Artificial Viscosity

Artificial viscosity (AV) is Exasim's shock-capturing and stabilization extension. It is configured by app flags and, when needed, model callbacks.

Conceptually, AV modifies the conservation law as

\[ \frac{\partial u}{\partial t} + \nabla \cdot F = S + \nabla \cdot F_{\mathrm{AV}}. \]

In the current implementation, AV is not an automatic universal flux. The application enables AV-related flags and the model author uses avfield, v/odg, and flux definitions to include artificial-viscosity effects.

Relevant User-Facing Fields

The pdeapp.txt reference documents these AV-related fields:

Field	Meaning
AV	Artificial-viscosity flag.
AVdistfunction	Distance-function option used by AV workflows.
AVsmoothingIter	Number of AV smoothing iterations.
frozenAVflag	Freeze/update behavior for AV fields.
avparam1, avparam2	AV parameter vectors.

Frontend models may define avfield(u,q,w,v,x,t,mu,eta). Text2Code uses the corresponding Avfield function.

How AV Is Used In Examples

Examples such as examples/Euler/mach8cylinder, examples/NavierStokes/flaredplate2d, and examples/NavierStokes/flaredplatecut2d compute AV fields and use those fields inside flux routines. Several examples store AV data in mesh.vdg / v.

Typical AV workflows include:

• Build a sensor from solution gradients or flow variables.
• Smooth or freeze the AV field according to app flags.
• Pass the AV field through v/odg or avfield.
• Add AV contributions to the physical flux.

Practical Guidance

• Use AV for shocks, contact discontinuities, or under-resolved steep fronts.
• Keep physical diffusion and artificial diffusion conceptually separate.
• Document which component of v or w stores the AV field.
• Verify conservation and shock thickness when changing AV parameters.