How to calculate 'drag coefficient' in CFX? I am working on Dish antenna to calculate 'cd', but am unable to get for different angle of attack. I have created Domain, and used this expression:
dragcoeff: force_x()@dish/(0.5*airrho*speed^2*area)
airrho: 1.2250[kg/m^3]
speed: 16.67[m/s]
area: which area I have to choose for my model? Whether dish front area or surface area? and for different angle of attack, when the flow is on x-axis, how to change the area values. (as if the flow is on x axis, and the dish is facing upward, then which area should be consider?
These are the expressions I have given for my model..whether these expressions are correct or not? Please suggest the expression for finding drag coefficient
Dear Muhammad,
This area used in the mentioned equation will be used for non-dimensionalization of the coefficient, so any area (even area = 1) can be a correct choice, but maybe unconventional. Generally the cross section area (or the area you can see from top view of the dish) has been used.
regards.
Typically, the surface area in the drag force equation is the effective surface area perpendicular to the flow. So, in your case it will depend on how the dish is oriented with respect to the direction of the flow.
Drag force is evaluated for shear area, while lift force is determined with normal area. Angle of attack is one, how object is placed with direction of flow . Drag force is evalated with measuring device like wind tunnel and from the relation,
Drag force = 0.5 xDrag coeff x surface areax (relative velocity) ^2 x density of fluid
Knowing drag coeff and angle of attack, the optimum angle of attack can be found out with graph drag coeff Vs angle of attack.
The absolute value of the drag is unable to compare the aerodynamic performance of two different solids: a solid twice as big can have a force twice as high. Similarly, the faster the flow, the higher the drag (everything else being equal). Non-dimensionalizing this force by characteristics of the flow and of the geometry is the purpose of employing drag coefficients. The characteristic of the flow is the freestream dynamic pressure which can be expressed as 0.5*rho*V^2. As for the reference surface, for cars it is usually the frontal area while for aircraft it is the projection of the wing surface area.
To conclude, drag coefficients are necessary for comparing the aerodynamic performance of different geometries (and flow conditions). If you wish to compare the aerodynamic efficiency of two dish antenna, you could take the area of the projection of the antenna perpendicular to the incoming flow.
The surface area exposed / in contact / normal to fluid flow is considered in the determination of the pressure drag. In the case of inclined surface, the area normal to flow (which is reduced due to inclination) is to be considered.
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