Variational calculus or fluid dynamics for fluid rotating in a cup

Click For Summary

Discussion Overview

The discussion revolves around the problem of determining the curve that describes the surface of a rotating liquid in a cylindrical cup, specifically in the context of whether to approach the problem using variational calculus or fluid dynamics. The scope includes theoretical considerations of fluid behavior under rotation and the mathematical modeling of the surface shape.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Lawrence introduces the problem of finding the surface curve of a rotating liquid and questions whether variational calculus or fluid dynamics is the appropriate method to solve it.
  • Another participant asks for a more specific problem statement to clarify the discussion.
  • Lawrence provides a detailed description of the setup, including parameters like the rate of rotation, density, and dimensions of the cup, and reiterates the question about the appropriate mathematical approach.
  • A participant suggests that the problem can be approached using hydrostatics, mentioning the pressure equation and the forces involved, while also referencing a variational principle for ideal-fluid mechanics.
  • Lawrence clarifies that the problem is not a homework question and expresses intent to consider the provided hints.
  • A later reply asserts that the problem is fundamentally a fluid dynamics issue and suggests it involves more complexity than just hydrostatics.

Areas of Agreement / Disagreement

Participants express differing views on whether the problem should be approached through variational calculus or fluid dynamics, indicating that no consensus has been reached on the best method to solve the problem.

Contextual Notes

The discussion highlights the need for clear definitions and assumptions regarding the forces acting on the fluid, as well as the potential complexity of the problem beyond basic hydrostatics.

LawrenceJB
Messages
3
Reaction score
0
my first post having just joined!
Problem statement - what curve describes the surface of a rotating liquid? Stirring my cup of coffee years ago sparked this thought.
Question - is the way to solve this problem to use variational calculus, or fluid dynamics? I have always thought the former but recently someone suggested to me that it's the latter.
Any thoughts greatly appreciated.
Lawrence
 
Physics news on Phys.org
Do you have a specific, precisely stated, problem in mind?
 
Hi. Here's my problem, stated with more detail.
Imagine a cylindrical cup of coffee, being stirred and where the surface of the stirred coffee has reached some steady state. In terms of the rate of rotation of the liquid and its density, define the equation of the surface of the rotating liquid S(r,h), where r is the distance of a point on the surface of the rotating liquid from the centre of rotation and h is its height above the lowest point on the surface of the rotating liquid. Ignore friction effects of the sides and bottom of the cup which has height H and radius R. The cup is half full of coffee before being stirred.

Hope that about does it. I'd like to know if this is a variational calculus problem or a fluid dynamics problem.

Thanks
 
Is this a homework problem? If so, please post in the homework section next time.

Some hint: In hydrostatics the equation for the pressure reads
$$\vec{\nabla} p=\vec{F},$$
where ##\vec{F}## is the force per volume. At the surface of the fluid you have ##p=0##, where ##p## is measured relative to the atmospheric pressure. Now you have just to specify the forces (gravity and centrifugal forces in the rotating reference frame) to get the equation of the surface. It's not too difficult!

The variational principle for ideal-fluid mechanics can be found in

A. Sommerfeld, Lectures on theoretical physics, vol. 2.
 
Thanks for the response. It's not a homework question - just a problem I thought was interesting.

Let me digest what you've suggested and I'll get back with some clarification questions.
 
LawrenceJB said:
Hi. Here's my problem, stated with more detail.
Imagine a cylindrical cup of coffee, being stirred and where the surface of the stirred coffee has reached some steady state. In terms of the rate of rotation of the liquid and its density, define the equation of the surface of the rotating liquid S(r,h), where r is the distance of a point on the surface of the rotating liquid from the centre of rotation and h is its height above the lowest point on the surface of the rotating liquid. Ignore friction effects of the sides and bottom of the cup which has height H and radius R. The cup is half full of coffee before being stirred.

Hope that about does it. I'd like to know if this is a variational calculus problem or a fluid dynamics problem.

Thanks
It's a fluid dynamics problem, and involves much more than just hydrostatics.
 

Similar threads

  • · Replies 6 ·
Replies
6
Views
2K
  • · Replies 3 ·
Replies
3
Views
1K
  • · Replies 20 ·
Replies
20
Views
10K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 29 ·
Replies
29
Views
5K
  • · Replies 4 ·
Replies
4
Views
3K
  • · Replies 2 ·
Replies
2
Views
4K
  • · Replies 3 ·
Replies
3
Views
3K
  • · Replies 6 ·
Replies
6
Views
4K
  • · Replies 13 ·
Replies
13
Views
1K