DEVELOPMENT OF A CFD MODEL FOR SIMULATION OF SELF-PROPULSION TESTS

Transcription

1 DEVELOPMENT OF A CFD MODEL FOR SIMULATION OF SELF-PROPULSION TESTS Alexandre T. P. Alho Laboratório de Sistemas de Propulsão DENO/POLI, UFRJ

2 INTRODUCTION 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

3 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.

4 HULL PARTICULARS Displacement hull (Bulk carrier, C B = 0.86) Twin screw propulsion system: 2 x B-Series propellers Main Dimensions: Length (Loa): Length (Lpp): Breath (B): Design draught (T): Service Speed (V S ): Shallow draught: Lower T/D ratio: ( ) 73.4 m 70.6 m 14.8 m 2.6 m 9.5 knt m

5 COMPUTATIONAL FLUID DOMAIN Configuration Box-shaped domain in full scale with the real ship. No scale effects 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). Boundary conditions are immediately defined.

6 GRID CONFIGURATION 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!!!

7 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.

8 GRID CONFIGURATION Mesh Generation Approach: Propeller (III) Non-structured mesh with fine refinement near leading and trailing edges: 3.7k elements (tetra and prism).

9 NUMERICAL MODEL RANS Code ANSYS CFX 14. Flow Regime Steady state design applications. Free Surface Volume of Fluid model (VOF): Propeller Frozen rotor model Turbulence Model F Fu i 0 t Two-equation SST model with the scalable wall function approach.

10 PROPELLER MODEL VALIDATION Simulation of an Open Water Test Propeller: Series B, 4 blade, 1.4 m dia. N = 450 rpm; V A = 3.150, and m/s. V A =4.2 m/s

11 KT, 10 KQ PROPELLER MODEL VALIDATION Hull+Propeller Curve Discrepancies: 8..10% 0,35 0,30 0,25 KT (Exp) 10 KQ (Exp) KT (CFD) 10 KQ (CFD) 0,20 0,15 0,10 0,05 High tangential velocities demand fine grid refinement compromise between quality and computational effort. 0,00 0,00 0,10 0,20 0,30 0,40 0,50 0,60 0,70 0,80 J

12 TOWING TEST SIMULATION Hull Performance Test speed (V S ): 9.5 knt Total resistance (R T ): 50.6 kn Wake coefficient (w): 0.153

13 SELF-PROPULSION TEST SIMULATION Test Results Propeller revolutions (N): Propeller thrust (T req ): 433 rpm 65.3 kn N = 420 rpm

14 SELF-PROPULSION TEST SIMULATION Results Evaluation Comparison against statistical estimation. Wake fraction, thrust deduction fraction and relative-rotative efficiency predictions based on Holtrop & Mennen (1984). Statistical Numerical Dif. Propeller Revolutions % rpm Propeller Thrust % kn Wake Fraction % --- Thrust Deduction Fraction % --- Relative-rotative Efficiency % ---

15 CONCLUSIONS 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 selfpropulsion 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.