# Why are there two fans in open circuit wind tunnel in laboratory?

In an experiment in hydraulics lab of my college, very recently someone asked me, why were there two fans in the open circuit wind tunnel.

That wind tunnel is used for conducting experiment for finding boundary layer thickness over a flat plate.

Momentum thickness, displacement thickness and energy thickness, over a flat plate.

probably because one fan would not develop enough pressure rise to drive the tunnel to its specified operating condition. This would be true if the fans were installed in tandem i.e., one fan in front of the other fan.

It might be true if a single fan could not develop enough mass flow rate to meet its specifications. This would be true if the fans were installed side by side.

• Yeah, the fans are installed in Tandem. Now that I think I should add one picture of that wind tunnel. Will edit this question... Apr 4, 2023 at 18:45

There were two fans to demonstrate the conditions of laminar as well as turbulent flow, when only one fan is on it represents laminar flow, while we turn both of the fans we consider it for turbulent flow, as in your laboratory for boundary layer experiment you will be asked to do calculations and plot the graphs for laminar as well as turbulent flow, this is the reason for having two fans.

• Laminar flow is defined as below 2200 Re or sometimes 1700 Re and there are other factors. Apr 3, 2023 at 17:28
• yeah so when one fan is on, the velocity is low so we consider it to be laminar although it may or may not be laminar ( should instead be checked by the manufacturer instead) i can say this with certainty because i myself have performed this exp. in my lab with the same question asked before to me in the lab Apr 3, 2023 at 17:31
• You are not the only one to have played in wind tunnels, my university had 3, two subsonic and one supersonic… Apr 3, 2023 at 18:02
• @SolarMike Actually turbulence triggering at Re 2200 would be the case for internal flows, not for flow over a flat plate. Apr 4, 2023 at 18:51
• @Shreepati Thanks, actually a very good idea, this could be checked from manufacturer. Maybe I will try to fetch some details of that equipment, after all that doesn't seem very old... Apr 4, 2023 at 18:54