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

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    Reynold's number
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Discussion Overview

The discussion focuses on calculating Reynold's Number for an infinite plate, exploring the definition and implications of the characteristic length scale used in the calculation. It encompasses theoretical considerations and conceptual clarifications regarding fluid dynamics.

Discussion Character

  • Exploratory, Conceptual clarification, Debate/contested

Main Points Raised

  • One participant suggests that Reynold's Number is defined as ##U L / \nu## but questions the definition of ##L##, proposing it could be ##U \delta / \nu## where ##\delta## represents boundary layer thickness.
  • Another participant indicates that the choice of characteristic length depends on the specific application, mentioning boundary layer thickness, momentum thickness, and distances along the plate as possible options.
  • A subsequent participant questions whether the choice of length scale is arbitrary or if there is a more systematic approach to understanding it.
  • One participant responds that the choice is somewhat arbitrary and relates to how governing equations are normalized, which is contingent on the physics being studied.
  • A later reply reiterates the idea of normalization, clarifying that it refers to reducing equations to dimensionless form.

Areas of Agreement / Disagreement

Participants express differing views on the arbitrariness of the characteristic length scale in calculating Reynold's Number, with no consensus reached on a definitive approach.

Contextual Notes

The discussion highlights the dependence on specific applications and the potential variability in defining the characteristic length, but does not resolve the implications of these choices.

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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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.
 
So is it arbitrary or is there a better thought process behind what is happening?
 
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.
 
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
 

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