UNRESOLVED: Inlet/outlet geometry not allowing transfer of flow URGENT HELP

[Anonymous]

Dear Autodesk Help.

In regards to my last post (this is becoming very urgent as I have tried multiple different ways) I rebuilt my Inventor model to first extrude holes for the inlets, and then extrude solids to mark the inlets. In CFD, when highlighting using volumes, these inlets are highlighted as one volume. The flow is still contained in this volume when the model is run. I have attached my latest inventor and CFX files, as well as another print screen of what i am experiencing in this model.

Assistance would be much appreciated, as I am going the same way as described and found previously and have no idea why this fluid isn't crossing over.

Kind Regards,

JAck

[Xiaojin.Zhang]

Hello @Anonymous,

There are some problems on your boundary conditions.

For inlets of your model, you only need to assign a volume flowrate and scalar. 

For outlets, only assign P=0.

CFD will calculate pressure on the inlet, scalar and flowrate on the outlet. Otherwise, the model will be over constraint.

Besides, for scalar analysis, you need to change the fluid from fixed to variable to allow the properties to change.

Thank you!

[Anonymous]

Hello Xiaojin

I changed the fluid volume to variable, and removed the pressure value from the inlets. The same thing has happened, the flow hasn't transferred over. 

I have cut holes in the walls and floors and then modeled the solid extrudes for the inlets/outlets like I was advised, this took a considerable amount of time to no avail also.

I tried to redraw a very simple model in Inventor trying to model basically a hollow box with inputs and outputs, but it seems when i import this into CFD, CFD groups all of the extrudes together, even though they were modeled as separate "create new solid". Is there something wrong with using inventor in conjunction with this program? 

Regards,

Jack

[Anonymous]

Screenshot of the updated result, I put a 0 pressure on the outlet, but this outlet actually has fluid leaving the volume. I set it to pressure = 0 to see if I could generate a reasonable result. Completely stumped now 

[Anonymous]

Attached is my latest CFZ file

[Xiaojin.Zhang]

Hello @Anonymous,

I just found some other setting problems on your model which are causing the issue here.

1. Scalar analysis is not enabled. The diffusion coefficient also needs to be input for scalar mixing analysis.

Please refer to the link below about how to enable scalar.

http://help.autodesk.com/view/SCDSE/2018/ENU/?guid=GUID-DCEC1333-ABBD-477F-9E33-FA3EE7A7EC0E

2. Please try to use volume flowrate instead of mass flowrate on the inlets. Inlet velocity is not properly calculated by mass flow rate boundary condition for scalar analysis.
3. Mesh at inlets and through the thickness direction is a little coarse. It should be refined.

4. Sometimes, the transient analysis is more difficult to converge due to big time step or mesh size, etc. For this case, I would suggest you try to reduce the time step or by running the analysis by two steps:

a. Disable scalar analysis and run a flow only steady state calculation.

b. After flow calculation converges, disable flow analysis and enable scalar analysis and run a scalar only transient analysis.

It should you get a stable result easily by this method. Please adjust your model and let me know if you have any questions.

Thank you!

[Anonymous]

Hi Xiaojin.Zhang.

In regards to Jons post also, I have enabled the scalar analysis. I have also input the correct diffusion coefficient.

I will convert the g/h into L/s and input these into the model. I am still having a fair few issues that i can't figure out:

I feel like the outlets are not modeled correctly, as it seems that the scalar variable is diffusing into the top inlets and not being relieved (see below screenshot. As you can see in the screenshot, the top red areas are completely covered in the scalar variable, and these 4 fluids are meant to be simulating exhaust from the car park. I set a few different BC's but none seemed to work (There are differing opinions on this it seems). IF  i need to model a flow outward, to i set a volume flow rate, scalar 0 and pressure 0? or just pressure 0 and flow rate, or just flow rate and scalar? seeing as there are 3 variables and at least 2 need to be on it seems like there are 6 different ways to do it. I am still confused by this.

"

4. Sometimes, the transient analysis is more difficult to converge due to big time step or mesh size, etc. For this case, I would suggest you try to reduce the time step or by running the analysis by two steps:

a. Disable scalar analysis and run a flow only steady state calculation.

b. After flow calculation converges, disable flow analysis and enable scalar analysis and run a scalar only transient analysis.

It should you get a stable result easily by this method. Please adjust your model and let me know if you have any questions."

I am very unsure what you mean by number 4. if i turn off scalar and run it for a little while the scalar flow will bulk up in the inlet extrusions just like i did previously, and because there is no mixing it will just be the volume flow rates in the model - but I cannot check this because I still cannot get an outlet to model the exhaust leaving the model properly. 

I have attached a screen of the outlet configuration, please note the zero pressure outlets vs the ones that I am trying to model a flow with, 4 are ramps going into other carparks (P = 0) and the other 4 openings are meant to simulated ducted exhaust out of the carpark to remove the CO buildup 

I have also attached my newest model for your input.

Thanks for your help.

Jack

[Xiaojin.Zhang]

Hi @Anonymous,

Regarding the outlet setting, there is not need to apply a volume flowrate or scalar.

Please try to change mass flow rate to volume flow rate. My testing result it that the mass flow rate doesn't work well for this model.

Regarding my suggestion about running the the analysis by two steps, 

You are correct that scalar flow will bulk up in the inlet extrusions during the steady-state calculation in step1.

However Step 2 will allow you to start the scalar mixing calculation. This intends to begin the scalar mixing after the flow gets converged.

Thanks!

[Anonymous]

Hi XiaoJin,

I think you are misinterpreting my outlets, I need to supply air to the space to mix with the contaminant, and then this mixture needs to be exhausted out of the system, otherwise the volume will just have air/CO being pumped into it and it would over pressurize/increasingly increase carbon monoxide. 

I have attached a PDF describing the way that the carpark exhaust systems usually work. I need an airflow to leave the volume that has been mixed, hence wondering if it needs a volume flow as well as a scalar, just showing out of the model. I have set up a model that appears from solid fill view to be showing this (The top of the outlets with the negative velocity show blue, which seems right, with red leading up into it as if it is being exhausted.

Although, when i draw a plane that cuts slightly deeper into the volume, it appears there is a large amount of red (Scalar 1). to the rough scale of 0.5. This means that concentration in the carpark has reached (0.5 x 1e6) = 500000 ppm, which is not right.  the same thing has happened in this persons post:

https  // forums.autodesk.com/t5/cfd-forum/scalar-mixing-concentration-ppm/td-p/5057312 - note i had to break the hyperlink to paste this 

where a large majority of the car park is almost showing ONE MILLION ppm. the concentration to satisfy the requirements needed in my area are 50ppm, this person had accepted the solution of a million ppm.. I believe his model is showing something very wrong.

Bit lost with how its being diffused/concentrations/whether any fluid is actually being extracted from the model like it should (in reality there are big fans in shafts that drag the air out of the volume and to the outside atmosphere at  a suitable discharge location)..

Attached are some screenshots of what I have described, please note the scalar values in the outlets like i have mentioned, as well as the plane cut showing much red up in the north of the model (Signifying a 500 000 ppm result apparently) - this is assuming previous posts are correct (that state scalar 1 = 1000 000 ppm) I don't understand that conversion, it must be something specific to ACAD CFD?

another previous post the user had a similar result to me, so he times his mass flow rate of the CO inlet by 500 and then divided his scalar by 500, meaning he had a 0.02 scalar as his input BC. I am using my original calculated mass flow rate, with a scalar of 1. Again not sure at all if this is right. Will also be converting to L/s for the inlets to see if that changes the result at all.

Sorry for so much information, I believe these points I am raising question further the validity of the past users' results/highlights some common areas people will have problems in as I have.

Regards,

Jack 

[Anonymous]

Attached model for your reference 

[Xiaojin.Zhang]

Hi @Anonymous,

From the P=0 boundary condition, the mixture will be exhausted. The mass flow rate exhausted from the P=0 boundary condition ideally will be equal with the mass flow rate coming into the system. It is based on mass conservation. There is no need to assign volume flow rate to P=0.

However, you might need to extend the outlet to avoid backflow from the outlet.

Before you check the scalar result, I would suggest you to make sure that the result is converged. If the analysis doesn't converge, the result is pointless.

Thanks!

[Anonymous]

Hi Xiaojin

Thank you for all your replies, I appreciate it very much.

Interesting point you have brought up there! you say that any outlets are assigned a P=0 and then CFD figures out the resultant flow rates out. I understand how that works in a closed system where everything balances to zero.

The difference in this particular case is that usually the air supplied to the carpark is less than that exhausted, this is to ensure there is a negative pressure in the volume so that the contaminants can be exhausted  (at a faster rate also).

There is no way to create it "imbalanced" so you'd say? because at the moment it appears like there isn't enough CO being exhausted, or any at all. I will remove the outlets to represent stairwells and only have outlets representing exhaust and see what the result is, but i dare say if CFD creates a volume flow out equal to that in, then there be much reticulation of CO around the carpark (i.e it will not exhaust properly if there isn't sufficient negative pressure). 

[Jon.Wilde]

If we pump in more air than is coming out, what is happening? We are losing air somehow?

We cannot create or destroy mass, so it must actually be leaking out somewhere else, like door seals etc.

I know what you mean though, we do usually have a higher pressure inside so that flow is only exiting, we should still see this with the setup we have, although if you need more detail, we might have to consider modelling things more accurately down the road. 

I suggest getting the basics under your belt first and then stepping up to the additional details (where you might need resistances on the outlets to try to force the flow out of other leakage paths, which you would then also need to model). Often this simply is not necessary 🙂

[Anonymous]

I see what you mean about the mass conservation, but I suppose in reality what is happening is the fans on either side of the system are effectively changing the pressure in the volumes creating a negative pressure in the compartment (the pressure causing stored work on the mixture, effectively creating a flow in the direction of a pressure lesser to its own, done by both creating a positive pressure forcing inwards and one pulling outwards, just stronger.

I think for this case the extraction of the gases is the main concern and reason for this study to be done in the first place, as the normal regulatory guidelines cannot be met for a carpark of such size - and engineered solution is required, hence the CFD!

This more detailed version - I'm guessing I'm going to need to get on board this from what it sounds like, and these resistances you speak of, are they a negative resistance per-say? because otherwise it would cause even less mass to leave the volume, The results are already showing an unreasonable amount of contaminant. Please advice as to what the next best path of action is. I will remodel the inventor file as required. Is it possible to model a negative pressure at the outlet and create a larger flow through the boundary?

Cheers for the guidance,

Jack

[Anonymous]

*BUMP. I still need assistance as to this car park exhaust model. none of the models that have been submitted here have been anywhere close to correct. As stated in my previous posts, a concentration of 500 000 ppm is not acceptable. I need a way to model an exhaust for this system, Still unsure about what result will come out of P=0 for the outlets

[Xiaojin.Zhang]

Thank you for the detailed information.

If the volume flowrate of the exhaust flow is a known condition, we can change the P=0 boundary condition to the volume flowrate

However, to keep the model balanced in mass, you need to create an additional P=0 boundary to allow the air to leak out. The boundary conditions should be like the picture below.

Do you think this setting make sense?

Thank you!

Xiaojin

[Anonymous]

Hi XiaoJin,

I set up the model with some P = 0 outlets as well as volumetric flow rates,

The model that has these BC's doesn't seem to be exhausting contaminant, as i said before the values for CO increase rapidly, and then just fill the entire volume.

I dont believe this works. 

I will again set my model up in this fashion and send it to you so you can see what is happening - That illustration is what i made to describe the scenario previously. I know this isn't working because if a volume flowrate out of the model is set as the boundary condition, CFD doesn't recognise these as outlets, I can tell because it still knows the P= 0 outlets are outlets, I.E the message window reads: 

18 Inlets 2 Outlets 0 Unknowns

So i cant just model a volumetric flow rate out, I have tried all the different combinations and I am completely stuck, it looks the same every time, the model along with the outlets just fill up completely with scalar 1.

Is it even possible for Autocad CFD to model exhaust systems?

Attached is my file with the described layout and a print screen of what happens when I use the arrangment described.

I have been losing sleep over this analysis. It is actually modelling a very large carpark to be built soon. I need a solution or to know if this program is capable of the simple system I am trying to model, I know for a fact other software's have modeled these systems in a way that shows no more than a 50ppm rise (as opposed to any of the models I've seen on here showing a 500 000 ppm rise as an accepted result).

Thanks for the assistance.

Regards,

Jack

[Xiaojin.Zhang]

Hi Jack,

Setting the P=0 as well as the volume flowrate to the same boundaries doesn't make sense to calculation.

There is difference between the real word and the calculation word. CFD needs some known boundaries to solve the governing equation.

Below is my result. The outlet is P=0.

You can see that air is exhausting from the P-0 boundaries. You can also see that there is also air entering into this boundary. That is why we suggest intending the outlet to avoid the back flow.

The inlet and outlet number in the log is not the physical inlet or outlet. Volume flow or mass flow rate boundaries are recognized as inlet and P=0 is recognized as outlet in the log. 

I hope it helps to you.

Thank you!

Xiaojin

[Xiaojin.Zhang]

Hi Jack,

I am attaching my model for your reference.

Thanks!

[Anonymous]

Hi Xiao,

In this model you haven't stated any volume flow rates at the outlets?

So How do I know for sure if any Carbon monoxide is leaving at the P = 0 outlet? I feel like I have so much conflicting information about what to do now, and did you do a conversion to increase the flow rates of the scalar and hence reduce its value? ( you have the units as m^3/h). 

I have also been able to get vectors at the P = 0 boundaries, but I dont know how much this means. I need to be able to make sure the amount of CO in the car park is less than 0.00005 scalar, or doesn't exceed this amount in any area. 

In regards to how many inlets/outlets, i thought this may be applicable as there is a massive amount of CO in my model regardless of how i set up the initials.

I have graphed your model after the first few iterations, as you can see the majority of the area is already 5e-05, in other words everyone in that carpark is suffering from carbon monoxide poisoning - attached is the screenshot.

The way these systems work in real life is that a fan pulls (for this case 100 000 L/s) out of the car park, this is 100 000 litres every second. All this model is doing is throwing in carbon monoxide into a box, allowing it to slowly leak out an opening. Can autocad CFD do exhaust modelling?

Basically what I'm trying to say is, something is fundamentally wrong with the program or the model setup, either its a limitation of the program or there is something we are missing.

The reason i say this, is that every time we change something or something new comes up as an idea, the EXACT SAME THING happens to the model, it fills up with huge, unrealistic amounts of carbon monoxide, with what appears to be very very little exhaust if ANY at all, i guess that's impossible to tell how much is leaving as according to what i have heard, it can only model a conserved flow through the fluid in question. when in reality the composition of the volume is constantly changing. maybe not its size, but definitely its concentrations.

In the other model we had sent off to do the CFD (a payed for analysis - yes we require actual professional results here), using a different program, the user had inlets with a positive pressure and outlets either a zero or negative (I'm not sure on this).

Trying a positive pressure at the inlets ends up with CO infiltrating the inlet extrusions. So it definitely cannot be modeled this way in autocad CFD.