Suggestion / Idea Changing Medium Parameters for Cloth Physics in ObiCloth: From Air to Water

[jacobwoord]

Hello everyone,

I have been working on a project in Unity that requires me to create a realistic simulation of a trawler net in water. To achieve this, I've successfully defined a proper mesh for my trawler net and utilized the ObiCloth plugin to apply cloth physics, which has produced impressive results thus far. However, I've encountered an issue that I'm hoping to get some guidance on.

The default settings of the ObiCloth plugin seems to imply that the cloth physics operate in air. Given that my simulation takes place underwater, I would like the physics to more accurately reflect this environment. Therefore, my question is: is there a way to modify the medium parameters in ObiCloth such that the cloth physics act as if they are operating in water rather than in air?

If this is possible, could someone please provide guidance on how to adjust these parameters appropriately to mimic water physics? If it's not possible within the scope of ObiCloth, does anyone have recommendations for other plugins or methods I could use to achieve this effect?

I appreciate any insight you can offer. Thank you in advance for your assistance.

Link from current result: https://www.youtube.com/watch?v=ISbrbxXkut4

Best regards,
Jacob Woord

[josemendez]

Hello everyone,

I have been working on a project in Unity that requires me to create a realistic simulation of a trawler net in water. To achieve this, I've successfully defined a proper mesh for my trawler net and utilized the ObiCloth plugin to apply cloth physics, which has produced impressive results thus far. However, I've encountered an issue that I'm hoping to get some guidance on.

The default settings of the ObiCloth plugin seems to imply that the cloth physics operate in air. Given that my simulation takes place underwater, I would like the physics to more accurately reflect this environment. Therefore, my question is: is there a way to modify the medium parameters in ObiCloth such that the cloth physics act as if they are operating in water rather than in air?

If this is possible, could someone please provide guidance on how to adjust these parameters appropriately to mimic water physics? If it's not possible within the scope of ObiCloth, does anyone have recommendations for other plugins or methods I could use to achieve this effect?

I appreciate any insight you can offer. Thank you in advance for your assistance.

Link from current result: https://www.youtube.com/watch?v=ISbrbxXkut4

Best regards,
Jacob Woord

Hi Jacob,

The medium in which the cloth exists isn't simulated at all, neither air nor water.

However, some aerodynamic effects was well as isotropic velocity damping are simulated.
You can independently adjust the drag an lift coefficient of aerodynamic constraints:
http://obi.virtualmethodstudio.com/manua...amics.html

You can also adjust velocity damping, which should help sell the effect that the cloth is underwater. See the "damping" parameter of the solver:
http://obi.virtualmethodstudio.com/manua...olver.html

kind regards,

[jacobwoord]

Hi Jacob,

The medium in which the cloth exists isn't simulated at all, neither air nor water.

However, some aerodynamic effects was well as isotropic velocity damping are simulated.
You can independently adjust the drag an lift coefficient of aerodynamic constraints:
http://obi.virtualmethodstudio.com/manua...amics.html

You can also adjust velocity damping, which should help sell the effect that the cloth is underwater. See the "damping" parameter of the solver:
http://obi.virtualmethodstudio.com/manua...olver.html

kind regards,

Hi,

Thank you for your prompt response. I have noticed the ability to adjust drag and lift coefficients in ObiRope, and this does seem like a promising path.

However, in our pursuit of creating a highly realistic trawler net simulation, we aim to factor in the specific properties of the net’s material and the underwater medium as accurately as possible. Given this, I wanted to ask if there is a recommended method for calculating these drag and lift coefficients to closely mirror real-world conditions?

If this isn’t feasible within the scope of cloth physics in ObiCloth, would it perhaps be more suitable to use a line renderer coupled with custom physics scripting to achieve our desired level of realism? 

Thank you in advance for your assistance.

Best regards,
Jacob Woord

[josemendez]

Hi,

Thank you for your prompt response. I have noticed the ability to adjust drag and lift coefficients in ObiRope, and this does seem like a promising path.

However, in our pursuit of creating a highly realistic trawler net simulation, we aim to factor in the specific properties of the net’s material and the underwater medium as accurately as possible. Given this, I wanted to ask if there is a recommended method for calculating these drag and lift coefficients to closely mirror real-world conditions?

If this isn’t feasible within the scope of cloth physics in ObiCloth, would it perhaps be more suitable to use a line renderer coupled with custom physics scripting to achieve our desired level of realism? 

Hi!

This isn't related to cloth simulation as much as it is to fluid simulation.

Obi assumes wind direction is the result of accumulating all force areas affecting the cloth. For instance, if you add a ObiAmbientForceZone and a ObiSphericalForceZone, the resulting wind blows in the same direction/magnitude everywhere in space (due to the ambient force) except in a spherical area, where it gets modified due to the spherical force zone.

This is simple to understand and easy to control, but doesn't behave at all like wind -or fluids in general- do in the real world. More specifically, the resulting velocity field is not incompressible and may have divergence. To accurately simulate wind or underwater currents you'd want to have a divergence-free fluid velocity field that takes pressure/density into account, which is far beyond the scope of cloth or rope simulation. This can be achieved using an eulerian (grid based) fluid simulator, either 2D or 3D depending on your needs. Simplest method is probably Stam's stable fluids.

Once you've got a proper fluid simulation for the wind, you just need to extract the wind direction/magnitude at each cloth particle position and write it into the  solver's "wind" data array, which is where particles in the simulation get their wind values from. Drag and lift forces resulting from wind are pretty accurately simulated in ObiCloth.

Note that if you want two-way coupling between the fluid and the cloth, you'd also need to take cloth geometry into account as a boundary condition for the fluid simulation.

kind regards,