Difference between 1D, 2D and 3D Flow

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Discussion Overview

The discussion focuses on the differences between 1D, 2D, and 3D flows in hydraulics, exploring the definitions, examples, and implications of each type of flow. Participants seek to clarify the concepts and their applications in real-life scenarios.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant inquires about the differences between 1D, 2D, and 3D flows, specifically in the context of hydraulics.
  • Another participant explains that the terms refer to the number of spatial or time dimensions over which flow variables vary.
  • A participant suggests that 2D flow occurs in a plane, while 1D flow is linear.
  • It is noted that 1D flow may be considered less interesting, and 2D flow can arise in axisymmetric situations.
  • Examples of axisymmetric flows are provided, including the flow around a cone at zero angle of attack and the flow around a missile's front end.
  • A participant presents equations to illustrate the dimensionality of flow, suggesting that certain equations represent 3D or 1D flows.
  • It is proposed that while all flows are inherently 3D, they can be simplified to 2D or 1D for calculations without significant loss of accuracy.
  • Examples are given, such as studying an airplane wing section in 2D and flow in a pipe in 1D.

Areas of Agreement / Disagreement

Participants generally agree on the definitions and implications of 1D, 2D, and 3D flows, but there are nuances in the examples and applications discussed. No consensus is reached on the specific conditions under which flows can be simplified to lower dimensions.

Contextual Notes

Some assumptions about the conditions for axisymmetry and the accuracy of simplifications in flow analysis remain unresolved. The discussion does not delve into the mathematical rigor behind the equations presented.

amck
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Hey guys,

I'm new to this forum and was hoping to get a clear answer regarding the difference between 1D, 2D and 3D flows in hydraulics (ex. 2D Numerical Model...) ?

Thanks a bunch!
-A.
 
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It usually refers to the number spatial (or time) dimensions over which the flow variables vary in a given situation or problem.
 
Thanks for the response! So if you say 2D flow - it's the flow in a plane whereas 1D would be linear?
 
Pretty much, though a 1D flow would be awfully uninteresting. A 2-D flow may also arise if, say, the flow is axisymmetric.
 
Fair enough. What kind of "real-life" flow would be considered axisymmetric?
 
The flow around a cone at zero angle of attack, the flow around the Apollo space capsule if it is at zero angle of attack the flow around the front end of a missile, the flow around a non-rotating sphere (in the Stokes limit), and the flow out of a rocket engine would all be examples of axisymmetric flows.
 
so an equation like v=1xt i + 2xt j +6xt k would be 3d

but V= 1xt i +6x j+7 k wouldbe 1D?
 
A 3D flow assumes that a particle of fluid can go either forward or backward, up or down, left or right.

All flows are 3D, but some can be estimated to a 2D or even 1D flow to simplified the calculations without loosing to much accuracy.

For example, you can study an airplane wing section to evaluate its lifting potential in 2D (particles goes from front to rear and can also go up or down):
wing-aerodynamics-mathspig.png
You can also study flow in a pipe in 1D situation (from inlet to outlet). In the above example, instead of considering the particles moving up and down when entering the enlarged pipe, we can just consider the area change:

790px-BernoullisLawDerivationDiagram.svg.png


More info about One, Two and Three Dimensional Flows.
 

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