Calculating Reynold's Number for an Infinite Plate | Understanding the Basics

Hi PF! Can you help me out with determining a Reynold's Number over an infinite plate? I know it to be ##U L / \nu## but ##L## isn't exactly defined. Would it be something more like ##U \delta / \nu## where ##\delta## is a vertical distance, say, the distance of the BL?"PF: "It depends on your application. Boundary layer thickness is used sometimes. Momentum thickness is used sometimes. The distance along the plate is common. Sizes of small feature can be used if they exist. It just depends the problem at hand."Chet: "So is it arbitrary or is there a better thought process behind what is happening?"PF: "It is a
  • #1
member 428835
Hi PF!

Can you help me out with determining a Reynold's Number over an infinite plate? I know it to be ##U L / \nu## but ##L## isn't exactly defined. Would it be something more like ##U \delta / \nu## where ##\delta## is a vertical distance, say, the distance of the BL?

Thanks!
 
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  • #2
It depends on your application. Boundary layer thickness is used sometimes. Momentum thickness is used sometimes. The distance along the plate is common. Sizes of small feature can be used if they exist. It just depends the problem at hand.
 
  • #3
So is it arbitrary or is there a better thought process behind what is happening?
 
  • #4
It is a little bit arbitrary. Really it's all about how the governing equations are normalized, and that all depends on the physics you hope to study.
 
  • #5
boneh3ad said:
It is a little bit arbitrary. Really it's all about how the governing equations are normalized, and that all depends on the physics you hope to study.
I think by "normalized,", boneh3ad means "reduced to dimensionless form."

Chet
 

Related to Calculating Reynold's Number for an Infinite Plate | Understanding the Basics

1. What is Reynold's number and why is it important?

Reynold's number is a dimensionless number used in fluid mechanics to determine the type of flow (laminar or turbulent) in a fluid. It is important because it helps predict the behavior of fluids in various situations, such as around objects or through pipes.

2. How do you calculate Reynold's number for an infinite plate?

To calculate Reynold's number for an infinite plate, you will need the fluid density, fluid velocity, and viscosity of the fluid. Then, you can use the formula Re = (ρ * v * L) / μ, where ρ is the fluid density, v is the fluid velocity, L is the characteristic length (in this case, the length of the infinite plate), and μ is the fluid viscosity.

3. What is the significance of an infinite plate in Reynold's number calculations?

An infinite plate is used in Reynold's number calculations because it represents a simplified scenario of fluid flow. It allows for easier and more accurate calculations compared to more complex shapes and objects.

4. What is considered a high Reynold's number for an infinite plate?

A high Reynold's number for an infinite plate is typically considered to be above 5 million. This means that the flow around the plate is turbulent and the fluid is less affected by viscosity.

5. How does Reynold's number affect the behavior of fluid flow around an infinite plate?

If the Reynold's number is low (below 5 million), the flow around the infinite plate will be laminar, meaning the fluid particles move in parallel layers with little mixing. If the Reynold's number is high (above 5 million), the flow becomes turbulent, with chaotic mixing of fluid particles. This can result in increased drag and changes in pressure distribution around the plate.

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