I’ve been trying to get rid of this non manifold edge in order to water tight it and put it into cfd. The non manifold edge is seen in pink after edgeanalysis command. However I am unable to do repair automatically using the meshrepair command. Would appreciate some help thank you very much. I’ve attached the file and photos here for reference.
Hi @Nicholas_Wong
First of all, your object is 2 km’s away from world 0 in mm’s, which is just asking for trouble. Move it to 0,0,0 and avoid a world of pain!
Other than that, the area with the non-manifold edge also has a bunch of overlapping poly’s. My best advice would be to delete the offending faces and re-build the area manually with Bridge and 3DFace (or whatever tools you see fit). Maybe something like this, depending on your design intent 
Could easily be optimized/smoothed out/look nicer by sliding a few vertices here and there - just a sketch-proposal 
HTH, Jakob
