Used to determine the normalized flow rate V_n at
peak efficiency eta.
It is used in the function
internalCoefficientsFromMeasurements.
function res_detadV extends Modelica.Math.Nonlinear.Interfaces.partialScalarFunction; input Real ch[3] "Coefficients of head polynomial"; input Real cp[3] "Coefficients of power polynomial"; end res_detadV;