two separation bubbles

Josh · December 13, 2010, 12:54

It looks like you posted an instantaneous Cp distribution. Those "multiple laminar separation bubbles" could be start-up/shedding vortices. If you average the Cp graph over time, you should obtain a better Cp distribution (the sharp spikes in the Cp graph should average to a plateau).

ghorrocks · December 13, 2010, 16:24

And a point from my previous post - what do you mean by PISO? This is not an option available in CFX.

aqib · December 14, 2010, 01:53

PISO is actually present in Fluent 12.0.16 CFX.
The Pressure-Implicit with Splitting of Operators (PISO) pressure-velocity coupling
scheme, part of the SIMPLE family of algorithms, is based on the higher degree of the
approximate relation between the corrections for pressure and velocity. One of the limitations of the SIMPLE and SIMPLEC algorithms is that new velocities and corresponding fluxes do not satisfy the momentum balance after the pressure-correction equation is solved.

Josh · December 14, 2010, 02:28

This is the CFX forum. Your question should be posted on the Fluent forum.

Any luck with the averaging?

aqib · December 14, 2010, 03:48

How the averaging is being taken? I don't know about that!

aqib · December 14, 2010, 03:49

now my graph of Cp is like that after taking the time step size of 0.001.

ghorrocks · December 14, 2010, 04:47

Yes, Aqib, I am well aware of what PISO and SIMPLE is. CFX only uses a coupled solver, very similar to the coupled solver in Fluent (the coupled solver in Fluent is from CFX technology, and for ancient history buffs the coupled solver in CFX came from Tascflow which was purchased by CFX years ago.)

aqib · December 14, 2010, 05:11

i think you can't understand....

ghorrocks · December 14, 2010, 05:27

Can't understand what?

aqib · December 14, 2010, 05:37

About PISO scheme

ghorrocks · December 14, 2010, 05:43

I am well aware of what the PISO scheme is. PISO is an option in Fluent, and is not available in CFX - and this is a CFX forum. So what don't I understand?

aqib · December 14, 2010, 05:59

mine mistake :-)

Josh · December 14, 2010, 13:38

That Cp graph looks much better. Did you average the Cp values over time?

aqib · December 22, 2010, 02:11

are you talking about the single value averaged on the whole blade? Are you asking about the mean value of Cp plot over the complete flow time?
If you are talking about the first case then should i average the value on the vertex or facet?

Josh · December 22, 2010, 11:02

I'm talking about Cp averaged over time.

aqib · December 22, 2010, 13:53

i can't understand how to get that?
Did i take Facet Average over Blade?
How to take the Average Cp over time?

Josh · December 22, 2010, 16:30

You'll have to export the Cp data at each timestep, which can be done with a batch file, then write a macro that averages all the Cp files.

aqib · December 24, 2010, 01:20

I calculate the Pressure co-efficient value by two methods tell me which is right....
1. I record the Pressure co-efficient value and select the "Integral" in Report type option(In surface Monitor) and after 4.4239 sec my average value is: 0.467477514
2. I record the Pressure co-efficient value and select the "Facet Average" in Report type option(In surface Monitor) and after 4.4239 sec my average value is: 1.063549129
Tell me which method is right and which value is more accurate?

aqib · December 24, 2010, 01:21

My Velocity Ratio is:
Vout/Vin = 1.779
is it ok?

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December 13, 2010, 12:54		#21
Josh Senior Member Joshua Counsil Join Date: Jul 2009 Location: Halifax, Nova Scotia, Canada Posts: 366 Rep Power: 18	It looks like you posted an instantaneous Cp distribution. Those "multiple laminar separation bubbles" could be start-up/shedding vortices. If you average the Cp graph over time, you should obtain a better Cp distribution (the sharp spikes in the Cp graph should average to a plateau).

December 13, 2010, 16:24		#22
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,949 Rep Power: 145	And a point from my previous post - what do you mean by PISO? This is not an option available in CFX.

December 14, 2010, 01:53		#23
aqib Member Muhammad Aqib Chishty Join Date: Nov 2010 Posts: 50 Rep Power: 16	PISO is actually present in Fluent 12.0.16 CFX. The Pressure-Implicit with Splitting of Operators (PISO) pressure-velocity coupling scheme, part of the SIMPLE family of algorithms, is based on the higher degree of the approximate relation between the corrections for pressure and velocity. One of the limitations of the SIMPLE and SIMPLEC algorithms is that new velocities and corresponding fluxes do not satisfy the momentum balance after the pressure-correction equation is solved.

December 14, 2010, 02:28		#24
Josh Senior Member Joshua Counsil Join Date: Jul 2009 Location: Halifax, Nova Scotia, Canada Posts: 366 Rep Power: 18	This is the CFX forum. Your question should be posted on the Fluent forum. Any luck with the averaging?

December 14, 2010, 03:48		#25
aqib Member Muhammad Aqib Chishty Join Date: Nov 2010 Posts: 50 Rep Power: 16	How the averaging is being taken? I don't know about that!

December 14, 2010, 04:47		#27
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,949 Rep Power: 145	Yes, Aqib, I am well aware of what PISO and SIMPLE is. CFX only uses a coupled solver, very similar to the coupled solver in Fluent (the coupled solver in Fluent is from CFX technology, and for ancient history buffs the coupled solver in CFX came from Tascflow which was purchased by CFX years ago.)

December 14, 2010, 05:11		#28
aqib Member Muhammad Aqib Chishty Join Date: Nov 2010 Posts: 50 Rep Power: 16	i think you can't understand....

December 14, 2010, 05:27		#29
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,949 Rep Power: 145	Can't understand what?

December 14, 2010, 05:37		#30
aqib Member Muhammad Aqib Chishty Join Date: Nov 2010 Posts: 50 Rep Power: 16	About PISO scheme

December 14, 2010, 05:43		#31
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,949 Rep Power: 145	I am well aware of what the PISO scheme is. PISO is an option in Fluent, and is not available in CFX - and this is a CFX forum. So what don't I understand?

December 14, 2010, 05:59		#32
aqib Member Muhammad Aqib Chishty Join Date: Nov 2010 Posts: 50 Rep Power: 16	mine mistake :-)

December 14, 2010, 13:38		#33
Josh Senior Member Joshua Counsil Join Date: Jul 2009 Location: Halifax, Nova Scotia, Canada Posts: 366 Rep Power: 18	That Cp graph looks much better. Did you average the Cp values over time?

December 22, 2010, 02:11		#34
aqib Member Muhammad Aqib Chishty Join Date: Nov 2010 Posts: 50 Rep Power: 16	are you talking about the single value averaged on the whole blade? Are you asking about the mean value of Cp plot over the complete flow time? If you are talking about the first case then should i average the value on the vertex or facet?

December 22, 2010, 11:02		#35
Josh Senior Member Joshua Counsil Join Date: Jul 2009 Location: Halifax, Nova Scotia, Canada Posts: 366 Rep Power: 18	I'm talking about Cp averaged over time.

December 22, 2010, 13:53		#36
aqib Member Muhammad Aqib Chishty Join Date: Nov 2010 Posts: 50 Rep Power: 16	i can't understand how to get that? Did i take Facet Average over Blade? How to take the Average Cp over time?

December 22, 2010, 16:30		#37
Josh Senior Member Joshua Counsil Join Date: Jul 2009 Location: Halifax, Nova Scotia, Canada Posts: 366 Rep Power: 18	You'll have to export the Cp data at each timestep, which can be done with a batch file, then write a macro that averages all the Cp files.

December 24, 2010, 01:20		#38
aqib Member Muhammad Aqib Chishty Join Date: Nov 2010 Posts: 50 Rep Power: 16	I calculate the Pressure co-efficient value by two methods tell me which is right.... 1. I record the Pressure co-efficient value and select the "Integral" in Report type option(In surface Monitor) and after 4.4239 sec my average value is: 0.467477514 2. I record the Pressure co-efficient value and select the "Facet Average" in Report type option(In surface Monitor) and after 4.4239 sec my average value is: 1.063549129 Tell me which method is right and which value is more accurate?

December 24, 2010, 01:21		#39
aqib Member Muhammad Aqib Chishty Join Date: Nov 2010 Posts: 50 Rep Power: 16	My Velocity Ratio is: Vout/Vin = 1.779 is it ok?