# simpleBc (1d, 2d, 3d)¶

Sets the boundary condition for MHD type equations

## Parameters¶

model (string)
The hyperbolic equation that describes the system being modelled. This should be one of the options available in Hyperbolic Equations and should match that used in (e.g) the classicMusclUpdater (1d, 2d, 3d) updater used to evolve the system.
coefficients (integer vector)
The size of the coefficients vector must be the same as the number of elements in the input vector and the hyperbolic equation refered to by model. For models containing vector fields, the components of the vector are rotated into the coordinate system of the vector normal to the boundary. Then, the components of the vector are multiplied by the coefficients vector to set the boundary condition.

## Example¶

In the example below, the hyperbolic equation is eulerEqn. This corresponds to a 5 component system with primary variables $$[rho, mx, my, mz, en]$$. Components 2-4 of this equation system correspond to the momentum vector, $$(mx,my,mz)$$, which are rotated so that $$mx$$ is aligned with the normal to the boundary. The coefficients for this example are $$[1.0,-1.0,1.0,1.0,1.0]$$ and so the sign of the normal momentum is reversed at the boundary, which is corresponds to a wall boundary condition. Other boundary conditions can therefore be created by manipulating the coefficient vector.

<Updater fluidWall>
kind = simpleBc2d
model = eulerEqn
coefficients = [1.0,-1.0,1.0,1.0,1.0]
onGrid = domain
out = [q]
entity = bottom
</Updater>