Ideal planar fluid flow over a submerged obstacle: Review and extension

A classical problem in free-surface hydrodynamics concerns flow in a channel, when an obstacle is placed on the bottom. Steady-state flows exist and may adopt one of three possible configurations, depending on the fluid speed and the obstacle height; perhaps the best known has an apparently uniform flow upstream of the obstacle, followed by a semiinfinite train of downstream gravity waves. When time-dependent behaviour is taken into account, it is found that conditions upstream of the obstacle are more complicated, however, and can include a train of upstream-advancing solitons. This paper gives a critical overview of these concepts, and also presents a new semianalytical spectral method for the numerical description of unsteady behaviour. doi:10.1017/S1446181121000341

Design and Dynamic Simulation Analysis of a Wheel–Track Composite Chassis Based on RecurDyn

The traditional chassis has the problems of low trafficability and poor stability under complex and changeable unstructured conditions. Thus, a wheel rail composite chassis is proposed. The chassis had a tracked travel mechanism at the front wheels and a wheeled travel mechanism at the rear wheels. This study presents the design, theoretical analysis and dynamic simulation analysis of the chassis. The maximum values of the passability of the wheel–track composite chassis that can be passed were calculated according to the relevant parameters. Furthermore, the chassis was modeled and simulated using RecurDyn to verify whether the values were reasonable. According to different values of the terrain, slope, vertical obstacle height and trench width, the change regularity of the track tension and driving torque of the chassis were obtained. The chassis is designed to improve the vehicle’s ability to operate under complex and diverse unstructured conditions.

Effect of obstacle height on the nanofluid convection patterns inside a hollow square enclosure

This paper contains natural convection of Ag–MgO/water micropolar hybrid nanofluid in a hollow hot square enclosure equipped by four cold obstacles on the walls. The simulations were performed by the lattice Boltzmann method (LBM). The influences of Rayleigh number and volume fraction of nanoparticle on the fluid flow and heat transfer performance were studied. Moreover, the effects of some geometric parameters, such as cold obstacle height and aspect ratio, were also considered in this study. The results showed that when the aspect ratio is not large ([Formula: see text] or 0.4), at low Rayleigh number (103), the two secondary vortices are established in each main vortex and this kind of secondary vortex does not form at high Rayleigh number (106). However, at [Formula: see text], these secondary vortices occur again in the middle two vortices at [Formula: see text], which is similar to that at [Formula: see text]. At [Formula: see text], the critical Rayleigh number, when the dominated mechanism of heat transfer changes from conduction to convection, is 104. However, the critical Rayleigh number becomes 105 at [Formula: see text] or 0.6. When the cold obstacle height increases, the shape of the vortices inside the enclosure changes due to the different spaces. Besides, at [Formula: see text], for different cold obstacle heights, the location of the thermal plume is different, owing to the different shapes of vortices. Accordingly, the average Nusselt number increases by increment of the Rayleigh number, nanoparticle volume fraction, cold obstacle height and aspect ratio.

Higher-Level Cognitive Function and Obstacle Attributes: An fNIRS Study in Older Adults With Parkinson’s Disease

Older adults with Parkinson’s disease (PD) rely on prefrontal activation to compensate for impaired motor function during the performance of complex mobility-related activities such as obstacle negotiation. However, the influence of the properties of the obstacles on prefrontal activation has not been systematically evaluated. Here, we examined the effects of obstacle height and anticipation time on prefrontal activation in patients with PD and older adults. 34 patients with PD (age: 67.4±5.7 years; 14 women) and 26 older adult controls (age: 71.3±8.9 years; 11 women) walked in an obstacle course while negotiating anticipated and unanticipated obstacles at heights of 50 mm and 100 mm. Prefrontal activation was measured using functional Near-Infrared Spectroscopy (fNIRS); obstacle negotiation performance was measured using Kinect cameras. PD patients showed greater increases in prefrontal activation during and after obstacle crossing compared to the older adults (p<0.001). Obstacle height affected prefrontal activity only when crossing anticipated obstacles (time x height interaction, p=0.011); in that case, higher obstacles were accompanied by higher prefrontal activity. PD patients showed higher levels of activation during unanticipated obstacles, compared to anticipated obstacles (p=0.015). Different correlations between prefrontal activation and obstacle negotiation strategies were observed in the patients and the controls. These results point to the use of prefrontal activation as a compensatory mechanism in PD. Moreover, the higher activation of prefrontal regions during more challenging obstacles suggests that there is a greater reliance on cognitive resources in these demanding situations that may contribute to the higher risk of falls in patients with PD.

Numerical Investigations of a Passive Micromixer Based on Minkowski Fractal Principle

This paper is mainly to study the mixing efficiency and pressure drop of the Minkowski fractal obstacle micromixers. The mixing efficiency of primary Minkowski fractal obstacle (PMFO) micromixer and secondary Minkowski fractal obstacle (SMFO) micromixer are compared at five kinds of Reynolds numbers. With the increase of obstacle height and the decrease of distance, the chaotic convection in the microchannel is enhanced. Especially at obstacle height (h) = 0.2 mm, obstacle distance (D) = 0.15 mm, and Re = 100, the vortex caused by the Minkowski fractal obstacle structure is more obvious. In addition, vortex phenomenon increases the contact area of two fluids and enhances chaotic convection. It shows that the flow direction of the fluid in the microchannel varies significantly.