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How to consider the weight of the flying body by CFD simulations?

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Old   November 28, 2017, 11:32
Default How to consider the weight of the flying body by CFD simulations?
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Nazanin Ansari
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Hey everyone,
i am using a CFX simulation of a wingsuit flyer in order to obtain the lift and drag values to compare them with the wind tunnel experimental data. The question that encountered to me today is that, i have experimental results of two pilots, same size and form, but different weights (about 15 kg) that wear the same suit in experiments and generate different results (i know that the position held by the pilot is different and each small detail matters). My question is when the body is extruded from the computational domain (by Boolean) how could the weight of the flyer be considered or have an influence in the calculations??Is there any way to define the density of the pilot, so the gravity force is different for each of them?
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Old   November 28, 2017, 17:27
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Glenn Horrocks
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The normal way of approaching this type of modelling is to do CFD models to get lift and drag only. The mass of the object does not come into it.

Then if you want to work out the different flight dynamics of objects with different masses you do a simple force balance (ie pen and paper, not CFD) use the lift and drag numbers from the CFD and add the mass force and any other forces you want to add. If you are looking for the case when both of these masses are in level flight you will find the heavier one will need more lift (obviously), which means he will probably have to pitch the wing up slightly to generate the extra lift. This will generate extra drag, and so on.
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Old   November 29, 2017, 03:07
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Nazanin Ansari
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thank you very much Gerrocks. It hit my mind for many days now and you brightened that up for me. So thankfull for your reply So the lift and drag calculated by CFD is just dependant on the angle of attack, velocity and the flying form posture itself?
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Old   November 29, 2017, 17:40
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Yes, the lift and drag should be just a function of the geometry of the device - AOA, posture; and air speed as well of course.

Of course this is a simplification, in the real situation the loads imposed by aerodynamic forces will deform the structure (ie buffeting will cause the person to wobble around a bit) and that buffeting may have a significant effect on the flight performance. But this is the nature of engineering - you have to decide how much of the physical details you wish to account for and what is "close enough".
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