FYFD

Celebrating the physics of all that flows. Ask a question, submit a post idea or send an email. You can also follow FYFD on Twitter and YouTube. FYFD is written by Nicole Sharp, PhD.


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Posts tagged "streamlines"

After visiting an aquarium or snorkeling near a reef, you may have wondered why fish come in so many different shapes. Given that all fish species need to get around underwater, why are some fish, like tuna, incredibly streamlined while others, like the box fish, are so, well, boxy? There are several major groupings for fish based on their shape and how they propel themselves, whether it’s by undulating their body and tail or primarily by flapping their fins. Which grouping a fish tends toward depends largely on its environment and needs. Open-water swimmers tend to use their bodies and tails. Their bodies are better streamlined, too, allowing them to outrace even some ships! Fish that live in more complicated environments, like along the seafloor or in a reef, tend to favor maneuverability over speed. These fish – which include rays, pufferfish, and surgeonfish – use their fins for their main propulsion. Many of these species are still faster swimmers than you or I, but their slower speeds have reduced their need for hydrodynamic streamlining, allowing these fish to evolve a wide variety of odd body shapes. (Video credit: TED-Ed)

One of the challenges of dealing with objects at the microscale is finding ways to manipulate them. This is what techniques like optical tweezers or magnetic traps are used for. The downside to these methods is that they often require complex experimental set-ups or place restrictions on the kinds of particles that can be manipulated. Recently, however, researchers have developed a new hydrodynamic alternative: the Stokes trap.

Using a six-channel microfluidic device like the the ones shown in A) and B) above, scientists can alter the flow in the device in such a way that they trap and manipulate two particles at the same time. The simultaneous inflow and outflow in the device creates streamlines like those shown in C) and D) above. The large white areas where the streamlines converge and diverge are stagnation points–areas of little to no velocity. The scientists trap their particles at the stagnation points and then carefully shift the flow rates into and out of the device to move the stagnation points–with particles in tow–wherever they want them. In the animation, you can see part of a movie where they use the particles to write out a capital I (for University of Illinois). The researchers hope the technique will be used in the future for studying the physics of soft materials and biologically-relevant molecules like DNA. For more, check out the full paper or the group’s website.  (Image credit and submission: C. Schroeder et al.)

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)

Here’s a potential flow field with heart-shaped streamlines, made just for you. Thank you to everyone for having helped made FYFD such a success over these 700 posts, whether by liking, reblogging, tweeting, or telling a friend. Happy Valentine’s Day!

For the curious among you, the flow is a superposition of uniform flow, two sources, and two sinks. The Matlab code is here. Have fun!

Flow visualization is a powerful design tool for engineers. When Google was interested in determining optimal configurations for their heliostat array, they turned to NASA Ames’ water tunnel facility to test upstream barriers to deflect flow off the heliostats.   In each photo, flow is from left to right and fluorescent dye is used to mark streamlines and reveal qualitative flow detail. Upstream of the obstacles, the streamlines are coherent and laminar, but after deflection, the flow breaks down into turbulence. In this case, such turbulence is desirable because it lowers the local fluid velocity and thus the aerodynamic loads experienced by each heliostat, potentially allowing for a savings in fabrication. For more, see Google’s report on the project. (Photo credits: google.org)

A smoke wire shows the deformation of streamlines around a swept-winged micro air vehicle (MAV). These crafts typically feature wingspans smaller than one foot and, thus, never develop the type of flow fields associated with larger fixed-wing airplanes. This complicates theoretical predictions of lift and drag for MAVs as well as making them difficult to control. MAVs have numerous commercial and military applications, including search and rescue operations. (Photo credit: Tom Omer)

Flow visualization in a water tunnel shows what the flow around a line of traffic looks like. Note the progressively more turbulent flow around each car as it sits in the wake of the car before it. Turbulent flow is usually associated with increased drag forces, but because turbulence can actually help prevent flow separation it is sometimes desirable as a method for decreasing drag. In the case of these cars drafting on one another, it is clear that the cars further back in the line cause less effect on the fluid–and thus have less drag to overcome–than the front car.  (Photo credit: Rob Bulmahn)

One of the topics in fluid dynamics almost everyone has come across is the explanation of how airplanes produce lift. Using Bernoulli’s principle–which relates velocity and pressure–and a picture of an airfoil, your average science text will say that a bit of air going over the top of the airfoil has to travel farther than a bit of air going under the airfoil, and that, therefore, the air over the top travels faster than the air under the airfoil.

Unfortunately, this is misleading and, depending on the wording, outright wrong! The hidden assumption in this explanation is that air that goes over the top and air that goes under the bottom have to reach the trailing edge of the airfoil at the same time. But why would that be? (As one of my profs once said, “There is nothing in physics that says there is Conservation-Of-Who-You-Were-Sitting-Next-To-When-You-Started.”)

Take a look at the video above. It shows an airfoil in a wind tunnel using smoke visualization to show how the air moves. Around the 0:25 mark, the video slows to show a pulse of smoke traveling over the airfoil. What happens at the trailing edge? The smoke going over the top of the airfoil is well past the trailing edge by the time the smoke going under the airfoil reaches the trailing edge!

It’s true that air goes faster over the top of the airfoil than the bottom and that this causes a lower pressure on top of the airfoil (as Bernoulli tells us it should) and that this causes an upward force on the airfoil. But which causes which is something of a chicken-and-egg problem.

A more straightforward way, in my opinion, of explaining lift on an airplane is by thinking about Newton’s 3rd law: for every action, there is an equal and opposite reaction. Take a look at the air’s movement around the airfoil as the angle of attack is increased around 1:00 on the video. Just in front of the airfoil, the air is moving upward. Just after the airfoil, the air is pointed downward. A force from the airfoil has pushed the air down and changed its direction. By Newton’s 3rd law, this means that the air has pushed the airfoil up by the same amount. Voila! Lift!