How to find force from volumetric flow rate and pressure

AI Thread Summary
The discussion focuses on calculating thrust for a hovercraft using a fan's performance data, specifically relating volumetric flow rate and pressure to force. The key equation to consider is Fthrust = F0 + γ*v, where γ represents the slope of the relationship between force and velocity. The participant has determined a slope of -0.285 [m3/s/mmH20] and is exploring how to interpret the graph correctly to find F0, the y-intercept, which represents thrust at zero pressure. There is confusion about the correct orientation of the graph, with some suggesting it is already correctly oriented with pressure on the vertical axis. The conversation emphasizes the need to clarify how pressure relates to force and volumetric flow in this context.
bl965
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1. Homework Statement
Hi. This is a problem related to designing a RC hover craft. We are given a datasheet for a fan used as thrust.
http://imgur.com/6liZSid Sorry image post no working, url: http://imgur.com/a/6XLwL
We have to approximate the equation to a straight line through the curve. Then relate it to Fthrust = F0+ γ*v. We have to find γ and what F0 represents.

Homework Equations


Q = velocity * Area
F = mass * acceleration

The Attempt at a Solution


I found that the slope is -0.285 [m3/s/mmH20]. To find F0, I think the graph has to be turned on its side: pressure is the independent variable and volumetric flow is the dependent. If pressure is zero, I can find F0 which is the y intercept. But how to relate force to volumetric flow or velocity.
 
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If P is the pressure and A is the area that the pressure acts on, what is the force?
 
If pressure is 0, volumetric flow is high, and small pressure * area is small force. As pressure increases, there will be more force with less flow. Makes sense. From start the fan is operating at atmosphere. I get high results when I search for one atmosphere in mm of water. Am I on the right track?
 
bl965 said:
If pressure is 0, volumetric flow is high, and small pressure * area is small force. As pressure increases, there will be more force with less flow. Makes sense. From start the fan is operating at atmosphere. I get high results when I search for one atmosphere in mm of water. Am I on the right track?
I have no idea. Can you please provide an exact problem statement?
 
(b) We can relate this linear relationship to Fthrust = F0-γv [Note: γ is a constant representing the slope of the F(v) line]. For each fan, based on your approximation in (a), what are the values for F0 and for γ?
 
bl965 said:
I think the graph has to be turned on its side:
No, it looks the right way around already, with pressure on the vertical axis. Seems to me you flipped it around in order to get:
bl965 said:
I found that the slope is -0.285 [m3/s/mmH20].

bl965 said:
how to relate force to volumetric flow or velocity.
What do you think relates volumetric flow through an aperture to the linear flow rate through it?
As Chester asks, what relates pressure to force?
 
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