Problem with CD and CDv for an airship envelope

Luis Díaz

I'm studying the aerodynamics of the envelope of an airship and, although I thought I got correct results, further analysis showed that something is wrong and I don't know why.

Literature states that the drag produced by an airship envelope is mainly caused by skin friction, and pressure drag can be not considerated. It defines CD as:

CD=D/(0.5*density*V^2*A), where A is the envelope surface aerea. This CD is known as CDa. An airship is a buoyant craft and its lift is related to the buoyant volume, so another CD parameter is defined as:

CDv=D/(0.5*density*V^2*Volume^(2/3))

All correct by now?

I started simulating the envelope of 2 airships that I know the CD value. I used the K-epsilon viscous model, with a turbulence intensity of 7.5% and an angle of attack of 0º and I got results that fitted the CD values -> 'Report>Forces>(1 0 0)', setting as 'Report>Refernce values' the surface area and the inlet velocity in area and velocity boxes.

First question: This value that I got by that procedure, is the real CD value? Did I do something wrong?

Later I simulated an ellipsoid as envelope shape, changing the volume, the fineness (length/max diameter)and the 'position of the max. diameter/length', and the results were completely wrong: the CD is reduced in increasing the surface, it has not a minimum with a fineness of 3 or 4, etc.

Questions: If I don't change the default 'refence values', the number I get reporting the forces (1 0 0) for an angle of attack of 0º is the Drag?

Reporting a surface integral of the envelope shape and setting the value I get and the veloc. inlet value as 'reference values' for area and velocity, I get the CD value (I guess it is CDa value)?.

How can I get the CDv value with Fluent? Can I get it changing the 'reference values' or I have to calculate it externally using the Drag value?

One last question: In all the simulations I didn't got a correct Cp values (setting only as 'reference values' the envelope surface and the v_inlet value. I didn't change the values of length, pressure and the other ones). The behaviour of the Cp was correct but the values were not (I got a Cp higher than 1 at the stagnation point). Which reference values I have to set in order to get correct Cp values?

Thanks to anyone that can help me, because I have not a lot of time to present my study and I don't know how to solve this problem and continue.