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What is THERMODYNAMIC pressure?

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Old   November 24, 1998, 13:05
Default What is THERMODYNAMIC pressure?
Xinwen Xiao
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According to the assumption for Newtonian fluid, in static state, normal stress(Pm) is EQUAL to the value of its thermodynamics pressure(Pt). In physics, they are not SAME, just happen to be equal in value in static state. While fluid moves, they are not equal any more.

My question is: what is really thermodynamic pressure? How to measure it? If you just use a tube with one end sealed and filled with mercury, which is the Toriceri(?)'s idea to measure atmosphere pressure, then what you can get is actually normal stress(Pm) because it is this normal stress that mechanically supports the mercury to prevent it to keep falling.

Ok, If you can not measure Pt, then how do you know Pm is equal to Pt in static state?

Somebody may say, enen if I can not measure, I still can calculate it by Pt=rho*R*T. Then I want to ask, if you could not measure it, how do you know you formulus is correct? How can you determine the R? When you try to build up this formulus, actually you calculate the R based on Pt, rho, and T. So actually you just recircle here if you can not measure Pt. You have to measure Pt!!! How??

Thank you.
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Old   November 24, 1998, 13:50
Default Re: What is THERMODYNAMIC pressure?
John C. Chien
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After reading your letter, I am really very confused. First of all, p=rho * R * T is an equation of state which characterize the ralationship between p, rho, and T for a particular gas. ( where p is the static pressure, rho is the density, and T is the static temperature in absolute unit,degree Rankine or Kelvin.) For moving fluid flow, you can measure both p and T directly. Two thermodynamic properties such as p and T are enough to specify the thermodynamic state uniquely. In this case the density rho has to be related to p and T through the equation of state.The total pressure and total temperature are derived quantities in fluid flow, just for convenience.
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Old   November 28, 1998, 13:46
Default Re: What is THERMODYNAMIC pressure?
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Refer to Schlichting.H , Boundary layer theory for an explanation of what is bulk vicosity and thermodynamic pressure. Actually ,the fact the navier-stokes equations have been derived based on these assumptions , and the fact that Navier-Stokes equations can also be derived from the boltzmann equation of the kinetic theory of gases , and the fact that the navier-stokes equations a valid for air and water have been proved experimenatlly proves that stokes approximation is a correct one. But the physical interpretation of bulk viscosity is not very clear to me, and it seems , is still a contraversial topic.
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Old   December 2, 1998, 14:19
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I have a miniature compressor for a cryocooler that provides a pressure pulse by moving a diaphragm. the diaphragm is placed in a mid section of a sealed/rigid volume. initially the pressure on both side of it is constant 200 psi (no strain). during operation one side of the diaphragm is kept at constant 200 psi while the opposite end produces a pulse of 80 psi ie 280 abs. the displacement of the diaphragm is .08 (0, .08,0,.08.....) is the the compression work equals expansion (ie. is this similar to a perfect elastic spring?) if so , my input power=0? assuming adibatic compression what is the net input power at 20hz ? same for isothermal. can anyone help?

thanks uri
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Old   April 22, 2009, 15:55
Default Fluid pressure vs thermodynamic pressure?
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I'd really appreciate it if you them me what's the diffrence between Fluid pressure And thermodynamic pressure?
I have to khnow it exactly in a few days.thanks
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Old   October 31, 2009, 17:43
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it looks like the thermodynamic pressure is the "hydrostatic pressure"
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Old   May 26, 2014, 13:33
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fluid pressure refer to the hydrostatic pressure. It is the pressure exerted by the liquid weight. Can be calculated using ro. g. h.

Thermodynamic pressure represent the total energy of the fluid represented in terms of pressure head.

When the fluid is static, both are same as there is no additional source of energy in fluid. But when in motion, the liquid also has kinetic energy which is in terms of velocity. So thermodynamic pressure is the sum of hydrostatic pressure plus the pressure head by velocity. (0.5XroXv2).

To measure thermodynamic pressure, place a tube bent at 90 deg angle. When the fluid enters the tube and reaches end of 90 deg bend, it comes to stop and kinetic energy is converted into pressure energy.

Hope this helps.
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Old   June 6, 2014, 16:25
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I usually see "thermodynamic pressure" is discussions regarding the stress tensor. Thermodynamic pressure is considered the stress perpendicular to flow direction. The other "pressures" being the shear stress components. So in the 3x3 2nd order stress tensor the thermodynamic pressures are the main diagonal.
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