![]() ![]() (B)Reduced frequency of flutter as a function of U air. N=1 for all feather types except those underlined, for which N=2 samples (there is almost complete overlap for the replicate Chaetocercus mulsant R4 and Calypte anna R5 samples). The abbreviated species names are Philodice mitchellii, Selasphorus calliope, Philodice bryantae and Calothorax e. a jump from one type of tip mode to another), but this could not be unambiguously diagnosed from our data. We hypothesize that these represent mode jumps within a mode type (e.g. Abrupt changes in frequency are indicated with arrows. ![]() from a tip mode to a trailing vane mode) have been omitted see the companion paper (Clark et al., 2013) for a description of this phenomenon. Mode jumps that switch between types of modes (e.g. Color indicates the type of mode exhibited by the feather: blue, trailing vane red, tip yellow, torsional and green, white-bellied woodstar (Chaetocercus mulsant) R4. (A)Fundamental frequency of vibration (from SLDV data) and (B) reduced fundamental frequency as a function of U air, for 30 feathers of 26 types. An extended version of this figure is presented in supplementary material Fig.S2, which shows sound spectrograms and SLDV vibration spectra corresponding to the data plotted here plotted points reflect the peak velocity of the fundamental frequency of vibration.įig.5. The scanning laser Doppler vibrometer (SLDV) had a maximum velocity of 3ms −1 (dashed line) that results in a downward bias of the points in range III, as the graphed points represent averages of all points across the feather that could be scanned. The transition from range II to range III occurs at a critical velocity (U*) characterized by an abrupt increase in vibration velocity. Asterisk indicates the sound first detected. Range II: at intermediate U air, small amplitude vibrations were recorded, but no sound was detected (braces). Range I: at low U air, no motion was detected (no data present). All feathers exhibited varying magnitudes of flutter that fitted into three ranges. Sound production and average flutter velocity as a function of airspeed (U air ), for Costa's hummingbird (Calypte costae) R5 (blue), volcano hummingbird (Selasphorus flammula) R2 (red) and black-chinned hummingbird (Archilochus alexandri) R5 (green). As a result, this feather's orientation did not change, even though airflow-induced bending may result in a large shift in the feather's longitudinal axis (L). ![]() (D)Under this coordinate system, angles α and β did not shift when feathers bent or twisted in airflow these are changes in aeroelastic deformation. (C)The sting could be rotated from outside the wind tunnel, changing α, whereas β was changed by bending the pin to which the feather was glued. The lasers (described in Clark et al., 2013) and camera recorded the feather through the acrylic walls of the tunnel. The top and bottom surfaces of the tunnel were lined with 2.54cm acoustic foam. The microphone was not in the aerodynamic wake of the feather. The feather was glued to a pin, which was mounted in a pin vise attached to the end of the sting. A sting projected into the working section from the top of the wind tunnel, and could be rotated about its longitudinal axis from outside the tunnel (rounded arrow). (B)Experimental setup used to record sounds, vibrations and video of feathers in the working section of a wind tunnel. These angles were defined at the feather's calamus, where aeroelastic deformation was negligible. Rotation about Y was angle α and rotations about the Zaxis (projects out of the page) was angle β. (A)Lab-based coordinate system (side view). Experimental setup and coordinate system. Flutter is instead aeroelastic, in which structural (inertial/elastic) properties of the feather interact variably with aerodynamic forces, producing diverse acoustic results.įig.2. This, along with the presence of strong harmonics, multiple modes of flutter, and several other nonlinear effects indicates that flutter is not simply a vortex-induced vibration, and that the accompanying sounds are not vortex whistles. Reduced frequency of flutter varied by an order of magnitude, and declined with increasing Uair in all feathers. Loudness increased with airspeed in most but not all feathers. At low airspeeds (Uair) feather flutter was highly damped, but at a threshold airspeed (U*) the feathers abruptly entered a limit-cycle vibration and produced sound. All feathers tested were capable of fluttering at frequencies varying from 0.3 to 10 kHz. We investigated the underlying mechanics of flutter and sound production of a series of different feathers in a wind tunnel. Flutter may be the result of vortex shedding, or aero-elastic interactions. Males in the 'bee' hummingbird clade produce distinctive, species-specific sounds with fluttering tail feathers during courtship displays. ![]()
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