I'm not a technician, but about tho flow-vis on the F138's floor i think:
1) flow around the sidepods is not bad;
2) problem come out when the front flow has a strong impact with exhaust gasses: looking at the following image (i only have evidenced the flow-vis better to see where flow goes), you can see big turbolence when front flow impact with gasses. So, the exhaust gasses don't "energized" the flow, but stop it! You can see the flow-vis go on the left and on the right of the exhaust flow.
3) the exhaust flow hits a too big area on the floor; he should go only between tyre and diffuser, to seal it.
By the way, it's my opinion and, as i told, i'm not a technician...