Abstract
Our understanding of animal flight benefits greatly from specialized wind tunnels designed for flying animals. Existing facilities can simulate laminar flow during straight, ascending and descending flight, as well as at different altitudes. However, the atmosphere in which animals fly is even more complex. Flow can be laminar and quiet at high altitudes but highly turbulent near the ground, and gusts can rapidly change wind speed. To study flight in both laminar and turbulent environments, a multi-purpose wind tunnel for studying animal and small vehicle flight was built at Stanford University. The tunnel is closed-circuit and can produce airspeeds up to 50 m s−1 in a rectangular test section that is 1.0 m wide, 0.82 m tall and 1.73 m long. Seamless honeycomb and screens in the airline together with a carefully designed contraction reduce centreline turbulence intensities to less than or equal to 0.030% at all operating speeds. A large diameter fan and specialized acoustic treatment allow the tunnel to operate at low noise levels of 76.4 dB at 20 m s−1. To simulate high turbulence, an active turbulence grid can increase turbulence intensities up to 45%. Finally, an open jet configuration enables stereo high-speed fluoroscopy for studying musculoskeletal control in turbulent flow.
Highlights
Wind tunnels are essential tools for studying both vehicle and animal flight in laminar and turbulent flows
The exchanger is positioned just downstream of the diffuser following the fan. This position was chosen for three reasons: (i) the resulting back pressure helps to keep the flow attached in the diffuser, (ii) any resulting turbulence has time to decay before reaching the test section and (iii) the resulting noise and pressure drop are minimized by placing the exchanger in a section with wide area and low airspeed
We obtained streamwise turbulence levels less than or equal to 0.030% throughout the (a) umax (r.p.m.)
Summary
Wind tunnels are essential tools for studying both vehicle and animal flight in laminar and turbulent flows. The key advantage of wind tunnels for biological research is that airspeed can be finely controlled over an animal that remains stationary in the laboratory frame This fixed frame allows direct measurements of forces, kinematics and flow fields that would otherwise be unattainable. Inspired by these advantages, several laboratories have built open-circuit wind tunnels designed for studying bird flight in the past 50 years [1,2,3,4,5,6,7,8,9]. To advance the capabilities of animal flight wind tunnels for biomechanics and aerodynamics studies, we built a multi-purpose tunnel that produces low streamwise turbulence (less than or equal to 0.028% at the centreline over the full operating range), and high turbulence using an active grid of spinning vanes To define the performance metrics for the wind tunnel, we used the KTH MTL wind tunnel as our primary reference, because it performs well and is relatively well documented [21,22]
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