Drag results and contours - Help

Franke · March 20, 2020, 06:08

Hi,
I ran my simulation though 2 of the 3 I had running failed but even the one that was successfull my drag coefficient results are arounf -45000! Please help me with suggestions for this I cannot understand or find a way to fix them

vinerm · March 20, 2020, 06:54

All the coefficients rely upon some normalizing factor, a value in the denominator for a fraction where predicted forces are the numerators. The values for these factors or denominators are taken from the Reference Values in Fluent. If those values are wrong, coefficients are wrong, too. However, this does not mean the simulation is wrong. Reference values do not affect the simulation until and unless user modifies something on the basis of coefficients. So, check your Reference Values, modify those as the current case, and then report coefficient.

Franke · March 20, 2020, 07:09

Are the reference values, e.g. area, to be of the entire domain or for the specific part to be considered? Its possible to compute these values from a boundary condition, should I do this? would it be best to input from one of the inlet conditions?

vinerm · March 20, 2020, 07:13

Usually, if you wish to compare your results against some available data or simulation, you need to use same reference values as used in the existing work. However, to make it a global standard, for most of the work, these values are standardized, such as, chord length of airfoil, frontal area for any bluff body, properties of free stream, etc. As far as drag coefficient is concerned, you need to ensure you specify density, velocity, and area. Density and velocity should be of the free stream and area should be frontal area, i.e., cross-section area normal to the freestream

Franke · March 20, 2020, 07:23

My body is essentially a semicircle (due to symmetry) made of 2 pipes, connection parts and uprights to handrail, should I simply input the widest point?, as it is pipes there is flow in the 'middle' of the circle also

Franke · March 20, 2020, 07:26

as in should the area be as though it is a block shape? as that is what you would see from head one? also is it area both above and below the waterline?

vinerm · March 20, 2020, 07:36

If it is a symmetric model, then you should provide half the area, i.e., the area that really exists in your model. As mentioned earlier, the area has to be the area of the object normal to the freestream direction. E.g., if the flow is in the x-direction, then take the projection of the object on the y-z plane and the area of this projection is to be used as reference value.

When Fluent predicts drag or any force coefficient in a VOF, it is considering both the phases. Therefore, whole of the area has to be taken into consideration.

Franke · March 20, 2020, 09:45

Hi, thanks am getting much closer results now, made it to -2 the flow is in x direction is the length it requires in the x direction also?

Franke · March 20, 2020, 09:47

Also is the viscosity it wants to be of the water? I put the density to that of water rather than air also

vinerm · March 20, 2020, 09:57

The reference values required to determine drag coefficient are density, velocity, and area. Nothing else matters.

Franke · March 27, 2020, 08:02

Hello,
I was wondering if you could advise on some of my results, I am still receiving negative results for both drag force and coefficient, any suggestions of possibilities for this? When initialising the solution I am inputting the x velocity as negative as it should be traveling in the opposite direction of the x-axis, is this correct? My global axis is also located at the centre of the body could this be having an effect? Is it also suitable that I am using the density of water in my reference values? the body is partially submerged between water and air but if using air density the values appear much much too high. Thanks so much for any help on this.

Franke · March 30, 2020, 12:49

Hello,
I have been getting results for my simulations, i am running 3 simulations to compare a slightly altered body within each.
I am getting much higher results for those where lower results were expected and these are also around a magnitude of 2/3 for drag coefficient. Do you have any suggestions of particular reference areas to check?

I also have walls set as my upper and lower bounds though I would like these to have as little effect as possible on the domain would you simply reccommend the use of no slip walls? or with shear? I have tried several options and don't appear to be finding much difference between them as they are

Thanks in advance!

March 20, 2020, 06:54	Coefficients	#22
vinerm Senior Member Vinerm Join Date: Jun 2009 Location: Nederland Posts: 2,946 Blog Entries: 1 Rep Power: 36	All the coefficients rely upon some normalizing factor, a value in the denominator for a fraction where predicted forces are the numerators. The values for these factors or denominators are taken from the Reference Values in Fluent. If those values are wrong, coefficients are wrong, too. However, this does not mean the simulation is wrong. Reference values do not affect the simulation until and unless user modifies something on the basis of coefficients. So, check your Reference Values, modify those as the current case, and then report coefficient. __________________ Regards, Vinerm PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.

March 20, 2020, 07:13	Reference for Reference Values	#24
vinerm Senior Member Vinerm Join Date: Jun 2009 Location: Nederland Posts: 2,946 Blog Entries: 1 Rep Power: 36	Usually, if you wish to compare your results against some available data or simulation, you need to use same reference values as used in the existing work. However, to make it a global standard, for most of the work, these values are standardized, such as, chord length of airfoil, frontal area for any bluff body, properties of free stream, etc. As far as drag coefficient is concerned, you need to ensure you specify density, velocity, and area. Density and velocity should be of the free stream and area should be frontal area, i.e., cross-section area normal to the freestream __________________ Regards, Vinerm PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.

March 20, 2020, 07:36	Area	#27
vinerm Senior Member Vinerm Join Date: Jun 2009 Location: Nederland Posts: 2,946 Blog Entries: 1 Rep Power: 36	If it is a symmetric model, then you should provide half the area, i.e., the area that really exists in your model. As mentioned earlier, the area has to be the area of the object normal to the freestream direction. E.g., if the flow is in the x-direction, then take the projection of the object on the y-z plane and the area of this projection is to be used as reference value. When Fluent predicts drag or any force coefficient in a VOF, it is considering both the phases. Therefore, whole of the area has to be taken into consideration. __________________ Regards, Vinerm PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.

March 20, 2020, 09:57	Drag Coefficient	#30
vinerm Senior Member Vinerm Join Date: Jun 2009 Location: Nederland Posts: 2,946 Blog Entries: 1 Rep Power: 36	The reference values required to determine drag coefficient are density, velocity, and area. Nothing else matters. __________________ Regards, Vinerm PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.

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March 20, 2020, 06:08		#21
Franke New Member Join Date: Mar 2020 Posts: 26 Rep Power: 6	Hi, I ran my simulation though 2 of the 3 I had running failed but even the one that was successfull my drag coefficient results are arounf -45000! Please help me with suggestions for this I cannot understand or find a way to fix them

March 20, 2020, 07:09		#23
Franke New Member Join Date: Mar 2020 Posts: 26 Rep Power: 6	Are the reference values, e.g. area, to be of the entire domain or for the specific part to be considered? Its possible to compute these values from a boundary condition, should I do this? would it be best to input from one of the inlet conditions?

March 20, 2020, 07:23		#25
Franke New Member Join Date: Mar 2020 Posts: 26 Rep Power: 6	My body is essentially a semicircle (due to symmetry) made of 2 pipes, connection parts and uprights to handrail, should I simply input the widest point?, as it is pipes there is flow in the 'middle' of the circle also

March 20, 2020, 07:26		#26
Franke New Member Join Date: Mar 2020 Posts: 26 Rep Power: 6	as in should the area be as though it is a block shape? as that is what you would see from head one? also is it area both above and below the waterline?

March 20, 2020, 09:45		#28
Franke New Member Join Date: Mar 2020 Posts: 26 Rep Power: 6	Hi, thanks am getting much closer results now, made it to -2 the flow is in x direction is the length it requires in the x direction also?

March 20, 2020, 09:47		#29
Franke New Member Join Date: Mar 2020 Posts: 26 Rep Power: 6	Also is the viscosity it wants to be of the water? I put the density to that of water rather than air also

March 27, 2020, 08:02		#31
Franke New Member Join Date: Mar 2020 Posts: 26 Rep Power: 6	Hello, I was wondering if you could advise on some of my results, I am still receiving negative results for both drag force and coefficient, any suggestions of possibilities for this? When initialising the solution I am inputting the x velocity as negative as it should be traveling in the opposite direction of the x-axis, is this correct? My global axis is also located at the centre of the body could this be having an effect? Is it also suitable that I am using the density of water in my reference values? the body is partially submerged between water and air but if using air density the values appear much much too high. Thanks so much for any help on this.

March 30, 2020, 12:49		#32
Franke New Member Join Date: Mar 2020 Posts: 26 Rep Power: 6	Hello, I have been getting results for my simulations, i am running 3 simulations to compare a slightly altered body within each. I am getting much higher results for those where lower results were expected and these are also around a magnitude of 2/3 for drag coefficient. Do you have any suggestions of particular reference areas to check? I also have walls set as my upper and lower bounds though I would like these to have as little effect as possible on the domain would you simply reccommend the use of no slip walls? or with shear? I have tried several options and don't appear to be finding much difference between them as they are Thanks in advance!