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Hybrid and Conservative Variable Values

The ANSYS CFX-Solver calculates the solution to your CFD problem using finite volume elements, which are NOT the same as mesh elements. Each node in the mesh is at the center of a finite volume element. Additional information on how the Finite Volume elements are used in the solution process is available. For details, see Numerical Discretization.

The result of this is that the values of some variables on the boundary nodes (i.e., on the edges of the geometry) are not precisely equal to the specified boundary conditions (e.g., the value of velocity on a node on the wall will not be precisely zero).

For visualization purposes, it can be more helpful if the nodes at the boundary contain the specified boundary conditions. Hence, hybrid values should usually be used when displaying a plot of a variable. ANSYS CFX-Post uses hybrid values (“corrected boundary node values”) by default for most variables. Hybrid values are obtained by taking the results produced by the ANSYS CFX-Solver and over-writing the results on the boundary nodes with the specified boundary conditions. This ensures, for example, that the velocity is displayed as zero on no-slip walls.

The original values, known as conservative values, do not contain exactly the specified boundary conditions on the boundary nodes. For quantitative calculations, the conservative values give more accurate results and should normally be used. If you wish to use these values in ANSYS CFX-Post, you can select them from the Variables Editor dialog box as described above. By default, ANSYS CFX-Post uses conservative values when the Calculate command is used.

The difference between hybrid and conservative values at wall boundaries can be demonstrated using the following figure:

Using velocity as an example, the velocity value calculated at a mesh node is based upon the ‘average' in the control volume surrounding that node. For calculation purposes, the entire control volume is then assumed to possess that velocity. At a boundary node, its surrounding control volume includes an area in the bulk of the fluid (this area is highlighted around the boundary node marked 1). Hence, the conservative velocity calculated at the wall is not zero, but an ‘average' over the control volume. This works well for calculation purposes since quantities such as mass flow are calculated correctly — a velocity of zero would produce zero mass flow through the control volume which is clearly inaccurate.

Hybrid values correct the wall node velocity to its true value of zero. These values are used to produce plots so that when you view a plot on a wall, the correct value is seen.

Solid-Fluid Interface Variable Values

Conservative Values at 1:1 Interface

At a solid-fluid 1:1 interface duplicate nodes exist. The conservative value for the solid-side node is the variable values averaged over the half on the control volume that lies inside the solid. The conservative value for the fluid-side node is the variable values averaged over the half of the control volume that lies in the fluid.

Consider the example of heat transfer from a hot solid to a cool fluid when advection dominates within the fluid. If you create a plot across the solid-fluid interface using conservative values of temperature, then you will see a sharp change in temperature across the interface. This is because values are interpolated from the interface into the bulk of the solid domain using the value for the solid-side node at the interface, while values are interpolated from the interface into the bulk of the fluid domain using the value for the fluid-side node at the interface. This results in a temperature discontinuity at the interface.

Hybrid Values at 1:1 Interface

When creating plots using hybrid variable values (the default in ANSYS CFX-Post), the 1:1 interface is single valued and takes the solid-side conservative value. You can therefore expect to see the same plot within the solid, but the temperature profile between the interface and the first node in the fluid interpolates between the solid-side interface value and the first fluid node value. In this case, a discontinuity does not exist since all nodes are single valued.

Conservative values should be used for all quantitative calculations.

Conservative Values on a GGI Interface

At a GGI interface, the CFX Solver calculates both fluid-side and solid-side temperatures based on heat flux conservation. These values are representative of the temperature within the half-control volumes around the vertices on the interface. The fluid-side and solid-side temperatures are generally not equal. As a result, a plot of conservative values of temperature will generally show a discontinuity across a GGI interface.

Hybrid Values on a GGI Interface

At a GGI interface, the CFX Solver calculates a "surface temperature" based on a flux conservation equation for the 'control surfaces' which lie between the fluid side and the solid side. The surface temperature is usually between the fluid-side and solid-side temperatures. Hybrid values of temperature on a GGI interface are set equal to the surface temperature. As a result, there is no discontinuity in hybrid values of temperature across a GGI interface.