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Wave Speed - Model Differs from Hand Calculation

Hello,

I attempted to confirm the wave speeds calculated by HAMMER using the Wave Speed Calculator Tool versus calculating them by hand. I calculated wave speeds by hand using the same exact equation that HAMMER uses (provided here: Equation used in Wave Speed Calculator tool - OpenFlows | Hydraulics and Hydrology Wiki - OpenFlows | Hydraulics and Hydrology - Bentley Communities) and values taken from the materials library within the model. The values I calculated by hand do not match the values calculated using the Wave Speed Calculator Tool. Is there an explanation why HAMMER does not reproduce exactly values I calculate by hand? Below are tables of data used to calculate the wave speeds and a comparison of hand calculated versus model calculated values.

  Units Water at 4C Cast-in-place concrete Steel
a = Wave speed (ft/s,m/s)  
Ev  = Bulk modulus of Elasticity (lbf/ft2, Pa) 45699927.92  
ρ = Liquid density (slugs/ft3, kg/m3) 1.94  
D = Diameter (in, mm)  
e = Wall thickness (in, mm)  
E = Young's Modulus (lbf/ft2, Pa) 417708000 4323277800
Ψ = pipeline support factor 0.925 0.85
μ = Poisson's ratio     0.15 0.3

Pipe Diameter (feet) Design Thickness (in) Material Hand Calculated Wave Speed (ft/s) HAMMER Wave Speed (ft/s) Ratio Difference (ft/s)
1 13 16 Cast-in-place concrete 3443.42 3,375.83 1.02002 67.59
2 13 16 Cast-in-place concrete 3443.42 3,375.83 1.02002 67.59
3 13 16 Cast-in-place concrete 3443.42 3,375.83 1.02002 67.59
4 13 16 Cast-in-place concrete 3443.42 3,375.83 1.02002 67.59
5 13 16 Cast-in-place concrete 3443.42 3,375.83 1.02002 67.59
6 13 16 Cast-in-place concrete 3443.42 3,375.83 1.02002 67.59
7 13.333 0.375 Steel 2207.62 2,067.49 1.06778 140.13
8 13.333 0.4375 Steel 2344.43 2,200.12 1.06559 144.31
9 13.333 0.5 Steel 2465.54 2,364.31 1.04282 101.23
10 13.333 0.625 Steel 2671.73 2,520.97 1.0598 150.76
11 13.333 0.6875 Steel 2760.62 2,609.05 1.05809 151.57
12 13.333 0.75 Steel 2841.88 2,689.95 1.05648 151.93
13 9 0.625 Steel 3037.83 2,836.15 1.07111 201.68
14 9 0.625 Steel 3037.83 2,836.15 1.07111 201.68
15 9 0.625 Steel 3037.83 2,836.15 1.07111 201.68
16 8.5 0.625 Steel 3090.50 2,886.61 1.07063 203.89
17 8.5 0.625 Steel 3090.50 2,886.61 1.07063 203.89
18 8.5 0.625 Steel 3090.50 2,886.61 1.07063 203.89
19 13.333 0.75 Steel 2841.88 2,689.95 1.05648 151.93
20 9 0.625 Steel 3037.83 2,836.15 1.07111 201.68
21 9 0.625 Steel 3037.83 2,836.15 1.07111 201.68
22 9 0.625 Steel 3037.83 2,836.15 1.07111 201.68
23 8.5 0.625 Steel 3090.50 2,886.61 1.07063 203.89
24 8.5 0.625 Steel 3090.50 2,886.61 1.07063 203.89
25 8.5 0.625 Steel 3090.50 2,886.61 1.07063 203.89
  • Hi Michael,

    I took a deep look at this for one of your example pipes (pipe 13, 9 ft diameter steel) and got the same numbers for a hand calc and from HAMMER. You had noted a pipeline support factor of 0.85 for this, which appears to correspond to "expansion joints throughout". However if you re-do the hand calc assuming 1.0 for the pipeline support factor, your hand calc will exactly match the resulting wave speed, so this is what HAMMER appears to be doing.

    I then selected "Anchored throughout" which works out to a pipeline support factor of 0.91 for the same pipe, and my hand calcs also matched HAMMER's result. I then did it a third time, choosing "supported at one end", which works out to a pipeline support factor of 0.95 for the same pipe, and my hand calcs again. So, it appears that HAMMER is assuming 1.0 when choosing the "expansion joints" option, which goes against what we had documented. However, I found a few references that actually mentioned 1.0 as an assumption in this case. So, I will check with our developers to confirm the expected behavior and will then update the wiki article.

    Note that you always have the option of entering your own wave speeds based on your own approach (and they can even be imported via ModelBuilder if you have them in a spreadsheet for example).

    Also note (as mentioned in the article) that the Korteweg equation is only considered valid for thin walled pipes, and it looks like your 16" thick concrete pipes do not meet that criteria.

    If in doubt, consider a sensitivity analysis whereby a range of wave speeds are tested and the transient response is compared to determine if the model is sensitive (or if you do not need to worry about getting it perfectly accurate).


    Regards,

    Jesse Dringoli
    Technical Support Manager, OpenFlows
    Bentley Communities Site Administrator
    Bentley Systems, Inc.

  • Thank you for your help, Jesse.

  • I appears you're using inputs with from 1 to 8 significant figures and reporting the results to 6 significant figures. You need to look at the true precision of measurements.

    With wave speeds in particular,  slight amount of dissolved gas in the water can dramatically change the wave speed.

    Plus, is the steel or concrete you will be using the exact same steel or concrete used to develop values of Poisson ratio or Young's modulus? As steel corrodes and concrete deteriorates, these values can change.

    Bottom line is that in working on a transient analysis for a large pipeline like this, engineers build in a lot of safety factors such that the difference between these wave speeds are insignificant. If you are unsure of the precision of a value, a good engineer would perform a sensitivity analysis and look at the impact of varying the wave speed. I would guess it would be insignificant but you as the engineer would need to judge that.

  • Michael as a follow-up to my previous reply, I have confirmed from our developers that the "expansion joints throughout" option assumes a pipeline support factor of 1.0. I have updated the related wiki article accordingly.


    Regards,

    Jesse Dringoli
    Technical Support Manager, OpenFlows
    Bentley Communities Site Administrator
    Bentley Systems, Inc.