Concave Obstacles Research Articles

Effective mixing in a microfluidic oscillator using an impinging jet on a concave surface

In this study, we present a microfluidic oscillator design that employs an impinging jet on a concave surface to enhance the microscale mixing process. The Coandă effect along with the Gortler instability proves to incite sustainable flapping motion beyond the obstacle and mixing is profoundly improved. From the flow visualization results, four different regimes are identified and we find that the primary enhancement of mixing performance is always linked to the transition of flow regime. Moreover, incorporating a sudden-expansion confluence provokes flow three dimensionality and elevates the mixing level significantly at low Reynolds numbers. For a Reynolds number as low as 70, the tail flow behind the concave obstacle successfully exhibits a periodic oscillation and Hopf bifurcation is induced, leading to a drastic augmentation in the time-average mixing efficiency. By utilizing the spectrum analysis, the characteristic frequency of flapping motion is found to vary linearly with the throat velocity, resulting in a constant Strouhal number of 3.8 × 10−5.

Active Route-Guiding Protocols for Resisting Obstacles in Wireless Sensor Networks

In wireless sensor networks, a geographic region without the functionality of sensing and communication can generally be treated as an obstacle, which significantly impacts the performance of existing location-based routing. An obstacle can dynamically be formed due to unbalanced deployment, sensor failure, or power exhaustion, animus interference, or physical obstacles such as mountains or buildings. This paper proposes novel algorithms that enable the existing location-based routing protocols that resist obstacles. Applying the proposed active route-guiding protocol for single obstacles (S-RGP), border nodes that surround the obstacles will actively establish a forbidden region for concave obstacles and make the obstacle information transparent. Then, packets will be guided to overcome the obstacle and move along the shorter path from the encountered border node to the sink node. In addition, the proposed active route-guiding protocol for multiple obstacles (M-RGP) takes multiple obstacles into consideration and integrates their information to help the packets overcome multiple obstacles. Simulation results show that the proposed S-RGP and M-RGP create low overhead and significantly reduce the average route length, and, therefore, improve the energy consumption and end-to-end delay for a wireless sensor network with obstacles.

Constructing a solution in time semidiscretization method to an equation of vibrating string with an obstacle

This article treats a problem which describes the movement of a string that hits an obstacle. In [M. Schatzman, A hyperbolic problem of second order with unilateral constraints: The vibrating string with a concave obstacle, J. Math. Anal. Appl. 73 (1980) 138–191] M. Schatzman solves this problem in a slightly classical way. In [K. Maruo, Existence of solutions of some nonlinear wave equation, Osaka J. Math. 22 (1985) 21–30; On certain nonlinear differential equations of second order in time, Osaka J. Math. 23 (1986) 1–53] K. Maruo constructs a solution to this problem by the use of the Yosida approximation. The purpose of this article is to construct a solution to this problem in a time semidiscretization method. In general approximation by the time semidiscretization method is different from the Yosida approximation. A simple example is presented in Appendix.

Three-Dimensional Interaction of a Supersonic Gas-Droplet Jet and Obstacles with Regard for Phase Transitions

The inleakage of a non-single-phase non-single-component jet consisting of a passive gas, evaporating droplets, and vapor on solid surfaces is investigated. In the physical model of the jet, the diffusion mechanism of interphase mass transfer is assumed, along with the presence of the velocity and thermal nonequilibrium between droplets and the carrier binary mixture of gases. This relaxing two-phase medium is treated as locally monodispersed, with the droplet integrity being defined by the Weber number. The droplets colliding with the surface break down into small fragments which vaporize instantaneously to produce the injection of the vapor mass equal to the mass of incident droplets into the flux. The numerical investigations are performed in the region of determined parameters corresponding to the steady-state regime of inleakage of a jet with a dispersed annular screen onto flat and concave cylindrical obstacles at different angles.

Solving the local minima problem for a mobile robot by classification of spatio‐temporal sensory sequences

The local minima problem occurs when a robot navigating past obstacles towards a desired target with no priori knowledge of the environment gets trapped in a loop. This happens especially if the environment consists of concave obstacles, mazes, and the like. To come out of the loop the robot must comprehend its repeated traversal through the same environment, which involves memorizing the environment already seen. This paper proposes a new real-time collision avoidance algorithm with the local minima problem solved by classifying the environment based on the spatio-temporal sensory sequences. A double layered classification scheme is adopted. A fuzzy rule base does the spatial classification at the first level and at the second level Kohonen’s self-organizing map and a fuzzy ART network is used for temporal classification. The robot has no prior knowledge of the environment and fuzzy rules govern its obstacle repulsing and target attracting behaviors. As the robot traverses the local environment is modeled and stored in the form of neurons whose weights represent the spatio-temporal sequence of sensor readings. A repetition of a similar environment is mapped to the same neuron in the network and this principle is exploited to identify a local minima situation. Suitable steps are taken to pull the robot out of the local minima. The method has been tested on various complex environments with obstacle loops and mazes, and its efficacy has been established. 2000 John Wiley & Sons, Inc.

Navigation of an Autonomous Mobile Robot by Coordination of Behaviors

In this paper we propose a sensor-based navigation algorithm based on the coordination of elementary behaviors, mainly built on fuzzy inference systems. The proposed navigator combines two types of obstacle avoidance behaviors, one for the convex obstacles and one for the concave ones. To avoid the convex obstacles the navigator fuses a "reaching the middle of a collision-free space" behavior and a "goal-seeking" behavior. The fust one is based on a fuzzy inference system and the second one uses a force derived from an attractive potential field. The concave obstacle avoidance behavior results of the mixture of a "wall-following" behavior with a creation of transition subgoals. An automatically on-line tuned fuzzy wall-following system using a neuro-fuzzy structure is designed. The effectiveness of the proposed method is verified by a series of experiments on the miniature mobile robot Khepera®.

Obstacle avoidance motion planning for mobile robots in a dynamic environment with moving obstacles

A new real-time obstacle avoidance method for mobile robots has been developed. This method, namely the vector-distance function method, permits the detection of obstacles (both moving and stationary) and generates a path that can avoid collisions. The proposed approach expresses the distance information in a vector form. Then the notion of weighting is introduced to describe relationship between sensors of mobile robots and the target to be reached. Furthermore, R-mode, L-mode and T-mode are introduced to generate a safe path for the mobile robot in a dynamic environment filled with both stationary and moving obstacles. The algorithm can deal with a complicated obstacle environment, such as multiple concave and convex obstacles. Simulation results are included to demonstrate the applicability and effectiveness of the developed algorithm.

Robot navigation in unknown generalized polygonal terrains using vision sensors

This paper considers the problem of navigating a point robot in an unknown two-dimensional terrain populated by disjoint generalized polygonal obstacles. A generalized polygon consists of a connected sequence of circular arcs and straight-line segments. The terrain model is not known a priori, but the robot is equipped with a vision sensor. A discrete vision sensor detects all visible (from a single position) portions of the obstacle boundaries in a single scan operation. The navigation problem deals with moving the robot through the terrain from a source position to a destination position, and the terrain model acquisition problem deals with autonomously building a model of the terrain. A complete solution to either problem is shown to require an infinite number of scan operations in cusp regions formed by a pair of convex and concave obstacle edges. Either problem is considered solved with a precision /spl epsiv/ if the points that have not been scanned are those in a cusp region with a clearance less than /spl epsiv/ from two obstacle edges. Three methods are proposed to solve both problems with a precision /spl epsiv/ based on extensions of the generalized visibility graph, the generalized Voronoi diagram, and the trapezoidal decomposition. Then simplified versions of these structures are proposed to exactly solve the navigation and terrain model acquisition problems using a continuous vision sensor that detects all visible obstacle boundaries as the robot navigates along a path. >

Experimental Investigation of the Hydrodynamic Interactions between a Sphere and a Large Spherical Obstacle

The hydrodynamic interactions between a spherical particle embedded in a very viscous fluid and a large close spherical obstacle are investigated experimentally. The displacement of the particle is followed by laser interferometry with a 100 nm resolution. The large obstacle is either convex, plane or concave. The experimental results for the drag coefficient on the particle are compared to the theoretical results written as three terms expansions for small gaps, viz. the result of Cox and Brenner (1967) for a plane obstacle, the result of Jeffrey (1982) and Jeffrey and Onishi (1984) for a convex obstacle and an extension of Cooley and O'Neill (1969) for a concave obstacle. Excellent agreement is found between the experimental and the theoretical results for all cases.

A Real-time Algorithm for Obstacle Avoidance of Autonomous Mobile Robots

SUMMARYA real-time obstacle avoidance algorithm is proposed for autonomous mobile robots. The algorithm is sensor-based and consists of a H-mode and T-mode. The algorithm can deal with a complicated obstacle environment, such as multiple concave and convex obstacles. It will be shown that the algorithm is more efficient and more robust than other sensor-based algorithms. In addition, the algorithm will guarantee a solution for the obstacle avoidance problem. Since the algorithm only takes up a small computational time, it can be implemented in real time.

A hyperbolic problem of second order with unilateral constraints: The vibrating string with a concave obstacle

The motion of a vibrating string constrained to remain above a material concave obstacle is studied. It is assumed that the string does not lose energy when it hits the obstacle. A set of natural inequations describes this model; an energy condition in an ad hoc form must be added to ensure uniqueness. Existence and uniqueness are proved for the Cauchy problem; the case of an infinite string and the case of a finite string with fixed ends are considered.