Explain the difference between species diffusion and turbulent diffusion.

Explain the difference between species diffusion and turbulent diffusion.

• A. Molecular diffusion arises from temperature gradients; turbulent diffusion is driven by concentration gradients.
• B. Molecular diffusion arises from concentration gradients; turbulent diffusion models enhanced mixing due to turbulent eddies via an eddy diffusivity approach.
• C. They are the same phenomenon described at different scales.
• D. Turbulent diffusion replaces molecular diffusion completely in all cases.

Answer: (C)

The main idea being tested is how diffusion is treated in CFD as a diffusion process that can encompass both molecular motion and turbulent mixing, depending on the scale and flow conditions. The best answer reflects that they are the same kind of transport process described at different scales. At the molecular level, diffusion comes from random molecular motion driven by concentration differences. In a turbulent flow, chaotic eddies mix material much more rapidly, and we capture that effect by introducing an eddy diffusivity. In the governing equations, you often use an effective diffusion coefficient that combines molecular diffusivity and turbulent diffusivity, so the overall transport is described by the same diffusion form, just with different contributions depending on scale and turbulence intensity. This unifies the two under one mathematical framework: diffusion, with the dominant mechanism determined by the flow regime. A note on the other ideas: diffusion is driven by concentration gradients, not temperature gradients in the standard molecular diffusion sense; turbulent diffusion is represented as enhanced mixing via eddy diffusivity rather than a completely separate phenomenon; and turbulent diffusion does not always replace molecular diffusion in all cases—both can be present, with one typically dominating depending on the situation.

The main idea being tested is how diffusion is treated in CFD as a diffusion process that can encompass both molecular motion and turbulent mixing, depending on the scale and flow conditions.

The best answer reflects that they are the same kind of transport process described at different scales. At the molecular level, diffusion comes from random molecular motion driven by concentration differences. In a turbulent flow, chaotic eddies mix material much more rapidly, and we capture that effect by introducing an eddy diffusivity. In the governing equations, you often use an effective diffusion coefficient that combines molecular diffusivity and turbulent diffusivity, so the overall transport is described by the same diffusion form, just with different contributions depending on scale and turbulence intensity. This unifies the two under one mathematical framework: diffusion, with the dominant mechanism determined by the flow regime.

A note on the other ideas: diffusion is driven by concentration gradients, not temperature gradients in the standard molecular diffusion sense; turbulent diffusion is represented as enhanced mixing via eddy diffusivity rather than a completely separate phenomenon; and turbulent diffusion does not always replace molecular diffusion in all cases—both can be present, with one typically dominating depending on the situation.