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Pressure ratio calculation in a transient run

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Old   January 14, 2019, 09:30
Default Pressure ratio calculation in a transient run
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I am doing a transient simulation of a compressor to evaluate the pressure ratio.
How should pressure ratio be calculated in a transient run?
For example, I am running for 5 passing periods and each one has 100 time steps.
Should it be calculated as the arithmetic average of pressure ratio in each of 100 time steps or it should be evaluated at the end of each passing periods?
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Old   January 14, 2019, 10:42
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Evaluating the pressure ration at the end of a passing period will only give you the instantaneous pressure ratio, which I am not sure what use it has.

However, you can compute a running average using the same number of timesteps per passing period if the time average (based on such period) if of interest to you. By doing a running average, there is no influence on when the period starts/finish, and the value should steadily converge to a single number as the simulation progresses.
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Old   January 15, 2019, 13:39
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I have two more questions:

First: is the running pressure ratio different than the actual pressure ratio?
Should the running pressure ratio ultimately reach the actual pressure ratio developed by the compressor?

Second: I have written an expression (massFlowAve(Total Pressure)@Outlet / massFlowAve(Total Pressure)@Inlet) which computes the pressure ratio in a steady run. Does this give me the true running pressure ratio in a transient run or I should use “Trn Stats”?

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Originally Posted by Opaque View Post
Evaluating the pressure ration at the end of a passing period will only give you the instantaneous pressure ratio, which I am not sure what use it has.

However, you can compute a running average using the same number of timesteps per passing period if the time average (based on such period) if of interest to you. By doing a running average, there is no influence on when the period starts/finish, and the value should steadily converge to a single number as the simulation progresses.
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Old   January 15, 2019, 14:46
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I can only suggest how to evaluate a given calculation, i.e. how to realize a formula using ANSYS CFX expression language. However, I cannot advise on the meaning or significance of what you are trying to evaluate since that it is up to you.

The software will evaluate what it is told to do.

Here is a list of questions that may help you

What are you trying to achieve with such a calculation?

What is the physical interpretation of the pressure ratio? or where does the formula come from?

If you were in the laboratory with the machine, can you measure the pressure ratio directly or indirectly?

Many of those quantities are meant for steady state interpretations; therefore, for unsteady calculations, they must be revisited based on significance, or ability to measure in the laboratory (directly or indirectly).

Hope the above helps you
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