Experiments with “Air Castles”

An article based on an interview with Aleksander Borisovich Ayrapetov, certified expert in aerodynamics of buildings, architectural and building structures

Just imagine: upon completion of construction, the height of the East tower of the multi-functional complex “Federation Tower” located in Moscow City, the modern business center of the city, will be 373.7 m (according to citynext.ru). Todays architects and builders create real “air castles” — bizarre structures of very sophisticated shapes, dominating over the cityscape. But with the growth of height of those facilities, a question is becoming increasingly important: How to make them more stable in air flow and safe for the human environment?

In the mind of a majority of people, the Central AeroHydrodynamic Institute (TsAGI) is the center of aviation science dealing with research of aircraft for improvement of their performance. But the experimental base of the Institute is applicable not only for aircraft. The Central AeroHydrodynamic Institute also studies more ‘earthly’ subjects — building structures — which, however, have large height and in the language of specialists they are called “bluff body.” These are buildings and monuments which, as well as bridges, fans, pipelines, etc., are studied in terms of industrial aerodynamics.

In the first years after its establishment, TSAGI started to test such structures. One of the first examples of such research was the experiment with a model of the roof of the building accommodating the first air tunnel of the Institute — T-1-2 (Radio Street, Moscow). Later on, the Moscow Facility of TSAGI surveyed the most famous architectural structures — the Lenin sculpture for the design of House of Soviets (Moscow), the monument “Motherland is Calling” (Volgograd), a number of the works of Zurab Tsereteli — the Victory Monument on Poklonnaya Gora (Moscow), Columbus statue (Seville) and a similar project for Miami; the New Cathedral Mosque (Moscow), the sculpture “Worker and Collective Farmer Woman” (Moscow) and other structures. By the way, the famous work of Vera Mukhina, “Ideal and Symbol of the Soviet Era,” created in 1937, was mentioned last for a reason. Contrary to widespread beliefs, the distinguished monument was not tested in TsAGI in the years of its creation. It was subjected to experiments just in 2003, during the stage of reconstruction.

Besides cultural and historical heritage objects, the Institute’s scientists tested more practical structures; the Alma-Ata TV Tower, the above mentioned “Federation Tower,” a number of multi-functional facilities of Moscow and St. Petersburg, multiple chimney stacks of a new generation of district heating plants, etc. A curious fact is that the most advanced high-altitude building structures are tested in the oldest installation of the TsAGI made of wood.

“The sky above the capital is no longer the same as in 1971 when I came to work at the Moscow Facility of the Institute at Radio Street 17,” says Aleksander Borisovich Ayrapetov, TsAGI’s leading expert in aerodynamics of buildings, architectural and building structures. “Today’s city environment significantly differs from that in Soviet Union time. Just compare: a stand-alone five-storey Khruschev-era apartment block and the set of high-rise buildings of Moscow City. The phenomena which we can find in the current architectural space of a megalopolis gives ground for discussion about revision of the regulations of city development,” the scientist notes.

A bizarre architectural view generates new challenges for scientists. As an example, A.B. Ayrapetov mentioned the design of the modern multi-functional complex “Chinese Business Center of Huamin Park” (Moscow): “Actually, it’s a high-rise with a cross-section reminiscent of an aircraft wing profile. But such unusual architectural shapes cause irregular aerodynamic phenomena which are not stipulated in the current building codes.”

For the last decade (since 2005 when the temporary standards and rules of design of multi-functional high-rise buildings and complex buildings in Moscow were introduced) all the architectural and building structures having a height of more than 75 m were studied in the Institute. “The accumulated scientific and practical experience often allows us to unmistakably find a way to solve a problem even without model experiments. But this is applicable in case when the structure’s height is no more than seven times that of its base diameter,” emphasizes Aleksander Ayrapetov. If this index is exceeded (and such trend increasingly occurs in recent years in the architecture of a megacity), model aerodynamic instability tests are necessary. Such experiments clearly demonstrate what phenomena can take place with a high-rise under the action of wind flow.

But an even more important aspect which is in the focus of scientists’ attention is the mutual influence of building structures. A dense group of high-rise buildings creates its own climate, e.g., in the area of the Moscow City business center, where skyscrapers stand close to each other, it’s more windy than in adjacent areas. As an illustration of how dangerous the influence of structures to each other can be, A.B. Ayrapetov uses the experiment with chimney stack models. According to the codes adopted in 1950s, the distance between them should be no less than five diameters of the stacks; but, what will happen if that rule is violated, and chimneys are located to closer to each other? “We started to move the chimney models close to each other in our experimental setup, and at a certain moment, when the distance between them was a half of the initial one, one of the pipes started to vibrate so strongly that we had to immediately stop the test,” said Aleksander Ayrapetov. Thus, local meteorology of an air basin of a megacity is now a separate new science.

Despite many years of practical experience, today the specialist also manages to find unusual phenomena which were not seen previously. Aleksander Ayrapetov mentioned the tests performed during the construction of the Cathedral Mosque in Moscow: “Its model with the two minaret towers were set in the air tunnel, and one of minarets mounted on the hinged joint of the vibrating unit of the tunnel was intentionally given a low amplitude of oscillations. We noted that the oscillations didn’t attenuate, as desired, but continued with constant amplitude. We increase the amplitude; the tower ‘captured’ it and started to oscillate with that new, already higher amplitude and also without a trend to attenuation. Another attempt gave the same effect. This is a new phenomenon which does not have an approved name yet.”

Sometimes, the issue of experiments is not raised on early phases of design and construction, when undesirable effects of wind and mutual influence of structures in urban development can be prevented until well into the construction or even after completion of a project. “Builders necessarily conduct geologic exploration, to avoid soil settlement under house foundation, otherwise a building will “float.” But aerodynamic phenomena, which we warn of, can arise, perhaps not immediately but in 50 years or so. However, they can still be dangerous. If architects and builders applied to us at the phase of concept design, as should happen, (but not as often as we would hope for), many adverse consequences could be avoided, Aleksander Ayrapetov emphasized. Yet, on the other hand, based on the capabilities of aerodynamics, there are methods to provide longer life of a building, substantially “free of charge,” e.g., by correct orientation of a building according to local “wind flows.”

In most cases, it is possible to eliminate the problems related to aerodynamic instability of a building or structure by installing special devices — oscillation absorbers. The principle of this device is similar to a car shock absorber: there is a heavy mass oscillating in a viscous medium. Installed on a high-rise structure, it forms another oscillating system along with the building. “The main help we can offer to present-day builders of high-rise buildings is to provide the data that help to design oscillation absorbers for particular case,” noted A. B. Ayrapetov.

Today, the industrial aerodynamics specialists of TsAGI widely use numerical simulations. As the scientist noted, experiments cannot be conducted in all cases: “It cannot be made correctly, e.g., if these are surveys aimed at providing comfort in pedestrian areas. Today they are often densely surrounded by building development, saturated with advertisement boards, etc. We simply cannot place all the surveyed zone in the air tunnel: the scales of separate subjects will be so minimized that there’s no way to provide reliability in the experiment. These are the cases when calculations help us.” Furthermore, numerical simulation is invaluable is the case of designing models with surrounding development; it allows researchers to determine the number and scale of buildings on a key plan, the scale of the surveyed building in terms of obstruction of the working section of the air tunnel and providing the needed mode of flow around the whole complex of models.

Research enables scientists to see the consequences of ill-considered designs of high-rise structures located close to human activity, therefore all such subjects should meet all safety requirements, be stable in air flow and in a dense massif of “stone jungles.” According to A.B. Ayrapetov, high-rise building development in a city without taking into consideration these factors creates a potential technogeous disaster. “A technogeous disaster is an event of a cascading scale in a megacity, causing uncountable casualties,” he noted. And it’s not a hypothesis, remembering such events as the Chernobyl disaster and the tragedy of September 11, 2001 in New York.

As the scientist said, the accumulated experience is sufficient now to modernize the regulations applied today in city planning. And this is the correct way of raising the safety and comfort of urban life. High-rise building aerodynamics specialists are ready to assist in this matter, now it’s up to the willingness of the management of construction business and the authorities of the cities.

Photo: Archive of TSAGI, Internet

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