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### Operating Pressure

In the Fluent software, all pressures that must be entered or the pressure on Fluent to be calculated are Gauge Pressure. In other words, if we want to put pressure in the software, we need to set the pressure relative to the operating pressure. In Fluent, the default operating pressure is P= 101325pa (in fact, atmospheric pressure). So, if we want to apply pressure to a pressure equal to atmospheric pressure in Fluent, we need to enter the relative pressure value of zero.

In some cases, it is necessary to change the value of the operation pressure from its default value. In this case, some command is used to change the default value.

The value of the operating pressure is very important for the incompressible flows. Because the density of the ideal gas is calculated directly, it must be ensured that the operating pressure is appropriate.

The operation pressure in compressible flows with a small Mach number is significant because, in these flows, the pressure fluctuations in the flow are very small compared to the absolute static pressure. Therefore, the amount of this pressure difference may not be considered due to numerical rounding, and in Fluent, instead of using absolute pressures, relative pressure is used to counteract this problem, in order to avoid the effect of the large absolute static pressure against the pressure drop and, consequently, Avoid rounding errors. The operating pressure in compressible flows with a large Mach number does not matter, because in these flows the pressure drop is much larger than the pressure drop in condensing flows with a small Mach number. Therefore, in this case, there is no problem of rounding error and there is no need to use relative pressures. Usually, absolute pressure is used, so it is better to have the pressure value in this case equal to zero so that the pressure Relative and absolute. If the density is constant or is defined as a function of temperature, the operating pressure is not used in the density calculation and the determination of the value of operating pressure is not important. In general, to determine the value of the operating pressure, we need to pay attention to the Mach number and how to calculate the density. For example, if we use the ideal gas rule, we must consider the operating pressure value equal to the average flow pressure.

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