The world’s most capable full-scale automotive aeroacoustic wind tunnel just held its grand opening in East Liberty, Ohio, courtesy of the Honda Automotive Laboratories Ohio (HALO). Superlatives like “most capable” are generally claimed during every full-scale automotive wind-tunnel grand opening, as they only happen about once a decade, with each new one building on the experience and learning of all those that preceded it. And every wind-tunnel visit is memorable.
$124M HALO Facility
The Honda Automotive Laboratories Ohio (HALO) wind tunnel took about five years and $124 million to construct. It’s the corporation’s fourth full-scale wind tunnel, and its only one in North America, but it will greatly reduce time required to ship test vehicles either abroad or to rental wind tunnels in this country. It was also designed and constructed in such a way as to be easy for other companies—even competitors, be they in the car biz or in racing—to securely utilize the facility, ensuring maximum “up time” and return on investment for this important design and development tool.
What Sets It Apart From Other Wind Tunnels?
Every wind tunnel’s job is to simulate a car moving over a stationary road through the air. Making wind blow is comparatively easy, and decades of tunnel design and engineering mostly perfected the various scales and devices that measure lift and downforce long ago. The hardest bit to master is the “ground effect.” You’d ideally like the air not to be moving right down at the ground, so many tunnels employ slots just in front of the vehicle that siphon off this boundary layer of air. That helps, but the gold standard is the moving ground plane, which the HALO tunnel provides in one of two ways.
One Belt or Five
The lower a vehicle’s ride height, the more crucial the boundary layer becomes, so the best means of testing race cars and sports cars is a full-width moving roadway. Picture God’s own upside-down belt sander, only the belt is a 0.8-millimeter-thick stainless-steel-based material, and it can run at 193 mph.
Before a vehicle is positioned on the belt, its precise wheelbase and track measurements are programmed into the tunnel so that sensitive scales can be positioned just under the belt at each tire. Air bearings allow the belt to glide over the tops of each scale while still precisely measuring aerodynamic lift or downforce to the half-pound. Vehicles using this system are typically outfitted with equipment to set and maintain suspension height so that all force measurements translate through the tires at various body attitudes. A system of tethers is required to maintain the vehicle’s position, and these necessarily have some effect on airflow.
More mainstream vehicles with greater ground clearance use the five-belt setup, which places a small belt to spin each tire (again utilizing an air bearing) with a longer, wider belt running between the tires beneath the rest of the vehicle. Note that the HALO tunnel also has slots capable of siphoning off boundary layer air, which remains especially important in front of the wheels. Here four pillars support the body’s weight and measure any aerodynamic forces acting on it, so there is never any suspension compression.
Note that in either case, the vehicle is prepped by removing the halfshafts and brake pads so that the only friction the belts must overcome is from the wheel bearings.
The five-belt and single-belt ground-plane modules mount to an elaborate balance system housed within a 12-meter diameter (39.4-foot) turntable. The three-axis load-cell balance system can measure drag forces to a precision of about 9 ounces. The turntable itself has the rare capability of rotating a full 180 degrees (many can only swivel plus or minus 15 degrees). This permits aerodynamic and aeroacoustic study of pure crosswind effects. In all, the system is set up to monitor 2,700 channels of force/balance/load information.
The wind-tunnel test chamber’s back wall opens like a banquet hall divider, revealing the “infield” of the wind tunnel’s air circuit. In here is a giant crane setup that comes out, attaches to the belt system on the turntable, lifts and carries it into the infield, and places it on a carrier that slides it out of the way so the other belt system can be picked up and moved into place. This room also stores a replacement for each of the two larger belts. The swap from a five-belt to one-belt setup, or vice-versa, is said to take four hours and is expected to only happen about once per month.
A 4,160-volt General Electric motor generates 5 megawatts (6,705 hp) spinning an 8-meter-diameter (26-foot) fan. The fan has 12 hollow carbon-fiber blades, each with just 4mm clearance to the tunnel wall. The fan maxes out at 253 rpm to produce 193-mph wind in the 18-square-meter (194-sq-ft) test area. The blades are inspected daily, and two replacements are stored on hand.
Four-Turn, Eighth-Mile Wind Track
As with most mainstream wind tunnels, air circulates continuously. The fan is located roughly opposite the test section, with elaborately shaped turning vanes in each corner to redirect the air. Just ahead of “turn one” (after the fan) is a giant radiator filled with 16,000 gallons of water/glycol coolant capable of heating the air to 122 degrees F or cooling it down to 50 degrees in about 30 minutes. This allows wind-noise measurement of plastic and rubber parts at different temperatures and varies the density of the air. Then, after another turn, comes a giant honeycomb screen that serves to straighten all the air. A final screen ensures nothing can hit the test vehicle. Finally, a nozzle shrinks the air passage size from 25 to 18 square meters, which speeds the air from a max of 155 mph up to 193 mph as it enters the roughly 16.4- x 9.8-foot test section. The air then bends around two more turns before completing its eighth-mile circuit. Construction of the tunnel itself took two years, 10,000 cubic yards of cement, and 900 tons of rebar.
Elaborate sound-absorbing materials cover the sides and top of the test section and much of the rest of the tunnel to simulate a car driving down a still, silent roadway at speed. The tunnel itself generates just 56.5 dBA of noise at 87 mph so as not to drown out the slightest bit of wind noise being generated by a mirror housing or weatherstrip. That’s somewhere between the noise made by a running refrig
erator and an electric toothbrush, and it makes this tunnel the quietest in its class.
500-Plus Mics—Oh, My Aachen Head!
As the industry pivots to electrification, wind and road noise become more noticeable (and objectionable). HALO can’t measure road noise but can scrutinize wind noise with a series of acoustic arrays covering a vehicle’s top, front, sides, and interior. In all, 502 directional microphones paired with cameras help pinpoint the source, loudness, and frequency of sounds generated by wind rushing around a vehicle. Another 54 internal microphones on a spherical array help pinpoint the source of interior sounds. And finally, there are anthropomorphic Aachen heads capable of perceiving sounds as a passenger would for “psychoacoustic” research.
Aerodynamics Vs. Aeroacoustics
Aerodynamics is the study of the way air flows over a particular shape and the forces it exerts on the shape, while aeroacoustics is the study of the noise that results from air passing over a shape. The rolling roadway is highly important to aerodynamics, but utterly unimportant for aeroacoustics, so when focusing on the latter, the turntable is typically covered by a large sound-absorbing mat made of honeycomb aluminum. Obviously, the wheels do not spin in these tests. Note that Honda generally finalizes the overall styling shape and design detail work in its smaller-scale wind tunnels, as it’s easier to revise and retest smaller-scale clay models.
80-Ton Robotic Arm
To help understand the tiny wind vortices that may cause a drag or noise problem, there’s an overhead crane-mounted robotic arm that can place an anemometer (wind-speed-measuring) probe anywhere within the cell, with half-millimeter precision, to pick up the tiny variations in speed that occur in a vortex. It can also be used to develop a detailed “wake plot” of airflow aft of the vehicle.
Four Customer Bays, Fabrication, and Laser Frontal-Area Bays
The 110,000-square-foot HALO facility also includes four secure bays where tunnel renters can prep vehicles (the bays’ design and security measures were influenced by Honda’s experience renting tunnel time elsewhere), plus bays capable of fabricating various parts and brackets (including a water-jet table), and a carwash. Any vehicle headed for a rolling-road test gets every possible pebble meticulously picked from its tire treads as well. Finally, there’s a room with a turntable and a laser measuring system that projects a line of horizontal green laser light onto an angled white screen behind the vehicle as it moves across the front or side of said vehicle. Ceiling cameras record this laser profile to precisely measure the frontal (or side-profile) area in a matter of minutes.