Wind tunnel- test section design question?
An open jet wind tunnel has the advantage of giving a smaller effective blockage ratio than a closed jet wind tunnel. Because the jet is not constrained by walls around the jet in the test section, the airflow can distort more than a closed jet wind tunnel - giving the same flow results as a larger closed jet tunnel.
There are however disadvantages with the open jet tunnel, such as the effect of the collector on the flow (this creates a longitudinal pressure differential referred to as &quot;buoyancy&quot;), and the effect of the shear layer that forms at the jet nozzle (the Kelvin Helmholtz instability can create transient flow effects and can be a noise source). When standing in the test section of a running tunnel, there is a lot of secondary airflow outside of the jet because of the open section.
The test section does not need to have a pressure different to atmospheric pressure, and does not need to be sealed. There can be vents in the test or return section of the tunnel, but they may create flow disturbances as well as allowing noise into the test section.
The free shear flow around the edge of the jet can be a source of disturbance for flow in the test section, however tunnels are designed so the disturbance is still far away from the model. Of course there are pressure fluctuations that interact with flow over the model, which are difficult to control. Some wind tunnels are designed with adaptive and slotted walls, which combine the benefits of open/closed sections, however they need much more setup than just putting a model in and turning on the fans.