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The current research attempts to describe the association between the aerodynamic drag and flow structures induced by train running in time-averaged and time-dependant views. A ICE2 train model is used as the research object, with the flow field solved by Large Eddy Simulation (LES). The validation is carried out through comparison with existing research data, and grid independence study in terms of time-averaged and second-order statistics. Numerical results are firstly presented by description of the time-averaged near wake flow topology based on surface flow visualization. Then, the wake integration method is used both time-averaged and instantaneously to link the aerodynamic force with integrated flow quantities on different wake planes: from the time-averaged perspective, the contribution of different flow terms to total drag is presented and compared; from the instantaneous perspective, the overall wake variation tendency is linked to the fluctuating component of aerodynamic force, based on the wake convection velocity and downstream distance of wake planes. The dynamic features of flow quantities in different locations are presented, with cross-correlation method used to describe the correlation between fluctuating aerodynamic drag and wake motion. Approaches and findings presented in the current study could serve as fundamentals which support future study. • Large eddy simulation employed to solve flow field around a generic ICE2 high-speed train model. • Surface flow visualization techniques utilized to illustrate the time-averaged near wake flow topology. • Wake integration method applied to link the aerodynamic force with integrated flow quantities on different wake planes. • Dynamic features of flow quantities presented, with cross-correlation method used to describe correlation between aerodynamic drag and wake motion. [ABSTRACT FROM AUTHOR]

The improved delayed detached eddy simulation (IDDES) was used to study the influence of the platoon configuration of trains on its aerodynamic performance. CFD simulations were validated against the wind tunnel test and different mesh refinement to further verify the flow field around the high-speed train. The platoon configuration mainly affects the aerodynamic forces of the tail car of the front train (car3) and the head car of the rear train (car4) through flow interference. The surface pressure distribution of the train and the time domain and frequency domain characteristics of the aerodynamic force under different spacing are analyzed, and the changing law of the aerodynamic force and the lateral stability of the trains are summarized. The boundary layer and slipstream of trains in platoon are also compared with those of single trains. Meanwhile, both the time-averaged and instantaneous near-wake structures are compared for different cases. • IDDES k-w turbulence model is used to model the wake interaction and detailed flow field structure. • The aerodynamic force, slipstream, and boundary layer under different spacing are evaluated. • The dominant equivalent full-scale frequency (Hz) at 300 ​km/h lies between 1.87 and 3.33 ​Hz (0.09 ≤ S t ≤ 0.16). • The largest oscillating side forces for the car4 in platoon is 157.62 ​N. [ABSTRACT FROM AUTHOR]

The improved delayed detached-eddy simulation (IDDES) method based on the shear-stress transport (SST) κ-ω turbulence model was employed to investigate the ballast height influence on aerodynamic performance at Re ​= ​9.12 ​× ​106. Both time-averaged and instantaneous slipstreams and near-wake flow structures and associated slipstream velocity distributions are compared for four cases. Results reveal that the mean lift force coefficient decreases with increasing ballast height. At the Technical Specifications for Interoperability (TSI) trackside position, the maximum slipstream velocity with a ballast height of 1.825 ​m (Case 3) is 97.4%, 15.6% and 11.7% lower than with a ballast height of 0 ​m (Case 1), 0.825 ​m (Case 2) and 2.825 ​m (Case 4), respectively. The TSI values in Case 3 ​at both the trackside and platform positions are the lowest. In the wake region, increasing the ballast height greatly constrains the outward and downward movements of vortices. The slipstream velocity in the wake region in Case 3 is the lowest among all cases. In this study, we found that Case 3 greatly decreases the slipstream velocity around the train and improves the flow structure in the wake region, which reduces the risk to workers and equipment along the railway. • The CFD results were validated with wind tunnel experiment and found good agreement. • IDDES k-w turbulence model is used to model the detailed flow field structure. • The effect of height of ballast on the slipstream and wake dynamics of train is investigated. [ABSTRACT FROM AUTHOR]

The aerodynamic loads on the overhead bridge bottom surface induced by train passage are reported in this paper. Both moving model test and numerical simulation approaches at the 1:20 scale are used. The numerical work is validated through both mesh independence tests and comparison with experimental data. Typical pressure variation curves are plotted and compared with previous studies. The peak pressure values' dependence on the Reynolds number is considered through four sets of experiments with different train running speeds. The peak pressure coefficient distribution law for the bridge bottom surface is presented. Differences in the pressure distribution in different bridge bottom areas are explained based on more detailed flow field information. The influence of the bridge height on the aerodynamic load magnitude and time interval is presented. Moreover, the application of the CEN Standard to practical engineering issues is discussed. [ABSTRACT FROM AUTHOR]

The pressure variations induced by a CRH380A EMU on a 2.15 m high bridge noise barrier are investigated in a field measurement. The familiar fluctuating pressure time history curves and the peak-to-peak pressure ( ΔP ) distributions attributing to the train head on the barrier surfaces are presented. A comparison of the positive head pulse pressure is made between the measurement results and the data calculated by empirical equations in EN 14067-4. Furthermore, the influences of train speed, train running lines, locations of measurement points, train marshalling length and environmental wind speed on ΔP are analysed. The results indicate as the train speed increases, the corresponding time intervals of ΔP decrease gradually, whereas their slopes become increasingly steep. For a CRH380A EMU, the aerodynamic length of the train head is between 7.63 and 7.64 m, which differs from the physical length of a realistic train with 12.00 m. Along the noise barrier from bottom to top, the ΔP values on the inner surface decrease with the increase of height, while these values on the outer surface increase. Taking the wind direction into account, the ΔP values are a little higher when the high-speed trains run against the wind direction. [ABSTRACT FROM AUTHOR]

With an increase in train speed, the aerodynamic effects caused by the train could escalate, especially for a train running in a tunnel. A number of large transient pressure waves are generated owing to the confined spaces within the tunnel, resulting in possible damage to the vehicle structure and the facilities in the tunnel. Therefore, it is necessary to study the aerodynamic performance of a train running in a tunnel. A scaled moving model test system was constructed to facilitate the simulation of the aerodynamic effects caused by a train running in a tunnel. In this study, the influence of grid density, calculation time step, and turbulence model on the pressure caused by the train entering the tunnel was analyzed, which is helpful for choosing suitable values of the aforementioned parameters to simulate the aerodynamic performance of the train in the tunnel. The impacts of Reynolds number effect on the distribution of the surface pressure and peak of pressure wave along the train, and the pressure waveform were also studied through numerical simulation on three scaled model trains (full scale, 1/8, 1/20, and 1/32 scaled). The findings aid in understanding the relationship between the Reynolds and pressure amplitude, and the results of the scaled test can be applied to a full-scale train. [ABSTRACT FROM AUTHOR]

The objective of this study was to investigate the effects of oblique tunnel portals on train and tunnel aerodynamics using a 1:20 scale moving model device. Transient pressure and micro-pressure waves were measured using pressure sensors as the train model travelled through various tunnel models at a speed of 350 km/h. The mitigation physical mechanism of oblique tunnel portal on the initial compression wave was explained. Experimental results showed that oblique tunnel portals had obvious mitigation effect on- the pressure gradient and micro-pressure wave induced by the train model passing through the tunnel model. A hat oblique tunnel portal combined with a buffer structure with top holes was particularly effective. [ABSTRACT FROM AUTHOR]

Using a 1:20 scale moving model device, pressure distributions on train and tunnel surfaces, and the distribution of micro-pressure waves within 50 m of tunnel exits were investigated. In addition, the effects of train speeds on the transient pressures and micro-pressure waves were analyzed. The results revealed that the effect of the train speed on P min values was more significant than that on P max values, on the train surface. Significant differences between symmetrical measurement points located on the same cross section of the tunnel with double tracks could be observed. Moreover, similar differences between symmetrical points located on the same cross section in the streamlined zone of the model train were observed and analyzed. The pressure changes in the measurement points located on the same cross section of the train body, other than the streamlined zone, were approximately coincident. The differences between the P min values of measurement points located on different cross sections of the tunnel determined the differences between the ΔP values of these measurement points. The micro-pressure waves were approximately equal for measurement points located on the same cross section. Moreover, linearity decreased when the distance between the measurement points and the tunnel exit increased. • Pressure distribution on streamlined zone and the body of train model was measured and analyzed. • Pressure distribution on tunnel was measured and analyzed. • Distribution of the micro-pressure wave outside tunnel exit was measured and analyzed. • Effects of train speed on ΔP values of train and tunnel, and the pressure gradients were analyzed. • Effects of train speed on maximum values of micro-pressure wave were measured and analyzed. [ABSTRACT FROM AUTHOR]

With the increasing need for passenger transportation, the double-unit formation of high-speed trains (HSTs) has been widely employed. However, the dynamic characteristics of the flow around double-unit HSTs have rarely been studied. In this study, the Improved Delayed Detached Eddy Simulation (IDDES) method is employed to analyse the differences in flow fields around single- and double-unit trains. The numerical case is validated through wind tunnel experiments, mesh independence tests and IDDES assessments. The results show that the existence of the double region concentrates the Strouhal number of the aerodynamic force of the wake at 0.15. For the flow downstream of the double region, the slipstream velocity detected until the far-wake region is higher than that in the single-unit case. The standard deviation of the velocity, which reflects the intensity of the turbulence, is also higher for the double-unit case. For the near wake, the vortex dominate region is dominated by flow structure shedding from the snow plough thus similar for two cases; for the downwash dominate region, the double-unit wake exhibits more turbulent flow structures and lower dominant frequency for velocity. The proper orthogonal decomposition results also indicate more turbulent wakes within the downwash dominate region in the double-unit case. [ABSTRACT FROM AUTHOR]

Abstract The improved delayed detached eddy simulation (IDDES) was used to study the influence of the train's nose length on its aerodynamic performance. Both the time-averaged and instantaneous near-wake structures and the associated distribution of slipstream velocity are compared for three nose lengths. As the nose length increases, the mean and Std values of the drag and lift force are decreased. The shorter nose-length case results in a higher slipstream velocity. In particular, at the TSI track-side position, the TSI value U 2δ for the 5-m nose length case is 30% and 32% higher than the corresponding values for the 7.5-m and 10-m nose length cases, respectively. The dynamical flow topology in the wake reveals that the flow structures of the 5-m nose length are different from those of the other two cases in the tail streamline surface. As nose length increases, the longitudinal vortices are weaker, and the angle and distance between the longitudinal vortices are smaller. The shear production from the P x y caused by the separation of the boundary layer at the lateral wall of the tail train is greater than that of the P x z caused by the separation of the boundary layer at the top and bottom of the tail train. [ABSTRACT FROM AUTHOR]