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Flow over sphere low reynolds number

Posted Oct 28, 2010, 7:24 p.m. EDT Fluid & Heat 8 Replies

Kadir Gökhan Güler
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?nlet velocity is 200e-6 m/s and diameter of sphere is 5e-6 m . I want to calculate the drag force acting on the sphere but when i compare it with the analythical solution which is 6*pi*eta*U*R the results are completely different . Please help
8 Replies Last Post May 29, 2011, 9:32 a.m. EDT
Ozgur
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Posted: 1 decade ago Oct 28, 2010, 11:02 p.m. EDT
Kadir,
I had the same problem before when I tried to compare calculated drag to analytical solution. Two observations:
1- drag on a body is extremely sensitive to "wall effects". So your computational domain should be very large compared to the size of the sphere. To ensure that computational domain boundaries are not affecting your results, I suggest you do a domain enlargement study and watch how calculated drag asymptotes to a fixed value.
2- accurate calculation of forces is important. You may want to use reacf operator rather than integrating the stress tensor.

Good luck
Ozgur
Kadir, I had the same problem before when I tried to compare calculated drag to analytical solution. Two observations: 1- drag on a body is extremely sensitive to "wall effects". So your computational domain should be very large compared to the size of the sphere. To ensure that computational domain boundaries are not affecting your results, I suggest you do a domain enlargement study and watch how calculated drag asymptotes to a fixed value. 2- accurate calculation of forces is important. You may want to use reacf operator rather than integrating the stress tensor. Good luck Ozgur
Kadir Gökhan Güler
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Posted: 1 decade ago Oct 29, 2010, 4:34 a.m. EDT
Thanks Ozgur I will try but what you mean by reacf ?
Thanks Ozgur I will try but what you mean by reacf ?
Ozgur
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Posted: 1 decade ago Oct 29, 2010, 11:57 a.m. EDT
just run a search for reacf in the help documentation and you should be able to find plenty of information and examples.
just run a search for reacf in the help documentation and you should be able to find plenty of information and examples.
Mehrdad Bagheri
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Posted: 1 decade ago Apr 8, 2011, 6:53 p.m. EDT
Hi,
I want to find drag force over a sphere and I have the same problem, have you found the solution?
How can I find the equation of reacf?

Thanks
Hi, I want to find drag force over a sphere and I have the same problem, have you found the solution? How can I find the equation of reacf? Thanks
Nagi Elabbasi Facebook Reality Labs
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Posted: 1 decade ago Apr 11, 2011, 3:25 p.m. EDT
If your flow is say in the Z-direction then the variable you need is reacf(w). You should sum that variable over the surface of the sphere. If you want to get the Stokes drag force (the equation Kadir provided in the beginning of the thread) you should select “Creeping Flow” for the fluid flow, otherwise another drag contribution proportional to the square of the velocity could be more dominant.

Nagi Elabbasi
Veryst Engineering
If your flow is say in the Z-direction then the variable you need is reacf(w). You should sum that variable over the surface of the sphere. If you want to get the Stokes drag force (the equation Kadir provided in the beginning of the thread) you should select “Creeping Flow” for the fluid flow, otherwise another drag contribution proportional to the square of the velocity could be more dominant. Nagi Elabbasi Veryst Engineering
Eduardo Ortiz
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Posted: 1 decade ago May 26, 2011, 5:05 a.m. EDT
Hi, I am working with a 3D model of a flow past a sphere as well.
My computer lab doesn't have the CFD module, so I don't have the creeping flow option.
I have done a very large domain vs the sphere size, sphere (r=0.05m), U_0=0.001m/s, rho=1e3kg/m^3,
eta=0.9e-3Pa*s.
My problem is that I still have a problem calculating the drag force, doing the integration
over the sphere surface I always find exactly twice the analytic value of the drag force.
I can't find what's wrong, Nagi you mention that there is a drag contribution proportional to the square of the velocity could be more dominant.
My domain is a cylindric channel, I've tried with walls as boundaries and also with symmetric boundaries to make the model believe that the domain is essentially infinite and I always find the twice the analytic value.
Any ideas?
Thanks in advance.
Hi, I am working with a 3D model of a flow past a sphere as well. My computer lab doesn't have the CFD module, so I don't have the creeping flow option. I have done a very large domain vs the sphere size, sphere (r=0.05m), U_0=0.001m/s, rho=1e3kg/m^3, eta=0.9e-3Pa*s. My problem is that I still have a problem calculating the drag force, doing the integration over the sphere surface I always find exactly twice the analytic value of the drag force. I can't find what's wrong, Nagi you mention that there is a drag contribution proportional to the square of the velocity could be more dominant. My domain is a cylindric channel, I've tried with walls as boundaries and also with symmetric boundaries to make the model believe that the domain is essentially infinite and I always find the twice the analytic value. Any ideas? Thanks in advance.
Danial Tz
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Posted: 1 decade ago May 27, 2011, 3:51 a.m. EDT

Hi, I am working with a 3D model of a flow past a sphere as well.
My computer lab doesn't have the CFD module, so I don't have the creeping flow option.
I have done a very large domain vs the sphere size, sphere (r=0.05m), U_0=0.001m/s, rho=1e3kg/m^3,
eta=0.9e-3Pa*s.
My problem is that I still have a problem calculating the drag force, doing the integration
over the sphere surface I always find exactly twice the analytic value of the drag force.
I can't find what's wrong, Nagi you mention that there is a drag contribution proportional to the square of the velocity could be more dominant.
My domain is a cylindric channel, I've tried with walls as boundaries and also with symmetric boundaries to make the model believe that the domain is essentially infinite and I always find the twice the analytic value.
Any ideas?
Thanks in advance.


reacf() gives the most accurate solution, check. however, if you want to see if you've done a mistake or not, you can just integrate the fluid forces in the x direction, and have a rough estimate. The more finer mesh, the closer it is to reacf value (with large meshes it is far off as well). If still high, you may write your steps down and we can check (preferably in a new forum thread)



--
Comsol 4.1
Ubuntu 10.04.1
[QUOTE] Hi, I am working with a 3D model of a flow past a sphere as well. My computer lab doesn't have the CFD module, so I don't have the creeping flow option. I have done a very large domain vs the sphere size, sphere (r=0.05m), U_0=0.001m/s, rho=1e3kg/m^3, eta=0.9e-3Pa*s. My problem is that I still have a problem calculating the drag force, doing the integration over the sphere surface I always find exactly twice the analytic value of the drag force. I can't find what's wrong, Nagi you mention that there is a drag contribution proportional to the square of the velocity could be more dominant. My domain is a cylindric channel, I've tried with walls as boundaries and also with symmetric boundaries to make the model believe that the domain is essentially infinite and I always find the twice the analytic value. Any ideas? Thanks in advance. [/QUOTE] reacf() gives the most accurate solution, check. however, if you want to see if you've done a mistake or not, you can just integrate the fluid forces in the x direction, and have a rough estimate. The more finer mesh, the closer it is to reacf value (with large meshes it is far off as well). If still high, you may write your steps down and we can check (preferably in a new forum thread) -- Comsol 4.1 Ubuntu 10.04.1
Nagi Elabbasi Facebook Reality Labs
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Posted: 1 decade ago May 29, 2011, 9:32 a.m. EDT
Getting exactly twice the drag force is most likely due to an input error or typo. It’s not the square velocity term because you are using creeping flow. Can you post the drag force value that you’re getting and the expression you use in COMSOL to calculate it?

Nagi Elabbasi
Veryst Engineering
Getting exactly twice the drag force is most likely due to an input error or typo. It’s not the square velocity term because you are using creeping flow. Can you post the drag force value that you’re getting and the expression you use in COMSOL to calculate it? Nagi Elabbasi Veryst Engineering

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