Extracting hydrodynamic forces in the IBFE method

[Oscar Jin]

Hi there,

I'm currently using the IBFE method to simulate the self-propelled motion of a flexible fish body. 

I saw in IBFE/explicit/ex6 that we can get the net force on the body. But I believed that for fish swimming, it is a combination of thrust and drag forces. 

How can I extract the drag forces in IBFE, or more specifically, pressure and viscous forces?

Thanks.

[Boyce Griffith]

You can get net thrust and net drag by looking at the correct components of the net force. If you want point wise values, this is a little harder to do — I don’t think we have code that will do it, but at present, you will need essentially to extrapolate those from the fluid that is slightly away from the fluid-structure interface. The extrapolation should use only grid values that are not within the support of the regularized delta functions that link the structure and the background grid. You will need a very well resolved boundary layer to get accurate results. I have some ideas on how to do this in a more natural way that we are currently working with.

(One of the reasons that we have been working with the immersed interface method is that it gives a way to evaluate these kinds of forces accurately on the boundary of the structure, although that method seems to present some other practical difficulties.)

Thanks.

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[Oscar Jin]

Hi Prof. Griffith,

Thanks for the info! Could you please give me more explanations on obtaining net thrust and net drag by looking at the correct components of the net force? Does it mean positive/negative components on each node, or x/y components?

[Boyce Griffith]

On Nov 6, 2025, at 7:34 PM, Oscar Jin <oscarj...@gmail.com> wrote:

Hi Prof. Griffith,

Thanks for the info! Could you please give me more explanations on obtaining net thrust and net drag by looking at the correct components of the net force? Does it mean positive/negative components on each node, or x/y components?

I think you will want to use expressions based on discretized integrals of forces over the structure domain.

On Thursday, November 6, 2025 at 2:29:03 PM UTC-6 Boyce Griffith wrote:

On Nov 6, 2025, at 2:19 PM, Oscar Jin <oscarj...@gmail.com> wrote:

Hi there,

I'm currently using the IBFE method to simulate the self-propelled motion of a flexible fish body. 

I saw in IBFE/explicit/ex6 that we can get the net force on the body. But I believed that for fish swimming, it is a combination of thrust and drag forces. 

How can I extract the drag forces in IBFE, or more specifically, pressure and viscous forces?

You can get net thrust and net drag by looking at the correct components of the net force. If you want point wise values, this is a little harder to do — I don’t think we have code that will do it, but at present, you will need essentially to extrapolate those from the fluid that is slightly away from the fluid-structure interface. The extrapolation should use only grid values that are not within the support of the regularized delta functions that link the structure and the background grid. You will need a very well resolved boundary layer to get accurate results. I have some ideas on how to do this in a more natural way that we are currently working with.

(One of the reasons that we have been working with the immersed interface method is that it gives a way to evaluate these kinds of forces accurately on the boundary of the structure, although that method seems to present some other practical difficulties.)

Thanks.

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