Suction caused by an air jet acting on a plate, why?

[ritchT]

Homework Statement

We set up in a lab a vertical pointing air jet giving a constant output and gradually increased the height of the plate above the jet whilst measuring the force upon it as imparted by the jet using a Newton-metre. At a big distance the air jet was imparting an upthrust on the plate but when the plate was around 5cm from the exit of the air jet the plate was subjected to a suction force.

How can this be possible, is it something to do with the pressure above and below the plate?
Thanks for any explanation

 

[rcgldr]

Is this a flat plate? One possible effect is that the flow is directed radially outwards from the center of the plate. This causes a reduction in pressure of that flow as the radius of the flow increases. If the pressure of that radial flow were reduced below ambient, then it would tend to draw the surrounding air due to pressure differential and viscosity, and the radial flow past the edges could result in vortices. Somehow all of these combined effects are reducing pressure.

Is there some way you can put a small streamer at the end of a thin rod and position the streamer at various points near the plate to visualize the flow?