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Lift and Drag at Transonic Speeds are Orders of Magnitude from realistic values

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Old   March 22, 2021, 15:51
Default Lift and Drag at Transonic Speeds are Orders of Magnitude from realistic values
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I am relatively new to CFD, though I have done some before. This is my first time using Fluent, which I'm using to test lift and drag at a variety of Mach numbers and the results I'm getting are very odd (e.g. Drag force of 1e8). I will start at the beginning to explain what I'm doing.

Following a Youtube tutorial, I started by building an aerofoil in Solidworks and importing it into the DesignModeler. The airfoil was rotated to a certain angle to represent the angle of attack. I created a reference volume around the aerofoil (extruded to the same depth as the airfoil itself) and used a Boolean operator to subtract the aerofoil from the reference volume.

i.e. [IMG] [/IMG]

I then created a mesh for this geometry. The mesh itself just a basic automated one, since at this point, I'm just looking for a rough answer.

Following the tutorial, I setup the simulation in Fluent. The viscous model used is Spalart-Allmaras and the fluid is air, with the air's density determined using the ideal gas equations. The speed of the airflow at the velocity inlet was set as a input parameter so I could see how the lift and drag vary with the Mach number of the airflow. The output parameters were the lift and drag force evaluated across the aerofoil walls.

I then set this simulation to run for a range of Mach numbers from 0.5 to 1.5. The simulation runs fine at Mach=0.5 and Mach=0.6, returning drag values of 98N and 238N respectively after a run time of 5-10 minutes. Unfortunately at Mach=0.7, the simulation takes well over an hour to run and returns a drag of 3000N, and for Mach=0.8, it says the drag is around 1x10^8N, which is quite obviously wrong.

Any advise? I have added a link at the bottom to a zip file containing the simulation if you wish to look over it yourself.


Link: Fluent - 34.8 MB
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