Sensing of minute airflow motions near walls using pappus-type nature-inspired sensors

Abstract

This work describes the development and use of pappus-like structures as sensitive sensors to detect minute air-flow motions. We made such sensors from pappi taken from nature-grown seed, whose filiform hairs’ length-scale is suitable for the study of large-scale turbulent convection flows. The stem with the pappus on top is fixated on an elastic membrane on the wall and tilts under wind-load proportional to the velocity magnitude in direction of the wind, similar as the biological sensory hairs found in spiders, however herein the sensory hair has multiple filiform protrusions at the tip. As the sensor response is proportional to the drag on the tip and a low mass ensures a larger bandwidth, lightweight pappus structures similar as those found in nature with documented large drag are useful to improve the response of artificial sensors. The pappus of a Dandelion represents such a structure which has evolved to maximize wind-driven dispersion, therefore it is used herein as the head of our sensor. Because of its multiple hairs arranged radially around the stem it generates uniform drag for all wind directions. While still being permeable to the flow, the hundreds of individual hairs on the tip of the sensor head maximize the drag and minimize influence of pressure gradients or shear-induced lift forces on the sensor response as they occur in non-permeable protrusions. In addition, the flow disturbance by the sensor itself is limited. The optical recording of the head-motion allows continuously remote-distance monitoring of the flow fluctuations in direction and magnitude. Application is shown for the measurement of a reference flow under isothermal conditions to detect the early occurrence of instabilities.

Enhanced flow sensing with interfacial microstructures

“…The air leakage of the fabricated comb-like cantilever was smaller than that of the normal cantilever when the differential pressure was over 40 Pa, whereas the sensitivity to constant differential pressure was equal to each other. To detect minute airflow motions, the pappus-type sensor was developed to increase the drag force on the cantilever tip [79]. Experimental results indicated that hundreds of hairs of the pappus increased the sensitivity of the sensor by increasing the drag at the tip.…”

The multi-scale nature of Wall shear stress fluctuations in turbulent Rayleigh-Benard convection

“…The sensor including its calibration in the BOI is described in detail in [6] . The interested reader is referred to the reference, while we give here only a short review.…”

“…Recently (in 2017), Bruecker and Mikulich developed a novel sensor particularly designed to measure the wall shear stress in low-speed air flows [6]. As reported in [6], its sensitivity and dynamic response were sufficiently good to apply it in the so-called Barrel of Ilmenau (BOI), a large-scale RB experiment, which will be described in detail below. The flow in such a convection cell is known to generate a large-scale circulation (LSC), whose shape mainly depends on the diameter-to-height ratio of the test cell.…”

Novel strategy for wide-range wind vector measurement using the hybrid CP/CTD heating mode and sequential measuring and correcting

“…The wind field measurement for natural environment is characterized by large wind speed dynamics and fast wind direction changes [ 7 , 8 ]. While in artificial environment, accurate wind speed and direction measurements are needed even the wind speed is small and the wind direction is stable [ 9 , 10 ].…”