### 16.1.1. Solid-Fluid Interface Variable Values

#### 16.1.1.1. 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.

#### 16.1.1.2. Hybrid Values at 1:1 Interface

When creating plots using hybrid variable values (the default in CFD-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 because all nodes are single valued.

Conservative values should be used for all quantitative calculations.

#### 16.1.1.3. 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.

#### 16.1.1.4. 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' that 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.

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