low pressure at front of buildings

CFT · March 1, 2010, 01:26

Dear all

I am simulating flow over a small low rise building. When I look at the dynamic pressure at the front of the building center, i get very low velocity and dynamic pressure. However this is contrary to experimental reults. Any suggestions as to why ?

I am using a k epsilon rng with wall functions, hexahedral grid, double digit precision.

Any help is much appreciated

regards

CFT

tony00 · March 1, 2010, 11:55

Hi,
two recommendations:
1) maybe this is a silly question but, are you sure your simulation is converged?
I know you might be expert and know it but someone else might find this useful.
The default convergence criteria set by Fluent are not sufficient and maybe misleading.
I recommend to get rid of the residual checking:
solve-> monitors->residuals->convergence criterion: NONE.
Create a couple of points in your domain where you monitor a significant quantity, this could be pressure or velocity component in the prevailing direction of your flow.
I would consider the simulation converged when the residuals are 10^-4 AND the monitored pressure or velocity components are independent of the iteration.

2) If you are using k-epsilon with STD wall function check the y+ on the walls: 30 <y+<300. If you are using k-epsilon with Enhanced Wall treatment y+<5 and ideally y+~1.

Good luck

CFT · March 5, 2010, 04:37

Hi

I checked my residuals and are all below 1e-6. Also the yplus is in the range of 0<yplus<300 for wall functions.

I compare the pressure coefficent and compare with experiemntal results and get less than 0.3. Which cannot be right ?

Is it something to do with my reference pressure location ? I place it at 0,0,0 as recommended in manual.

Any other suggestions

thanks

CFT

tony00 · March 7, 2010, 17:25

Hi Have you done this?

1) Create a couple of points in your domain where you monitor a significant quantity, this could be pressure or velocity component in the prevailing direction of your flow.
I would consider the simulation converged when the residuals are 10^-4 AND the monitored pressure or velocity components are independent of the iteration.

2) this is the correct range of y+ for wall functions: 30<y+<300 instead of 0 <y+<300.

March 1, 2010, 01:26	low pressure at front of buildings	#1
CFT New Member CFT Join Date: Mar 2009 Posts: 15 Rep Power: 17	Dear all I am simulating flow over a small low rise building. When I look at the dynamic pressure at the front of the building center, i get very low velocity and dynamic pressure. However this is contrary to experimental reults. Any suggestions as to why ? I am using a k epsilon rng with wall functions, hexahedral grid, double digit precision. Any help is much appreciated regards CFT

March 1, 2010, 11:55	Convergence criteria, wall fuction vs enhanced Wall treatment	#2
tony00 New Member Join Date: Nov 2009 Posts: 22 Rep Power: 16	Hi, two recommendations: 1) maybe this is a silly question but, are you sure your simulation is converged? I know you might be expert and know it but someone else might find this useful. The default convergence criteria set by Fluent are not sufficient and maybe misleading. I recommend to get rid of the residual checking: solve-> monitors->residuals->convergence criterion: NONE. Create a couple of points in your domain where you monitor a significant quantity, this could be pressure or velocity component in the prevailing direction of your flow. I would consider the simulation converged when the residuals are 10^-4 AND the monitored pressure or velocity components are independent of the iteration. 2) If you are using k-epsilon with STD wall function check the y+ on the walls: 30 <y+<300. If you are using k-epsilon with Enhanced Wall treatment y+<5 and ideally y+~1. Good luck

March 5, 2010, 04:37	low pressure at front of building	#3
CFT New Member CFT Join Date: Mar 2009 Posts: 15 Rep Power: 17	Hi I checked my residuals and are all below 1e-6. Also the yplus is in the range of 0<yplus<300 for wall functions. I compare the pressure coefficent and compare with experiemntal results and get less than 0.3. Which cannot be right ? Is it something to do with my reference pressure location ? I place it at 0,0,0 as recommended in manual. Any other suggestions thanks CFT

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March 7, 2010, 17:25		#4
tony00 New Member Join Date: Nov 2009 Posts: 22 Rep Power: 16	Hi Have you done this? 1) Create a couple of points in your domain where you monitor a significant quantity, this could be pressure or velocity component in the prevailing direction of your flow. I would consider the simulation converged when the residuals are 10^-4 AND the monitored pressure or velocity components are independent of the iteration. 2) this is the correct range of y+ for wall functions: 30<y+<300 instead of 0 <y+<300.