OveRlapping-grids in ReFRESCO Issues, plans

IMPROVED MODELLING OF SHEET CAVITATION DYNAMICS ON DELFT TWIST11 HYDROFOIL

Guilherme Vaz, Thomas Lloyd, Arun Gnanasundaram

INTRODUCTION • • • • •

Cavitation modelling activities in the last 4 years using ReFRESCO at Marin, Chalmers, Southampton. 2D Foils: NACA0015. OW propeller: E779A, PPTC, CRS-Sharcs-I,-II. OW Current Turbines: Southampton. In-behind propeller: E779A. Ship+propeller: CRS-Sharcs-I, CRS-Sharcs-II.

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Focus:

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Numerics; Verification and Validation (V&V); Cavitation (inception) modelling; Turbulence & transition modelling; Pressure fluctuations; Acoustics;

INTRODUCTION • •

3D twisted hydrofoil, “Delft Twist11 Foil”, is a simplified geometry case with some complexities of a propeller (more difficult though :-)). Data from Foeth-2008. SMP-2011: a workshop organized by MARIN on the Delft Foil. • • • •

Cavitating and non-cavitating flow. Five participants using RANS, DES, and LES. Results varied significantly even when using the same turbulence/cavitation models. MARIN results: coarse grids; RANS: stable behavior; RANS+eddyVisc-corr: shedding (1calc!).

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Since then Li, Bensow, Asnaghi have also addressed this case.

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Objectives: • • • •

Readdress this case using finer and “better” grids. Analyze the differences between RANS, RANS+eddyVisc corr and SRS model. Study their numerical requirements, understand their background principles. (Start) Performing thorough V&V studies.

DELFT TWIST11 HYDROFOIL • • • •

Twisted hydrofoil with a NACA0009 profile, spanwise varying twist angle for 0deg (sides) to 11deg (midpsan). AoA=-2deg. Tested at Delft cavitation tunnel. U0=6.97 m/s; Tu=1%. Sig=1.07.

CFD CODE •

ReFRESCO is a community based open-usage/source CFD code for the Maritime World.

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Multiphase incompressible viscous flows using the Navier-Stokes equations. Free-surface, cavitation models and n-phases volume-fraction transport equations. Segregated and coupled time-implicit solvers (SIMPLE based). The implementation is face-based (all types of grids) and MPI parallelized. Moving, sliding, deforming and automatic grids (adaptation). Coupling with rigid (6DOF) and flexible (FSI) structural equations-of-motion. RANS/URANS, SRS turbulence approaches. Several couplings: acoustic models, propeller BEMs, XMF tools, and wave codes.

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ReFRESCO (v2.3.0) is developed, verified and its several applications validated at MARIN, IST, USP-TPN, TUDelft, Soton, UTwente, Chalmers, UMaine, TAMU, UPB, WUR, WavEC, IOWA, UDE.

www.refresco.org

MODELLING •

Homogenous (Euler-Euler) cavitation model: • •

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Reboud/eddyViscosity correction •

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Modified (numerics) Sauer model. no: 1e8 m-3 , Rmin: 1e-5 m.

Ad-hoc correction to reduce eddy viscosity in cavitation areas.

Turbulence models • •

KSKL (already slightly SAS/SRS) => less eddy viscosity. SST DDES (SRS model) => reduces eddy viscosity.

COMPUTATIONAL SETUP • • •

Domain + bcs: inflow, pressure outlet, symmetry, slip walls, no-slip Grids: structured with (nested) refined suction side. Settings: • • • • •

2nd order in space and time; Quick, Upwind, Upwind for convection schemes; Cavitation number in steps (pv modified; bc fixed); n=4 and dt=5E-5s (RANS); dt=1.5E-5s (DDES);

RESULTS: WETTED FLOW • • • • • •

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Optimal iterative convergence. Difference in CL between coarsest and finest grid of 0.6% (RANS) and 0.8% (DDES). Lower than 0.5% for CD and both approaches. Numerical uncertainty for CL of 1.0% and CD of 1.5% (only for RANS). RANS (results for G2): CL=0.425, CD=0.0146 DDES (results for G2): CL=0.428, CD=0.0143 Experimental uncertainty on lift (CL=0.47!) and drag too large (according to Foeth around 10%). Cp agreement with experiments looks good.

RESULTS: CAVITATING FLOW • •

More difficult iterative convergence! Limiting-streamlines, eddy-viscosity and vapour iso-surfaces (αv=0.1). RANS

RANS-Reboud

DDES

RESULTS: CAVITATING FLOW •

Axial velocity and off-plane vorticity RANS-Reboud

DDES

RESULTS: CAVITATING FLOW •

Effect of grid resolution

RESULTS: CAVITATING FLOW •

Effect of grid resolution RANS-Reboud G4

G2

DDES

RESULTS: CAVITATING FLOW •

Cavity dynamics RANS-Reboud

DDES

eddyVisc

eddyVisc

f(α)

1-lt/lRANS

RESULTS: CAVITATING FLOW •

Cavity Dynamics RANS-Reboud

DDES

RESULTS: CAVITATING FLOW •

Comparison with experiments

CONCLUSIONS • • • •

Low numerical uncertainties for the wetted-flow calculations. But large difference between the numerical results and the experimental lift coefficient (like in almost all studies in the literature). The numerical demands in terms of grids, time-step and non-linear iterations increase significantly in the case of cavitating-flow. For the RANS-Reboud correction case, these are even more significant. For the same numerical settings, DDES calculations delivered better iterative convergence at lower computational costs than the RANS-Reboud ones. The effect of grid refinement is different for the three methods used: • • •

1) for RANS, the finer the grid the higher the values of eddy-viscosity and the worse the numerical predictions terms of cavity dynamics; 2) for DDES, the finer the grid the more turbulent structures are solved, the lower the eddyviscosity levels, and the better the cavity dynamics; 3) for RANS-Reboud correction, refining the grid leads to non-monotonic convergence behaviour, which increases the numerical difficulties (robustness) of applying this correction.

CONCLUSIONS/FUTURE WORK • • • •

The cavity growth, shrinking and detachment was better captured using a RANSReboud correction approach than DDES. This is due to the shielding of the boundary layer from turbulence resolution intrinsic to this SRS model. Reboud-correction may be effective but it is undesirable in terms of iterative and grid convergence, and alters the physics in an unrealistic way… The loads on the wing, and associated cavitation extents, were underpredicted when compared with the available experiments (?). Future Work: • • • • •

Finish all grids and perform thorough V&V (including time-step). Complete wing and larger spanwise resolution. EARSM, XLES, IDDES, PANS calculations. Automatic grid adaptation. PhD student starting…

QUESTIONS?

www.refresco.org

TURBULENCE MODELLING

WETTED FLOW SIMULATIONS

CAVITATING FLOW SIMULATIONS RANS • No Shedding. • Vv decreases with grid refinement.