OrcaFlex also offers a choice, on the line data form, of formulation of the way in which drag force components vary with the incidence angle. If wake interference effects are being modelled for a given line, then these normal drag coefficients represent the undisturbed drag coefficients: the drag force will be calculated based on a drag coefficient modified according to the wake model used. Full details of the drag calculation are given in the line theory section. For very smooth cylinders the drag coefficient falls rapidly to 0.
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Box , AD Emmeloord, The Netherlands Abstract Various new types of trains were investigated in a wind tunnel at relatively high Reynolds numbers. The testing involved large-scale models in combination with high wind speeds. Local pressures, local temperatures and overall forces and moments were measured.
This paper discusses the various relevant parameters from the perspective of wind tunnel testing. Further, some test results and the economic benefits of drag reduction are discussed. Some data points concerning drag and local pressure were available from full-scale testing; unlike the pressure values, the drag values compared fairly well. Introduction During the last decade Dutch Railways has developed three new types of trains to carry a very rapidly increasing number of passengers in a quick and comfortable manner.
The designs involved a rapid wide-body two-coach train set for short distance, a double-deck three- or four-coach train set and a three-bogies m o t o r coach with AC traction engines. Wind tunnel experiments were defined to gather data for the optimization of train schedules, minimize the aerodynamic drag and corresponding energy costs within practical limits, optimize ventilation inlet and outlet positions and avoid overheating of the cooling systems in the engine bay of the m o t o r coach.
The LST is a closed-circuit wind tunnel with test section dimensions of 3. Test set-up and model configurations Three different train types were tested on scale in a wide variety of config- urations of train head, track clearer, roof, b o t t o m and side skirts. All rights reserved. To overall wind loads were measured with a highly accurate six-component, platform-type balance above the test section. The train configurations were defined for one-coach and two- coach trains in such a way that the drag of longer trains could be derived from the available test results.
F o r the temperature measurements the hot exhaust gases from the ventilators of the various cooling systems in the engine bay were scaled dynamically.
In each system the mass flow can be adjusted in steps by changing the diameter of the sonic venturi or continuously by variation of the total pressure before the venturi. The supplied air was transported through a heat exchanger towards the simulated active thermal sources. The temperatures were measured with copper-constantan thermocouples. Pressures at 46 tap positions in the tunnel walls were measured to investigate the wind tunnel blockage effects. Data reduction 3.
Temperatures The measured temperatures at an immission point i as a result of the emission at a point j were reduced in such a way that the temperature effects from the separate thermal sources could be calculated. In this way, adverse sources and wind conditions could be determined and more promising model modifications be defined.
The applied formulae were obtained from a straightforward heat balance analysis. Wind Eng. The ambient air will absorb heat by the amount of cp tl - - t a m a. Wind tunnel conditions 4. Blockage effects When a model is mounted in the test section, the free lateral displacement of the air flow near the model is obstructed by the adjacent solid walls, resulting in higher local wind speeds than in the full-scale condition.
The mirror-imaged sources and sinks are responsible for the wall-induced wind velocity in the test section . The strength of the various sources and sinks may be calculated by measuring wall pressures upstream, near and downstream the model.
Especially in yawed conditions, the scaled train models caused strong blockage effects. Therefore, the corrections at model centre were applied in the calculations of the corrected overall wind load coefficients. Reynolds number effects Equal Reynolds number between two bodies with identical geometrical shape ensures similar flow patterns, i.
The Reynolds number is a flow parameter following from dimensional analysis theory and represents the relation between viscous forces and inertia forces. Reynolds number effects can best be illustrated with a circular cylinder .
Reynoldsnumber effectsfor a two-dimensionalcylinderwith surface roughness. For a train the characteristic length is not known, but it is related to the dimensions of the train head roundings. Normally, the Re numbers of the full-scale train are in the supercritical range and the Re numbers of the train model in the subcritical or early critical range.
ELL 0,75' BLT i i The surface friction and thus the drag is then increased Fig. On the other hand, it is uncertain which surface roughness is required for a successful Reynolds number jump. Stationary wheels and ground-plane effects The similarity principle demands that the relative motion between vehicle and ground and the rotation of the wheels need to be simulated in wind tunnel tests. In relatively calm wind conditions the layer of air above the ground has no vertical gradient.
In the wind tunnel the downstream development of a boundary layer along the tunnel floor will results in a noticeable vertical wind speed gradient near the model. The boundary layer is accompanied by a momentum deficit and a dis- placement thickness, affecting the pressure distribution on especially the vehicle underbody.
The free space between the investigated train models and the upper rail level was 4 to 9 cm; the distance from train to wind tunnel floor was further increased by modeling a 50 cm full scale height ballast bed. The boundary layer effects may be counteracted by boundary layer suction, by tangential blowing or by testing with a moving ground plane. More detailed information can be found in various SAE papers . It was found that as long as the drag of the tested vehicles was comparatively high, the errors in the drag from a fixed ground floor and stationary wheels were in general negligible in the order of 0.
The considerations given above justify the conclusion that the applied test set-up with stationary ground, stationary wheels and no tangential blowing results in adequate and reliable aerodynamic coefficients.
Turbulence effects Testing a scale model of a vehicle in a wind tunnel under low-turbulent uniform- flow conditions may affect the flow similarity between model and full-scale vehicle. This is particularly true for the flow over the fore-running train head and for the location of the reattachment zones after flow separation. In the full-scale situation, the relative wind speed is a vector composition of train induced wind with no turbulence and actual wind with a turbulence level of e.
The turbulence intensity level in the LST wind tunnel is very low: less than 0. Ind Aerodyn. In Section 4. This is confirmed by the presented pressure data on the train head; it is only a very local effect, visible in some local pressures but not in the overall wind load coefficients Fig.
This also indicates that the train head drag contribution is not dominant and the other train parts e. As a whole, no significant adverse effects on the test results are expected from testing under low turbulent, oncoming flow conditions. Results 5. Wind tunnel results The tests covered a whole spectrum of train modifications.
Just a few results will be discussed here. The possible differences in drag related to the shape of a fore-running train head is already illustrated in Fig. The E L L head was characterized by an elliptical ground form and considered to be very good streamlined head. The BLT was not streamlined at all. Full-scale gap values of 0. G a p values of 1. Rather straightforward modifications in the track clearer could result in a drag coefficient reduction of 0.
WillemsenfiZ Wind Eng. Full-scale results The a e r o d y n a m i c drag for three- and four-coach trains was derived from the wind tunnel test results on various configurations of one-coach and t w o - c o a c h trains.
The corresponding full-scale tests yielded Cvx values fairly close to the wind tunnel data: 0. Full-scale tests gave pressure difference coefficients of - 0. Various e x p l a n a t i o n s are possible for the differences, e. Conclusions The following conclusions m a y be d r a w n with regard to wind t u n n e l tests o n trains: a high R e y n o l d s n u m b e r is necessary to o b t a i n reliable full-scale results, - the described test set-up yielded good results, blockage effects could be well corrected for, - a e r o d y n a m i c o p t i m i z a t i o n m a y be very worthwhile from an e c o n o m i c p o i n t of view.
References  J. Hacket, D. Wilsden, D. Awbi, Wind-tunnel-wall constraint on two-dimensional rectangular section prisms, J. Szechenye, Supercritical Reynolds number simulation for two-dimensional flow over circular cylinders, J.
Fluid Mech. Pearcy, R. Cash, I. Berndtsson, W. Eckert, E. Mercker, The effect of groundplane boundary layer control on automotive testing in a wind tunnel, SAE Paper , Mercker, H. Knape, Ground simulation with moving belt and tangential blowing for full-scale automotive testing in a wind tunnel, SAE Paper , Mercker, J.
Wiedemann, Comparison of different ground simulation techniques for use in automotive wind tunnels, SAE Paper , Mercker, N. Breuer, H. Berneberg, H.
Line types: Drag & lift data
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