I think there is something written in the Mathworks, but there is no detailed explanation at all:(
So I faced the same problem in AppDesign...
Try changing the code as follows!
simInp = simInp.setVariable('head',app.ConstantHeadMPaKnob.Value,'Workspace','head_loss');
And before creating the App, you can use the Matlab code
to see that the Simulink results change depending on the value you set.
For example)
>> ConstantHeadMPaKnob = 10;
>> simInp = Simulink.SimulationInput('head_loss');
>> simInp = simInp.setVariable('head', ConstantHeadMPaKnob,'Workspace', 'head_loss');
>> simOut = sim(simInp);
Thank you, and good Luck!
