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  • Tour de France Physics: Time Trials

    Tour de France Physics: Time Trials

    Unlike road stages in which cyclists can draft off one another to reduce drag, in the time trial a cyclist is on a solo race against the clock with nowhere to hide. As a result, the event features lots of technologies designed to reduce both pressure drag and skin friction on the cyclist. For time trials, cyclists wear skinsuits and shoe covers to eliminate any sources of flapping fabrics and to reduce skin friction. They ride bicycles designed to be as light and aerodynamic as possible. Instead of rounded tubing in the frames, these bikes consist of elongated airfoil profiles that direct air past and prevent separation that may increase pressure drag. The rims of their tires are wider and the back wheel is replaced with a disc wheel that allows no airflow aross the wheel. Like the airfoil tubing, these changes help prevent separation. Similarly, riders wear elongated helmets designed to be as aerodynamic as possible while the rider is in the “aero” position, with arms directed out over the wheels, head level, elbows tucked, and back flat. In wind tunnel tests, the rider best able to hold this position will experience the least drag. Even the addition or subtraction of a water bottle is not left to chance, with many time trial bikes designed to be more aerodynamic with a water bottle onboard (though you probably won’t catch the cyclists breaking their aero position to get a drink)! (Photos by Veeral Patel)

    FYFD is celebrating the Tour de France with a weeklong exploration of the fluid dynamics of cycling. See previous posts on drafting in the peloton, and pacelining and echelons, and the art of the lead-out train.

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    Tour de France Physics: Breakaways

    In cycling, a small group of riders often leave the protection of the peloton in a breakaway. These riders will often spend 80% or more of a stage or race outside of the peloton, trying to reach the finish line before they’re caught. Because the pressure drag is so draining on a lone cyclist, it’s vital that breakaway riders work together. When the wind comes predominantly from the front or back, riders will form one or two lines, riding with their wheels within a foot of one another (see ~0:23). This paceline rotates so that every rider takes a turn at the front, bearing the brunt of the effort while other cyclists recover in their wake, where they experience less drag.

    If the wind blows predominantly across the riders, they will form a diagonal line with the frontmost rider rotating behind for shelter from the wind after a pull. This drag reduction technique is called an echelon (see ~1:40). As seen above, for experienced riders the echelon can protect individuals even in bike-stealingly high winds.

    FYFD is celebrating the Tour de France with a weeklong exploration of the fluid dynamics of cycling. See part one on drafting in the peloton.

  • 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. #

  • Whipping Instability

    Whipping Instability

    A droplet of glycerol coalescing in silicone oil while subjected to strong electric fields exhibits a whip-like instability reminiscent of fireworks. Check out videos of the phenomenon or see the paper for more information. Happy Independence Day to our American readers!

    For more fun, holiday-themed high-speed video, check out PopSci’s fireworks videos.

  • Stall-Sensing Hairs

    Stall-Sensing Hairs

    Bats use tiny hairs on their wings to sense the direction and speed of air flow. Researchers found that removing these hairs caused bats to fly faster and make wider turns, likely because the bat believed it was on the verge of stalling and losing lift. Engineers are considering whether artificial versions made of flexible polymers that respond to strain could provide improved stall sensing on fixed-wing aircraft. # (Photo credit: justynk)

  • X-51A Scramjet Test Flight

    X-51A Scramjet Test Flight

    The X-51A Waverider hypersonic aircraft had its second test flight earlier this week. Unfortunately, its supersonic combustion ramjet (scramjet) engine failed to transition from its start-up fuel to its primary fuel. According to the US Air Force Research Laboratory:

    A US Air Force B-52H Stratofortress released the experimental vehicle from an altitude of approximately 50,000 feet. After release the X-51A was initially accelerated by a solid rocket booster to a speed just over Mach 5. The experimental aircraft’s air breathing scramjet engine lit on ethylene and attempted to transition to JP7 fuel operation when the vehicle experienced an inlet un-start. The hypersonic vehicle attempted to restart and oriented itself to optimize engine start conditions, but was unsuccessful. The vehicle continued in a controlled flight orientation until it flew into the ocean within the test range. #

    Un-starting is the term used when supersonic flow is lost in an engine or wind tunnel. If the pressure or temperature in the engine deviates too far from the ideal conditions, the upstream mass flow through the engine will be greater than the downstream mass flow and the engine will choke (video). A shock wave forms and travels upstream, leaving subsonic flow in its wake. Loss of supersonic flow inside the engine would likely also result in losing ignition of the fuel/air mixture, resulting in flameout. #

    If you haven’t guessed already, engineers like to make up words.

  • Cloud Wakes Off Islands

    Cloud Wakes Off Islands

    This satellite image shows the cloud wakes of two small, volcanic islands off the coast of Chile. The disruption in airflow caused by the islands results in 100 km long cloud-free V-shaped wakes, even though the islands themselves are barely visible in the image. Such variation in the size of the obstacle compared to its effect is not unusual in fluid mechanics, but the scale, in this case, is impressive. #

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    Vortex Shedding from a Hot Cylinder

    This numerical simulation shows vortex shedding behind a hot cylinder. The behavior is very similar to what one sees behind an unheated cylinder, until the coefficient of thermal expansion increases and the von Karman vortex street is completely distorted. Describing the particulars of the computation, jessecaps writes (links added):

    I wrote an incompressible flow solver to simulate flow past a heated cylinder. The Navier-Stokes equations are discretized on a Cartesian grid and solved explicitly in time. The pressure-Poisson equation is solved implicitly using a bi-conjugate gradient method. The Boussinesq approximation was used (density is constant everywhere except for the gravity term) to account for buoyancy. Reynolds number = 250, Froude number = 1 (gravity is pointing down). The two simulations show the effect of the coefficient of thermal expansion. Each video shows a plot of velocity and temperature.

    (submitted by jessecaps)

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    Coronal Waves

    NASA’s Solar Dynamics Observatory has found evidence of Kelvin-Helmholtz waves in the sun’s corona. These waves, which occur between two fluids of different densities or moving at different speeds, are similar to the iconic waves surfers ride. Researchers suspect that this turbulent motion may help explain why the corona is 1,000 times hotter than the surface of the sun. #

  • Brazilian Barrier Islands

    Brazilian Barrier Islands

    Barrier islands are in a constant state of flux due to the currents, tides, and winds that surround and shape them. This satellite image of islands off the Brazilian coast shows meandering waterways and the mixing of sediment from the land into the sea. Often, secondary flows are responsible for shaping of these sorts of geographic features. #

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