CFX Post - expressions / math. operations

meister-hubert · May 7, 2018, 17:38

Hi,

I need to calculate second derivatives in CFX Post.
I created an expression which works for something like $\frac{\partial v_x}{\partial x}$ with
"Velocity u.Gradient X"

Basically I want to calculate something like $\frac{\partial^2}{\partial x_i \partial x_j}u_i u_j$

I tried different things like Velocity u.Gradient X.Gradient X, but even (Velocity u * Velocity v).Gradient X doesnt work. Also creating an expression for lets say "grad_x_v_x = Velocity u.Gradient X" and then creating a new expression "grad_x_v_x.Gradient X" doesn't work.

I am using version 18.2.

Can anyone help? Thank you guys so much!

Opaque · May 8, 2018, 16:21

Insert a User Defined variable, select scalar type.

Then, set equal to Velocity u.Gradient X

MyUGradX = Velocity u.Gradient X

Now, insert a new variable (any name)

Select Gradient, and choose you MyUGradX, press Apply

You should get MyUGradX.Gradient which represents

MyUGrad.Gradient X, MyUGrad.Gradient Y, MyUGrad.Gradient Z

Effectively, they should correspond to

d2u/dx2, d2u/dydx, d2u/dzdx

May 7, 2018, 17:38	CFX Post - expressions / math. operations	#1
meister-hubert New Member Join Date: May 2018 Posts: 1 Rep Power: 0	Hi, I need to calculate second derivatives in CFX Post. I created an expression which works for something like $\frac{\partial v_x}{\partial x}$ with "Velocity u.Gradient X" Basically I want to calculate something like $\frac{\partial^2}{\partial x_i \partial x_j}u_i u_j$ I tried different things like Velocity u.Gradient X.Gradient X, but even (Velocity u * Velocity v).Gradient X doesnt work. Also creating an expression for lets say "grad_x_v_x = Velocity u.Gradient X" and then creating a new expression "grad_x_v_x.Gradient X" doesn't work. I am using version 18.2. Can anyone help? Thank you guys so much!

May 8, 2018, 16:21		#2
Opaque Senior Member Join Date: Jun 2009 Posts: 1,880 Rep Power: 33	Insert a User Defined variable, select scalar type. Then, set equal to Velocity u.Gradient X MyUGradX = Velocity u.Gradient X Now, insert a new variable (any name) Select Gradient, and choose you MyUGradX, press Apply You should get MyUGradX.Gradient which represents MyUGrad.Gradient X, MyUGrad.Gradient Y, MyUGrad.Gradient Z Effectively, they should correspond to d2u/dx2, d2u/dydx, d2u/dzdx aero_head likes this.

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