So you have only those two pictures, not a test mission where it could be repeated and measured ? From this one case ? I'm sorry but I have done not one, but dozen of tests, in different scenarios and different ways to measure noise level and I'm absolutely sure it was constant in 1.01. There were also later available some ways to measure this directly, something like Debugoutput showing platform noise level, results were the same.
The difference could be because of cavitation, but you said it wasn't.
I see there are two pictures, how were they made ? Was it a Kilo that run at 12kts and then increased speed to 17kts ? Or was it the same mission run two times, once with Kilo set to 12kts, and second time with Kilo set to 17kts ? Because you can't compare results from 2 different runs even of the same scenario. Water conditions are different each time, layer depth is different and detection ranges can vary, so the strentgh of the signal.
If you can replicate the situation and send me the mission I could check it, at least in few ways. But that would have to be two Kilos in same mission, one going 12kts and the other 17kts and generating different amount of noise. Or Kilo going 12, and after reaching a waypoint speeding to 17kts but in the same mission.
About the noise becoming stronger with turbulent flow - of course but I'm not sure the difference is so sharp UNDERWATER, where is no air which can mix with water causing bubbles, splashing and noise. And the speed at which flow becomes turbulent can be different at differents part of the sub, just like local Mach number can be different around differents parts of a plane.
The flow probably becomes turbulent on low scale (close to submarine skin) much sooner than 20kts, only the shapes of hull are designed to keep the flow smooth and produce less noise. It's my guess of course but it's hard for me to imagine fully laminar water flow around sub surface at speed of 15kts or more. I think the noise level of flow would increase more or less smoothly, if the shape of sub is designed properly (so modern sub).
The really sharp increase in noise level underwater is connected with cavitation rather, which occurs only at much higher speeds, like for propeller blade or Shkval torpedo.
But as I said you could probably clearly see on the graph of broadband noise where the flow noise becomes to predominate other noises - the plot would become to raise faster from there.
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