[zdunol]

Hi, this question regards fluid dynamics in general not cfd, if I shouldn't post in here then I am sorry :c

I am wondering why and if reducing inlet diameter to some volume (rectangular cavity for example) would result in different (higher?) turbulence. First answer is obvious - for constant volumetric flow rate and reduced inlet diameter velocity must go up and therefore Reynolds number and turbulence will go up. However Reynolds number is a ratio of inertia and viscous forces and they do not depend on inlet diameter (amount of momentum is constant i think). I wonder if this is related to Kolmogorov's spectrum of energy(where energy density drops with eddies size growth), but I do not understand this concept to much, so I am not sure.

[FMDenaro]

Originally Posted by zdunol

Hi, this question regards fluid dynamics in general not cfd, if I shouldn't post in here then I am sorry :c

I am wondering why and if reducing inlet diameter to some volume (rectangular cavity for example) would result in different (higher?) turbulence. First answer is obvious - for constant volumetric flow rate and reduced inlet diameter velocity must go up and therefore Reynolds number and turbulence will go up. However Reynolds number is a ratio of inertia and viscous forces and they do not depend on inlet diameter (amount of momentum is constant i think). I wonder if this is related to Kolmogorov's spectrum of energy(where energy density drops with eddies size growth), but I do not understand this concept to much, so I am not sure.

if you are talking about inlet in a pipe or channel or flat plate, you have to consider that the local Reynolds number increases depending on the increased longitudinal position. Thus, you will encounter transition to turbulence at some position

[virendra_p]

If the velocity increases won't the momentum carried by fluid increase? After all momentum is mass x velocity (mass constant on account of constant flow rate)....thus the flow accelerates and the inertia force increase.....but if you are correlating it with eddy length scale...i don't know much...but interesting view...some expert on turbulence should clarify further please...

[zdunol]

Originally Posted by FMDenaro

if you are talking about inlet in a pipe or channel or flat plate, you have to consider that the local Reynolds number increases depending on the increased longitudinal position. Thus, you will encounter transition to turbulence at some position

It does not matter (at least too much) if flow at inlet pipe is turbulent or not, because when fluid arrives at the volume it changes its direction (turns 90 deg.) and turbulence in this area of volume(cavity) in my opinion depends on an inlet size because we have constant energy flux density [J/s] but less space