Obstacle Configurations Research Articles

Tangential Gap Flow (TGF) navigation: A new reactive obstacle avoidance approach for highly cluttered environments

This paper presents a novel reactive collision avoidance method for mobile robots moving in dense and cluttered environments. The proposed method, entitled Tangential Gap flow (TGF), simplifies the navigation problem using a divide and conquer strategy inspired by the well-known Nearness-Diagram Navigation (ND) techniques. At each control cycle, the TGF extracts free openings surrounding the robot and identifies the suitable heading which makes the best progress towards the goal. This heading is then adjusted to avoid the risk of collision with nearby obstacles based on two concepts namely, tangential and gap flow navigation. The tangential navigation steers the robot parallel to the boundary of the closest obstacle while still emphasizing the progress towards the goal. The gap flow navigation safely and smoothly drives the robot towards the free area in between obstacles that lead to the target. The resultant trajectory is faster, shorter and less-oscillatory when compared to the ND methods. Furthermore, identifying the avoidance maneuver is extended to consider all nearby obstacle points and generate an avoidance rule applicable for all obstacle configurations. Consequently, a smoother yet much more stable behavior is achieved. The stability of the motion controller, that guides the robot towards the desired goal, is proved in the Lyapunov sense. Experimental results including a performance evaluation in very dense and complex environments demonstrate the power of the proposed approach. Additionally, a discussion and comparison with existing Nearness-Diagram Navigation variants is presented.

Comparative study of solar air heater performance with various shapes and configurations of obstacles

An investigation is performed to find an optimum shape of obstacles attached to a solar air heater using three-dimensional Reynolds-averaged Navier–Stokes analyses of heat transfer and fluid flow. The Reynolds number, which is based on the hydraulic diameter of the channel, is in the range of 6800–10,000. The Nusselt number and friction factor are used to measure the thermal and aerodynamic performances of the solar air heater, respectively. Four different obstacle shapes (U-shaped, rectangular, trapezoidal, and pentagonal) and three arrangements of obstacles were tested to determine their effects on performance of the solar air heater. The results show that the performance factor (defined by a ratio of thermal to aerodynamic performance) was above unity for all the cases tested, and the pentagonal obstacle shape indicates the highest performance regardless of the Reynolds number. Detailed analyses of the thermal and flow fields are performed in order to obtain a better understanding of the heat transfer characteristics.

Energy-Bounded Caging: Formal Definition and 2-D Energy Lower Bound Algorithm Based on Weighted Alpha Shapes

Caging grasps are valuable as they can be robust to bounded variations in object shape and pose, do not depend on friction, and enable transport of an object without full immobilization. Complete caging of an object is useful but may not be necessary in cases where forces such as gravity are present. This letter extends caging theory by defining energy-bounded cages with respect to an energy field such as gravity. This letter also introduces energy-bounded-cage-analysis-2-D (EBCA-2-D), a sampling-based algorithm for planar analysis that takes as input an energy function over poses, a polygonal object, and a configuration of rigid fixed polygonal obstacles, e.g., a gripper, and returns a lower bound on the minimum escape energy. In the special case when the object is completely caged, our approach is independent of the energy and can provably verify the cage. EBCA-2-D builds on recent results in collision detection and the computational geometric theory of weighted $\alpha$ -shapes and runs in $O(s^2 + s n^2)$ time where $s$ is the number of samples, $n$ is the total number of object and obstacle vertices, and typically $n \textless\textless s$ . We implemented EBCA-2-D and evaluated it with nine parallel-jaw gripper configurations and four nonconvex obstacle configurations across six nonconvex polygonal objects. We found that the lower bounds returned by EBCA-2-D are consistent with intuition, and we verified the algorithm experimentally with Box2-D simulations and RRT* motion planning experiments that were unable to find escape paths with lower energy. EBCA-2-D required an average of 3 min per problem on a single-core processor but has potential to be parallelized in a cloud-based implementation. Additional proofs, data, and code are available at: http://berkeleyautomation.github.io/caging/.

Swimming patterns of a polarly flagellated bacterium in environments of increasing complexity

The natural habitat of many bacterial swimmers is dominated by interfaces and narrow interstitial spacings where they frequently interact with the fluid boundaries in their vicinity. To quantify these interactions, we investigated the swimming behavior of the soil bacterium Pseudomonas putida in a variety of confined environments. Using microfluidic techniques, we fabricated structured microchannels with different configurations of cylindrical obstacles. In these environments, we analyzed the swimming trajectories for different obstacle densities and arrangements. Although the overall swimming pattern remained similar to movement in the bulk fluid, we observed a change in the turning angle distribution that could be attributed to collisions with the cylindrical obstacles. Furthermore, a comparison of the mean run length of the bacteria to the mean free path of a billiard particle in the same geometry indicated that, inside a densely packed environment, the trajectories of the bacterial swimmers are efficiently guided along the open spacings.

Low completion rate of adjuvant chemotherapy in resected pancreatic cancer in clinical routine care

The microscale transport and stream model MITRAS has been improved and a new technique has been implemented to improve numerical stability for complex obstacle configurations. Results of the updated version have been compared with wind tunnel data using an evaluation method that has been established for simple obstacle configurations. MITRAS is a part of the M-SYS model system for the assessment of ambient air quality. A comparison of model results for the flow field against quality ensured wind tunnel data has been carried out for both idealised and realistic test cases. Results of the comparison show a very good agreement of the wind field for most test cases and identify areas of possible improvement of the model. The evaluated MITRAS results can be used as input data for the M-SYS microscale chemistry model MICTM. This paper describes how such a comparison can be carried out for simple as well as realistic obstacle configurations and what difficulties arise.

Dynamics of cylindrical converging shock waves interacting with aerodynamic obstacle arrays

Cylindrical converging shock waves interacting with an array of aerodynamic obstacles are investigated numerically for diverse shock strengths and for different obstacle configurations in air in standard conditions. The considered number of obstacles N is 4, 6, 8, 16, and 24. Obstacles are lenticular airfoils with thickness-to-chord ratios tc of 0.07, 0.14, and 0.21. The distances of the airfoil leading edge from the shock focus point rLErLEref are 1, 2, and 2.5, where rLEref=7 is the dimensionless reference distance from the origin. Considered impinging shock Mach numbers Ms are 2.2, 2.7, and 3.2 at the reference distance from the origin. The reference experimental configuration (N=8,tc=0.14,rLE=7,Ms=2.7) was proposed by Kjellander et al. [“Thermal radiation from a converging shock implosion,” Phys. Fluids 22, 046102 (2010)]. Numerical results compare fairly well to available one-dimensional models for shock propagation and to available experimental results in the reference configuration. Local reflection types are in good agreement with the classical criteria for planar shock waves. The main shock reshaping patterns are identified and their dependence on the shock strength and obstacle configuration is exposed. In particular, different shock patterns are observed after the leading edge reflection, which results in polygonal shock wave with N, 2N, 3N, and 4N sides. The largest temperature peak at the origin is obtained for the 8- and the 16-obstacle configurations and for the smallest thickness to length ratio, 0.07, located at distance from the origin of 2rLEref. In terms of compression efficiency at the origin, the 16-obstacle configuration is found to perform slightly better than the reference 8-obstacle configuration—with an efficiency increase of about 2%-3%, which is well within the model accuracy—thus confirming the goodness of the obstacle arrangement proposed by Kjellander and collaborators.

Fast marching tree: A fast marching sampling-based method for optimal motion planning in many dimensions

In this paper we present a novel probabilistic sampling-based motion planning algorithm called the Fast Marching Tree algorithm (FMT*). The algorithm is specifically aimed at solving complex motion planning problems in high-dimensional configuration spaces. This algorithm is proven to be asymptotically optimal and is shown to converge to an optimal solution faster than its state-of-the-art counterparts, chiefly PRM* and RRT*. The FMT* algorithm performs a ‘lazy’ dynamic programming recursion on a predetermined number of probabilistically drawn samples to grow a tree of paths, which moves steadily outward in cost-to-arrive space. As such, this algorithm combines features of both single-query algorithms (chiefly RRT) and multiple-query algorithms (chiefly PRM), and is reminiscent of the Fast Marching Method for the solution of Eikonal equations. As a departure from previous analysis approaches that are based on the notion of almost sure convergence, the FMT* algorithm is analyzed under the notion of convergence in probability: the extra mathematical flexibility of this approach allows for convergence rate bounds—the first in the field of optimal sampling-based motion planning. Specifically, for a certain selection of tuning parameters and configuration spaces, we obtain a convergence rate bound of order O(n−1/d+ρ), where n is the number of sampled points, d is the dimension of the configuration space, and ρ is an arbitrarily small constant. We go on to demonstrate asymptotic optimality for a number of variations on FMT*, namely when the configuration space is sampled non-uniformly, when the cost is not arc length, and when connections are made based on the number of nearest neighbors instead of a fixed connection radius. Numerical experiments over a range of dimensions and obstacle configurations confirm our theoretical and heuristic arguments by showing that FMT*, for a given execution time, returns substantially better solutions than either PRM* or RRT*, especially in high-dimensional configuration spaces and in scenarios where collision-checking is expensive.

Numerical Study of Shock Wave Attenuation in Two-Dimensional Ducts Using Solid Obstacles: How to Utilize Shock Focusing Techniques to Attenuate Shock Waves

Research on shock wave mitigation in channels has been a topic of much attention in the shock wave community. One approach to attenuate an incident shock wave is to use obstacles of various geometries arranged in different patterns. This work is inspired by the study from Chaudhuri et al. (2013), in which cylinders, squares and triangles placed in staggered and non-staggered subsequent columns were used to attenuate a planar incident shock wave. Here, we present numerical simulations using a different obstacle pattern. Instead of using a matrix of obstacles, an arrangement of square or cylindrical obstacles placed along a logarithmic spiral curve is investigated, which is motivated by our previous work on shock focusing using logarithmic spirals. Results show that obstacles placed along a logarithmic spiral can delay both the transmitted and the reflected shock wave. For different incident shock Mach numbers, away from the logarithmic spiral design Mach number, this shape is effective to either delay the transmitted or the reflected shock wave. Results also confirm that the degree of attenuation depends on the obstacle shape, effective flow area and obstacle arrangement, much like other obstacle configurations.

Investigation of concentration effects on the flame acceleration in vented channels

Propane–air gas explosion experiments were performed in two vented channels of dimensions 1.5 m×0.3 m×0.3 m (lab-scale) and 6 m × 1.2 m×1.2 m (medium-scale). The pressure–time development and flame speed were recorded. Tests were performed with several obstacle configurations. The equivalence ratio ϕ was varied between 0.7 and 1.7, to study the corresponding effects on the flame acceleration and maximum explosion overpressure. The experimental results were compared to numerical simulations performed with the computational fluid dynamics (CFD) tool FLACS, employing two different burning velocity models: (i) the standard burning velocity model in FLACS, (ii) an alternative burning velocity model that incorporates Markstein number effects. Both models gave acceptable predictions of the experimental maximum overpressures for ϕ<1.4. For fuel-rich mixtures, ϕ>1.4, the standard burning velocity model in FLACS generally under-predicted the maximum overpressures. The Markstein number-dependent burning velocity model gave improved results, consistently predicting overpressures within ±10% of the experimental values.

Methane–air explosion characteristics with different obstacle configurations

To investigate the flame and overpressure characteristics of methane–air explosion with different obstacle configurations, an experimental study has been conducted, taking account of the number of obstacles, obstacle distance from ignition source, and stream-wise and cross-wise obstacle positions. The results show that the flame speed and peak overpressure increase with the increasing number of obstacles, while the time to reach the peak is not fully determined by it. And the configuration having the farthest obstacle produces a higher overpressure and takes a longer time to reach the peak, but a slower flame propagation speed is obtained. Similar explosion characteristics are also observed in the configurations with two obstacles fixed at different stream-wise positions. Furthermore, the experimental results demonstrate that the peak overpressures and flame speeds in configurations with central or staggered obstacles are relatively higher, which should to be avoided in practical processes to minimize the risk associated with methane–air explosion.

Distributed estimation using online semi‐supervised particle filter for mobile sensor networks

This study proposes an improved particle filter by incorporating semi-supervised machine learning for location estimation in mobile sensor networks (MSNs). A time-varying prior model is learned online as the likelihood of particle filter in order to adapt to dynamic characteristics of state and observation. Thanks to semi-supervised learning, the proposed particle filter can improve efficiency and accuracy, where the amount of available labelled training data is limited. The authors compare the proposed algorithm with the particle filter based on supervised learning. The algorithms are evaluated for received signal strength indicator (RSSI)-based distributed location estimation for MSN in which communication bandwidth and accuracy of the range measurement are limited. First, experimental results show that the semi-supervised algorithm can learn suddenly-changed RSSI characteristics while the supervised learning cannot. Second, the proposed particle filter is more accurate and robust against variations of the environment such as new obstacle configurations. Furthermore, the suggested particle filter shows low statistical variability during repeated experiments, confirmed by much smaller error deviation than the compared particle filter.

DDT and detonation propagation limits in an obstacle filled tube

Experiments with hydrogen–air and ethylene–air mixtures at atmospheric pressure were carried out in a 6.1 m long, 0.1 m diameter tube with different obstacle configurations and ignition types. Classical DDT experiments were performed with the first part of the tube filled with equally spaced 75 mm (44% area blockage ratio) orifice-plates. The DDT limits, defining the so-called quasi-detonation regime, where the wave propagates at a velocity above the speed of sound in the products, were found to be well correlated with d/λ = 1, where d is orifice-plate diameter and λ is the detonation cell size. The only exception was the rich ethylene limit where d/λ = 1.9 was found. In a second experiment detonation propagation limits were measured by transmitting a CJ detonation wave into an obstacle filled (same equally spaced 44% orifice plates) section of the tube. An oxy-acetylene driver promptly initiated a detonation wave at one end. In this experiment the quasi-detonation propagation limits were found to agree very well with the d/λ = 1 correlation. This indicates that the d/λ = 1 represents a propagation limit. In general, one can conclude that the classical DDT limits measured in an orifice-plate filled tube are governed by the wave propagation mechanism, independent of detonation initiation (DDT process) that can occur locally in the obstacles outside these limits. For rich mixtures, transmission of the quasi-detonation into the smooth tube resulted in CJ detonation wave. However, in a narrow range of mixtures on the lean side, the detonation failed to transmit in the smooth tube. This highlights the critical role that shock reflection plays in the propagation of quasi-detonation waves.

NON-STATIONARY FLOW AROUND BUILDINGS DURING RUN-UP OF TSUNAMI WAVES ON A PLAIN BEACH

This paper presents an experimental investigation that focuses on some predominant flow features that arise around surface mounted vertical obstacles which are exposed to a transient flow. The flow under investigation is caused by a tsunami-like long wave that climbs up a 1:40 sloping plain beach. In this study the wave height in offshore waters is varied. A single obstacle of 10 cm width as well as side-by-side arrangement of two identical square obstacles with different spacing are considered at an approximate length scale of 1 in 100. The analysis reveals important flow features around the various obstacle configurations. Particular emphasize is laid on the spatiotemporal evolution of the wake angle that linearly increases over time irrespective of the obstacle spacing. The growth rate of the wake angle reciprocally depends on the gap ratio over the investigated range of g* = 0.0 to g* = 3.0.

Panel method for mixed configurations with finite thickness and zero thickness

Panel methods are well-known methods for solving potential fluid flow problems. However, mixed configurations of obstacles with finite thickness and zero thickness have not been solved with these methods. Such configurations arise naturally in delta wings, sailing boats, and even in complete aircraft aerodynamics. In this work, a new numerical approach is proposed for solving 2D mixed configurations of obstacles with finite thickness and zero thickness. The method is based on the Dirichlet and Neumann formulations and is checked by comparison with analytical results.

Deflagration to detonation transitions (DDTs): Predicting DDTs in hydrocarbon explosions

This paper discusses our efforts to expand CFD calculations to include the prediction of deflagration to detonation transitions (DDTs) for various fuels including hydrogen, ethylene, propane and natural gas. The work includes validation against experiments conducted in a variety of configurations including: (1) closed pipes with obstacles; (2) other congested lab-scale geometries; (3) medium to large-scale 3-D obstacle configurations; and (4) large-scale geometries where DDT occurred upon flame propagation from confined to more open configurations. Validation of CFD models against experimental data is a critical step in verifying their accuracy in predicting DDT phenomena. In addition, an extensive set of experiments, which were performed under a variety of conditions that influence the transition to detonation, should be utilized in the validation to demonstrate the robustness of a CFD model and ensure its use for actual large-scale scenarios, i.e., petrochemical facilities where the likelihood of DDTs need to be evaluated for siting studies.The CFD model FLACS has been recently extended to identify whether DDT is likely in a given scenario and indicate the regions where it might occur. The likelihood of DDT has been expressed in terms of spatial pressure gradients across the flame front. This parameter is able to visualize when the flame front captures the pressure front, which is one of the main mechanisms that may initiate a DDT. Reasonable agreement was obtained with experimental observations in terms of explosion pressures, transition times, and flame speeds. The DDT model is currently being extended to include a criterion for estimating the likelihood of DDT by comparison of the geometric dimensions with the detonation cell size of the fuel-air mixture. This paper will provide a summary of some of the results.

Gait algorithm of personal mobility vehicle for negotiating obstacles

Purpose: Personal mobility vehicles (PMVs) are under active development. Most PMVs are wheel-driven, a mode of transport notable for its efficiency. However, wheeled PMVs tend to have poor mobility against negotiating obstacles. The four-wheeled vehicle RT-Mover PType 3 has been developed featuring wheeled legs capable of leg motion. This allows the PMV to overcome uneven terrains, including a step approached at an angle, which ordinary wheelchairs cannot negotiate. Method: This article discusses a gait algorithm in which a leg executes the necessary leg motion when optionally presented with obstacles on a road. In order to lift a wheel off the ground and perform a leg motion, the support wheels must be moved to support points to ensure that the vehicle remains stable on three wheels. When moving towards the target support point, a wheel may encounter another obstacle, and a response method for this case is also described. Results: To assess the gait algorithm, several configurations of obstacles were used for performance tests with a passenger. The capabilities of the PMV were demonstrated through experiments. Conclusion: We proposed a novel gait algorithm for our PMV and realised the proposed motion pattern for PMV-based negotiating obstacles.Implications for RehabilitationOur single-seat personal mobility vehicle, RT-Mover PType 3 features wheels attached on legs capable of performing leg motion, which allows the vehicle to traverse rough terrains in urban areas.We proposed a gait algorithm for RT-Mover PType 3 consisting of a series of leg motions in response to rough terrain. With this algorithm, the vehicle can traverse not only randomly placed obstacles, but also a step approached at an oblique angle, which conventional powered wheelchairs cannot navigate.Experiments with a passenger demonstrated the effectiveness of the proposed gait algorithm, suggesting that RT-Mover PType 3 can expand the mobility and range of activities of wheelchair users.

Power-law fluid flow and heat transfer in a channel with a built-in porous square cylinder: Lattice Boltzmann simulation

The lattice Boltzmann method (LBM) has been established as an efficient technique for solving a fluid dynamics problem in a complex porous medium. In this paper, the power-law fluid flow and heat transfer are studied numerically in a channel partially filled with an anisotropic porous block for three power-law indices, n=0.8, 1 and 1.2. Combined pore level simulations of flow and heat transfer are performed for a 2D channel that is partially filled with square obstacles in both ordered and random arrangements. A step by step verification procedure is taken to ensure the accuracy and the physical correctness of the numerical simulation. The effects of the different arrangements of obstacles, Reynolds number, power index n, blockage ratio and porosity on the velocity and temperature profiles are studied. The local and averaged Nusselt numbers are also calculated on the channel walls. It is found that pseudo plastic fluids generate the highest heat transfer rate for all configurations of obstacles. For constant porosity and block size, the increase is noticeable when the arrangement of square obstacles is random. Also by decreasing the porosity, the value of averaged Nusselt number is increased. Two correlations for regular and random obstacle arrangements between the Nusselt number, Reynolds number, power index n, blockage ratio and porosity are presented. The values of averaged Nusselt number with the respective confidence interval are also reported in the case of random arrangement of obstacles.

Effects of cross-wise obstacle position on methane–air deflagration characteristics

A vented chamber, with internal dimensions of 150 mm × 150 mm × 500 mm, is constructed in which the premixed methane–air deflagration flame, propagating away from the ignition source, interacts with obstacles along its path. Three obstacle configurations with different cross-wise positions are investigated. The cross-wise obstacle positions are found to have significant effects on deflagration characteristics, such as flame structure, flame front location, flame speed, and overpressure transients. The rate of flame acceleration, as the flame passes over the last obstacle, is the highest at the configuration with three centrally located obstacles, whereas the lowest is observed at the configuration with three obstacles mounted on one side of the chamber. Compared with the side configuration, the magnitude of overpressure generated increases by approximately 80% and 165% for the central and staggered configurations, respectively. Furthermore, flame propagation speeds and generated overpressures for both the central and staggered configurations are greater, which should to be avoided to reduce the risk associated with turbulent premixed deflagrations in practical processes.

Numerical study of obstacle configuration for droplet splitting in a microchannel

The droplet splitting in a microfluidic channel by the use of an obstacle, which can be applied to lab-on-a-chip devices for biochemical testing and synthesis, is investigated numerically. The droplet deformation is calculated by a sharp-interface level-set method which is modified to treat the immersed solid surface of an obstacle. The numerical results demonstrate that obstacle configurations, such as obstacle width, length, location and inclination, in a microchannel determine the droplet splitting pattern with or without re-merging. The effects of obstacle configurations on the droplet motion are quantified to obtain the optimal conditions for droplet splitting with even volume distribution.

The structure of convective flows driven by density variations in a continuously stratified fluid

The paper is devoted to an investigation of specific fluid motions observed around motionless objects immersed in a quiescent stratified medium. Such fluid motions, called diffusion-induced flows, may essentially influence the mixing processes and passive substance transport to a great distance and also the formation of stratified medium fine structures. The calculation results for diffusion-induced flow structures are presented in the paper for different geometrical configurations of impermeable obstacles, including a sloping plate of finite thickness, a wedge, a rectangular box, a disc and a cylinder. Even very weak stratification essentially affects a fine flow's structure and dynamics. An especially complicated flow structure is formed near the sharp edges and poles of impermeable obstacles, where the turning-points of the main jets serve as sources of dissipative-gravitational waves observed in both calculations and laboratory experiments in the form of extended horizontal streaky structures. Diffusion-induced flows always exist in non-homogeneous media at arbitrary orientations of an obstacle and non-exist in a homogeneous fluid.