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I've done an experiment where we direct an air jet from a fan onto a set of electronic scales. By measuring the velocity of the air jet at the exit of the fan nozzle, and applying the conservation of linear momentum, I've found the force the jet exerts on the scales, and found this agrees pretty well with the reading on the scales.

Next, I put a large wooden plate about 10cm behind the scales, normal to the top plate of the scales. This causes the reading on the scales to increase. My understanding is that this is due to a lower pressure in the area inbetween the back of the scales and the added wooden plate, which then exerts an extra pressure force on the scales.

My question is, why/how? This is one of those (in my opinion) awful "you don't know this yet, but make something up" questions that some professors seem to like, so I'm more interested in an idea of the phenomena at play than great detail.

An additional minor confusion; I used a thin metal rod with a piece of light string to hold in the flow in order to see the direction of flow. Without the additional plate, the flow direction was in the same plane as the scales. With the extra plate added, the flow direction now starts in the same plane at the edge of the top plate of the scales, but then starts to curve in towards the extra plate. How can, seemingly, a plate placed in a place where there is no very little flow affect the direction of flow upstream?

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Could you add a sketch or photo to clarify the configuration? – Ghillie Dhu Mar 18 '13 at 19:47
This is a guess since there is no figure, but I doubt your professor was making things up arbitrarily. It sounds like you increased the effective drag on your instrument by adding the wooden plate (WP). Normally, the air would flow around your sensor plate and create the typical turbulent wake in the downstream. However, with the added WP, you disrupt this flow. If the WP is close enough to the sensor, you can create a vacuum-like region behind the sensor, thus increasing the pressure. – honeste_vivere Oct 18 '14 at 14:29

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