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Best way for different flow directions

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Old   November 5, 2015, 02:45
Default Best way for different flow directions
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Hello

I'm simulating a case with flow over a couple of buildings and want to change the wind direction between 0 and 360 degrees.

What would be the best way to do this? Is it to rotate the STL file and make a new mesh for each wind direction, or should I change the inflow and outflow BC locations, and if so how to deal with all angles except 0 90 180 and 270 where the flow will be angled towards the boundaries.

For a westerly flow I now have an inflow and outflow boundary and symmetry conditions on the south and north wall.

Thanks in advance,
Anders
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Last edited by strofse; November 5, 2015 at 02:46. Reason: Misspelled title
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Old   November 5, 2015, 03:19
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Quote:
Originally Posted by strofse View Post
Hello

I'm simulating a case with flow over a couple of buildings and want to change the wind direction between 0 and 360 degrees.

What would be the best way to do this? Is it to rotate the STL file and make a new mesh for each wind direction, or should I change the inflow and outflow BC locations, and if so how to deal with all angles except 0 90 180 and 270 where the flow will be angled towards the boundaries.

For a westerly flow I now have an inflow and outflow boundary and symmetry conditions on the south and north wall.

Thanks in advance,
Anders
If you are automating the setting of the boundary conditions, there is absolutely nothing special about 0, 90, 180 and 270 degrees. It is trivial to write a groovyBC boundary condition that sets a vector according to a specified angle. Something alone the lines "valueExpression = (1,0,0)*cos(alpha)*windspeed + (0,1,0)*sin(alpha)*windspeed;". You can wrap this around a bash script that sets alpha using sed and runs your simulation in a loop.
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Old   November 5, 2015, 03:32
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Dear akidess,

Thanks for your reply. But how would I change the BC conditions in an angled flow as essentially there will be two inflow boundaries and two outflow boundaries (see my poorly drawn figure). It appears to me that angled flows can't have symmetry conditions on the walls? Would this not affect the solution?

(see attached fig)

bc.png

Last edited by strofse; November 5, 2015 at 03:36. Reason: changed to figure instead
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Old   November 5, 2015, 03:49
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Call me Anton. Why would you remove the symmetry conditions?

http://imgur.com/nQ784to
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Old   November 5, 2015, 03:58
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Well, that is a valid question I guess. My thought was that at a flow nearly orthogonal to the inflow it would be a strange setting, but maybe I am wrong? for a strictly northerly flow for example, it would be purely orthogonal to the inflow and outflow? Would it then not make more sense to move the inflow and outflow BC to the southern and northern wall?

Last edited by strofse; November 5, 2015 at 03:59. Reason: spelling
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Old   November 5, 2015, 04:15
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Orthogonal = Normal = 90 degree to the wall. Do you mean parallel to the boundary? If you like you can move the boundaries in that case, but there is nothing particularly strange about it. People use that for lid driven cavities all the time.
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Old   November 5, 2015, 04:25
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Yeah, sorry, parallell to the boundary, in a case as my picture below. Would this not be a very strange placement of the inflow and outflow boundary and would in not give a different solution compared to if I moved the BCs? After this point I would also need to switch position of the inflow and outflow of course.

bc2.png
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Old   November 5, 2015, 05:49
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Originally Posted by strofse View Post
Would this not be a very strange placement of the inflow and outflow boundary and would in not give a different solution compared to if I moved the BCs? After this point I would also need to switch position of the inflow and outflow of course.

Attachment 43272
Again I ask - why is this a strange placement of the boundaries? What do you think is so fundamentally odd about having your flow parallel to a boundary?
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Old   November 5, 2015, 06:03
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Well, I guess it is my view of an inflow is that something is actually flowing in a direction normal to the boundary. In this case there would be no flow through the outflow boundary, but maybe I have the wrong impression of the symmetry boundary condition.

But say if I switch the sym-bc to solid walls, my impression is that if I would want to simulate an angled flow, it will give a very different solution in my pictured case if I had the inflow outflow bc at the in the west-east direction compared to south-north.

Please correct me if I'm wrong.
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Old   November 5, 2015, 08:22
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Indeed I wouldn't call it inflow any more, but if you specify a fixed value for U it wouldn't be a major difference compared to other angles.

Something else I just realized is that you probably do not want to use a symmetry boundary condition, but a periodic one?


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But say if I switch the sym-bc to solid walls, my impression is that if I would want to simulate an angled flow, it will give a very different solution in my pictured case if I had the inflow outflow bc at the in the west-east direction compared to south-north.
I did not understand what you mean by this. An image might help.
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Old   November 5, 2015, 09:16
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OK, I've made another wonderful drawing with my question to try to illustrate what I perceive as the problem. Please see below.

scene.png

Also, what I mean by the the north-south inflow outflow vs west-east I tried to illustrate in the picture below.

scene2.png

Thanks for your efforts to help me with this, it might just be the case that I don't fully understand how to set up the simulation.
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Old   November 5, 2015, 14:31
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About the first image - you still didn't explain why you apply symmetry instead of periodic.

About the second image - the two setups are not equivalent. The first case would lead to two counter-rotating vortices, the second one to something like a pipe flow (thinking in 2D).
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