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Tag: wind tunnel testing

  • Derecho-Induced Skyscraper Damage

    Derecho-Induced Skyscraper Damage

    Derechos are short-lived, intense wind storms sometimes associated with thunderstorms. Last spring, such a storm passed through Houston, leaving downtown skyscrapers with more damage than a hurricane with comparable wind speeds. Now researchers believe they know why a derecho’s 40 meter per second winds can badly damage buildings built to withstand 67 meter per second hurricane winds.

    In surveying the damage to Houston’s skyscrapers, the team noted that broken windows were concentrated in areas that faced other tall buildings. In a wind facility, the team explored how skyscrapers interfered with each other, based on their separation difference. They looked both at conditions that mimicked a hurricane’s winds as well as the downbursts — strong downward wind bursts — that are found in derechos.

    The researchers found that downbursts in between nearby buildings caused extremely strong suction forces along a building’s face — even compared to the forces seen with higher hurricane-force winds. Currently, these buildings are designed for hurricane-like conditions, but the team suggests that — at least in some regions — designers will need to take into account how downburst wind patterns affect a skyscraper, too. (Image credit: National Weather Service; research credit: O. Metwally et al.; via Ars Technica)

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  • Paris 2024: Cycling in Crosswinds

    Paris 2024: Cycling in Crosswinds

    Wind plays a major role in cycling, since aerodynamic drag is the greatest force hampering a cyclist. In road racing, both individual cyclists and teams use tactics that vary based on the wind speed and direction. Crosswinds — when the apparent wind comes from the side in the cyclist’s point of view — are some of the toughest conditions to deal with. In races, groups will often form echelons to minimize the group’s overall effort in a crosswind. Alternatively, racers looking to tire their competitors out will position themselves on the road so that the rider behind them gets little to no shelter from the wind; this is known as guttering an opponent.

    In this study, researchers put a lone cyclist in a wind tunnel and measured the effects of crosswind from a pure headwind to a pure tailwind and every possible angle in between. From that variation, they were able to mathematically model the aerodynamic effects of crosswind on a cyclist from every angle. With rolling resistance (a cyclist’s second largest opposing force) included, they found relatively few conditions where a crosswind actually helped a cyclist. Most of the time — as any cyclist can tell you — hiding from the wind is beneficial. (Image credit: J. Dylag; research credit: C. Clanet et al.)

    Related topics: The physics of the Tour de France, how the peloton protects riders aerodynamically, track cycling physics, and a look inside wind tunnel testing bikes and cyclists

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  • Saving Energy By Following a Leader

    Saving Energy By Following a Leader

    Scientists have long suspected that birds save energy by following a leader — think of the V-shaped flight formation used by geese — but a new study captures that savings directly. The team studied starlings, flying singly or in groups of two or three, in a special wind tunnel. Each bird wore a tiny backpack with sensors and lights that captured its motion and helped researchers identify it individually in videos. And, using before and after metabolic measurements, the researchers could pin down exactly how much energy each bird used when flying.

    They found that birds who spent most of the flight in a “follower” position used up to 25% less energy than they did when flying solo. That’s a major incentive to follow someone else. Interestingly, they also found that the most efficient solo fliers were the birds most likely to take on the “leader” position. The team notes that these “leaders” tend to use a lower wing-flapping frequency, but a full explanation of how they save energy will require a follow-up study. (Image credit: R. Gissler and S. Hao; research credit: S. Friman et al.; via Physics World)

  • Crocodilian-Inspired Aerodynamics

    Crocodilian-Inspired Aerodynamics

    Inspired by crocodilians, young scientist Angela Rofail designed attachments to reduce wind loads on high-rise buildings. When crocodilians swim, the ridges on their back help hide their motion from observation above the surface. Rofail wondered whether similar ridges would reduce the wind-induced swaying of high-rise buildings. Using a scale-model and crocodile-inspired knobs, the Year 10 student (read “high-school freshman” for U.S. readers) conducted wind tunnel tests that showed her modifications reduced drag on the model and kept it from moving in windy conditions. (Image credit: H. Roettger; video credit: CSIRO; via CSIRO; submitted by Kam-Yung Soh)

  • The Protection of the Peloton

    The Protection of the Peloton

    It’s well-known by professional cyclists that sitting in the middle of the peloton requires little effort to overcome aerodynamic drag, but now, for the first time, there’s a scientific study to back that up. Researchers built their own quarter-scale peloton of 121 riders to investigate the aerodynamic effect of cycling in such a large group versus riding solo. Through wind tunnel studies and numerical simulation, they found that riders deep in the peloton can experience as little as 5-10% of the aerodynamic drag of a solo cyclist. 

    Tactically, this means teams should aim to position their protected leader or sprinter mid-way in the pack, where they’ll receive lots of shelter without risking one of the crashes common near the back of the peloton. It also suggests that teams wanting to isolate another team’s leader should try to push them toward the outer edges of the peloton rather than letting them sit in the middle. It will be interesting to see whether pro teams shift their race strategies at all with these numbers in hand.

    Of course, this study considers only a pure headwind. But other groups are looking at the effects of side winds on cyclists. (Image credit: J. Miranda; image and research credit: B. Blocken et al.; submitted by 1307phaezr)

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    Hummingbird Hovering

    Hummingbirds have a unique way of flying among birds. By flapping in a figure-8 motion, they generate lift on both the upstroke and the downstroke, which enables them to fly forward, backward, and even hover for extended periods. Such mid-air acrobatics are necessary for a species that feeds on flower nectar. What is especially impressive about the birds, though, is how they hold up even in adverse conditions like wind or rain. By placing birds in a wind tunnel and filming with high-speed video, researchers can see how hummingbirds maintain their feeding position even in 20 mph (32 kph) winds. By fanning out their tail feathers like a rudder, they can control their body orientation despite turbulent gusts. Not even rain stops them. The birds will periodically shake themselves dry, much like a dog if a dog could manage to fly while shaking itself. (Video credit: Deep Look; submitted by entropy-perturbation)

  • Testing a Supersonic Car

    Testing a Supersonic Car

    How do you test a supersonic car like the Bloodhound SSC in a wind tunnel? With free-flying objects like airplanes, wind tunnel testing is relatively straightforward. Mounting a stationary model in a supersonic flow gives an equivalent flow-field to that object flying through still air at supersonic speeds. The same does not hold true for the supersonic car, though, because you need to account for the effect of the ground on airflow. One option is to build a moving wall in the wind tunnel. For low-speed applications, this is feasible but incredibly complicated and very expensive. For supersonic speeds, it’s impossible. You could achieve the same moving-wall effect at supersonic speeds with a rocket sled, but that is also expensive and difficult to fit in most experimental facilities. The simplest solution is the one you see above – build two models and mount them belly-to-belly. Reflecting the models makes the plane of symmetry a stagnation plane, which, fluid dynamically speaking, acts like an imaginary ground plane relative to the model. For more on the project and the technique, check out this article.  (Photo credit: B. Evans; via ThinkFLIP; submitted by G. Doig)