gorbax asks: I’ve been wondering for a while, actually, how do we know when the method of flow visualization doesn’t actually alter the flow of a fluid itself?
This is a great question and one that fluid dynamicists have to deal with all the time. Ideally, we’d love to measure everything we want from a flow at all points at all times without doing anything to affect it. In reality, however, that just doesn’t happen. Some measurement techniques are less intrusive than others, but just about everything risks having some effect. This raises two questions: 1) How small can we make that effect? and 2) Do we even care if we’re affecting the flow?
With regards to the first, the onus is typically on the experimentalist to show that whatever visualization technique he/she uses is not significantly affecting the flow. For something like particle image velocimetry, which requires seeding the flow with particles, this means selecting particles that follow the flow rather than changing it and considering carefully how and where to seed the flow such that any added vorticity from the injection does not alter the flow significantly. Checking for this can be done many ways, for example with comparisons to other measurement techniques (with and without seeding) or by comparing to simulation.
The second question–do we care?–is also a significant consideration. Because the purpose of flow visualization is often to get a qualitative feel for the flow field rather than quantitative information, it is often not a significant concern if there is some slight effect from the visualization technique. This can often be the case with smoke-wire and dye visualizations where we just want to see what’s going on.
Finally, there are some instances of flow visualization which are completely unobtrusive to the flow. Schlieren photography and infrared thermography are two examples. Both are optical techniques that act from a distance and take advantage of extant flow properties to make certain features visible. The real key is knowing what technique(s) will work for the flow you have and will give you the information you want. After that, it’s all about proper and thorough execution. (Photo credits: N. Vandenberge et al., T. Omer, M. Canals, P. Danehy et al., A. Wilkens et al., W. Saric et al.)
Reader Question: Standing Waves
captainandry asks:
What would happen to a fish or swimmer in a standing wave?
First of all, check out the video that inspired this question, which shows a standing water wave created in a wave tank. Before we tackle the standing wave, it’s helpful to know what motion exists in a typical water wave. For deep water waves, the motion of a particle as the waves pass is circular, with a decreasing radius with increasing depth. Below a certain depth the energy of the surface wave doesn’t penetrate. Here’s an animation, where the red dots represent massless particles and the blue circles show their paths:
In shallower waters, the circular paths get compressed into ellipses. The image below shows pathlines for particles at different depths as a water wave passes. Notice how the paths are circular near the surface, where the depth is much greater than the wavelength, while close to the bottom, the pathlines are elliptical.
So what about motion for a standing water wave? Such a wave has no apparent horizontal motion, as seen in the animation below:
Similar to the way that decreasing the depth compresses the circular particle motion into an ellipsoid, creating a standing wave compresses the horizontal motion of any particle near the surface. What this means is that anything floating near the surface of the standing wave will simply bob up and down. Unless it’s located at one of the nodes (marked by red dots), in which case it won’t move at all! As with the other types of water waves, the amount of displacement will decrease with depth. People and fish, of course, are not massless particles, so their motion will be damped by inertia, but the same principles apply.
(Photo credits: P. Videtich; R. L. Wiegel and J.W. Johnson; Wikipedia)
Reader Question: Frosty Cars
Reader Mike L asks:
Why do I never see frost on my car when I park in a detached garage or under a carport?
Great question! Frost forms on surfaces when their temperature drops below the freezing point of water and the dew point of the surrounding air. The water vapor in the air gets deposited as a solid directly; this is called deposition. This means that the surface–in this case your car–has to be colder than the nearby air. Neither conduction nor convection of heat between your car and the surrounding air can cause this drop; heat transfer between your car and the surrounding air would tend to make them the same temperature, not make the car colder than the air. The third–and typically least effective–type of heat transfer, radiation, is the answer because it allows heat transfer between two objects that are not in direct contact like the air and car are.
Frost typically forms on still, clear nights with little clouds or wind. A car sitting beneath a clear night sky will radiate heat out into space. Since space is much, much colder than the air, this radiation cooling to space allows the car’s surface temperature to drop below that of the surrounding air, which is not a good radiator by comparison. On a night with little wind (and thus little convection), this radiation cooling can be quite effective. Frost will tend not to form on one’s car under a carport because the car is sheltered from the night sky, blocking such radiative cooling. Having a tree or house blocking the car from the night sky is also effective at preventing frost formation. (Photo credit: N. Sharp; with thanks to Keri B and Jerry N for the meteorological assistance)
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.
Getting Ketchup to Flow
Most everyone is familiar with the difficulty of getting ketchup out of its bottle. Part of the trouble is that ketchup is a shear-thinning fluid, meaning that its viscosity decreases with an increasing rate of shear. Thus, a shear-thinning fluid flows better once it starts moving. This is why the ketchup moves much faster once it is initially disturbed. LiquiGlide, a new coating material demonstrated above, has gained a lot of popular attention in the press recently for solving the difficulty of the stuck condiments. It appears that the coating reduces the static coefficient of friction between the food and the bottle, meaning that the ketchup starts sliding down the wall even before an increase in shear stress starts the flow. (submitted by @szescstopni)
Reader Question: Drafting in Cycling
jonesmartinez asks:
As a cyclist, I’m curious about drafting. How fast do I need to be going for there to be a measurable benefit? Additionally, often in a time trial a single rider is often followed by the team car and I’ve heard the rider can be pushed by the air around the team car. Any truth to this rumor? Thanks, I love the blog.
Drafting plays a major role in cycling and its tactics (check out our previous series on cycling). In general, drag increases with the square of velocity and data show this holds for cyclists. The rule of thumb I’ve heard given is that aerodynamic drag doesn’t play a large role below 15 mph, but I have not seen the numbers that inform that claim. Moreover, you have to consider the resultant airspeed around the cyclist. For example, a cyclist moving 13 mph into a 15 mph headwind (28 mph effective) will be experiencing more drag than a cyclist moving 20 mph with a 10 mph tailwind (10 mph effective). With drag being reduced 25-40% by drafting a leading rider, it is almost always beneficial to get behind someone.
That said, I have seen no measurable benefit for a leading rider with a paceline behind him, even though this should, in theory, reduce the drag on the lead rider by closing out his wake. With a large object like a car behind a solo rider, there might theoretically be some benefit. However, the car would have to be driving extremely close to the rider–far closer than they do in reality.
That said, with the prevalence of power meters in the amateur market these days, I think it would be a neat project to go out and try a few of these things firsthand and see whether such tactics actually result in a measurable difference in a cyclist’s performance–though I don’t recommend riding a foot off the front or back of a car!
Reader Question: Fire as a Fluid?
Reader David L asks:
I understand that fire is a form of energy rather than a fluid in the physical/tangible sense. However, is it possible for fire to exhibit fluid-like behaviours to a certain extent.
In other words, could the dynamic properties of fire be described with pseudo-variables analogical to variables that describe a physical fluid (i.e. viscosity, density, Re, etc.)?
Actually, combustion is a major topic of research among fluid dynamicists. Since the part of fire that we identify as visible flame is a reacting mixture of gas and some solid particles, it moves according to the same equations of motion as any other gas. However, when studying combustion thermodynamical equations and chemical reactions must also be tracked in addition to mass and momentum, which makes modeling fire very difficult. Combustion plays a major role in internal flows like those in car, jet, and rocket engines. (Photo credit: master.blitzy)
Reader Question: Creeping Flow
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David asks:
I’m taking an undergraduate fluid dynamics course, and I’m having trouble understanding what a Creeping Flow exactly is. The only thing I understand about that is that the Re should be 0 or close to 0 for the flow… Could you post an example of a creeping flow please? Thank you!
Absolutely! Creeping flow, also called Stokes flow, is, like you said, a very low Reynolds number flow. It would be hard to say that the Reynolds number is zero because that would seem to imply no flow at all. Think of it instead as a Reynolds number much, much less than one. When the Reynolds number is very low, it means that viscous forces are dominating the flow. The video above shows creeping flow around a cylinder; notice how the streamlines stay attached all the way around the surface of the cylinder. There’s no separation, no turbulent wake, no von Karman vortex street. Viscosity is so dominant here that it’s damped out all of that inertial diffusion of momentum.
We’ve posted some other great examples of creeping flow, as well, though not by that name. There are the reversible laminar flow demos and various experiments in Hele-Shaw cells, all of which qualify as creeping flow because of their highly viscous nature. If you have the time, there’s also a great instructional video from the 1960s called “Low Reynolds Number Flow” (Parts 1, 2, 3, 4) starring G. I. Taylor (a famous fluid dynamicist) that is full of one demo after another.
Reader Question: Rocket Propulsion
staunchreality-deactivated20120 asks:
Hey there – Love the blog. Most interesting science blog I follow 🙂 This may be a silly question – is propulsion through space purely a function of exit velocity and catching gravity slingshots around planets, or is there enough of anything to push against for rocket propulsion?
Thanks! Glad you enjoy the blog. And your question is not silly at all.
Whether in the atmosphere or not, rocket engines always operate on the same principle: Newton’s 3rd law. For every force exerted, there is an equal and opposite reaction force. For a rocket, this means that the momentum of the rocket exhaust provides forward momentum–thrust–for the rocket. When acting in an atmosphere, the exhaust doesn’t push against the atmosphere in order to move the rocket–in fact, rockets have to overcome aerodynamic drag when in the atmosphere, which opposes their thrust.
While the operating principle of a rocket remains the same regardless of its surrounding, the ambient pressure (essentially zero in space and non-zero in an atmosphere) does affect the efficiency of the rocket’s nozzle, which can affect the exit velocity of the exhaust, and, thus, the efficiency of the rocket. Under ideal conditions, the exhaust should exit the nozzle at the same pressure as the ambient conditions–whatever they are. If the exhaust pressure is lower than the ambient, the exhaust can separate from the nozzle, causing instabilities in the flow and potentially damaging the nozzle. On the other hand, if the exhaust pressure is too high, then there is exhaust that could be turned into thrust that is going to waste. Unfortunately, matching the exhaust pressure to the ambient pressure is a function of the geometry of the nozzle, which is usually fixed. Engineers of rockets intended to fly from within the atmosphere to space usually have to pick a particular altitude to design around and deal with the inefficiencies while the rocket flies at other ambient conditions.
Outside of the physical mechanics of how thrust is produced, propulsion in space is dominated by the influence of orbital mechanics. Once in an orbit, a spacecraft will stay on that orbital path without expending any thrust. To change between orbits, it is necessary for the spacecraft–rocket or otherwise–to change its velocity–typically referred to as delta-v–by firing an engine or thruster. It’s also possible to change orbits using the gravity of other celestial bodies (Jupiter is a popular one) to change a spacecraft’s delta-v without expending propellant. However, fluid dynamics don’t play a big role in the process aside from the problems of fuel sloshing aboard the spacecraft and the actual mechanism by which thrust is produced.
That said, if anyone is interested in getting a better feel for how orbit mechanics work, I have two recommendations. The first is to watch this video of water droplets “orbiting” a charged knitting needle aboard the ISS. And the second is to play the game Osmos. It is like rocket propulsion and orbit mechanics in action!
(Photo credits: NASA, The Aerospace Corporation, Hemisphere Games)
Reader Question: How Airfoils Produce Lift
doughboy3-deactivated20120305 asks:
I’m a Undergrad Aeronautical Engineering student. I’m curious as to your opinion as to how airfoils produce lift. I know the usual theory told in this situation. However my aerodynamics professor says that there are many things going on during the flow around an airfoil. I’m hoping to get a better idea of the different mechanisms responsible for lift.
There’s a common misconception of Bernoulli’s principle that’s often used to explain how an airfoil creates lift (which I assume is the “usual theory” to which you refer), and while there are many correct (or, perhaps, more correct) ways of explaining lift on an airfoil, I think the only opinions involved are as to which explanation is best. After all, opinions don’t keep a plane in the air, physics does!
I tackled the air-travels-farther-over-the-top misconception and presented one of my preferred ways of looking at the situation in a previous post; in short, the airfoil’s shape causes a downward deflection of the flow, which, by Newton’s 3rd law, indicates that the air has exerted an upward force on the airfoil. There’s a similar useful video from Cambridge on the topic here.
Another explanation I have heard used concerns circulation and its ability to produce lift (see the Kutta-Joukowski theorem for the math). In this case, it’s almost easier to think about lift on a cylinder instead of lift on a more complicated shape like an airfoil. If you spin a cylinder, you’ll find that the circulation around that object results in a force perpendicular to the flow direction. This is called the Magnus effect and, in addition to explaining why soccer balls sometimes curve strangely when kicked, has been used to steer rotor ships. One of my undergrad aero professors used to do a demonstration where he’d wrap a string around a long cardboard cylinder and demonstrate how, by pulling the string, the cylinder’s spinning produced lift, making the cylinder fly up off the lectern and attack the unsuspecting students.
An airfoil doesn’t spin, but its shape produces the same type of circulation in the flow field. Without delving into the mathematics, it’s actually possible through conformal mapping and the Joukowski transform to show that the potential flow field around a spinning cylinder is identical to that around a simple airfoil shape! Although that mathematical technique is not all that useful in a world where we can calculate the inviscid flow around complicated airfoils exactly, it’s still pretty stunning that we can analytically solve potential flow around (and thus estimate lift for) a host of airfoil shapes on the back of an envelope.
In short, your aerodynamics professor is right in saying that there are many things going on during the flow around an airfoil. If you get a roomful of aerodynamicists together and ask them to explain how airfoils generate lift, you would be faced with a lively discussion with about as many competing explanations as there are participants. As you learn more in your classes, you’ll gain a better intuitive feel for how it works and you’ll learn more of the nuances, which will help you understand why there is no one simple-to-understand explanation that we use!**
** Lest I confuse someone into thinking that aerodynamicists don’t know how airfoils produce lift, let me add that the argument here is over how best to explain the production of lift, not over how the lift is produced. We have the equations to describe the flow and we can solve them. We know that lift is there and why. We simply like to argue over how to explain it to people without all the math.