Uppsala University Department of Information Technology

Technical Report 2004-030

Time Step Restrictions using Semi-Implicit Methods for the Incompressible Navier-Stokes Equations

Wendy Kress and Per Lötstedt

July 2004

The incompressible Navier-Stokes equations are discretized in space and integrated in time by the method of lines and a semi-implicit method. In each time step a set of systems of linear equations has to be solved. The size of the time steps are restricted by stability and accuracy of the time-stepping scheme, and convergence of the iterative methods for the solution of the systems of equations. The stability is investigated with a linear model equation derived from the Navier-Stokes equations. The resolution in space and time is estimated from turbulent flow physics. The convergence of the iterative solvers is studied using the same model equation. The stability constraints and the convergence rate obtained from the model equation are compared to results for a semi-implicit integrator of the Navier-Stokes equations with good agreement. The most restrictive bound on the time step is given by accuracy, stability, or convergence depending on the flow conditions and the numerical method.

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