Choosing laminar or turbulent model

nived · November 17, 2018, 07:24

Iam doing a CFD simulation on horizontal cylinders placed vertically above one another. The top cylinder is rotating while the bottom cylinder is stationary. Iam facing difficulty in whether to choose laminar or turbulent model. Iam attaching my geometry here.
2 cylinders one above another.
Both are heated cylinders 800W/m2K
Diameter 0.0254m
Distance between the cylinders is varied as 2D, 4D, 6D
Top cylinder is rotated at 200 rpm, 400 rpm, and 600rpm.
Please somebody help....

nived · November 18, 2018, 02:50

Pls somebody help....

Maddin · November 19, 2018, 08:23

Google Reynolds.

acalado · November 20, 2018, 10:57

I would try seeing if with the RPM you are proposing, if there is any characteristic Reynolds based on this that would case turbulent flow around the cylinder.

Otherwise, just try running laminar and see if you have any issues with convergence. If you do you can try changing models.

nived · November 20, 2018, 14:16

Iam trying to simulate natural convection in a cylinder. Iam considering rectangular domain around cylinder. To study the influence of domain, i tried increasing the domain size...but the temperature on cylinder increases continuously as the domain size increases.
Cylinder dia 25.4cm
Heat flux 800W/m2K
Why does this happen?? What to do?

Jiricbeng · November 22, 2018, 02:10

Do you consider radiation? What type of boundary condition is the rectangle domain? Wall, far-field, inlet and outlet?

ztdep · November 26, 2018, 07:52

dear sir, the flow regime of natural convection is determined by the Grasholf number, so compute the Gr number first based on the heat flux of the cylinder.

November 19, 2018, 08:23		#3
Maddin Senior Member Join Date: Mar 2009 Posts: 260 Rep Power: 18	Google Reynolds. nived likes this.

November 20, 2018, 10:57		#4
acalado Senior Member André Join Date: Mar 2016 Posts: 133 Rep Power: 10	I would try seeing if with the RPM you are proposing, if there is any characteristic Reynolds based on this that would case turbulent flow around the cylinder. Otherwise, just try running laminar and see if you have any issues with convergence. If you do you can try changing models. nived likes this. __________________ Sapere aude!

November 20, 2018, 14:16	Natural cconvection in cylinder 2D	#5
nived New Member Nived Sankar Join Date: Mar 2018 Posts: 27 Rep Power: 8	Iam trying to simulate natural convection in a cylinder. Iam considering rectangular domain around cylinder. To study the influence of domain, i tried increasing the domain size...but the temperature on cylinder increases continuously as the domain size increases. Cylinder dia 25.4cm Heat flux 800W/m2K Why does this happen?? What to do?

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November 18, 2018, 02:50		#2
nived New Member Nived Sankar Join Date: Mar 2018 Posts: 27 Rep Power: 8	Pls somebody help....

November 22, 2018, 02:10		#6
Jiricbeng Senior Member Jiri Join Date: Mar 2014 Posts: 218 Rep Power: 13	Do you consider radiation? What type of boundary condition is the rectangle domain? Wall, far-field, inlet and outlet?

November 26, 2018, 07:52		#7
ztdep Senior Member p ding Join Date: Mar 2009 Posts: 427 Rep Power: 19	dear sir, the flow regime of natural convection is determined by the Grasholf number, so compute the Gr number first based on the heat flux of the cylinder.