DEVELOPMENT OF A CFD MODEL FOR

SIMULATION OF SELF-PROPULSION TESTS

Alexandre T. P. Alho

Laboratório de Sistemas de Propulsão

DENO/POLI, UFRJ

Motivation

▪ Growing demand for high efficiency propulsion systems.

▪ Demand for accurate power predictions in less time and at low costs.

Accurate CFD models: designers can rely on as an effective design tool.

CFD model must be developed based on a good compromise between

the quality of the numerical result and the computational effort.

▪ Numerical investigation of propeller-hull interaction.

Pitot-based experimental methods are virtually impossible.

Velocimetry methods still have challenges to overcome.

“Green” ship

Domain and grid configuration

Numerical models adopted

INTRODUCTION

Objective

▪ Develop a CFD model dedicated to estimate the propulsion factors

and to simulate the self-propulsion test of a hull.

Methodology

▪ The flow around a typical displacement hull, equipped with a

standard marine propeller, is simulated by means of commercial

CFD code (ANSYS CFX, release 14).

Focus

▪ Design applications.

INTRODUCTION

Displacement hull (Bulk carrier, CB = 0.86)

Twin screw propulsion system: 2 x B-Series propellers

Main Dimensions:

▪ Length (Loa): 73.4 m

▪ Length (Lpp): 70.6 m

▪ Breath (B): 14.8 m

▪ Design draught (T): 2.6 m

▪ Service Speed (VS): 9.5 knt

Shallow draught:

▪ Lower T/D ratio: 0.176 m

(0.35..0.48)

HULL PARTICULARS

Configuration

▪ Box-shaped domain in full scale with the real ship.

▪ Fluid domain width and depth are proportional to typical dimensions

observed in towing tanks.

▪ Transversal symmetry (port-starboard type symmetry) of the flow

was considered (L/B = 4.77) Half model.

▪ Domain length: 7 hull lengths (2.5 ahead and 3.5 astern of the hull).

No scale effects

Boundary conditions are

immediately defined.

COMPUTATIONAL FLUID DOMAIN

Requirements

▪ CFD model as an effective design tool grid configuration must

minimize error sources and its propagation with less computational

load.

▪ Classical strategy: anisotropic meshes with a fine grid in directions of

high gradients of flow properties (relatively coarse mesh in other

directions).

Mesh Generation Approach: Hull (I)

▪ Displacement hulls: viscous and wave-making resistance are the

major resistance components a fine grid in the near-wall region of

the hull and a good discretization of the free surface must be

simultaneously implemented. Usually results in high

storage and runtime

requirements!!!

GRID CONFIGURATION

Mesh Generation Approach: Hull (II)

▪ Hull lines: predominantly flat surfaces viscous drag estimated

based on flat plate approach.

Non-structured mesh with a refinement

approach focused on the discretization

of the free surface: 2.7k tetra elements.

GRID CONFIGURATION

Mesh Generation Approach: Propeller (III)

▪ Non-structured mesh with fine refinement near leading and trailing

edges: 3.7k elements (tetra and prism).

GRID CONFIGURATION

RANS Code

▪ ANSYS CFX 14.

Flow Regime

▪ Steady state design applications.

Free Surface

▪ Volume of Fluid model (VOF):

Propeller

▪ Frozen rotor model

Turbulence Model

▪ Two-equation SST model with the scalable wall function approach.

0

iFu

t

F

NUMERICAL MODEL

Simulation of an Open Water Test

▪ Propeller: Series B, 4 blade, 1.4 m dia.

▪ N = 450 rpm; VA = 3.150, 3.675 and 4.200 m/s.

PROPELLER MODEL VALIDATION

VA =4.2 m/s

Hull+Propeller Curve

▪ Discrepancies: 8..10%

PROPELLER MODEL VALIDATION

0,00

0,05

0,10

0,15

0,20

0,25

0,30

0,35

0,00 0,10 0,20 0,30 0,40 0,50 0,60 0,70 0,80

KT,

10

KQ

J

KT (Exp)

10 KQ (Exp)

KT (CFD)

10 KQ (CFD)

High tangential velocities demand fine

grid refinement compromise between

quality and computational effort.

Hull Performance

▪ Test speed (VS): 9.5 knt

▪ Total resistance (RT): 50.6 kN

▪ Wake coefficient (w): 0.153

TOWING TEST SIMULATION

Test Results

▪ Propeller revolutions (N): 433 rpm

▪ Propeller thrust (Treq): 65.3 kN

SELF-PROPULSION TEST SIMULATION

N = 420 rpm

Results Evaluation

▪ Comparison against statistical estimation.

▪ Wake fraction, thrust deduction fraction and relative-rotative efficiency

predictions based on Holtrop & Mennen (1984).

SELF-PROPULSION TEST SIMULATION

Statistical Numerical Dif.

Propeller Revolutions 456 433 -5.2% rpm

Propeller Thrust 70.4 65.3 -7.8% kN

Wake Fraction 0.181 0.153 -16.7% ---

Thrust Deduction Fraction 0.243 0.184 -32.3% ---

Relative-rotative Efficiency 1.028 1.024 -0.4% ---

The overall performance achieved suggests that the numerical

model was able to resolve the physics of the flow related to

hull&propeller interaction at full scale.

The comparison against statistical predictions showed that:

▪ The numerical model was able to provide reasonable

predictions for propeller thrust and revolutions under self-

propulsion conditions.

▪ The numerical model was able to provide more realistic

predictions of the effects of hull&propeller interaction.

Future investigations are needed concerning the validation of

the numerical model against experimental data.

CONCLUSIONS