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The **Surface data** result control item calculates statistics of result fields over the specified faces of the CAD model, such as average value or surface integral. The settings panel appears as follows:

The parameters to be specified in the settings panel for the surface data calculations are:

**Surface data:**Select the type of computation to perform, between*Area average*and*Area integral*(see below for details).**Write control**: Control the way in which you want the results to be written in the output:**Time step**: It corresponds to the value of*Delta t*in the simulation control settings panel for fluid flow analysis.**Synchronize with field output**: The output will synchronize with other*Write control*types specified under simulation control for fluid flow analysis.

**Write interval**: Specify the number of*Write control*intervals to be skipped between two successive writes of results. For example, a write interval value of 3 for a time step length*Delta t*of 2 seconds means that the results will be written every 3 x 2 = 6 seconds.

Tip

To evaluate numerical convergence of a result field, writing intervals up to 20 is sufficient. It’s important to have enough data points to visualize the convergence patterns, so defining large values for write interval is not recommended in this case.

The available surface data statistics are computed as follows:

This function calculates the average of the scalar result variable over the assigned face. If the result field is vectorial, the average is reported for each component. The user can choose a number of faces to perform the operation on, and the statistic will be computed over all of them separately.

For instance, for an example scalar quantity \( u \), the average is weighted and normalized using the cell surface areas:

$$ \bar{u} = \frac{1}{A} \int_{S} udA $$

Where \(A\) is the surface area of the set of assigned faces \(S\). This value is computed numerically over the surface mesh cells:

$$ \bar{u} = \frac{1}{A} \sum_{i} u_i A_i $$

This function calculates the integral of the scalar result variable over the assignment. If the result field is vectorial, the integral is computed for each component. The user can choose a number of faces to perform the operation on, and the statistic will be computed over all of them separately.

For our example quantity:

$$ Q_u = \int_{S} udA $$

This value is again computed numerically:

$$ Q_u = \sum_{i} u_i A_i $$

To visualize the application of the *Surface data* control items, let’s take the example of a pipe flow simulation, with statistics to be computed at the inlet face:

After the run is complete, we have the computed values available in the simulation tree:

We use the surface data to compute two quantities: the average pressure at the inlet, which is useful to compute the pressure drop across the pipe, and the flow rate. First for the pressure:

Second, for the flow rate we select the integral of *Uy*, the y-component of the flow velocity field:

The following are some of our tutorials where you can practice the use of the surface data result control item:

- Fluid Flow Through a Non-Return Valve
- Fluid Flow Simulation Through a Centrifugal Pump
- Compressible Flow Simulation Around a Wing

Last updated: August 11th, 2021

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