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First published online December 14, 2005
Journal of Experimental Biology 209, 43-56 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.01958
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Induced airflow in flying insects II. Measurement of induced flow

Sanjay P. Sane* and Nathaniel P. Jacobson

Department of Biology, University of Washington, Seattle, WA 98195, USA



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  		Fig. 1. Anemometer calibration. (A) A servo motor driven at up to 300 Hz drives a balsa wood plate. The air disturbance generated as a result of this oscillation is clearly registered as a pulse in the anemometric record (B). Thus, the anemometer has a frequency response of at least 300 Hz. (C) The time taken by the anemometer to reach its steady value has a time constant {tau} of 0.0103 s, nearly 1/4 of the wing period (ca. 0.04 ms). A=airspeed; the red line indicates the exponential function A0(1-et/{tau}); see text for details.

 


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  		Fig. 2. Measurement of induced air flow in a flight chamber. (A) Two anemometers, an axial inflow anemometer and a radial outflow anemometer, placed between the antennae and behind the wings at roughly 3/4 of the wing length from the base, respectively, recorded the induced airflow due to the flapping wings. An optical force sensor simultaneously recorded the vibrations generated by the flapping moth on its tether. A computer-controlled visual display motivated the moth to fly and modulate its wing kinematics. In the experiments reported here the visual display was turned off after the moth initiated flight and the chamber was left dark. (B,C) The frequency peaks of the vertical measurements and the two anemometric records (only axial inflow data are shown) always matched the primary stroke frequency (asterisks) and a secondary peak at double stroke frequency.

 


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  		Fig. 3. Raw anemometric data. (A-C) Axial inflow. The data are shown here at time scales of (A) 102 s, (B) 101 s and (C) 100 s for axial inflow in a sample flight bout for Moth 7. (D-F) Radial outflow. The data are shown here at time scales of (D) 102 s, (E) 101 s and (F) 100 s. In these graphs, the red lines are averages over each second of recording. B and E are expanded records of the insets in A and D, respectively, and C and F are expanded records of the insets in B and E, respectively.

 


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  		Fig. 4. Spectral characteristics of the induced flow. (A-D) Axial inflow and (E-H) radial outflow spectral data for the sample case of Moth 7 (A,B,E,F) and Moth 12 (C,D,G,H). For each moth, the top panels show standard Fourier plots (Moth 7: A,E; Moth 12: C,G) and lower panels show the Welch's Overlapped Segment Averaging (WOSA) plots (Moth 7: B,F; Moth 12: D,H). A vertical bar in each of the WOSA plots shows the 95% confidence interval for the spectral estimate.

 


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  		Fig. 5. Mean axial flow as a function of wing beat frequency. The anemometric time series from all bouts from Moth 5 (A), Moth 6 (B), Moth 7 (C), Moth 11 and Moth 12 (B) were divided into 1 s bins and the peak wing beat frequency for each bin was plotted against the mean value of the airflow over that bin. The grey area in the background describes the range of values predicted by Eqn 1.

 


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  		Fig. 6. Driven oscillations of artificial flexible and rigid wings. A mechanical lever attached to a servomotor and driven at 50 Hz (A) oscillated a flexible paper plate and a rigid plate made of balsa wood of the same dimensions and weight. Recordings from an anemometer placed near the paper plate (B) and balsa wood plate (C) show that the flexible plate has several frequency peaks for the driving frequency of 50 Hz, whereas the balsa wood plate has only a single peak at exactly 50 Hz.

 


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  		Fig. 7. Aerodynamic signature of wing flutter using anemometry. (A,B) Values replotted from the data of Combes and Daniel (2003Go) show the wing movements in terms of wing position and wing bending (excursion of trailing edge) as a function of time (A). These data were taken for a wing oscillated at frequencies typical of Manduca sexta. A Fourier analysis of the wing bending reveals frequency peaks at wing beat frequency and twice the wing beat frequency (B) similar to the frequency peaks measured in the trailing edge anemometric record (C).

 

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© The Company of Biologists Ltd 2006