Tag: bernoulli

Fluids Round-up – 2 November 2013
Fluids round-up time! Here are your latest links:
- Over at PhysicsFocus, Colin White discusses the Bernoulli fallacy and other zombie myths of physics. (Via @JenLucPiquant)
- Aviation Week has an exclusive look at Skunk Works’ SR-72 next-gen hypersonic aircraft.
- MinutePhysics asks if it’s better to walk or run through rain. This post has another take on the question.
- io9 describes why bubbles lose their color as they pop.
- Physics Buzz looks at knotted fluid vortices. They also have a nice write-up on the foaming of a struck beer, which we talked about last week.
- Enjoy the beauty of mathematics next to the physics they describe. (via io9)
- More fun fluids from Physics Buzz, this time looking at new tiny jellyfish-like flying robots.
- Remember the Chelyabinsk meteor from February? Discovery reports on an analysis of the air burst and its probability.
- Is there fluid mechanics in neck cracking? (?!?)
- New research shows that mesoscale self-assembly can be achieved using capillary charges.
- Finally, our lead image shows a simulation of turbulent flow in a tightly packed lattice of spheres. It’s an entry from Argonne National Laboratory’s annual “Art of Science” contest. Take a look at the entries and vote for your favorites!
While not strictly fluid dynamical, I want to take a moment to talk about education. I receive a lot of stunned reactions and self-deprecation when people learn I study aerospace engineering. Many people say, “Oh, I could never do that!” or “You must be some kind of genius.” I’m not. It’s true that studying engineering and fluid dynamics involves a lot of math and some it is complex (no pun intended). There’s a lot of unfounded fear about science and math in our society, when really they are just skills that any of us can improve with practice and effort. So, for those out there who have ever thought, “I can’t do that, there’s too much math,” please watch this young woman address mathphobia. She sums up just about everything I’ve always wanted to tell you.
(Photo credit: Argonne National Laboratory)
November 2, 2013
Fluid Juggling
It’s that time of the year – the 2013 APS Division of Fluid Dynamics meeting is not far off, and entries to this year’s Gallery of Fluid Motion are starting to appear. This week we’ll be taking a look at some of the early video submissions, beginning with one that you can recreate at home. This video demonstrates a neat interaction between a slightly-inclined liquid jet and a lightweight ball. The jet can stably support–or, as the authors suggest, juggle–the ball under many circumstances, as seen in the video. Initially, the jet impacts near the bottom of the ball and then spreads into a thin film over the surface. This decrease in thickness between the jet and the film is accompanied by an increase in speed due to conservation of mass. That velocity increase in the film corresponds to a pressure decrease because of Bernoulli’s principle. This means that there is a region of higher pressure where the jet impacts the ball and lower pressure where the film flows around the ball. Just as with airflow over an airfoil, this generates a lift force that holds the ball aloft. (Video credit: E. Soto and R. Zenit)
October 14, 2013
Streamlines in Oil
Bernoulli’s principle describes the relationship between pressure and velocity in a fluid: in short, an increase in velocity is accompanied by a drop in pressure and vice versa. This photo shows the results left behind by oil-flow visualization after subsonic flow has passed over a cone (flowing right to left). The orange-pink stripes mark the streamlines of air passing around the Pitot tube sitting near the surface. The streamlines bend around the mouth of probe, leaving behind a clear region. This is a stagnation point of the flow, where the velocity goes to zero and the pressure reaches a maximum. Pitot tubes measure the stagnation pressure, and, when combined with the static pressure (which, counterintuitively, is the pressure measured in the moving fluid), can be used to calculate the velocity or, for supersonic flows, the Mach number of the local flow. (Photo credit: N. Sharp)
August 20, 2013
Egg-Spinning Fun
If you have any leftover hard-boiled eggs, you can recreate this bit of fluid dynamical fun. Spin the egg through a puddle of milk, and you’ll find that the egg draws liquid up from the puddle and flights it out in a series of jets. As the egg spins, it drags the milk it touches with it. Points closer to the egg’s equator have a higher velocity because they travel a larger distance with each rotation. This variation in velocities creates a favorable pressure gradient that draws milk up the sides of the egg as it spins, creating a simple pump. To see the effect in action check out this Science Friday video or the BYU Splash Lab’s Easter-themed video. (Photo credit: BYU Splash Lab)
April 1, 2013
Stalling
[original media no longer available]
At high angles of attack, the flow around the leading edge of an airfoil can separate from the airfoil, leading to a drastic loss of lift also known as stall. Separation of the flow from the surface occurs because the pressure is increasing past the initial curve of the leading edge and positive pressure gradients reduce fluid velocity; such a pressure gradient is referred to as adverse. One way to prevent this separation from occurring at high angle of attack is to apply suction at the leading edge. The suction creates an artificial negative (or favorable) pressure gradient to counteract the adverse pressure gradient and allows flow to remain attached around the shoulder of the airfoil. Suction is sometimes also used to control the transition of a boundary layer from laminar to turbulent flow.
February 15, 2013
Reader Question: Dry Rear Windshields in the Rain
Reader sheepnamedpig asks:
I was driving through the rain down the highway when I noticed something strange: though the rain was heavy enough to reduce visibility to a quarter mile, the rear windshield of my Corolla was bone dry except for the streams of water flowing off the roof. There was no wind so far as I could tell, but I had to slow down all the way to ~20-25 mph for rain to start falling on the rear windshield. Why is that?
That’s a wonderful observation! Like many sedans, your Corolla has a long, sloped rear window that acts much like a backward-facing step with respect to the airflow while the car is moving. Note the smoke lines in the photo above. At the front of the car, we see closely spaced intact lines near the hood and windshield, indicating relatively fast, smooth airflow over the front of the vehicle. At the back, though, there is a big gap over the rear windshield. This is because flow over the car has separated at the rear windshield and a pocket of recirculating air. This recirculation zone is, for the most part, isolated from the rest of the air moving over the car; that’s why the smoke lines continue relatively unaffected a little ways above the surface. This same pocket of recirculating air is protecting your rear windshield from rainfall. It’s an area of low-speed, high-pressure fluid, and the raindrops are preferentially carried by the high-speed, low-pressure air over the recirculation zone. This is one reason why many sedans don’t have rear windshield wipers. (Photo credit: F-BDA)
ETA: Reposted by request to make it rebloggable.
November 29, 2012
Magnus Force
Physics students are often taught to ignore the effects of air on a projectile, but such effects are not always negligible. This video features several great examples of the Magnus effect, which occurs when a spinning object moves through a fluid. The Magnus force acts perpendicular to the spin axis and is generated by pressure imbalances in the fluid near the object’s surface. On one side of the spinning object, fluid is dragged with the spin, staying attached to the object for longer than if it weren’t spinning. On the other side, however, the fluid is quickly stopped by the spin acting in the direction opposite to the fluid motion. The pressure will be higher on the side where the fluid stagnates and lower on the side where the flow stays attached, thereby generating a force acting from high-to-low, just like with lift on an airfoil. Sports players use this effect all the time: pitchers throw curveballs, volleyball and tennis players use topspin to drive a ball downward past the net, and golfers use backspin to keep a golf ball flying farther. (Video credit: Veritasium)
October 1, 2012
The Backward-Facing Step
This photo collage shows vortices shed off a backward-facing step. The flow is left to right. Here the flow is visualized using dye released in water. Initially, the vortex forms near the bottom of the step in the recirculation zone. Because flow over the top of the vortex is much faster than the flow beneath the vortex, a low pressure zone forms over the vortex and gradually draws it up toward the top of the step. Eventually the vortex will rise to the point where the upstream flow pushes it downstream and the process begins anew. (Photo credit: Andrew Carter, University of Colorado)
September 26, 2012
London 2012: Soccer Aerodynamics
Corner kicks and free kicks are tough to defend in football (soccer for Americans) because the ball’s trajectory can curve in a non-intuitive fashion. Known as the Magnus effect, the fluid dynamics around a spinning ball cause this curvature in the flight path. When an object spins while moving through the fluid, it drags the air near the surface with it. On one side of the spinning ball, the motion opposes the direction of freestream airflow, causing a lower relative velocity, and on the opposite side, the spin adds to the airflow, creating a higher velocity. According to Bernoulli’s principle, this causes a lower pressure on the side of the ball spinning with the flow and a higher pressure on other side. This difference in pressure results in a force acting perpendicular to the direction of travel, causing the unexpected curvature in the football’s path. In the case of the corner kick above, the player kicks the ball from the right side, imparting an anti-clockwise spin when viewed from above. As the ball travels past the goal, air is moving faster over the side nearest the goal and slower on the opposite side. The difference in velocities, and thus pressures, creates the sideways force that drives the ball into the goal even without touching another player. The same effect is used in many other sports to complicate play and confuse opponents. In tennis and volleyball, for example, topspin is used to make the ball drop quickly after passing the net.
ETA: Check out this other great example of a free kick sent in by reader amphinomos.
FYFD is celebrating the Olympics by featuring the fluid dynamics of sport. Check out some of our previous posts including the flight of a javelin, how divers reduce splash, and what makes a racing hull fast.
August 9, 2012
Circulation Around an Airfoil
As a followup to yesterday’s question about ways to explain lift on an airfoil, here’s a video that explains where the circulation around the airfoil comes from and why the velocity over the top of the wing is greater than the velocity around the bottom. Kelvin’s theorem says that the circulation within a material contour remains constant for all time for an inviscid fluid. Before the airplane moves, the circulation around the wing is zero because nothing is moving. As shown in the video, as soon as the plane moves forward, a starting vortex is shed off the airfoil. As the plane flies, our material contour must still contain the starting position and thus the starting vortex. However, in order to keep the overall circulation in the contour zero, the airfoil carries a vortex that rotates counter to the starting vortex. This is the mechanism that accelerates the air over the top of the wing and slows the air around the bottom. Now we can apply Bernoulli’s principle and say that the faster moving air over the top of the airfoil has a lower pressure than the slower moving air along the bottom, thus generating an upward force on the airfoil. (submitted by jessecaps)
February 1, 2012