Closed Circuit Wind Tunnels (CCT)

AEROLAB specializes in custom-made closed circuit wind tunnels (CCT). The use of steel for construction offers complete design freedom to meet your unique specifications and needs (ex. low overhead clearance or building support columns). Most AEROLAB CCT designs are traditional single-return, horizontally arranged circuits. However, any configuration is possible! As with all AEROLAB wind tunnels, exceptionally steady test section flow with close uniformity of velocity is standard. Turbulence level is typically below 0.10 percent throughout the full tunnel speed range. Contact AEROLAB with your ideas (size, speed, shape) for a free, custom-made quotation.

Advantages of Closed Circuit Tunnels with comparison to Open Circuit Tunnels:

  • Power requirement for a given speed is lower.
  • Particulate matter can be contained within the circuit.
  • Noise is significantly lower.
  • Laboratory air movement (air vents, doors, windows, etc.) does not affect wind tunnel flow.
  • Air entering the test section is free of laboratory dust.
  • Fan blades are not as vulnerable to damage from model failure.

Disadvantages of Closed Circuit Tunnels with comparison to Open Circuit Tunnels:

  • Cost is generally three times greater for a given test section size.
  • Air supply is recycled which can be prohibitive when working with combustion engines.
  • Footprint is much larger and requires more overall space.
  • Increasing air temperature can become an issue during prolonged use.

Common Applications:

Aerodynamic testing, aerodynamic demonstrations

Performance specifications:

  • Airspeed Range : customer-defined
  • Turbulence Level: typically < 0.1%
  • Contraction - Contraction ratio (inlet-to-outlet area ratio) is a major driver in the overall size and complexity of a closed circuit tunnel. Power requirements, accordingly, follow. AEROLAB uses either a fifth, or, ninth-order polynomial to define a smooth transition between the settling chamber (contraction inlet) and the test section entrance.
  • Honeycomb Flow Straightener - High quality, high-aspect-ratio aluminum honeycomb flow straightener is installed in all AEROLAB closed circuit tunnels.
  • Turbulence-reducing Screens - All AEROLAB tunnels are supplied with two 20 x 20 (mesh) stainless steel turbulence-reducing screens. Also, sufficient space is provided in the screen area to fit two additional screens at a later date should the customer need extremely smooth flow for sensitive experiments. A full-length door allows removal for cleaning.
  • Static Pressure “Rings” - Static pressure “rings” are fitted immediately downstream of the turbulence-reducing screens and at the entrance of the test section. Both rings consist of at least four pressure taps (one centered on each wall) sharing a common manifold. The pressure measured within the "rings" is used to calculate airspeed without the need of an invasive probe or sensor. Depending upon tunnel size, additional taps may be installed.
  • Test Section - Anodized 6061 aluminum is used to construct the load-bearing frame and flanges of AEROLAB test sections. Typically, three standard Plexiglas ® windows are installed– top and two hinged sides. To compensate for boundary layer growth, the walls of the test section diverge slightly. If parallel test section walls are desired, fillets can be added to the contraction, test section, and diffuser, instead.
  • Breathers - “Breathers” are installed at the beginning of the first diffuser. These breathers increase test section static pressure to atmospheric and prevent unwanted pressure oscillations in the tunnel circuit.
  • Diffusers - To prevent separation in the diffusers, and hence diffuser-related effects in the test section, AEROLAB restricts included diffuser divergence angles to 6º. An exception to this rule would be the need for high-angle diffusers when overall space is limited.
  • Turning Vanes - Closed circuit wind tunnels typically use four 90° turns to return aiflow to the test section. In order to do this smoothly and efficiently, each corner must employ turning vanes. Simple, curved plates are effective. However, vanes with a thicker “airfoil” shape are superior. AEROLAB manufactures two types of hollow aluminum extrusions. Although more costly, these can be plumbed and injected with chilled water to also server as heat exchangers.

turning vane cascade

  • Cooling - Friction with the walls inside a CCT can cause a temperatures rise for the re-circulated airflow. This is undesirable for most testing. Standard AEROLAB treatments for the problem are: cooled hollow turning vanes (as mentioned above), large radiators in the slowest section of the tunnel circuit and a cooling air “jacket” for the motor. Other solutions are available depending upon circumstances.

AEROLAB proprietary hollow-core turning vane profile

  • Modular Design - For ease of shipment and installation, tunnel sections are flanged. Sections are bolted together on-site.
  • Modular Design - For ease of shipment and installation, tunnel sections are flanged. Sections are bolted together on-site.
  • Steel Support Structure - Tubular steel legs and cross members support the tunnel.
  • Hatches - Hatches provide internal access to every section of the tunnel.