Taming OpenFOAM for Ship Hydrodynamics Applications

Transcription

1 Taming OpenFOAM for Ship Hydrodynamics Applications Sung-Eun Kim, Ph. D. Computational Hydromechanics Division (Code 5700) Naval Surface Warfare Center Carderock Division

2 Background Target Applications Issues Examples Underwater bodies Surface ships Propulsors Concluding remarks Outline 2

3 Background Complex geometry and complex physics (high Re TBL, vortices, multiphysics) Increasing emphasis/pressure on delivering engineering solutions to real-world problems in a timely manner. High-fidelity of CFD solutions commensurate with computational cost We are using several commercial CFD packages and inhouse codes. Can an open source CFD software be tamed as a production code for industrial applications? 3

4 Target Applications Resistance and propulsion of underwater vehicles and surface ships Cavitation on hydrofoils and propulsors Fluid structure interaction Maneuvering Seakeeping 4

5 Numerical Issues Spatial discretization Gradients Interpolation schemes Advection schemes for interface capturing (volume fraction transport) Solution algorithms Implicit iterative time-advancement Non-iterative fractional-step method for high-level simulation of turbulence (e.g., LES) Moving body problems - meshing strategy (single-grid, overset grids, deforming) n 1

6 Spatial Accuracy on Unstructured Grids Drag predictions on an ellipsoidal body using structured and unstructured meshes Very low profile (form) drag Hybrid mesh with 500K cells

7 Evolution of a Leading Commercial CFD Code CB Grad NB Grad NB Grad + HORC Grad B + HORC+MUSCL

8 Heat Transfer in a Duct - Tet Mesh Tet vs. Hex cell-based node-based node-based + HORC

9 Heat Transfer in a Duct - Prism + Tet cell-based node-based node-based + HORC

10 Physical Modeling Issues Turbulence modeling Wall boundary conditions for turbulent quantities (EVMs and RSTMs) Source term linearization High-order RANS models (EARSM, DRSM) Dynamic SGS models for LES Cavitation modeling Bubble dynamics modeling Mass transfer models

11 ONR Body-1 Results Body 1 Grid Characteristics (Half Body) No. of Points: 841,438 No. of Tets: 699,851 No. of Prisms: 1,380,128 y + 1.0

12 ONR Body-1 Results Longitudinal Distribution of Pressure (C p ) and Skin Friction (C f ) Coefficient

13 ONR Body-1 Results Boundary Layer Profiles

14 Series 66 Drift Angle Study (ONR) The negative experimental drift angles are believed to be less accurate as the strut was mounted on the side and the body was in the wake of the strut OpenFOAM with the SST turbulence model providing more accurate predictions than our traditional unstructured solver (Tenasi) on the same grids

15 SUBOFF Body Bare hull and fully appended cases Computations are underway with various meshing and turbulence modeling strategies 6M cell Hexpress mesh near-wall resolving mesh (y+ ~ 1)

16 SUBOFF Body Hexpress Grid - Stern Appendages

17 SUBOFF Body - RANS Solutions (Re L = 1.2 x 10 7 ) K-ω EARSM SST k-ω U contour at x/l =0.978

18 KVLCC - Double-Body Tanker Measured Axial velocity contours at propeller plane

19 KVLCC - Nominal Wake Prediction (Kim, 2001) Predicted axial velocity contours at the propeller plane

20 KVLCC2 Double-Body Tanker Model (Kim et al., 2010, Gothenburg Workshop) Hybrid unstructured mesh SnappyHexMesh Contour of axial velocity at the propeller plane 20

21 Background Target Applications Issues Examples Underwater bodies Surface ships Propulsors Concluding remarks Outline 21

22 Issues with Surface Ships Advection scheme for volume fraction is critical for solution accuracy and stability A suite of advection schemes (CICSAM, HRIC, MHRIC, intergamma, InterGamma-M) for volume-fraction equation have been implemented and validated. Large time-step size for steady or quasi-steady applications Jun-11

23 Zalesak s Rotating Disk Coarse mesh: 400 x Contours of volume fraction after one revolution Jun-11

24 DTMB 5415 One of the test problems for the 2010 Gothenburg CFD Workshop on Ship Hydrodynamics (Kim et al, 2010) Re L = 1.2 x 10 7, Fr = 0.28 Computations done for fixed and free sinkage and trim Two-phase RANS computations on systematically refined hexahedral grids using combinations of Advection schemes Turbulence models 24

25 DTMB 5415 Fixed Sinkage and Trim Mesh Dependency 0.01 meas. 13 Million 6 Million 3 Million y/l=0.082 z/l PP x/l PP 0.01 meas. 13 Million 6 Million 3 Million Y=0.172 z/l PP x/l PP

26 DTMB 5415 Fixed Sinkage and Trim (UCR1) Impacts of Turbulence Models y/l=0.082 z/l PP meas. SST RKE HRW x/l PP meas. SST RKE HRW Y=0.172 z/l PP x/l

27 DTMB 5415 Impacts of Convection Scheme x/l = z/l PP x/l PP x/l = EFD (Longo et al. 2007) 6million-cell-SST-HRIC 6million-cell-SST-MHRIC z/l PP x/l PP EFD (Longo et al. 2007) 6million-cell-SST-HRIC 6million-cell-SST-MHRIC

28 Parallel Scalability DTMB 5415 Speed-Up NavyFOAM Computational Performance Parallel Scalability on Harold at ARL Using Pure MPI for 13 Million Cells SGI Altix ICE ,752 GHz Intel Nehalem (8 CPUs on a node) 32 TB Memory 4X DDR Infiniband Ideal NavyFOAM All the results shown were run fully-dense; namely, one prcocess per CPU (e.g., 8 processes on a node) Processors 28

29 DTMB 5415 Axial velocity contour at x/l = 0.935

30 Background Target Applications Issues Examples Underwater bodies Surface ships Propulsors Concluding remarks Outline 30

31 Continuum Approach Locally homogeneous mixture formulation (Kim and Brewton, 2008; Kim, 2009) Phase compositions are represented by volume-fraction. Incompressible gas (vapor) & liquid phases Implicit time-advancement scheme Pressure-based projection method Mass transfer models Merkle Kunz Schnerr & Sauer Validations Modified NACA-66 foil Clark-Y hydrofoil Unsteady sheet/cloud cavity on a NACA-0015 hydrofoil Propeller (P4381, P4383, P4990) Waterjets (AxWJ1, AxWJ2) 31

32 Effects of Cavitation Number (α = 8, σ = 1.0) α = 8, σ = 1.0 LES result on a 3.3M cell mesh Schnerr and Sauer s mass transfer model 32

33 NACA-0015 Hydrofoil Lift and Drag CL C D (exp.) C L (exp.) RANS - C D RANS - C L DES - C D DES - C L LES - C D LES - C L CD fc/u Measured (Obernach) Measured (SAFL - 7 ppm) Measured (SAFL - 13 ppm) Predicted (DES) Predicted (LES) σ(cavitation number) Mean lift & drag coefficients σ/2α Shedding frequency 33

34 P4381- Thrust Breakdown at J = Thrust, Κ Τ Torque, Κ Q σ= 0.6 σ= 1.0 σ=

35 ONR AxWJ-2 Thrust Breakdown Unsteady RANS Computation N = 2000 RPM, Q* = 0.76, σ = movie from 36 in tunnel

36 ONR AxWJ-2 Cavitation Computation, σ =

37 ONR AxWJ-2 Thrust Breakdown Prediction Predicted using Wilcox k-w model on a 2.2M cell (very coarse) mesh for 360 domain

38 Concluding Remarks We have been evaluating OpenFOAM for years, and benchmarking it against other CFD codes. A number of projects have been successfully carried out using OpenFOAM at NSWCCD for naval applications. Language (C++) barrier and object-oriented programming (OOP) make the learning curve stiff. Not all implementations in OpenFOAM are verified and validated. Thank you! 38