Open Circuit
The open circuit wind tunnel is the simplest and most affordable to build. In these tunnels air is expelled directly into the laboratory and typically reingested after circulating through the lab, though some tunnels utilize instead a compressed gas source. In addition to their low costs, open circuit tunnels are also advantageous because they have are relatively immune to temperature fluctuations and large disturbances in return flow, provided that the volume of the laboratory is much greater than that of the tunnel.
There are two basic types of open circuit tunnels, “suckdown” and “blower,” and the two are most easily differentiated by the location of the fan. Blower tunnels are the most flexible because the fan is at the inlet of the tunnel, so the test section can be easily interchanged or modified with seriously disrupting flow. These tunnels are so forgiving that exit diffusers can often be completely omitted to allow easier access to test samples and instruments, though the omission often results in a noticeable power loss. Suckdown tunnels are typically more susceptible to low frequency unsteadiness in the return flow than blowers, though some claims have been made that intake swirl is less problematic in these tunnels because it does not pass through the fan before entering the test section.
Closed Circuit
As the name implies, closed circuit tunnels (also called closed return) form a enclosed loop in which exhaust flow is directly returned to the tunnel inlet. These tunnels are usually larger and more difficult to build. They must be carefully designed in order to maximize uniformity in the return flow.
These tunnels are powered by axial fan(s) upstream of the test section and sometime include multistage compressors, which are often necessary to create trans- and supersonic air speeds.
Closed circuit wind tunnel.
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