253 EEDI reduction by investigating the capability of RANSE CFD for propeller, propeller– hull form performance calculation during ship optimization process Dr Tran Ngoc Tu (1), Msc Nguyen Manh Chien[.]
EEDI reduction by investigating the capability of RANSE CFD for propeller, propeller– hull form performance calculation during ship optimization process Dr Tran Ngoc Tu (1), Msc Nguyen Manh Chien (2) Vietnam Maritime university, tutn.dt@vimaru.edu.vn Vietnam Maritime university, chiennm.@vimaru.edu.vn Abstract In recent years, the concerning about environment protection has grown significantly, especially about global warming and reduction of CO2 emission Besides, there are considerable development in marine transportation and activities: from offshore installation supply to the exploitation of marine resources It leads to the high increasing of fuel consumption for ship operation on the ocean Moreover, in 2010, International Maritime Organization (IMO) introduced Energy Efficiency Design Index (EEDI) as a technical measure to limit pollution of the environment resulted by marine engines [1] EEDI is expressed by CO2 gram per ship’s capacity So smaller EEDI means smaller CO2 exhausting to the environment With that reason, many efforts have been made to optimize ship’s fuel consumption, to save the operation cost, on the one hand and to reduce the CO2 emission, or reduce EEDI on the other hand From the EEDI equations [2], according to Bazari & Longva, 2011 and IMO MEPC 63 (2011) [2], there are 15 methods of EEDI reduction Within these 15 methods, hull form and propulsion optimization are common approaches for many designers and researchers To optimize the hull form and propeller, the designers need to carry many designs then select the best one based on their performance Estimation of hull form and propeller performance usingmodel tests widely accepted as most reliable means, and could be considered as the closest method to reality However, due to time and cost for making testing models, it is not suitable for optimization process; it is just only used to validate the result of optimization Besides, with the rapid improvement of computational resources, Computational Fluid Dynamic (CFD) is getting to become a useful tool in ship design and power prediction CFD method is able to look into local flow properties and providing a room for designers to improve the design In this paper, the authors will investigate the capability of CFD method for propeller and propeller – hull form performance calculation, during ship optimization process The approach of CFD here is Reynolds-averaged Navier–Stokes equations (RANSE) During the optimization process, many designs have to be analyzed, so the level of accuracy and computational time of the calculation have to be taken into account The paper has two major parts For the propeller calculation in open water, the authors will perform methods to model the rotation of propeller and select the best one in terms of accuracy and time consumption Later, the self-propulsion simulation is carried out That is a setup with full rotating propeller behind a ship The ISIS - CFD code, integrated in the commercial software Numeca Fine Marine is used The simulation results will be compared with model test results Keywords: propeller, hull, CFD, optimization, RANSE, EEDI reduction, ISIS code Introduction Propeller calculation using CFD method is not a new topic for researchers Many authors have predicted the performance of the propeller, both in open water and behind condition Giulio Dubbioso et al [3] has performed the open water simulation with INSEAN E779A propeller with fine mesh (1.31 million cells) and in-house solver χ navis - a finite volume uRaNSe (unsteady Reynolds-averaged Navier–Stokes 253 equations) solver To investigate “the effect of turbulence models on RANSE computation of propeller vortex flow”, Hongxuan (Heather)Peng,WeiQiu n, ShaoyuNi [4]did the simulation on David Taylor Model Basin (DTMB) 5168 propeller Three mesh sizes (1.92, 2.4 and 2.74 million cells) and 10 turbulence models (k-ε, k-ω, SST, Omega RSM …) has used during the simulation In terms of Propeller and hull interaction simulation, G Dhinesh [5] used RANSE solver Star CCM+ with k-ε turbulence model and sliding interfaces between propeller domain and ship domain All the authors have presented good simulation result in comparison with experiment result However, almost the simulations has just concentrated on the accuracy of the simulation, the computational time as well as the practical use of the method has not been studied., although it plays an important role during ship optimization process because many designs have to be considered in short period of time Thus, this paper also presents the balance between computational time and level of accuracy of propeller calculation Some methods for open water simulation are studied to choose the best one The solver using in this paper is commercial RANSE code ISIS Solver, integrated in Numeca Fine Marine software The turbulence model which mainly uses is k-ω SST All the simulations are performed on cluster over 16 up to 96 cores The first part of this paper deals with open water simulation over different methods: Sliding Grid, Rotating Reference Frame, and the last one is whole calculation domain rotating with propeller (called Rotating Domain in this paper) After selecting the best method to open water simulation, the authors are going to the second part: simulation of propeller working behind the hull At the end, the authors give the assessments and evaluations about the computational resources, level of accuracy and the practical use of simulation Literature Review 2.1 ISIS Flow Solver The ISIS flow solver is a solver based on incompressible unsteady Reynolds – averaged Navier-Stokes equation (RANSE) and developed by Laboratoire de Mécanique des Fluides, Ecole Centrale de Nantes, France Finite volume method is used in the solver for discretization of fluid domain The velocity field and pressure field are obtained by solving momentum and mass conservation equation [6] 2.2 Method for open water simulation As stated above, the study of methods using for open water simulation is carried out: Sliding Grid, Rotating Reference frame and Rotating domain Sliding Grid is the common approach to describe the rotational motion of fluids In this method, there often have two parts which are connected together: stationary part and rotating part The rotating part rotates each every time steps, and the connection between two parts is also re-calculated each time steps For the standard cells (non – rotating cells), we have to calculate fluxes in and out the cells For the cell and face at sliding interface, we search the cell centre (in the other part) that is best match the face This cell will be used for flux computation as the same as for the standard cells Another approximately approach to describe the rotating motion is the Rotating Reference Frame The mesh of rotational part does not have to change its position each time step Instead of that, there are coordinates system: the stationary and the moving one The propeller viewed from the rotating reference frame will be stationary This method can be considered as “a steady approach” for rotating motion, therefore, compared with Sliding Grid, it takes less computational resources The last one is the classical approach for open water simulation: the rotating domain method It means that there is only one domain (the fluid around the propeller) rotates with the same revolution of propeller 254 The open water simulation is carried out with all methods The authors are going to compare in terms of level of accuracy and computational time, then select the best one Open water simulation 3.1 Propeller Test Case To evaluate the result of open water test, the well – known propeller test case is used It is Potsdam propeller test case [7] The Potsdam propeller is blades, right handed propeller (look from the pressure side) with some basic dimension as follows: diameter 0.25m, area ratio: 0.77896; skew angle: 18.837 degree 3.2 Mesh generation As stated above, the open water simulation is carried out by different methods: Rotating Reference Frame, Sliding Grid and the classical approach: whole domain rotating with propeller (in this paper, we call Rotating Domain) The same mesh can be used for Rotating reference frame and rotating domain method The difference between two methods is the simulation setup For Sliding Grid method, we need to generate different mesh, because there are domains: propeller domain and fluid domain 3.2.1 Mesh generation for Rotating Domain (RD) and Rotating Reference Frame (RRF) method The mesh is hexahedral and mesh is generated by using Hexpress Detail characteristic of calculation Domain is described in Figure The Domain is a cylinder with the Diameter equaling 10 times the Propeller Diameter L =4.3m Outlet Va Inlet Calculation Domain: a Cylinder D = 2.5m Diameter = 2.5m Figure 1: Calculation Domain for RRF and RD method The Leading Edge, Trailing Edge and Tips of propeller are much more refined compared to other areas due to complex geometry at these areas The mesh size for RRF and RD method is around 3.9 million cells 255 Figure 2: Typical mesh of propeller 3.2.2 Mesh generation for Sliding Grid method As mentioned above, with sliding grid method, there are two domains: the rotating domain inside the fixed domain (Figure 3) The outer domain has same dimension as RRF method, and the inner one is just small enough to cover whole propeller inside Between two domains there are common faces - “Non matching connection face” The grid of common face between two domains is not required point-topoint matching each other This connection enables the solver to compute flux through two domains For each time step, the inside domain rotates and changes its position, therefore the solver has to recalculate this connection each time step Propeller Domain: a Cylinder Diameter = 0.28 m Length = 0.52 m Va Outlet Inlet Calculation Domain: a Cylinder Diameter = 2.5m Length = 4.3m Figure Calculation domain for Sliding Grid method The mesh size after generation and inserting viscous layer is 3.9 million cells, similar to other methods 3.3 Computational Setup The open water simulation is carried out with different advance coefficient J We keep constant revolution n = 15 rps for the propeller, J is changed by varying advance velocity Va Particularly, advance coefficients J is simulated: Advance velocity Va (m/s) Advance coefficient J 2.25 0.6 3.00 0.8 3.75 1.0 4.50 1.2 5.25 1.4 Turbulent models: k-ω SST The same boundary condition is applied for all three methods as follows: • Inlet and External boundary: Far field with advance velocity (Va) imposed; • Outlet boundary: Prescribed pressure (frozen pressure); • Solid parts: Wall function approach When selecting this option, ISIS solver automatically calculates the y+ to apply appropriate model: wall function or low Reynold number approach (low y+) 256 The major differences in setup of methods are the time step and the number of iteration per time step This setup directly influences to time consumption or computer resources during simulation The Rotating reference frame method can be considered as a steady approach for open – water test, therefore large time step and small numbers of iteration is used Detail setup of time step is as follows: Table Time step setup for open water simulation Number of Iteration per time step Time step Rotating Domain 0.0003333s (200 time steps per round) Rotating Reference Frame 0.00667s (10 time steps per round) Sliding Grid 0.00013333s (500 time steps per round) Method Computation of the simulation is performed parallel on cluster with 16 cores 3.4 Result and discussion The result is achieved by measuring the force in X direction (thrust) and the moment through X axis (torque) on propeller blades and hub when convergence is reached The thrust and torque are expressed in non-dimensional forms by KT and KQ After that, the open water efficiency ηO is also calculated Figure Open water curves obtained from different methods, comparing with experiment result (EFD) General view, compared to experiment data, the simulation results of three methods are good at J from 0.6 to 1.0 particularly, from 3% to 6% difference for all KT, KQ, and ηO The result of KQ is also good for all J, less than 5% The difference just gets higher for KT, with J from 1.2 to 1.4, up to 7% and 13%, respectively The reason for that could be because the magnitude of KT is getting very small with increasing J There is not much difference in terms of numerical result among methods The Rotating Reference Frame method shows very good estimation of KQ, giving the best result compared to two other methods For KT, the Sliding Grid is the closest to experiment The details of computational result are described in the Table below: 257 Table Open water simulation result of different methods Experiment Sliding Grid Rotating Domain Rotating Reference Frame J 10KQ 10KQ ΔKQ 10KQ ΔKQ 10KQ ΔKQ 0.6 1.396 1.451 3.94% 1.466 4.98% 1.432 2.53% 0.8 1.178 1.224 3.88% 1.242 5.41% 1.208 2.52% 1.0 0.975 1.002 2.78% 1.019 4.57% 0.988 1.35% 1.2 0.776 0.791 1.92% 0.803 3.50% 0.779 0.33% 1.4 0.559 0.559 0.10% 0.546 -2.36% 0.546 -2.38% J 0.6 0.8 1.0 1.2 1.4 Experiment KT 0.629 0.510 0.399 0.295 0.188 Sliding Grid KT ΔKT 0.630 0.13% 0.506 -0.74% 0.388 -2.97% 0.277 -6.06% 0.166 -11.37% Rotating Domain KT ΔKT 0.630 0.12% 0.508 -0.33% 0.390 -2.35% 0.278 -5.72% 0.162 -13.69% Rotating Reference Frame KT ΔKT 0.623 -0.99% 0.501 -1.74% 0.383 -4.08% 0.273 -7.39% 0.162 -13.71% J 0.6 0.8 1.0 1.2 1.4 Experiment ηO 0.430 0.551 0.652 0.726 0.749 Sliding Grid ηO ΔηO 0.414 -3.66% 0.527 -4.44% 0.616 -5.59% 0.669 -7.82% 0.663 -11.44% Rotating Domain ηO ΔηO 0.410 -4.63% 0.521 -5.44% 0.609 -6.62% 0.661 -8.91% 0.662 -11.59% Rotating Reference Frame ηO ΔηO 0.415 -3.43% 0.528 -4.15% 0.617 -5.34% 0.670 -7.69% 0.662 -11.59% In terms of computational time, the simulation for all methods is performed in parallel with 16 cores The mesh sizes are 3.9 million cells The average computational time is follows: Table Computational time of different methods Method Sliding Grid Rotating Reference Frame Rotating Domain Computational time (average) 58.3 h 15 h 40h Percentage (compared to Sliding Grid method) 100% 25.% 68.6% It is clear that Rotating Reference Frame takes least computational time, by less than one-third compared to two other methods Therefore, Rotating Reference Frame method has big advantage in practical and daily use 3.5 Assessment and conclusion of result for open water simulation Rotating reference frame method proves that it is suitable method for open water simulation, concerning computational time and level of accuracy, as well as convergence of result However, this method is only suitable for simulation with domain, it cannot be used for simulation of propeller behind the ship In this case, Sliding Grid approach should be used The investigation of setup for sliding grid approach 258 in this section is very useful for doing simulation of propeller behind the ship in the next part of this paper Propeller behind ship simulation To have consistency with experiment, the simulation is carried at model scale for ship and propeller The ship is bulk carrier, with a 4-blade propeller [8], from a Chinese shipyard The experiment result is provided by China ship scientific research center (CSSRC) [8] The output is wake fraction (wT), thrust deduction factor (t), relative rotative efficiency (η R), and hull efficiency (ηH) Besides, the factors that represents performance of propellers also need to be taken into account: thrust coefficient (KT), torque coefficient (KQ) (note that these two coefficients are calculated in the case of propeller behind the hull, different from open water case) In order to get all the output, it is necessary to use open water curve from open water test simulation Hence, the simulation steps and result of open water for this propeller will be shortly presented 4.1 Ship and propeller geometry Basic dimension of ship and propeller are described below: Table Basic dimension of ship and propeller Ship (bulk carrier) Length overall Length between Perpendicular Breadth moulded Design draft Displacement Block coefficient CB 7.5 7.233 1.0753 0.4067 2.708 0.855 m m m m m3 Propeller Diameter 0.2333 Chord length at 0.75R 0.0502 Expanded blade ratio 0.3766 Number of blades Direction of turning Right handed m m 4.2 Open water test result The mesh generation and calculation setup for open water case has been described completely in the previous chapter Therefore, only brief information about this simulation is presented The method using is Rotating Reference Frame method, mesh size 2.1 million cells, turbulence model: k-ω SST The open water curve is presented in Figure below Figure Open water Curve - Self propulsion test 259 ... search the cell centre (in the other part) that is best match the face This cell will be used for flux computation as the same as for the standard cells Another approximately approach to describe the. .. After selecting the best method to open water simulation, the authors are going to the second part: simulation of propeller working behind the hull At the end, the authors give the assessments... Method Computation of the simulation is performed parallel on cluster with 16 cores 3.4 Result and discussion The result is achieved by measuring the force in X direction (thrust) and the moment through