HONDA The Power of Dreams

Why Are Wind Tunnels Needed?

A full-scale wind tunnel is a facility that can create a flow of air around a static vehicle driven on a ground plane system to measure the forces acting on the vehicle. Honda has announced that it will “Honda is striving to achieve carbon neutrality for all products and corporate activities by 2050. ” To achieve carbon neutrality by 2050, it is important to improve fuel and electricity efficiency for both internal combustion engine vehicles and electric vehicles. Reducing aerodynamic drag directly improves fuel and electricity efficiency, so aerodynamic development is becoming increasingly important.

Measuring aerodynamic drag on a moving vehicle requires a lot of effort. It is also difficult to achieve good test efficiency because the repeatability of measurements is not high under changing weather conditions, such as air temperature and wind speed. Therefore, aerodynamic forces and airflows around the vehicle are often measured in wind tunnels using actual vehicles or full-scale models.

Honda Full-Scale Wind Tunnel

Honda currently conducts purpose-specific full-scale wind tunnel testing in automobile and motorcycle development.

In 1991, a full-scale fixed-ground wind tunnel became operational at Honda R&D facility adjacent to the Tochigi Proving Ground in Haga District, Tochigi Prefecture, Japan. This wind tunnel is now being used mainly in development for reducing wind noise.

In 2009, a full-scale wind tunnel with a single belt system became operational at HRC Sakura (Sakura City, Tochigi Prefecture), about 30 kilometers north of the Honda R&D facility (Haga District, Tochigi Prefecture). Normally, when a vehicle is driving, the air and ground seem to move toward it, from the front to the rear, from the perspective of the vehicle. In the case of a moving belt wind tunnel, the vehicle is fixed and the belt moves from front to rear at the same speed as the air. For this reason, a moving belt wind tunnel can simulate airflows that are more like actual driving conditions, especially near the ground, than in a fixed-ground wind tunnel. Therefore, this wind tunnel is mainly used for aerodynamic development of racing cars.

In 2020, a full-scale wind tunnel with a 5-belt system became operational within the same site as the fixed-ground full-scale wind tunnel.

Aerodynamic Development Using the Full-Scale 5-Belt Wind Tunnel

Honda built this full-scale 5-belt wind tunnel to achieve collect high precision measurement and improve the efficiency of aerodynamic development. It is mainly used for aerodynamic development that aims to reduce aerodynamic drag. During the initial stage of product development, full-scale clay models are used to test a variety of ideas. Styling designers from Honda R&D Design Center in Wako City, Saitama Prefecture and mechanical and aerodynamic engineers from Tochigi R&D facility participate in the wind tunnel testing together. While referring to the discussing the results of the testing, they discuss and modify the shape of the clay model in the plenum (measurement chamber) and, through a series of trial and error, optimize the shape based on the measurement results.

Full-scale 5-belt wind tunnel

During the latter stages of development, the prototype model is brought into the plenum to confirm that target performance has been achieved.

A traverse system, with a probe at its tip, is used in various stages of development in the wind tunnel to help create the next ideas. The probe, which looks like a comb placed vertically, has a number of vertically arranged tubes for measuring air pressure. The traverse system enables the probe to be moved to any position around the vehicle to measure airflow. The tubes are placed at narrow intervals in order to capture the airflow to a high degree of granularity near the ground surface, where the aerodynamic impact is significant. The measurement results can be visualized and checked as contour diagrams, which helps engineers to generate new ideas in the development process. In the clay model testing stage, ink is also placed on the vehicle surface to visualize and evaluate airflow close to the body surface.

In addition to being used for aerodynamic development, this 5-belt wind tunnel was also built to conduct wind tunnel testing to comply with the Worldwide harmonized Light vehicles Test Procedures (WLTP).

WLTP (Worldwide harmonized Light vehicles Test Procedures)

The WLTP set forth a new testing standard to measure fuel efficiency that came into effect in 2018. Established under the leadership of the United Nations Economic Commission for Europe (UNECE) and others, it is accepted by many countries, mostly in a Europe, but also in China and Japan.

The key difference between the WLTP and other fuel efficiency testing standards is that driving resistance measurements can be taken in a test laboratory environment instead of on-road driving. Previously, Honda measured driving resistance on proving grounds with actual vehicles driven in coast-down mode. With the WLTP, it became possible to measure rolling resistance using a chassis dynamometer and aerodynamic drag in a full-scale wind tunnel, and then submit the combined value of these resistances as the driving resistance value for certification application.

Characteristics of the full-scale 5-belt wind tunnel

One of the unique features of this wind tunnel is the 5-belts system. While the wind tunnel at HRC Sakura uses a single moving belt that is wider than the vehicle body, this full-scale 5-belt wind tunnel uses five belts, comprising one belt under the vehicle that is narrower than the vehicle width and four belts under each wheel to reproduce tire rotation. The belt located under the vehicle is called the center belt and the belts under the wheels are called the wheel drive units (WDUs). The center belt and WDUs are compatible with a wide range of vehicles, accommodating different tracks and wheelbases.

As the air density changes with temperature, so does aerodynamic drag also change with the temperature. To obtain highly repeatable aerodynamic force measurements, it is better to maintain a consistent temperature of the air. The duct walls of the full-scale 5-belt wind tunnel use a double-skin construction to reduce the influence of outside temperatures. This enables the air temperature inside the wind tunnel to be controlled. Airflow temperature can be then maintained at 23 degrees Celsius throughout the year, as required by the WLTP regulations.

Double-skin construction

The full-scale 5-belt wind tunnel is equipped with a number of devices that enable highly efficient measurements, including an automatic vehicle attachment (AVA) system for clamping the vehicle. This is done using rocker panel restraints (RPRs). The positions of the RPRs are different for each vehicle, but the AVA detects the positions by camera and automatically moves the RPRs into place.

Automatic adjustment of RPR positions enables the time required for test preparation to be reduced compared to manual adjustment, and also greatly improves the efficiency of measurements. While it is not possible to change tires or floors in single belt wind tunnels, the vehicle can actually be jacked up with the RPRs in 5 belt wind tunnels, so parts can be replaced on the spot. As a result, work efficiency has also been dramatically improved.

Another device that is making a major contribution to more efficient wind tunnel operation is the exhaust gas removal (EGR)*¹ system. With conventional measurement methods, the drive shaft had to be removed when conducting measurements to eliminate load on the transmission. In general, in wind tunnels where moving belts are used, tires are rotated by belts rather than by the vehicle itself. In order to test without removing the drive shaft, the engine must be running while measuring. The EGR system makes it possible to run the vehicle’s engine within the wind path. The hose connecting the tailpipe to the system is hidden behind the rear tire so that it does not interfere with airflow.

EGR(Exhaust Gas Removal)

*1 While also abbreviated as EGR, exhaust gas recirculation used in engine technologies is different from exhaust gas removal.

Configuration of the full-scale 5-belt wind tunnel

The full-scale 5-belt wind tunnel facility is comprised of a number of elements, including the wind tunnel building itself, a control room from which the wind tunnel is operated, and a maintenance building for vehicle maintenance and other uses. The wind tunnel building measures approximately 91 meters by 47 meters. The plenum is located on one of the long sides, and a 5 belt rolling road is situated on an 11 meter diameter turntable.

(Image supplied by WBI)

The turntable can be rotated to change the direction of the vehicle relative to the wind direction. This enables changes in aerodynamic forces due to crosswinds to be measured. The rolling road also uses air bearings under the steel belt, with high pressure air blown from the bearings lifting the belt slightly during operation.

Load cells are located below the turntable to measure six force; forces along three orthogonal axes (Fx, Fy, Fz) and their three associated moments (Mx, My, Mz). In the wind tunnel, highly accurate force measurements are required, but the load cells that measure the forces are sensitive to temperature. Therefore, an air handling unit is used to maintain a constant air temperature in the room where the load cells are located.

RRS(Rolling Road Belt System)

A boundary layer removal system is located in front of the moving belt, with a silver-colored suction plate as part of the system. A layer of slow moving air, called the boundary layer, occurs near the surface of the fixed floor, but because this does not occur when a vehicle is actually moving, it is suctioned off through the plate.

Another unique characteristic of the full-scale 5-belt wind tunnel is the long 18-meter distance between the nozzle and collector in the plenum. The longer this distance, the flatter the static pressure gradient (pressure change), creating an ideal measurement environment that is close to actual driving conditions.

Distance from nozzle to collector

At this facility, the plenum is located in the middle of one of the longer sides of the rectangular-shaped closed-circuit type wind tunnel (bottom side in the diagram), with the corner immediately downstream of the plenum called corner 1, and the next corner in the clockwise direction called corner 2. After passing through corners 3 and 4, the air is returned to the plenum. On each corner, corner vanes are installed to assist smooth turning of the airflow within the tunnel.

Full-scale 5-belt wind tunnel

(Image supplied by WBI)

The main fan, which creates the airflow, is installed downstream of corner 2. This fan has a diameter of 9 meters, 18 carbon fiber reinforced plastic (CFRP) rotor blades and 23 steel stator blades. Powered by a motor with maximum output of 2.4-megawatt, the fan generates a maximum wind velocity of 200 kilometers per hour, with a maximum rotational speed of 187 revolutions per minute (rmp).

A heat exchanger, measuring 15 meters wide by 11.4 meters high, is installed downstream of corner 3. Comprised of pipes and fins, it looks just like a giant radiator. The heat exchanger is used to make necessary adjustments to maintain constant air temperature.

A rectifying grid with a honeycomb structure is installed downstream of corner 4 to smooth out the swirling motion of the air. The grid is then followed by two airflow rectifying steel mesh screens that reduce turbulence to make a uniform airflow. Finally, a nozzle with a flow contraction ratio of 6 to 1 is situated downstream of the screens to increase airspeed and supply a uniform airflow into the plenum.

In this way, through repeated testing in a full-scale 5-belt wind tunnel facility that incorporates the latest technologies, Honda is contributing to the creation of even more appealing products.