FreeCAD CFD- External Flow Analysis Example|JOKO ENGINEERING|

i've been asked about doing external flow analysis and i will say as i've said in the past i'm not a career analyst but this should be enough to get your feet wet so here i have a corsair i've made this in the curve shapes workbench uh this the video of actually making this from scratch is already on the channel and if you'd like to follow along you can download this part from my grab cad let's get started uh by getting a setup for an analysis now any time you do an analysis i don't care if it's finite element computational fluid dynamics whatever you're doing you want to test the simplest version of your model possible details equal computation and possible errors so as far as this goes i don't think this propeller is necessary because i'm going to be flowing past the body of the aircraft and the propeller is just going to get in the way especially because it will be simulated like it's stationary so let's get rid of the propeller first and make sure that our model is as simple as we can get it let's go to uh sketcher and i care about the z y plane so i'll create a new sketch on the y z plane here i'm going to draw a rectangle and we'll import a part or part of the body here and coincident here excellent now let's take this crazy high and in an ideal world i would fully constrain this sketch if you're at this level and you can't fully constrain then i don't suggest proceeding on you know what let's just fully constrain it 2.5 millimeters looks good and then vertical 30 right so let's close that and then from the part workbench we'll do a revolved cut and of course you don't really have the cutting on the workbench but i can say revolve a shape and of course that will be my very last and we would want to do it around the y so under there it is we put a 0 for z and a 1 for y and we're going 360 degrees so that's looking pretty good then we simply select the body of the plane which in the tree here is called fill it hold control select revolve in that order and then we choose this make it cut no more propeller so now that this is all set for um and analyzing let's talk about what kind of mesh that we're going to need to have if you recall in the previous flow example we did an internal flow study and we modeled the internal part of a pipe meshed that and floated so whatever um we model for the meshing we want to have a solid there now that's not always true in solidworks and other platforms you have a different take where installed works you need a sealed volume to flow but in freecad we want to have a solid where we're going to flow so to accomplish that i'm going to import a cube so with our cube also known as our geometric primitive i can bump this out a little bit so we want to do a length of let's say 100 this is in millimeters by 200 by about 100 and what we're setting ourselves up for is to create this as the envelope of our flow area and then we cut away the parts that we wish to flow and then we have you know like a negative of it the same way that you make an injection mold tool or something like that and so we can use this um as a negative so let's go with a position and let's say y um move it to negative 50 millimeters and then let's do another negative 50 on the z that might have gone to negative 50 meters it did so make it millimeters there we go so our plane is right in the middle now a few topics for discussion here first off is the triad right so the triad is not only telling us that our x-axis is along this line or parallel to this line because you can see the axises are parallel there and our z axis is vertical and our y axis is over here but it also tells us the positive and negative directions these arrows point into the positive x direction and going against the arrow will be into the negative x direction positive z negative z and so on and so forth so that's how we orient things by knowing what is positive and negative x y and z additionally if we look at the envelope of our flow area this is for a quick computation right our flow area here is pretty short in front of the plane and it doesn't go very far behind the plane we generally want to take the length of the plane or the length of whatever you wish to flow externally and make it 10 times that length and put it right in the middle and that's a general rule of thumb and so we're not that far you know this is for a real quick computation it'll be less than ideal as far as the best envelope that we could use keep that in mind right but i'm not going to spend like 10 hours you know figuring this out i'm just going to do a real quick flow and with that let's take our cube highlight our cut in that order we'll do a boolean subtraction and now we have half the plane in this boolean subtraction now you might ask why not do the whole plane well because the plane is symmetric we can cut the computation in half by doing half of it ideally you can use a symmetric constraint and have it compute both sides of the flow evenly i actually haven't gotten that to work but i know it does work so when we get there i'll show you what you can do even though i haven't actually got it to work and then uh we'll set up our analysis so we'll go to part c of the open foam we'll set up a cfd envelope and we'll open up this cfd analysis here our physics model we want to make sure that this is viscous with laminar flow our flu property should be air and that's looking good we want to initialize fields from potential flow and then we have our solver so next let's build a totally awesome mesh i'll stand on my cut that i wish to mesh and i'll click the mesh button and this should be pretty simple i'm going to match it pretty tight because i've got a small area and i can afford to let's go with something like 1.8 millimeters and i'm going to make sure to specify that unit and uh cf mesh is awesome if you have trouble with cf mesh i've also had good luck with gmesh and i bet snappy hex mesh is good too but i'm going to stick with cf mesh but you can do any of the above we'll write the mesh case and run i will also note that this plane is much smaller in scale so make sure that you mesh appropriately according to your scale because if this is an actual size plane 1.8 millimeters is way too tight so looks like our meshing is completed as before we can load our surface mesh and check it out and that's looking pretty good we can clear our mesh as well now there's one point that i'd like to make also you can play around with a mesh refinement and that's just incredible technology these developers are just knock your socks off amazing i'm continually impressed as to what they're able to do in free cad i'm going to call this an internal volume and it wants us to select something from a list i'll choose select from list and i want to choose my regular cut right so the mesh that is around this we're going to select and say we're going to refine it to 0.75 relative to the other mesh and i'm going to say done and okay so now under my mesh i got this little drop down and i have my little sub mesh refinement and of course i'm not showing what i'm actually meshing anymore but that's okay i'm going to run my measure and i'm just going to show my body again with the space bar so you can get a little bit better of an idea of what's going on and we're continuing to mesh here all right meshing is completed let's load the surface mesh and again if you care about really visually looking at your mesh choose pair of view loading surface mesh isn't as visually as accurate but as you can see now around the object that i've created i have a tighter mesh and and the idea behind the mesh refinement is to say i've got a lot of volume and area that i'm just flowing air through and i don't need that much definition on so i can mesh tighter around the objects where i need more resolution but i can save all the computation and keep it looser around here so that's how you refine a mesh and i also want to point out that it might be more standard practice to use a surface mesh instead and you can specify aspects of your boundary layers i think four boundary layers is pretty standard with a 1.2 expansion rate is pretty standard and then you just select from list again but so that's how you do different mesh refinements right your refinement thickness is up to you i don't know four millimeters or something so something to play around with i think the mesh refinement for this specific study over complicates it a bit so uh i won't run the analysis with this i mean mesh mesh refinement can give you a lot of definition it can also be a source of an error so you know be aware that it's a double-edged sword and it's more computation of course but at the end of the day it saves you a lot of computation if you need a tight mesh so we've got our mesh here and i'll just rewrite my mesh case and while that's writing i can probably sneak into some of the other topics here once we have our mesh we're going to be ready to solve oh actually no we'll have to add boundary layers so i have to determine what is input and output and i want to use kind of a unique direction on our flow to illustrate um ways that you can tackle inlets and outlets and stuff to be a little bit more realistic all right um yep meshing is completed so we'll close that and then let's work on boundary layers i'm going to hide my cut here so you can get a more realistic view of what we're actually cutting so if i go to my left view i want not just to flow straight to back right front to back but i want to flow front to back with the air coming upwards a little bit at a slight angle so let's work on how to do multiple directions right first i'm going to select this face and i'm going to select this bottom face and we're going to come here to boundary conditions and we want to have an inlet uniform velocity is fine and this is where we use that triad trick we can see the positive y direction so we're going to specify a positive y value let's go with like 15 mil 50 meters per second and then a positive z value for a slight upward velocity and our uz let's go with half a meter per second right so we're coming up at a slight angle probably won't be that noticeable but that is how you can select your faces because i didn't just select my front face i selected my bottom face and that's for a uniform flow across the cross section if i were to just specify the front face and have an upward angle of the air and have this bottom face be a wall then it just won't flow quite right it won't you won't have like a steady state right it'll be flowing up and then it'll try to go against the wall again and it just introduces more turbulence and chaos so we're flowing from all the faces that our direction originates from because we're going up a little bit and mostly from front to back let's do the same for these back faces right so these other faces are going to be outlets so i've got my faces highlighted we choose outlet and that's really all we need right no pressure no velocity they're just outlets and so we're flowing from the blue faces to the red faces and then we want to add a wall right to and it's it gets a little bit strange trying to select some of these faces because they're transparent but i've got that that you know far side wall face selected and we select this guy there's our face we want a slip wall so that we won't create any turbulence as we flow along it and then what to do here well this is the part where i get a little bit stuck usually to my understanding you can add a constraint and make it symmetrical and then whatever flow you have on one side of the face it will flow on the other side of the face so if i add that no no i say okay because that was already added then i've got my symmetry face but i haven't gotten it to actually be symmetrical and if you just want to do half make it a slip wall and of course you can just make this cube encompass the entire plane and flow it just for at the cost of more computation that's an option as well but i think flowing half the plane here is good maybe this time we'll get lucky and it will uh mirror across with the symmetry constraint but we'll see either way we have all of our boundary conditions we need let's go to our solver and i'm going to write my case and i'm going to run my case and we'll take a look at our residuals it looks like we exited with error and it looks like we have a floating point exception down here the floating point exception usually means that you divided by zero and that often has been mesh size for me so that doesn't necessarily mean you need to have a smaller or larger mesh but change the mesh size in one direction right i'm going to say 1.5 i'll write a mesh case run the measure and after it's done meshing we'll rerun this and see if we don't get a floating point exception all right so our meshing is completed we'll close i'll close my failed plot there we'll go to cfd solver we'll write and run [Music] and there are simulation finished successfully my ears are perked up with this upward trend here but we ended up to be at 10 to the negative fourth which is generally good results so for the purposes of this video i'm going to go with it let's open up peer review and take a look at the results so paraview is a very complex uh visualizing system and as you can see we're not looking too bad this is looking about right let's create a stream tracer and i'm going to show the stream tracer and hide my cube and of course lines can be very helpful i'm going to use a point cloud and we'll change this to about a thousand instead of 100 and apply all right so it's a little bit hard to read with just a stream tracer but as you can tell we have our blue wing here and we are indeed getting lift on the wing because we have a pressure drop over what about is the zero point so we have zero pressure around here and we dip into blue as we go over the top of the wing meaning that we indeed have a do we indeed do have a drop in pressure also known as lift and the reason is and i i think there's still some debates as to exactly the dynamics that caused this but the air moving faster to my understanding over the top of the wing than the air moving slower creates a pressure drop because any time you have a moving fluid there's a drop in pressure so that's where your lift would come from and we can i think the red white and blue is the best viewing method but i can also imply another color method if i try to get a little more color out of it and that's pretty cool to be able to see that over the wing i'll move my viewing cube a little bit more over here and apply the new view there you go so you can kind of see the body of the plane right so we'd hit the propeller up here we have high pressure and then we have a pressure drop into the green as we go over the body and especially the wing and um i can always kind of toggle on and off right so you can see exactly what you're seeing over the stream tracer there are way better ways to view this i am absolutely sure my big thing is i'm not that expert on pair of view so this is about as good as i'm going to get it in this video [Music] but let me actually add some resources into the description that has some step-by-step instructions on how to go even further so i hope this was helpful if it was please subscribe and i'll see you in the next one [Music] you

Channel: Joko Engineeringhelp

Views: 3,181

Keywords: joko, engineering, help, jokoengineering, engineeringhelp, ACADEMY, jokoengineeringhelp, tutorial, how, to, howto, engineer, JMT, EXPLAIN, EXPLAINED, GOENGINEER, KHAN

Id: JVO2kqO3D1Q

Length: 21min 32sec (1292 seconds)

Published: Fri Dec 17 2021