I am building a remote-controlled aircraft for a project and need to know the pressure distribution, forces and moments etc. The pressure above and below the wing is similar and negative, I am using a NACA aerofoil meaning this should not be the case (pressure should be positive below). When I animate, the iterations jump from 0 to 9442 instead of increasing continuously.
Hello, and thanks for using SimScale Forum!
Please note that we only deal with gauge pressures in an Incompressible simulation type in the platform. I understand that you would expect to have positive pressure values on lower surfaces of the wing in order to obtain a resultant lift. However, please be aware that air is still accelerating on the lower surface of the wing, consequently resulting in negative pressure values. The pressure drop on the lower surface will not be as high as it is on the upper surface due to camber of the airfoil which will in fact still generate a lift force in positive Z direction.
Velocity contours on a cutting plane through the wing:
Pressure contours on a cutting plane through the wing:
Regarding your second questions about the jumps in iterations, this is related with the Write Interval settings under the Simulation Control. As it is now set to 1.5e+4 which is also the maximum number of iterations, simulations results obtained in final iteration will be printed and saved only.
Since the simulation is completed with a "The specified maximum runtime of the job was exceeded. " message, the results are saved in 9442th iteration. If you’re interested in intermediary results, you can adjust Write Interval parameters as you wish.
Hope these are helpful,
Best.
Thank you this was very helpful. Was a bit uncertain when the forces in the z direction were below 1N.
edit: I forgot that the lift coefficient for this airfoil is slightly positive when the AOA is 0 hence the low lift.
I understand. As far as I can see the simulation setup seems alright.
What is the expected lift force in this configuration and weight of the vehicle? Does this configuration represent a cruise condition - I see there is a negative incident angle applied to the horizontal tail which will significantly reduce the force in Z direction.
The negative incident angle was a bit of an experimental setup, the expected weight of the vehicle is around 0.8kg, lift force should be around 11.5N. I am using another software to find out the predicted stall speed for this vehicle (which is the higher freestream velocity I set of 8.5ms^-1). This was my first run so just wanted to see how it would perform. I’m going to make the horizontal stabiliser level and increase AOA and see how that goes.