physea said:
The velocity downstream and upstream a bend may be equal, but IN the bend? Won't it get affected?
And the turbulence created after the bend, won't affect the mean velocity nearly after the bend?
So, I understand that any losses inside a pipe will be exhibited as pressure drops and not velocity drops? (again I mean average velocity).
Think of it this way. Let's say you have a pipe network with no branches (one inlet, one outlet) and you are pumping in water at a steady volumetric flow rate of ##Q##. Along the entire length of that pipe network, regardless of the fittings, the volumetric flow rate must still be ##Q##, otherwise you would have mass missing (total mass flow out has to equal total mass flow in).
Now, volumetric flow rate is related to cross-sectional area, ##A##, and the
average velocity, ##V_{avg}##, over that area by ##Q = V_{avg}A##. So, regardless of fittings or turbulence or anything else, in order to make sure mass is conserved, the average velocity is
always ##V_{avg} = Q/A##. If you pick two positions with the same cross-section, then the average velocity will be the same, even if there is a fitting between them, since ##Q## is constant.
Now, across any cross-section of the pipe, the velocity will not be constant. it will vary from zero at the wall to some maximum at the centerline (or at least near the centerline), and this fact is not reflected in the use of ##V_{avg}##. Perhaps this is where you are getting confused. The relationship between the velocity at a specific point at that average velocity is
V_{avg} = \dfrac{\oint_A V(r,\theta)\;dA}{\oint_A dA} = \dfrac{1}{A}\oint_A V(r,\theta)\;dA.
So, in short, fittings and turbulence can affect the distribution of ##V(r,\theta)##, but they won't change the value of ##V_{avg}## because that would break the law of conservation of mass.
For example, a laminar flow in a straight pipe will have a parabolic ##V(r,\theta)## profile centered at the centerline with no ##|theta## dependence. A turbulent flow would have a smaller maximum ##V(r,\theta)## but the profile would be "fuller", i.e. it would have a sharper gradient near the wall and a flatter profile near the centerline. A bend would tend to move the maximum point off of the centerline. Fittings could do any number of things depending on the type. The one universal law here is that ##Q## is constant but ##V(r,\theta)## is not.