Often it's desirable to reduce the turbulence level in an internal flow without changing the Reynolds number or fluid. This could be measured by the turbulence intensity, though other measures might be better in different circumstances. I haven't seen a good compilation of these techniques, and I believe such a compilation could be very useful to engineers.
Below are the ways I'm aware of:
Flow straighteners reduce turbulence and swirl, but have an associated pressure drop. You can also get similar effects with meshes, porous media, and fibers.
It's commonly believed that turbulence transition begins in pipe flows at a Reynold numbers of about $2300$ in general, but laminar flows at far higher Reynolds numbers can be obtained with better quality experimental setups as discussed in this review paper on turbulence transition in pipe flows:
Experimental evidence suggests that the laminar state can be achieved in pipe flows over a wide range of $Re$ with the record standing at $Re = 100,000$ by Pfenniger (1961). Reynolds himself managed to achieve $Re = 13,000$, and Ekman (1911) later improved on this to $∼50,000$ using Reynolds’ original apparatus. There have been many others (Barnes & Coker 1905, Draad et al. 1998, Gilbrech & Hale 1965, Goldstein 1965, Hof et al. 2003, Nishi et al. 2008, Paterson & Abernathy 1972, Schiller 1921, Wygnanski & Champagne 1973) who have achieved Hagen-Poiseuille flow over a wide range of $Re$ greater than 20,000, which is the upper value in Figure 2. Achieving laminar flows at high values of $Re$ is an indication of the quality of an experimental facility and gives some confidence that the observations will not be contaminated by extraneous background disturbances such as entrance flow effects, convection, and geometrical irregularities.
From what I understand, in addition to the effects listed above, surface roughness and vibration can also trigger turbulence, so you can delay the development of turbulence with smoother surfaces and/or vibration damping in some cases. The standard transition criteria appears to be correct, but for systems that don't take special precautions to eliminate disturbances.
Relaminarization is when a turbulent flow becomes laminar, and is a stricter version of what I'm after. I'm interested in reducing the turbulence level. The flow could still remain turbulent, just less so. Here are a few ways to do this discussed by this review article:
- dissipation of turbulence (could happen from enlargement in a pipe)
- curved or rotating flows (could introduce swirl, though)
- acceleration (could happen from a contraction in a pipe)
There are a number of papers on suppression of turbulence via sound emission or vibration. I don't know much about this, but I'm aware that it exists. If anyone knows more about this, or knows a good place to learn more, I'm interested.
If you can change the fluid entirely, that could be a good option to reduce turbulence by reducing the Reynolds number. There also are a variety of additives (mainly for water?) which can suppress turbulence (polymers, fibers, viscous liquids, etc.), but that's an entire other subject.
What other methods exist?