I say you again that original data was ZERO, not any noise increase with speed at all :-D. The SCS noise-speed fix changed this to 10 points (so 20dB) of linear function of speed for SSNs.
I'm not sure about what "gap" you are talking here. If you have water drag in mind (which is quite complicated function of the speed in the water) or difference between laminar and turbulent flow maybe ?
The noise generated is related to both drag and flow kind (turbulent being more noisy) but don't know if it's related so directly to make same plot and to make assumptions about noise created by an object in water from the knowledge of it's drag characteristics. And even if you have plotted function of object's drag in the water vs speed on logarythmic scale for drag, you'd be surprised how the graph looks :-). It bends in the other side than you expect :-).
For sure the Seawolf (and every modern sub) is designed to reduce turbulence and reduce flow noise (or rather to increase the speed at which the flow noise becomes noticeable). And if the machinery noise is low and not increase much with speed, and the flow noise is reduced so that it becomes to show up only at 20kts, then you get a quieter sub than can run faster being quiet. But this would also mean being quieter from competition at lower speeds :-). The situation you describe would occur if there is a minimum amount of machinery noise that CAN'T be reduced even at stop (why?), but on the other hand it is dampened so good that it don't increase with speed almost at all and stays on this lowest level up to 20kts. And then at 20kts the flow noise shows up and raises further with the speed. So base noise level at 0, 5, 10, 15 kts, and then flow noise becomes to show and kicks in at 20kts and higher. Well little strange, if there is some machinery that makes some noise at idle (and you can't reduce it to non-measurable value) then the machinery would make increased noise at half speed or full speed... Or do we assume that every piece of Seawolf's propulsion (reactor and it's cooling, steam generators and turbines, reduction gear and shaft, propulsor) is working or rotating with same speed at 5kts than when at 25kts ?
What is true that the machinery noise raises slower than flow noise and that's why the flow noise predominate at some speed. But I don't believe that it's 110db, 110db, 110db and suddenly starts to raise 120 130 140. It may raise slower at low speeds, and become to raise faster when flow noise predominates. It may be at 20kts in Seawolf and 10kts with 688I maybe, and maybe not. We don't know at what speed the flow noise predominates for modern subs. But I have assummed that it's about 20kts for Seawolf and about 10kts for 688I and set the machinery and flow noise graphs to reflect this. Currently I can't model this in speed-noise curves (because they ARE linear in DW from the very start, it was SC were they were non-linear) but I know it CAN be done, because the FFG has non-linear curve, or more precisely have some flat area on the beginning of the later-linear graph. I can't make sub curves same way because this affects sub minimum speed. It can be compensated and it is compensated for FFG (it can stop) but I don't know exactly how and can't do this. But I plan to find how it was done for FFG and then I could add flat area on beginning of sub's noise graph, even 15kts of it :-). I will plot supposed sub noise profile and then find what possible to achieve DW noise curve would fit best to it. Possible to achieve in DW are linear with some flat area on beginning. Non linear noise function was in Sub Command only and was not very fortunate because even flow noise curve raises sharply at some speed range, but then becomes to flatten in dB scale. In SC it raised faster and faster with speed hitting max already before flank speed.
I think current model is better and more accurate, if I knew how to compensate the minimum RPM value effect on minimum speed it would be even better with flat beginning possible.
|