Flexible Obstacles Research Articles

Mechanism of accelerating premixed hydrogen/methane flame with flexible obstacles

Low-velocity laminar flames gradually accelerates under the joint action of various mechanisms, and develops from deflagration to detonation, and the mechanism in this process is the focus of explosion dynamics research. This article presents experimental results of the propagation of premixed hydrogen/methane flames in square-section channels with flexible and rigid obstacles at a blockage ratio (BR) of 0.3. Theory and experiment show that the acceleration effect of flexible obstacles on flame is weaker than that of rigid obstacles. The results show that there are many differences in the physical mechanism of flame acceleration. It is mainly reflected in the difference between the two obstacles on turbulence guidance, and the difference in the peak pressure of flame propagation. The flexible obstacles create smaller turbulence and absorbs the transverse waves within the pipe that continuously accelerate the flame. At the same time, compared with rigid obstacles, flexible obstacles can effectively reduce the peak flame tip speed Vmax, and the peak flame tip speed reduction rate is the highest by 22.86%. In addition, unburned pockets formed by obstacles play a key role in the acceleration of the flames. The expansion of gas in the flexible obstacle pocket expands the obstacle spacing, accelerates the fuel combustion in the pocket, and avoids the concentrated release of fuel energy. The intensity of the turbulence and the velocity of the flame are lower after passing through the obstacle area, showing a lower explosion risk than rigid obstacles.

The effect of flexible obstacles with different thicknesses on explosion propagation of premixed methane-air in a confined duct

The effect of flexible obstacles with varying thicknesses on the explosion characteristics of combustible gas in a simulated confined duct (cross section 80 mm × 80 mm, length 3 m) was experimentally investigated, aiming to reduce the huge losses caused by gas explosion accidents in the process industries and mining industries. In this paper, plant fiber membranes with an opening area of 0 and thicknesses of 0.105 mm, 0.210 mm, 0.315 mm, 0.420 mm, 0.525 mm, and 0.630 mm were selected as flexible obstacles. The thickness of the flexible obstacle determines the strength of its compressive resistance. The characteristics of overpressure and flame during methane explosions are analyzed and conclusions are drawn. Results indicate that several shock wave reflection processes occur before the diaphragm ruptures, resulting in turbulent flames. In addition, the explosion wave generated numerous shock reflections during the rupture process of the diaphragm, which was gradually discharged downstream of the pipe by ejection as the pressure wave accumulated in front of the diaphragm. It should be noted that the thickness of the flexible obstacle determines the pressure accumulation in front of the membrane. Generally, the thinner the flexible obstacle, the less intense the turbulent flame is induced by the flexible obstacle, decreasing the contact area between the unignited gas downstream of the pipeline and the turbulent flame area. In conclusion, with an increase in the thickness of the flexible barrier, it exhibits a mechanism of initially suppressing and subsequently enhancing the impact on methane explosions. The increase of the thickness of the flexible obstacle motivates the flame propagation speed, which leads to an increase of turbulence intensity and explosion intensity.

DeforMoBot: A Bio-Inspired Deformable Mobile Robot for Navigation Among Obstacles

Many animals can move in cluttered environments by conforming their body shape to geometric constraints in their surroundings such as narrow gaps. Most robots are rigid structures and do not possess these capabilities. Navigation around movable or compliant obstacles results in a loss of efficiency—and possible mission failure—compared to progression through them. In this letter, we propose the novel design of a deformable mobile robot; it can adopt a wider stance for greater stability (and possible higher payload capacity), or a narrower stance to become capable of fitting through small gaps and progressing through flexible obstacles. We use a whisker-based feedback control approach to match the amount of the robot's deformation with the compliance level of the obstacle. We present a real-time algorithm which uses whisker feedback and performs shape adjustment in uncalibrated environments. The developed robot was tested navigating among obstacles with varying physical properties from different approach angles. Our results highlight the importance of co-development of environment perception and physical reaction capabilities for improved performance of mobile robots in unstructured environments.

The influence of flexible/rigid obstacle on flame propagation and blast injuries risk in gas explosion

ABSTRACT The interaction between flame, shock wave and obstacle is an important content in the study of fuel combustion and safety. In this paper, the performance difference between flexible and rigid obstacles in the process of explosion flame propagation was studied. Five barriers with different blocking rates（BR） were tested in the pipeline for methane premixed explosion. The high-speed photography system collects the flame image, the flame front velocity is calculated, and the dynamic pressure changes in the upstream and downstream pipelines during flame propagation are recorded by the high-frequency pressure acquisition system. The results show that the obstacle affects the unburned flow field and generates compression waves and shear layers. The explosion promotion degree of the two obstacles presents different trends, and the peak pressure decreases. At the same time, obstacles reflect pressure waves, causing instability in the front of the flame. With the increase of plugging rate, the difference of peak pressure rises to 30% at BR = 0.4. The flame velocity of flexible obstacles is also more flat, and the peak velocity at BR = 0.2 and BR = 0.4 is close to 43 m/s. During the study, it was found that the flexible material can effectively absorb the reflected wave, and the pressure oscillation is absorbed. The average amplitude of the rigid oscillation rises from 4 kPa to 10 kPa, and the flexible oscillation decreases to nearly zero. The difference of barrier materials makes the impact of shock wave on flame and primary explosive injury (PBI) lighter, which is more conducive to assessing the explosion risk of combustible gas.

Effect of flexible obstacles on gas explosion characteristic in underground coal mine

Gas explosion is one of the fatal accidents in underground coal mine, threatening the lives of miners and causing significant economic losses. Gas explosion characteristics are affected by many factors, including flexible obstacles such as flexible duct systems (DS) and line brattices (LB) in the auxiliary ventilation system. In order to explore the influence of flexible obstacles in the tunnel on the propagation law of gas explosion, this study established a numerical simulation model by using OpenFOAM that is an open-source CFD source code. The process variable in XiFoam tools was used for deflagration reaction. Gas explosion simulations were carried out for analyzing the impacts of DS and LB on the evolution of flame and shock wave propagation inside the tube. The results showed that the effect of flexible obstacles on the flame propagation law was evident. The flame shape easily caused many wrinkles in the tube with flexible obstacles. Meanwhile, the maximum pressure and flame propagation speed changed obviously comparing with the empty tube. In the model of DS and LB, the maximum pressure increased by 29 % and 77.8 % than the maximum pressure of empty tube, and the maximum flame propagation speed increased by 6.8 % and 20 % than the flame propagation of empty tube, respectively. The results can provide guidance for gas explosion prevention in underground coal mine.

Interaction between granular flows and flexible obstacles: A grain-scale investigation

Flexible obstacles are used to reduce the impact load exerted by granular flows. However, grain-scale interactions between granular assemblies and flexible obstacles remain poorly understood, making it difficult to estimate the required resisting force. In this fundamental study, a flexible obstacle was simplified as a grain intruder and dragged through a static granular assembly. A new physical intruder apparatus was used to calibrate a discrete element model. The calibrated model was then used to evaluate the effects of the stiffness and loading rate on the force experienced by the intruder and the energy dissipated by the granular assembly. The response of the intruder and the granular assembly depended significantly on both the drag rate and the obstacle stiffness. Softer obstacles caused longer sticks between slips. At least 60% of the work was transferred from the intruder into the granular assembly regardless of the drag rate and obstacle stiffness. Stick-slip behaviour was not observed for stiff obstacles and high drag rates (>1 m/s), where the bulk granular assembly fluidised. The results of this study suggest that rigid obstacles are better for causing energy dissipation in the granular assembly, while flexible obstacles are better for reducing the resisting forces required.

Adaptive Neuro-Fuzzy control of Autonomous Ground Vehicle (AGV) based on Machine Vision

Development of self-autonomous robot that has the capability to deal with dynamic obstacles as well as static obstacles and keep finding its optimal path represent big challenge to many of the researchers nowadays. Utilization of artificial intelligence (AI) in developing a robust algorithm for nonlinear system was proved in some applications. The present work focus on excessive analysis and investigation of the optimal AI algorithm technique for controlling the path planning of four -wheel autonomous robot to achieve its set target through both static and dynamic obstacles with the optimal path without human interference. Fuzzy logic, Neural Network, and Adaptive Neuro-Fuzzy Inference System (ANFIS) were tested and evaluated as controlling algorithm for this navigation task. The performance evaluation was done both theoretically and experimentally using vision-based feedback sensor, Kinect camera, on a real four-wheel mobile robot. Results prove that ANFIS successfully control the automatic path planning of the robot with the optimal path by reaching its target with %48.01 less than Neural Network and %58.51 less than Fuzzy Logic. ANFIS controller also has given safer path as well as flexible obstacles avoidance through its path to final destination with smooth time-efficient path in the cluttered environment.

Wall-mounted flexible plates in a two-dimensional channel trigger early flow instabilities.

A high level of mixing by passive means is a desirable feature in microchannels for various applications, and use of flexible obstacles (or plates) is one of the prime choices in that regard. To gain further insight, we carry out two-dimensional numerical simulations for flow past one or two flexible plates anchored to a channel's opposite walls using a fluid-structure interaction framework. For the inlet flow Reynolds number vs the Strouhal number plane, we observe a sudden flow change from a laminar to a time-periodic vortex shedding state when flexible plates are present in the channel. We found the critical Reynolds number to be Re_{cr}≈370 when a single plate is anchored on the channel wall and Re_{cr}≈290 or even lower when two plates are anchored. With an increase in the inlet flow Reynolds number (up to 3200), we found that vortices detach regularly at the plates' tips, which causes the flow to meander in the channel. In a two-plate anchored configuration, primary vortices generated at the first plate are constrained by the second plate and result in an energetic secondary vortex generation in the downstream side. The overall flow features and the energy dissipation in the channel are mainly controlled by the separation gap between the plates. At high-inlet-flow Reynolds numbers (≥1600), the probability density function (F) of the kinetic energy dissipation in a flexible plate configuration shows a stretched exponential shape in the form F(Z)∼1/sqrt[Z]e^{-pZ^{q}}, where Z is the normalized kinetic energy dissipation and the constants p=0.89 and q=0.86. The observed increase in energy dissipation comes at the cost of an increase in pressure loss in the channel, and we found that the loss is inversely related to the plates' separation gap. From our simulations, we found that if high mixing levels are desired, then two flexible plates anchored to the channel walls is a better choice than a channel flow without obstacles or flow past a single plate. The two-plate configuration with zero separation gap between the plates is best suited to achieve a high mixing level.

Experimental study of flame propagation across flexible obstacles in a square cross-section channel

A comparative study was performed to investigate flame propagation in a square-cross section channel filled with either flexible- or rigid-obstacles with blockage ratio (BR) of 0.429. Experiments were conducted in premixed hydrogen-air mixtures with different equivalence ratios, at initial conditions of 100 kPa and 298 K. High-speed Schlieren photography was used to obtain the detailed flow structure, flame front evolution and the flame tip velocity. Also, pressure transducers were employed to monitor the pressure around the obstacles. Flame propagation across the obstacles was found to be strongly affected by flow contraction induced by obstacles and separated flow pattern downstream of obstacles. Flame propagation with rigid obstacles is mainly governed by the turbulent burning of the fresh gas in the pockets. For the flexible cases, the flow structure is characterized by the shear layer coming off the obstacles leading-corner and the vortex downstream from the obstacles. These special flow structures together provide a flow contraction and constrict flame propagation in the obstacles-free channel, and therefore the flame maintains acceleration. Most notable, the gas flow ahead of the flame purges the flexible obstacle to tilt, yielding an increase in BR, which is correlated with the stronger acceleration as the flame propagates through the obstacles. However, exposing the obstacles to the overpressure for a long period also induces too much deformation. Therefore, the instantaneous BR (BRreal) will also decelerate slightly. Interestingly, BRreal is closely related with the overpressure level.

Flame propagation across a flexible obstacle in a square cross-section channel

Flame propagation across a single flexible fence-type obstacle was studied experimentally in a square cross-section channel, and compared to results obtained using similar blockage ratio (BR) rigid obstacles. The experiments were carried out with different BRs in premixed stoichiometric hydrogen-air mixtures, at initial conditions of 101 kPa and 298 K. High-speed Schlieren photography was employed to gain insight into the flame front structure and the flame tip velocity. Pressure transducers were used to measure the pressure at different axial positions near the obstacle. Flame propagation was found to be strongly influenced by the flow contraction of the unburned gas upstream of the obstacle, and the separated flow downstream of the obstacle. The most significant effect of the flexible obstacle, compared to the rigid obstacle, was observed for BRs above 0.71. The flame front evolution was dominated by the shear layer coming off the obstacle leading-edge and the vortex downstream from the obstacle. For the rigid obstacle BRs tested, the shear layer coming off the obstacle leading-edge reattached to the top of the obstacle, resulting in a vortex (i.e., recirculation-zone) downstream of the obstacle. For the high BR flexible obstacles (BR > 0.43), significant obstacle deformation (downstream tilt and vertical compression), and an associated decrease in BR, resulted in slightly lower flame tip velocities past the obstacle. The downstream obstacle tilt resulted in a different type of separated flow, compared to that observed in the rigid obstacle, where the shear layer didn't reattach to the top of the obstacle. The resulting vortex and strong shear layer confined the flame tip to the top part of the channel, delaying the consumption of the unburned gas immediately downstream of the obstacle. The deformation of the flexible obstacle reduced the peak pressure and the rate of pressure rise compared to that obtained with the rigid obstacles.

Active Particles with Soft and Curved Walls: Equation of State, Ratchets, and Instabilities.

We study, from first principles, the pressure exerted by an active fluid of spherical particles on general boundaries in two dimensions. We show that, despite the nonuniform pressure along curved walls, an equation of state is recovered upon a proper spatial averaging. This holds even in the presence of pairwise interactions between particles or when asymmetric walls induce ratchet currents, which are accompanied by spontaneous shear stresses on the walls. For flexible obstacles, the pressure inhomogeneities lead to a modulational instability as well as to the spontaneous motion of short semiflexible filaments. Finally, we relate the force exerted on objects immersed in active baths to the particle flux they generate around them.

A Novel Volume-of-Solid-Based Immersed-Boundary Method for Viscous Flow with a Moving Rigid Boundary

The present study adopts a volume-of-solid (VOS)-based immersed-boundary method (IBM) to simulate viscous incompressible flows interacting with moving solids using a numerical model based on the Navier-Stokes equations. The flow equations, with velocity–pressure variables, are discretized by a finite–element method on a nonuniform Cartesian grid, with the solutions obtained using a decoupled numerical scheme. Geometries featuring flexible solid obstacles in the flow are embedded in the Cartesian grid, with special treatments inside the embedded cell to ensure the accuracy of the solutions in the cut cells. In order to satisfy the no-slip condition of the body surface, a volume fraction is estimated to calculate the discretized body force inside the cut cell. More reasonable results for flow problems, including flows past a non-control/control circular cylinder and two cylinders moving against each other, are obtained by the present method. The time histories of drag and lift coefficients, as well as the vortex shedding frequencies, are extensively examined to demonstrate that the proposed method can be suitably combined with the fractional-step algorithm. Moreover, the temporal variations of velocity and vorticity fields are presented to demonstrate the capability of the present formulation in solving flow problems involving complex geometries, and the significance of the solid body forces on the flows.

Numerical simulation of fluid–structure interaction with the volume penalization method

We present a novel scheme for the numerical simulation of fluid–structure interaction problems. It extends the volume penalization method, a member of the family of immersed boundary methods, to take into account flexible obstacles. We show how the introduction of a smoothing layer, physically interpreted as surface roughness, allows for arbitrary motion of the deformable obstacle. The approach is carefully validated and good agreement with various results in the literature is found. A simple one-dimensional solid model is derived, capable of modeling arbitrarily large deformations and imposed motion at the leading edge, as it is required for the simulation of simplified models for insect flight. The model error is shown to be small, while the one-dimensional character of the model features a reasonably easy implementation. The coupled fluid–solid interaction solver is shown not to introduce artificial energy in the numerical coupling, and validated using a widely used benchmark. We conclude with the application of our method to models for insect flight and study the propulsive efficiency of one and two wing sections.