A mm-Scale Aeroelastic Oscillation-Based Anemometer

By combining the aeroelastic and vortex-forced flutter modes of a thin plastic strip, its oscillation frequency can be confined to scale monotonically with fluid velocity. This principle has been used to produce a low-cost, mm-scale anemometer that measures air speed to ±(5% + 0.5m/s) from 118m/s. The device uses a 2mm slot-type photointerrupt detector to monitor the fundamental frequency of a 7μm thick Kapton strip suspended parallel to air flow. This paper describes the prototype and three of the experiments that informed its design. These investigated the effect of a bluff body on flutter onset velocity, the effect of filament geometry on bending position, and the effect of the superposition of the vortex-forced and aeroelastic flutter modes on discretely-measured flutter frequency. The experiments demonstrate that a trapezoidal filament in the wake of a similarly-sized bluff body is wellsuited for this novel flow measurement strategy. INTRODUCTION A streamer suspended in a moving fluid often flutters at a frequency that increases with the velocity of the fluid. Most people are familiar with this phenomenon from watching flags; on calm days, flags wave slowly back and forth, while on windy days they flap much more quickly. This type of aeroelastic flutter is a well-studied problem in the interaction of structures and fluids. Various efforts have clarified most aspects of flutter behavior, including the origin of the flutter instability and the tendency of the oscillation frequency to increase with fluid velocity [1, 2]. But while fluttering flags are commonly used as an informal indicator of wind speed, the possibility of an anemometer based on aeroelastic flutter hasn't been evaluated. This omission is striking because fluttering filaments ('flags') have no mechanical joints, are simple to manufacture, and have the potential to indicate fluid velocity at very small scales. These factors make aeroelastic flutter a promising basis for a new class of low-cost, miniature flowmeters. Typical anemometers rely on lift or drag forces to indicate wind speed. For rotary vane or ball-and-cup anemometers, the lift and drag coefficients CL and CD, respectively, each scale with the characteristic length L of the rotor or blade, CD,L ~ 1/ L. By contrast, although lift and drag forces are also involved, the scaling factors that define the flutter of a flag involve only the aspect ratio, not the absolute size of the flag. As derived by Argentina and Mahadevan [3], the critical wind velocity UC for the flutter of a flag scales with the flag thickness h and length L as UC~ (h/L). As a result, an anemometer based on flutter phenomena could conceivably work at smaller scales than traditional systems based on lift or drag forces. In this work the aeroelastic oscillation-based flowmeter consists of a small filament suspended parallel to fluid flow. As the filament flutters, its fundamental frequency is measured and compared to a frequency-fluid velocity correlation developed for the filament. The result is a small, low cost anemometer with reasonable accuracy; a system based on a 5mm filament reliably measures air flow to ±(5%+0.5m/s) from 1-18m/s, and costs less than $5. The small size and low cost could make a similar system useful in various applications unsuitable for traditional anemometers. Though simple in principle, an aeroelastic oscillation-based flowmeter requires a filament with both a low and consistent flutter onset velocity and a monotonic flutter frequency – air velocity correlation. Practically, these challenges require a design that is both inherently unstable and inclined to oscillate in one dominant mode over a range of fluid velocities. This paper describes three experiments that informed the anemometer design. These measured the effect of a bluff body on flutter onset velocity, the effect of filament geometry on flutter measurability, and the effect of the superposition of the