4 research outputs found
Adaptive boundary conditions for exterior stationary flows in three dimensions
Recently there has been an increasing interest for a better understanding of
ultra low Reynolds number flows. In this context we present a new setup which
allows to efficiently solve the stationary incompressible Navier-Stokes
equations in an exterior domain in three dimensions numerically. The main point
is that the necessity to truncate for numerical purposes the exterior domain to
a finite sub-domain leads to the problem of finding so called "artificial
boundary conditions" to replace the conditions at infinity. To solve this
problem we provide a vector filed that describes the leading asymptotic
behavior of the solution at large distances. This vector field depends
explicitly on drag and lift which are determined in a self-consistent way as
part of the solution process. When compared with other numerical schemes the
size of the computational domain that is needed to obtain the hydrodynamic
forces with a given precision is drastically reduced, which in turn leads to an
overall gain in computational efficiency of typically several orders of
magnitude.Comment: 17 pages, 3 tables, 11 figure
Artificial boundary conditions for stationary Navier-Stokes flows past bodies in the half-plane
We discuss artificial boundary conditions for stationary Navier-Stokes flows
past bodies in the half-plane, for a range of low Reynolds numbers. When
truncating the half-plane to a finite domain for numerical purposes, artificial
boundaries appear. We present an explicit Dirichlet condition for the velocity
at these boundaries in terms of an asymptotic expansion for the solution to the
problem. We show a substantial increase in accuracy of the computed values for
drag and lift when compared with results for traditional boundary conditions.
We also analyze the qualitative behavior of the solutions in terms of the
streamlines of the flow. The new boundary conditions are universal in the sense
that they depend on a given body only through one constant, which can be
determined in a feed-back loop as part of the solution process
Simulating an exterior domain for drag force computations in the lattice Boltzmann method
The simulation of a stationary fluid flow past an obstacle by means of a lattice Boltzmann method is discussed. The problem of finding appropriate boundary conditions on the boundaries of the truncated numerical domain is addressed by a method recently discussed in the literature, based on a truncated expansion of the solution. The iterative process at the heart of this method is coupled with the iteration steps of a progressive grid refinement technique that allows a rapid convergence towards a well resolved stationary state. It is shown that this combination results in a highly efficient numerical tool which can speed up the resolution process in a substantial manner