I won't be able to check your case files as ANSYS Stuff. Just a question: do you need a radiation model in your case? Why are you using explicitely P1 model (generally used for thick media).
Just help your self by debugging the case by switching off radiation, just applying the heat from solar radiation and paying more attention to the flow dynamic settings. Check the pressure resulting from your UDF. The pressure returned will be the static gauge pressure at your outlet (pressure-outlet). Reversal flow means that either it recirculate due to non-proper pressure field or it's physical. Turbulence is quite high at your outlet where I assume free-stream (stagnant flow) if the boundary is well far away..
Be aware this is an open community so everyone would might access your case. I will remove the link in your posting. You will then just share with the community members who wants to help you directly.
I hope the community will chime in here.
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