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Beautiful fluid dynamics images on an earthly scale!

Jun 29, 2011

I am always struck by the often beautiful world of flow measurement. Even though we design and manufacture precision flow meters and controllers for nearly any gas, liquid or steam flow application, the images linked below explain why there are so many different measurement technology solutions (Vortex, Thermal, Ultrasonic, Magnetic, Coriolis…etc.) to handle the variety of fluid flow applications found on planet earth.  Enjoy these images I found that demonstrate the beauty and often challenging dynamics of fluid flow.

>>>View Images…

[Courtesy of WiReD.com]

Here is my take on the images…

Fluid dynamics, be them internal or external flows, are generally dictated by one of a few sets of related single or multi-variable systems of differential equations. While the math, quite obviously, boggles the mind of most, the patterns they make are by no means alien. In the pictures shown in WiReD’s article, we see the basic principles of flow dictated in vivid living color. While some explanation is provided, I wanted to share some more thoughts with you on each image:

1. SUPERSONIC SHOCK WAVE: The seemingly blunt tip of the bullet slug produces a stagnant air pressure point at the bow of the projectile, whereupon, just like our continental divide, all fluids flow in the positive or negative direction of each dimension. Because the bullet is moving faster than the speed of sound, and thus, so is the wave, the sound waves created trail off the sides of the bullet (pictured), as evident by the micro-disturbances of air (visible portions). The bullet is also creating a large amount of heat energy due to friction, leaving a turbulent, vortex-like trail behind it.

2. VON KARMAN VORTEX STREETS: This water-vapor thermal image of two Alaskan Aleutian islands illustrate the vortex flow principle, as common in nature. This is shown in smaller patterns on the right. As the wind encounters the island, it creates a negative pressure on the backside, pulling the wind, which was split by the obstacle, in from the sides. As the wind from one side fills the void, it creates a vortex curl behind the island, and then breaks off, bounding along in the wake the island created. This alternates from side to side, creating an alternating pattern of oppositely spinning vortices. In contrast, the left side shows two mushroom-like flow patterns common when a large bulk of air runs into another bulk of air, such as wind blowing into a cloud bank. The force of the air hitting a literal wall, causes the wall to push back slightly as the wind pushes, and diverts to either side into eddy currents.

3. The Von Karman Vortex Street principle is illustrated, again, this time in natural color. The clouds themselves illustrate the pattern majestically.

4. INVERTED DRAFTING: Drafting is the principle by which, when two objects are in a flow stream, the follower encounters less resistance, or drag, than the leader.  A classic example of this is the flying ‘V’ pattern exhibited by migratory birds. In said case, the birds are riding on the wake/stream interface, working off the same flow disturbance-wave angle as the other birds on each side of the V. There need not be two sides to the V, however, utilizing such makes for a tighter bunch.   We also see this phenomenon illustrated for us at high speeds as a key competitive advantage in NASCAR where races are often won and lost based on the drivers ability to draft.  In inverted drafting, the contextual nature of the disturbance the follower makes provides for less resistance on the leader. In the case shown (two flags), the vortices from the front flag are absorbed by the disturbances of the back flag, thereby effectually eliminating the leader’s wake before it has a chance to propagate and build on itself to create significant pull on the offending object. The reduction in drag provides for an actualized reduction in wind resistance.

5. SAFFMAN-TAYLOR INSTABILITY: Unlike fluids do not mix, and do not mix they do… with style! One of the two fluids, generally the one added to the other, will seemingly reach for escape, creating long, narrow deltas and finger-like patterns across the existing substrate. The patterned fingers and continual splitting and reorganization of such, often with congruences, are a function of both static and dynamic viscosity.

6. Same Saffman-Taylor Instability principle of viscosity, different example.

7. CONVECTION: Fluids and heat flow much in the same way; so much so, in fact, they are often taught at universities as a combined subject: Heat and Mass Transfer. Here we have a goblet filled with warm liquid. As the air inside and around the glass heats up, the air rises, creating a column of heat. As the stream dissipates, and the environment surrounding it cools the air it contains, said air streams back down along the outside of the column, into the environment. It is interesting to note that our thermal mass flow meters operate based on these same general principals of heat transfer found in nature–however we are isolating out what is called forced convection (the mass flow of the gas in the pipe and the measurement we want) from natural convection, radiation and conduction, it is interesting to see it illustrated in this format.

8. Same principle of Convection, but with a cold ice cube in warm liquid. The cold liquid melting off the ice sinks in the warmer environment, dissipating along the bottom of the container.

9. CAVITATION: When objects cut through a liquid with enough speed, they create a void on the liquid. The void is absent of atmosphere. After forming, the cavity, lacking outward pressure to support it, collapses, creating a resulting jet and back jet. In this picture, a laser is used to accelerate photons against the water (object at high speed). This creates a bubble just under the surface, which, in turn, collapses, sending a jet of water upward.

10. This video illustrating the principle of Cavitation, using a water droplet at 0G and an electrode, gives a live demonstration of cavitation without the obvious object in the way.

11. KELVIN-HELMHOLTZ INSTABILITY: When two flows of different speeds move in parallel, they create a friction boundary layer between them. In the picture provided, this change in temperature and pressure at the boundary layers has created clouds, visually illustrating this principle for us.

12. The Kelvin-Helmholtz Instability is evident in space, where we see the same mathematically-based flow principle appear between atmospheric bands on Saturn.

13. BOUNCING JET: Surface tension and viscosity give the surface of liquids an effectual ‘skin.’ We can see this when a drop of water sits still on a flat surface. Given a tuned jet, we can literally bounce one body of liquid off another. In order to increase the force needed to move the jet and impart a significant amount of frictional resistance against it, the medium pool is flowing. As the liquid moves, it translates the plunging jet horizontally, changing the flow dynamics of the path, and given the frictional force created by the flowing liquid, and that of the jet pushing air against the flowing liquid, a pocket is formed on top of the flow which curls the jet back upward.

14. See the Bouncing Jet principle in motion. Flow never ceases to amaze me.

Related Post: http://www.sierrainstruments.com/blog/?p=443 discusses the need to manage flow profile to optimize flow measurement and gives solutions & a video.

Morgan Zealear, Engineer
Written By:
Morgan Zealear, Engineer
Sierra Instruments

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