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Transient Thermal Analysis of arm-band model using System Coupling

Speos Consumer Electronics Healthcare

This article explores how to couple Ansys Speos with other Ansys tools using System Coupling for a multi-physics analysis focused on photoplethysmography (PPG). PPG is a non-invasive optical method commonly used in wearable heart rate sensors to monitor pulse through the skin. In this study, transient thermal analysis is performed to observe temperature changes on the human body over time under continuous light exposure. The results will help to improve the design of optical devices in direct contact with the skin, ensuring both performance and safety.

Authored By Alborz Ehteshami, Sara Karami

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Prerequisites

  • Speos (Enterprise) 2024 R2 or higher
  • Mechanical 2024 R2 or higher
  • Speos integration for System Coupling

Overview

Understand the simulation workflow and key results.

The Ansys simulation suite enables integrated Multiphysics analysis through System Coupling , which automates two core tasks:

  • Co-simulation : connects multiple physics solvers (e.g., thermal + optical) to solve a coupled problem simultaneously.
  • Mapping : automatically transfers data (such as heat sources or power density) between solvers with different meshes or domains—no manual alignment needed.

This workflow demonstrates how to link optical simulations in Speos with thermal simulations in Mechanical using System Coupling. The goal is to study how light energy absorbed by skin tissue leads to heat generation over time—a critical factor for safe and effective wearable optical devices.

System Coupling optical–thermal workflow

The workflow is divided into three main steps:

  • Step 1 – Optical Simulation Setup: Set up a 3D light-tissue interaction using Speos. Define the anatomical structure of the arm, assign optical properties to tissue layers, and use a 3D energy density sensor to capture how light energy is absorbed and distributed.
  • Step 2 – Thermal Simulation Setup: In Mechanical, assign thermal properties to biological layers and apply boundary conditions. Insert a System Coupling Region to allow incoming data (from Speos) to act as heat sources.
  • Step 3 – Co-simulation in System Coupling: Import both simulations into System Coupling. Define the coupling interface, configure transient settings (duration, step size), and launch the co-simulation to evaluate temperature rise over time.

Run and Results

Instructions for running the model and discussion of key results.

Step 1: Optical simulation setup

The optical armband setup involves the integration of 3D models of the human arm and armband, along with their respective optical properties. The simulation includes detailed volume and surface definitions of the skin layers, ranging from the epidermis, papillary dermis, and blood network, all the way to the bone. Tissue deformation and the specific optical properties of each biological layer have been modeled based on data from the Zemax Knowledgebase.

In this model:

  • The armband is treated as an opaque body , and surface reflections from the band are not considered.
  • A Lambertian LED light source is placed on the backside of the armband to represent a heart rate sensor. It emits at 525 nm with a normalized power output of 1 W .
  • A 3D energy density sensor is used to calculate energy absorption in the tissue volume (in W/m³). This data captures how light diffuses through biological tissue, which is then passed to the Mechanical solver via System Coupling .

The sensor region is dimensioned to fully encompass the area under the armband. A direct simulation is performed that includes the geometry, light source, and energy sensor. Local meshing is applied to areas that require more detail to optimize simulation time without compromising accuracy.

  1. After downloading and extracting the example package, open the 3DEnergyDensitySens-Armband.scdocx Speos project file to begin the optical simulation setup.
  2. Insert a 3D energy density sensor covering the armband region to capture volumetric power distribution.
  3. Run a direct simulation that includes:
    1. Light source
    2. Arm geometry
    3. Energy density sensor
    4. Use local meshing in high-interest areas.
  4. Right-click the simulation > perform a Linked Export.

  5. Save the exported results in a dedicated folder for use during co-simulation.

This sets up foundation for accurately analyzing light source–tissue interaction and calculating the energy distribution in a human arm model, which can then be used for transient thermal analysis in Ansys Mechanical.

Step 2: Thermal Simulation set up

In Ansys Mechanical Enterprise , the thermal simulation begins with preparing the CAD geometry of the wrist , including key biological layers such as bone , epidermis , and fat . Each component is assigned appropriate material properties, like thermal characteristics, to support accurate Multiphysics analysis.

  1. Open the Armband.wbpj project file to set up the thermal simulation in Ansys Mechanical.
  2. Import or prepare the CAD geometry

  3. Assign material properties to each component

In addition, meshing parameters are defined to ensure that the model supports integration with the optical setup and maintains sufficient resolution for thermal analysis. Once the geometry and materials are fully defined, co-simulation can be performed through System Coupling . To do this, a System Coupling Region must be inserted.

  1. Insert a System Coupling Region

    1. Select the body in the graphics window.
    2. Go to Mode in the graphics window.
    3. Open the Selection Mode Toolbar > choose Box Select. 
      Select the wrist or arm geometry and apply it to the System Coupling Region.

    4. Apply it to the System Coupling Region.

This step defines the portion of the model that will exchange data during the co-simulation process.

Once the coupling region is defined, proceed to the Transient Thermal module.

  1. Right click on the Transient Thermal Solution

  2. Write System Coupling Files

    System Coupling export files dialog

     

This step generates the necessary configuration and data files for integration. These files (.scp and .dat) can later be imported during the co-simulation phase in System Coupling, ensuring a seamless connection between the thermal and optical domains.

Step 3: Co-simulation in System Coupling 

Set Up the Working Environment

  1. Open System Coupling.
  2. Set the working directory to the folder named "System Coupling Analysis".
  3. Add Simulation Participants
  4. Add Mechanical (Thermal) Simulation

  5. Click Add Participant.

  6. In the pop-up window:
    1. Select Input Files.
    2. Browse to the .scp file in the Thermal sub-folder.
    3. Click Add.
  7. Add Speos (Optical) Simulation 
    Click Add Participant again. 
    In the pop-up window:
    1. Change input type from Input Files to Python Script.
    2. script located in the Ansys installation folder (.../Ansys Inc/v251/System Coupling/Participants/SpeosServer/syscspeosserver.py)
    3. Set the Working Directory to the Optical sub-folder containing the exported simulation.

    4. Click Add.

    5. In the next interface:  Select the .speos file.
    6. Check the Set Transient box.

    7. Click Ok.

      You can rename each participant by right-clicking its name in the participant list.

  8. Create the Coupling Interface: Click Add Coupling Interface.

  9. In the new interface (CouplingInterface1):
    • Set Side 1 to Speos
      • For the region, select 3D Energy Density.
    • Set Side 2 to MAPDL Transient Thermal.
      • For the region, select the thermal region defined in the Transient Thermal module.
  10. Right-click CouplingInterface1, then select Add Data Transfer.
  11. In DataTransfer1: 
    Set MAPDL as the Target Side (meaning Speos sends heat data to Mechanical). 
    Configure Transient Analysis Settings 
    Go to Solution Control. 
    Set:
    • Duration Option: End Time
    • End Time: 3600 seconds (1 hour)
    • Time Step Size: 600 seconds (10 minutes)

  12. Launch the Co-Simulation

    System Coupling run button

  13. Save the project.
  14. Click Start Solve to begin the co-simulation. 
    Progress can be monitored in the Command Console tab. 
    Viewing Temperature Map in Insight

  15. Open Insight.

    Insight launch button

  16. Go to File > Open, click on keep currently loaded data and press ok, then locate the .RTH file under the Mechanical folder. 
    This file contains the saved temperature map.

    Insight temperature map viewer

  17. In the Parts window, locate Case 2.

  18. Right-click on Case 2 and choose Color By > Variable.

  19. In the Scalars section, select Temperature and click OK.
  20. The temperature map will now be displayed across all time steps.
  21. To visualize the data over time, use the playback controls to play all results.

In this study, system coupling is used to automate the multiphysics workflow. As a result, temperature data was computed across all layers of the wrist at various time steps. The images below illustrate the temperature distribution at the initial and final time steps.

Temperature distribution at final time step

These findings offer valuable insights for optimizing the safety of optical medical devices that are in direct contact with the skin. Specifically, they help ensure such devices operate within safe thermal limits, minimizing the risk of burns or other tissue damage.

Important Model Settings

Description of important objects and settings used in this model

This section highlights the key parameters that govern the behavior of the coupled optical–thermal simulation. These settings can be adjusted to explore different design conditions or device configurations.

The optical setup uses a Lambertian LED source emitting at 525 nm with 1 W nominal power , chosen to represent the wavelength range used in photoplethysmography (PPG) sensors. The illumination is applied to a multilayer human wrist model that includes skin, blood, and bone tissues, each assigned realistic optical and thermal properties based on literature data.

For the thermal analysis, the simulation assumes an initial temperature of 298 K (25 °C) and applies natural convection at the outer skin boundary to simulate heat dissipation into the environment. The thermal material properties—conductivity, specific heat, and density—are tuned to match human biological tissues. Mesh refinement is focused around the illuminated area to ensure accurate heat distribution while maintaining efficient run times.

In the co-simulation environment, System Coupling transfers the absorbed optical energy field from Speos to Mechanical as a distributed heat load. The transient analysis is configured with a 1-hour duration and 10-minute time steps , allowing temperature evolution to be tracked throughout the exposure period.

Consistent units (W, m, s, °C) are used across all models, and data exchange between tools is fully automated. These parameters can be easily modified to study different illumination powers, wavelengths, or boundary conditions without changing the core workflow.

Updating the Model With Your Parameters

Instructions for updating the model based on your device parameters

This workflow is designed to be easily adapted to your own product geometry, optical sources, and material data. By updating a few key parameters in the provided project files, you can apply the same coupled optical–thermal process to any similar design.

To customize the optical model , open the Speos project file and modify parameters such as light source wavelength, power, and emission pattern. You may also replace the human wrist geometry with your own device model or target surface. When doing so, ensure that a 3D Energy Density Sensor is correctly positioned to capture absorbed power in the region of interest. Once the updated simulation is complete, export the new energy density data for use in the thermal analysis.

Next, in the Mechanical project, update material properties or geometry to match your design. Adjusting thermal conductivity, density, or convection coefficients allows you to simulate different skin types, materials, or environmental conditions. Confirm that the System Coupling Region covers the same area as in the optical setup to ensure correct data mapping.

Finally, in System Coupling , import the new optical and thermal configurations. You can modify the simulation duration, time-step size, or participant names to reflect your updated files. Once configured, rerun the co-simulation to generate results specific to your parameters.

Taking the Model Further

Information and tips for users that want to further customize the model

The same workflow demonstrated with the armband can be extended to any optical medical device that interacts with the human body or biological tissue. Devices such as wearable sensors, skin-contact diagnostic probes, or implantable light-based systems can all be modeled using this approach.

By leveraging this automated optical–thermal coupling, designers can evaluate both performance and safety without the need for manual data transfer between solvers. This ensures consistent, repeatable simulations even when testing multiple configurations or operating conditions.

The framework also supports integration with Ansys optiSLang , enabling full design optimization for thermal safety, light efficiency, or material selection. This makes it possible to explore a wide range of design parameters systematically and identify optimal solutions with minimal manual effort.

Additional Resources

Additional documentation, examples and training material

See also

Ansys Learning Hub Courses

 

Associated files

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