When
Just two years before, he constructed a 1-foot-by-1-foot wind tunnel for the Westinghouse Science Talent Search that earned him a visit to the
'It was my very first day on
Since its dedication in 1938, the
Today,
'If I had one word to describe the state of the old tunnel after 80 years, it would be decrepit. The tunnel shell and supporting foundations, the instrumentation, and the drive motor and fan were all in a state of decay. The airflow quality was poor, and the tunnel was extremely loud and power-inefficient,' says Drela. 'It just wasn't holding up against our modern standards of wind tunnel testing. Our goal was to bring our vintage tunnel into the 21st century and beyond, and we did that.'
Go with the flow
Wind tunnels have been in use for more than 150 years - even Wilbur and
Wind tunnel measurements can determine how much fuel an aircraft will consume, how slowly it can fly during landing, or how much control it has in maneuvers. But wind tunnels are not limited to aerospace applications. They can also measure the aerodynamic loads on ground vehicles, such as cars and bicycles, or wind loads on stationary objects, such as bridges and buildings. Scientists and engineers also use wind tunnels for fundamental research, like studying how the air behaves when it interacts with an object to understand the science of fluid mechanics.
Uses for the
Beyond its use in research, educators also used the tunnel extensively for coursework and public outreach. But after nearly eight decades, the aged equipment became a challenge to use. A full replacement was in order, and thanks to its urban campus home, the project presented several unique challenges.
'To have the best facility possible, we knew we needed a large test section with very good airflow quality and a maximum speed of at least 200 miles per hour, which dictated a large tunnel size and a powerful drive motor,' says Drela. 'But since the tunnel sits right in the middle of campus, we had to achieve these goals while making it compatible with our urban environment. When your goals massively conflict with your constraints, you get an incredibly challenging project.'
Innovating convention
In general, nearly all wind tunnels aim to generate 'clean' airflow, which means uniform flow with a constant velocity, free from distortion or turbulence. The convention would dictate a large tunnel for the required test-section size, which paradoxically requires less power to produce higher airflow quality while generating less noise. But for the
'Like any engineering project, size and cost were major considerations. We couldn't just take the design of a conventional tunnel and size it to fit into the old tunnel's relatively small space and expect it to work,' says Drela. 'We had to design an entirely new architecture with many innovations to the fan, diffusers, contraction, and the corner vanes to give the new tunnel our desired capabilities within the limits of the old tunnel's existing footprint.'
Both the old and new
One of the most distinctive visual differences between old and new is the design of the fan itself. The old fan followed convention still commonly seen today: a 13-foot diameter with six blades made of wood that resembled boat oars. The 2,000-horsepower motor could only run at four fixed speeds, and the operator adjusted the airflow speed by varying the fan's pitch mechanically. As a result, the system was complex, and the fan was noisy to operate. To mitigate these issues in the new tunnel, Drela worked with wind tunnel vendor Aerolab to conceive and manufacture an entirely new design: the Boundary Layer Ingesting (BLI) fan.
Air flowing over an object has a layer of slow-moving air over the object's surface caused by fluid friction called a boundary layer. Consequently, the airflow inside a wind tunnel has boundary layers over the entire inner surface of the shell. In the test section, where the airflow is cleanest, the boundary layer is only a few inches thick, but it grows as the airflow moves downstream. By the time it enters the fan, the airflow has a thick boundary layer extending over approximately half the length of each fan blade. Traditional wind tunnel fan design typically ignores the boundary layer, opting to eliminate it by mixing it with the rest of the flow farther downstream. But with 17 uniquely-shaped blades and a 16-foot diameter, the BLI fan is specifically designed not only to accommodate this inflow nonuniformity, but to exploit it.
'The flared tips of the fan blades add extra work to the boundary layer where the velocity is lowest, near the wall,' says Drela. 'Using the fan to remove this velocity nonuniformity requires less power than the downstream mixing in all other wind tunnels. The resulting flow that exits the fan is uniform, further reducing the power losses in the downstream portion of the tunnel.'
The BLI fan is driven directly by a 2,500-horsepower motor, so the overall drive system in effect has only one moving part - a significant improvement over the mechanically complex variable-pitch drive of the old tunnel. A variable frequency drive controls the motor speed, making it more power-efficient and quieter than the old tunnel's system.
The fan pressurizes most of the tunnel flow circuit, resulting in the tunnel's far wall opposite the fan withstanding up to 80 tons of load when the tunnel operates at full speed, equivalent to the force of a 240-mph hurricane. To accommodate the resulting elastic flexing of the walls, the only parts of the
After the flow leaves the test section, it turns through corners one and two, then passes through the fan, after which it goes through a heat exchanger to regulate the air temperature, which is then followed by corner three. Up until this point, this is a standard process in most current wind tunnels, but according to Drela, the final corner four 'is where the real magic happens' in the
While the first three corners have vanes that only turn the airflow 90 degrees, corner four not only turns the flow but also expands its area while slowing it down significantly, enabled by a screen and aluminum honeycomb diffusers installed in the passages between the vanes. Performing the same flow-deceleration and straightening in a conventional tunnel requires more space and separate honeycomb filters and screens. By combining these components into the single corner vane row, the
'If we didn't have the screen expanding turning vanes suppressing the wall boundary layers in corner four, they would 'burst' or separate after the corner, thus filling the entire flow path and making the air slosh around like in a washing machine. The resulting flow going into the test section would be very messy and unusable for aerodynamic tests,' says Drela. 'The screened expanding turning vanes at corner four are arguably the most important components of the new tunnel because it allows for a large flow area expansion in no added space while maintaining a nearly uniform flow.'
Although the airflow exiting corner four is relatively clean, it next passes through four flow-conditioning screens to make it even more smooth and uniform. Immediately after the final screen, the air enters the contraction, the widest part of the tunnel that accelerates the flow into the test section. A key parameter to indicate the efficiency and quality of a wind tunnel is the contraction ratio, which is the ratio of the airflow velocity between the test section and after the flow-conditioning screens. The old tunnel had a contraction ratio of 4.5:1, but Drela wanted to reach the 'sweet spot' by increasing the ratio in the new one to 8:1.
'For the new tunnel, we used computational fluid dynamics to carefully design a minimum-length contraction by combining it with the usual settling chamber after the screens,' says Drela. 'This combination saved us about eight feet of space, which was significant for a tunnel that is only 96 feet in total length.'
In the test section, an object is mounted on a slender post connected to the main force balance, which is the instrument installed immediately under the test section floor that senses and reads the aerodynamic forces as the airflow interacts with the model. The test section size and shape in the
The new tunnel also features a new MATLAB-based tunnel control and data acquisition system. This system combines the typical functions of manual tunnel operation, control, and data collection into a streamlined, fully customizable platform. The test section's glass walls and ceiling windows give extensive optical access, which enables laser-doppler velocimetry and particle-image velocimetry measurements as well as optical model motion tracking. Safety and security features are also built directly into the tunnel control system, monitoring tunnel health parameters such as temperatures, pressures, and vibration levels. The system automatically switches to rapid shutdown mode if any health parameter exceeds its preset physical limit, or in the event of a manual emergency stop.
'You can control everything through this interface - tunnel speed, model positioning, instrument interrogation, data display, logging, and more - all from the same place,' says Drela. 'It removes as much human error from the process as possible. Since the system is watching your back, you literally cannot do anything to break the tunnel from the keyboard, which is very comforting from the user's perspective.'
Breaking new ground
Construction for the
The Building 17 renovation overhauled these spaces, combined them with the
'Safety is always a top priority on any construction site. The coronavirus situation took it to another level, especially with the Cambridge-wide moratorium on construction projects that lasted for weeks,' says
'
In addition to support from
Even though the cranes and bulldozers have left the site, the team continues to make final calibrations to the instrumentation and other finishing touches in order to reach full operational capacity by midsummer. At that time, the
In keeping with its predecessor, the new tunnel will carry forward an important legacy representing AeroAstro in outreach efforts across
'We're looking forward to bringing this sense of excitement back to campus since it's been on hiatus due to construction and the pandemic,' says
According to Drela, even in the age of advanced computing, simulation, and modeling, practical testing in wind tunnels is just as valuable as ever, especially when paired with these advanced techniques.
'Even with the most advanced computer, we can't calculate flow with adequate precision or confidence or without significant margins of error, which could be catastrophic in some circumstances. For example, if you significantly underestimate stall speed, a crucial aspect of airplane performance, it's the difference between life or death,' says Drela. 'While there are situations where I wouldn't trust calculations over measurements, wind tunnel testing and computation are extremely complementary. Experimental data obtained in wind tunnels will always be indispensable for validating a theoretical and computational fluid flow model.'
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