CFD SIMULATION
1 The most important component for the successful completion of the project is the CFD simulation. Among the number of CFD simulation software like fluent, simFlow, Xflow etc. open FOAM was chosen for our simulation. The version of the openFOAM is 17.06. The three dimensional model of the Bell 412 main rotor after meshing with snappyHexMesh meshing utility, was put for running simulation.
5.1 SCALE
2 The solid model of Bell 412 main rotor was imported to Open FOAM from Solid works in STL format in units of meters. Moreover the blockMesh was also scaled in units of meters in Open FOAM where the solid model of the Bell 412 main rotor was adjusted.
5.2 MATERIAL AND REFERANCE VALUE
3 The fluid in our simulation is taken as air. It was assumed to be under the standard atmospheric condition with standard air pressure of 1.0125 Kpa, density of 1.225 kg/m3 and temperature of 300 k.
4 The reference values for initial conditions and other standard parameters were same for all cases in forward flight , HOGE and HIGE except that the forward speed of 5 m/s was allocated for forward flight whereas not for others. The viscosity value was 1.4028E-4 m2/s. Other parameters values were assumed that of standard sea level conditions. The Reynolds number was fixed to 500000.
5.3 OPERATING CONDITION
5 The Bell 412 main rotor was assumed to be operated under standard atmospheric conditions under the action of gravity. The rotor was given the RPM of 123 and rotated about Y axis in case of HOGE and HIGE but in case of forward flight the rotational axis was taken by considering tilt angle of 7 degrees. The gravitational field was set to 9.8 m/s2 in opposite direction of Y axis.
5.4 TURBULANCE MODEL
6 For our research project we have made selection of k-e turbulence model. It is a two equation model which gives a general description of turbulence by means of two transport equations (PDEs). We have made it as our choice because of its good convergence ability and low memory requirement. Furthermore it can consider the effects of free-shear layer flows with relatively small pressure gradients. It gives good compromise between computational cost and memory requirements. Moreover it also account for vortices formation also.
5.5 CALCULATION OF THE Y+ VALUE FOR k-EPSILON TURBULANT MODEL
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7 Based on the turbulence model selected the value of the parameters were been defined. As our turbulence model selected is k-epsilon turbulence model. The determination of the y+ value was very critical. It is because the based on the value of the y+ we can define the boundary condition for the wall, which in our case is the Bell 412 main rotor. The y+ value for our problem is calculated approximately 272. Since this value was in the range between 30 and 300, our use of k-epsilon turbulence model was justified.
Skin friction coefficient (Cf) = 0.058*Re-0.2
= 0.058*500000-0.2 = 4.20E-
Wall shear stress (?w) = 0.5* Cf*?* U2
= 0.06437
Friction velocity (U?) = ?(?w/?)
= 0.2292
Now,
y+ = (?*U?*y)/?
= (1.225*0.2292*0.1667)/(1.71E-4)
= 272
5.6 CALCULATION FOR INITIAL CONDITIONS FOR K-EPSILON TURBULANT MODEL
The value for the k and epsilon are also calculated for our problem:
Turbulence kinetic energy (k)
k = 3/2(UI) 2
Where, I=5% for medium Reynolds number
= 1.5(5*0.05)2
= 0.09375
Rate of dissipation of turbulence energy (?)
We know;
Turbulent length scale (l) = 0.07*Length of problem (L)
Where, L=14.028 m (Diameter of Bell 412 main rotor)
? = C?0.75 k^1.5/l where, C? = 0.09
= 0.090.75 ?0.09375?^1.5/0.98196
= 0.004803
