[nea]

Hello everyone,
For a project I'm studying an airflow around a building (in my case this is just a 5x5x5 m cube), particulary the recirculation zone above the building. The idea is to see how the vortex changes when the temperature difference between the air and the building is 5, 15 and 30 K. The building is inside of a domain with the proper distances from the inlet and the outlet and from the top (based on scientific papers). The outlet is set to pressure outlet, left, right and top planes are symmetry planes. The velocity at the inlet is set to be constant at 0,5 m/s, k-epsilon model is used with enhanced wall treatment (y+ value is really high) and boussinesq model for the thermal effects. I'm using PISO algorithm with second order schemes.
I've encountered a problem, the vortex above the building does not appear when the temperature difference is 15 or 30 K but it is there when the temperature difference is 5 K. I've even tried 10 K but it is still not there.
If anyone has an idea why this is happening, I would greatly appreciate your input. Thank you in advance.