When water flows through the converging side of a Venturi, why is it that the total head (experimental and calculated) is not constant? Could it be due to losses?

Also, why was it that a lot more total head was lost when the fluid came out of the diverging side? Could it be due to turbulent flow?

And why does volume flow rate effect steadiness of flow?

Also why is the total head different for the experimental and calculated data as soon as it enters the Venturi too?

This is my data, sorry it doesn’t let me upload a picture as it’s too big: Axial position: 0, 60.28, 68.68, 73.18, 81.08, 141.54 Static head: 230, 194, 158, 121, 75, 130 Total head measured: 235, 230, 230, 228, 224, 149 Total head calculated: 234, 239, 244, 249, 242, 134

I don’t get why the total head please starts at 1mm difference and how it remains the same at 230mm for the experimental one and how the calculated total head is increasing?

enter image description here

  • $\begingroup$ Please show the experiment, the theoretical model, and your calculations. $\endgroup$
    – Phil Sweet
    Commented Apr 15, 2021 at 10:06
  • $\begingroup$ Is this a sealed round pipe, or a channel? Pressures, flow rate, atmospheric pressure? Are the transitions sharp, inset (like a weld), or rounded? What is the intention of this test? If something is not constant, describe its relationship over time in cycles per seconds, amplitude, etc. $\endgroup$
    – ericnutsch
    Commented Apr 15, 2021 at 21:36

1 Answer 1


Hi sorry I was posting as a guest before and it signed me out so I have to answer instead of comment.

It’s a duct where both ends are open and water is flowing in from converging end. Transitions are smooth and we are trying to find validity of Bernoulli’s equation.

I’ll delete this message after you see it


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