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By Jerry Lim,Fluent Inc.
Introduced as a beta feature in FLUENT 6.0, the dynamic mesh model, part
of the moving and deforming mesh capability, extends the capacity of the
FLUENT solver to handle problems that involve unsteady moving geometry.
After successfully completing several industrial strength test cases,
and featuring several enhancements, the dynamic mesh model is being formally
released to all users in FLUENT 6.1. In addition to significant robustness
improvements, the model will be fully parallelized.
Valve motion shown in two steps above; below, the first step is shown
in cyan (light blue) the second is overlayed in magenta, and grey denotes
no change to the mesh between the steps
In order to accommodate a
wide range of motion types, FLUENT
6 offers three modes of mesh deformation:
dynamic layering, spring
smoothing, and local remeshing. The
first two approaches are similar to
mesh motion schemes that have been
widely used for many years. Dynamic
layering is useful for linear motion.
Layers are added and deleted to
accommodate the specified boundary
motion. The term “dynamic”
means that the process is handled
internally by the FLUENT solver, based
only on a specified ideal cell height,
and factors that govern when a cell
should split and when two should
coalesce. These parameters define
upper and lower cell height limits.
When the cell height limits are exceeded,
FLUENT automatically detects this
condition, and splits or coalesces the
layer as needed. Because cells are
added and deleted, neighbor connectivity
changes are made as well.
This approach may be utilized for
quadrilateral (2D volume cells and
3D boundary faces), prismatic, and
hexahedral element types.
The spring smoothing method is useful for relatively small deformations.
The assumption in FLUENT is that the mesh nodes are connected like a network
of springs. By performing a force balance on each of the “spring
elements”, an equilibrium balance is sought which provides a smooth
(minimum energy) mesh. If two elements (nodes) are too close, the spring
force will repel the nodes away from each other. Since each nodal position
depends on its neighbor nodes, and the neighbor nodal positions are dependent
on their own neighbor nodes, spring smoothing is accomplished through
an iterative process, like that used by other elliptic mesh generators.
The spring smoothing process does not result in any connectivity changes
since all node/cell relationships are preserved. Used as a stand-alone
scheme, spring smoothing is limited, since excessive deformation will
result in highly skewed cells. The spring smoothing algorithm may be used
for triangular (2D volume cells, 3D boundary faces) and tetrahedral elements.
Pressure contours on a 3D valve
The third approach, local remeshing, represents a departure from traditional
mesh motion schemes. In this approach, the cell size and quality (skewness)
limits are prescribed. As mesh motion occurs, cells will eventually exceed
the prescribed limits. FLUENT detects these cells and marks them for remeshing.
In addition to marking the offending cells, several neighbor cells are
also marked. This collection of cells represents a subdomain, which is
automatically remeshed using the TGrid algorithm that is now built into
FLUENT. After remeshing, the CFD solution is interpolated onto the new
cells. Thus, rather than generate a completely new mesh for the entire
volume, remeshing and interpolation works on a local basis. As with dynamic
layering, local remeshing implies connectivity changes. Typically (but
not necessarily), it is used in conjunction with spring smoothing. The
local remeshing algorithm may be used for triangular (2D volume cells,
3D boundary faces) and tetrahedral elements.
The mesh around two Ahmed bodies deforms as one (grey) overtakes another
(red)
A dynamic calculation requires an initial mesh and description of the
boundary motion. A model with several independently moving parts can be
treated using different zones to represent the different parts. Independent
motions for these parts can be specified, and the regions surrounding
them will be remeshed using whichever technique is appropriate at the
time. The flexibility of the model makes it well suited to address a number
of different application areas, as described in the articles that follow.
Contours of exhaust gas mass fraction are used to illustrate the launch
of a rocket, solved using the dynamic mesh model in FLUENT
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