I'm working on my master thesis in Fluid mechanics and turbomachinary field. My project is an axial water turbine called Kaplan turbine that besides having main runner has a sub-runner in series with itself which identifies upstream flow characters from upstream main runner and uses them to adjust main runner blade angles through a control mechanism. I obtained blade angles in 3 radial spans for both runners through a design code in MATLAB software and designed the turbine blades geometry in ANSYS BLADEGEN software and meshed it in ANSYS TURBOGRID. Finally, I simulated my meshed geometry in ANSYS CFX solver. The run proceeded well and the solution converged to the prescribed resiguals for the flow equations. But my problem is that obtained Head from cfd isn't equal to the pre-determined Head at the beginning of the design but close with it, although the net power and ratio of the two runners Head is approximately equal to its pre-determined value. I'll be glad to help me solve this issue.