Hi.... I need some advice for the leakage problem in my experimental model. I found a very tiny crack on the model that suck the air flow from outside (it's a suction type system). Since the crack is on the upper surface of a diffusing bend, will this increase or decrease the flow separation already existed in the region?

Thank you for any advice here.

There is a chance that the crack doesn't effect the flow too much. That depends on the size of the crack, the mass flow going through it, and the main flow over the crack. Do you actually observe separation? If yes: where is the crack located relative to the separation point (upstream, downstream...)?

Hi Mani... I use the tuft to visualize the flow separation. Reverse flow already presented upstream of the crack. I'm wonder if the data that I collect previously can still be used (I've already returned some of the measuring equipment). I'm really not sure when this crack start to develop and I hope someone here can give me some hints of how to evaluate this problem. It's just a very tiny crack anyway.

The only way that you will ever be sure - & not subject to guessing - is to plug the crack, borrow the measuring equipment again & run a few cross-check experiments.

diaw...

If the separation point is upstream of the crack, as you say, there is a good chance that the crack has no influence on separation. If it was downstream of the crack it would be a different issue, so you may be lucky. This is all you can say, without rerunning your experiments. If you do have a chance to rerun the tests, fill the crack with epoxy and carefully polish the surface when it's hardened. Then run just one of the tests that you did before, preferably one that's critical to your investigation. However, if the crack is very tiny compared to other scales of your experiment (for example compared to the size of the separation region), and the wall is otherwise smooth, I find it very unlikely that it should have any influence on the flow. The crack presents a small cavity with flow passing over it almost as over a smooth wall.