KeywordsDrag Reduction, Trim Optimization, Numerical Simulation, Computational Fluid Dynamics, Fuel Consumption, Ansys Fluent.
1. IntroductionIn the past years the computer speed has been increased exponentially and more sophisticated RANS codes have been developed. More realistic simulations are able to be performed. All these advances have been well documented in the proceedings of several international conferences on the CFD techniques applications to ship flows. These conferences have been held every year since 1990. The most famous are those in Tokyo in 1994 , Gothenburg in 2000  and Tokyo in 2005 . Remarkable information on these application of CFD codes to naval ships and submarines can be found in recent studies shown in recent proceedings of the ‘Symposium on Naval Hydrodynamics’. This symposium is sponsored by the US Office of Naval Research (ONR) and was first held in 1956. Some examples of the advancement of current specialized CFD codes for the flow simulation around naval hulls can be found in the work of Burg et al.  reported at the 24th Symposium on Naval Hydrodynamics. It was used in Burg’s study a three-dimensional unstructured, paralyzed CFD code named U2NCLE, developed by the Computational Simulation and Design Centre at Mississippi State University to simulate the free surface flow around a fully-appended model of a US naval ship. The results found by using this code realistically captured the turbulent flow and the vortices arising from the bulbous bow and the tips of the propulsors and rudders. Also it was found that to accurately model the propulsor rotation. Near-field wave shapes were successfully simulated by a nonlinear free surface algorithm. CFDShip-Iowa code was developed at the University of Iowa’s Institute of Hydraulic Research (IIHR) under funding provided by ONR, can be used as another example of a RANS code. This code was developed to work specifically on surface-ship interface and marine propulsor flow problems. General application in naval hydrodynamics include the prediction of resistance that include friction and pressure drag prediction, wave profiles, sea keeping using a code permitting a six-degree of freedom, and manoeuvring. Wilson et al.  recently compared the performance of CFDShip-Iowa code with two commercial CFD codes. Fluent, developed by Fluent Inc. and Comet, developed by CD-Adapco, in order to predict the ship generation of waves. It was found that each of the codes has different advantages and disadvantages respecting the other one, and that each code has certain specific parameters to be considered for obtaining accurate solutions of a surface ship wave fields. It was also found that commercial solvers may present and advantages when complex geometries shapes involving appendages and propulsors are being considered because of the flexibility to use unstructured create mesh contain unstructured as well as hybrid grids. Both code, Fluent and Comet, also presented the advantages of allowing solution based grid adaption techniques to provide finer grid resolution in critical regions of the grid. It can be very useful when an interface region is considered as in this case, and hence may offer a more detailed and computationally economical way to provide accurate results of the free surface prediction problems in the vicinity of surface ship hulls. Ship trim optimization operations are considered as one of most economically wise methods for both ship performance optimization and reduction of fuel consumption. There are no requirements of shape modification or puissance upgrade in such operation. It can be achieved by preparing a proper loading plan and efficient ballasting/de-ballasting process. Operationally, Trim optimization are not considered as important as other conventional power performance test procedures. Nevertheless, it was proven that trim optimization can maintain potential savings. Also, depending on the type of ship and its operation, trim optimization could result a return on investment between one to six months. These outcomes have been proven also by Hansen and Freund in 2010 , describing depth water influence on gains. Trim optimization test procedures can be employed in towing tank model testing or in computational fluid dynamic (CFD) simulation. Hansen and Hochkirch (2013)  demonstrated possible power gains by CFD tests and gave some CFD method comparisons with tests at model and full scale. Regarding the tendency of power requirement with respect to trim, both methods were found to be in reasonable agreement. A leading consultant in trim optimization as Force Technology showed possible fuel savings of up to 15% at specific work conditions. In overall fleet operation, these savings can arrive until 2% or 3%. Extensive research campaigns have been made by Force Technology to understand physical effects to reduce propulsive power. Lemb Larsen et al. in 2012  presented an analysis of the origin factors in resistance and propulsive fields and the influence on power requirement. Even though resistance causes the most of power gain, overall performance can also be affected by variation in the propulsive coefficients, which could be considered as well.  In this paper, it was not taken in count operational constrains that should be considered in shipping, slamming or green water, strength and stability, manoeuvrability and overall safety. It was principally studied the trim optimization.
2. IMO: Ship Energy Efficiency Management PlanRecently, energy and environment issues have been studied carefully in all around the world. The objective is to promote the energy efficiency of transportation vehicles such as ships and airplanes. Nowadays, global temperature has risen about 2°C above the pre-industrial level and it is expected that it would be cause several destruction on global scale. The only way to overcome this problem is to decrease global greenhouse gas emissions. Studying future scenarios show that if preventive actions are not taken, levels of CO2 emissions from ships would be doubled by 2050. European Union (EU) and United Nations framework convention on Climate Change (UNFCCC) are working hard to get control of global emissions as shown by Kim, Hizir, Tuan, Day and Incecik, in 2017 . International Maritime Organization (IMO) is currently controlling to set GHG commands for international shipping with industrial, operational, and market-based policy tools. It seems clearly IMO regulations concerning to CO2 emissions from shipping will remain to develop in coming years. Thus, fuel prices have ever been increasing and anticipated to continue growing in the future as demonstrated by Kim, Hizir, Tuan, Day and Incecik, in 2017 . Several operators are promoting ship’s performance should be conduct to do constant routine maintenance and control fuel consummation efficiencies. Every day offers the opportunity to optimize speed, find the safest route and make sure the ship is sailing at the best draft and trim that is tuned to keep course safely and efficiently. These efforts are compatibles directly to the goals of recent IMO Guideline on Ship Energy Efficiency Management Plans, a framework that captures the corporate commitment to energy conservation. Most important factors that should be considered for energy conservation on ships in service are as listed below: 1. Voyage Speed Optimization 2. Weather Routing – Safe Energy Efficient Route Selection 3. Hull Roughness and Its Impact on Resistance 4. Trim/Draft Optimization In this report Trim Optimization was studied carefully to find optimal points for a simplified geometry of a ship
3. Trim OptimizationFirstly, trim is defined as draught at AP and FP difference, as defined in the following expression: Trim = TA –TF In positives values trim results to the aft. Moreover, the displacement and speed keep constant when the ship is trimmed. There is not extra ballast added and power consumption varies only if resistance varies with the trim. Global goal of trim optimization is to minimize the required power at specific displacement and velocity of the ship. Physical effects of reducing propulsive power (PD) when a ship is trimmed can be caused by the hull resistance ( ) and total propulsive efficiency ( ) as shown below: PD = RT.VηD Ship speed (V) keeps constant, as said before. So, it can be concluded that the aim is to reduce the resistance and increase the total efficiency.
3.1 Reduction of ResistanceAccording to ITTC standards, still water ship resistance is written by: RT=1/2.ρ.V2.S.CT Ship resistance changes can be a wetted surface area (S) and total resistance coefficient ( CT) function. Both parameters have to be decreased in order to get a trim gain. Wetted surface is calculated for the ship at rest, without dynamic sinkage and trim. Variation of wetted surface are caused by trim relates to the large flat stern area and it is relatively small. It can arrive to 0.5% of the even keel wetted surface and it causes that total resistance varies because of the linear proportionality. Total resistance coefficient can be achieved by reducing all parameters mentioned before. It can be described by the following expression: CT= CR+1+k.CF0+CA Allowance coefficient ( CA) keeps constants unless for ships with big variation in the draught. Friction resistance coefficient (CF0) can vary with the Reynolds number (Re) for the flow along the hull: CF0=0.075(log10Re-2)2 Where Reis the Reynolds number defined by: Re=V.Lwlν From friction resistance coefficient and Reynolds number, it can be deduced that friction resistance coefficient is a function of the water line length ( Lwl), and this relation is inversely proportional. Although water line length can vary 5% from the even keel condition, inverse proportionality results by increase or decrease propulsive power of 0.5%. This effect comparing with overall possible savings is negligible. Form factor, ( 1+k), is often kept invariable at each draught to optimize the experimental program cost in the towing tank. In practice, there is no influence giving by this factor on the resistance changes for different trimmed conditions. Residual resistance coefficient ( CR) is often the parameter most affected by the ship trim. It was seen before that residual resistance coefficient at trimmed conditions can rise up to 150% from even keel condition values. That is reflected in power requirement changes up to 20%.
3.2 Increase of Total Propulsive EfficiencyPropulsive efficiency is the product result between the hull efficiency ( ηH), the open water propeller efficiency ( ηo) and the relative rotating efficiency ( ηrr). ηD=ηH.ηo.ηrr When the ship is trimmed none of these parameters are constant. Hull efficiency is a function of the thrust deduction ( t) and the effective wake fraction ( w). ηH=1-t1-w It is obvious that the thrust deduction decreases and effective wake fraction increases to achieve a gain trimming. Thrust deduction is a function of the propeller thrust ( T) and the hull resistance. t=T-RTT Also, it is known that hull resistance varies when the ship is trimmed and it causes that propeller thrust changes as the speed stays constant. Nevertheless, hull resistance is not constant. Thrust deduction changes as the trim changes and sometimes a peak, when propeller submergence decreases until arrive to a critical level, as it can be observed. The peaks localization depends on the dynamic sinkage and stern wave. Thrust deduction changings can produce values up to 15%. That means propulsive power changes up to 3%. However, the thrust deduction changes should be relatives to effective wake changes. Effective wake fraction is related to the ship velocity and the propeller inflow velocity (VA). w=V-VAV If the ship speed keeps constant, the effective wake fraction changes can only be related to the propeller inflow velocity. As mentioned before, the effective wake fraction increases for bow condition of trimming and decreases for stern condition of trimming. Wake fraction increasing for bow trims can catch up to 20% and the stern trims decreasing can rise up to 10%: Wake fraction differences can results in a power gain up to 2%. Propeller open water efficiency depends on the advancing ratio ( J), on the water inflow velocity of the propeller ( VA) and on the revolutions ( η). J=VAn.D Where Dis the propeller diameter. As concluded, propeller inflow velocity is affected by the trim. Since open water curve for the propeller efficiency is inclined for the actual ratio of advancing, with minor changes in the advance ratio result in a propulsive power changings. These changes can vary up to 2% of the even keel power demand. Relative rotating efficiency is described as the relation between the open water propeller torque coefficient (KQow) and the propeller torque coefficient behind the ship (KQship). ηrr=KQOWKQship It can be reach up to 2% from even keel condition and influencing the power requirement.
4. Experimental DataThere exist extensive database for CFD validation models. Some tests were done by the Resistance Committee of the 22nd International Towing Tank Conference to create this data base . The focus was on modern ship forms. Forms as tanker (KVLCC2), container ship (KCS), and surface combatant (DTMB 5415) were recommended for use. These results were shown at the workshop of Gothenburg 2000 on CFD for ship hydrodynamics  and other conferences. The bare hull trim results of this study were compared with the results of Olivieri et al.  and a combined effort between Istituto Nazionale per Studi ed Esperienze di Architettura Navale (INSEAN, Italian ship model basin ) and the Iowa Institute of Hydraulic Research (IIHR) to show experimental towing tank data and then compare to the CFD results. The CFD results used for compare appended model were the existing towing tank results performed by David Taylor Naval Ship Research and Development Centre . This model has fixed pitch shafts and struts and is designated Model 5415-1.
5. CFD ModelComputational Fluid Dynamics or CFD is a computational tool developed for studying the behaviour of the fluid flows in many different fields. It is impressive how the computer performance in this era gives us the possibility to recreate fluid flow models perfectly reals and the possibility of examining different equipment designs or compare performance under different operating conditions.
5.1 Ansys Fluent CodeFluent code is a cell centred of finite volume general resolution for CFD problems developed by Fluent, Inc. and now marketed by ANSYS, Inc. which is a provider of engineering simulation software that acquired Fluent, Inc. in 2006. Ansys Fluent has been used by the Hydrodynamics Research Group within MPD for several years to recreate problems involving submarine related flow fields. A big difference between the simulation described in this study and those reported by the Hydrodynamics Research Group is the presence of the air/water interface in the simulation. To estimate this interface there are two main approaches, either surface fitting approaches or surface capturing approaches. Ansys Fluent implemented the surface capturing approach by using VOF (Volume of Fluid) scheme for general multiphase flow modelling. This approach involves defining a volume fraction of each phase defined throughout the domain and then convecting the volume fraction of each phase with the average fluid flow. The interface between both fluids is defined from the volume fraction function for each of phase defined before in the near of the interface. The simulation of the surface ship motion not only requires maintaining a sharp interface between the water and the air phases but requires the specification of correct boundary conditions as well at the inlet and outlet of the domain for each of the phase defined in the model. One method to do this is to impose a User Defined Function to specify the total pressure and the fluid volume profile for both fluids at both inlet and outlet conditions. Another way is to use the Open Channel Boundary Condition which allows the inflow and outflow conditions to be specified from the inflow velocity and the free surface level and then the pressure and fluid volume profile can be automatically calculated. This could be simplified the simulation. Ansys Fluent offers a package of different RANS turbulence models. These include the Sparlart-Allmaras model, the k-ε model, and the k-ω model, all of which are based on the Boussinesq approximation. Boussinesq approach assumes the calculated turbulent viscosity coefficient is isotropic. Furthermore for complex flows involving streamline curvature, swirl, rotation, and rapid changes in strain rate the Boussinesq approach is not a good approximation. For these cases Ansys Fluent provides the Reynolds Stress Model (RSM), which solves the RANS equation by solving additional transport equations for each of the individual Reynolds stresses. This means that there are five additional transport equations that are solved in all three-dimensional flows and the simulation time increases substantially. In this study the simulation described involves only straight-ahead motion at minimal angles of attack. It is expected that the flow to remain largely attached to the ship surface, hence it was employed the standard k-ε turbulence model and standard wall functions for this campaign of simulations.
5.2 BackgroundThe three fundamental principles govern the physical aspects of any fluid flow are the conservation of mass, the conservation of energy and Newton’s second law. These fundamental principles are expressed in mathematical equation forms, which in a general form are integral or partial differential equations. In most cases these expressions cannot be solved analytically. Computational Fluid Dynamics (CFD) uses appropriate discretized algebraic forms to replace the integral or partial derivatives in the principles equations. These discretized algebraic forms can be solved easier. The CFD solution outcome is numbers that describe the flow filed at discrete points in time and space. Navier-Stokes equations refer to the complete system of flow equations, which solves momentum, continuity and energy equations. The complete flow system equation for the solution of an unsteady, three-dimensional, compressible, viscous flow is: Continuity Equation expression: DρDt+∇.ρ.V=0 Momentum Equation expressions: ∂(ρu)∂t+∇.ρuV=-∂p∂x+∂Txx∂x+∂Tyx∂y+∂Tzx∂z+ρfx ∂(ρv)∂t+∇.ρvV=-∂p∂y+∂Txy∂x+∂Tyy∂y+∂Tzy∂z+ρfy ∂(ρw)∂t+∇.ρwV=-∂p∂z+∂Txz∂x+∂Tyz∂y+∂Tzz∂z+ρfz Energy Equation expression: ∂∂tρe+v22+∇.ρe+v22V=ρq+∂∂xk∂T∂x+∂∂yk∂T∂y+∂∂zk∂T∂z-∂up∂x-∂vp∂y∂wp∂z+∂uTxx∂x+∂uTyx∂y+∂uTzx∂z+∂vTxy∂x+∂vTyy∂y+∂vTzy∂z+∂wTxz∂x+∂wTyz∂y+∂wTzz∂z+ρf.V Governing conservation form of equations derives from a control volume fixed in space with a fluid flow through it. Nonconservation form corresponds to the control volume moving with the fluid such that the same fluid particles are always in the same control volume. The distinction between conservation and nonconservation form was triggered by CFD development and the problem equations was more favourable to use for a given CFD application. Conservation form of the equations is more favourable for a numerical and computer programming perspective. Equation forms for an inviscid flow more known as Euler equations can be derivate from Navier-Stokes equations given above and dropping all terms corresponding to friction and thermal conduction. Navier-Stokes equations describe any fluid flow, but not all flows in the same way. The boundary conditions, and often the initial conditions, determine the particular solutions of the general governing equation for a given problem.
5.3 Discretization MethodMost of CFD codes use a finite volume method of discretization, and Ansys Fluent code, used for this study, is not an exception. As that was presented above, the governing equation or Navier-Stokes equations are composed by differential equations. It represents a problem and it is necessary transform these equation into numerical equations that contain only numbers (not expression). This process produces a numerical analogue to the derivate is named numerical discretization. The discretization is made for a simple spatial domain into finite control volumes. A control volume can decompose in many elements of a mesh and, therefore, the governing equations are solved for each element of this mesh, as shown in Figure 1. Figure 1. Volume finite method mesh element. The resulting integral equation laws would be exactly satisfied for each element and likewise for the entire domain.
- Hydrofoil (aerofoil adapted geometry) type: NACA0020 (Symmetrical)
- Hydrofoil Chord: 63 mm
- Hydrofoil Wing Span: 49mm
6. Model SimulationAn implicit steady-state cell based solution procedure was used to solve the governing equation in this study. The SIMPLE algorithm was used for the pressure-velocity coupling and a PRESTO scheme was imposed to result the pressure interpolation. A 2nd order upwind scheme was choose for the solution of the momentum equations and the modified HRIC scheme for the solution of the volume fraction equation. The relaxation factors were typically set to 0.2 and the standard K-Epsilon turbulence model with equilibrium wall functions was used to simulate turbulent flow regime. The y+ values for the wall adjacent cells over the solid geometry were in the range of 80 and 100. That allows being comfortable within the guidelines for the use of the wall function approach. Before running the simulation, it is very important to know that Ansys determines to stop the simulation taking in consideration the iteration number and convergent limit that were specified. It means that once the maximum iteration number is reached or the convergence limit is satisfied the computation is terminated. It was imposed for every calculation a convergence limit of 10E-6 for each monitor and a maximum iteration number of 1495. Figure 4. Convergence plot (stopped at 1495 iteration). Simulation initial convergence was found to be considerably enhanced. It was due to the initialization procedure that it has to be careful attended. It is recommended that the primary phase is set to the lower density fluid, as in this case it was the air. The specific operating density should be set to that of the primary phase and the reference pressure location has to be set to the region of the primary phase. Also it is necessary and helpful to initialize the entire water domain with the correct hydrostatic pressure profile and to initialize both phase domains at the same velocity.
6.1 Model ValidationAccording to experimental results which were derived from literature experimental tests, total drag coefficient is shown in figure 5. The results of series of simulation with Ansys Fluent were completely compared with the given results of experimental results. The simulation was set up to get the same situation and the same size as the ones which were utilized in theoretical experiences and good agreement could be found. Figure 5a.CFD Pressure Coefficient. Figure 5b.Experimental Pressure Coefficient. It was found that the main problem encountered in this study and independently was the air/water interface became considerably disrupted during the course of the simulation run. In some cases large drops of water were found to be suspended in the air in completely irregular patterns. It was necessary to repatching the appropriate part of the domain with the air phase during the course if the simulation. The values for the pressure coefficient predicted by theoretical methods are smaller than those predicted by the CFD model. It can be conclude that the bad alignment of the struts (attack angle - 10°) in the CFD simulation can explain the relatively large values of the pressure coefficient given by the simulation. The pressure coefficient values of CFD simulation using an angle of attack perfectly aligned are closer than those attended from theoretical experiences. Nevertheless, the CFD values of this condition do not capture the effect of the interaction between water and air. Figure 6a.Experimental results. Figure 6a.Experimental results Olivieri et al.  provide figures of the wave pattern and the wave profile along the ship at Fr=0.41 from the towing tank measurements. In figure 7, the experimental results from Olivieri et al. for the wave pattern on the free surface at Fr=0.41, and at the bottom half is the wave pattern that was found from the CFD code at the same physics work conditions. It is noticed that there is a good agreement with the CFD code results for the general form of the wave pattern from the towing tank test. Figure 7. Experimental and CFD wave pattern at Fr=0.41. Figure 8. Dynamic Pressure Contour. Figure.9 below shows the velocity contour at Fr=0.41. The zones with larger values of velocity (drawn with a red or closer to red colour) have a greater contribution of the frictional and pressure resistance values. It can be seen that the pressure coefficient at the tip edges receives larges values as has previously been attended. Figure 9. Velocity Contour. In the towing tank report from literature  there is a photo of the bow pressure contour at Fr=0.41 The visual comparison between the CFD and the towing tank bow contour shows a similar maximum point of pressure in the geometry as shown in figure 8. The goal of this study was to find a CFD simulation that allows compare theoretical results to these given by the theoretical values. In general the results, at the lower speeds, were not good enough. One reason of this result is that for smaller parameter values at lower speeds more accurate numerical predictions relative to the higher speeds are required to get 5% accuracy compared to the experimental data. Olivieri et al.  provide figures of the wave pattern and the wave profile along the ship at Fr=0.41 from the towing tank measurements. It can be seem in figure.7 the experimental results from Olivieri et al. for the wave pattern on the free surface at Fr=0.41, and at the bottom half is the wave pattern that was found from the CFD code at the same physics work conditions. It is noticed that there is a good agreement with the CFD code results for the general form of the wave pattern from the towing tank test.
7. ConclusionThe present study presents how possible it is to numerically create a functionally model of a container ship from a simplified geometry. This approximation gives a pretty good estimation of the results attended from theoretical experiences. Some of the scientific papers related to this problem consider the flow as unsteady flow, attached to a cavitation model at high ship velocities. Using that large eddy simulation model as turbulence model could also capture the free surface effect on the hydrofoil as the hydrofoil position during service is around to the surface water. Nevertheless, it was proved that using a K-Epsilon model and considering the flow as two dimensional and fully turbulent flow gives well estimation of the physical working. Numerical approach was used to calculate the free-surface flow around a simplified geometry of a DTMB 5415-1 naval ship model. The simulations were performed using hexahedral mesh after doing some mesh sensibility tests. The results given by the simulation show that this numerical approach is able to accurately simulate the total pressure coefficient, near-field wave shapes, and the pressure and velocity field in the propeller plane. Using Ansys Fluent it can be found that the software is particularly sensitive to the attack angle of the grid respect to the free surface water line in simulation of this kind. This can represent a problem, but I can be solved by re-meshing the geometry for each individual sinkage and trim calculation. The pressure coefficient calculated on the simulation within an attack angle null was found agree with the experimental value to within 8.3%. The simulated velocity and pressure contour shapes along the surface of the ship were found to be in good qualitative agreement to those of experimental profiles. Regarding the possible power saving it should be conclude that for medium speed ranges the results are not changing, while for the angle of attack deviation reach from 0.7 up to 14. Thus, this could be a recommended for better power saving prediction in the speed range considered. Therefore, the predicted power saving are generally lower, but it exhibit a good approximation of real experiences. The above results are encouraging and indicate that Ansys Fluent code is a viable tool and it could be considered for use in more demanding real problems.
8. Recommendation for Future WorkThis study was focused on optimizing trim of ship simplified geometry. Next step would be to simulate a more realistic geometry to compare the results obtained in this study. In the direct power saving, that could be considered to predict power levels a wake scale effect and correlation allowance coefficient. References
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