Fully Developed Flow: Laminar vs Turbulent

In summary, the definition of fully developed flow may differ for laminar and turbulent flows due to the different entrance lengths. However, the general definition is that the velocity profile and momentum remain constant with respect to the stream coordinate. There is also consideration for the effects of temperature and concentration gradients, which may prevent the flow from being fully developed. There is no specific mention of this definition in textbooks, but it is important to note that the viscosity must be constant for the flow to be fully developed.
  • #1
Kensiber
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Does the definition of fully developed flow is different for laminar and turbulent?
I understand the fact that the entrance length are different in laminar and turbulent flows, but I believe the definition of fully hydrodynamically developed flow means that the velocity profile (hence momentum) doesn’t change with respect to the stream coordinate.
Please give some insights?
Thanks

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  • #2
That sounds correct to me. Why did you think something else was possible?
 
  • #3
I thought this definition of "fully developed flow" may change with the temperature gradient and concentration gradient between the fluid and pipe wall. Otherwise, it should be defined as follows.
"Except the pressure gradient in the pipe section (which balances shear resistance to sustain uniform velocity profile) all fluid parameters such as temperature and concentration difference must be zero to have a fully developed flow." I couldn't find something similar to this definition in any textbooks. [there are explanations about concentration and thermal boundary layers separately, but couldn't see one description including all these phenomena]
 
  • #4
Well, if the temperature is varying radially, it must also be changing axially, so the flow can't be fully developed unless the viscosity is independent of temperature. At least, that is my take on this.
 
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1. What is fully developed flow?

Fully developed flow is a type of fluid flow where the velocity profile remains constant along the flow direction. This means that the fluid particles move at the same speed and in the same direction at any given point in the flow. It occurs when the fluid has traveled a sufficient distance to reach a steady state and there are no changes in the flow conditions.

2. What is the difference between laminar and turbulent flow?

Laminar flow is characterized by smooth and orderly movement of fluid particles in a straight line, with little to no mixing between layers. Turbulent flow, on the other hand, is characterized by chaotic and irregular movement of fluid particles, with mixing and eddies present. Laminar flow is more common in low velocity and highly viscous fluids, while turbulent flow is more common in high velocity and less viscous fluids.

3. How do you determine if a flow is laminar or turbulent?

The Reynolds number (Re) is used to determine if a flow is laminar or turbulent. It is calculated by multiplying the fluid velocity, characteristic length, and fluid density, and dividing it by the fluid viscosity. If the Re is less than 2300, the flow is laminar, and if it is greater than 4000, the flow is turbulent. In between these values, the flow can be transitional, meaning it exhibits characteristics of both laminar and turbulent flow.

4. What are the main factors that affect flow transition from laminar to turbulent?

The main factors that affect flow transition from laminar to turbulent include fluid velocity, fluid viscosity, and pipe diameter. Higher fluid velocities and lower viscosities promote turbulent flow, while lower velocities and higher viscosities promote laminar flow. Smaller pipe diameters also promote laminar flow, while larger diameters promote turbulent flow.

5. What are the practical applications of understanding fully developed flow?

Understanding fully developed flow is important in various industries, such as in hydraulic and fluid systems, where the efficiency and performance of pumps, valves, and pipes are affected by flow characteristics. It is also important in the design and optimization of heat exchangers and other industrial processes that involve fluid flow. Additionally, understanding the transition from laminar to turbulent flow is crucial in predicting and controlling flow patterns and turbulence-induced phenomena, such as pressure drop and flow separation.

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