Report
Teaser, summary, work performed and final results
Periodic Reporting for period 1 - LiftTrain (Aerodynamic Lift force of Trains subjected to cross winds—get it right!)
Teaser
Currently there are different methodologies for the estimation of train aerodynamic forces including full-scale measurements, physical modelling techniques and numerical modelling using computational fluid dynamics (CFD) techniques. The aerodynamic assessment of trains was...
Summary
Currently there are different methodologies for the estimation of train aerodynamic forces including full-scale measurements, physical modelling techniques and numerical modelling using computational fluid dynamics (CFD) techniques. The aerodynamic assessment of trains was based mainly on physical and CFD modelling and these are the approved methods in the EU standard for train aerodynamic assessment in crosswinds.
From literature it has been found that both physical and CFD modelling are reliable in estimating the side forces in a good accuracy. However, there the two methods provide different values for the lift force and thus for the rolling moment. The aim of this innovative Fellowship was thus to investigate the source of discrepancies between the different methods and in particular to develop an accurate numerical technique based on the steady Reynolds Average Navier Stokes (RANS) capable of accurately predicting the aerodynamic forces. The methodology was based on wind tunnel experiments, moving model testing and different types of steady and unsteady CFD techniques. In addition, the effect of surface roughness on the lift force prediction of a train subjected to cross wind is also investigated. The objectives of this research involved:
• Carrying out wind tunnel tests on an idealised, smoothed roof train model and a rough train model and repeating the tests on a moving model to measure surface pressures and velocity fields at a 30° yaw angle
• To use the results from wind tunnel experiments to develop CFD turbulence models for the flow around roof and the ground for different train models
• To develop CFD models based on different RANS models with and without wall functions
• To investigate the influence of different simulation parameters such as inlet boundary conditions, turbulence modelling, ground movement and discretization schemes on the train surface pressure.
• To analyse the data obtained from the different simulations and physical modelling to investigate the source of discrepancies.
Most trains have irregular surfaces, which can be represented as roughness. It was revealed from the experimental work that added roughness on the roof was able to reduce the minimum surface pressure on the roof and leeward side of the train and thus affecting the aerodynamic lift and side forces. In terms of the numerical research, the choice of turbulence models is a key factor in numerically exploring the flow around trains. Results of the numerical study showed that both, Shear Stress Transport (SST) and Improved Delayed Detached Eddy Simulation (IDDES) turbulence models predict similar trends in the mean flow field around the train with slight differences found in the size of the vortices and the position of separation points. Furthermore, the effects of uniform and non-uniform crosswinds were also explored. It was observed that uniform crosswinds tend to overestimate the pressure coefficients.
Work performed
Preparatory work for the project consisted of two main aspects; developing Dr Rashidi’s competence in train aerodynamics and crosswind assessment and developing his competence in the tools and methodologies used in the project. The first was achieved through studying the European standard (CEN code for train assessment in crosswinds) and indeed he developed a summary report of the aerodynamic assessment of trains in crosswinds including the different approved methods for such assessment. The latter was achieved through training on the different computational fluid dynamic tools for train aerodynamics including both the Reynolds Averaged Navier Stokes (RANS) methods and the more advanced CFD tool Dethatched Eddy Simulations (DES). Dr Rashidi also received training for the pre and post processing tools for performing CFD simulations from the supervisor Dr Hemida and he got assistance from Dr Li and a RS, Miss Anam Hashmi.
A one and half vehicle was used in the work similar to the moving train experiment. The Fellow produced CFD models based on both RANS and DES. For the RANS simulations, different parameters were investigated and their contribution to the surface pressure and the flow around the train was studied in detail. The effect of inlet flow was investigated through using two types of inlet boundary conditions in the simulations; uniform and non-uniform inlet velocity. The latter was similar to the one used in the experiment. Also, different types of turbulence modelling have been used in the simulations. The effect of discretization schemes was investigated through repeating the same RANS simulation with different discretization schemes and the results were compared with those from the experiment. In the second year of the project Dr Rashidi studied the effect of wall function through performing simulations with and without wall functions. Moreover, the effect of the ground movement was investigated through performing two simulations with stationary ground and with moving ground. The results were compared with the moving ground experiment and with stationary ground experiment (wind tunnel). In addition to the RANS simulations, detailed time varying simulations were performed using the advanced numerical technique DES. The results obtained from the DES were rich enough to visualize the flow around the train in both time averaged and instantaneous sense and the results were compared with those from the experiment and the RANS simulations. The effect of surface roughness on the surface pressure were investigated in the second half of the second year of the project through conducting wind tunnel experiments on the same train model with and without surface roughness. Two yaw angles were investigated; at 90 degrees and 30 degrees. The surface roughness was achieved through sticking longitudinal stripes on the surface of the first car of the train. The stripes were concentrated on the roof of the train. Both surface pressure and velocity around the train were measured with and without roughness.
Final results
The results produced by this fellowship are really novel and provided an improved understanding of the flow around high-speed trains subjected to crosswinds. The parameters investigated in this project added new knowledge to the field of train aerodynamics. This research will also give confidence to both train manufacturer and operators on the method used in assessing the stability of their trains in cross winds. The impact of the research will not only be immediate in both academia and industry by exploring a fundamental issue and providing the best practice of CFD, but also will be felt far into the future by providing state-of-the-art knowledge.