First Derivative of Periodic Tube Profile | Get Help Now

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Discussion Overview

The discussion centers around calculating the first derivative of a periodic tube profile represented by a set of data points in axial and radial coordinates. Participants explore methods for deriving the first derivative from discrete data, with a focus on the specific context of the user's work.

Discussion Character

  • Technical explanation, Mathematical reasoning, Debate/contested

Main Points Raised

  • One participant requests assistance in calculating the first derivative of a periodic tube profile given a set of data points.
  • Another participant asks for clarification on the specific derivative to be calculated and requests a sketch of the setup, as well as sample data.
  • A participant provides a data set and specifies the derivative they wish to calculate, dh/dz.
  • Concerns are raised about the consistency between the provided data and the graph, prompting a discussion on the definition of the derivative and methods for calculating it from discrete datasets.
  • A proposed method for calculating the derivative is presented, using a central difference formula.
  • One participant warns about the unreliability of numeric derivatives and suggests fitting the data to a sinusoidal function to obtain more accurate derivatives.

Areas of Agreement / Disagreement

Participants express varying opinions on the best method to calculate the derivative, with some supporting the numeric approach and others advocating for data fitting. There is no consensus on the optimal method, and the discussion remains unresolved regarding the best approach.

Contextual Notes

Participants note potential issues with the accuracy of numeric derivatives and the importance of data fitting, highlighting limitations in the reliability of results derived from discrete datasets.

nazmulislam
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Hi,

I have some points say, 100 points which come from a periodic tube profile, i.e., (z,r), where z and r are the axial and radial coordinates, respectively.

Now, I need to calculate the first derivative at each point.

Could you please help me in this regard?

Cheers
 
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nazmulislam said:
Hi,

I have some points say, 100 points which come from a periodic tube profile, i.e., (z,r), where z and r are the axial and radial coordinates, respectively.

Now, I need to calculate the first derivative at each point.

Could you please help me in this regard?

Cheers

Can you post a sketch of the setup? And you want to calculate the derivatave of what with respect to what? Can you show some sample data?

Is this for schoolwork, or is a data set from your work?
 
Hi, I have given the profile below

upload_2015-3-25_12-58-25.png


I want to calculate dh/dz. Some data are below:

z h
-1 1
-0.8 0.823664
-0.6 0.714683
-0.4 0.714683
-0.2 0.823664
0 1
0.2 1.176336
0.4 1.285317
0.6 1.285317
0.8 1.176336
1 1

The data set from my work.

Thanks
 

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nazmulislam said:
I want to calculate dh/dz

It looks to me like your data do not match your graph...

But in any case, are you aware of the definition of the derivative? Or implementations in discrete datasets?

One way to do it is:

[tex]h'(z) =\frac{h(z+1)-h(z-1)}{2z}[/tex]

Okay, LaTeX isn't working for me tonight... In clear text:
Note from Mark44: It's fixed now @Berkman -- there was an extra brace that I removed.

h'(z) = ( h(z+1) - h(z-1) ) / 2z

Where 2z is the distance between the 2 bracketing datapoints (in the units of z). Does that make sense?
 
Last edited by a moderator:
Yes, I understand the facts. Many thanks.

Cheers
 
Numeric derivatives are notoriously bad, so take your results with a grain of salt. A better approach would be to attempt to fit your data to a sinusoidal function. Then get your derivatives from that.
 
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