Skewness after duct-bend

[es335]

Hi.

I have a duct with a 90° bend. After the bend I want to have a symmetric velocity profile.
But if I do nothing to the flow, the velocity distribution will be skew.

What can one do to even out the distribution?
And how far from the bend will the skewness remain?


Flow details: u=10m/s. d=200mm

 

[quark]

Generally splitters(for round elbows) and turning vanes(for square elbows) are used to reduce turbulence at the bends.

Regards,


Believe it or not : Eratosthenes, a 3rd century BC true philologist, calculated circumference of earth with the help of a stick and it's shadow. The error was just 4% to the present day calcuated value.

 

[vanstoja]

Flow through a single 90 degree elbow (not a lobsterback bend as in air ducts)usually produces twin, counter-rotating vortices which decay and merge as they proceed downstream. Experimental data for decay of the swirl momentum has been published by Baker & Sayre, Senoo and Nagata and Murakami. Expressions are of the form K/K0=e^-beta*x/D where K is swirl momentum, x is decay distance, d is pipe inside diameter. e is the Naperian and beta is the decay rate constant. Murakami reported beta values of 0.037 and 0.155 for smooth and rough-wall piping. B&S reported beta values of 0.02 to 0.07 for experiments in water. A plot of these curves show swirl momentum ratios at 15 pipe diameters downstream to be 0.74, 0.35 and 0.10 for beta values of 0.02, 0.07 and 0.155, respectively. This data shows that it takes more than 15 pipe diameters to dissipate swirl from a single 90 degree elbow and the axial velocity profile will not likely return to normal anytime sooner. For a discussion of elbow turning vanes to remedy such a problem refer to Idelchick (1986),"Handbook of Hydraulic Resistance" 2nd Edition Chapter 6.