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Negative propulsive efficiency of a moving airfoil

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Old   March 3, 2007, 07:09
Default Negative propulsive efficiency of a moving airfoil
  #1
zonexo
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Hi,

I'm doing my simulation on moving airfoil. I found that for some configuration, I got negative propulsive efficiency. Is that possible?

Power input is defined as the energy required to move the airfoil. I've got a positive value.

Power output is defined as Thrust * free flow velocity. In these cases, drag was mostly generated instead of thrust, hence resulting in the average Power output over 1 cycle negative.

I also wonder if there's something wrong with my solver instead, ie such results are definitely wrong...

Btw, I'm using a cambered airfoil. Previous test with naca0012 resulted in only postive efficiency.
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Old   March 3, 2007, 13:05
Default Re: Negative propulsive efficiency of a moving air
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H
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are you using the relative velocty of your moving arifoil?

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Old   March 4, 2007, 07:59
Default Re: Negative propulsive efficiency of a moving air
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zonexo
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what do you mean? The energy required to move the airfoil was given by P(in)=-lift force* dh/dt - Moment* d(theta)/dt

where dh/dt is the absolute up/down velocity of the airfoil.

P(out)= thrust*absolute free flow velocity, which is 1 in this case

thank you.
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Old   March 6, 2007, 09:33
Default Re: Negative propulsive efficiency of a moving air
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Mani
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The way the propulsive efficiency is defined, there is nothing fundamentally wrong with a negative value. It simply means you create negative thrust (or drag). If you think this shouldn't be the case, check on the motion you are assigning (is it backwards?) and compare the flow solution with your previous solution on the symmetric airfoil. Anyway, it's very possible to create negative thrust, and hence, negative propulsive efficiency, as any hummingbird can tell you.
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Old   March 7, 2007, 01:16
Default Re: Negative propulsive efficiency of a moving air
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zonexo
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Thanks everybody. I found that I've made some mistakes in the motion, which resulted in drag being produced most of the times. In the correct motion, thrust is generated ;-). However, as what Mani says, it's definitely possible, based on the definition.

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