The Gimli Glider

In class today we discussed fluid properties such as viscosity and density. It's a good idea to have a 'gut' knowledge of the magnitude of these quantities so you have a better chance of knowing when your calculations are off - a sort of "Wait a minute, that doesn't make any sense" ability. For a classic example of what can happen if you - don't - I offer you the Gimli Glider, described here.

Acoustics in the Seas

In class we talked about the speed of sound. While we won't be doing much acoustics in this class, it's a fun topic. While we usually think of sound propagating through air, in fact it is just as important how sound propagates through water - particularly if you are on a submarine! For just about everything you ever wanted to know about sound in the sea, visit the website here. The history of underwater acoustics near the bottom of this page is particularly fascinating.

The Boston Molasses Flood

One of the fluid properties we talked about last Thursday was the coefficient of thermal expansion. To see an extreme example of what can happen if this property is ignored, check out the link given here. We'll look at this case again a little later, calculating the velocity profile in a homework problem.

Most of an iceberg is underwater - so what does it look like?

In class we talked about the fraction of an iceberg which was below the waterline. Since the part above the waterline collects dirt, debris, and snow you get the usual "white" iceberg. Every so often, one flips over. To see what the underside looks like, go here.

The Johnstown Flood

For homework you solved for the tremendous forces on a dam well below waterline. A famous (and tragic) example of what can happen if these forces get a little too large is the Johnstown, West Virginia dam failure late in the 19th century. For contemporary New York Times articles on the disaster, click here. A summary of the disaster can be found here.

Cosmic Evolution

In class we've been dealing with conservation of mass and the continuity equation. Perhaps the ultimate in matter conservation problems is the study of cosmology - the evolution of the universe we live in. An interesting guide to the current theory of cosmic evolution from the Big Bang to the present is given here.

Chaos Theory

In our class demonstration today we looked at how a thin drop of dye (water) breaks up when dropped onto a concentrated surfactant (Triton X-100). The water drop was torn apart forming streamers and several generations of a fractal pattern. Fractals are the result of non-linear dynamical systems, and are often found in fluid mechanics. For a website with lots of interesting videos of fractals, you can go here. The fractal we saw in class is the same as the "Bloom" fractal, the fourth one down on the page. The site also provides an elementary course on fractals given here.

Pneumatics From a Looong Time Ago!

In class today we demonstrated "Hero's Fountain", an example of hydrostatics which goes back to Hero of Alexandria over two millenia ago - fluids research has been going on a long time! To learn more about this early fluid mechanic read the translation of his work from Greek given here. The Editor's and Translator's Prefaces are also interesting.

Crookes Radiometer

In our class demonstration today we looked at Crookes' Radiometer, which was initially thought to be a demonstration of the momentum associated with quanta of light. It took many years before the noted hydrodynamicist Osborne Reynolds (1879) was able to show that it was actually a demonstration of the migration of molecules due to thermal gradients around the edges of the vanes. For a detailed explanation of the phenomenon and its history, go here.

Pyroclastic Flows

In class today we demonstrated the phenomenon of gravity driven flow down an inclined plane. There are many examples of this, both in the laboratory and in nature. Perhaps the most spectacular, however, (and certainly the most deadly) are the pyroclastic flows associated with volcanic activity. A good description of such flows may be found here.

Osborne Reynolds

The transition to turbulence for Poiseuille flow was first identified and described by Osborne Reynolds, a famous fluid mechanic of the late 19th century. He showed that the transition occurred when the ratio of inertial forces to viscous forces exceeded a critical value, now named the Reynolds number in his honor. The importance of this parameter to fluid mechanics cannot be overstated, since as we shall see it controls the flow pattern found in nearly every geometry. A good biography of Reynolds may be found here. A description of how Reynolds determined this dimensionless group is given here.

G. I. Taylor

Sir Geoffrey Ingram Taylor is generally regarded as the preeminent fluid dynamicist of the 20th century. His genius was to combine detailed mathematical treatment of complex phenomena with simple, precise experimental results. In class we demonstrated one of the many phenomena associated with his name - the Taylor-Couette instability. For a discussion of this and other phenomena associated with G. I. Taylor, go here. The reference to Taylor-Couette flow is about half-way through the article (download the pdf file). A brief biographical sketch of G.I. Taylor is given here.

Ship Modeling at the David Taylor Model Basin

A classic example of the use of dynamic similarity and the laws governing scale-up is the design of ships. In the U.S. the main ship model design and testing center is the former David Taylor Model Basin (now the Carderock Division of the Naval Surface Warfare Center). While you can't really keep the Reynolds number constant between model and the full scale ship, you can keep the Froude number constant. This allows you to see how hull design, etc., affects bow waves, wakes, and drag. Provided the Reynolds number is "high" for both model and full scale (so that viscous effects can be neglected), the drag on the model can be related to that on the full scale ship. The history of the model basin may be found in an ASME brochure given here.

Dimensional Analysis and the Bomb

In class we talked about the example of estimating the yield of a point explosion through dimensional analysis, solved for early atom bomb tests by G.I. Taylor. A nice discussion of his result is provided here and the complete paper (publication of which caused a bit of a stir, as the yield was still classified at the time) can be found here. It's a good illustration of the power of dimensional analysis to solve complicated problems.

Lab-on-a-Chip

Lubrication flows are just one example of flows at low Reynolds numbers where inertia may be neglected. One active area of research in low Re system is the field of microfluidics, in which chemical laboratories are reduced to a microscale and reactants are pumped around through microchannels etched on a chip. Wikipedia does a decent job of describing some of the uses of this technology. That article can be found here.

Ice Skating - Why You Slide

One of the most interesting examples of lubrication flows is that found in ice- skating. In that winter sport, the friction between the blade and the ice melts a small quantity of water, forming a thin lubricating layer. The old story that it's the pressure of the blade which melts the ice isn't true - and the later theory that it's the frictional heating that does the trick isn't quite right either. Instead, the current theory is that there is a very thin layer of water at the surface of ice even well below freezing due to thermodynamic effects. A good discussion of the effects of a thin layer of water on ice (taken from a Physics Today article) is given here.

Snowflakes

Winter is coming, and with it snow! One subject which has always been of interest to me is the wonderful shapes of snowflakes. If you've ever wondered why they grow as they do, you should explore the link given here - an entire website devoted to the detailed understanding of ice crystals.

Snapping Shrimp

Fluid mechanics is important in a wide range of biological systems. At the cellular level the Reynolds number is usually quite low, and viscous forces are dominant. At the organism level the Reynolds number is usually high and inertial forces are important. Occasionally flows are so fast that cavitation effects are important! A really cute example of such a flow is the phenomenon of the snapping shrimp, described here.

A Really Big Flow Meter

In class today we talked about Bernoulli's Equation and how it could be used to solve for pressure distributions - and how at high Re the pressure goes down when the velocity goes up! A classic example of how this can be employed is the Venturi meter. Basically, the flow rate of a fluid is measured by determining the pressure differential produced by a slight contraction of a pipe. While usually little beasties (e.g., the diameter of a pipe), sometimes they can grow to truly gargantuan proportions. A press release describing what is billed as the world's largest flow meter is given here. You can also go directly to a picture of the thing here. The meter itself is a bit too big to put in the undergrad lab - in fact it approaches the size of Fitzpatrick!

Insect Flight

Today we talked about inviscid flow past a cylinder, and by analogy invicid flow past a wing. The wings on airplanes are really very simple things in comparison to the complex dynamics associated with the flight of living organisms such as birds and insects. The key difference is that the wings of planes pretty much stay fixed in orientation while air blows over them, while for animals the wing is in usually in constant motion. This is particularly true for insects: it has often been said that a bumblebee really shouldn't be able to fly, yet somehow it manages anyway. For a fascinating look at the world of insect flight, check out the research being done at Berkeley described here.

Vortex Shedding

Boundary layer flows lead to fascinating phenomena. One key problem in flow past a bluff body at high Re is boundary layer separation, and the transient wakes which form behind the body. These vortices often become detatched from the object leading to what is called a von Karman Vortex Street. As the eddies are shed from alternate sides of the body, there are often large lateral forces exerted on the body. To see a little movie of the vortex shedding process, go here. These lateral forces due to vortex shedding can interact with resonant frequencies of a structure leading to an effect called aeroelastic flutter. Probably -the- classic example of this is the Tacoma Narrows bridge disaster. A nice collection of movies of the bridge is given here. The aeroelastic instability isn't all bad, though - a company in Spain has developed into a new method for harvesting energy from the wind that they claim is much cheaper than conventional windmills. A description of the device may be found here.

Super Soakers

A fun application of Bernoulli's equation to summer fun is the Super Soaker! To see detailed information on how such squirt guns work, check out the link given here.

Boundary Layer Control

For homework this week you are asked to solve for the velocity profile in boundary layer flow along a flat plate with suction - one of the techniques used to control boundary layer growth and separation. A Physics Today article on the history of Prandtl's boundary layer theory and experiments is given here. A really lovely picture of the effect of suction on boundary layer separation over a wing may be found here. Another link at the same site (given here) has some interesting examples of experimental planes where the technique was used.

Drag Reduction via Turbulence

In class we discussed the boundary layer separation which occurs as a result of the adverse pressure gradient on the back side of a sphere or other bluff body. This separation may be delayed, and the drag reduced via pressure recovery, when the boundary layer becomes turbulent. For a nice discussion of this transition, and associated drag reductions, check out the link given here. In addition to general information on drag reduction, this website also contains just about everything you would want to know about the aerodynamics of bicycle design!

Boundary Layer Separation and the Frisbee

An interesting example of boundary layer separation, turbulence, and angular momentum is the design of a frisbee. Basically, the angular momentum produced by the spin of the frisbee (and the weighted edges) keeps the frisbee stable (constant orientation) while the aerodynamic lift produced by its motion relative to the air keeps it up. Boundary layer control is important too: The ridges around the edge of the frisbee promote turbulence and delay boundary layer separation, reducing drag. The first frisbees tended to be unstable and didn't fly terribly well. It wasn't until the mid-60's that the frisbee as we now know it was developed. The patent on the ridges can be found here.

How to Size a Pump

There are many sources of information on sizing pumps and designing piping networks available on the web (although they do tend to disappear over time!). A comprehensive one is given here. It would be a -really- good idea to review it if you are ever in charge of sizing a pump! Pump curves are also available on-line. As as example, those for Sykes pumps (such as we looked at in class today) are given here. Other links are available on the web for pipe fittings. A useful one is given here. Examination of the data shows the K factors -do- change a little bit with diameter, but not very much!