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

Transcription

1 STAR Global Conference 2014 Vienna, Austria, March Numerical Modeling of Ship-Propeller Interaction under Self-Propulsion Condition Vladimir Krasilnikov Department of Ship Technology, MARINTEK Trondheim, Norway Norsk Marinteknisk Forskningsinstitutt

2 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

3 1) Examples of research problems involving ship-propeller interaction

4 Design of wake adapted propeller Nominal wake 1-W T N = 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. Effective wake: 1-W T E = 0.769

5 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 recirculated 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.

6 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.

7 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 breakingup, 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.

8 2) Approaches to numerical modeling of ship-propeller interaction

9 Challenges associated with numerical modelling 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.

10 Approaches and software employed in ship-propeller interaction simulations Approaches: 1) Iterative coupled viscous/potential method with Actuator Disk Hull 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 effect Hull RANS, Propeller RANS (Sliding Mesh), Free surface not included (symmetry plane «double-body model»). 3) Fully unsteady RANS method with free surface Hull RANS, Propeller RANS (Sliding Mesh), Free surface VOF. Software: RANS solver: Panel method solver: Lifting surface solver: Actuator Disk setup: STAR-CCM+ (CD-Adapco) AKPA (MARINTEK, in-house propeller analysis program) AKPD (MARINTEK, in-house propeller design program) ADM (MARINTEK, in-house)

11 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.

12 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 Free surface *) VOF, 2nd Order, FlatVofWaves Blended HRIC; Pure HRIC Time-accurate 1st order, t=( ) L PP /V DOF *) Turbulence *) Fixed position 2DOF (free sinkage and trim) SST k-, All Y+ Treatment (used routinely) Other turbulence models (RSM, DES investigated) *) Also apply with the Coupled Method

13 Meshing considerations: Ship hull (1) Coarse mesh: Hex trimmed mesh, (5 7) prism layers, stretching factor ( ), 1.5 mio cells per half ship without appendages, 30<Y + <90.

14 Meshing considerations: Ship hull (2) Free surface treatment 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.

15 Meshing considerations: Propeller Fine mesh: Poly mesh, (10 30) prism layers, stretching factor ( ), ( ) mio cells per blade passage, Y + <1. Coarse mesh: Tet mesh, no BL mesh, ( ) mio cells per blade passage, 30<Y + <250.

16 3) Validation example of the benchmark KCS container ship

17 Main particulars of ship and propeller KRISO container ship KCS KRISO Propeller KP505 Main particulars Model scale Length between PP L PP, [m] Maximum beam at B WL, [m] WL Depth D, [m] Draft T, [m] Wetted surface area S W, [m 2 ] Block coefficient C B Midship section C M coefficent Coordinates of propeller center *) (x/l PP, y/l PP, z/l PP ) (0.4825, 0.0, ) *) Origin of coordinate system at CP, midship, WL; x - downstream Propeller elements Model scale Propeller diameter D P, [m] 0.25 Hub ratio d H /D P 0.18 Number of blades Z 5 Blade area ratio A E /A Pitch ratio P(0.7R)/D Sections NACA66/a=0.8 Conditions Calm water, Fixed position and Free motion Without rudder Froude number Fr=V/(g*L PP ) 1/ Reynolds number Re=(V*L PP )/ν 1.4*10 7 Ship speed V, [m/s] Propeller RPS *) n, [Hz] 9.5 *) Measured during self-propulsion tests

18 Resistance calculation: Ship Resistance Influence of interface scheme Experiment with Rudder (KRISO) without Rudder (SRI) friction line Ct Cp Cf Ct Cp -residual Cf Cf0 (ITTC-57) Calculations with blended HRIC scheme Time step, interface scheme Cp Cv Ct dt=0.02 [s], blended HRIC dt=0.03 [s], blended HRIC dt=0.05 [s], blended HRIC Solution appears dependent on time step due to the Courant number limits in the blended HRIC scheme Calculations with pure HRIC scheme Time step, interface scheme Cp Cv Ct dt=0.01 [s], pure HRIC dt=0.02 [s], pure HRIC dt=0.03 [s], pure HRIC dt=0.04 [s], pure HRIC dt=0.05 [s], pure HRIC Solution is independent on time step *) SST k- turbulence model is used in this exercise

19 Resistance calculation: Wave profiles

20 Resistance calculation: Pressure distribution on the hull

21 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) Calculations with blended HRIC scheme, Time step dt=0.02 [s] Turbulence model Cp Cv Ct SST k-w Real k-e RSM *) SST k-w + RSM *) Calculations with pure HRIC scheme, Time step dt=0.02 [s] Turbulence model Cp Cv Ct SST k-w Real k-e RSM *) *) RSM Model: Linear Pressure Strain, High-Re

22 Resistance calculation: Nominal wake field Objectives of the study 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.

23 Resistance calculation: Free sinkage and trim, different Froude numbers Observations 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.

24 Calculation of open water propeller characteristics

25 Self-propulsion calculation: Ship resistance and propeller characteristics Calculation results at «ship point», SFC=30.3 [N] from model tests Ct, S-P RPS K TB K QB Coupled method , % Unsteady RANS method , % Experiment Coupled method Blended 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. *) 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.

26 Self-propulsion calculation: Pressure distribution on the hull *) Results obtained with Unsteady RANS method

27 Self-propulsion calculation: Velocity field downstream of propeller

28 4) Aspects of numerical modeling that require closer attention

29 VOF: Interface capturing scheme 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?

30 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.

31 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.

32 Thank you! Technology for a better society Norsk Marinteknisk Forskningsinstitutt