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

RESEARCH ARTICLE Sensing of minute airflow motions near walls using pappus-type nature-inspired sensors Christoph H. Bruecker1*, Vladimir Mikulich2 1 Department of Mechanical Engineering and Aeronautics, City, University of London, London, United Kingdom, 2 Institute of Mechanics and Fluid Dynamics, TU Bergakademie Freiberg, Freiburg, Germany * a1111111111 a1111111111 a1111111111 a1111111111 a1111111111 OPEN ACCESS Citation: Bruecker CH, Mikulich V (2017) Sensing of minute airflow motions near walls using pappustype nature-inspired sensors. PLoS ONE 12(6): e0179253. https://doi.org/10.1371/journal. pone.0179253 Editor: Vanesa Magar, Centro de Investigacion Cientifica y de Educacion Superior de Ensenada Division de Fisica Aplicada, MEXICO Received: December 31, 2016 Accepted: May 28, 2017 Published: June 28, 2017 Copyright: © 2017 Bruecker, Mikulich. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All relevant data are within the paper and its Supporting Information files. Funding: The position of Professor Christoph Bruecker is co-funded by BAE SYSTEMS and the Royal Academy of Engineering (Research Chair no. RCSRF1617\4\11) as the Chair in Aeronautical Engineering at City, University of London. The funders had no role in the study design nor did they have acted as an expert witness in relevant legal proceedings or have sat or currently sit on a 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 naturegrown 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 nonpermeable 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. Introduction Sensing of low-speed air motions is of critical importance in nature for prey detection [1,2]. The term “minute” is understood as a small velocity magnitude in the order of several hundreds of μm s-1 which are signalling the presence of instabilities or disturbances in an otherwise calm situation or as addition to an otherwise quasi-steady flow situation. In nature, crickets are capable of sensing low-frequency flows by their sensory hairs down to a threshold of air velocities of about 100 μm s-1 [1,2] which can be seen as a lower bound of these minute air motions. In technical application, this is also important for monitoring minute amounts of air fluctuations as, e.g. for neonatal incubators to monitor infants in intensive care units [3]. Comfort of human ventilation is another field where it is necessary to ensure low air-speeds PLOS ONE | https://doi.org/10.1371/journal.pone.0179253 June 28, 2017 1 / 17 Pappus air-flow sensor committee for an organization that may benefit from publication of the paper. Competing interests: The position of Professor Christoph Bruecker is funded by BAE SYSTEMS and the Royal Academy of Engineering as the Chair in Aeronautical Engineering at City, University of London. This does not alter our adherence to PLOS ONE policies on sharing data and materials. along the body for well-being, as e.g. for passengers in cars or aircrafts. Most of the measurement principles to measure air-flow speed use the method of hot wire [4] or optical flow detectors such as Laser Doppler Anemometry [5]. In the last decade, the method of a flexible fibretype flow sensor has been developed [3,6] which uses the bending signal of the filament that is arranged normal to the flow as a measure proportional to the drag force acting along the filament which is proportional to the airflow velocity [3,6]. Attached to walls, those filaments sense the wall-shear stress when the assumption of a linear velocity profile is valid as a first approximation [6–8]. Especially for the comfort measurements, the direct sensing near the body is relevant since the flow near the hull of the body is already affected by the presence of the body itself, the location of the ventilation device as well as other internal objects in the room. Another field where the velocity detection near the wall is of importance is the investigation of near-wall turbulence. Herein, not only the mean flow but also the frequency content linked with instabilities of different time- and spatial scales is important to understand the physics of heat and momentum transfer in such flows. Instabilities in flows are typically evolving in a certain frequency range as waves with very low disturbance amplitude. Such instabilities in their early stage are difficult to be detected because of the low disturbance amplitude. This requires the sensors to be on one hand sensitive to small fluctuations in magnitude and direction of the air motions and on the other hand to have a constant frequency response over a broad range of frequencies. For the flexible single-hair wall-shear sensors reported above increasing sensitivity goes with the disadvantage of reduced bandwidth [6,7]. The pappus-type sensor as developed and used herein extends the single hair sensor with a tip which is built from multiples of filiform hairs as radial protrusions from the stem. The hundreds of hairs of the pappus increase the sensitivity of the sensor by increasing the drag at the tip. We make use of the availability of pappus structures of Dandelion seed in nature, which is a suited sensor size for our focus of research in large-scale convection flows. Until t (...truncated)