Introduction to Turbulence Modeling in Ansys Fluent — Lesson 1

Introduction

This tutorial provides a comprehensive overview of turbulence modeling in Ansys Fluent, focusing on turbulent flows, their applications, and various modeling approaches. By understanding how to calculate the Reynolds number and select the appropriate RANS turbulence model, you can effectively analyze complex turbulent flows commonly encountered in industrial applications.

Step 1: Understanding Turbulent Flows

• Definition: Turbulent flow is characterized by chaotic changes in pressure and flow velocity. It is common in high-speed flows and is critical for many engineering applications.
• Applications:
  • Aerospace engineering (aircraft design)
  • Automotive industry (vehicle aerodynamics)
  • Environmental engineering (pollutant dispersion)

Step 2: Calculating the Reynolds Number

• Purpose: The Reynolds number helps determine whether the flow is laminar or turbulent.
• Formula: [ Re = \frac{\rho v L}{\mu} ] Where:
  • (Re) is the Reynolds number
  • (\rho) is the fluid density
  • (v) is the flow velocity
  • (L) is a characteristic length (e.g., diameter of a pipe)
  • (\mu) is the dynamic viscosity of the fluid
• Interpretation:
  • (Re < 2000): Laminar flow
  • (Re > 4000): Turbulent flow
  • (2000 < Re < 4000): Transitional flow

Step 3: Selecting the RANS Turbulence Model

• RANS Overview: The Reynolds-Averaged Navier-Stokes (RANS) equations provide a time-averaged representation of turbulence. This approach is economical for complex turbulent flows.

• Common RANS Models:

  • k-epsilon model: Suitable for general-purpose simulations and simple flows.
  • k-omega model: Better for flows with strong adverse pressure gradients.

• Selection Tips:

  • Assess the flow characteristics and geometry of your simulation.
  • Consider computational resources and required accuracy.
  • Experiment with different models to determine which best fits your specific case.

Step 4: Practical Implementation in Ansys Fluent

• Setting Up Your Simulation:

  1. Open Ansys Fluent and create a new project.
  2. Define the geometry and mesh your model.
  3. Select the appropriate fluid properties (density, viscosity).
  4. Choose the turbulence model from the settings:
     • Navigate to Models > Turbulence > select your RANS model.

• Running the Simulation:

  1. Set boundary conditions (inlet, outlet, wall).
  2. Initialize the solution.
  3. Run the simulation and monitor convergence.

Conclusion

In this tutorial, we explored the fundamentals of turbulence modeling in Ansys Fluent, including calculating the Reynolds number and selecting the appropriate RANS turbulence model. Understanding these concepts will enable you to analyze turbulent flows effectively. As a next step, consider experimenting with different turbulence models in Ansys Fluent to see how they impact your simulation results, and explore additional resources for advanced topics in turbulence modeling.