Custom wall-damping functions in K-epsilon Turbulence model: ANSYS UDF Tutorial

Engineers write custom programming code to predict chaotic fluid motion inside industrial pipes accurately. This ANSYS Fluent CFD tutorial uses a K-epsilon turbulence model UDF to calculate kinetic energy and solid wall friction. Mastering this CFD simulation course improves your design skills, and you can discover more advanced coding methods in our UDF CFD simulation tutorials.

Figure 1: Custom turbulent viscosity UDF in viscous model panel

Simulation Process: ANSYS Fluent UDF & K-Epsilon Modeling

The k-epsilon realizable CFD simulation in ANSYS Fluent solves flow in a 2D axisymmetric pipe using a custom UDF (User Defined Function) that implements the k-epsilon turbulence model with wall-damping corrections. The UDF code defines two User-Defined Scalars (UDS): turbulent kinetic energy (TKE) and dissipation rate (TDR), which transport through the axisymmetric domain using modified convection-diffusion equations. The k-epsilon realizable model in this Fluent CFD setup includes damping functions that correct near-wall behavior, ensuring accurate turbulent viscosity prediction at pipe walls and centerline.

The UDF code hooks into ANSYS Fluent via four main mechanisms:

• k_source calculates TKE generation from strain rate and applies dissipation through damping factor, controlling energy production and loss
• d_source computes dissipation rate evolution using coefficients C1_D and C2_D, scaled by damping functions for wall effects
• ke_diffusivity defines turbulent diffusion using Schmidt numbers plus molecular viscosity, governing scalar transport
• ke_mut calculates turbulent eddy viscosity from TKE and TDR, closing the momentum equation. The wall boundary condition UDF enforces Neumann-type dissipation at pipe walls based on wall distance.

Figure 2: Structured grid – 40000 quad cells

Post-processing: Analysis of Turbulent Boundary Layers

We examine the physical fluid behavior by analyzing the visual contours of the pipe flow. As the fluid enters and travels through the long channel, physical friction from the solid walls slows it down. The velocity contour proves this physical phenomenon clearly. The fluid speed drops to 0 at the top and bottom boundaries due to the no-slip condition. Because the outer fluid layers drag against the metal, the fluid in the open center must accelerate to push the same mass through the pipe. This restriction creates a high-speed central core reaching a maximum velocity of 0.177.

This massive difference in speed between the fast center and the stagnant walls creates intense shear forces. These tearing forces generate chaotic spinning fluid circles called eddies. We track the energy of these spinning eddies using the Scalar-0 contour, which represents the turbulent kinetic energy, or parameter k. The contour shows the kinetic energy growing as the flow develops, reaching a peak value of 0.001 in the main flow region. Taking the entire pipe volume into account, the solver calculates a global mass-average turbulent kinetic energy of 0.00082586359. Near the solid walls, the physical space is too tight for large eddies to spin. The solid boundary suppresses the motion, forcing the kinetic energy to drop back down to 0.

Figure 3: The kinetic energy distribution proving the turbulent forces develop strongly in the central flow area.

Figure 4: The dissipation rate map showing the turbulent eddies breaking apart against the solid boundaries.

Figure 5:  The velocity contour revealing a fast central core and stagnant fluid near the solid edges.

While the kinetic energy dies near the wall, the destruction of that energy spikes significantly. The Scalar-1 contour visualizes the turbulence dissipation rate, known as parameter e. When the chaotic eddies crash into the solid pipe surface, they break apart into microscopic heat. The contour displays a bright red zone along the boundaries, indicating a maximum dissipation rate of 0.007. In the open center of the pipe, the fluid flows much more freely, and the dissipation drops near 0. For the overall system, the software computes a mass-average dissipation rate of 0.0028828698. This physical relationship proves that solid boundaries simultaneously crush forward fluid momentum and destroy internal turbulent energy.

Frequently Asked Questions (FAQ)

• What does turbulent kinetic energy mean?
  • It is a measurement of how much chaotic energy exists inside a flowing fluid. High kinetic energy means the fluid is swirling and mixing violently. Low kinetic energy means the fluid is calm or physically blocked from moving.
• Why is velocity zero at the pipe walls?
  • Fluid molecules physically stick to the microscopic rough bumps on a solid metal wall. This creates a no-slip condition. The fluid touching the wall stops moving completely, which creates friction that slows down the rest of the liquid.