Two-dimensional Obstacle Research Articles

Fixed-wing UAV obstacle avoidance algorithm based on real terrain

Abstract This paper presents an obstacle avoidance algorithm for UAVs based on real terrain constraints. The common obstacle avoidance algorithm often abstracts the obstacle as a sphere or a circle or studies the UAV’s two-dimensional obstacle avoidance on a raster map. With the development of fixed-wing UAVs, UAVs have longer and longer time in the air, longer and longer range, and the low-altitude environment they face is often difficult to simulate with simplified regular shape. Thus, this paper constructs the UAV flight terrain environment using open elevation data and designs an obstacle avoidance algorithm accordingly. When the UAV is tracking the track point, it can better control the UAV to avoid obstacles and fly to the target track point.

A path planning algorithm for three-dimensional collision avoidance based on potential field and B-spline boundary curve

Path planning is one of the key technologies in unmanned aerial systems. The path-planning algorithm for UAVs in this study incorporates a three-dimensional obstacle model, addressing the limitations of existing research that primarily focuses on the two-dimensional plane. This approach elevates traditional two-dimensional obstacle constraints to a three-dimensional space, fulfilling the requirements for precise obstacle avoidance. Utilizing B-spline curves, an obstacle boundary model is proposed, which, in combination with an improved artificial potential field accounting for localized self-locking oscillations, achieves smooth path planning for obstacle avoidance from the starting point to the destination. The simulation results show the effectiveness of this method in collision-free path planning within three-dimensional environments containing static single or multiple obstacles. At the points of maximum curvature in the two-dimensional coordinate system, the path planned by the proposed algorithm exhibits a smoothness improvement of 68 % and 98 %, respectively, as compared to the traditional artificial potential field algorithm with the equivalent three-dimensional obstacle model and the tangent point method. The proposed algorithm enables drones to achieve precise obstacle avoidance along surfaces, generating smoother collision-free flight paths in a shorter period of time.

Intensification of heat transfer behind the backward-facing step using tabs

The influence of tab size and location on the flow structure, distribution of pressures on the surface, and heat transfer in the separation region behind the backward-facing step at Reynolds number Re = 4000 was experimentally investigated. The effect of tabs on the distribution of the local pressure and heat transfer coefficients in the longitudinal and transversal directions was studied. The presence of tabs significantly intensifies the heat transfer directly behind the separation point and brings the region of maximum heat transfer closer to the step. There is a strong uneven distribution of pressure coefficients and Nusselt numbers in the transverse direction due to the formation of vortical wake. The results of measurements are compared with the case of using two-dimensional obstacles in the form of solid ribs; the features of the development of the flow and heat transfer behind 3D tabs are analyzed. It is shown that the main difference in the installation of two-dimensional and three-dimensional obstacles is observed mainly in the immediate vicinity of the flow separation point at X/H < 6.

Graph neural networks for laminar flow prediction around random two-dimensional shapes

In recent years, the domain of fast flow field prediction has been vastly dominated by pixel-based convolutional neural networks. Yet, the recent advent of graph convolutional neural networks (GCNNs) has attracted considerable attention in the computational fluid dynamics (CFD) community. In this contribution, we proposed a GCNN structure as a surrogate model for laminar flow prediction around two-dimensional (2D) obstacles. Unlike traditional convolution on image pixels, the graph convolution can be directly applied on body-fitted triangular meshes, hence yielding an easy coupling with CFD solvers. The proposed GCNN model is trained over a dataset composed of CFD-computed laminar flows around 2000 random 2D shapes. Accuracy levels are assessed on reconstructed velocity and pressure fields around out-of-training obstacles and are compared with that of standard U-net architectures, especially in the boundary layer area.

Robust deep learning for emulating turbulent viscosities

From the simplest models to complex deep neural networks, modeling turbulence with machine learning techniques still offers multiple challenges. In this context, the present contribution proposes a robust strategy using patch-based training to learn turbulent viscosity from flow velocities and demonstrates its efficient use on the Spalart–Allmaras turbulence model. Training datasets are generated for flow past two-dimensional obstacles at high-Reynolds numbers and used to train an auto-encoder type convolutional neural network with local patch inputs. Compared to a standard training technique, patch-based learning not only yields increased accuracy but also reduces the computational cost required for training.

An experimental investigation of turbulent flow over a two-dimensional obstacle on a flat plate

Aeroacoustic and aerodynamic characteristics of the turbulent boundary layer encountering a large obstacle are experimentally investigated in this paper. Two-dimensional obstacles with a square and a semi-circular cross-section mounted on a flat plate are studied in wind tunnel tests, with particular interests in the shear layer characteristics, wall pressure fluctuations, and far-field noise induced by the obstacles. Synchronized measurements of the far-field noise and the wall pressure fluctuations were conducted using microphone arrays in the far-field and flush-mounted in the plate, respectively. Additionally, the streamwise and wall-normal velocity fluctuations behind the obstacle were measured using the X-wire probe. The measured velocity profiles, spectra, and wall pressure spectra are compared, showing that the rectangular obstacle has a significant impact on both the turbulent flow and far-field noise. The large-scale vortical structures shed from the obstacles can be identified in the wall pressure spectra, the streamwise velocity spectra, and the wall pressure coherence analysis. Within the shear layer, the pairing of vortices occurs and the frequency of the broadband peak in the velocity spectra decreases as the shear layer grows downstream. Further eddy convective velocities of large-scale vortical structures inside the shear layer were analyzed based on the wall pressure fluctuations.

Modulation of viscous planar jump by an obstacle in the flow path—Interrogation through shallow water equations and numerical analysis

The paper investigates a planar hydraulic jump during a thin film flow over a two-dimensional obstacle spanning the entire width of a horizontal channel. The shallow water analysis supported by numerical simulations identifies a hydraulic drop and simultaneous jump-drop-jump phenomenon under certain flow conditions. A hydraulic drop although observed for flow over obstacles in the macro-domain has never been reported for a thin film flow. The study, thus, establishes the efficacy of shallow water analysis which, with a novel solution methodology, predicts jump and drop characteristics over obstacles with much less computational effort compared to computational fluid dynamics based simulations. In addition, analytical expressions for jump efficiency and head loss across jump along with energy dissipation analysis are provided for a viscous jump. The numerical simulations reveal recirculation zones close to the channel floor and at the free surface, and the energy dissipation analysis provides a quantitative criterion for formation of recirculation zone pair. The observations are consolidated as a phase diagram which characterizes a myriad of free surface profiles based on operating and geometrical parameters.

Radon measure solutions for steady hypersonic-limit Euler flows passing two-dimensional finite non-symmetric obstacles and interactions of free concentration layers

Radon measure solutions for steady hypersonic-limit Euler flows passing two-dimensional finite non-symmetric obstacles and interactions of free concentration layers

Skyrmion dynamics and topological sorting on periodic obstacle arrays

We examine skyrmions under a dc drive interacting with a square array of obstacles for varied obstacle size and damping. When the drive is applied in a fixed direction, we find that the skyrmions are initially guided in the drive direction but also move transverse to the drive due to the Magnus force. The skyrmion Hall angle, which indicates the difference between the skyrmion direction of motion and the drive direction, increases with drive in a series of quantized steps as a result of the locking of the skyrmion motion to specific symmetry directions of the obstacle array. On these steps, the skyrmions collide with an integer number of obstacles to create a periodic motion. The transitions between the different locking steps are associated with jumps or dips in the velocity–force curves. In some regimes, the skyrmion Hall angle is actually higher than the intrinsic skyrmion Hall angle that would appear in the absence of obstacles. In the limit of zero damping, the skyrmion Hall angle is 90°, and we find that it decreases as the damping increases. For multiple interacting skyrmion species in the collective regime, we find jammed behavior at low drives where the different skyrmion species are strongly coupled and move in the same direction. As the drive increases, the species decouple and each can lock to a different symmetry direction of the obstacle lattice, making it possible to perform topological sorting in analogy to the particle sorting methods used to fractionate different species of colloidal particles moving over two-dimensional obstacle arrays.

Reverse time migration for inverse obstacle scattering with a generalized impedance boundary condition

ABSTRACT We consider the inverse scattering problem of determining the shape of an impenetrable two-dimensional obstacle with a generalized impedance boundary condition from near-field measurements. We propose the reverse time migration method where the imaging functional is defined as the imaginary part of the cross-correlation. Our analysis shows that, on the one hand, the imaging functional has contrast at the boundary of the scatterer and decays outside the scatterer; On the other hand, the imaging functional is always positive thus may have better stability properties. These imply that the approach has powerful imaging quality which is confirmed in our numerical experiments.

Two-dimensional obstacle avoidance control algorithm for snake-like robot in water based on immersed boundary-lattice Boltzmann method and improved artificial potential field method

In order to study the adaptability of a multi-redundancy and multi-degree-of-freedom snake-like robot to underwater motion, a two-dimensional (2-D) obstacle avoidance control algorithm for a snake-like robot based on immersed boundary-lattice Boltzmann method (IB-LBM) and improved artificial potential field (APF) is proposed in this paper. Firstly, the non-linear flow field model is established under the framework of LBM, and the IB method is introduced to establish a fluid solid coupling of a 2-D soft snake-like robot. Then, the obstacle avoidance of a snake-like robot in a flow field is realized by optimizing the curvature equation of the serpentine curve and eliminating the local minimum in APF method. Finally, the effects by exerted different control parameters on a snake-like robot’s obstacle avoidance capability are analyzed via MATLAB simulation experiment, by which we can find the optimal parameter of the obstacle avoidance and testify the validity of the proposed control algorithm.

Uticaj prisustva prepreka na disperziju emisije iz savijenog dimnjaka

It is proposed to study numerically the dispersion of polluting emissions in turbulent conditions around a two-dimensional obstacle. The effect of the presence of obstacles on the flow from a Bent chimney is mainly addressed. A numerical simulation of the dispersion of pollutants emitted by the chimneys was performed using the simulation code. The numerical method used to solve the equations describing the flow is the finite volume method (FVM) and the mesh size adopted is non-uniform, very tight near the stack and obstacle. The results found essentially show that the presence of obstacles modifies the flow.

Asymptotic analysis of unilateral contact problems for linearly elastic shells: Error estimates in the membrane case

We consider a family of linearly elastic shells all sharing the same middle surface and in unilateral contact with a rigid foundation on the lower face. The shells are elliptic and their lateral face is clamped. Under these conditions, when the thickness tends to zero, the solution of the three-dimensional problem converges to the solution of a two-dimensional obstacle problem for an elastic membrane shell. In this paper we provide error estimates for this convergence. The proof uses a corrector method.

On-Line Obstacle Detection, Avoidance, and Mapping of an Outdoor Quadrotor Using EKF-Based Fuzzy Tracking Incremental Control

The unknown two-dimensional obstacles, including different shapes, numbers, locations, and in the vicinity of the planning path for an outdoor quadrotor, are on-line detected by a 360-degree Li-Dar Sweep Scanse. Based on the comparison among detected triangles containing extreme points of each obstacle, the central orientation of the triangle with the maximum detected distance and the angle larger than the specific threshold is assigned to fly a specific distance for obstacle avoidance. Nevertheless, the quadrotor probably encounters a dead end and is then navigated to a specific altitude for the continuous procedure of obstacle detection. By on-line detection at different waypoints for the same obstacle, an obstacle mapping is also established by all the obstacle detections. Since the sensing signals of an outdoor quadrotor are easily affected by stochastic noise and the quadrotor is operated in “x” configuration, an extended Kalman filter based fuzzy tracking incremental control (EKF-FTIC) is employed to simultaneously execute the path tracking, obstacle avoidance, and target approach. Finally, the experiments with 5 unknown obstacles in the neighborhood of a planning path connecting start point with end point validate the effectiveness and feasibility of the proposed method.

Effect of length of two-dimensional obstacles on characteristics of separation and reattachment

The flow over a forward-facing step (FFS) and a backward-facing step (BFS) have been extensively studied separately, but the interaction between both obstacles has received little attention. It is believed the distance between the two faces of the step is a critical parameter governing the overall behaviour of the flow. This could have implication on the effect an obstacle would have on its surroundings, such as wind disturbance, or spread of pollution. Consequently, we investigate in detail the flow over a two-dimensional obstacle (also referred to as a rib), that consists of a FFS followed by a BFS. The flow field in such configuration is a result of the interaction between the multiple separation regions that appear upstream, above and downstream of the ribs. Our experimental model is submerged in a fully turbulent boundary layer (δ/H=1.37, where δ and H are respectively incoming boundary layer thickness and rib height), and the Reynolds number based on rib height is ReH=20,000. Rib length (distance between the two vertical faces) varied between L/H=0.1 and L/H=8. In order to describe the general features of such a flow, we carried out flow field velocity and surface pressure measurements. Results show that two trends exist according to the length of the rib. Short ribs (L/H≤4) produce one large recirculation region from the leading edge which results in higher levels of large scale turbulence which propagate far downstream. On the contrary, the FFS portion of long ribs (L/H≥4) is decoupled from the BFS portion. Two separate shear layers are formed which decay quicker resulting in lower levels of turbulence propagating downstream.

Numerical Simulations of Two-Layer Flow past Topography. Part I: The Leeside Hydraulic Jump

Abstract Laboratory observations of the leeside hydraulic jump indicate it consists of a statistically stationary turbulent motion in an overturning wave. From the point of view of the shallow-water equations (SWE), the hydraulic jump is a discontinuity in fluid-layer depth and velocity at which kinetic energy is dissipated. To provide a deeper understanding of the leeside hydraulic jump, three-dimensional numerical solutions of the Navier–Stokes equations (NSE) are carried out alongside SWE solutions for nearly identical physical initial-value problems. Starting from a constant-height layer flowing over a two-dimensional obstacle at constant speed, it is demonstrated that the SWE solutions form a leeside discontinuity owing to the collision of upstream-moving characteristic curves launched from the obstacle. Consistent with the SWE solution, the NSE solution indicates the leeside hydraulic jump begins as a steepening of the initially horizontal density interface. Subsequently, the NSE solution indicates overturning of the density interface and a transition to turbulence. Analysis of the initial-value problem in these solutions shows that the tendency to form either the leeside height–velocity discontinuity in the SWE or the overturning density interface in the exact NSE is a feature of the inviscid, nonturbulent fluid dynamics. Dissipative turbulent processes associated with the leeside hydraulic jump are a consequence of the inviscid fluid dynamics that initiate and maintain the locally unstable conditions.

Inverse scattering for shape and impedance revisited

The inverse scattering problem under consideration is to reconstruct both the shape and the impedance function of an impenetrable two-dimensional obstacle from the far field pattern for scattering of time-harmonic acoustic or E-polarized electromagnetic plane waves. We propose an inverse algorithm that is based on a system of nonlinear boundary integral equations associated with a single-layer potential approach to solve the forward scattering problem. This extends the approach we suggested for an inverse boundary value problem for harmonic functions in Kress and Rundell(2005) and is a counterpart of our earlier work on inverse scattering for shape and impedance in Kress and Rundell(2001). We present the mathematical foundation of the method and exhibit its feasibility by numerical examples.

Pollutant fluxes in two-dimensional street canyons

The turbulent dispersion of a passive tracer emitted by a line source simulating vehicular traffic in an idealized urban canyon in neutral conditions is studied in the water channel by means of simultaneous measurements of velocity and concentration fields. The experiments are conducted for two geometrical arrangements of two-dimensional obstacles reproducing the skimming flow (AR = 1) and the wake-interference regime (AR = 2), where AR is the canyon aspect ratio. The results show a strict connection between the dynamics of the shear layer developing at the top of the canyon and the vorticity field. For AR = 1, it is found that the shear layer flaps upwards and downwards according to two different frequencies. The greatest of them matches the vortex shedding frequency as measured at the canyon top, while the lower is comparable to H/u∗ (H is the building height and u∗ a reference friction velocity). The shear layer flapping modulates in time the pollutant exchange rate between the canyon and the outer layer. The different characteristics of the shear layer found for the two flow regimes also explain the larger pollutant re-entrainment observed for AR = 1, which turns out to be greater than the emission rate at the source. Sweep and ejection modes are identified via quadrant analysis and used to quantify the weights of the factors involved in the turbulent exchanges of tracer and momentum between the canyon and the outer layer. It is found that the venting of polluted fluid at the canyon top increases substantially passing from AR = 1 to 2, while an opposite trend is observed for the entrainment of polluted fluid. The vertical flux of pollutant at the canyon top for AR = 2 is largely of turbulent nature, with the contribution of the mean flux being practically negligible. On the other hand, the latter becomes comparable or even exceeds the magnitude of the turbulent flux when AR = 1. The maps of the first three statistical moments of the pollutant concentration for the two geometrical arrangements are also reported and discussed.

Comparison of turbulent forced convection between wall jet and channel flow over a heated obstacle

Turbulent forced convection of a wall jet of thickness H impinging on a two-dimensional obstacle is investigated numerically in this study. The influences of two types of turbulent incoming flows (the wall jet and the channel flow) are studied in order to deepen the understanding of the effect of the wall jet outer layer on the reattachment process and heat transfer over an elongated obstacle. The finite volume method is used to solve the governing equations. In the case of the wall-jet incoming flow, the effect of the nozzle thickness ranging from H/4 to 2H is examined as a parameter. The obtained results showed that increasing of the thickness of the nozzle improves the heat transfer and considerably modifies the location of the stagnation point. Both turbulence kinetic energy and temperature contours evidence the effect of the external shear layer of the jet. The average obstacle Nusselt number is observed to increase with the Reynolds number ranging from 10,000 to 50,000 and decrease with the augmentation of the thickness of the incoming flow. A correlation is also proposed for the average Nusselt number of the obstacle as a function of the Reynolds number and the obstacle aspect ratio.

A Review on Micromixers.

Microfluidic devices have attracted increasing attention in the fields of biomedical diagnostics, food safety control, environmental protection, and animal epidemic prevention. Micromixing has a considerable impact on the efficiency and sensitivity of microfluidic devices. This work reviews recent advances on the passive and active micromixers for the development of various microfluidic chips. Recently reported active micromixers driven by pressure fields, electrical fields, sound fields, magnetic fields, and thermal fields, etc. and passive micromixers, which owned two-dimensional obstacles, unbalanced collisions, spiral and convergence-divergence structures or three-dimensional lamination and spiral structures, were summarized and discussed. The future trends for micromixers to combine with 3D printing and paper channel were brought forth as well.