Reduction of a mass flow by using an expression

todeisen · June 9, 2021, 13:09

Hello all,
I am currently working on the simulation of a centrifugal compressor. The model consists of an inlet section, the compressor blade, and the diffuser. The simulation is stationary.

An exit corrected mass flow rate is specified at the diffuser. This mass flow is currently reduced manually when convergence occurs. I would like to create an expression that "automates" this reduction.

In the first idea, I captured the total pressure at the outlet via an expression and then created a monitor under the Qutlet Control tab. Here I used the Monitor Statistics function. With the Interval option "Previous Complete Interval" I wanted to look at the standard deviation over 50 iterations. With the function/command "probe" I wanted to include the standard deviation in an expression, e.g. if the standard deviation is less than 10 [Pa] the Exit Corrected Mass Flow Rate is reduced.
But apparently, the solver cannot process the function "probe", at least I get an error message here.
Is there another way to include the standard deviation from the monitor point into an expression? It would be my preferred idea. In the documentation, I only found a command for a transient calculation here.

Another idea is to look at the difference between the mass flow rate at the inlet and the outlet. If this was zero or close to zero, the Exit Corrected Mass Flow Rate would also be reduced. The current problem here is that this difference oscillates strongly, which is due to the oscillation of the mass flow rate at the outlet. As a second criterion in this idea, I have created an additional variable to compare the current value of the Mass Flow Rate with the previous one. Here I subtract the previous value from the current value. This corresponds to the slope of a differential equation if I am not mistaken. If this difference is equal to 0, there would also be convergence. Unfortunately, the problem here is that the values oscillate very strongly,

I would be very happy to receive new ideas or food for thought, as I have been working on this problem for a while and am not making any progress at the moment. If you need any diagrams or files, I would be happy to post them.

Best regards,
Tobi

Opaque · June 9, 2021, 13:58

May I ask what are you trying to achieve by automating different exit corrected mass flows?

If you are trying to build a machine map, there is already a built-in feature to do that for you. Look for Operating Maps functionality in the ANSYS CFX documentation.

If you specify the range of operating "exit corrected mass flow", or any other variables, you will obtain the machine map for the selected parameters directly in CFD-Post.

In addition, if you have plenty of computational resources it can parallelize the map computation with multiple concurrent simulations (also in parallel)

todeisen · June 9, 2021, 14:14

Hi Opaque,

the mass flow should be reduced to generate speed lines of the compressor. Here the speed is specified and the exit corrected mass flow is reduced to approach the surge limit of the compressor. For the speed lines, the total pressure ratio is then plotted against the mass flow.

Can the operating maps functionality be used here?

Opaque · June 9, 2021, 14:43

As far as I understand, the operating map feature should be able to build a full compressor map given the range of exit correct mass flow, and machine speed with a single simulation setup, and multiple runs behind the scenes.

The workflow I understand is
1 - Setup your standard simulation,
2 - Select which parameters you want to change, say mass flow and angular speed.
3 - Give a range for those parameters, or a table for which points in the map you are interested in.
4 - Write definition file
5 - Submit simulation and define the approach: sequential or concurrent based on your resources
6 - Wait for the simulation to finish. Notice the ANSYS CFX solver will be starting/stopping several times within the single simulation
7 - Open CFD-Post and visualize your compressor map

That is at least a plot of Total Pressure ratio vs ExitCorrMassFlow with rotational speed as a parameter. You can then create other variants.

Are you trying to determine the surge line numerically?

https://ansyshelp.ansys.com/account/...783372294.html

https://ansyshelp.ansys.com/account/...033781803.html

June 9, 2021, 13:09	Reduction of a mass flow by using an expression	#1
todeisen New Member Tobias Join Date: Jun 2021 Posts: 2 Rep Power: 0	Hello all, I am currently working on the simulation of a centrifugal compressor. The model consists of an inlet section, the compressor blade, and the diffuser. The simulation is stationary. An exit corrected mass flow rate is specified at the diffuser. This mass flow is currently reduced manually when convergence occurs. I would like to create an expression that "automates" this reduction. In the first idea, I captured the total pressure at the outlet via an expression and then created a monitor under the Qutlet Control tab. Here I used the Monitor Statistics function. With the Interval option "Previous Complete Interval" I wanted to look at the standard deviation over 50 iterations. With the function/command "probe" I wanted to include the standard deviation in an expression, e.g. if the standard deviation is less than 10 [Pa] the Exit Corrected Mass Flow Rate is reduced. But apparently, the solver cannot process the function "probe", at least I get an error message here. Is there another way to include the standard deviation from the monitor point into an expression? It would be my preferred idea. In the documentation, I only found a command for a transient calculation here. Another idea is to look at the difference between the mass flow rate at the inlet and the outlet. If this was zero or close to zero, the Exit Corrected Mass Flow Rate would also be reduced. The current problem here is that this difference oscillates strongly, which is due to the oscillation of the mass flow rate at the outlet. As a second criterion in this idea, I have created an additional variable to compare the current value of the Mass Flow Rate with the previous one. Here I subtract the previous value from the current value. This corresponds to the slope of a differential equation if I am not mistaken. If this difference is equal to 0, there would also be convergence. Unfortunately, the problem here is that the values oscillate very strongly, I would be very happy to receive new ideas or food for thought, as I have been working on this problem for a while and am not making any progress at the moment. If you need any diagrams or files, I would be happy to post them. Best regards, Tobi

June 9, 2021, 13:58		#2
Opaque Senior Member Join Date: Jun 2009 Posts: 1,880 Rep Power: 33	May I ask what are you trying to achieve by automating different exit corrected mass flows? If you are trying to build a machine map, there is already a built-in feature to do that for you. Look for Operating Maps functionality in the ANSYS CFX documentation. If you specify the range of operating "exit corrected mass flow", or any other variables, you will obtain the machine map for the selected parameters directly in CFD-Post. In addition, if you have plenty of computational resources it can parallelize the map computation with multiple concurrent simulations (also in parallel) __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

June 9, 2021, 14:43		#4
Opaque Senior Member Join Date: Jun 2009 Posts: 1,880 Rep Power: 33	As far as I understand, the operating map feature should be able to build a full compressor map given the range of exit correct mass flow, and machine speed with a single simulation setup, and multiple runs behind the scenes. The workflow I understand is 1 - Setup your standard simulation, 2 - Select which parameters you want to change, say mass flow and angular speed. 3 - Give a range for those parameters, or a table for which points in the map you are interested in. 4 - Write definition file 5 - Submit simulation and define the approach: sequential or concurrent based on your resources 6 - Wait for the simulation to finish. Notice the ANSYS CFX solver will be starting/stopping several times within the single simulation 7 - Open CFD-Post and visualize your compressor map That is at least a plot of Total Pressure ratio vs ExitCorrMassFlow with rotational speed as a parameter. You can then create other variants. Are you trying to determine the surge line numerically? https://ansyshelp.ansys.com/account/...783372294.html https://ansyshelp.ansys.com/account/...033781803.html __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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June 9, 2021, 14:14		#3
todeisen New Member Tobias Join Date: Jun 2021 Posts: 2 Rep Power: 0	Hi Opaque, the mass flow should be reduced to generate speed lines of the compressor. Here the speed is specified and the exit corrected mass flow is reduced to approach the surge limit of the compressor. For the speed lines, the total pressure ratio is then plotted against the mass flow. Can the operating maps functionality be used here?