3.1 RESEARCH PROCEDURE
1 Our research methodology will begin from modeling the SOLIDWORKS solid model of the main rotor of the Bell 412 helicopter. A surface and volume mesh continuum will be generated that will contain approximately millions polyhedral cells, where the Finite Volume Method (FVM) will be chosen as a discretization technique. The software to generate the rotating mesh will be Gambit software. The subsequent CFD simulations will be conducted with open Foam 17.06 software in subsonic flow regimes. Also an implicit unsteady flow solver, with an ideal gas and a SST K-Omega turbulence model will be used. Forward flight case will be examined. At last we will compare the theoretical and simulated results. In step wise it will go like this;
• Solid modeling
• Mesh generation
• CFD Simulation
• Turbulence model
• Solutions and Calculations.
3.1.1 SOLID MODELING
2 The solid model of the main rotor of Bell 412 helicopter will be created using Solid Works 2015 drawing software. The airfoil data and other required data about dimensions and profile configurations of Bell 412 helicopter will be taken from the Sri Lanka Airforce (SLAF).
3.1.2 MESH GENERATION
3 A mesh is a discretization of the geometric domain. The accuracy of the CFD simulation strongly depends on the quality of the grid. A good quality grid considering the flow physics leads to faster convergence and better solution. Thus design and construction of a quality grid is crucial to the success of the CFD analysis. For the mesh generation we will implement the Gambit Software because it is suitable for generating polyhedral cells and also it’s relatively easy accessible. In our research we will create the structured mesh because of requirement of less computational memory and cost, data locality, available solution algorithms, high degree of control and alignment leading to better convergence. Also polyhedral cells will be taken into consideration because polyhedral meshes showed better accuracy, lower memory demand, shorter computational speed and faster convergence behavior than other shaped cells.
4 Finally, surface mesh and subsequently volume mesh which is generated will be made as rotating mesh using Gambit software. We will select Finite Volume Method (FVM) for the discretization method. More cells can give higher accuracy. The downside is increased memory and CPU time. Millions of cells are huge and should be avoided if possible. However, they are common in aerospace and automotive applications. Thus we also will be also generating nearly 3-5 million polyhedral cells in the volume mesh continuum.
Then our next step will be to set boundary conditions. After generating the mesh by using the gambit software we will allocate the boundary types and continuum types for the box domain which was used for all three simulations.
3.1.3 CFD SIMULATION
5 Our next step will be CFD simulation. The simulation will be carried out in openFoam17.06 solver software. The CFD simulation will be done for Forward flight. For that we will take the meshed volume continuum from Gambit. For simulation purpose we will consider the working fluid as air and will assume that the main rotor operating in the standard atmospheric conditions. During our simulation we will be requiring reference values for quantities like air density, temperature, pressure, viscosity, enthalpy etc. Therefore in such cases we will consider respective values at standard atmospheric sea level conditions.
We will use turbulent model in CFD simulation.
3.1.4 TURBULANCE MODELS
6 Turbulence flows are three dimensional, fluctuating and chaotic (full of eddies and wakes).Governing equations cannot be solved for 3D turbulent flows of engineering interest. Turbulence model describes turbulent motion, allow calculation of mean flow variables and do not require calculations of the entire time history at spatial locations. Therefore a turbulence model is a computational procedure to close the system of mean flow equations. Turbulence models allow the calculation of the mean flow without first calculating the full time-dependent flow field. We only need to know how turbulence affected the mean flow.
7 We are considering Reynolds-Averaged Naiver-Stokes (RANS) model for computing the turbulent flow. These models simplify the problem to the solution of two additional transport equations and introduce an Eddy-Viscosity (turbulent viscosity) to compute the Reynolds Stresses. There are several turbulent models under it. For a turbulence model to be useful, it must have wide applicability, be accurate, simple and economical to run. Therefore we will choose k-? SST model because this model has proved to be a very good turbulence model for many engineering applications that provides a good trade-off between computational cost and accuracy. However, it requires a good resolution of the near-wall region which is a memory intensive case. But still its accuracy is not compromised. We will employ the k-? or the k-? model to compute the flow field and use it as initial conditions for the k-? SST as it exhibits sensitivity to the initial conditions.
3.1.5 SOLUTION AND CALCULATIONS
8 Finally, from the CFD simulation we will determine the tip velocity of rotor, thrust required, power required, coefficient of lift and coefficient of drag and coefficient of moment. And for theoretical calculations, on the basis of blade element theory and momentum theory, we will again calculate tip velocity of rotor, thrust required, power required, coefficient of lift, coefficient of drag and coefficient of moment by using the maintenance manual and other necessary data documents from SLAF about Bell 412 helicopter.
1 Here we have selected Boeing VR-7 airfoil and we start to design the solid work 1:1 model. First we have designed the model in MMGS (millimeter, grams, and seconds) unit system. But when this model run in the Open Foam it will take unlimited time to analyze the details, because it take 1 mm as a one part so there will be many number of parts. So we redesigned the model with MKS unit system and analyze it in Open Foam. So after that we have crated Mesh using Open Foam software and finally simulate it by using the same software.
4.1 SOLID MODELLING
2 The solid model of the main rotor was 1:1 and it was created using the solid work 2015 software. We found the co-ordinates of Boeing VR-7 (Vertol 7) airfoil which used as the airfoil in Bell 412 helicopter. For that we used Aerofoil.com website. 19 From there the DAT file which had the co-ordinate of Boeing vr-7 has been downloaded. Then the co-ordinates of x and y inserted in to MS Excel 2013 software and changed it with adding Z co-ordinates as “0”. In DAT file it contained X and Y co-ordinates only. It helped to draw the airfoil in XY plane in solid work. Throughout the modeling part we have used the Meter, Kilogram, and Second (MKS) as the Unit System.