FYFD

Search results for: “drag”

  • Tour de France Physics: Pelotons

    Tour de France Physics: Pelotons

    July is well underway and for cycling fans around the world that means it’s time for the Tour de France. This week at FYFD we’re going to do something a little different: in honor of cycling’s biggest race, every post this week will focus on some of the fluid dynamics involved in the sport.

    On a bicycle, except when climbing, the majority of a rider’s energy goes toward overcoming aerodynamic drag. Riders wear close-fitting clothes to reduce skin friction and loss to flapping fabric, but most of their drag is pressure-based. A blunt object disturbs the airflow around it, usually resulting in separated flow in its wake. A high pressure region forms in front of the rider and a low pressure region forms in the separated flow behind them. This pressure difference literally pulls the rider backwards. Since drag goes roughly as speed squared, adding a headwind makes matters even worse for a cyclist.

    In races, especially on flat stages, the majority of the riders will stay in a large group called a peloton in order to counteract these aerodynamics. By riding in the wakes of those in the front, riders in the peloton experience a much smaller front-to-back pressure difference and thus much less drag. For a rider in the midst of the peloton, the drag reduction can be as great as 40% (#). This allows riders to conserve energy for solo efforts near the end of the race or stage, like breaking away from the peloton in the final kilometers or winning a sprint for the finish line. (Photo credit: Wade Wallace)

  • Wind Tunnel Testing

    Wind Tunnel Testing

    A scale model of the Space Shuttle attached to its modified 747 carrier hangs in a NASA wind tunnel. Wind tunnel tests can be used for flow visualization, lift and drag measurements, control system checks and so forth, but mounting models correctly and safely in the tunnel is crucial. Many models use sting mounts that project forward, as this one does, in order to expose the model to freestream flow unimpeded by the mounting mechanism. Any mounts and models must also be sturdy enough that all or part of them does not break off mid-test and fly into the wind tunnel’s fans. #

  • Drafting Flags

    Drafting Flags

    Wired Science has published a gallery of fluid dynamics photos and videos, several of which have been featured here previously. There’s some neat stuff there, well worth checking out. #

    This image shows two flags oriented in line with a film flowing top to bottom. The second flag interrupts the wake of the first one, which reduces the drag experienced by the first flag and increases that on the second. This is called inverted drafting and occurs because the flags are passive objects that bend to every change in the flow. #

  • Featured Video Play Icon

    Ground Effect Vehicles

    Ground effect vehicles (a.k.a. wing-in-ground-effect vehicles) rely on their proximity to a flat surface to inhibit the wingtip vortices that create lift-induced drag. This effectively increases the lifting capabilities of the vehicle in comparison to regular flight, but only so long as the vehicle remains close enough to the ground. This video features many model gliders that rely on ground effect.

  • Featured Video Play Icon

    Hot Spheres Sink Faster

    New research shows that the Leidenfrost effect–which causes water droplets to skitter across a hot pan–can drastically reduce the drag on objects moving through a liquid. When raised to a high enough temperature, a sphere falling water will be coated in a protective layer of vapor (see video above) that acts like a lubricant as the sphere moves through the water. If the temperature of the object drops too low, the vapor layer will dissolve into a mess of bubbles (~35 secs into video). One way that this mechanism reduces drag is by keeping flow attached to the sphere for longer as shown in this video. Preventing this flow separation increases the pressure recovered after the point of lowest pressure (the shoulders of the sphere), which reduces overall drag.

    See also:

  • Featured Video Play Icon

    Feathering on SpaceShipTwo

    Virgin Galactic and Scaled Composites recently performed their first feathered flight with SpaceShipTwo, which is on track to be the first commercial spaceship. Feathering is a re-entry technique devised by Scaled Composites founder Burt Rutan:

    Once out of the atmosphere the entire tail structure of the spaceship can be rotated upwards to about 65º. The feathered configuration allows an automatic control of attitude with the fuselage parallel to the horizon. This creates very high drag as the spacecraft descends through the upper regions of the atmosphere. The feather configuration is also highly stable, effectively giving the pilot a hands-free re-entry capability, something that has not been possible on spacecraft before, without resorting to computer controlled fly-by-wire systems. The combination of high drag and low weight (due to the very light materials used to construct the vehicle) mean that the skin temperature during re-entry stays very low compared to previous manned spacecraft and thermal protection systems such as heat shields or tiles are not needed. During a full sub-orbital spaceflight, at around 70,000ft following re-entry, the feather lowers to its original configuration and the spaceship becomes a glider for the flight back to the spaceport runway. #

    Though it works well for decelerating from sub-orbital speeds, feathering is sadly not useful for orbiting spacecraft due to the much higher kinetic energies that have to be dissipated.

  • Laminar Flow Control

    Laminar Flow Control

    On Wednesday, March 30, 2011 at 3:00 EDT NASA engineers are holding an online chat about a current project to achieve laminar flow control on business jet-class airplanes. Keeping flow over an airplane’s wings laminar could decrease the total drag on an airplane by as much as 15%. In particular, this project involves placing tiny hockey-puck-shaped discrete roughness elements (DREs) along the front of the wing. These DREs are positioned such that they perturb the mean-flow over the wing at a higher frequency than the naturally most unstable frequency; as a result, flow actually remains laminar over a greater extent of the wing than would normally be the case. For more on the technical ideas, see this NASA blog post or feel free to ask questions in the comments. #

    Full disclosure: This project is being conducted in joint with professors with whom I work, and the subject matter is related to my own research.

  • Featured Video Play Icon

    Aerodynamics with Bill Nye and Samuel L. Jackson

    Bill Nye, Samuel Jackson, golf balls, Reynolds number, dimples, and boundary layers. It doesn’t get much better than this. – Khristopher O (submitter)

    It definitely beats Jackson’s other foray into aerodynamics! The dimples on a golf ball cause turbulent boundary layers, which actually decrease drag on the ball and make it fly farther. Why bluff bodies experience a reduction in drag as speed (and thus Reynolds number) increases was a matter of great confusion for fluid mechanicians early in the twentieth century, but it’s not too hard to see why it happens with some flow visualization.

    On the top sphere, the laminar boundary layer separates from the sphere just past its shoulder. This results in a pressure loss on the backside of the sphere and, thus, an increase in drag. On the bottom sphere, a trip-wire placed just before the shoulder causes a turbulent boundary layer, which separates from the sphere farther along the backside. This late separation results in a thinner wake and a smaller pressure loss behind the sphere, thereby reducing the overall drag when compared to the laminar case. (Photo credit: An Album of Fluid Motion)

  • Discovery Wingtip Vortices

    Discovery Wingtip Vortices

    Wingtip vortices mark the path of Discovery as she makes her final landing. Though not always visible, these vortices are generated by any lifting body planform and can be a major source of induced drag on the craft. Here the vortices are visible because the low pressure in the core of the vortex caused a local temperature drop below the dew point, thus causing condensation. Such vortices persist for significant lengths of time in the wake of aircraft; they are a major source of wake turbulence, which limits how frequently aircraft can take-off or land on a single runway. (Photo by Jen Scheer)

  • Wright Brothers’ Wind Tunnel

    Wright Brothers’ Wind Tunnel

    A large part of the Wright Brothers’ ultimate success in creating the first powered heavier-than-air craft came as a result of work done in their homemade wind tunnel, shown above. In the aftermath of the failure of their 1901 Glider, the brothers decided that the lift and drag data they had used from Otto Lilienthal must be inaccurate. They built this wind tunnel and its force balances to measure lift and drag on two hundred different airfoils themselves and were rewarded with far more successful flights with their 1902 Glider, which led directly to the Wright Flyer in the following year. #

More posts