# Transport phenomenon

## Main Question or Discussion Point

My book states that when a flow around object is considered,

Non dimensional momentum flux is defined as the drag coefficient

In case of flow through tubes it states

The non dimensional momentum flux is defined as the friction factor

What do these statements mean? What do they practically define?

## Answers and Replies

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Chestermiller
Mentor
Many books on transport processes, in order to emphasize the analogy between heat transfer, mass transfer, and momentum transfer, typically regard pressure and stress mechanistically as being equivalent to momentum transfer. (There is certainly valid molecular basis for treating pressure and stress in this way).

For flow past an object, the relationship between the drag force F and the dynamic pressure ##\frac{1}{2}\rho v^2## is expressed as $$\frac{F}{A}=C_D\left(\frac{1}{2}\rho v^2\right)$$where A is the projected area of the object. Since dynamic pressure has units of momentum flux (and, mechanistically, can be regarded as a momentum flux), and since F/A also has units of momentum flux, ##C_D## is thereby sometimes regarded as a dimensionless momentum flux. I personally don't like this interpretation, and it does nothing for me.

In the case of fluid flow in a tube, the relationship between the shear stress at the wall ##\tau## and the dynamic pressure ##\frac{1}{2}\rho v^2## is expressed as $$\tau=f\left(\frac{1}{2}\rho v^2\right)$$where f is the Fanning friction factor. Since dynamic pressure has units of momentum flux (and, mechanistically, can be regarded as a momentum flux), and since the wall shear stress ##\tau## also is interpreted as momentum flux, f is thereby sometimes regarded as a dimensionless momentum flux. I personally don't like this interpretation, and it too does nothing for me.

Rahulx084
Sir , when do we use momentum flux and when do we use dimension less momentum flux?? In a problem if we are required to find momentum flux, which one of both is needed to find? And what will be going to be the difference in both lf them?

Chestermiller
Mentor
Sir , when do we use momentum flux and when do we use dimension less momentum flux?? In a problem if we are required to find momentum flux, which one of both is needed to find? And what will be going to be the difference in both lf them?
Don't worry about that now. You'll get the idea once they teach you how to approach problems. Your time is too valuable to worry about this for now.

My teacher,she doesn't give a dang about what she teaches . Our university have appointed her just because of her approaches to dean . Thats why I was asking you sir .

Chestermiller
Mentor
My teacher,she doesn't give a **** about what she teaches . Our university have appointed her just because of her approaches to dean . Thats why I was asking you sir .
Sorry to hear that. You can count on us at Physics Forums to help you in any way we can.

I was reading about fields in transport phenomenon and it states that fields are defined as continuously varying functions of position . What does this statement implies to the velocity,momentum and temperature fields?

Chestermiller
Mentor
I was reading about fields in transport phenomenon and it states that fields are defined as continuously varying functions of position . What does this statement implies to the velocity,momentum and temperature fields?
When you begin a new topic, please start a new thread.

When you begin a new topic, please start a new thread.
Actually the question is from the same topic the thread I started few days ago , so I thought maybe its okay to ask here

Chestermiller
Mentor
Actually the question is from the same topic the thread I started few days ago , so I thought maybe its okay to ask here
In my judgment as a Physics Forums moderator, it's not, so please start a new thread.