referenceVelocity

• version=0: \( \mathbf{v}_{ref} = 0\). fixed observer.
• Version=1: currently, the \( \mathbf{v}_{ref}\) is chosen by the mean movement velocity of the boundary of any cluster (given by %ind_cluster% ). later, we will choose \( \mathbf{v}_{ref}\) such that it forms a reference system in which the sum of the instantionary term changes vanish, that is \( \frac{\partial \mathbf{v} }{ \partial t } + ( \mathbf{v}_{ref}^T \nabla ) \mathbf{v} = 0\) in the strong form, or at least or at least \( \int \limits_{\Omega_{i_\text{Cluster}}} \left( \frac{\partial \mathbf{v} }{ \partial t } + ( \mathbf{v}_{ref}^T \nabla ) \mathbf{v} \right) = 0\) in the weak form.