Ground Vehicle
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GROUND VEHICLE AERODYNAMICS The VKI is active, mainly through academic research with students from Belgian universities and high-schools, in ground vehicle aerodynamics. The projects are related to the determination of the drag of an eco-marathon car, the design of a solar racing car, and the determination of the performance of new spinnakers for sailing boats. Another major project for the Institute in recent years is the investigation of the pressure waves generated by a high-speed train entering a confined area. The L-1 wind tunnel was also used to study the drag of cyclists and triathlon athletes.
© 2006, von Karman Institute for Fluid Dynamics, Rhode-St-Genèse, Belgium
69
Ground Vehicle Aerodynamics
AERODYNAMICS OF TRAINS The emergence of commercial high-speed trains travelling at speeds up to 360km/h led to the development of an efficient short-range transport system competing with air transport. It is well known that a highspeed train entering a tunnel generates pressure waves that propagate backward and forward, at the speed of sound, to the portals where they are reflected. The combination of these waves with the motion of the train creates a very complex flow where the resulting transient pressure may affect passenger comfort and cause damage on elements of the train. Considerable efforts have been made to study the velocity field and pressure pattern induced by such a train entering a tunnel in order to develop new methods to weaken the pressure variations. For several years the von Karman Institute has been investigating the aerodynamics of a train entering confined areas such as single bore tunnels and stations. A 1:87 scale-model facility has been designed with dynamic and geometric similitude to enable quantitative studies of this phenomenon (Figure 1). A launcher based on crossbow technology provides the energy required for propelling the vehicle at high speed (up to 160km/h). The train model slides on paired steel wires and passes through a six-meter long tunnel. Static pressure and flow velocity fluctuations due to the passage of the train have been measured in the tunnel for several geometrical configurations of the vehicle (i.e., conic-shaped nose, variable length and cross-section) and of the tunnel (i.e., ground effect, flared entry portals, airshafts…). Tests performed for different train cross-sections (circular and square) gave the same pressure trend. The pressure waves propagating through the tunnel are essentially plane waves. This finding is consistent with the propagation of acoustic waves in long pipe systems. Large increases of the pressure rise time have been obtained with a flared tunnel entrance or with airshafts. These airshafts can be distributed along the first part of the tunnel in such a way that the initial rise time of the compression wave can be increased (Figure 2). This geometry provides a decrease of the pressure gradient by a factor of three. Such airshafts may be the best approach for reducing pressure wave amplitudes to acceptable values. 70
In this project, an extensive numerical research has also been initiated. The simulations are done with the commercial code Fluent. A special methodology has been developed to handle this problem. The first step required cleaning the CAD file of a high-speed Thalys train in order to import it into Fluent. At this level two grids are prepared: one mesh models the steady environment around the tunnel and a part of the tunnel itself, the second mesh contains the train itself and all the cells moving with it.
Figure 1: Experimental facility The two meshes are glued and prepared to support the resolution of an Euler set of equations with Fluent. A total of more than 1 million cells are used. At every time step the moving mesh slides inside of the fixed mesh, i.e. a new mesh is obtained by deforming-collapsing-creating cells at the interface between moving and steady mesh. The CFL number of the simulation is kept much less than one to allow a fine description of the pressure compression/propagation process. The compressible solver is used in parallel on two nodes of a dedicated shared memory 64-bit machine. A fine convergence of the residues is obtained with a high number of iterations by time step.
Figure 2: Pressure pattern with and without airshafts (train velocity = 150km/h)
© 2006, von Karman Institute for Fluid Dynamics, Rhode-St-Genèse, Belgium
Ground Vehicle Aerodynamics The overall simulation represents more than 150m of displacement of the train at a velocity of 97m/s. The attenuation of the pressure rise in the tunnel is obtained by a pre-tube equipped with open windows on its top. With a special distribution of the open surfaces, an attenuation of the pressure gradient inside of the tunnel
is expected. This attenuation is illustrated in Figure 3, where the pressure wave transforms from an overpressure attached to the train nose to a 1D compression pressure wave travelling in the tunnel at the sonic velocity.
Figure 3: History of the train entering in the tunnel equipped with 6 windows. Yellow iso-surface displays the mechanism of creation of a 1D compression pressure wave. Velocity of the train is 97m/s © 2006, von Karman Institute for Fluid Dynamics, Rhode-St-Genèse, Belgium
71
Ground Vehicle Aerodynamics
AERODYNAMICS OF ROAD VEHICLES The von Karman Institute is equipped with several low speed wind tunnels available for research on road vehicle aerodynamics, among which are the L-1 and L-2b tunnels. During the last few years, investigations of an eco-car, a solar car and racing cyclists were performed. For the development of an eco-car, VKI is collaborating with a Belgian high-school which participates every year to the Eco-Marathon and is developing all the elements of the car that do not deal with the aerodynamics of the vehicle, while VKI is taking care of this last aspect by conducting wind tunnel experiments on different geometries.
Figure 1: Eco-Marathon car in the VKI L-1 wind tunnel The drag coefficient of the real car has been measured in the open-jet test section of the L-1 wind tunnel equipped with a ground floor including a drag force balance (Figure 1). Then, measurements to reduce the aerodynamic drag were planned on sub-scale models in the L-2b wind tunnel. A new test section has been first designed specifically to limit the blockage effect. It also includes a moving ground, a boundary layer suction system and a strain gage balance for measurement of the drag and lateral forces. The new test section can now be used for testing commercial road vehicles, or elements of them, and to develop new experimental techniques that can be of interest to the automotive industries. Figure 2 shows the sub-scale model of the Eco-Marathon in the new test section. Several tests to investigate the effect of the side mirrors, the car shape, the roughness, etc have already been performed. The study of the aerodynamic performance of a solar car designed and built for the World Solar 72
Figure 2: Sub-scale model in the new VKI L-2b test section Challenge 2005 was also performed at VKI in collaboration with another Belgian high-school. Students, as part of their final year thesis, have measured the performances of the solar car in the closed test section of the L-1 wind tunnel equipped with a six-component floor balance for drag measurements. Flow visualizations were also performed to improve the design of the solar car. Figure 3 shows a typical result of an oil flow visualization test on a scale-model of the car during a side-wind investigation. Finally, the full-size car was constructed. and was entered in the World Solar Challenge. The team was the first from Belgium to participate in this event and finished in 11th place. Using the L-1 wind tunnel, force measurements were also performed on cyclists from a Belgian professional racing team and with a professional triathlon athlete.The goal of both projects was to establish a database of aerodynamic characteristics (drag resistance) of a top sport cyclist on his bicycle, at constant speed (for example 60km/h).The driving parameters were the position of the cyclist on his bicycle and his gear.Tests were performed with real cyclists and with a dummy such as that shown in Figure 4.
Figure 3: Oil flow visualization – scale model car of the Solar team
© 2006, von Karman Institute for Fluid Dynamics, Rhode-St-Genèse, Belgium
Ground Vehicle Aerodynamics
Figure 4: Dummy on bicycle inside L-1
At the VKI, the research activity on the aerodynamics of sailing yachts is focused on wind tunnel tests on spinnakers. Students, during their final year thesis, have measured the performance of spinnakers, both symmetrical andasymmetrical, in the L-1 wind tunnel since 1999. A model at a scale 1/25 of a typical America’s Cup class has been designed, built and continuously improved for the wind tunnel tests.
AERODYNAMICS OF SAILING YACHTS The sailing performance of a yacht depends on the balance of the hydrodynamic and aerodynamic forces acting on the hull and the sails. The improvement of the performance of sailing yachts is the main objective of High Performance Yacht Design. New ideas and concepts of new designs are tested using towing tanks, wind tunnels, the Velocity Prediction Program (VPP) and Computational Fluid Dynamics (CFD).
Figure 2: Modified spinnaker The model with a classical spinnaker, placed on a sixcomponent floor balance to measure the overall forces and moments, is shown in Figure 1. The sails could be trimmed during the tests from outside the wind tunnel using three winches, driven by electrical motors, located under the model’s deck. The pole position was set manually before switching on the wind tunnel, but the influence of the main sheet, the spinnaker sheet and the spinnaker guy on the driving force, the side force and on the vertical force could be observed in real time. During the tests, the trimming parameters could be optimized continuously to maximize the driving force for each configuration.
Figure 1: VKI sailing yacht model
Figure 3: Zoom of the appendage inside the spinnaker
© 2006, von Karman Institute for Fluid Dynamics, Rhode-St-Genèse, Belgium
73
Ground Vehicle Aerodynamics Current investigations deal with the modification of the shape of the spinnaker by adding a wing inside a horizontal opening in the spinnaker. This new concept of spinnaker, developed by a German company, combines the traditional spinnaker theory with paraglider technology. Based on this idea, an independent investigation was carried out at the VKI on the modification of a spinnaker by the addition of an appendage. Figure 2 shows a typical modified spinnaker and a more detailed view of the appendage is shown in Figure 3. The study has shown that a horizontal slit located inside a spinnaker has a strong negative impact on the driving force of the sailing boat. The use of an appendage to recover the loss induced by the opening is not straightforward. Different geometries have been tested and all the experimental results have shown that the driving force was less than that exhibited by a standard spinnaker. However, the vertical force was increased with different appendages and this will affect the overall balance of the yacht and might lead to an improvement in its overall performance.
74
© 2006, von Karman Institute for Fluid Dynamics, Rhode-St-Genèse, Belgium
© 2006, von Karman Institute for Fluid Dynamics, Rhode-St-Genèse, Belgium
69
Ground Vehicle Aerodynamics
AERODYNAMICS OF TRAINS The emergence of commercial high-speed trains travelling at speeds up to 360km/h led to the development of an efficient short-range transport system competing with air transport. It is well known that a highspeed train entering a tunnel generates pressure waves that propagate backward and forward, at the speed of sound, to the portals where they are reflected. The combination of these waves with the motion of the train creates a very complex flow where the resulting transient pressure may affect passenger comfort and cause damage on elements of the train. Considerable efforts have been made to study the velocity field and pressure pattern induced by such a train entering a tunnel in order to develop new methods to weaken the pressure variations. For several years the von Karman Institute has been investigating the aerodynamics of a train entering confined areas such as single bore tunnels and stations. A 1:87 scale-model facility has been designed with dynamic and geometric similitude to enable quantitative studies of this phenomenon (Figure 1). A launcher based on crossbow technology provides the energy required for propelling the vehicle at high speed (up to 160km/h). The train model slides on paired steel wires and passes through a six-meter long tunnel. Static pressure and flow velocity fluctuations due to the passage of the train have been measured in the tunnel for several geometrical configurations of the vehicle (i.e., conic-shaped nose, variable length and cross-section) and of the tunnel (i.e., ground effect, flared entry portals, airshafts…). Tests performed for different train cross-sections (circular and square) gave the same pressure trend. The pressure waves propagating through the tunnel are essentially plane waves. This finding is consistent with the propagation of acoustic waves in long pipe systems. Large increases of the pressure rise time have been obtained with a flared tunnel entrance or with airshafts. These airshafts can be distributed along the first part of the tunnel in such a way that the initial rise time of the compression wave can be increased (Figure 2). This geometry provides a decrease of the pressure gradient by a factor of three. Such airshafts may be the best approach for reducing pressure wave amplitudes to acceptable values. 70
In this project, an extensive numerical research has also been initiated. The simulations are done with the commercial code Fluent. A special methodology has been developed to handle this problem. The first step required cleaning the CAD file of a high-speed Thalys train in order to import it into Fluent. At this level two grids are prepared: one mesh models the steady environment around the tunnel and a part of the tunnel itself, the second mesh contains the train itself and all the cells moving with it.
Figure 1: Experimental facility The two meshes are glued and prepared to support the resolution of an Euler set of equations with Fluent. A total of more than 1 million cells are used. At every time step the moving mesh slides inside of the fixed mesh, i.e. a new mesh is obtained by deforming-collapsing-creating cells at the interface between moving and steady mesh. The CFL number of the simulation is kept much less than one to allow a fine description of the pressure compression/propagation process. The compressible solver is used in parallel on two nodes of a dedicated shared memory 64-bit machine. A fine convergence of the residues is obtained with a high number of iterations by time step.
Figure 2: Pressure pattern with and without airshafts (train velocity = 150km/h)
© 2006, von Karman Institute for Fluid Dynamics, Rhode-St-Genèse, Belgium
Ground Vehicle Aerodynamics The overall simulation represents more than 150m of displacement of the train at a velocity of 97m/s. The attenuation of the pressure rise in the tunnel is obtained by a pre-tube equipped with open windows on its top. With a special distribution of the open surfaces, an attenuation of the pressure gradient inside of the tunnel
is expected. This attenuation is illustrated in Figure 3, where the pressure wave transforms from an overpressure attached to the train nose to a 1D compression pressure wave travelling in the tunnel at the sonic velocity.
Figure 3: History of the train entering in the tunnel equipped with 6 windows. Yellow iso-surface displays the mechanism of creation of a 1D compression pressure wave. Velocity of the train is 97m/s © 2006, von Karman Institute for Fluid Dynamics, Rhode-St-Genèse, Belgium
71
Ground Vehicle Aerodynamics
AERODYNAMICS OF ROAD VEHICLES The von Karman Institute is equipped with several low speed wind tunnels available for research on road vehicle aerodynamics, among which are the L-1 and L-2b tunnels. During the last few years, investigations of an eco-car, a solar car and racing cyclists were performed. For the development of an eco-car, VKI is collaborating with a Belgian high-school which participates every year to the Eco-Marathon and is developing all the elements of the car that do not deal with the aerodynamics of the vehicle, while VKI is taking care of this last aspect by conducting wind tunnel experiments on different geometries.
Figure 1: Eco-Marathon car in the VKI L-1 wind tunnel The drag coefficient of the real car has been measured in the open-jet test section of the L-1 wind tunnel equipped with a ground floor including a drag force balance (Figure 1). Then, measurements to reduce the aerodynamic drag were planned on sub-scale models in the L-2b wind tunnel. A new test section has been first designed specifically to limit the blockage effect. It also includes a moving ground, a boundary layer suction system and a strain gage balance for measurement of the drag and lateral forces. The new test section can now be used for testing commercial road vehicles, or elements of them, and to develop new experimental techniques that can be of interest to the automotive industries. Figure 2 shows the sub-scale model of the Eco-Marathon in the new test section. Several tests to investigate the effect of the side mirrors, the car shape, the roughness, etc have already been performed. The study of the aerodynamic performance of a solar car designed and built for the World Solar 72
Figure 2: Sub-scale model in the new VKI L-2b test section Challenge 2005 was also performed at VKI in collaboration with another Belgian high-school. Students, as part of their final year thesis, have measured the performances of the solar car in the closed test section of the L-1 wind tunnel equipped with a six-component floor balance for drag measurements. Flow visualizations were also performed to improve the design of the solar car. Figure 3 shows a typical result of an oil flow visualization test on a scale-model of the car during a side-wind investigation. Finally, the full-size car was constructed. and was entered in the World Solar Challenge. The team was the first from Belgium to participate in this event and finished in 11th place. Using the L-1 wind tunnel, force measurements were also performed on cyclists from a Belgian professional racing team and with a professional triathlon athlete.The goal of both projects was to establish a database of aerodynamic characteristics (drag resistance) of a top sport cyclist on his bicycle, at constant speed (for example 60km/h).The driving parameters were the position of the cyclist on his bicycle and his gear.Tests were performed with real cyclists and with a dummy such as that shown in Figure 4.
Figure 3: Oil flow visualization – scale model car of the Solar team
© 2006, von Karman Institute for Fluid Dynamics, Rhode-St-Genèse, Belgium
Ground Vehicle Aerodynamics
Figure 4: Dummy on bicycle inside L-1
At the VKI, the research activity on the aerodynamics of sailing yachts is focused on wind tunnel tests on spinnakers. Students, during their final year thesis, have measured the performance of spinnakers, both symmetrical andasymmetrical, in the L-1 wind tunnel since 1999. A model at a scale 1/25 of a typical America’s Cup class has been designed, built and continuously improved for the wind tunnel tests.
AERODYNAMICS OF SAILING YACHTS The sailing performance of a yacht depends on the balance of the hydrodynamic and aerodynamic forces acting on the hull and the sails. The improvement of the performance of sailing yachts is the main objective of High Performance Yacht Design. New ideas and concepts of new designs are tested using towing tanks, wind tunnels, the Velocity Prediction Program (VPP) and Computational Fluid Dynamics (CFD).
Figure 2: Modified spinnaker The model with a classical spinnaker, placed on a sixcomponent floor balance to measure the overall forces and moments, is shown in Figure 1. The sails could be trimmed during the tests from outside the wind tunnel using three winches, driven by electrical motors, located under the model’s deck. The pole position was set manually before switching on the wind tunnel, but the influence of the main sheet, the spinnaker sheet and the spinnaker guy on the driving force, the side force and on the vertical force could be observed in real time. During the tests, the trimming parameters could be optimized continuously to maximize the driving force for each configuration.
Figure 1: VKI sailing yacht model
Figure 3: Zoom of the appendage inside the spinnaker
© 2006, von Karman Institute for Fluid Dynamics, Rhode-St-Genèse, Belgium
73
Ground Vehicle Aerodynamics Current investigations deal with the modification of the shape of the spinnaker by adding a wing inside a horizontal opening in the spinnaker. This new concept of spinnaker, developed by a German company, combines the traditional spinnaker theory with paraglider technology. Based on this idea, an independent investigation was carried out at the VKI on the modification of a spinnaker by the addition of an appendage. Figure 2 shows a typical modified spinnaker and a more detailed view of the appendage is shown in Figure 3. The study has shown that a horizontal slit located inside a spinnaker has a strong negative impact on the driving force of the sailing boat. The use of an appendage to recover the loss induced by the opening is not straightforward. Different geometries have been tested and all the experimental results have shown that the driving force was less than that exhibited by a standard spinnaker. However, the vertical force was increased with different appendages and this will affect the overall balance of the yacht and might lead to an improvement in its overall performance.
74
© 2006, von Karman Institute for Fluid Dynamics, Rhode-St-Genèse, Belgium
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