 2000 he started at the Swedish Defence Material Administration (FMV) as Submarine Systems Engineer involved in procurement of the new generation submarines for Sweden, Denmark and Norway (the Viking Project). Since 2002 he is working at Kockums, as Naval Architect at the Conceptual Design Department, involved in hydrodynamics and conceptual design.