What is the meaning of reverse thrust of airplanes when braking?
Most thrust reversers do turn the flow slightly forward – maybe 20 degrees or so. But the velocity at that point is pretty low, so the actual thrust in the forward direction created by that flow is pretty small. The way that the reverser works is to convert the gigantic momentum induced by the fan into a sudden stop in the axial sense relative to the engine case. This works essentially the same way if you just turn the flow straight out to the side without directing it forward at all. You don’t need to turn the flow back forward. You just need to pull that flow in like gangbusters with the fan – which it does very well when you run the engine up – and then stop it instead of pushing it out the back even faster. The clamshells, and the vents and the baffles do that. They put up a wall for that flow to hit as it is pulled in by the fan. Sometimes they turn it slightly forward in a pretty inefficient manner, but that is secondary (ADDED: This is why reversers do not really work very well
