Inspired by the reproductive success of plant species that employ bristled seeds for wind-borne dispersal, this study investigates the gust response of milkweed seeds, selected for their near-spherical shape. Gust-response experiments are performed to determine whether these porous bodies offer unique aerodynamic properties. Optical motion-tracking and particle image velocimetry (PIV) are used to characterize the dynamics of milkweed seed samples as they freely respond to a flow perturbation produced in an unsteady, gust wind tunnel. The observed seed acceleration ratio was found to agree with that of similar-sized soap bubbles as well as theoretical predictions, suggesting that aerodynamic performance does not degrade with porosity. Observations of high-velocity and high-vorticity fluid deflected around the body, obtained via time-resolved PIV measurements, suggest that there is minimal flow through the porous sphere. Therefore, despite the seed’s porosity, the formation of a region of fluid shear, accompanied by vorticity roll-up around the body and in its wake, is not suppressed, as would normally be expected for porous bodies. Thus, the seeds achieve instantaneous drag exceeding that of a solid sphere (e.g. bubble) over the first eight convective times of the perturbation. Therefore, while the steady-state drag produced by porous bodies is typically lower than that of a solid counterpart, an enhanced drag response is generated during the initial flow acceleration period.

No abstract

The effective natural transport of seeds in turbulent atmospheric flows is found across a myriad of shapes and sizes. However, to develop a sensitive passive sensor required for large-scale (in situ) flow tracking measurements, systems suffer from inertial lag due to the increased size and mass needed for optical visibility, or by carrying a sensor payload, such as an inertial measurement unit (IMU). While IMU-based flow sensing is promising for beyond visual line-of-sight applications, the size and mass of the sensor platform results in reduced flow fidelity and, hence, measurement error. Thus, to extract otherwise inaccessible flow information, a flow-physics-based tracer correction is developed through the application of a low-order unsteady aerodynamic model, inspired by the added-mass concept. The technique is evaluated using a sensor equipped with an IMU and magnetometer. A spherical sensor platform, selected for its symmetric geometry, was subject to two canonical test cases including an axial gust as well as the vortex shedding generated behind a cylinder. Using the measured sensor velocity and acceleration as inputs, an energized-mass-based dynamic model is used to back-calculate the instantaneous flow velocity from the sensor measurements. The sensor is also tracked optically via a high-speed camera while collecting the inertial data onboard. For the 1D test case (axial gust), the true (local) wind speed was estimated from the energized-mass-based model and validated against particle image velocimetry measurements, exhibiting good agreement with a maximum error of 10%. For the cylinder wake (second test case), the model-based correction enabled the extraction of the velocity oscillation amplitude and vortex-shedding frequency, which would have otherwise been inaccessible. The results of this study suggest that inertial (i.e. large and heavy) IMU-based flow sensors are viable for the extraction of Lagrangian tracking at large atmospheric scales and within highly-transient (turbulent) environments when coupled with a robust dynamic model for inertial correction.

In-situ flow-tracking measurements at scales on the order of 10 m3 and larger remain a challenge. The large size of the tracers required for optical visibility results in an inertial lag and inherently low seeding density. For instance, natural snowfall, fake snow and soap bubbles on the order of 2 cm have been used as tracers for field measurements and extracted statistical quantities (Nemes et al., 2017; Wei et al., 2021; Rosi et al., 2014). There is also growing interest in networks of sensors for remote- measurement where optical access is impossible (Bolt et al. 2020; Villa et al. 2016). Onboard inertial measurement units (IMU) are a promising tool for high-resolution measurements over large spatial domains without optical access. However, due to the intrinsic lag, a dynamic-model-based correction is required for the tracking of transient phenomena, sketched in figure 1. In the present study, the tracer-velocity correction is evaluated by quantifying the residual error in measured flow velocity after the method of Galler et al. (2021) is applied.

<div class="section abstract"><div class="htmlview paragraph">Road vehicles in the real world experience aerodynamic conditions that might be unappreciated and omitted in wind-tunnel experiments or in numerical simulations. Precipitation can potentially have an impact on the aerodynamics of road vehicles. An experimental study was devised to measure, in a wind tunnel, the impact of rain on the aerodynamic forces of the DrivAer research model.</div><div class="htmlview paragraph">In this study, a rain system was commissioned to simulate natural rain in a wind-tunnel environment for full-scale rain rates between about 8 and 250 mm/hr. A 30%-scale DrivAer model was tested with and without precipitation for two primary configurations: the notch-back and estate-back variants. In addition, mirror-removal and covered-wheel-well configurations were investigated.</div><div class="htmlview paragraph">The results demonstrate a distinct relationship between increasing rain intensities and increased drag of the model, providing evidence that road vehicles experience higher drag when travelling in precipitation conditions. At the lowest rain rates examined, representing moderate rain conditions equivalent to about 8 and 29 mm/hr full scale, drag increases on the order of 2% to 4% were measured. Drag increases in excess of 10% were observed for the highest rain rate tested, which represented an unrealistically-high precipitation condition equivalent to about 250 mm/hr full scale. A non-linear increase in drag with rain rate was observed, suggesting that multiple mechanisms of rain-induced drag were present during the experiments. Lift- and side-force variations did not manifest any trends that were beyond the estimated experimental uncertainty. Therefore precipitation is likely not an influence towards aerodynamic stability. Within the experimental uncertainty, the results did not show any evidence of rain affecting the drag changes associated with modifications to the vehicle shape.</div></div>