Zuojin Zhu

A new continuum model for traffic flow and numerical tests

A new continuum traffic flow model is developed in this paper based on an improved car-following model. In this new continuum model, the speed gradient replaces the density gradient in the equation of motion, and this replacement guarantees the property that the characteristic speeds are always less than or equal to the macroscopic flow speed. This new model also overcomes the backward travel problem that exists in many high-order continuum models. Shock waves, rarefaction waves, stop-and-go waves, and local cluster effects can be obtained from this new model and are consistent with the diverse nonlinear dynamical phenomena observed in the freeway traffic.

A new dynamics model for traffic flow

As a study method of traffic flow, dynamics models were developed and applied in the last few decades. However, there exist some flaws in most existing models. In this note, a new dynamics model is proposed by using carfollowing theory and the usual connection method of micro-macro variables, which can overcome some ubiquitous problems in the existing models. Numerical results show that the new model can very well simulate traffic flow conditions, such as congestion, evacuation of congestion, stop-and-go phenomena and phantom jam.

Numerical study of three-dimensional flows around two identical square cylinders in staggered arrangements

With the aim to understand air pollutant dispersion among high-density, high-rise buildings, this paper presents the numerical results of three-dimensional flows around two identical square cylinders TISCs in staggered arrangements at a Reynolds number of 250 and zero incident angle when the angle between the incoming velocity vector and the line connecting the centers of the cylinders is 45°. The dependence of the drag and lift, their root mean square values, and the Strouhal number on the horizontal spacing have been evaluated. It was found that the correlation coefficients between the drag and lift of the upstream square cylinder SC-I were close to unity, with those for the downstream square cylinder SC-II exhibiting a wavy form along the spanwise direction. The wavelength and the magnitude are closely related to the cylinder spacing. Secondary vortex coherent structures as well as the primary vortex structures were reported and discussed.

Discrete Hilbert transformation and its application to estimate the wind speed in Hong Kong

Abstract Discrete Hilbert Transform (DHT) has been applied to estimate the wind speed with the sample data sequence selected from the data record observed by the observatory in Hong Kong in June 1989, during which the data pertain to deep valleys and sharp crests due to manifold weather conditions in this region. To confirm the performance of the discrete Hilbert transformer, two harmonic input sequences were used to inspect the output signals, whether good agreement with the theoretical results is obtained. It was found that the energy spectrum and the outputs for the two different harmonic discrete waves are certainly correct. After the inspection of the DHT filter, the sample data for wind speed in Hong Kong were used for wind speed forecasting. For zero mean input sequence, the variance of the output is the same as that of the input signals, and so is the energy spectrum. The DHT of an individual input sample can really reflect the local variation performance, since it is the convolution with the reciprocal of time and the input data sequence, but there exists phase shift. For harmonic signals, the output signal holds a 90° phase delay.

NUMERICAL STUDY ON TRANSIENT LAMINAR NATURAL CONVECTION IN AN INCLINED PARALLEL-WALLED CHANNEL

The transient laminar natural convection in an inclined parallel-walled channel when the bottom wall is heated was investigated by direct numerical solution of the governing equations using the accurate projection method, PmIII. Comparison of the investigation results with other published experimental results for the overall Nusselt numbers was made

Numerical simulation of thermal fluid instability between two horizontal parallel plates

The buoyancy-induced thermal fluid instability between two parallel horizontal plates has been investigated numerically by solving the governing equations of base and perturbation fields using a fractional algorithm, in which the fourth-order Adams scheme and the Bi-CGSTAB scheme are embedded. An initial assumption of the perturbed temperature was made by a Gaussian random number generator. A primitive static fluid with Prandtl number 0.73 was induced to flow in the gap between the two horizontal parallel plates for three diAerent cases: top heating, bottom heating and heating from both plates. The kinetic energy of induced flow in each vertical section and in the whole domain was calculated. The results can demonstrate whether the thermal fluid flow is stable or unstable. A linear decaying behavior is found after the initial stage. The total fluid kinetic energy in the domain converges for the top heating case, but diverges for the other two heating cases. ” 2001 Elsevier Science Ltd. All rights reserved.

Numerical study of convective heat transfer from two identical square cylinders submerged in a uniform cross flow

This article presents numerical results of convective heat transfer from two identical square cylinders (TISC) arranged at close proximity in a uniform cross flow of air (Pr = 0.71) at the particular Reynolds number of 250. The proximity arrangement of the TISC is defined by longitudinal spacing ranging from zero to unity, with the transverse spacing ranging from 0.125 to unity. Results are obtained by the finite-difference method for 25 cases, divided into five groups for convenience of discussion. It is found that the time-averaged or mean convective heat transfer rates from the TISC reach their maxima when the transverse spacing value is about 0.3 from the view of the group averaging at a given transverse spacing. The heat transfer rates from inner faces of the TISC are generally greater than those of the outer sides, by a factor of about 2 to 3. Depending on the TISC arrangement, a bifurcation sometimes occurs, with the bifurcated lower St value corresponding to the shedding from the outer shear layer, and the higher one corresponding to the shedding from the inner shear layer. The significant effect of the TISC arrangement on the flow-induced forces is also reported and discussed.

Numerical Analysis of Freeway Traffic Flow Dynamics for Multiclass Drivers

A first-order multiclass model is presented for illustrating freeway traffic dynamics, especially regarding the temporal and spatial variation of both densities and flow rate along freeway segments without the disturbance from ramp flows. The proposed macroscopic model is grounded on the assumption that freeway traffic may consist of multiple classes of drivers who, characterized by their unique speed–density relation, are likely to react differently under the same driving environment. The distribution of various classes of drivers and their differences in responding to perceived driving conditions may contribute significantly to the observed freeway traffic dynamics that remain to be better explained by existing traffic flow theories. The numerical solution and simulation results reported in this study, however, indicate that this proposed first-order multiclass model offers the potential to explore the complex interactions between freeway drivers and their collective impact on traffic flow patterns.

Exploring peculiarities of traffic flows with a viscoelastic model

In this paper, a visco-elastic traffic flow model is described and applied to explore peculiarities of traffic flows on a loop road with ramp effects numerically. Based on different expressions for traffic pressure, sound speed and relaxation time, the viscoelastic model is derived from mass and momentum conservations and a linear viscoelastic constitutive relation. Numerical simulations of loop traffic flows have been carried out, with the ramp flow rate being assumed to be randomly dependent on the local main flow at the intersection. The results revealed that the on-ramp effect can cause the occurrence of traffic shock waves, the off-ramp effect can lead to the decrease of traffic density on the loop road. The viscoelastic effect does cause the significant changes of traffic flow pattern, indicating that self-organisation of loop traffic is a crucial impacting feature.

Simulation of the behaviour of transparent insulation materials in buildings in northern China

A simulation model has been developed to analyze the potential of the application of transparent insulation for passive solar buildings in northern China. The potential advantages of using transparent insulation materials in buildings are estimated, in which the effects of variations in solar radiation have been taken into account. By simulating the monthly auxiliary energy requirement of the building and the solar fraction variation due to transparent insulation, it is found that if the traditional south brick walls are covered by a 10Â cm thickness layer of honeycomb material in the Beijing area, an increase of solar fraction will be around 39%, corresponding to a solar heat gain of around 137 kW/m2 year.