Hello, we can only output the velocity magnitude in the example file “cavity2d”, and I want to output the vector field.

could you please give me some suggestions?

```
template<class BlockLatticeT>
void writeVTK(BlockLatticeT& lattice,
IncomprFlowParam<T> const& parameters, plint iter)
{
T dx = parameters.getDeltaX();
T dt = parameters.getDeltaT();
VtkImageOutput2D<T> vtkOut(createFileName("vtk", iter, 6), dx);
vtkOut.writeData<float>(*computeVelocityNorm(lattice), "velocityNorm", dx/dt);
vtkOut.writeData<2,float>(*computeVelocity(lattice), "velocity", dx/dt);
}
```