-Fundamentals of Fluid Mechanics- Bruce Munson, Donald Young, Theodore Okiishi, Wade Huebsch

Transcription

1 -Fundamentals of Fluid Mechanics- Bruce Munson, Donald Young, Theodore Okiishi, Wade Huebsch Fluids in the News (All Fluids in the News contained here are in the print edition as indicated) Table of Contents 1. Hilsch Tube (Ranque Vortex Tube) (5 th and 6 th Edition) 2. Sonification (5 th and 6 th Edition) 3. Pistol Shrimp Confound Blast Detectors (5 th and 6 th Edition) 4. Supersonic and Compressible Flows in Gas Turbines (5 th and 6 th Edition) 5. Liquid Knife (5 th and 6 th Edition) 6. Rocket Nozzles (5 th Edition)

2 munson 6e ch11.qxd 12/18/08 5:54 PM Page 1 Hilsch tube (Ranque vortex tube) Years ago (around 1930) a French physics student (George Ranque) discovered that appreciably warmer and colder portions of rapidly swirling air flow could be separated in a simple apparatus consisting of a tube open at both ends into which was introduced, somewhere in between the two openings, swirling air at high pressure. Warmer air near the outer portion of the swirling air flowed out one open end of the tube through a simple valve and colder air near the inner portion of the swirling air flowed out the opposite end of the tube. Rudolph Hilsch, a German physicist, improved on this discovery (ca. 1947). Hot air temperatures of 260 F (127 C) and cold air temperatures of 50 F ( 46 C) have been claimed in an optimized version of this apparatus. Thus far the inefficiency of the process has prevented it from being widely adopted. (See Problems )

3 munson 6e ch11.qxd 12/18/08 5:54 PM Page 2 Sonification The normal human ear is capable of detecting even very subtle sound patterns produced by sound waves. Most of us can distinguish the bark of a dog from the meow of a cat or the roar of a lion, or identify a person s voice on the telephone before they identify who is calling. The number of things we can identify from subtle sound patterns is enormous. Combine this ability with the power of computers to transform the information from sensor transducers into variations in pitch, rhythm, and volume and you have sonification, the representation of data in the form of sound. With this emerging technology, pathologists may soon learn to hear abnormalities in tissue samples, engineers may hear flaws in gas turbine engine blades being inspected, and scientists may hear a desired attribute in a newly invented material. Perhaps the concept of hearing the trends in data sets may become as commonplace as seeing them. Analysts may listen to the stock market and make decisions. Of course, none of this can happen in a vacuum.

4 munson 6e ch11.qxd 12/18/08 5:54 PM Page 3 Pistol shrimp confound blast detectors Authorities are on the trail of fishermen in Southeast Asia and along Africa s east coast who illegally blast coral reefs to rubble to increase their catch. Researchers at Hong Kong University of Science and Technology have developed a method of using underwater microphones (hydrophones) to pick up the noise from such blasts. One complicating factor in the development of such a system is the noise produced by the claw-clicking pistol shrimp that live on the reefs. The third right appendage of the 2-in.-long pistol shrimp is adapted into a huge claw with a moveable finger that can be snapped shut with so much force that the resulting sound waves kill or stun nearby prey. When near the hydrophones, the shrimp can generate short-range shock waves that are bigger than the signal from a distant blast. By recognizing the differences between the signatures of the sound from an explosion and that of the pistol shrimp blast, the scientists can differentiate between the two and pinpoint the location of the illegal blasts.

5 munson 6e ch11.qxd 12/18/08 5:54 PM Page 4 Supersonic and compressible flows in gas turbines Modern gas turbine engines commonly involve compressor and turbine blades that are moving so fast that the fluid flows over the blades are locally supersonic. Density varies considerably in these flows so they are also considered to be compressible. Shock waves can form when these supersonic flows are sufficiently decelerated. Shocks formed at blade leading edges or on blade surfaces can interact with other blades and shocks and seriously affect blade aerodynamic and structural performance. It is possible to have supersonic flows past blades near the outer diameter of a rotor with subsonic flows near the inner diameter of the same rotor. These rotors are considered to be transonic in their operation. Very large aero gas turbines can involve thrust levels exceeding 100,000 lb. Two of these engines are sufficient to carry over 350 passengers halfway around the world at high subsonic speed. (See Problem )

6 munson 6e ch11.qxd 12/18/08 5:54 PM Page 5 Liquid knife A supersonic stream of liquid nitrogen is capable of cutting through engineering materials like steel and concrete. Originally developed at the Idaho National Engineering Laboratory for cutting open barrels of waste products, this technology is now more widely available. The fast moving nitrogen enters the cracks and crevices of the material being cut then expands rapidly and breaks up the solid material it has penetrated. After doing its work, the nitrogen gas simply becomes part of the atmosphere which is mostly nitrogen already. This technology is also useful for stripping coatings even from delicate surfaces.

7 munson 6e ch11.qxd 12/18/08 5:54 PM Page 6 Rocket nozzles To develop the massive thrust needed for space shuttle liftoff, the gas leaving the rocket nozzles must be moving supersonically. For this to happen, the nozzle flow path must first converge, then diverge. Entering the nozzle at very high pressure and temperature, the gas accelerates in the converging portion of the nozzle until the flow chokes at the nozzle throat. Downstream of the throat, the gas further accelerates in the diverging portion of the nozzle (area ratio of 77.5 to 1), finally exiting into the atmosphere supersonically. At launch, the static pressure of the gas flowing from the nozzle exit is less than atmospheric and so the flow is overexpanded. At higher elevations where the atmospheric pressure is much less than at launch level, the static pressure of the gas flowing from the nozzle exit is greater than atmospheric and so now the flow is underexpanded, the result being expansion or divergence of the exhaust gas as it exits into the atmosphere. (See Problem )