Shock wave formation.
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optical effect at the boundary of Hi/Lo pressure isn't to do with relative pressure, But HEAT.
In a shock wave the properties of the fluid (density, pressure, temperature, velocity, Mach number) change almost instantaneously.
Brian
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Sorry airfoil, but pressure changes are the driving force behind the whole process. As an object moves through a gas, the gas molecules are deflected around the object. If the speed of the object is much less than the speed of sound of the gas, the density of the gas remains constant and the flow of gas can be described by conserving momentum, and energy. As the speed of the object approaches the speed of sound, we must consider compressibility effects on the gas. The density of the gas varies locally as the gas is compressed by the object.
For compressible flows with little or small flow turning, the flow process is reversible and the entropy is constant. The change in flow properties are then given by the isentropic relations (isentropic means "constant entropy"). But when an object moves faster than the speed of sound, and there is an abrupt decrease in the flow area, shock waves are generated in the flow. Shock waves are very small regions in the gas where the gas properties change by a large amount. Across a shock wave, the static pressure, temperature, and gas density increases almost instantaneously. The changes in the flow properties are irreversible and the entropy of the entire system increases. Because a shock wave does no work, and there is no heat addition, the total enthalpy and the total temperature are constant. But because the flow is non-isentropic, the total pressure downstream of the shock is always less than the total pressure upstream of the shock; there is a loss of total pressure associated with a shock wave. Because total pressure changes across the shock, we can not use the usual (incompressible) form of Bernoulli's equation across the shock. The Mach number and speed of the flow also decrease across a shock wave.
For compressible flows with little or small flow turning, the flow process is reversible and the entropy is constant. The change in flow properties are then given by the isentropic relations (isentropic means "constant entropy"). But when an object moves faster than the speed of sound, and there is an abrupt decrease in the flow area, shock waves are generated in the flow. Shock waves are very small regions in the gas where the gas properties change by a large amount. Across a shock wave, the static pressure, temperature, and gas density increases almost instantaneously. The changes in the flow properties are irreversible and the entropy of the entire system increases. Because a shock wave does no work, and there is no heat addition, the total enthalpy and the total temperature are constant. But because the flow is non-isentropic, the total pressure downstream of the shock is always less than the total pressure upstream of the shock; there is a loss of total pressure associated with a shock wave. Because total pressure changes across the shock, we can not use the usual (incompressible) form of Bernoulli's equation across the shock. The Mach number and speed of the flow also decrease across a shock wave.
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If the light is in the right direction you can see a very faint shock wave on the upper surface of an A320 wing in cruise.
I assume the pressure changes cause some refraction effects which appear as a shadow of the shock wave.
I assume the pressure changes cause some refraction effects which appear as a shadow of the shock wave.
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Many of you will have seen the following photo, some thing similar either in photo or in person at an airshow. What you are seeing is not a Mach cone, but a Prandtl-Meyer expansion fan. The angle of the fan can be used to calculate the Mach of the airflow producing the fan, but not the Mach of the aircraft - important difference. When an object moves faster than the speed of sound, and there is an abrupt decrease in the flow area, shock waves are generated. If the flow area increases, however, a different flow phenomenon is observed. If the increase is abrupt, we encounter a centered expansion fan. You can see an expansion fan on top of the canopy as the flow area suddenly increases. The expansion fan on the fuselage/wing is due once again to the flow area increasing. The word "expansion" denotes that the area is increasing. There are some marked differences between shock waves and expansion fans. Across a shock wave, the Mach number decreases, the static pressure increases, and there is a loss of total pressure because the process is irreversible. Through an expansion fan, the Mach number increases, the static pressure decreases and the total pressure remains constant.
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You say there is no increase in energy? That wants explanation. I am limited in the extent of my mathematical prowess, but believe I understand the inviscid nature of "air" below the defined value of ~260 knots. First, an increase in pressure produces an immediate increase in temperature, (Boyle?), and pressure waves are present attached to any A/C travelling through air faster than 260 knots, well below Mach 1 (IAS). Consider that in the case of the Hornet flying over San Francisco Bay, it is manouvering in ground effect, which impacts the compressibility of the ambient air to begin with. Thirty feet above its position, the Pressure drop might well have been invisible. My simple understanding is this: As a mass transits an air mass, it compresses it, the air having less time to maintain its density (low) before establishing a high pressure Flow about the airframe. As the air re-enters the "hole" caused by the A/C in the air behind it, it accelerates and produces an immense release of the energy it was forced to contain during the brief transit of the A/C. This "Thunderclap" is one of two separate events precipitated by the energy of the passing A/C. "Shock" by definition means an abrupt change in "stasis" and is accompanied by all manner of fascinating phenomena, begging varying degrees of explanation, from simple to complex. This isn't a defense of my interpretation, merely my description of my understanding of a sonic event from my perspective. If you like, I will certainly admit that Pressure Differential caused that neat picture.
At my age, I'm stuck in the way I learn. I don't use Wiki, I think it diminishes the quality of an exchange. Besides, I have seen glaring errors in it, and stubbornly hold to my method of writing my interpretation, and then subjecting myself to the challenges of others, having to defend or acquiesce as necessary. Further, I don't hesitate to challenge data presented by others; I've had to eat humble pie more times than I care to count. This may be one of those times.
BRGDS, Airfoil
At my age, I'm stuck in the way I learn. I don't use Wiki, I think it diminishes the quality of an exchange. Besides, I have seen glaring errors in it, and stubbornly hold to my method of writing my interpretation, and then subjecting myself to the challenges of others, having to defend or acquiesce as necessary. Further, I don't hesitate to challenge data presented by others; I've had to eat humble pie more times than I care to count. This may be one of those times.
BRGDS, Airfoil
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All the refraction effects come from the change of air refractive index.
The refractive index depends on two things - density and composition.. And density depends on temperature and pressure.
If you look at the air above an open fire, or any heated object, the column will refract light. You can see it in distortion of objects behind, as well as in a shadow which it casts in external light.
The pressure is almost the same, and when the object is heated from its heat capacity, electricity etc., the composition also is the same. Different temperature means different density and refractive index.
But when you deal with air affected by wing - well, the temperature certainly changes. In moist air, fog forms in the parts where air is rarefied and cooled, like above wing and in engine inlets. Obviously the density of air decreases - and in clear dry air, the visual distortion should also be seen. But so long as the speeds are low and airflow subsonic, the distortion is continuous and hard to see.
The refractive index depends on two things - density and composition.. And density depends on temperature and pressure.
If you look at the air above an open fire, or any heated object, the column will refract light. You can see it in distortion of objects behind, as well as in a shadow which it casts in external light.
The pressure is almost the same, and when the object is heated from its heat capacity, electricity etc., the composition also is the same. Different temperature means different density and refractive index.
But when you deal with air affected by wing - well, the temperature certainly changes. In moist air, fog forms in the parts where air is rarefied and cooled, like above wing and in engine inlets. Obviously the density of air decreases - and in clear dry air, the visual distortion should also be seen. But so long as the speeds are low and airflow subsonic, the distortion is continuous and hard to see.
Afraid not. The Mach cone develops when the aircraft itself reaches Mach 1.
Brian Abraham, M 1.17---is still sonic because every part of the fighter has to reach M 1.00
I forgot the exact range but supersonic isn't until like M 1.5???...So, yeah a
Mach cone---
PA
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"Every part of the fighter...."??
Woh. Now I'm right out of my comfort zone. I don't want to get into Einstein, not here, not now. Which part of the fighter is super, which part not? Are you saying that Mach 1 is a different velocity for different parts of the A/C?. Even if that is so, by definition, Mach 1 has a discrete value relative to the whole A/C, No? Boggling. Anyone for Focal Calculus?
Airfoil
Airfoil
Hey, Airfoilmod--not exactly--I'm saying different part become sonic at different speeds-high perf.aircraft..DC-9s, P51 Mustangs, F18's will achieve upon parts of its airframe both regions of sonic and supersonic flow...as long as they fly near sonic velocities...-- --the main reason [regarding the wing] is sweep which decreases the length of the velocity vector along the wing-----an important concept is the M-crit which is when the very first supersonic flow is encountered on the airframe--[they learn this i n the wind tunnel]---it is similar, in concept, to a propeller tip or chopper blades where [in this case ] the angular velocity is higher at the tip than the roots--as a result---speed is limited because the supersonic flow creates a good deal of drag--
Well because of this all of the defined regions have a value higher than the minimum--to ensure that the whole airframe is 'immersed' in a sonic/supersonic or hypersonic flow---
PA
Well because of this all of the defined regions have a value higher than the minimum--to ensure that the whole airframe is 'immersed' in a sonic/supersonic or hypersonic flow---
PA
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Hmmm
Pug. But you describe "regions" of airflow, of course airflow has different velocities as it transits the Airframe. The airframe has one single velocity because it is a solid, and cannot by definition have more than one. I read your "parts of the fighter" as implying there was a possibility of a plural Mach value. That is by definition ridiculous. Do I misunderstand your reply? I don't find the "plural value of Mach" addressed in it relative to the A/C, not the Airflow.
Airfoil
Aside. How can Mach 1 be "About Mach 1.5"?
Airfoil
Aside. How can Mach 1 be "About Mach 1.5"?
I'm not takin about Mach meter indications--I'm mean the airframe parts of your ERJ-145 737/A320---are sonic at M values at an IMN less than M 1.00,
with that in mind reread my post, I'm just an aerodynamicist
Back to my my Anna Gabriel'
'Mexico Lindo'
ay ay ay ay canta y no llores, porque cantando se alelgran cielito lindo los corazones---- ese lunar que tienes cielito lindo----
Old Smokey or MFS Help!
PA
with that in mind reread my post, I'm just an aerodynamicist
Back to my my Anna Gabriel'
'Mexico Lindo'
ay ay ay ay canta y no llores, porque cantando se alelgran cielito lindo los corazones---- ese lunar que tienes cielito lindo----
Old Smokey or MFS Help!
PA
Last edited by Pugilistic Animus; 31st May 2008 at 19:56. Reason: to fix up mi espanol
Exactamente!----te llevE aqui
busques en Pprune por 'critical Mach number'
also Old Smokey has written extensively--about Mach meter calibration---I can do no justice--to his posts--on that topic--- so--I wont even attempt
Cuidete
PA
busques en Pprune por 'critical Mach number'
also Old Smokey has written extensively--about Mach meter calibration---I can do no justice--to his posts--on that topic--- so--I wont even attempt
Cuidete
PA
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I don't use Wiki, I think it diminishes the quality of an exchange
M 1.17---is still sonic because every part of the fighter has to reach M 1.00
I'm just an aerodynamicist
As the air re-enters the "hole" caused by the A/C in the air behind it, it accelerates and produces an immense release of the energy it was forced to contain during the brief transit of the A/C. This "Thunderclap" is one of two separate events precipitated by the energy of the passing A/C
For today's supersonic aircraft in normal operating conditions, the peak overpressure varies from less than one pound to about 10 pounds per square foot for a N-wave boom. Peak overpressures for U-waves are amplified two to five times the N-wave, but this amplified overpressure impacts only a very small area when compared to the area exposed to the rest of the sonic boom.
The strongest sonic boom ever recorded was 144 pounds per square foot and it did not cause injury to the researchers who were exposed to it. The boom was produced by a F-4 flying just above the speed of sound at an altitude of 100 feet.
In recent tests, the maximum boom measured during more realistic flight conditions was 21 pounds per square foot. There is a probability that some damage -- shattered glass, for example, will result from a sonic boom. Buildings in good repair should suffer no damage by pressures of less than 16 pounds per square foot. And, typically, community exposure to sonic boom is below two pounds per square foot. Ground motion resulting from sonic boom is rare and is well below structural damage thresholds accepted by the U.S. Bureau of Mines and other agencies. (USAF FACT SHEET 96-03)
For students interested in aerodynamics NASA have a good site here
http://www.grc.nasa.gov/WWW/K-12/airplane/guided.htm
With the caveat that even the experts can get it wrong (I’m no expert, just an interested bystander) I think even airfoilmod will find it an acceptable source of information. If NASA can't get it right we really are in big trouble.
I reference this site because of the facility to play around with numbers to see what occurs.
You say there is no increase in energy? That wants explanation
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"...there is no addition of heat..."
"The total enthalpy and the total temperature are constant."
I think it's fair to say that picking apart your post would not be helpful. I mean no disrespect, but you seem to stylishly increase complexity while attempting to characterize a simple concept. I prefer a straightforward characterization. This is a technical forum, and I may be out of place. I enjoy metaphor and global inference. Mathematics is like a boggy swamp when I want to blaze a hole in the ether. I get what you write; I can't be sure if your prose is proprietary, paraphrase or quote.
"We cannot use the incompressible form of Bernoullis equation across the Shock". Well, of course, that is precisely what I said.
"The density of the gas varies locally as the gas is compressed by the object." Again, intuitively (and textually) obvious.
I wouldn't correct you, what you say is enlightening and accurate, but I think you want me to be wrong, or somehow in disagreement with technical prose. Any Natural phenomenon can be explained in a number of ways, from the entry level to the arcane and hopelessly circuitous. I am not expert in aerodynamics by any means and made that clear at the beginning. I won't learn anything if I always have to be right or the most learned in the room.
Basically, I like the pictures more than the text, always have. Never grew up I guess.
All the Best (Is Australia as beautiful as I imagine it to be?)
Airfoil
I think it's fair to say that picking apart your post would not be helpful. I mean no disrespect, but you seem to stylishly increase complexity while attempting to characterize a simple concept. I prefer a straightforward characterization. This is a technical forum, and I may be out of place. I enjoy metaphor and global inference. Mathematics is like a boggy swamp when I want to blaze a hole in the ether. I get what you write; I can't be sure if your prose is proprietary, paraphrase or quote.
"We cannot use the incompressible form of Bernoullis equation across the Shock". Well, of course, that is precisely what I said.
"The density of the gas varies locally as the gas is compressed by the object." Again, intuitively (and textually) obvious.
I wouldn't correct you, what you say is enlightening and accurate, but I think you want me to be wrong, or somehow in disagreement with technical prose. Any Natural phenomenon can be explained in a number of ways, from the entry level to the arcane and hopelessly circuitous. I am not expert in aerodynamics by any means and made that clear at the beginning. I won't learn anything if I always have to be right or the most learned in the room.
Basically, I like the pictures more than the text, always have. Never grew up I guess.
All the Best (Is Australia as beautiful as I imagine it to be?)
Airfoil
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but I think you want me to be wrong, or somehow in disagreement
The peculiar evil of silencing the expression of an opinion is, that it is robbing the human race; posterity as well as the existing generation; those who dissent from the opinion, still more than those who hold it. If the opinion is right, they are deprived of the opportunity of exchanging error for truth: if wrong, they lose, what is almost as great a benefit, the clearer perception and livelier impression of truth, produced by its collision with error…….We can never be sure that the opinion we are endeavouring to stifle is a false opinion; and if we were sure, stifling it would be an evil still
I can't be sure if your prose is proprietary, paraphrase or quote
I like the pictures more than the text
BTW I don't expect you to believe that. 
Re Australia and beautiful. Depends on the viewer I guess, it does range from barren desert to tropical jungle so there should be some thing to keep most people happy - if you can put up with the flies, poisonous spiders, snakes and jellyfish.
Definitely not. Your Airbus or Boeing airliner never gets close to Mach 1,
Ooh no---I thought all airliner's move at 1000mph---almost as fast as a freight train--right---well roger tower--- ready to get cleared for TO---from the tarmac
good book ---can't think of the author's not home---but the title is unique
try 'Engineering Supersonic Aerodynamics'---by, some
Warning it gets a lil rough in the partial differential equations