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This guide to the design of wind tunnels has been developed so that the reader can easily jump between sections - for example, the section dealing with a particular tunnel component or design feature is reached simply by clicking on that component's name in the diagram above. It is intended to serve all wind-tunnel (and water-tunnel) designers, from K-12 teachers to industrial users. Each section is linked to references extracted from a bibliography of over 10000 selected papers on turbulence and related subjects (such as experimental or computational techniques). The complete bibliography is available at wind-tunnel design references.
Structural design, force balances and instrumentation are not considered in detail, being too specific to the size and purpose of the tunnel, but some suggestions are offered for construction of small tunnels by schools or private individuals. An old but not seriously outdated paper on wind tunnel design is one by Mehta and Bradshaw, "Design rules for small low speed wind tunnels" Aero. Journal (Royal Aeronautical Society), Vol. 73, p. 443 (1979) is available in PDF format.
The components of a wind tunnel are arranged to deliver, to the test section, a flow whose velocity is as near as possible uniform in space and independent of time. Flow nonuniformities are discussed in the section on contractions (nozzles), the last component before the test section. There are two main wind tunnel configurations:
Open-circuit tunnels(i) "Suckdown" tunnels, with an entry open to the atmosphere (laboratory) and an axial fan or centrifugal blower downstream of the test section. This type of wind tunnel is not a good idea - the entry has to re-ingest the exit flow after it returns through the laboratory, probably with significant swirl and low-frequency unsteadiness. The traditional, simple, "K-12" project "suck-down" wind tunnel is not necessarily the best for users. |
(ii) "Blower" tunnels, with a fan or blower upstream of the test section (usually a true centrifugal blower). Blower tunnels are the most flexible type - any desired test-section can be attached to the end of the contraction. Entry swirl is again a possible problem, but in general blower tunnels are much less sensitive to entry conditions than suckdown tunnels. The exit flow from a centrifugal blower is nonuniform and turbulent, but without the low-frequency unsteadiness of flow entering directly from a room.
Closed-circuit tunnels(iii) Closed-circuit ("racecourse", "closed-return", usually with an axial fan, or a multi-stage axial compressor in the case of a transonic/supersonic tunnel). Closed-circuit tunnels have more uniform flow, in principle, than type (ii), and are the usual choice for large tunnels, but care is needed to maintain good flow at the entrance to the contraction. The flow at exit from the fourth corner (counting from the test section) is typically not much better than the exit flow from a centrifugal blower, although the corner vanes themselves have some effect in reducing turbulence (they can be regarded as honeycombs with walls in one direction only).
The first author owes much of his basic knowledge of wind tunnels and experimental techniques to the written work, and kindly personal supervision, of the late Dr. R.C. Pankhurst ("Pank") of the National Physical Laboratory, Teddington, England, perhaps best known as the senior author of the 1952 book "Wind Tunnel Technique." Our 1964 review "The design of low-speed wind tunnels" (Prog. Aero. Sci. vol. 5, p. 1) is, alas, not seriously outdated. Comments can be e-mailed to bradshaw@vk.stanford.edu but please do not send technical inquiries to Bradshaw or Mehta: all we know is here! |
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