CFD VALIDATION FOR SURFACE COMBATANT 5415 STRAIGHT AHEAD AND STATIC DRIFT 20 DEGREE CONDITIONS USING STAR CCM+

Transcription

1 CFD VALIDATION FOR SURFACE COMBATANT 5415 STRAIGHT AHEAD AND STATIC DRIFT 20 DEGREE CONDITIONS USING STAR CCM+ by G. J. Grigoropoulos and I..S. Kefallinou 1. Introduction and setup 1. 1 Introduction The current report describes the results of the CFD simulations for 5415(5512) hull for the straight ahead and static drift 20 degrees conditions using the STAR CCM+ commercial package. STAR CCM+ is a general purpose CFD code that employs a number of tools useful in ship hydrodynamics such as Dynamic Fluid Body Interaction, overset meshing for large angles of heel, moving reference frames and actuator disk theory for modeling the propeller, along with morphing and 6-DOF model, and calculates accurately the hydrodynamic forces in turbulent flows. The simulations run on two Dell Alienware systems -i7 at 3.2Gz- with 6-core hyper-threading INTEL processors of 6GB and 12GB RAM respectively. The number of cells is between 3M and 7M, the simulation time is within 40-60sec and the computational time varied from up to 8 days. 1.2 Numerical Methods The code supports unstructured grids (face to face) and uses the finite volume method for the discretization of the integral form of the equations. The implicit-unsteady scheme is employed along with the segregated flow model for the Dynamic-Fluid-Body-Interaction (DFBI). The coupling of the pressure and the velocity equations is achieved by using a Rhie-and-Chow-type coupling combined with a SIMPLE-type algorithm. With the Volume Of Fluid (VOF) model in use with the Eulerian Multiphase mixture for incompressible flow- each cell near the contact area of the water-air mixture consists of a volume fraction of both phases. This spatial distribution of the phases in any given time introduces the free surface in the simulation. First and second order temporal discretization schemes can be used (second order in our case). Tetrahedral, hexahedral and polyhedral cells can be used for the volume mesh, with the hexahedral mesher (trimmer) giving the option for anisotropic cell size in 3-directions. For proper analysis of the flow solution the prism layer model can be used, where orthogonal prismatic cells are generated next to wall boundaries to obtain better resolution of the boundary layer region. 1.3 Simulation setup The computational domain extends from 2.5Lpp aft of the stern, 1.5Lpp forward of the bow, 1.5Lpp from the centerline in both directions and 1.0Lpp from the baseline also in both directions. In order to avoid possible reflections from the boundaries and maintain the domain dimensions within limits, it was decided to keep the hull in 0 degrees and have the flow entering the region with 20 degrees angle in accordance with the instructions. With this configuration the boundaries in front of the ship and on the starboard side are assigned as velocity inletsthe same applies for the top and bottom boundary-while the boundary behind the ship and on the portside, where the fluid leaves the region, are pressure outlets.

2 The 5512 hull and bilge keels are simulated according to the experimental conditions and the ship is set in the fixed sinkage and trim in the earth-fixed coordinate system. The ship is tested on straight ahead and static drift 20 degree condition. The RANS approach is used with the realizable k-epsilon model, wall functions and all-y+ wall treatment that allows the gradual coarsening of the mesh as we move far from the body. The y wall distance was taken as m resulting in y+ between 30~40 for the most of the hull, apart from the stagnation and the separation points especially in the bow and the bilge keels. 2.Grid generation and results 2.1.General At first, the surface remesher tool is applied in order to optimize the quality of the imported surface and prepare it for the volume mesh. The volume is meshed with the trimmer model, where hexahedral cells are created on the previously retriangulated surface and are refined/ trimmed on the hull surface. This provides higher accuracy of the solution and at the same time reduces the computational cost by minimizing the required number of cells far from the body. Volumetric controls are assigned within the region to control the size of the cells. 2.2.Static drift 20 degree condition For the static drift 20 degree condition 3 different grids are generated. The dimension of the cells surrounding the hull is reduced by a step of 50% in each refinement with the rest of the domain far from the body slowly following the changes. The majority of the cells are located around the free surface and the bilge keels to capture as effectively as possible the fluid motion. The resulted grids consist of 4.6M-5.7M-7.0M cells. The simulations run until the residuals were normalized and the computed forces were oscillating around a steady value.

3 Computational domain and grid generation (7M cells). Residuals from the static drift condition simulations. On the top row residuals from the 4.6M grid, on the left of the bottom row the 5.7M residuals and on the right the 7.0M. The calculated Global Forces and Moments are given below.

4 CFD X Y N Scheme/ EFD Test 10-3 E%D 10-3 E%D 10-3 E%D EFD Data PPM RANS (4.6M grid) RANS (5.7M grid) RANS (7.0M grid) While in all three set-ups the error is below 5%, the 5.7M and the 7.0M provided results more close to each other and from the residuals is apparent that these simulations ran more smoothly, so we consider grid independence at least for this level of grid sizes. 2.3 Straight ahead condition The same grid configuration is used for the straight ahead condition. The grid is refined by cutting down the size of cells by 50% in the area close to the hull, and placing the majority of the cells close to the free surface and bilge keels. The use of the symmetry plane allows the modeling of the half of the domain. Due to the high level of error obtained for this condition (5.5%~ 7.0%) various different grids were used without any change, at least for the time being. In general, with STAR CCM+ the expected error for the straight ahead calm-water resistance is below 1.5%. The set-up of the physical conditions (Reynolds number, dynamic sinkage and trim, etc.) remained the same. The only observation is that this might be due to the wall boundary. Here, are presented the results from 4 grids with 2.5M 3.0M 3.5M - 4.4M. The slight change in the number of cells in each refinement is explained by the fact that most of the cells are already placed around the bilge keel and the free surface. We keep as final the 3.0M cells grid because most of the simulations were around this value. The smaller error in the 4.4M grid is rather expected because with a high level of refinement the resistance values slightly rises at least with the 5415M geometry. The computational domain for the symmetrical problem.

5 Residuals from the straight ahead condition simulations. On the top row and left residuals from the 2.5M grid and on the right from the 3.0M grid. On the bottom row and left the residuals from the 3.5M grid and on the right the 7.0M grid. The calculated Global Forces and Moments are given below. CFD X Y N Scheme/ EFD Test 10-3 E%D 10-3 E%D 10-3 E%D EFD Data PPM RANS (2.5M grid) RANS (3.0M grid) RANS (3.5M grid) RANS (4.4M grid) Due to the small number of cells, the isosurfaces in the volume plots were of very low resolution, thus volume plots are not submitted. To provide a level of the flow resolution around the hull with grid refinement, contour plots for two different grids are provided.

6 For the straight ahead condition the grids of 3.0M and 3.5M cells are compared and for the static drift 20 degree condition the grids used are the 5.7M and 7.0M cells. The following plots show some differences among the two grids for the static drift case, meaning that the 7M grid resolves better the flow around the hull from the 5.7M grid in comparison with the experimental results and the submitted CFD results, but for the straight ahead condition regardless the similar level of refinement the plots share very small differences. Plots from the 4.4M grid will be submitted in order to further explore the straight ahead condition. 3. Grid Topology Static drift 20 degree condition Station x= Station x= Station x=0.12

7 3.2.4 Station x= Station x=0.3

8 3.2.6 Station x= Station x= Station x=0.8

9 3.2.9 Station x= Station x= Local Flow Field Static drift 20 degree condition Station x=0.06 1) Axial velocity and cross-plane streamlines

10 2) Turbulent kinetic energy 3) Vorticity A. Axial component,ω x. B. Transverse component, ω y

11 C. Vertical component, ω z Station x=0.1 1) Axial velocity and cross-plane streamlines 2) Turbulent kinetic energy 3) Vorticity A. Axial component,ω x

12 B. Transverse component, ω y C. Vertical component, ω z

13 4.2.3 Station x=0.12 1) Axial velocity and cross-plane streamlines 2) Turbulent kinetic energy 3) Vorticity A. Axial component,ω x

14 B. Transverse component, ω y C. Vertical component, ω z Station x=0.2 1) Axial velocity and cross-plane streamlines

15 2) Turbulent kinetic energy 3) Vorticity A. Axial component,ω x B. Transverse component, ω y

16 C. Vertical component, ω z Station x=0.3 1) Axial velocity and cross-plane streamlines 2) Turbulent kinetic energy

17 3) Vorticity A. Axial component,ω x B. Transverse component, ω y C. Vertical component, ω z

18 4.2.6 Station x=0.4 1) Axial velocity and cross-plane streamlines 2) Turbulent kinetic energy 3) Vorticity A. Axial component,ω x

19 B. Transverse component, ω y C. Vertical component, ω z Station x=0.6 1) Axial velocity and cross-plane streamlines

20 2) Turbulent kinetic energy 3) Vorticity A. Axial component,ω x B. Transverse component, ω y

21 C. Vertical component, ω z Station x=0.8 1) Axial velocity and cross-plane streamlines 2) Turbulent kinetic energy

22 3) Vorticity A. Axial component,ω x B. Transverse component, ω y C. Vertical component, ω z

23 4.2.9 Station x= ) Axial velocity and cross-plane streamlines 2) Turbulent kinetic energy 3) Vorticity A. Axial component,ω x

24 B. Transverse component, ω y C. Vertical component, ω z Station x=1.0 1) Axial velocity and cross-plane streamlines

25 2) Turbulent kinetic energy 3) Vorticity A. Axial component,ω x B. Transverse component, ω y

26 C. Vertical component, ω z