T. Guiard, Head of Energy-Saving Devices Full-Scale Self-Propulsion Calculations Including Roughness Effects and Validation with Sea Trial Data STAR Global Conference 2017 – Berlin March 6 - 9 T. Guiard, Head of Energy-Saving Devices

Outline: Motivation & Background Test Case for Self-Propulsion CFD Setup Postprocessing and Results Validation Summary

1) Motivation & Background Who are we? Founded in 1993 in Rostock 100% Owned by Becker Marine Systems (BMS) Our main services for BMS: Research & Development: General CFD and FEM services for research and development. Customization of rudders (powering, cavitation, maneuvering) Energy Saving Devices: Hydrodynamic design of ESDs ESD product development

1) Motivation & Background Situation in ESD design: The design of ESDs is strongly depending on the inflow field (ship wake). Validation of CFD calculations for ship wake fields is typically limited to model scale. A selected set of physical models, validated for model scale, is not automatically valid for full scale. Full-scale wake, same ship & speed Different model selection for turbulence and roughness Question: What does the full scale wake look like, and what models should we use to reliably predict it?

1) Motivation & Background Validation studies for full-scale wake calculations: Empirical scaling used as reference: Validation with measured data: Results: A combination of RST turbulence with a roughness height of ~0.2 mm shows the best correlation. But: Using roughness is often said to lead to too high resistance. Summary of validation studies in: Guiard, T. (2014): ‘Challenges with Respect to Scaling and the Prediction of the Full-Scale Wake Field in the Context of the Becker Mewis Duct®’, Jahrbuch der Schiffbautechnischen Gesellschaft, 108. Band, pp. 148-156, Hamburg, Germany

2) Test Case for Self-Propulsion In 2016 Lloyd’s Register was inviting to a workshop on full-scale simulation. Test case: 16.9k DWT general cargo ship “REGAL” (IMO 8908868) Hull, rudder and propeller geometry available from laser scan Draught, trim and list, as well as water and air properties measured during the sea trial and made available for CFD. Figure 1, Table 4, Table 7 from: Ponkratov, D. (2016): ‘Workshop on CFD in Ship scale hydrodynamics: Description of cases’ Lloyds Register, June 2016

3) CFD Setup - Basic Idea Approach for the Workshop: Using the Experience with wake prediction and propulsion modelling. Concentrating on the calculation of propulsion and propulsive coefficients. Gaining experience in calculating ship resistance, addressing the problem of too high resistance with roughness. Problem: Which models to select for turbulence and wall modelling? Solution: Splitting of the setup into resistance and propulsion.

3) CFD Setup - Structure Setup 1 Resistance Setup 2 Prop Open Water RANSE, SST k-ω, high y+ Roughness? Free surface Trim & sinkage Setup 2 Prop Open Water RANSE, RST, high y+ Smooth Setup 3 Propulsion RANSE, RST, high y+ Rough, ks = 188 µm No free surface No trim & sinkage Propeller modelled as in setup 2. “Setup” 4 Excel Interpolating self-propulsion points

3) CFD Setup – Resistance General Setup: RANSE, transient, SST k-ω, Free surface, VOF Trim & sinkage Number of cells: 17 Mio Wall functions, y+ ~ 100 to 500 Question: Should we use roughness for the resistance calculation? How to find validation possibilities / references? Reference to model ship measurements and corresponding trial predictions. Reference to flat plate friction measurements with ‘ship-like’ surface conditions.

3) CFD Setup - Resistance Reference to ship model measurements and corresponding trial predictions: Total viscous resistance comparison for two test cases: very good correlation at model scale Values scaled from model test are about 12 to 18% higher than calculated (SST k-ω, smooth). A roughness of ks = 188 μm leads to ~ 25% increase in CV.

3) CFD Setup – Resistance Reference to flat plate friction measurements with ‘ship-like’ surfaces: Table from: Schultz, Michel P. (2007): ‘Effects of coating roughness and biofouling on ship resistance and powering’, Biofouling, 23:5, 331-341 Result: According to Schultz (2007) a value ks = 188 μm corresponds to a hull condition between “Deteriorated coating…” and “Heavy slime”. With respect to calculated viscous resistance and guidelines ks = 188 μm seems realistic for a ‘mid-aged’ ship.

3) CFD Setup – Propeller Open Water General Setup: RANSE, steady, RST, moving reference frame Number of cells (propeller domain): 4.8 Mio Wall functions, y+ ~ 100 to 450 Smooth Problem: Propeller needs to run with RST turbulence for propulsion setup, SST k-ω would be preferred. Question: How does the propeller perform with RST? Result: Difference in open water data between RST and SST k-ω is considered acceptable. RST is used for open water and propulsion without further corrections.

3) CFD Setup - Propulsion General Setup: RANSE, transient, RST Ship: No trim and sinkage Free surface approximated with a symmetry plane Rough, ks = 188 μm Wall functions, y+ ~ 150 to 700 Propeller: Rotating SM Propeller mesh identical to open water setup Smooth Wall functions, y+ ~ 100 to 600 Problem: Free Surface very close to (or actually intersecting with) the propeller. Question: How should we position the symmetry plane?

3) CFD Setup - Propulsion Solution: Positioning the free surface in order to get the same nominal wake like in the resistance calculation. Wake from resistance setup SST k-ω, smooth Wake from propulsion setup SST k-ω, smooth Wake from propulsion setup RST, ks = 188 µm 9 kn: 14 kn:

4) Postprocessing and Results Preliminary Results Postprocessing: Get function RT(VS) from resistance setup Get functions KT(J), KQ(J), η0(J) from open water setup Get RT(VS) from propulsion setup without propeller Get T(n, VS), Q(n, VS), RT(n, VS) from propulsion setup with propeller Get t (from RT(n, VS), RT(VS), T(n, VS)), w and ηR t, w, ηR assumed constant for small ΔVS (ΔCTH ) Interpolating on Rt(VS), Kt(J), Kq(J), η0(J) for self-propulsion point.

5) Validation Sea trials performed under very good weather conditions Very close agreement between CFD simulations and measurement (approx. within measurement accuracy) Sea trial results & figure from: Ponkratov, D. (2016): ‘Workshop on Ship Scale Hydrodynamic Computer Simulation: Draft Proceedings’ Lloyds Regiser, LR Reference: Ref. 8428

6) Summary Based on this limited set of full-scale results it seems that: using ‘reasonable’ roughness values does not lead to too high resistance. the SST k-ω model should be preferred for resistance, while the RST model should be preferred for wake. (which would be consistent with experience from model scale) roughness should be taken into account with a ‘reasonable’ roughness value set. (which would be consistent with experience from model scale as ship models should be hydraulically smooth) splitting the CFD setup can turn out to be beneficial by offering the possibility to use the ‘right tools’ for each individual job.

Thank You for Your Attention!