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• # Convection – diffusion using FV

Posted by on March 13, 2024 at 5:08 pm

Confusion regarding the definition of convection: In the lecture, it has been emphasized many times that convection is advection + diffusion. However, if we look at the governing equation of the convection-diffusion problem, we can see that diffusion is an independent term, and convection is an independent term. So, how can we say convection is advection plus diffusion? I can easily remove the diffusion term from the governing equation, and it will become just a pure convection problem that does not have diffusion. I hope I was able to explain my query. Thanks in advance.

2 Members · 3 Replies
• 3 Replies
• ### Sujith FTL

Member
March 18, 2024 at 8:33 am

Hello,

In the context of the lecture, the emphasis on “convection is advection plus diffusion” is to highlight the combined effects that contribute to the overall transport of a quantity, such as heat or mass. While the governing equation for convection-diffusion problems does indeed separate the terms for convection and diffusion, the essence of convection lies in both advection (bulk motion) and diffusion (spreading or smoothing).

When you remove the diffusion term from the equation, you’re simplifying the problem to one of pure advection, where there’s no molecular diffusion influencing the transport of the quantity. This distinction is important for understanding different scenarios and for mathematical modeling purposes.

Regards,

Sujith

• ### DEEPANKER CHAUDHARY

Member
March 19, 2024 at 9:09 am

Hi, Thanks for your response. But I still confused. Should not be the convective term be called as advective term?

• ### Sujith FTL

Member
March 19, 2024 at 9:34 am

Hello,

• Lecture: The focus is on the bigger picture – convection as the entire heat/mass transfer process, including both bulk flow (advection) and random particle movement (diffusion).
• Equation: It separates these for mathematical reasons. Diffusion acts on a microscopic level, while advection describes large-scale motion. Separating them allows for easier analysis.

Think of soup heating on a stove. Broth circulation is advection, but heat also spreads due to particle movement (diffusion) even without the flow. The equation isolates these for calculation, but in reality, they often work together.

Don’t worry if “convective term” doesn’t directly translate to “advection” in the equation. It considers the bulk motion’s effect (advection) on transporting a property, along with some diffusion.

In cases with negligible diffusion, the convective term might behave very similarly to advection.

Regards,

Sujith