Norsk Marinteknisk Forskningsinstitutt

Numerical Modeling of Ship-Propeller Interaction under Self-Propulsion Condition

Vladimir KrasilnikovDepartment of Ship Technology, MARINTEK

Trondheim, Norway

STAR Global Conference 2014

Vienna, Austria, March 17-19

Content of the presentation

1) Examples of research problems involving ship-propeller interaction

2) Approaches to numerical modeling of ship-propeller interaction

3) Validation example of the benchmark KCS container ship

4) Aspects of numerical modeling that require closer attention

1) Examples of research problems involving

ship-propeller interaction

Nominal wake 1-WTN = 0.736

Effective wake: 1-WTE = 0.769

Design of wake adapted propeller

In order to achieve desired high propulsive efficiency and ensure favorable cavitation and acoustic characteristics of propeller, one has to design the propeller well adapted to the wake field behind ship hull.

Interaction between ship hull and propeller results in effective wake field on propeller that may differ considerably from nominal wake, which is normally measured during model tests.

At MARINTEK we employ a coupled viscous/potential method to extract the effective wake field and optimize propeller design, using our in-house propeller design and analysis software. In this coupled method, STAR-CCM+ performs as a viscous flow solver.

Analysis of propeller characteristics under extreme off-design conditions

These studies are relevant to the problems of low-speed maneuvering of ships, backing and crash-back situations.

Off-design propeller analysis involves extremely complex flows, where the blade back side performs as a pressure side, and the whole blade is stalled.

Extended domains of separated and re-circulated flows exist, giving rise to unsteady vortex shedding.

The example shown on this slide presents the comparsion between the experimental data and numerical predictions obtained with STAR-CCM+(unsteady RANS method) for the B-series propeller operating in the entire 1st quadrant.

Investigations into scale effect on ducted propellers

Interaction between propeller and duct is a crucial mechanism behind scale effect.

The regions of blade tip clearance and duct T.E. are of particular importance.

Ducted propeller flow is most adequately solved in the unsteady formulation, by employing the Sliding Mesh method.

Scale effect depends significantly on the duct type, propeller geometry, and radial loading distribution towards blade tip, which complicates greatly the application of simplified engineering scaling methods.

Within the frameworks of the ongoing R&D project “PROPSCALE” we use STAR-CCM+to quantify scale effect on ducted propellers of different types.

Studies on formation and development of blade vortices

The physical mechanisms associated with the formation and development of blade tip and leading edge vortices are still not investigated to a sufficient degree.

In particular, unsteady phenomena, such as vortex bursting and breaking-up, represent substantial interested from the point of view of propeller noise, erosion and induced pressure impulses.

In this example, we used an unsteady RANS method of STAR-CCM+ to study the behavior of the leading edge vortex that caused erosion on the blades of a pulling podded propeller operating at bollard condition.

2) Approaches to numerical modeling of

ship-propeller interaction

Unsteady (time-dependent) nature of the problem due to the interaction between

the rotating parts (propeller) and stationary parts (hull, appendages, rudder).

Presence of free surface of unknown geometry.

Flow turbulence of various scales that need appropriate modeling assumptions.

Scale effects, including those related to the presence of laminar and transient flow

regimes in model scale.

Challenges associated with numerical modelling of ship-propeller interaction

Approaches:

1) Iterative coupled viscous/potential method with Actuator DiskHull – RANS, Propeller – Panel method or Lifting surface, Coupling – Actuator Disk (Circumferential-averaged volumetric momentum source model), Free surface – VOF.

2) Unsteady RANS method with simplified account for free surface effectHull – RANS, Propeller – RANS (Sliding Mesh), Free surface – not included (symmetry plane – «double-body model»).

3) Fully unsteady RANS method with free surfaceHull – RANS, Propeller – RANS (Sliding Mesh), Free surface – VOF.

Software:

RANS solver: STAR-CCM+ (CD-Adapco)Panel method solver: AKPA (MARINTEK, in-house propeller analysis program)Lifting surface solver: AKPD (MARINTEK, in-house propeller design program)Actuator Disk setup: ADM (MARINTEK, in-house)

Approaches and software employed in ship-propeller interaction simulations

Coupled method: Main principles

Actuator disk model

Disk thickness: 0.01D. Constant loading along the disk axis.

Radial distribution of elemental thrust identical to realistic propeller calculated by PM – dT(r).

Equivalent, but not identical distributions of circulation and elemental torque - (r)and dQ(r).

Circumferential averaged distribution of momentum sources (axial and tangential).

Effective wake field

All-component wake field . AD induced velocities are defined from an

additional Open Water calculation with the AD, having the same dT(r) as the AD behind hull.

Velocities are sampled at the wake control section 0.1D upstream of propeller plane.

Unsteady RANS method: Main principles

Hull-propeller interaction

Rotation motion of propeller region

Sliding interface mesh Time-accurate 1st order,

t2 of propeller revolution Two-stage solution MRF+SM

Turbulence *)

SST k-, All Y+ Treatment (used routinely)

Other turbulence models (RSM, DES – investigated)

Free surface *)

VOF, 2nd Order, FlatVofWaves Blended HRIC; Pure HRIC Time-accurate 1st order,

t=(0.005÷0.01)×LPP/V

DOF *)

Fixed position 2DOF (free sinkage and trim)

*) Also apply with the Coupled Method

Meshing considerations: Ship hull – (1)

Coarse mesh: Hex trimmed mesh, (5÷7) prism layers, stretching factor (1.25÷1.35), 1.5 mio cells per half ship without appendages, 30<Y+<90.

Meshing considerations: Ship hull – (2)

About 30 cells in vertical direction, 3 volumetric controls.

About 100 cells per wave length near ship hull, about 30 cells per wave length in the far field.

Wave damping at the Inlet, Outlet and Side boundaries; Damping length is chosen so that damping begins in the refinement zone.

The size of the domain in transverse direction is large enough to avoid intersection of the Kelvin’s wake with the side boundaries.

Free surface treatment

Meshing considerations: Propeller

Fine mesh: Poly mesh, (10÷30) prism layers, stretching factor (1.1÷1.2), (1.3÷1.5) mio cells per blade passage, Y+<1.

Coarse mesh: Tet mesh, no BL mesh, (0.3÷0.5) mio cells per blade passage, 30<Y+<250.

3) Validation example of the benchmark

KCS container ship

Main particulars Model scaleLength between PP LPP, [m] 7.2786Maximum beam at WL

BWL, [m] 1.019

Depth D, [m] 0.6013Draft T, [m] 0.3418Wetted surface area SW, [m2] 9.4379Block coefficient CB 0.6505Midship section coefficent

CM 0.9849

Coordinates of propeller center *)

(x/LPP, y/LPP, z/LPP)

(0.4825, 0.0, -0.02913)

*) Origin of coordinate system at CP, midship, WL; x -downstream

Propeller elements Model scalePropeller diameter DP, [m] 0.25Hub ratio dH/DP 0.18Number of blades Z 5Blade area ratio AE/A0 0.8Pitch ratio P(0.7R)/D 0.9967Sections NACA66/a=0.8

ConditionsCalm water, Fixed position and Free motionWithout rudderFroude number Fr=V/(g*LPP)1/2 0.26

Reynolds number Re=(V*LPP)/ν 1.4*107

Ship speed V, [m/s] 2.19663Propeller RPS *) n, [Hz] 9.5*) Measured during self-propulsion tests

KRISO container ship KCS

KRISO Propeller KP505

Main particulars of ship and propeller

Time step, interface scheme Cp Cv Ct

dt=0.01 [s], pure HRIC 0.000633 0.002840 0.003473dt=0.02 [s], pure HRIC 0.000630 0.002842 0.003472

dt=0.03 [s], pure HRIC 0.000628 0.002842 0.003470dt=0.04 [s], pure HRIC 0.000630 0.002842 0.003472

dt=0.05 [s], pure HRIC 0.000630 0.002842 0.003472

Time step, interface scheme Cp Cv Ct

dt=0.02 [s], blended HRIC 0.000711 0.002843 0.003554dt=0.03 [s], blended HRIC 0.000726 0.002846 0.003572

dt=0.05 [s], blended HRIC 0.000756 0.002849 0.003605

with Rudder (KRISO) without Rudder (SRI) friction line

Ct Cp Cf Ct Cp -residual Cf Cf0 (ITTC-57)

0.003557 0.003534 0.000689 0.002845 0.002832

Resistance calculation: Ship Resistance – Influence of interface scheme

Calculations with blended HRIC scheme

Calculations with pure HRIC scheme

Experiment

Solution appears dependent on time step due to the Courant number limits in the blended HRIC scheme

Solution is independent on time step

*) SST k- turbulence model is used in this exercise

Resistance calculation: Wave profiles

Resistance calculation: Pressure distribution on the hull

Resistance calculation: Ship Resistance – Influence of turbulence model

Experiment

with Rudder (KRISO) without Rudder (SRI) friction line

Ct Cp Cf Ct Cp -residual Cf Cf0 (ITTC-57)

0.003557 0.003534 0.000689 0.002845 0.002832

Calculations with blended HRIC scheme, Time step dt=0.02 [s]

Turbulence model Cp Cv Ct

SST k-w 0.000711 0.002843 0.003554

Real k-e 0.000717 0.002853 0.003570RSM *) 0.000697 0.002972 0.003669

SST k-w + RSM *) 0.000703 0.002971 0.003674

*) RSM Model: Linear Pressure Strain, High-Re

Turbulence model Cp Cv CtSST k-w 0.000630 0.002842 0.003472

Real k-e 0.000636 0.002852 0.003488RSM *) 0.000618 0.002970 0.003588

Calculations with pure HRIC scheme, Time step dt=0.02 [s]

Resistance calculation: Nominal wake field

Influence of turbulence model. Symmetry of calculated wake field: Half

ship and full ship. Influence of the inclusion of a new region:

Propeller block and Actuator Disk block.

Objectives of the study

Resistance calculation: Free sinkage and trim, different Froude numbers

Oscillatory convergence is observed for all conditions.

At lower Fr, oscillations show larger amplitude, and levels of residuals are higher.

The presented results are obtained with blended HRIC, dt=0.02 [s]. Calculation done with pure HRIC reveal large oscillations and become unstable at lower Fr.

Observations

Calculation of open water propeller characteristics

Self-propulsion calculation: Ship resistance and propeller characteristics

Ct, S-P RPS KTB KQB

Coupled method 0.003991 9.55 0.1703 0.02942

, % +0.63 +0.53 +0.18 +2.15

Unsteady RANS method 0.003907 9.53 0.1650 0.02933

, % -1.48 +0.32 -2.94 +1.84

Experiment 0.003966 9.50 0.170 0.0288

Coupled methodBlended HRIC, dt=0.05 [s]; SST k-; 5 iterations between the RANS and panel method solvers.

Unsteady RANS method:MRF+SM; Pure HRIC and Blended HRIC *), dt=0.02 [s] at MRF stage, dt 2 at SM stage; SST k-; About 50 propeller revolutions are performed at the SM stage.

Calculation results at «ship point», SFC=30.3 [N] from model tests

*) Both the calculation with pure HRIC scheme and blended HRIC scheme result in very close predictions of resistance and propeller forces, since the pure HRIC scheme is used effectively at the SM stage due to small time step.

Self-propulsion calculation: Pressure distribution on the hull

*) Results obtained with Unsteady RANS method

Self-propulsion calculation: Velocity field downstream of propeller

4) Aspects of numerical modeling that

require closer attention

Pure HRIC Offers solution independent on time step, which is advantageous in self-propulsion simulations using realistic propeller.

Blended HRIC Shows more stable performance in simulations involving free motion.

What is the best practice for self-propulsion simulations with free motion?

VOF: Interface capturing scheme

Wetted transom flow and vortex separation

A small wetted area is predicted at the transom, in the vicinity of CP.

Wave elevation is over-predicted at the stern end, but agrees well with the measurements aft of the transom.

The flow in this region is influenced by vortex separation that occurs at the transom, both below and above the free surface.

Resolution of vortices in propulsor slipstream

With standard two-equation turbulence models the computed slipstream vortices are excessively diffusive, and they dissipate too soon downstream of propulsor.

Before making the final shift toward the use of LES and DES methods, one should explore the possibilities of improvement offered by: Anisotropic turbulence models (RSM); Vorticity confinement method; Curvature correction model.

Norsk Marinteknisk Forskningsinstitutt

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