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

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

Sketch of the wall-jet apparatus.

<p>Air flow is generated with a planar nozzle flow that is tilted at an angle of <i>β</i> = 45° towards the bottom wall of the research facility and generates a wall-jet in direction of the sensor. The sensor is located at the centre of the bottom wall and the nozzle exit is at a distance of 20 slot heights <i>H</i> away. The 2D jet flow exiting the nozzle is pointing towards the sensor in <i>x</i>-direction.</p

Velocity profile.

<p>Velocity profile of the wall-jet at <i>x =</i> 20<i>H</i> downstream of the nozzle (blue circles: measurements; solid line: fit with self-similar solution for a laminar wall jet, maximum velocity is <i>U</i>₀)</p

Time-trace of sensor tip motion in the wall-jet flow.

<p>Tip motion in streamwise direction after start of the fan of the wall-jet facility at <i>t</i> = 8<i>s</i> (<i>U</i>₀ = 0.5 <i>ms</i>⁻¹).</p

Pictures of the pappus sensor fixated with the stem at the wall.

<p>Pictures show the side view (left) with a scale bar and the top view with higher magnification (right).</p

Sensor step response.

<p>a) tip motion back to rest position after abrupt unloading. b) sensor frequency response.</p

Power spectrum in the wall-jet flow.

<p>Power spectrum of the tip motion signal after start of the jet-flow in the quasi-steady state shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0179253#pone.0179253.g006" target="_blank">Fig 6</a>.</p

Polar plot of the directional tests.

<p>Directional response of the sensors for four different wind-directions generated with the wall-jet facility (north N, east E, south S, west W). The plot shows at the cross-symbols the mean response of the qualified sensor and the diameter of the small circles indicate the variation of all others being tested, partly failing to comply with our quality criterion. The larger dashed circles indicate the tested velocity of the jet flow. Note, that the tip displacement coordinates of Q are related to the x-y plane of the camera sensor parallel to the bottom wall. The coordinate system of the camera was rotated such that the direction to North is parallel to the positive y-direction in the plot.</p

Foot fixation of the stem.

<p>The stem of the pappus sensor is fixated at the bottom wall in a thin silicone rubber membrane. Sensor at rest (left) and under strong wind load (right). Note that the stem keeps its straight shape under load.</p