Elastic Reconfiguration Research Articles

Flutter limitation of drag reduction by elastic reconfiguration

Through experiments, we idealize a plant leaf as a flexible, thin, rectangular plate clamped at the midpoint and positioned perpendicular to an airflow. Flexibility of the structure is considered as an advantage at moderate flow speed because it allows drag reduction by elastic reconfiguration, but it can also be at the origin of several flow-induced vibration phenomena at higher flow speeds. A wind tunnel campaign is conducted to identify the limitation to elastic reconfiguration that dynamic instability imposes. Here, we show by increasing the flow speed that the flexibility permits a considerable drag reduction by reconfiguration, compared to the rigid case. However, beyond the stability limit, vibrations occur and limit the reconfiguration. This limit is represented by two dimensionless numbers: the mass number and the Cauchy number. Our results reveal the existence of a critical Cauchy number below which static reconfiguration with drag reduction is possible and above which a dynamic instability with important fluctuating loads is present. The critical dimensionless velocity is dependent on the mass number. Flexibility is related to the critical reduced velocity and allows defining an optimal flexibility for the structure that leads to a drag reduction by reconfiguration while avoiding dynamic instability. Furthermore, experiments show that our flexible structure can exhibit two vibration modes: symmetric and anti-symmetric, depending on its mass number. Because the system we consider is bluff yet aligned with the flow, it is unclear whether the vibrations are due to a flutter instability or vortex-induced vibration or a combination of both phenomena.

Enhancing low‐priority traffic reconfiguration designs in mixed‐critical avionics networks

AbstractIn avionics system networking, mixed‐critical networks are commonly used to integrate system applications with different levels of guarantees for various traffic classes on a shared resource platform, effectively addressing issues such as cabling, cost, weight, volume, power consumption, etc. However, current fault recovery and reconfiguration techniques mainly focus on high‐priority time‐triggered (TT) traffic, often neglecting the impact of TT traffic reconfiguration on low‐priority traffic. This paper proposes an avionics mixed‐critical networking approach based on the IEEE 802.1CB protocol, which is shown to be more effective for the post‐failure recovery of Rate‐Constrained (RC) traffic (a typical type of low‐priority traffic in TT networks). Furthermore, an elastic reconfiguration algorithm is presented to ensure quality of service for RC traffic during and after reconfiguration, preventing arbitrary application and traffic shutdown triggered by TT traffic reconfiguration. Simulations demonstrate that the proposed approach and algorithm can significantly improve the post‐failure recovery ratio of RC traffic, as well as guarantee quality of service and network throughput of RC traffic during and after reconfiguration to a certain extent.

Dynamic Cloud Network Control Under Reconfiguration Delay and Cost

Network virtualization and programmability allow operators to deploy a wide range of services over a common physical infrastructure and elastically allocate cloud and network resources according to changing requirements. While the elastic reconfiguration of virtual resources enables dynamically scaling capacity in order to support service demands with minimal operational cost, reconfiguration operations make resources unavailable during a given time period and may incur additional cost. In this paper, we address the dynamic cloud network control problem under non-negligible reconfiguration delay and cost. We show that while the capacity region remains unchanged regardless of the reconfiguration delay/cost values, a reconfiguration-agnostic policy may fail to guarantee throughput-optimality and minimum cost under nonzero reconfiguration delay/cost. We then present an adaptive dynamic cloud network control policy that allows network nodes to make local flow scheduling and resource allocation decisions while controlling the frequency of reconfiguration in order to support any input rate in the capacity region and achieve arbitrarily close to minimum cost for any finite reconfiguration delay/cost values.

Equilibrium position of a rigid sphere in a turbulent jet: A problem of elastic reconfiguration.

The position of floating spheres trapped within an immersed turbulent water jet is investigated. Using the self-similarity properties of the jet velocity profile, the equilibrium problem is formulated in a rescaled space where the sphere is static and deformable. This approach is found to be related to a problem of elastic reconfiguration where elasticity arises here from the geometry of the flow instead of an actual deformation of a body.

Guest Editorial: Special Issue on Cloud-Integrated Cyber-Physical Systems

The advances in wireless sensor devices, big data, mobile computing, and cloud computing offer tremendous opportunities to realize the seamless integration between the physical world and the cyber space. The cloud-integrated cyberphysical system (CCPS) refers to virtually representing physical system components, such as sensors, actuators, robots, and other devices in clouds, accessing (e.g., monitoring, actuating and navigating) those physical components through their virtual representations, and processing/managing/controlling the large amount of data collected from physical components in clouds in a scalable, real-time, efficient, and reliable manner. Particularly, integrating cloud computing techniques (e.g., virtualization, elastic re-configuration, and multi-tenancy of resources) with CPS techniques (e.g., real-time scheduling, adaptive resource management and control, and embedded system design) will bring hope to advance the state of the art, and allow previously unachievable systems such as cloud-integrated internet of vehicles to be built, deployed, managed, and controlled effectively. This Special Issue on CCPS solicits the manuscripts on rigorous research on theories, methodologies, tools, and testbeds for CCPS. In this special issue, we selected ten papers. Each paper was carefully reviewed by peer review and guest editors. In the following, we will overview the accepted papers that reflect recent advances.

Drag reduction by elastic reconfiguration of non-uniform beams in non-uniform flows

Flexible systems bending in steady flows are known to experience a lesser drag compared to their rigid counterpart. Through a careful dimensional analysis, an analytical expression of the Vogel exponent quantifying this reduction of drag is derived for cantilever beams, within a framework based on spatial self-similar modelling of the flow and structural properties at the clamped edge of the structure. Numerical computations are performed on various situations, including systems involving more complex distributions of the flow or structural parameters. The scaling of drag versus flow velocity for large loadings is shown to be well predicted by fitting the system properties by simple power laws at the scale of the length on which significant bending occurs. Ultimately, the weak sensitivity of the Vogel exponent to the parameters of the system provides an explanation to the rather reduced scattering of the Vogel exponents around −1 observed on most natural systems in aquatic or aerial vegetation.

Strong and weak, unsteady reconfiguration and its impact on turbulence structure within plant canopies

Flexible terrestrial and aquatic plants bend in response to fluid motion and this reconfiguration mechanism reduces drag forces, which protects against uprooting or breaking under high winds and currents. The impact of reconfiguration on the flow can be described quantitatively by introducing a drag coefficient that decreases as a power-law function of velocity with a negative exponent known as the Vogel number. In this paper, two case studies are conducted to examine the connection between reconfiguration and turbulence dynamics within a canopy. First, a flume experiment was conducted with a model seagrass meadow. As the flow rate increased, both the mean and unsteady one-dimensional linear elastic reconfiguration increased. In the transition between the asymptotic regimes of negligible and strong reconfiguration, there is a regime of weak reconfiguration, in which the Vogel number achieved its peak negative value. Second, large-eddy simulation was conducted for a maize canopy, with different modes of reconfiguration characterized by increasingly negative values of the Vogel number. Even though the mean vertical momentum flux was constrained by field measurements, changing the mode of reconfiguration altered the distribution, strength, and fraction of momentum carried by strong and weak events. Despite the differences between these two studies, similar effects of the Vogel number on turbulence dynamics were demonstrated. In particular, a more negative Vogel number leads to a more positive peak of the skewness of streamwise velocity within the canopy, which indicates a preferential penetration of strong events into a vegetation canopy. We consider different reconfiguration geometry (one- and two-dimensional) and regime (negligible, weak, and strong) that can apply to a wide range of terrestrial and aquatic canopies.

Flexible body with drag independent of the flow velocity

AbstractThe drag of a rigid object is expected to increase with flow velocity. For wide ranges of velocities commonly encountered, the drag increases as the square of the relative velocity of the fluid. This strong dependence of the load on the velocity accounts for specific survival strategies adopted by passive living systems such as plants in wind or algae in marine environments: through elastic reconfiguration, the drag on plants is reduced when compared to a rigid configuration and the velocity exponent for the drag is typically found to be between 1 and 1.5. In this work, a membrane configuration is presented that exhibits a drag force that is almost independent of the free-stream velocity. This surprising result is shown to be remarkably robust as it is experimentally observed for a range of geometries. This study opens the way for the design of devices subjected to a drag that is independent of the flow velocity. This possibility constitutes a key point in various fields involving flexible structures that are towed or subjected to wind.

Flow-induced pruning of branched systems and brittle reconfiguration

Whereas most plants are flexible structures that undergo large deformations under flow, another process can occur when the plant is broken by heavy fluid-loading. We investigate here the mechanism of such possible breakage, focusing on the flow-induced pruning that can be observed in plants or aquatic vegetation when parts of the structure break under flow. By computation on an actual tree geometry, a 20-yr-old walnut tree ( Juglans Regia L.) and comparison with simple models, we analyze the influence of geometrical and physical parameters on the occurrence of branch breakage and on the successive breaking events occurring in a tree-like structure when the flow velocity is increased. We show that both the branching pattern and the slenderness exponent, defining the branch taper, play a major role in the breakage scenario. We identify a criterion for branch breakage to occur before breakage of the trunk. In that case, we show that the successive breakage of peripheral branches allows the plant to sustain higher flow forces. This mechanism is, therefore, similar to elastic reconfiguration, and can be seen as a second strategy to overcome critical events, possibly a widespread solution in plants and benthic organisms.