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How to tell COMSOL to balance the drag force and gravity?
Posted 5 apr 2012, 21:28 GMT-4 Fluid & Heat, Computational Fluid Dynamics (CFD) Version 3.5a, Version 5.2 5 Replies
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Hi all,
I have a question related to how COMSOL calculates the drag force and gravity force in a "parachute" model. Imagine that an object is falling down to the ground at a high velocity because of the gravity. When its parachute is opened, the gravity is still the same, but a drag force which is against gravity is generated. Based on stokes law, the drag force can be calculated as "-6*pi*viscosity_air*radius_object*velocity_object". The higher the velocity of the falling object, the higher the drag force will be. After a short while, the drag force will balance the gravity so that the object is considered to be force equibalance. The object will fall down in a constant velocity, and will be finally landed.
However, this situation is not happening in COMSOL simulation. When COMSOL calculates the drag force from the velocity produced by gravity, the drag force is even higher than gravity, which means that the object is driven by drag force and moving up against the gravity. The reason for that, I think, is COMSOL doesn't know how to simulate the situation when two forces are balanced. What should we do to sovle this problem?
Thank you very much in advance!
Sam
I have a question related to how COMSOL calculates the drag force and gravity force in a "parachute" model. Imagine that an object is falling down to the ground at a high velocity because of the gravity. When its parachute is opened, the gravity is still the same, but a drag force which is against gravity is generated. Based on stokes law, the drag force can be calculated as "-6*pi*viscosity_air*radius_object*velocity_object". The higher the velocity of the falling object, the higher the drag force will be. After a short while, the drag force will balance the gravity so that the object is considered to be force equibalance. The object will fall down in a constant velocity, and will be finally landed.
However, this situation is not happening in COMSOL simulation. When COMSOL calculates the drag force from the velocity produced by gravity, the drag force is even higher than gravity, which means that the object is driven by drag force and moving up against the gravity. The reason for that, I think, is COMSOL doesn't know how to simulate the situation when two forces are balanced. What should we do to sovle this problem?
Thank you very much in advance!
Sam
5 Replies Last Post 3 ago 2017, 15:17 GMT-4
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Posted:
1 decade ago
Hi Sam,
I think what COMSOL is calculating makes sense. If the parachute is going down at a high velocity and then opens the initial drag force will be higher than the gravitational force causing deceleration, and it will decrease with decreasing velocity until it is equal to the gravity force.
Nagi Elabbasi
Veryst Engineering
I think what COMSOL is calculating makes sense. If the parachute is going down at a high velocity and then opens the initial drag force will be higher than the gravitational force causing deceleration, and it will decrease with decreasing velocity until it is equal to the gravity force.
Nagi Elabbasi
Veryst Engineering
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Posted:
1 decade ago
Hi Nagi,
Thank you very much for your reply. I generally agree with what you said. But there is one thing I don't understand. You said that if the parachute is going down at a high velocity and then opens the initial drag force will be higher than the gravitational force causing deceleration, then in COMSOL the object will move up again, instead of falling down because the drag force overcomes the gravitational force. Finally, the object is not falling down, it will keep going up until it hits the boundary and stops. So, I don't think what COMSOL is calculating makes sense.
Sam
Thank you very much for your reply. I generally agree with what you said. But there is one thing I don't understand. You said that if the parachute is going down at a high velocity and then opens the initial drag force will be higher than the gravitational force causing deceleration, then in COMSOL the object will move up again, instead of falling down because the drag force overcomes the gravitational force. Finally, the object is not falling down, it will keep going up until it hits the boundary and stops. So, I don't think what COMSOL is calculating makes sense.
Sam
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Posted:
1 decade ago
When the drag force exceeds the gravitational force the object decelerates not moves up. Its velocity will drop but not until it reaches zero. Way before that, the drag force (which is dropping as it depends on velocity) will be equal to the gravitational force and at that point you should see no further change in velocity.
Nagi Elabbasi
Veryst Engineering
Nagi Elabbasi
Veryst Engineering
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Posted:
1 decade ago
Hi Nagi,
Thanks for your answer. I now fully understand your answer. But actually, my situation is a little bit more complicated. The "gravitational force" in my case is decreasing during the whole process. And this force is set to be position sensitive. It will even change direction (point up to the sky or point to the ground) at different location in COMSOL model. If the "gravitational force" is pointing up to the sky and coupled with the drag force. It will finally make the object go up, right?
Sam
Thanks for your answer. I now fully understand your answer. But actually, my situation is a little bit more complicated. The "gravitational force" in my case is decreasing during the whole process. And this force is set to be position sensitive. It will even change direction (point up to the sky or point to the ground) at different location in COMSOL model. If the "gravitational force" is pointing up to the sky and coupled with the drag force. It will finally make the object go up, right?
Sam
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Posted:
7 years ago
Hi Sam ^^
I wanted to know since you did already calculates the Drag Force how did you do that I can't simulate it right now and I am using as a physics " Fluid Flow " and " Particle Tracing for Fluid Flow " so I have searched in many topics but they don't give more details into the steps to calculate it ?
By the way I want to use it in order to know the Drag force that exert it the Fluid on the particules on it ^^
I would really appreciate your help since I have this problem for months ^^
Thanks for your attention.
Kind Regards,
Imène Oualid.
I wanted to know since you did already calculates the Drag Force how did you do that I can't simulate it right now and I am using as a physics " Fluid Flow " and " Particle Tracing for Fluid Flow " so I have searched in many topics but they don't give more details into the steps to calculate it ?
By the way I want to use it in order to know the Drag force that exert it the Fluid on the particules on it ^^
I would really appreciate your help since I have this problem for months ^^
Thanks for your attention.
Kind Regards,
Imène Oualid.