Aeroacoustic analysis minimizes the risk of wind-induced building noise
The National Museum of African American History and Culture is the latest addition to the National Mall in Washington, D.C. The museum documents the legacy of the 10 million to 15 million Africans brought as slaves to The New World, a practice that only ended in the early 19th century.
Taking its place on the last large open plot on the mall, between the Washington Monument and the National Museum of American History, the 350,000+ square foot structure is rich in symbolism. The above-ground structure features a three-tier design reminiscent of the Yoruban corona, or crown, motif from West Africa. Although half the structure is below ground, the upper five stories feature a glass curtain wall surrounded by a bronze-colored, scrim-like aluminum latticework mesh, creating a stunning presence at a focal point on the mall. The latticework is a key visual element in the building’s design. Made up of hundreds of individual decorative panels, it pays tribute to the ironworking skills that many early African-Americans developed as they transitioned from agrarian life to professions in the cities.
Wind flow around buildings can lead to annoying noise levels, especially when it creates tonal noise, like whistling. Recognizing that the flat terrain and wide open spaces on the mall would make it possible for wind to flow quickly toward and around the museum building, the designers were concerned about the possibility of wind-induced noise. With all the intricate openings in the corona panels encircling the museum building just outside the glass curtain wall, the building’s designers and owners determined that it would be prudent to study the risks of wind-induced noise during the design stage instead of waiting to see whether problems arose and troubleshooting them after construction.
Many complex interactions between airflows and structures can cause wind-induced noise, but predicting every possible source of noise is difficult. Therefore, the approach we adopted in this case was to have one of our aeroacoustics specialists review the building’s exterior design to identify areas of concern. Our team then ranked these areas of concern according to their likelihood of generating unpleasant noise; the higher the risk, the more detailed our investigation of how each feature would behave.
There are several ways wind-induced noise can occur around a building. It can be caused by an edge tone, which occurs when a strong wind flow moves toward a sharp-edged obstacle, such as a sheet of glass with an exposed edge. As the air moves toward the sharp edge, it oscillates between flowing to one side of the obstacle and then the other, its frequency varying with the speed of the air and the geometry of the object. If the frequency of the edge tone coincides with either a cavity resonance or a structural frequency, then the tone can be amplified and become clearly audible. Another concern is vortex shedding, which occurs when wind creates audible vibration as it passes blunt cylindrical objects like the strands that compose the corona latticework.
To deliver a complete analysis, we needed to understand how each element of the building’s exterior might produce each of these phenomena. To determine the risk of wind-induced noise from the façade’s impressive corona panels, we investigated wind conditions in the area as well as the natural frequencies of the building’s external components.
To estimate local wind conditions, we relied on the recorded data from nearby Baltimore-Washington International Airport. Although we found that the greatest risk of wind-induced noise from the corona panels would come from vortex shedding, we also determined that at the building’s lower levels this risk would be mitigated by two factors. First, the framework supporting the panels from behind would disrupt the airflow parallel to the panels. Second, the close proximity of the glass curtain wall would keep wind from flowing directly through the panel perforations at high speed. Because these two mitigating factors were at work on the lower part of the building, only the top two rows of panels, where wind could flow freely through the panels and overtop of the building, presented a meaningful risk of wind-induced noise.
We performed an engineering analysis of the natural modes of vibration for the top two rows of panels and found that because their fundamental frequencies (their most “excitable” modes) are in the range of 10 Hz, any vibrations produced by local wind would be below the range of human hearing. The irregular perforation patterns and angled surfaces of the facade further reduced the risk of audible sound radiating from the panels.
Based on our desktop assessment, we concluded that there was low risk of audible wind-induced noise given the wind speeds that are typical around the building site. The building opened in September 2016 to numerous accolades. “Architecture often deals with grand statements rather than subtle emotions,” wrote Julie V. Iovine in the Wall Street Journal near the time of the museum’s opening. “But at the National Museum of African American History and Culture, the spirit of inclusion, celebration and even spiritual uplift is palpable in every space.”