Well, it does rather depend on what is upstream and what is downstream, and what is flowing through it.

Mechanical Engineers don't do process fluids very well....

I forget to mention that Helium gas is flowing from upstream (i.e. rotor side) to stator side (downstream) where pressure variation is required from low pressure to high pressure of 22 bar. During the start of operation, downstream pressure is 8 bar and due to intersection area between stator and rotor, area increases as time passes and so we want pressure to vary along with area. How much variation in pressure we get if we have known area variation? Is there any software in which this simulation can be done?

It is vital to know the flow rate as well as the actual shape and length of the pipework either side, it will be very non linear and may not lend itself to simulation.
I'd suggest making a model and testing it. I'd go for a thick rotor with smoothly blended edges.
My guess is most of the pressure change will be in the first few millimeters of opening.
Del

No temperatures stated yet and the forum still doesn't know what sizes are involved, either of the valve or the pipework either side. Is there anything else that has been forgotten?

So far, all that can be said is that pressure will vary from 8 to 22 bar.

http://www.parcol.com/docs/1-I_gb.pdf?bcsi_scan_03DEEAA357C9E78B=0&bcsi_scan_filename=1-I_gb.pdf

So is this thing intended to be a pressure control valve, or just a variable orifice?

What are the process conditions that would make one select one of these over and above anything else?

It sounds as though it is being designed as an exercise and without any true purpose in mind at the moment.