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Field Calculations

Under Field calculations, the user can define a series of additional outputs for their simulations, including total pressure, vorticity, amongst others.

New field calculations can be defined under the Result control tab, as in Figure 1:

creating a new field calculation result control
Figure 1: Click on the ‘+’ button next to Field calculations to add a new entry.

Important

If you are interested in one of the field calculation outputs, make sure to set it up before running the simulation.

Result controls are never applied retroactively to older simulation runs.

Below, we will go over each one of the Field calculation options available in SimScale.

Pressure Fields

The pressure field control is only available for the incompressible analysis type. With this field calculation, the user can define a series of different outputs, most notably Total pressure and Pressure coefficient, also known as \(C_p\). Find below some useful configurations.

Total Pressure

For incompressible analysis, the default pressure values shown in the post-processor are gauge static pressures. If you are interested in having one additional scalar showing total pressures, this is also possible. Find below one sample result control configuration:

total pressure pressure fields simscale
Figure 2: The reference pressure is the absolute pressure of your domain.

The platform will perform the following equation to show total pressure in the post-processor:

$$P_{Total} = P_{Gauge,\ static} + P_{Reference} + 0.5\rho U^2 \tag {1}$$

Where:

  • \(P_{Total}\) is the new scalar in the post-processor, representing total pressure
  • \(P_{Gauge,\ static}\) is the usual gauge static pressure in the post-processor
  • \(P_{Reference}\) is the Reference pressure from Figure 2. Note that one usual approach is to define here the absolute pressure of your system
  • \(\rho\) is the density of the fluid
  • \(U\) is the local velocity of the fluid

Pressure Coefficient

A result control for pressure coefficient can be created with the following configuration:

pressure coefficient result control simscale
Figure 3: Sample pressure coefficient result control for a system with a reference pressure of zero Pascal.

In Figure 3, the user should define their Free stream velocity based on the global coordinate system. Based on the input settings, SimScale uses the following formula to calculate the pressure coefficient \(C_p\):

$$C_p = \frac{P_{Gauge,\ static}\ – \ P_{Free\ stream}}{0.5\rho U^2} \tag {2}$$

Where:

  • \(C_p\) is the pressure coefficient
  • \(P_{Gauge,\ static}\) is the usual gauge static pressure in the post-processor
  • \(P_{Free\ stream}\) is the free stream pressure. For incompressible analysis, this value will usually be zero Pascal
  • \(\rho\) is the density of the fluid
  • \(U\) is the velocity of the fluid, calculated from the Free stream velocity defined in Figure 3

Turbulence

The Turbulence result control allows the user to evaluate the yplus values in the post-processor. This parameter is very important for applications such as external aerodynamics and turbomachinery.

This result control is available for incompressible, compressible, convective heat transfer, and conjugate heat transfer analysis types. Note that the turbulence model must not be Laminar for the Turbulence result control to be available.

For background information on yplus and useful formulas, please check this post.

Vorticity

From a physical standpoint, one can understand vorticity as a vector having a magnitude equal to the maximum “circulation” at each point. Furthermore, the vector is oriented perpendicularly to the plane of circulation for each point [1].

In the SimScale post-processor, vorticity is represented by vectors containing components in the x, y, and z directions, as well as a magnitude.

From a mathematical perspective, vorticity is defined as the curl of the velocity vector, as in Equation 3:

$$ Vorticity = \nabla \textrm{🗙} \vec{U} \tag{3}$$

Note

The vorticity result control is available for incompressible and incompressible (LBM) analysis types.

Wall Shear Stress

The wall shear stress result control can be set in incompressible, compressible, convective heat transfer, and conjugate heat transfer simulations. As an output, the user obtains wall shear stress components in the x, y, and z-directions. Furthermore, the resultant vector for wall shear stress is also available.

Thermal Comfort Parameters

This result control complies with the methodology of both ASHRAE 55 and ISO 7730 Standards. By setting up a thermal comfort parameters result control, the post-processor will contain two additional scalars:

  • Predicted Percentage of Dissatisfied (PPD)
  • Predicted Mean Vote (PMV)

A series of parameters may affect the PPD and PMV values, including the clothing coefficient, metabolic rate, and relative air humidity:

thermal comfort parameters setup
Figure 4: With the formulation from the ASHRAE 55 and ISO 7730 Standards, SimScale calculates the PMV and PPD values.

For more detailed insights about the set up of a thermal comfort parameter result control, please check this documentation page.

Note

The Thermal Comfort Parameters result control is available for convective heat transfer and conjugate heat transfer v2.0 analysis types.

Friction Velocity

This field calculations result control item computes the friction velocity \(u_\tau\), which can be used to determine wall shear stress:

$$u_\tau = \sqrt\frac{\tau}{\rho} \tag {4}$$

Where \(\tau\) is the wall shear stress and \(\rho\) is the fluid density. Since both friction velocity and wall shear stress are vectors, we can rearrange equation 4 to account for the direction of the vectors when calculating wall shear stress:

$$\vec {\tau} = \rho\ mag\ (\vec{u_\tau})\vec{u_\tau} \tag{5}$$

Where \(mag\ (\vec{u_\tau})\) indicates the magnitude of the friction velocity vector.

Note

This result control is only available in the incompressible (LBM) analysis type. To calculate the wall shear stress field, it’s possible to take the SimScale results to ParaView and use the Calculator filter.

Surface Normals

The surface normals result control is only available for the incompressible (LBM) solver. This result control item is mostly used for further post-processing in third-party software, such as ParaView. The surface normals are not available for visualization in the SimScale post-processor.

Last updated: May 28th, 2021

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