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Outlet pressure

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Old   August 18, 2015, 08:11
Default Outlet pressure
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Aja
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Hi,

I am simulating a turbine and I have a question about outlet boundary condition in my simulation. Uniform velocity is applied at the computational domain inlet, while outlet pressure boundary condition is
used at the domain outlet.

This type turbine is located next to the beach of ocean, so the value of pressure is equaled at the outlet to the atmospheric pressure value.

Now, How to set operating pressure and value of outlet pressure?

Thanks,

Aja

Last edited by aja1345; August 20, 2015 at 17:20.
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Old   August 18, 2015, 08:58
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By default the operating pressure is 101325 Pa. If you keep the operating pressure at the default, the outlet pressure (which is specified as a gage pressure) should be 0 Pa.
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Last edited by LuckyTran; August 18, 2015 at 10:31.
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Old   August 18, 2015, 09:24
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Quote:
Originally Posted by LuckyTran View Post
By default the operating pressure is 101325 Pa. If you the operating pressure at the default, the outlet pressure (which is specified as a gage pressure) should be 0 Pa.
Thanks,

In my case, compressibility effect is neglected because of the low Mach number flow and so, density is constant(=1.225 kg/m3)

Now, according to the following link, what is the exact meaning of 'If the density is assumed constant or if it is derived from a profile function of temperature, the operating pressure is not used in the density calculation.'?

According to above sentences and the following link, I think that the operating pressure is equal to 0. What is your opinion?

file:///C:/Fluent.Inc/fluent6.3.26/h...ug/node374.htm

I am grateful that guide me more about this problem.

Best,

Aja
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Old   August 18, 2015, 10:29
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"If the density is assumed constant or if it is derived from a profile function of temperature, the operating pressure is not used in the density calculation."

This is a simple statement of truth. If you specify a constant density, then operating pressure never affects the density (which you specified as a constant). Similarly, if the density is specified as a profile function of only temperature, then once again the operating pressure does not affect your density profile in any way since it is never used.

Operating pressure should be set as close to the mean pressure as possible if you want to minimize round-off error since Fluent always uses gage pressure under-the-hood. You can set the operating pressure to 0 for convenient but it doesn't help to deal with the roundoff errors which is a problem for incompressible or low-Mach number problems.

Even for compressible flows, you can still get more accurate results by using a non-zero operating pressure, except that the relative improvement is less compared to incompressible flows because pressure changes in compressible flows are naturally larger (they are less susceptible to roundoff because they are bigger numbers).
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Old   August 18, 2015, 10:56
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Quote:
Originally Posted by LuckyTran View Post
"If the density is assumed constant or if it is derived from a profile function of temperature, the operating pressure is not used in the density calculation."

This is a simple statement of truth. If you specify a constant density, then operating pressure never affects the density (which you specified as a constant). Similarly, if the density is specified as a profile function of only temperature, then once again the operating pressure does not affect your density profile in any way since it is never used.

Operating pressure should be set as close to the mean pressure as possible if you want to minimize round-off error since Fluent always uses gage pressure under-the-hood. You can set the operating pressure to 0 for convenient but it doesn't help to deal with the roundoff errors which is a problem for incompressible or low-Mach number problems.

Even for compressible flows, you can still get more accurate results by using a non-zero operating pressure, except that the relative improvement is less compared to incompressible flows because pressure changes in compressible flows are naturally larger (they are less susceptible to roundoff because they are bigger numbers).
Thanks your answer,

What is the roundoff?

you say that if I specify a constant density, then operating pressure never affects the density, it is right. But I think that the operating pressure affects the outlet pressure. Correct?

I should calculate Pressure drop coefficient(please see the following figure). this coefficient is related to the total pressure drop.


total pressure drop= P(inlet)-P(outlet)



thanks,

Best,

Aja
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Old   August 18, 2015, 12:02
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Roundoff error is a type of computing error where numerical accuracy is limited by the bit representation of the values (int,float & single vs double precision, etc). Results must be computed using a finite number of digits in their representation. Small differences in big numbers are harder to represent because of roundoff error. On the other hand, large differences in small numbers have less roundoff error. Hence, Fluent uses the gage pressure to limit roundoff error. The same problem occurs practically when trying to measure pressures (to the extent that most pressure transducers are differential pressure transducers).

The choice of operating pressure is immaterial except for roundoff purposes, the physics do not change regardless of the setting of the operating pressure. This is true up to rounding errors. If rounding errors are significant, of course you can get some non-physical results.

Quote:
Originally Posted by aja1345 View Post
you say that if I specify a constant density, then operating pressure never affects the density, it is right. But I think that the operating pressure affects the outlet pressure. Correct?
Yes and no. The operating pressure affects the "absolute pressure." However, for constant density / incompressible flows the driver is pressure differences and not absolute pressure. In that sense, the operating pressure does not play any dynamic role, so no. It depends on which pressure we are discussing, the absolute pressure or a gage pressure. The operating pressure affects the absolute pressure up to roundoff errors only.

You can still compute pressure differences using gage pressures since the same operating pressure appears in both. Nonetheless, the calculation of total pressure loss coefficient is fairly straightforward as long as you do don't mix up absolute and gage pressures.

Total pressure loss should be in terms of stagnation pressure, which is not the same as static pressure and may be an absolute or gage pressure. In other words P(inlet) - P(inlet) is the total pressure loss only if you are talking about stagnation pressures. A lot can go wrong if you happen to confuse any of them. If you are talking about static/mechanical/thermodynamic pressure than that is incorrect. Furthermore, stagnation & static pressure can be inferred from different combinations of absolute or gage pressures (the same way motion can be inferred from different reference frames).
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