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Homework Help: Work done on real thermodynamic data

  1. May 11, 2017 #1
    1. The problem statement, all variables and given/known data
    I have the data for a p-v diagram from experiment of a fluid (the fluid is unknown). The data is attached below as the crosses. It also has the approximated ideal cycle for that data. Does anyone know how to find the work done for this real experimental data? Maybe a MATLAB command or some other software? Also, would v2/v1 be the compression ratio for this cycle?

    3. The attempt at a solution
    I tried using the trapz function in MATLAB however unsure whether it is correct or not.

    Attached Files:

  2. jcsd
  3. May 11, 2017 #2
    Do you know how work is defined as the cyclic integral of P with respect to V? You may very well need to do some numerical integration, and you may (or may not) want to use some interpolation/extrapolation to augment the data to what you think is the true curve in regions that are poorly defined.
  4. May 11, 2017 #3
    Are there analytic expressions fitted to the ideal cycle?
  5. May 11, 2017 #4
    Yes there are. The power stroke is pv^-1.5=544.7 and the compression process is pv^-1=199.7
  6. May 11, 2017 #5
    Then you can integrate pdv for each of them from the lowest volume to the highest volume. Those exponents on v are not correct.
  7. May 11, 2017 #6
    Once you assume a functional form, you have abandoned the experimental data for the most part. If we want to work with the experimental data, we should do just that.
  8. May 11, 2017 #7
    I respectfully disagree if the analytic functional form is an excellent fit to the experimental data (as it appears to be in the figure).
  9. May 11, 2017 #8
    It seems that I need to do numerical integration to find the work done for the real data (because I'm specifically told to find the work done from the real data not from the ideal cycle). Does anyone know how to do that in excel say? or does it have to be in MATLAB or some other advanced software?
  10. May 11, 2017 #9
    Aldo, get a calc book or a numerical methods book, and look up "quadrature." You will likely want to look at the Trapezoidal Rule, or possibly Simpson's Rule (the later only if the data are evenly spaced). You can implement these methods in Fortran, BASIC, spead sheet, etc., just about anything that will enable the computer to do a lot of arithmetic for you.
  11. May 11, 2017 #10
    Why don't you just use the trapezoidal rule, and compare with the integration of the analytic fits.
  12. May 11, 2017 #11
    Is this homework or a course lab problem? Or is this from an industrial source? How to interpret the rules depends somewhat on the source of the problem.
  13. May 11, 2017 #12
    It's a course lab problem; yea I think I need to go and read a numerical methods book as you said. Thanks for your help
  14. May 11, 2017 #13
    Can you please provide the actual p-v data?
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