March 29, 2020 at 9:46 pmrbamSubscriber
I’m questioning about how do you define altitude in ANSYS Fluent.
My aim is to simulate the flow around an airfoil at Mach=0.73 at sea level and at an altitude for example 40 000 ft.
There are many discussions about it, but from my point of view, none of them answered properly and clearly about settings this parameter. I therefore create this discussion in order to receive answers to understand clearly how to define altitude by using examples at sea level and 40 000ft.
On my side, I’m confused between operating conditions, reference values, inlet-velocity BC with Supersonic/initial gauge pressure + outflow gauge pressure and Outlet-pressure BC with Gauge Pressure. It’s seem difficult to set correctly those values that’s why I am asking your help. (I’m using K-Omega SST as Turbulence Models)
Please find below settings for both cases/ examples :
Sea Level 40 000 ft
Altitude (ft) 0 ft 40 000
Pressure (Pa) 101325 19330
Temperature (K) 288,2 216,7
Air Density (kg/m3) 1,225 0,3109
Speed of Sound (m/s) 343 295
Kinematic Viscosity (m2/s) 1,46e-05 4,57e-05
Dynamic Viscosity (kg/m.s) 1,79e-05 1,42e-05
Can someone fill the attachement of what should be the values replacing interrogation marks for both cases and give us an explanation of how and why ?
Thank you in advance for your support,
March 30, 2020 at 11:00 amRobAnsys Employee
We just change the operating pressure and/or material properties if we can assume they're constant. You can't assume constant properties as gas is generally compressible over about 0.3M
I'd not worry too much about the solver for now. Review some of the videos on line and in the Fluent documentation. Then compare your mesh with those before returning to Meshing to fix yours.
March 30, 2020 at 11:53 amrbamSubscriber
When you said you change the operating pressure and/or material properties, so if I understand well you will define as 101300 Pa for Sea Level and 19330 Pa for 40 000 ft. and put 0 everywhere else ? I do not understand what I need to fix with my mesh ?
Could you expand upon that and explain more specifically what you mean with your previous answer,
April 1, 2020 at 10:37 amRobAnsys Employee
Pressure is about right, yes, and I assume you're using a compressible gas? Otherwise the change in operating pressure is pointless.
Have a look at the various aerofoil videos online (and via Help in Fluent) and compare the cell size, aspect ratio & growth rate with the tutorials.
April 1, 2020 at 12:56 pmrbamSubscriber
Thank you for your answer, I'll have a look for the meshing. Yes, I'm using ideal gas/compressible gas in material properties.
By reading Ansys Fluent Documentation and others, I understood that :
Gauge Pressure = Absolute Pressure of the System - Atmospheric Pressure (101325 Pa)
For example , Absolute Pressure of the System at 40 000ft will be, with previous data, 19330 Pa + 101325 Pa = 120655 Pa. Am I Right ?
And as the documentation says, if we set the operating pressure to zero instead of Sea Level (101325 Pa), gauge pressure and absolute pressure are equivalent. (Following picture helps me to understand ©engineeringtoolbox)
However, can you help me understand the following notions, supersonic/initial gauge pressure and outflow gauge pressure of the BC inlet-velocity ? And how to set them ? Are they equal ?
April 2, 2020 at 2:31 pmRobAnsys Employee
Operating pressure is the equivalent to gauge pressure. I think it's set to zero in most tutorials for aerofoils.
As you have compressible flow and nothing around the wing you'll also want to look at the far-field boundary: again it's demonstrated in the tutorials.
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