Jiasong Wang

Traffic Emission Transportation in Street Canyons

Spatial distributions of traffic-related pollutants in street canyons were investigated by field measurements and Computational Fluid Dynamics (CFD). Two typical street canyons were selected for field monitoring, and a three-dimensional numerical model was built based on Reynolds-averaged Navier-Stokes equations equipped with the standard k-ε turbulence models for CFD simulations. The study shows that the pollutant concentrations of vehicle emission correlate well with the traffic volume variation, wind direction and wind speed. The wind direction and speed at the roof level determine overwhellmingly the flow field and the distributions of pollutant concentrations in the street canyon. When the wind speed is equal to zero, the pollutant concentrations on the breath height of the both sides of the street canyon are almost the same. When the wind direction is perpendicular to the street, one main vortex is formed with a shape depending on the building structure on both sides of the street, the pollutant is accumulated on the leeward side, and the pollutant concentrations at the breath height on the leeward side are 2 to 3 times as those at the breath height on the windward side. If the wind direction makes some angles with the street canyon, the pollutant concentration will be higher on the leeward side because one main vortex will also be formed in the vertical section of the canyon by the perpendicular component of the wind. But pollutant concentrations decrease in the canyon because pollutants are dispersed along the axis of the street. Pollutants at different heights of the vertical section decrease with height, i.e. there are concentration gradients in the vertical section, and the pollutant concentrations on the leeward side of the upstream building are much higher than those on the windward side of the downstream building.

FLOW AROUND A CIRCULAR CYLINDER USING A FINITE-VOLUME TVD SCHEME BASED ON A VECTOR TRANSFORMATION APPROACH

Flows around a circular cylinder displaying an unsteady vortex shedding process at the Reynolds numbers of 1000, 3900 and 1 × 104 are studied using a finite-volume Total Variation Diminishing (TVD) scheme for solving the Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations. An Elemental Velocity Vector Transformation (EVVT) approach is proposed for the local normal and tangential velocity transformation at the interfaces of main and satellite elements. The presented method is validated by comparing with the available experimental data and numerical results. It is shown that the two-dimensional TVD finite volume method with the Renormalization Group (RNG) turbulence model can be used to determine hydrodynamic forces and captures vortex shedding characteristics very well.

Vortex-induced vibration on 2D circular riser using a high resolution numerical scheme

This paper presents a high resolution numerical method for vortex induced vibration (VIV) simulation on the fluid structure interaction (FSI) of circular cylinder which represents a two dimensional marine riser. For the VIV case, the cylinder is elastically mounted and is modeled as a spring-mass oscillation system. Based on a new proposed elemental velocity vector transformation (EVVT) method, a finite-volume total variation diminishing (TVD) approach developed recently for solving unsteady Reynolds Averaged Navier-Stokes (URANS) equation with the RNG turbulence model was used to simulate the key hydrodynamic parameters such as lift coefficients. The four-stage Runge-Kutta method is used to solve the dynamic response equation of the structure. The FSI prediction results are compared with the available experimental data and showed a good agreement in a wide range of Reynolds number, which provide a good picture of real physics of phenomenon including the Karman vortex streets with different vortex modes with regard to the reduced velocities.

Numerical Simulation of Flow Control on Marine Riser With Attached Splitter Plate

Attaching a splitter plate (SP) on the base of a riser wall is used to control the flow of risers and evaluated by using the CFD technique in this paper. A finite-volume total variation diminishing (TVD) approach for solving incompressible turbulent flow with renormalization group (RNG) turbulence model was used to simulate the hydrodynamic characteristics of the riser system with additional SP for the different aspect ratio of length to diameter L/D. It was shown that the present numerical method has high order of accuracy by comparing with the available experimental and numerical simulation data for typical circular cylinder flow. A riser system attached with SPs of L/D = 0.5∼2.0 for Reynolds number 1000, and 30000 respectively can obviously reduce the lift and drag coefficient and alter the vortex shedding frequency. The mean drag coefficient can be reduced up to 20% and 35% and the maximum lift coefficient can be reduced up to 94% and 97%, for Re = 1000 and 30000, respectively. The lift can be effectively suppressed after a relative long time. L/D = 0.5∼1.0 may be considered as more practical geometries considering the real conditions, which also have good flow control effect.Copyright