Hi You know draft Tube after a Turbine decreases velocity and increases head of flow (it changes kinematic energy of flow to pressure energy). It is used at outlet of turbine to increase pressure head from sub-atmospheric pressure to zero. Now this is the question: Why is pressure at the outlet of turbine, negative? Flow has a high pressure at inlet of turbine. it rotates turbine and then moves out of turbine. What happens here which results in negative pressure?