Sediment erosion in flexible canopies by vortex impact

A model experiment with a vortex impacting a flexible canopy filled with a thin homogeneous bed of particles is presented. Flexibility of the filaments increases the efficiency of resuspension by the amount it allows the effective canopy-height to reduce due to the reconfiguration of the flexible structures. Scaling of the results with the effective canopy height leads to a collapse of the observed resuspension in the history of several successive impacts. It is further shown that preferential pathways in the canopy play a large role in resuspension. When comparing a hexagonal arrangement to a random arrangement of the filaments at the same average porosity, one needs to double the amount of impacts to achieve the same average resuspension. Hence, it is concluded that the random path of the particles around the filaments is affected in a non-linear manner by the local resistance. Regions of locally sparse arrangements of the filaments cannot balance the trapping effect of particles within regions of dense arrangement in their travel history. Flexibility of the filaments again proves a better resuspension under such conditions

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

The PELskin project: part II—investigating the physical coupling between flexible filaments in an oscillating flow

The fluid-structure interaction mechanisms of a coating composed of flexible flaps immersed in a periodically oscillating channel flow is here studied by means of numerical simulation, employing the Euler-Bernoulli equations to account for the flexibility of the structures. A set of passively actuated flaps have previously been demonstrated to deliver favourable aerodynamic impact when attached to a bluff body undergoing periodic vortex shedding. As such, the present configuration is identified to provide a useful test-bed to better understand this mechanism, thought to be linked to experimentally observed travelling waves. Having previously validated and elucidated the flow mechanism in Paper 1 of this series, we hereby undertake a more detailed analysis of spectra obtained for different natural frequency of structures and different configurations, in order to better characterize the mechanisms involved in the organized motion of the structures. Herein, this wave-like behaviour, observed at the tips of flexible structures via interaction with the fluid flow, is characterized by examining the time history of the filaments motion and the corresponding effects on the fluid flow, in terms of dynamics and frequency of the fluid velocity. Results indicate that the wave motion behaviour is associated with the formation of vortices in the gaps between the flaps, which itself are a function of the structural resistance to the cross flow. In addition, formation of vortices upstream of the leading and downstream of the trailing flap is seen, which interact with the formation of the shear-layer on top of the row. This leads to a phase shift in the wave-type motion along the row that resembles the observation in the cylinder case

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

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

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

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

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

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