A Projection Method Based Fast Transient Solver for Incompressible Turbulent Flows

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Information about A Projection Method Based Fast Transient Solver for Incompressible...

Published on September 16, 2015

Author: DarrinStephens

Source: slideshare.net

1. logo.png Applied CCM Motivation PISO SLIM Results Summary A Projection Method Based Fast Transient Solver for Incompressible Turbulent Flows Chris Sideroff 08 June 2015 Applied CCM © 2012-2015 23rd CFD Society of Canada Conference

2. logo.png Applied CCM Motivation PISO SLIM Results Summary Applied CCM Specialize in the application, development and support of OpenFOAM® - based software Creators and maintainers of Caelus Locations: Canada, Australia, USA Applied CCM © 2012-2015 23rd CFD Society of Canada Conference

3. logo.png Applied CCM Motivation PISO SLIM Results Summary Motivation Why develop another transient solver? DES and LES attractive because RANS tends to be problem specific Low cost hardware + open-source software ⇒ DES and LES feasible Traditional transient, incompressible algorithms (PISO and SIMPLE) do not scale well for large HPC, GPU and Many Integrated Core (MIC) environments Let’s review PISO algorithm Applied CCM © 2012-2015 23rd CFD Society of Canada Conference

4. logo.png Applied CCM Motivation PISO SLIM Results Summary PISO Overview Pressure Implicit with Splitting of Operators (PISO)1 method: 1. Solve momentum equation (predictor step) 2. Calculate intermediate velocity, u∗ (pressure dissipation added) 3. Calculate momentum fluxes 4. Solve pressure equation: · ( 1 Ap p) = · u∗ 5. Correct momentum fluxes 6. Correct velocity (corrector step) Repeat steps 2 – 6 for PISO (1 – 6 for transient SIMPLE) 1Isaa, R.A. 1985, “Solution of the implicitly discretised fluid flow equations by operator splitting” J. Comp. Phys., 61, 40. Applied CCM © 2012-2015 23rd CFD Society of Canada Conference

5. logo.png Applied CCM Motivation PISO SLIM Results Summary Fractional Step Error Step 2 main issue with PISO Predicted velocity used only to update matrix coefficients: u∗ = 1 ap Σ anb unb − ( p − p) Pseudo-velocity, u∗, is used on the RHS of pressure equation Therefore requires at least two corrections to make velocity and pressure consistent Applied CCM © 2012-2015 23rd CFD Society of Canada Conference

6. logo.png Applied CCM Motivation PISO SLIM Results Summary Pressure Matrix Non-constant coefficients ( 1 ap ) in pressure matrix affects multi-grid solver performance Multi-grid agglomeration levels cached first time pressure matrix assembled Coefficients ( 1 ap ) only valid for the first time step Turning off caching of agglomeration too expensive Applied CCM © 2012-2015 23rd CFD Society of Canada Conference

7. logo.png Applied CCM Motivation PISO SLIM Results Summary SLIM Overview Semi Linear Implicit Method (SLIM), based on projection method1: decompose velocity into vortical and irrotational components. 1. Solve momentum equation (vortical velocity) 2. Calculate momentum fluxes (pressure dissipation added) 3. Solve pressure equation (irrotational velocity): ∆t 2(p) = · u 4. Correct momentum flux 5. Correct velocity (solenoidal) Use incremental pressure approach to recover correct boundary pressure 1Chorin, A.J. 1968, “Numerical Solution of the Navier-Stokes Equations”,Mathematics of Computation 22: 745-762 Applied CCM © 2012-2015 23rd CFD Society of Canada Conference

8. logo.png Applied CCM Motivation PISO SLIM Results Summary Fractional Step Error Velocity split into vortical and potential components - much smaller fractional step error Pressure and velocity maintain stronger coupling Continuity satisfied within one pressure solve because predicted velocity used directly in pressure equation Applied CCM © 2012-2015 23rd CFD Society of Canada Conference

9. logo.png Applied CCM Motivation PISO SLIM Results Summary Pressure Matrix Pressure matrix coefficients purely geometric Multi-grid agglomeration levels assembled during first step now consistent for all time steps Significantly improves parallel scalability for multi-grid solver Applied CCM © 2012-2015 23rd CFD Society of Canada Conference

10. logo.png Applied CCM Motivation PISO SLIM Results Summary 2D Periodic Hills Two dimensional, stream-wise, staggered hills of polynomial shape Reh = 10,595 Stream-wise and span-wise boundaries periodic. Hills and top boundaries no slip. Grid: ∼ 4.5 million hex cells; LES model: Smagorinsky Applied CCM © 2012-2015 23rd CFD Society of Canada Conference

11. logo.png Applied CCM Motivation PISO SLIM Results Summary Validation Experimental data of Rapp (2009) Mean and second moment components at 10 vertical rakes Applied CCM © 2012-2015 23rd CFD Society of Canada Conference

12. logo.png Applied CCM Motivation PISO SLIM Results Summary x/h = 2 Both compare favorably SLIM slightly closer than PISO Applied CCM © 2012-2015 23rd CFD Society of Canada Conference

13. logo.png Applied CCM Motivation PISO SLIM Results Summary x/h = 4 SLIM consistently closer than PISO at all locations Likely due to lower fractional step error Applied CCM © 2012-2015 23rd CFD Society of Canada Conference

14. logo.png Applied CCM Motivation PISO SLIM Results Summary Simulation Time SLIM on average about 30% faster on modest HPC system Fewer total iterations of pressure equation (SLIM: 10; PISO: 14) # cores PISO SLIM % diff. 1 2095 1550 26 5 988 711 28 10 419 302 28 20 330 231 30 40 219 147 33 60 216 138 36 Applied CCM © 2012-2015 23rd CFD Society of Canada Conference

15. logo.png Applied CCM Motivation PISO SLIM Results Summary Precursor Simulation Establish turbulent conditions to use as initial condition for wind park simulation Start from quiescent condition. Run until fully turbulent. Steam-wise and span-wise periodic Grid size: 50 million hex cells Results courtesy of Greg Oxley at Vestas using Firestorm super computer Applied CCM © 2012-2015 23rd CFD Society of Canada Conference

16. logo.png Applied CCM Motivation PISO SLIM Results Summary Mean Wind Profile SLIM slightly more accurate than PISO Fully turbulent condition reached sooner than PISO Applied CCM © 2012-2015 23rd CFD Society of Canada Conference

17. logo.png Applied CCM Motivation PISO SLIM Results Summary Scaling Consistent multi-grid agglomeration levels give SLIM significant advantage Applied CCM © 2012-2015 23rd CFD Society of Canada Conference

18. logo.png Applied CCM Motivation PISO SLIM Results Summary MPI Profiling Profiled MPI calls on 125 million cell mesh up to 4096 cores Applied CCM © 2012-2015 23rd CFD Society of Canada Conference

19. logo.png Applied CCM Motivation PISO SLIM Results Summary Future Work For static grids, pressure matrix construction may be pulled entirely from time loop to save assembly of pressure matrix every time step Advantageous for GPU and MIC computing. Compute pressure matrix once. Only need to transfer RHS vector For peta-scale core counts, solve momentum equations explicitly (Runga-Kutta). Combined with above, could perform close to fully explicit codes Solvers have been developed and are undergoing testing Applied CCM © 2012-2015 23rd CFD Society of Canada Conference

20. logo.png Applied CCM Motivation PISO SLIM Results Summary Summary SLIM significantly faster than PISO. Problem dependent but 30-100% is typical improvement and even more for very large HPC calculations. Exact velocity splitting improves both convergence and accuracy Geometric pressure matrix coefficients advantageous for parallel efficiency, particularly for multi-grid solvers Additional modifications enable scaling to very large number of cores (HPC, GPU, MIC) Applied CCM © 2012-2015 23rd CFD Society of Canada Conference

21. logo.png Applied CCM Motivation PISO SLIM Results Summary Strategic Perspective Select research and development projects that are unique and help transfer knowledge to industrial applications. Solvers: transient, compressible, multi-phase, combustion, acoustics Turbulence: RANS, DES and LES, VLES, wall models Sensitivity, design optimisation, and uncertainty propagation: adjoint, tangent Numerical acceleration and stabilisation Platforms and architectures: HPC, GPU, MIC Applied CCM © 2012-2015 23rd CFD Society of Canada Conference

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