Fluent Post-processing: Torque with Changing Center and Direction

Problem Description

There are cases involving relative motion where the moment center and the required axial direction may change over time when calculating torque.

For example, when analyzing the aerodynamic lateral torque during train passing, the moment center should be chosen as a fixed point on the train, such as the contact point between the train wheel and the track. Due to the relative motion between the trains, the moment center remains fixed relative to the train but changes in the absolute coordinate system based on the ground.

In Fluent simulations, how can torque values be obtained for such cases?

Mechanics Background

Torque is a vector and is calculated using the following formula:

The direction of the torque is perpendicular to both the position vector and the applied force, following the right-hand rule.

Method

Note: This article only suitable for problems with known motion. The steps are based on Fluent 2024R2 version and may differ in other versions.

The following model is used to illustrate the steps:

• The pink sphere in the model represents a solid moving part rotating around the Z-axis. The goal is to obtain the variation of fluid-induced torque during the rotation, with the moment center being the center of the pink sphere.
• The surrounding green cylinder represents a stationary computational domain, and the green spherical surface is the interface between the fixed and rotating domains.

In Fluent, when directly creating a torque report in the Report Definition, the moment center and direction can only be defined as constants and cannot be set as variables.

Due to the limitations of report creation, torque data can be obtained using expressions.

Step 1: Create an expression for the variation of the sphere's center coordinates

The expression for the sphere's X-coordinate is shown in the figure, derived from the motion dynamics.

The Y and Z coordinates of the sphere are defined similarly.

Step 2: Create the position vector

The position vector expression is shown in the figure, defined using the vector function.

The vector function is used to create vectors and requires three input variables for the X, Y, and Z components of the vector. The units of the variables must be consistent, and it is recommended to use length units.

The output of the vector function is a vector.

Step 3: Create a torque expression based on the sphere's center coordinates

The torque expression is shown in the figure.

The first letter of the Moment function must be capitalized; lowercase will not be recognized. Input variables include a defined vector and a reference position.

The output of the torque function is a vector.

Step 4: Create a report

In the Report Definition, only scalar objects are supported for report creation, meaning only numerical values can be output, and vectors cannot be output directly. To output vector results, they need to be split into components or output as magnitudes.

For a defined vector V, the following suffixes can be added to the expression:

• v.x: X-component of the vector.
• v.y: Y-component of the vector.
• v.z: Z-component of the vector.
• v.mag: Magnitude of the vector.

The results of the above four types of expressions are all scalars. The X-component of the torque report created using expressions is shown in the figure.