# density, viscosity expressions CFX

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July 23, 2013, 13:25
density, viscosity expressions CFX
#1
Member

Karthik
Join Date: Oct 2012
Location: Germany
Posts: 53
Rep Power: 13
Hello Friends,

I wanted to give expressions for temperature dependent fluid properties (density, viscosity etc.) for T junction flows in CFX. I fitted the trendline and got polynomial equations for each fluid property as function of temperature in Excel. But when I substituted those equations in CFX and tried seeing the plot, it is completely wrong from what is shown in my excel sheet. I could not understand how this happens. I am attaching the excel sheet along with this post. Kindly guide me as to what is happening/ what is the mistake I am doing?

Regards,
Karthick
Attached Files
 Temperature dependent properties for CFX (1).xlsx (16.4 KB, 260 views)

 July 23, 2013, 16:58 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,732 Rep Power: 143 And what do you get in CFX? Are you sure you have CFX using the correct units?

 July 23, 2013, 18:22 #3 Senior Member   Erik Join Date: Feb 2011 Location: Earth (Land portion) Posts: 1,171 Rep Power: 23 You are obviously not using enough decimals. format trendline and pick the equation and format it as text. you will get much more digits.

July 25, 2013, 08:45
expressions in cfx
#4
Member

Karthik
Join Date: Oct 2012
Location: Germany
Posts: 53
Rep Power: 13
Dear Glenn,

I tried again using the modified expressions in cfx. But it is not working. I am attaching a picture which shows the polynomial equations obtained from excel and implementing it in cfx. The results are drastically different. Visocsity becomes negative if I implement it in CFX. I don't understand how it works. Please give me suggestions as to what I am doing wrong.
Attached Images
 expressions.jpg (27.4 KB, 369 views)

Last edited by selvam2487; July 25, 2013 at 08:55. Reason: attaching picture

 July 25, 2013, 11:54 #5 Senior Member   Edmund Singer P.E. Join Date: Aug 2010 Location: Minneapolis, MN Posts: 511 Rep Power: 20 I think something is wrong with your trendline eqn. Type that exact equation in a cell with the temp cells as the input. I bet it doesnt match your actual data.

 July 25, 2013, 13:49 #6 Senior Member   Erik Join Date: Feb 2011 Location: Earth (Land portion) Posts: 1,171 Rep Power: 23 Wasn't I clear enough the first time? YOU NEED TO USE MORE THAN 1 DIGIT when you use higher order polynomials: Use This function for viscosity: 5.41924E-12x4 - 8.34074E-09x3 + 4.82854E-06x2 - 0.001248669x + 0.122192878 Not this: 5E-12x4 - 8E-09x3 + 5E-06x2 - 0.0012x + 0.1222 Use this for density: 5.36279E-06x3 - 0.008336901x2 + 3.261356376x + 624.9598808

 July 26, 2013, 12:12 Importing MS Excel data in ANSYS CFX #7 Member   Jan Join Date: Jul 2013 Location: Berlin - Germany Posts: 36 Rep Power: 12 Hi. I try to give you another solution. If you have an array of values in MS Excel, you should save them in a CSV File (comma separated values). The values of the columns are separated by a comma - values of different rows are separated by the line number. You can import such a file in ANSYS CFX by defining a user function. Its not so complicated: CFX Pre >> "Expressions, Functions and Variables" >> "User Functions" *right click* "Insert" >> "User Function". Now you have to define the units of the argument and the returned value of this function. If you want to calculate the density depending on temperature, the argument units are °C or K (in CFX [C] or[K]). The result units are kg/m³ (in CFX [kg/m^3]). Now keep on with a right click in the large listbox and choose "import data". Select the CSV File (just selecting, no double click), you exported in MS Excel (you have to change the selection filter to |all files *.*|). In column selection you have to specify the column number of the argument- and result values. Now, click Ok! In an expression, you can use the return value of this function by typing the name of the user function followed by brackets with the variablename of the depending value: Expression_Density = User Function1(T) In this example, the Function you defined is called "User Function1". T is the short name of the temperature in CFX. There are two disadvantages: You have no access to this data in CFD Post and you have no continuous function. But the advantage is, that you got clipped, linear interpolated values of the function. A polynom of an high order may fit the values of the function inside the domain of its definition quite well. Exceeding this range, the polynom runs to large (pos/neg) values. You also need to use much more positionis after devision in the coeffficient, when you multiply it with x^12 . Regards, Jan ---------------------------- Jan Smedseng CFX Berlin Software GmbH roman.gaspar likes this.

 Tags les, t junction