possibly something in the data?
I did simple driver oprogram and changes the 'x' values and had no run time errors
Option Explicit
Sub drv()
Dim y() As Double, dydx() As Double
ReDim y(1)
ReDim dydx(1)
y(0) = 12.3
y(1) = 23.4
Call Derivs(0, y, dydx)
MsgBox dydx(0)
MsgBox dydx(1)
End Sub
Sub Derivs(x As Double, y() As Double, dydx() As Double)
Const g As Double = 32.1740485564
Const Hr As Double = 100
Const h0 As Double = 80
Const fm As Double = 0.024
Const L As Double = 1500
Const dp As Double = 2
Const tc As Double = 5
Const k As Double = 25.7
Const Di As Double = 5
Dim u0 As Double
Dim Qv As Double
Dim Qv0 As Double
Dim hstar As Double
u0 = ((g * h0 / ((1 / 2) * fm * (L / dp))) * ((Hr / h0) - 1)) ^ (1 / 2)
Qv0 = (u0 * 3.14 * Di ^ 2) / 4
hstar = h0 - (Qv0 / k) ^ 2
If x >= 1 Then
Qv = 0
Else
Qv = k * (h0 ^ 0.5) * (1 - x) * (y(1) - hstar / h0) ^ 0.5
End If
dydx(0) = ((tc * g * h0) / (L * u0)) * (((Hr / h0) - y(1)) - ((Hr / h0) - 1) * y(0) * Abs(y(0)))
dydx(1) = ((dp / Di) ^ 2) * (u0 * tc / h0) * y(0) - ((4 * Qv * tc) / (3.14 * h0 * Di ^ 2))
End Sub
Sorry,
Paul