CFD Online Discussion Forums - How apply pressure gradient to get fully developed flow?

Hi, I'm working on a problem that deals with cylindrical obstacles being used as turbulence promoters in a laminar flow in a pipe. I need to achieve a fully developed flow at the pipe inlet and the base paper I'm working on says that they achieved this by "imposing a constant mean pressure gradient until the flow reached a statistically stable state".

So does anyone knows how to do this? I mean what kind of BC do I employ? I don't want to change the pipe length for every different Reynold's number simulation and change the geometry in GAMBIT and do the simulation all over again.

Please help me out guys :(

This is the excerpt from the paper where the "problem formulation" is given:

Quote:

A CFD code (FLUENT, v. 6.0.12), which employs the ﬁnite-volume method, was used. During each simulation the governing equations were integrated in time by imposing a constant mean pressure gradient until the ﬂow reached a sta-tistically steady state, that is when all mean variables such as velocities and Reynolds stresses reach steady values. Time was advanced by a second-order Adams–Moulton scheme and the convective terms were discretized by a second-order upwind scheme. In all the simulations the dimensionless
time step was less than 0.0015. The computational grid was chosen to be ﬁner near the cylinder and the solid surfaces, in order to closely capture boundary layers and high-shear zones. By numerical experimentation it was found that a grid containing 5400 nodes was adequate for the range of
Reynolds numbers covered in this study. This issue is dis-cussed in more detail in the next section.