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Old   August 10, 2007, 01:36
Default physical significance of....
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vijesh
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Hi friends... Do you hav any idea of the following??/

1. Physical significance of Mach number - whether it helps only in deciding the flow (sub, sonic, super, trans)??

2. What is the diff bet static pressure, total pressure, stagnation pressure and their physical significance??

3. Why do we neglect viscosity effects in hypersonic flow?? is it only because that pressure forces dominate viscous forces or what?/

PLZ reply ...

Thank you...

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Old   August 10, 2007, 05:52
Default Re: physical significance of....
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Hrvoje Jasak
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Write your own homework - your teachers also have internet.

Hrv
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Old   August 10, 2007, 11:44
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Lionel S.
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Hi Vijesh !

1. Physical significance of Mach number - whether it helps only in deciding the flow (sub, sonic, super, trans)??

>> It's the ratio: fluid velocity over sound velocity. The flow is subsonic at about 0 < Mach < 0.5, transsonic at about 0.5 < Mach < 1 supersonic at 1 < Mach < 5 and at Mach 5 or over, it's hypersonic.

2. What is the diff bet static pressure, total pressure, stagnation pressure and their physical significance??

>> The static pressure, p, is the most "standard" pressure. It's the pressure defined as the force per surface unit exerced by the fluid on a (virtual) surface. The total pressure is the static pressure plus the kinetic energy of the fluid , i.e. p_0 = p + 1/2 rho V^2. I don't know anything called stagnation pressure. The physical signifiance of the total pressure is the potential+kinetic energy of the fluid. p is a kind of potential energy, because compressed fluid acts like a kind of spring. The second term is a standard kinetic term.

3. Why do we neglect viscosity effects in hypersonic flow?? is it only because that pressure forces dominate viscous forces or what?/

The more the velocity is hign, the more it's negligeable, because the inertial force and the pressure forces are very bigger than viscosity. Also, hypersonic aircrafts flies at very high altitudes, and the viscosities are small because air is rarefied.

Hope it helps ! Bye
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Old   August 11, 2007, 02:15
Default Re: physical significance of....
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vijesh
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Thank you friend... But I really wanted to know when I have to go for static pressure and when for total pressure...

PLZ reply..

Thank you...
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Old   August 11, 2007, 06:59
Default Re: physical significance of....
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Lionel S.
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"when I have to go for static pressure and when for total pressure"

I don't know, it depends on what you are looking for. But usually, static pressure is the most common pressure, it gives for example the main lift force on a wing.
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Old   August 12, 2007, 05:41
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sudhakar
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hi vijesh,

when ever a body is placed in a fluid medium, its surface will be bombarded by fluid molecules moving at random. The intensity of this 'molecular bombardment force' is called static pressure..

totalpressure = static + dynamic pressure..

it can be viewed like this : dynamic pressure is the measure of kinetic energy of the flow and static pressure is like some virtual potential energy (not internal energy). so total pressure can be considered as the total energy contained with the fluid.

when you want to measure aerodynamic forces over the immersed body, go for static pressure and if you want to measure the amount of energy lost between two sections (say pipe inlet and exit), measure total pressure at those sections; energy lost is propotional to the total pressure difference.

Hope it will help you..
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Old   August 12, 2007, 22:32
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RR
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...little more to Sudhakar's post........just read any books which explain the working of a Pitot tube (both for compressible and incompressible flows).."Fundamentals of Aerodynamics" by Anderson is a good one..

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Old   August 13, 2007, 04:18
Default Re: physical significance of....
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Vijesh
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Thank you friends....
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