keun woo shin, rasmus møller bering -...
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17.03.2014 © MAN Diesel & Turbo Unsteady cavitation simulation on Kappel propeller with a hull wake field < 1 >
Unsteady cavitation simulation on Kappel propeller with a hull wake field
Keun Woo Shin, Rasmus Møller Bering MAN Diesel & Turbo Frederikshavn, Denmark
17.03.2014 © MAN Diesel & Turbo Unsteady cavitation simulation on Kappel propeller with a hull wake field
Innovative tip-modified propeller
Tip smoothly curved towards the suction side of blade - Tip swept along the vortex shedding
Non-planar lifting surface - Lifting surface is curved by orthogonal lifting line - Nose-tail line is vertically inclined
< 2 >
Introduction- Kappel propeller
17.03.2014 © MAN Diesel & Turbo Unsteady cavitation simulation on Kappel propeller with a hull wake field
Reduction of tip vortex loss in the same principle as airplane winglet → Higher propulsive efficiency
Reduction of pressure pulse on hull structure ← Tip bending blocks radial propagation
< 3 >
Introduction- Kappel propeller
17.03.2014 © MAN Diesel & Turbo Unsteady cavitation simulation on Kappel propeller with a hull wake field
Kappel propeller in EU project ’Kapriccio’ is handled Andersen P., Friesch J., Lundegaard L., Patience G., Development of a marine propeller with nonplanar lifting surfaces, Marine Technology, Vol.42, No.3, 2005 - Self-propulsion tests and cavitation tests for conventional & Kappel propeller s on a container ship and a tanker - Sea trial and cavitation observation of full-scale conventional & Kappel propellers
CFD simulation of a fully wetted flow on Kappel propeller has been made with a hull wake field Shin K.W., Andersen P., Bering R., CFD simulation on Kappel propeller with a hull wake field, Numerical Towing Tank Symposium, 2013
< 4 >
Introduction- Kappel propeller
17.03.2014 © MAN Diesel & Turbo Unsteady cavitation simulation on Kappel propeller with a hull wake field < 5 >
Propeller flow simulation
Propeller design phase: - Single-blade model with periodic boundary - Steady-state computation with MRF - Uniform inflow for open-water condition - Radially-varying and circumferentially uniform inflow for behind-hull condition - RANS solver with k-ω SST turbulence model and Gamma ReTheta transition model - Trimmed mesh with prism layer
17.03.2014 © MAN Diesel & Turbo Unsteady cavitation simulation on Kappel propeller with a hull wake field < 6 >
Propeller flow simulation
Propeller design phase: - Blade is designed by lifting-surface method and vortex lattice method - Blade geometry is prepared by external CAD software - Automatic replacement of blade model by JAVA macro - Domain and mesh size are adjusted according to propeller diameter (e.g. 0.2-0.4%D for blade surface mesh, 0.5%D for volume mesh of tip vortex trace)
17.03.2014 © MAN Diesel & Turbo Unsteady cavitation simulation on Kappel propeller with a hull wake field < 7 >
Propeller flow simulation
Propeller design phase: - 1.5 - 2.0 million cells → Less than 1 hour by 32 nodes of 3.06 GHz processor and infiniband → Practical propeller optimization
- Comparison with open-water model test: Discrepancies < 3.0% in KT, KQ, ηO in design condition
0.2 0.3 0.4 0.5 0.6
0.1
0.2
0.3
0.4
0.5
0.6
J
K T, 10K
Q, ηo
ηo
KT
10KQ
ExpCFD
17.03.2014 © MAN Diesel & Turbo Unsteady cavitation simulation on Kappel propeller with a hull wake field < 8 >
Propeller flow simulation
Final design phase: - Propeller with a rudder - Unsteady computation with rigid body motion - Δt corresponds to 5° rotation per Δt, until the flow is developed (5 revolutions) - 1° rotation per Δt - 5 inner-iterations - 8 – 10 million cells → 20 – 30 hours for 10 revolutions with 32 nodes
17.03.2014 © MAN Diesel & Turbo Unsteady cavitation simulation on Kappel propeller with a hull wake field < 9 >
Propeller flow simulation
Final design phase: - Axial wake field is applied 3D ahead of propeller plane ← Wake measurement is scaled by weff/wnom, where w = 1-Va/Vs, wnom w/o propeller and weff with propeller
- Upward flow corresponding to averaged transverse wake is added ←Transverse wake is characterised by upward flow ← Slanted stern hull shape
- Wake simulation without a propeller is compared to wake measurement → Good agreement in high wake at inner radii and upper part and bilge vortex
Measurement No upward flow Including Upward flow
17.03.2014 © MAN Diesel & Turbo Unsteady cavitation simulation on Kappel propeller with a hull wake field < 10 >
Propeller flow simulation
Final design phase: - Single-blade thrust is periodic and highest at 12 o’clock position ← wake peak
- Comparison with self-propulsion test → Wake fraction is increased by 0.6% to reach the same thrust as in model test → Torque is 4.1% higher → ηB is 4.0% lower → ηO and ηR are 1.3% and 3.6% lower, respectively
0 90 180 270 360
0.03
0.04
0.05
Blade angle [deg]
KTB
on
a si
ngle
bla
de
17.03.2014 © MAN Diesel & Turbo Unsteady cavitation simulation on Kappel propeller with a hull wake field < 11 >
Cavitation simulation
BEM (Boundary Element Method) - Used for cavitation estimation in blade design phase - All three wake components are applied - Pressure distribution on blade surface is calculated every 5° blade position - Low computational effort ← about15 min - Coarse mesh → insufficient resolution for blade edge curvature
Pressure from BEM Blade mesh in BEM Blade mesh in CFD
17.03.2014 © MAN Diesel & Turbo Unsteady cavitation simulation on Kappel propeller with a hull wake field < 12 >
Cavitation simulation
BEM (Boundary Element Method) - Comparison of chordwise pressure distribution in BEM and CFD - Pressure distribution deviates at leading and trailing edges - Deviation at inner radii ← No hub in BEM
0 0.2 0.4 0.6 0.8 1
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
x/C
-Cp
0.4R
0 0.2 0.4 0.6 0.8 1-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
x/C
-Cp
0.6R
0 0.2 0.4 0.6 0.8 1-3
-2
-1
0
1
2
3
4
5
6
x/C
-Cp
0.8R
BEM (Pressure)BEM (Suction)CFD (Prsessure)CFD (Suction)
17.03.2014 © MAN Diesel & Turbo Unsteady cavitation simulation on Kappel propeller with a hull wake field < 13 >
Cavitation simulation
CFD - RANS solver with k-ω SST turbulence model and Gamma ReTheta transition model - Hull wake field - Cavitation model (←Rayleigh-Plesset equation) and VOF model - Seed density = 1.5∙1012m-3 and diameter=5∙10-6m → Cavitation growth rate - Hydrostatic pressure → initial & outlet pressure - Ambient pressure (reference pressure) ← cavitation number - Fixed vapor pressure
Cavitation tunnel test - Ship model - High propeller revolutions of 30 Hz - Tunnel flow speed and pressure are adjusted for a given set of thrust and cavitation number
17.03.2014 © MAN Diesel & Turbo Unsteady cavitation simulation on Kappel propeller with a hull wake field < 14 >
Cavitation simulation
CFD - Iso surface of 10% vapor - Unsteady sheet cavitation in upper part of propeller disk - No vortex cavitation in CFD ← insufficient volume mesh refinement along vortex trace - Vorticity magnitude indicates tip vortex
17.03.2014 © MAN Diesel & Turbo Unsteady cavitation simulation on Kappel propeller with a hull wake field < 15 >
Cavitation simulation
- CFD overestimation at tip ← difference in thrust - CFD underestimation at 0.7R-0.8R ← difference in hull wake - BEM underestimation ← no cavitation convection
Blade angle
= 160°
Blade angle = 180°
Experiment CFD BEM
17.03.2014 © MAN Diesel & Turbo Unsteady cavitation simulation on Kappel propeller with a hull wake field < 16 >
Cavitation simulation
- CFD overestimation ← difference in thrust - Leading-edge cavitation at 0.7R-0.9R in BEM ← insufficient resolution of leading edge curvature
Blade angle = 200°
Blade angle = 220°
Experiment CFD BEM
17.03.2014 © MAN Diesel & Turbo Unsteady cavitation simulation on Kappel propeller with a hull wake field < 17 >
Cavitation simulation
- BEM has differences in cavitation inception and termination ← insufficient resolution of leading edge curvature - CFD has earlier inception and later termination ← difference in thrust
Experiment Blade angle=140°
CFD 130°
BEM 90°
Inception
Termination
240° 260° 300°
17.03.2014 © MAN Diesel & Turbo Unsteady cavitation simulation on Kappel propeller with a hull wake field < 18 >
Cavitation simulation
Conclusion - CFD simulation has acceptable accuracy for estimation of unsteady propeller cavitation behind hull - It needs more validation cases to ensure robustness and consistency for quantative estimation of cavitation - It is necessary to economize computational model and run it with more than 100 nodes for practical cavitation simulation on propeller design phase
Future work - Propeller optimization based on CFD with respect to propulsive efficiency and cavitation safety - CFD estimation of propeller noise & vibration by aeroacoustics model - Tip vortex cavitation simulation by Lagrangian multiphase model
17.03.2014 © MAN Diesel & Turbo Unsteady cavitation simulation on Kappel propeller with a hull wake field < 19 >
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