Please excuse a potentially stupid question. I'm quite new to CFD but have narrowed down my problem setup and I'm looking for some advice.
I'm modelling an external flow of M=2 for a simple lift and drag value of an aircraft design. I'm using the following setup:
- Density-based Solver
- Pressure Far-field
- K-w SST model
- Ideal Gas as air density
- Sutherland as Viscosity
There are obviously many other parameters, but my main question is: if I know the cruise altitude at which I'm trying to analyze (18,300m), and I therefore know the dynamic pressure and viscosity, and temperature of the air at that altitude, are these actually accounted for by the ideal gas and sutherland models?
It seems counter-intuitive to me that there would be no difference in the result if I were to be travelling at M=2 at sea level or at 18,300m. Am I missing something here? Can the program tell the difference?
Any help would be very valuable, I appreciate I may be lacking some knowledge of the fundamental physics at work here.
Kind regards
Ed
at your altitude static temperature is about 216.5 k, whereas at sealevel it is 288k. At M=2 your stagnation temperature is function of static temperature and given by relation To/T = ( 1+(gamma-1)*0.5*M^2); so stagnation temperature is different at sealevel and at your altitude. So change the values of boundary conditions of temperature.
Rest the models will take care. Enabling viscous heating will give better results for density based model. Far field model will do good outside boundary layers.
Using the ideal state equation, you can write M=v/a=v/sqrt(gamma*R*T). If you want to fix M=2 for every altitude, that drives to change v according to the temperature at the chosen altitude, to take constant the rate. But when you change v you will change also the Reynolds number and, therefore, the flow problem.
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