The numerical flow around freight wagons
Dominic Flynn, Doctoral researcher
School of Civil Engineering
University of Birmingham
E-mail: dcf147@bham.ac.uk
Supervisors: Dr Hassan Hemida, Prof. Chris Baker
Background
In recent years there have been numerous accidents involving freight trains that have shed their containers because of the aerodynamic loads imposed by side winds. These incidents have prompted numerous investigations into the aerodynamic forces and moments experienced by these vehicles.
Research aims and objectives
Aim
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To investigate the aerodynamic flow around freight wagons and the relation of flow structures to the forces incurred
Objectives
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To carry out a number of simulations using large-eddy simulation over various geometries and at various yaw angles
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Compare numerical results against experimental work conducted in order to validate simulations
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To investigate the flow structures developed using post-processing methods and relate them to the incurred forces
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To report the results
Method
The flow over bluff bodies such as cubes or square prisms causes large scale, unsteady wake shedding. The principal behind LES is that the large turbulent scales within the flow field are resolved directly and the smallest scales, which do not contribute to the forces, are simulated. The geometries used with this research are bluff bodies and hence, exhibit a similar large-scale shedding as the cubes. Large-eddy simulation also allows for the capture of instantaneous forces and the time-varying velocity field as well as producing more accurate time-averaged values.
Computational fluid dynamics is increasing in popularity as an analysis tool because with ever-increasing computational power more accurate simulation methods such as LES are possible. As well as this, the use of flow visualisation techniques to determine the values of selected variables within the flow field. This advantageous over experimental work because to gain such detailed results would require huge cost and effort and in many circumstances be impossible.
The work has been conducted using a combination of Ansys CFX and OpenFOAM which is an open-source C++ based text-user interface. Open source software allows scientists and engineers around the world to conduct a multitude of simulations from various industrial applications without being subjected to expensive licences.
Investigations
Flow around SL freight wagon
The flow around an isolated freight wagon was conducted using large-eddy simulations as well as two popular RAS models. Pressure iso-surfaces were developed using results from the different turbulence models and the results for the steady flow showed encouraging similarity. The use of these models for similar flow would drastically reduce the computational power required to get accurate time-averaged forces and pressure coefficients for a similar geometry.
Flow around a class 66 with containers
The aerodynamic flow over a class 66 train with freight wagons was conducted using large-eddy simulation. The simulations were conducted parallel to experimental tests using a moving model rig. The work captured both the instantaneous and time-averaged flow in order to gain a full perspective of the flow regimes around the vehicle. The CFD analysis confirmed a velocity peak above the nose of the train (around 1.35 times the freestream velocity) which was previously found in experimental data.
The flow around a double-stacked freight wagon
The numerical simulation of the flow around a double-stacked freight wagon was simulated. Previous experimental work conducted by Alam and Watkins investigated the forces and yawing moments incurred by an isolated freight wagon at a range of yaw angles (-90o to 90o). The numerical simulation showed the side force coefficient was within 2% of the experimental work. This high level of agreement gives credence to the notion of using large-eddy simulation for bluff body flows.
Future work
Further work will include the numerical simulation of the flow beneath a high-speed train in order to determine the effect the underbody complexities have on the flow properties. Furthermore, the results gained will be validated against current experimental work and from here assess the likelihood of ballast flight beneath the vehicle. The aim is to complete this assessment for several train geometries in order to determine the key causes of the flow behaviour.
Simulation of track switches using finite element methods
Collaborative research between the Birmingham Centre for Railway Research and Education (BCRRE) and Network Rail is being conducted to establish the effect of introducing new parts to two current track switch mechanisms. The work is being carried out using Abaqus and is aimed at determining whether a train going through the switch, if it is locked the wrong way, will de-rail the train or cause permanent damage to the mechanism. The simulations allow investigators to determine the stress levels on every deformable part within the mechanism and previous simulations have shown good agreement to experiment.
