[Mr.Goodcat]

Good day, everyone.

I'm trying to simulate the flow field sorrounding a helicopter rotor in hover for my thesis and i'm in a quandary as to how to get the values of Ct and Cq (thrust and moment coefficients) for the whole rotor, considering that the rotor is two-bladed and so i'm modeling just 1/2 of the domain (applying periodic BCs).

I understand that my question could seem a bit dumb to a veteran of CFD, but i'm learning now how to use Fluent, so bear with me, please.

I understood that:

-Fluent will compute the overall force/moment from viscous and pressure forces on the blade/wall

-from that, if the appropriate reference values are given in input, they will be used to non-dimensionalize the force/moment and obtain the coefficients Cl and Cm, by dividing the lift by 0.5*ro*v^2*A and the moment by 0.5*ro*v^2*A*L.

-the experimental data with which i'm going to confront my results consist in values of C_thrust and C_torque (Cl in the axial direction and Cm with respect to the rotor axis, i presume) , respectively calculated as:

Ct=T/(ro*(wR)^2*A_rotor) ; Cq=Q/(ro*(wR)^2*A_rotor*R)


My doubts are:

1) The formulae for the coefficients used in the reference paper are different from the "canonic ones" used in fluent: where has the 0.5 factor gone?
Would it be wrong of me to assume they are in that form because the T (Q) in the "canonic formulae" would be for the single blade (Tsingle and Qsingle), while in the reference paper they are considering the whole two bladed rotor (2*Tsingle, and 2*Qsingle) ?

2) As far as fluent is concerned, if i model only a slab of the domain (180° in this case) imposing the periodicity of the flow, will the force/moment i obtain in output be the force/moment of the whole system (of both the blades toghether), OR of the single blade (to then be multiplyed for the number of blades) ?
(The second option would make probably more sense, if my assumption for n. 1) is correct).

3)I've read that i could approach the problem in two ways:

-use appropriate reference values. This way, the Cl and Cm obtained from the monitors should be the correct adimensionalized ones, and i get to see the force/moment using an appropriate Force Report.
In this case, for reference area i should use the whole rotor disc area, shouldn't i?

-forget about reference values and use the force/moment (from the aforementioned Force Report) to perform personally the conversion to coefficients.
In this case the values of Cl and Cm obtained from the monitors loose
significance, don't they?


Any input/suggestion you may give me would be greatly appreciated!