[Julian121]

I would like to plot the RMS of pressure on a specified plane. I use this expression to create a variable:
sqrt(1/N*sum(Pressure-(Pressure.trnavg))^2)
Where N is the number of measurements during a passing period. However, there is a warning that sum needs a locator. Is there a way to fix it?

[Gert-Jan]

Do you want to do this during a run? Or afterwards in CFD-Post? This is unclear.

[Opaque]

Would you mind showing the expression specifics such as the error message can be interpreted?

[Julian121]

Originally Posted by Gert-Jan

Do you want to do this during a run? Or afterwards in CFD-Post? This is unclear.

I would like to plot the RMS of pressure distributions in CFD-Post on the blade surface.

[Julian121]

Originally Posted by Opaque

Would you mind showing the expression specifics such as the error message can be interpreted?

"ERROR The function 'sum', referenced by 'RMS of pressure', requires a locator to be specified."

[Gert-Jan]

what is n in your equation?

[ghorrocks]

Doesn't the error message say exactly what the problem is? The syntax is sum(variable)@locator. You need to add the "@locator" bit to say which region to do the sum over.

Also note you probably don't want to use the sum function. You probably want areaAve(variable)@locator instead.

[Gert-Jan]

@Glenn, the scientific equation cannot be used in Post directly. That should be obvious, not?
Therefore I asked a silly question, and tried to trigger Julian121 to think for him/her self.

@Julian121: Try to answer the question "What am I actually trying to do? And why is it not working?"

[Julian121]

Originally Posted by Gert-Jan

@Glenn, the scientific equation cannot be used in Post directly. That should be obvious, not?
Therefore I asked a silly question, and tried to trigger Julian121 to think for him/her self.

@Julian121: Try to answer the question "What am I actually trying to do? And why is it not working?"

I wanted to quantify the unsteadiness of pressure and plot related contours on blade surface or any other planes. The first thing came to my mind was to use standard deviation or root mean square of pressure. The thing I did not understand was that why a location should be specified when defining a variable in CFD-Post. Can't the unsteadiness exist in any plane? I even used the equation without "N". I know CFD-Post a little, so I do not know why it is not working indeed!

[Gert-Jan]

Given the variable Pressure.Trnavg, you apparently found in Pre how you can determine an average Pressure over time.
In the same menu, you can find the variable that you want. So, select the RMS there, perform a new calculation, and you have the result you want.

Then in Post, determine an areaAve(Pressure)@surface to find averages over a certain surface.
Btw, better use the function calculator and tick the "Show equivalent expression" to understand the syntax.

Also, please remind that you can have an RMS in time and in space. You look for an RMS in time, but your initial equation refers to something in space, making it somewhat unclear.

[Julian121]

Originally Posted by Gert-Jan

Given the variable Pressure.Trnavg, you apparently found in Pre how you can determine an average Pressure over time.
In the same menu, you can find the variable that you want. So, select the RMS there, perform a new calculation, and you have the result you want.

Then in Post, determine an areaAve(Pressure)@surface to find averages over a certain surface.
Btw, better use the function calculator and tick the "Show equivalent expression" to understand the syntax.

Also, please remind that you can have an RMS in time and in space. You look for an RMS in time, but your initial equation refers to something in space, making it somewhat unclear.

As you said, I have selected RMS in CFX-Pre transient statistics and performed a new calculation. However, I am not sure the result is correct since the values are just like pressure. I guess RMS values should be much lower than pressure. As I said previously, I would like to plot the RMS of pressure contours not on a single node, but rather on any turbo surface. Shouldn’t the numbers be on orders of 100 or 1000?

[ghorrocks]

Shouldn’t the numbers be on orders of 100 or 1000?

No. Make sure you understand what mathematical function RMS actually is.

The RMS of (10001, 10000, 9999) is 10000, not around 1.

[Julian121]

Originally Posted by ghorrocks

No. Make sure you understand what mathematical function RMS actually is.

The RMS of (10001, 10000, 9999) is 10000, not around 1.

Lets's say there are 100000, 100500, 100231, 100150, 100780. The average is 100332. Then, the RMS should be around 276. The RMS contour shows numbers around 100000!

I use [(1/N) sigma (P-P average)^2]^1/2 equation. Is it not correct?

[Gert-Jan]

I think your picture is a bit weird since the unit is missing.
It says absolute pressure. In what? Meters water column, torr, psi?
What did you change?

An other thing......why do you look at absolute pressure? Wouldn't it be more logical to just look to Pressure?.

[ghorrocks]

I use [(1/N) sigma (P-P average)^2]^1/2 equation. Is it not correct?

No, that is not correct. The mean of the numbers you list is 100332 as you say, and the RMS is also 100332.

The equation you state is the RMS of (P-Paverage). (Not the RMS of P).

See https://en.wikipedia.org/wiki/Root_mean_square

[Opaque]

https://en.wikipedia.org/wiki/Standard_deviation