Fluid comunity: A question: As rule of thumb and without making any calculation, What is the length required to establish a laminar flow (of gas = air ) through a 2" DIA tube downstream of the elbow, if it were 90 degrees?

(1). The flow will be turbulent all the way downstream of the elbow, if I understand your "laminar flow" definition correctly.

Hi: I thank if Re number is small ,It is possible that fluid flow a bend that still have laminer . But I thanks it will happens secondary flow.

(1). Well I guess, if he turns off the vlave, then, there will be no flow in the pipe. The flow will become laminar if one wait long enough. (2). The critical Reynolds number for flow to become turbulent in a smooth pipe is Re,cre=2300. For a 2"DIA pipe, and air flow, the mean flow velocity will have to be less than a couple of feet per second, which is about 1.5mile/hour wind. (3). I think, one can always make the flow laminar, if the flow rate is reduced such that the mean velocity in the pipe is below 2ft/sec.

John and olivia: Thanks to both! Point well taken, there are secondary flows and it is difficult to keep it laminar due to the velocities involved!