Slipstream Velocity Research Articles

Flow topology and unsteady features of the wake of a generic high-speed train

The unsteady wake of a high-speed train is investigated experimentally. From a practical point of view, the wake region is of considerable importance as it is where slipstream velocities—velocities induced by the vehicles movement through air—are largest. In turn, this can create a considerable risk for passengers and track-side workers as the train passes. The flow is quantified in a 1:10 scale wind-tunnel experiment using high-frequency 4-hole dynamic pressure cobra probes, surface-pressure measurements and flow visualisation. The dominant feature of the time-average wake topology consists of a clearly identifiable counter-rotating streamwise vortex pair. Although the wake structure and evolution should perhaps be considered as a whole, the near wake exhibits periodic unsteadiness, at a Strouhal number of 0.2, that could be attributed to periodic shedding from the sides and to a lesser extent the top surface. This periodicity feeds into the trailing vortices, consistent with lateral and vertical displacement of the cores as they advect downstream and thus affecting maximum slipstream velocities.

Effect on measurements of anemometers due to a passing high-speed train

The three-dimensional unsteady incompressible Reynolds-averaged Navier-Stokes equations and k-<TEX>${\varepsilon}$</TEX> double equations turbulent model were used to investigate the effect on the measurements of anemometers due to a passing high-speed train. Sliding mesh technology in Fluent was utilized to treat the moving boundary problem. The high-speed train considered in this paper was with bogies and inter-carriage gaps. Combined with the results of the wind tunnel test in a published paper, the accuracy of the present numerical method was validated to be used for further study. In addition, the difference of slipstream between three-car and eight-car grouping models was analyzed, and a series of numerical simulations were carried out to study the influences of the anemometer heights, the train speeds, the crosswind speeds and the directions of the induced slipstream on the measurements of the anemometers. The results show that the influence factors of the train-induced slipstream are the passing head car and tail car. Using the three-car grouping model to analyze the train-induced flow is reasonable. The maxima of horizontal slipstream velocity tend to reduce as the height of the anemometer increases. With the train speed increasing, the relationship between <TEX>$V_{train}$</TEX> and <TEX>$V_{induced\;slipstream}$</TEX> can be expressed with linear increment. In the absence of natural wind conditions, from the head car arriving to the tail car leaving, the induced wind direction changes about <TEX>$330^{\circ}$</TEX>, while under the crosswind condition the wind direction fluctuates around <TEX>$-90^{\circ}$</TEX>. With the crosswind speed increasing, the peaks of <TEX>$V_X,{\mid}V_{XY}-V_{wind}{\mid}$</TEX> of the head car and that of <TEX>$V_X$</TEX> of the tail car tend to enlarge. Thus, when anemometers are installed along high-speed railways, it is important to study the effect on the measurements of anemometers due to the train-induced slipstream.

Experimental investigation of the slipstream development around a container freight train using a moving model facility

Increases in the volume of trade within the UK rail freight industry have led to proposed increases in freight train speeds. There is a concern that the unsteady slipstream created around a moving freight train could have implications on efficiency and the safety of passengers waiting on platforms or trackside workers. This paper describes a series of moving model-scale experiments conducted at the University of Birmingham’s TRAIN rig facility. Experiments were undertaken to assess the slipstream development of a container freight train and draw conclusions on flow characteristics. In this paper the term ‘freight train’ refers to a series of flatbed wagons loaded with ISO standard shipping containers hauled by a Class 66 locomotive. In-depth analysis of slipstream velocity and static pressure ensemble average results at train side and above the roof identified a series of key flow regions. Results within the boundary layer region exhibit an influence from container loading configuration. Slipstream magnitudes are larger than typical high speed passenger train results, which it is suggested is related to the vehicle shape. The effect of train length and train speed was also considered. A detailed analysis of the nature of slipstream velocity components in specific flow regions is investigated, and conclusions drawn on characteristic patterns and factors influencing possible safety issues. The analysis highlighted differences created through decreased container loading efficiencies, creating increased boundary layer growth with a larger displacement thickness with higher turbulence intensities. Integral time and length scales calculated through autocorrelation indicate that proposed limits of human instability are exceeded for the container freight train with a lower loading efficiency. Overall the results from this paper offer for the first time a definitive experimental study on container freight slipstream characteristics, allowing the nature of the flow field around freight trains to be understood in far greater detail than before.

Marine propellers performance and flow-field prediction by a free-wake panel method

A Boundary Element Method (BEM) hydrodynamics combined with a flow-alignment technique to evaluate blades shed vorticity is presented and applied to a marine propeller in open water. Potentialities and drawbacks of this approach in capturing propeller performance, slipstream velocities, blade pressure distribution and pressure disturbance in the flow-field are highlighted by comparisons with available experiments and RANSE results. In particular, correlations between the shape of the convected vortex- sheet and the accuracy of BEM results are discussed throughout the paper. To this aim, the analysis of propeller thrust and torque is the starting point towards a detailed discussion on the capability of a 3-D free-wake BEM hydrodynamic approach to describe the local features of the flow-field behind the propeller disk, in view of applications to propulsive configurations where the shed wake plays a dominant role.

Wind tunnel analysis of the slipstream and wake of a high-speed train

The slipstream of high-speed trains is investigated in a wind tunnel through velocity flow mapping in the wake and streamwise measurements with dynamic pressure probes. The flow mapping is used to explain the familiar slipstream characteristics of high-speed trains, specifically the largest slipstream velocities in the near wake. Further, the transient nature of the wake is explored through frequency and probability distribution analysis. The development of a wind tunnel methodology for slipstream assessment is presented and applied, comparing the output to full-scale results available in the literature. The influence of the modelling ballast and rail or a flat ground configuration on the wake structure and corresponding slipstream results are also presented.

Detached-eddy simulation of the slipstream of an operational freight train

With increasing train speeds the subsequent increase in slipstream velocities can have a detrimental effect on the safety of persons in close proximity to the vehicle. Due to their uneven loading and bluff geometries, freight trains can produce higher slipstream velocities than passenger trains at given measurement locations. The highly turbulent non-stationary slipstream of a model-scale Class 66 locomotive and wagons was investigated using delayed detached-eddy simulation (DDES). The Reynolds number of the flow was 300,000 and results were compared for meshes of 25 and 34 million hexahedral cells. Good agreement was observed between the DDES and model-scale physical experiments. Slipstream velocities along the train side and roof were investigated and the bogie region was seen to produce the highest slipstream velocities. A comparison between time-averaged and ensemble-averaged data from the simulations gave comparable results. The technical standards for interoperability (TSI) analysis showed that the slipstream velocities generated were below half of the maximum permissible value of the standard whereas the pressure was 43% greater than the limiting value. Furthermore the presence of a periodic phenomenon is detected above the roof of the locomotive.

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).

Aerodynamic prediction tools for high-speed trains

With high-speed trains, the need for efficient and accurate aerodynamic prediction tools increases, since the influence of the aerodynamics on the overall train performance raises. New requirements on slipstream velocities and head pressure pulse in the revised Technical Specification for Interoperability (TSI) for train speeds higher than 190 km/h are more challenging to fulfil for wide-body trains, like the Green train concept vehicle Regina 250, as well as higher trains, like double-deck trains. In this paper, we give an overview of the results from a project within the Green train programme, where the objective was to increase the knowledge on slipstream air flow of wide body trains at high speeds, to understand the implications of the new requirements on the front shape and to develop a prediction methodology in order to take this into account early in the design cycle. In addition, the front design was in parallel optimized with respect to head pressure pulse and drag.

Full-scale measurement and analysis of train slipstreams and wakes. Part 1: Ensemble averages

This paper describes a series of extensive and unique full-scale measurements of the slipstreams of trains of various types that were carried out as part of the EU-sponsored AeroTRAIN project, together with the analysis of the experimental data. These experiments were carried out with the fundamental aim of seeking to reduce the complexity of the current technical specifications for interoperability (TSI) testing methodology. Experimental sites in Spain and Germany were used, for a range of different train types – high-speed single-unit trains, high-speed double-unit trains, conventional passenger units and locomotive/coach combinations. The data that was obtained was supplemented by other data from previous projects. The analysis primarily involved a study of the ensemble averages of the slipstream velocities, measured both at trackside and above platforms. The differences between the flows around different train types were elucidated, and the effect of platforms on slipstream behaviour described. A brief analysis of the effects of crosswinds on slipstream behaviour was also carried out. Through a detailed analysis of slipstream velocity components, the detailed nature of the flow around the nose and in the near wake of the train was investigated, again revealing differences in flow pattern between different trains. Significant similarity in the far wake flows was revealed. These fundamental results form the basis for the detailed discussion of the proposed TSI methodology that will be presented in Part 2 of this paper. Overall the results enable the nature of the flow field around trains to be understood in far greater detail than before, and also allow the developments of a revised TSI methodology which is more efficient than current practice.

Mode Decomposition and Slipstream Velocities in the Wakes of Two High-Speed Trains

Mode Decomposition and Slipstream Velocities in the Wakes of Two High-Speed Trains

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 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.

Phase-Locked Particle Image Velocimetry Measurements of a Flapping Wing

The unsteady aerodynamics of a biomimetic inspired flapping-wing mechanism has been analyzed by performing detailed phase-locked diagnostics of its flow field. Flow visualization and particle image velocimetry results have shown the presence of a shed dynamic stall vortex that spans across most of the wing span. The shedding of this type of leading-edge vortex was accompanied by the formation of another leading-edge vortex before the first vortex reached the midchord, resulting in multiple shedding leading-edge vortices on the top surface of the wing during each wing stroke. A strong starting vortex was also formed at the trailing edge of the wing during the early part of its translational stroke. This vortex continuously gained strength from shed vorticity as the wing accelerated into its stroke. The starting vortex remained close to the trailing edge until the wing reached midstroke. A pair of vortices that continuously trailed from the root and tip of the wing were identified, both of which induced a significant downwash velocity over the wing surface. These trailed vortices were found to exhibit a contracting wake structure as they convected into the wake below the wing, consistent with an increase in slipstream velocity. The evolution of the tip and root vortex pair showed rapid diffusive characteristics with an increase in time (wake age).

The slipstream and wake of a high-speed train

This paper describes the results of experimental work to determine the structure of the slipstream and wake of a high speed train. The experiments were carried out using a 1/25th scale model of a four-coach train on a moving model rig (MMR). Flow velocities were measured using a rake of single hot films positioned close to the model side or roof. Tests were carried out at different model speeds, with and without the simulation of a crosswind. Velocity time histories for each configuration were obtained from ensemble averages of the results of a number of runs. A small number of particle imaging velocimetry (PIV) experiments were also carried out, and a wavelet analysis revealed details of the unsteady flow structure around the vehicle. It was shown that the flowfield around the vehicle could be divided into a number of different regions of distinct flow characteristics: an upstream region, a nose region, a boundary layer region, a near wake region and a far wake region. If the results were suitably normalized, the effect of model speed was small. The effect of crosswinds was to add an increment to the slipstream and wake velocities, and this resulted in very high slipstream velocities in the nose region.

Numerical method for estimation of propeller efficiencies

Introduction D URING the conceptual design and preliminary performance analysis of propeller aircraft, frequently the propulsive efficiency and the corresponding thrust must be determined at various speeds. For given engine power and propeller diameter, for example, the thrust variation with airspeed is required for the calculation of takeoff distance, climb performance, cruising speed, and maximum level flight velocity. Typically at this point the designer is interested in simple and quick method that yields reasonably accurate results which can be used for parametrical studies of aircraft performance, assuming that an optimized propeller design will be found later in the process. In this context, absolute accuracy is not as important as consistency, relative simplicity, and the ability to show proper trends. The algorithm presented below was developed for these purposes and can easily be programmed. Equation (2) expresses the thrust developed by the propeller, with P being the power generated by the engine at the propeller shaft. Since the thrust is inversely proportional to velocity, it increases with decreasing speed, however, at lower speeds the propulsive efficiency decreases also. Inspection of Eq. (4) reveals that the ideal propulsive efficiency becomes zero at zero velocity, indicating the fact that the aircraft is not propelled (V — 0), and all the shaft power is converted into the flow of kinetic energy in the slipstream. Conversely, at high speeds the ideal efficiency approaches unity since the slipstream velocity decreases, Eq. (3). This behavior of 77, is shown as curve a in Fig. 1 and represents the theoretical maximum for 17. The thrust usually peaks at the static condition, where rj and V are both zero. Equation (2) becomes therefore meaningless, however, it can be shown that at the limit V—> 0 the static thrust becomes

Experimental Measurements of Multiple‐Jet Induced Flow

Experimental results are confirmed for theoretical predictions of a vortex model for the two-dimensional inviscid flow generated by a line of submerged, turbulent, shallow water jets in a coflowing current. The experiments are performed in a large shallow water basin in which a strong uniform current can be produced and boundary effects are negligible. The vortex model is validated against detailed velocity measurements at and downstream of the momentum source line. Observations of surface flow patterns show that, depending only on the ambient current to ultimate slipstream velocity ratio, distinct flow regimes ranging from a strong inwardly directed sink flow to a smooth contracting slipstream can result.

Propeller in Slipstream/Wing Interaction in the Transonic Regime

An inviscid model for the interaction between a thin wing and a nearly uniform propeller slipstream is presented. The model allows the perturbation velocities due to the interaction to be potential although the undisturbed slipstream velocity is rotational. A finite difference scheme is used to solve the governing equation. Numerical examples indicate that the slipstream has a strong effect on the aerodynamic properties of the wing section within the slipstream and lesser effects elsewhere. The slipstream swirling motion strongly affects the wing load distribution, however, its effect on the wing's total lift and wave drag is small. The axial velocity increment in the slipstream has a small effect on the wing lift, however, it causes a large increase in wave drag.

An Ultrasonic Altitude-Velocity Sensor for Airplanes in the Vicinity of the Ground

This paper shows the possibility of an ultrasonic sensor which detects the altitude and the vertical velocity of an airplane in the vicinity of the ground. The principle of the present technique depends on the measurement of time in which the ultrasonic wave propagates the distance between the airplane and the ground, because the sonic velocity is approximately constant at the usual atmospheric temperature. Furthermore, by differentiating the altitude signal with respect to time, it is also possible to detect the vertical velocity of the airplane. The fundamental performances of this sensor, i.e. the detectable altitude limit, effect of the power plant noise, effect of the attitude, slip stream and forward velocity of the airplane, effect of ground conditions and so on, are investigated with some experiments carried out in the laboratory and also by the flight tests using a helicopter and a conventional airplane.

Experimental Investigation of the Aerodynamics of a Wing in a Slipstream

An experimental study of a wing in a propeller slipstream was made in order to determine the span wise distribution of the lift increase due to slipstream at different angles of attack of the wing and at different free stream to slipstream velocity ratios. The results were intended in part as an evaluation basis for different theoretical treatments of this problem. The comparative span loading curves, together with supporting evidence, showed that a substantial part of the lift increment produced by the slipstream was due to a or boundary-layer-control effect. The integrated remaining lift increment, after subtracting this destalling lift, was found to agree well with a potential flow theory. An empirical evaluation of the destalling effects was made for the specific configuration of the experiment.

Discussion on "A Survey of Engine Development"

Mr. Nutt has given a very able and interesting paper on this subject. However, he has not covered one possibility which seems to me distinctly interesting at the present time. With the advent of the modern clean airplane with no external bracing and with retractible landing gear, the drag of air-cooled radial engines is likely to be from 20 to 30 percent of the total drag of the airplane in the high-speed condition. This is in spite of the use of the very best forms of cowling and the best engine and propeller position with reference to the wing, as determined by the work of the N.A.C.A. A consideration of these facts makes it of interest to consider the possibility of engines entirely within the wing structure. With the wing sizes now in vogue in multi-engined airplanes, it is obvious that the enclosed engine could not be of radial or Vee form. With a tractor propeller, the engine would normally be located near the thickest section of the wing and it might be possible to use a flat X cylinder arrangement. For pusher installations, however, the horizontal-opposed type seems preferable in order to minimize propeller-shaft length. This type could be made relatively thin at the rear, where it is housed near the trailing edge, and the accessories could be located forward where a greater wing thickness is available. The development of satisfactory power plants will depend not only on the development of satisfactory engines of proper form, but also on the satisfactory solution of the problems of cooling, mounting structure, accessibility, etc. Of these, cooling is probably the most difficult. If the engine is to be air-cooled, there is the problem of drawing in the air and discharging it where it will give minimum interference with wing and propeller performance. Tests made at M.I .T. -some years ago indicate that if circulation is obtained by virtue of the pressure differences existing between different parts of the wing surface, a prohibitive amount of drag would be involved. If air cooling is to be accomplished by utilizing airplane and slipstream velocities, the present radial engine with N.A.C.A. cowling probably involves as small an amount of over-all drag as any feasible arrangement. I t seems probable, therefore, that an air-cooled engine would require some sort of blower system. Such systems absorb a considerable portion of the engine power, and the net result might easily be poorer engine performance with respect to weight and drag than could be obtained by a suitable liquid-cooling system.