Allow setting the viscosity in the creeping flow model (#102)

The creeping flow model takes the dynamic viscosity as a coefficient,
coefficientFunctions: { viscosity }, a positive number or a function of
(x, y) evaluated at every integration point. It defaults to 1, so
existing models give the same results.

Tests: check that a viscosity (number and function) leaves the lid-driven
cavity velocity unchanged and scales the pressure, and that a varying
viscosity gives the same solution through the worker. Examples: set the
viscosity in the lid-driven cavity.

Closes #102

Author: nikoscham

dist/feascript-worker.esm.js

@@ -14,7 +14,11 @@ console.log("FEAScript Version:", printVersion);
 
 // 1. Create the model and select the physics
 const model = new FEAScriptModel();
-model.setModelConfig("creepingFlowScript");
+model.setModelConfig("creepingFlowScript", {
+  coefficientFunctions: {
+    viscosity: 1, // μ
+  },
+});
 
 // 2. Define the mesh
 model.setMeshConfig({

dist/feascript-worker.esm.js.map

@@ -364,7 +364,11 @@ export class FEAScriptModel {
       }
     } else if (this.solverConfig === "creepingFlowScript") {
       // Use regular linear solver methods for steady creeping flow
-      const creepingFlowResult = assembleCreepingFlowMatrix(meshData, this.boundaryConditions);
+      const creepingFlowResult = assembleCreepingFlowMatrix(
+        meshData,
+        this.boundaryConditions,
+        this.coefficientFunctions,
+      );
       jacobianMatrix = creepingFlowResult.jacobianMatrix;
       residualVector = creepingFlowResult.residualVector;