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MikeArmstrong1-Visitor
1-Visitor
September 7, 2015
Solved

Solve Block Issue - Improvement Required

  • September 7, 2015
  • 3 replies
  • 9394 views

I have a spreadsheet which calculates the reaction, deflection and slope of a HDPE pipeline as it is being flooded to the seabed for a given water depth. There is 7 no. simultaneous equations which have to be solved in order to obtain the results.

My issue is that the solve block is very sensitive to the guess values, and I often find myself tweaking them until the solve block returns a solution which can be extremely tedious. - Changing Ra to 50 in this instance.

So my question is, can this method be improved and could the equations be solved symbolically?

Cheers,

Mike

Best answer by AlanStevens

I think if the last equation in your solve block is replaced by one in which moments are taken about B instead of A you will have your last independent equation. I think it will be:  w1*L1^2/2 + w2*L2^2/2 - Ra*L1 - Rc*L2 = 0

This gives a robust solution  (though, since I don't know what the physical scenario is actually like, my equation might well be wrong!).

Alan

3 replies

V
24-Ruby IV
ValeryOchkov24-Ruby IV
24-Ruby IV
September 7, 2015

I have same problem (the solve block is very sensitive to the guess values) with my system 10 equations.

New input data and now solution

My solution of this problem.

When I have a solution of the system I copy it as a guess value.

Than I "little" change input data and copy and paste again.

If I have no solution I do a little step back.

We can do it only in Prime if we have different units in answer:

In Prime

FindCopy.png

    M
    MikeArmstrong1-VisitorAuthor
    1-Visitor
    September 7, 2015

    Valery Ochkov wrote:

    I have same problem (the solve block is very sensitive to the guess values) with my system 10 equations.

    New input data and now solution

    My solution of this problem.

    When I have a solution of the system I copy it as a guess value.

    Than I "little" change input data and copy and paste again.

    Again, I can imagine this being time consuming and cumbersome. I was hoping to get an automated solution or symbolic solution which will not rely on initial guess values.

    V
    24-Ruby IV
    ValeryOchkov24-Ruby IV
    24-Ruby IV
    September 7, 2015

    Mike Armstrong wrote:

    I was hoping to get an automated solution

    Use the Newton method

    http://twtmas.mpei.ac.ru/mas/Worksheets/Newton_3.mcd

    R
    RichardJ19-Tanzanite
    19-Tanzanite
    September 7, 2015

    There's no way you can get s symbolic solution. Your equations are a set of high order polynomials in your unknowns. You can eliminate L1 and L2, to reduce the number of equations, but when you do that it shows up a problem. It seems one of your equations is not independent. Unless I made a mistake, anyway, so you had better check what I did carefully.

    R
    RichardJ19-Tanzanite
    19-Tanzanite
    September 7, 2015

    Update:

    M
    MikeArmstrong1-VisitorAuthor
    1-Visitor
    September 7, 2015

    Even with the update, there still seems to be an error with EQ5.

    Mike

    A
    AndyWesterman12-Amethyst
    12-Amethyst
    September 7, 2015

    Hi Mike,

       I'm not sure, but do you have 7 independent equations?

    I've read through the attached & I can't see an obvious error.

    looks like equations 5 & 7 can be manipulated to give equation 6.

    Hope this helps

    Andy

    M
    MikeArmstrong1-VisitorAuthor
    1-Visitor
    September 7, 2015

    A Westerman wrote:

    Hi Mike,

       I'm not sure, but do you have 7 independent equations?

    I've read through the attached & I can't see an obvious error.

    looks like equations 5 & 7 can be manipulated to give equation 6.

    Hope this helps

    Andy

    Hi Alan,

    I believe I do have 7 independent equations, but unsure why it cannot be solved symbolically. Still digesting Richards sheet

    Mike

    A
    AlanStevens19-Tanzanite
    19-Tanzanite
    September 7, 2015

    The last three equations within the solve block are linear in Ra, Rb and Rc and can be solved symbolically to find them in terms of L1 and L2.  If you do this you find Rc is identically zero.  You can therefore reduce your number of equations/unknowns to 6, but this still doesn't help.  You can further reduce the number of equations to 2 (for L1 and L2) by defining thetab and deltab as functions of L1 and L2 (making use of Ra and Rb, also defined as functions).  You then have two equations/unknowns (L1 and L2), but this still doesn't really help!  Are you sure you have all your signs correct?

    Alan.

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