Learning Forum

[shanmugapriyansv]

Hello,

I am working with a 2D axisymmetric model in ANSYS Fluent and would like to clarify the coordinate system conventions used by the solver.

My understanding is that:

• The (x)-direction represents the axial direction.
• The (y)-direction represents the radial direction.
• The third direction corresponds to the azimuthal ((\theta)) direction.

However, I would like to confirm how Fluent defines the positive azimuthal direction and whether the axisymmetric formulation follows the standard right-handed cylindrical coordinate system ((r,\theta,z)).

Specifically, I would appreciate clarification on the following:

1. Does Fluent's 2D axisymmetric solver follow the standard right-handed cylindrical coordinate convention?
2. Does Fluent's 2D Axisymmetric mapping ($x$=axial, $y$=radial) impose a specific parity that requires this coordinate-basis transformation correction?
3. Is there a simple benchmark or test case that can be used to verify the orientation of the azimuthal direction and related vector quantities?
4. Are there any Fluent documentation references that describe the coordinate orientation and sign conventions used in axisymmetric calculations?

Thank you for your assistance.

[Rob]

Fluent typically uses the right hand rule, so I'd expect that in 2d-axi with swirl, but if you check the documentation it's likely covered there. https://ansyshelp.ansys.com/public/account/secured?returnurl=/Views/Secured/main_page.html?hl=f

Regarding a benchmark, why not it? 

[shanmugapriyansv]

Thank You Mr. Rob, I studied those documents, but what about the curl components in UDF. I am simulating 2D axisymmetric plasma model, wherein I solve for the magnetic vector potential components using User-Defined Scalars (UDSs) and then calculate the azimuthal magnetic field component (B_theta) from the curl of the vector potential. 

• Using the standard curl definition $B_{\theta} = \frac{\partial A_r}{\partial z} - \frac{\partial A_z}{\partial r}$ (mapped to Fluent's $x, r$ coordinates as $\frac{\partial A_y}{\partial x} - \frac{\partial A_x}{\partial y}$), I couldn't observe the elongated arc formation.

• By introducing an explicit minus sign (using $B_{\theta} = \frac{\partial A_x}{\partial y} - \frac{\partial A_y}{\partial x}$), the arc correctly forms toward the anode. 

Is there an implicit difference in how Fluent's internal MHD/3d udf modules while still using 2d axisymmetric model define the cross-product basis compared to standard 3D cylindrical curl definitions, necessitating this parity flip?

[Rob]

Unknown, are you expecting an axial arc? I would be surprised if there was an error at that level but do know there are some odd interactions between the various fields. 

Note, arcs tend to wander around a mean path so 2d-axi and 3d-sector models may not be suitable.