An Australian convective wind gust climatology using Bayesian hierarchical modelling.

To quantify the hazard or risks associated with severe convective wind gusts, it is necessary to have a reliable and spatially complete climatology of these events. The coupling of observational and global reanalysis (ERA-Interim) data over the period 2005-2015 is used here to facilitate the development of a spatially complete convective wind gust climatology for Australia. This is done through the development of Bayesian Hierarchical models that use both weather station-based wind gust observations and seasonally averaged severe weather indices (SWI), calculated using reanalysis data, to estimate seasonal gust frequencies across the country while correcting for observational biases specifically, the sparse observational network to record events. Different SWI combinations were found to explain event counts for different seasons. For example, combinations of Lifted Index and low level wind shear were found to generate the best results for autumn and winter. While for spring and summer, the composite Microburst Index and the combination of most unstable CAPE and 0-1 km wind shear were found to be most successful. Results from these models showed a minimum in event counts during the winter months, with events that do occur mainly doing so along the southwest coast of Western Australia or along the coasts of Tasmania and Victoria. Summer is shown to have the largest event counts across the country, with the largest number of gusts occurring in northern Western Australia extending east into the Northern Territory with another maximum over northeast New South Wales. Similar trends were found with an extended application of the models to the period 1979-2015 when utilizing only reanalysis data as input. This implementation of the models highlights the versatility of the Bayesian hierarchical modelling approach and its ability, when trained, to be used in the absence of observations.

Effects of uniform vertical inflow perturbations on the performance of flapping wings.

Flapping wings have attracted significant interest for use in miniature unmanned flying vehicles. Although numerous studies have investigated the performance of flapping wings under quiescent conditions, effects of freestream disturbances on their performance remain under-explored. In this study, we experimentally investigated the effects of uniform vertical inflows on flapping wings using a Reynolds-scaled apparatus operating in water at Reynolds number ≈ 3600. The overall lift and drag produced by a flapping wing were measured by varying the magnitude of inflow perturbation from J Vert = -1 (downward inflow) to J Vert = 1 (upward inflow), where J Vert is the ratio of the inflow velocity to the wing's velocity. The interaction between flapping wing and downward-oriented inflows resulted in a steady linear reduction in mean lift and drag coefficients, C ¯ L and C ¯ D , with increasing inflow magnitude. While a steady linear increase in C ¯ L and C ¯ D was noted for upward-oriented inflows between 0 < J Vert < 0.3 and J Vert > 0.7, a significant unsteady wing-wake interaction occurred when 0.3 ≤ J Vert < 0.7, which caused large variations in instantaneous forces over the wing and led to a reduction in mean performance. These findings highlight asymmetrical effects of vertically oriented perturbations on the performance of flapping wings and pave the way for development of suitable control strategies.

Performance of passively pitching flapping wings in the presence of vertical inflows.

The successful implementation of passively pitching flapping wings strongly depends on their ability to operate efficiently in wind disturbances. In this study, we experimentally investigated the interaction between a uniform vertical inflow perturbation and a passive-pitching flapping wing using a Reynolds-scaled apparatus operating in water at Reynolds number ≈3600. A parametric study was performed by systematically varying the Cauchy number (Ch) of the wings from 0.09 to 11.52. The overall lift and drag, and pitch angle of the wing were measured by varying the magnitude of perturbation fromJVert= -0.6 (downward inflow) toJVert= 0.6 (upward inflow) at eachCh, whereJVertis the ratio of the inflow velocity to the wing's velocity. We found that the lift and drag had remarkably different characteristics in response to bothChandJVert. Across allCh, while mean lift tended to increase as the inflow perturbation varied from -0.6 to 0.6, drag was significantly less sensitive to the perturbation. However effect of the vertical inflow on drag was dependent onCh, where it tended to vary from an increasing to a decreasing trend asChwas changed from 0.09 to 11.52. The differences in the lift and drag with perturbation magnitude could be attributed to the reorientation of the net force over the wing as a result of the interaction with the perturbation. These results highlight the complex interactions between passively pitching flapping wings and freestream perturbations and will guide the design of miniature flying crafts with such architectures.

Dynamics and flight control of a flapping-wing robotic insect in the presence of wind gusts.

With the goal of operating a biologically inspired robot autonomously outside of laboratory conditions, in this paper, we simulated wind disturbances in a laboratory setting and investigated the effects of gusts on the flight dynamics of a millimetre-scale flapping-wing robot. Simplified models describing the disturbance effects on the robot's dynamics are proposed, together with two disturbance rejection schemes capable of estimating and compensating for the disturbances. The proposed methods are experimentally verified. The results show that these strategies reduced the root-mean-square position errors by more than 50% when the robot was subject to 80 cm s-1 horizontal wind. The analysis of flight data suggests that modulation of wing kinematics to stabilize the flight in the presence of wind gusts may indirectly contribute an additional stabilizing effect, reducing the time-averaged aerodynamic drag experienced by the robot. A benchtop experiment was performed to provide further support for this observed phenomenon.