Error Using Solve Function

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Brian Robinson
Brian Robinson 2020-4-30
评论： darova 2020-5-1
Hello there,
I'm getting an error using the solve function. What is the reason for this and how do we correct it? See below.
Q = 5; s_0 = 0.005; n = 0.02; L = 35; k_e = 0.5; C_D = 0.65; H = 4; g = 9.81;
syms D deltaH
A = pi*D^2/4;
V = Q/A;
R_H = D/4;
eqn1 = deltaH == H - D + L*s_0;
eqn2 = deltaH == (k_e + 2*g*n^2*L/R_H^(4/3) + 1)*(Q^2/(2*g*A^2));
deltaH = double(solve(eqn1 == eqn2, D,"Real",true))
Thanks very much,
Brian
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Brian Robinson
Brian Robinson 2020-5-1
When I try it I get deltaH = 10 and D = 4.138. Which is the same answer as I got with no parameter.
Regardless, I want to get the other solution which is 1 < D < 2. I can see this solution when I graph the two equations.
darova
darova 2020-5-1
What about this
[deltaH, D] = vpasolve(eqn1 == eqn2, [deltaH, D],[2 1])
deltaH =
1.1885695516004419959725632337124
D =
2.0
[deltaH, D] = vpasolve(eqn1 == eqn2, [deltaH, D],[2 4])
deltaH =
4.1628911435565798210779440646728
D =
2.0

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