Hassan Hemida

Transient Simulation of the Aerodynamic Response of a Double-Deck Bus in Gusty Winds

The purpose of the research reported in this paper was to investigate the aerod ynamic response of a double-deck bus in gusty winds using a Detached-Eddy Simula tion (DES). The bus was subjected to three different scenarios of wind gusts: g ust in a wind tunnel, gust in a natural wind and gust behind the exit of a tunne l. The proposed scenarios of gusts are in the time domain and take into account the dynamic behavior of natural winds. The Reynolds number of the flow, based on the time-averaged speed of the side wind and a reference length of squre root of 0.1 [m], was 1300000. Detailed transient responses of the aerodynamic coe fficients and flow structures were investigated. Good agreement was found betwe en the DES results and the available experimental data. A comparison between the influence of the different gust scenarios on the aerodynamic coefficients shows that the gust behind the exit from a tunnel has a stronger influence on the aer odynamics than the other gust scenarios. Moreover, the influence of the gusts o n the time history of aerodynamics coefficients is found to be limited to the pe riod of the gust.

The calculation of train slipstreams using Large-Eddy Simulation

High-speed trains push air to the front, sides and over the top to form a train slipstream. The extension of the slipstream to the side, top and wake flow depends on train speed, train shape, ambient conditions and the environment in which the train operates. In this paper, the slipstream and wake flow of a 1/20th scale model of a simplified five-coach ICE2-shape train running in two different environments; in open air and when passing a platform, were obtained using large-eddy simulation (LES). The flow Reynolds number was taken to be 300,000; based on the speed and height of the train. The effect of the platform height on the train slipstream was investigated by performing simulations on a platform of different heights: 20, 60, 90 cm. To investigate the effect of mesh resolution on the results, two different computations were performed for the case of the flow around the train running in the open air using a different number of mesh nodes; a fine mesh consisting of 18,000,000 nodes and a coarse mesh consisting of 12,000,000 nodes. The results of the coarse mesh simulation were deemed to be comparable to those from the fine mesh simulation. The LES results were also compared with full-scale data and a good agreement obtained. A number of different flow regions were observed in the train slipstream: upstream region, nose region, boundary layer region, inter-carriage gap region, tail region and wake region. Localized velocity peaks were obtained near the nose of the train and in the near wake region. Coherent structures were formed at the nose, roof and inter-carriage gaps of the train. These structures spread in the slipstream and extend a long distance behind the train in the far wake flow. The maximum slipstream turbulent intensity was found in the near wake flow. The results showed that there is a significant effect of the platform height on the slipstream velocity and nose and tail pressure pulses. However, there is only a minor effect of the platform height on the static pressure along the body of the train compared with that on the nose and tail pressure pulses. In general, the slipstream velocity in the lower region of a train running in the open air was found to be larger than that around a train passing a platform. This has been related to the effect of the underbody complexities of the train.

LES Study of the Influence of a Train-Nose Shape on the Flow Structures Under Cross-Wind Conditions

Cross-wind flows around two simplified high-speed trains with different nose shapes are studied using large-eddy simulation (LES) with the standard Smagorinsky model. The Reynolds number is 300000 based on the height of the train and the free-stream velocity. The cross section and the length of the two train models are identical whilst one model has a nose length twice that of the other. The three-dimensional effects of the nose on the flow structures in the wake and on the aerodynamic quantities such as lift and side force coefficients, flow patterns, local pressure coefficient and wake frequencies are investigated. The short-nose train simulation shows highly unsteady and three-dimensional flow around the nose yielding more vortex structures in the wake. These structures result in a surface flow that differs from that in the long-nose train flow. They also influence the dominating frequencies that arise due to the shear-layer instabilities. Prediction of vortex shedding, flow patterns in the train surface and time-averaged pressure distribution obtained from the long-nose train simulation are in good agreement with the available experimental data.

Large-Eddy Simulation of the Flow Around a Simplified High Speed Train Under the Influence of a Cross-Wind

Large-eddy simulation (LES) is made to solve the flow around a simplified high speed train under the influence of a cross wind. The reynolds number of the flow is Re=300000 based on the height of the train and the incoming air velocity. The results are obtained at a yaw angle of 90 degree. Both the instantaneous and the time-averaged flows are explored. Compareson of the LES flow patterns and aerodynamic forces with experimental data is reported. It is found that the LES predicts the flow in agreement with the experimental observations. Aerodynamic forces are computed and ther time histories are used to find the characteristic frequencies of the flow motion around the train body. The results reveal that the dominated flow motion is very small and approches the resonance frquency of the real trains.

Comparison of RANS and Detached Eddy Simulation Results to Wind-Tunnel Data for the Surface Pressures Upon a Class 43 High-Speed Train

Currently, there are three different methodologies for evaluating the aerodynamics of trains; full-scale measurements, physical modeling using wind-tunnel, and moving train rigs and numerical modeling using computational fluid dynamics (CFD). Moreover, different approaches and turbulence modeling are normally used within the CFD framework. The work in this paper investigates the consistency of two of these methodologies; the wind-tunnel and the CFD by comparing the measured surface pressure with the computed CFD values. The CFD is based on Reynolds-Averaged Navier–Stokes (RANS) turbulence models (five models were used; the Spalart–Allmaras (S–A), k-e, k-e re-normalization group (RNG), realizable k-e, and shear stress transport (SST) k-ω) and two detached eddy simulation (DES) approaches; the standard DES and delayed detached eddy simulation (DDES). This work was carried out as part of a larger project to determine whether the current methods of CFD, model scale and full-scale testing provide consistent results and are able to achieve agreement with each other when used in the measurement of train aerodynamic phenomena. Similar to the wind-tunnel, the CFD approaches were applied to external aerodynamic flow around a 1/25th scale class 43 high-speed tunnel (HST) model. Comparison between the CFD results and wind-tunnel data were conducted using coefficients for surface pressure, measured at the wind-tunnel by pressure taps fitted over the surface of the train in loops. Four different meshes where tested with both the RANS SST k-ω and DDES approaches to form a mesh sensitivity study. The four meshes featured 18, 24, 34, and 52 × 106 cells. A mesh of 34 × 106 cells was found to provide the best balance between accuracy and computational cost. Comparison of the results showed that the DES based approaches; in particular, the DDES approach was best able to replicate the wind-tunnel results within the margin of uncertainty.

LES of the Slipstream of a Rotating Train

The slipstream of a high-speed train was investigated using large-eddy simulation (LES). The subgrid stresses were modeled using the standard Smagorinsky model. The train model consisted of a four-coach of a 1/25 scale of the ICE2 train. The model was attached to a 3.61 m diameter rotating rig. The LES was made at two Reynolds numbers of 77,000 and 94,000 based on the height of the train and its speed. Three different computational meshes were used in the simulations: course, medium and fine. The coarse, medium, and fine meshes consisted of 6×106, 10×106, and 15×106 nodes, respectively. The results of the fine mesh are in fairly agreement with the experimental data. Different flow regions were obtained using the LES: upstream region, nose region, boundary layer region, intercarriage gap region, tail region, and wake region. Localized velocity peak was obtained near the nose of the train. The maximum and minimum pressure values are also noticed near to the nose tip. Coherent structures were born at the nose and roof of the train. These structures were swept by the radial component of the velocity toward the outer side of the train. These structures extended for a long distance behind the train in the far wake flow. The intercarriage gaps and the underbody complexities, in the form of supporting cylinders, were shown to have large influences on the slipstream velocity. The results showed that the slipstream velocity is linearly proportional to the speed of the train in the range of our moderate Reynolds numbers.

Numerical calculation of the slipstream generated by a CRH2 high-speed train

Slipstreams caused by high-speed train movement through the atmosphere pose a safety risk to passengers, trackside workers and track infrastructure. The improved delayed detached eddy simulation (IDDES) approach, an improved version of the detached eddy simulation method, is adopted in this paper to calculate the slipstream of a four-coach 1/25th-scale model of the CRH2 high-speed train. Slipstream velocities and pressures at various lateral distances from the centre of rail (COR) position and vertical distances from the top of rail (TOR) position at trackside are calculated. Numerical results are compared with measurements obtained in a full-scale test and good agreement is obtained, which verifies the effectiveness and potential of the less costly IDDES method. It is found that the velocity and pressure distributions are similar to those obtained using different train types but with different peak values related to the difference in shapes. The peak velocities in the slipstream along the length of the train are found at the tail and in the near wake region. The magnitude of the peak decreases with an increasing distance from the COR and shows a relatively high value at about two thirds of the train height from the TOR. The maximum pressure coefficients are found in the upstream and nose regions. The results show that the value of these coefficients decreases with an increasing distance from the COR and TOR. Based on the suggested safe slipstream velocity in China, the IDDES results show that for a CRH2 high-speed train at a speed of 350 km/h, the safe standing distance should be greater than 3.4 m in the lower part of the train’s slipstream (up to about half of the train height from the ground) and 2.4 m for the top part of the train’s slipstream (above half the height of the train from the ground).

Integration of crosswind forces into train dynamic modelling

In this paper a new method is used to calculate unsteady wind loadings acting on a railway vehicle. The method takes input data from wind tunnel testing or from computational fluid dynamics simulations (one example of each is presented in this article), for aerodynamic force and moment coefficients and combines these with fluctuating wind velocity time histories and train speed to produce wind force time histories on the train. This method is fast and efficient and this has allowed the wind forces to be applied to a vehicle dynamics simulation for a long length of track. Two typical vehicles (one passenger, one freight) have been modelled using the vehicle dynamics simulation package ‘VAMPIRE®’, which allows detailed modelling of the vehicle suspension and wheel—rail contact. The aerodynamic coefficients of the passenger train have been obtained from wind tunnel tests while those of the freight train have been obtained through fluid dynamic computations using large-eddy simulation. Wind loadings were calculated for the same vehicles for a range of average wind speeds and applied to the vehicle models using a user routine within the VAMPIRE package. Track irregularities measured by a track recording coach for a 40 km section of the main line route from London to Kings Lynn were used as input to the vehicle simulations. The simulated vehicle behaviour was assessed against two key indicators for derailment; the Y/Q ratio, which is an indicator of wheel climb derailment, and the Δ Q/Q value, which indicates wheel unloading and therefore potential roll over. The results show that vehicle derailment by either indicator is not predicted for either vehicle for any mean wind speed up to 20 m/s (with consequent gusts up to around 30 m/s). At a higher mean wind speed of 25 m/s derailment is predicted for the passenger vehicle and the unladen freight vehicle (but not for the laden freight vehicle).

Exploring Flow Structures Around a Simplified ICE2 Train Subjected to a 30 Degree Side Wind Using LES

Abstract Large-eddy simulation (LES) is conducted to investigate the flow around a simplified ICE2 train under side wind conditions at 30° yaw angle. The Reynolds number based on the height of the train and the free stream velocity is 2×105. Two computations on two different meshes with different numbers of nodes are made to check the effect of the mesh resolution on the results. The fine and the coarse meshes give similar results meaning that the results are mesh independent. The results are also verified against available experimental data. Good agreement is obtained between the LES results and the experimental data. The LES results show that two flow regimes exist in the wake. The first flow regime consists of steady vortex lines in the upper part of the wake flow. It changes into unsteady shedding after a distance of about five train heights from their onset on the surface of the train. The second flow regime is the unsteady movement of the lower part of the wake vortices. They attach and detach from the surface of the train in a regular fashion. The time-averaged flow and the instantaneous flow around the ICE2 train at 30° yaw angle are explored.

The effect of time trial cycling position on physiological and aerodynamic variables

Abstract To reduce aerodynamic resistance cyclists lower their torso angle, concurrently reducing Peak Power Output (PPO). However, realistic torso angle changes in the range used by time trial cyclists have not yet been examined. Therefore the aim of this study was to investigate the effect of torso angle on physiological parameters and frontal area in different commonly used time trial positions. Nineteen well-trained male cyclists performed incremental tests on a cycle ergometer at five different torso angles: their preferred torso angle and at 0, 8, 16 and 24°. Oxygen uptake, carbon dioxide expiration, minute ventilation, gross efficiency, PPO, heart rate, cadence and frontal area were recorded. The frontal area provides an estimate of the aerodynamic drag. Overall, results showed that lower torso angles attenuated performance. Maximal values of all variables, attained in the incremental test, decreased with lower torso angles (P < 0.001). The 0° torso angle position significantly affected the metabolic and physiological variables compared to all other investigated positions. At constant submaximal intensities of 60, 70 and 80% PPO, all variables significantly increased with increasing intensity (P < 0.0001) and decreasing torso angle (P < 0.005). This study shows that for trained cyclists there should be a trade-off between the aerodynamic drag and physiological functioning.