Question

Which of these statements hold true?

(a) Diffusive flux is always positive

(b) Diffusive flux is positive in the direction of the positive gradient of fluid property

(c) Diffusive flux is positive in the direction of the negative gradient of fluid property

(d) Diffusive flux is always negative

This question was posed to me by my school teacher while I was bunking the class.

This interesting question is from General Transport Equation topic in portion Governing Equations of Fluid Dynamics of Computational Fluid Dynamics

Answer 1

The correct answer is:

(c) Diffusive flux is positive in the direction of the negative gradient of fluid property

In the context of the General Transport Equation, diffusive flux refers to the movement of a fluid property (such as temperature or concentration) from regions of higher to lower concentration or temperature. This occurs in the direction opposite to the gradient of the fluid property.

• When a property has a positive gradient (increasing in the direction of the gradient), the flux will be directed towards the lower concentration (opposite to the gradient).
• The negative gradient (decreasing in the direction of the gradient) indicates the direction in which diffusion naturally occurs.

Therefore, the diffusive flux is positive in the direction of the negative gradient of the fluid property, meaning the flux moves from high to low concentration.