SimScale CAE Forum

Viscous/Inviscid CFD Flow


My project has air flow in and out over a block and I see the post processing to show me that the material in our case-air is inviscid as the flow isn’t seperating/turbulence at the trailing edge of the block. Fairly new to CFD.

How can I ensure that flow is viscous and why is this happening as I looked into multiple settings and couldn’t find anything wrong?

Thank you for your support!

H. Mattoo

Hi H.Mattoo (@hmattoo)!

Please set your project to public first of all. @PowerUsers_CFD and I are then having a look at your project and get back to you.

All the best!


Hi Jousef

Thank you for your quick reply!
I have made it public.

Sorry about that

Hey @hmattoo
interesting project you have there
in main time if you expecting a flow separation you have to do more on your mesh when all solids pass through fluids or stationary objects subjected to moving fluid they acquire a boundary layer of a fluid around them when the viscous force occurs in the layer of fluid close to the solid surface, bcause Flow separation occurs when the boundary layer travels far enough against an adverse pressure gradient that the speed of the boundary layer relative to the object falls almost to zero, to capture this effect we add inflate boundary layer in our mesh but yours don’t have that. if you take a CFD master class course in the simscale academy you can find a nice example for meshing such problem in session one

and I am wondering why you model the rood as moving wall?


Hi @Anware

Thank you for the support, I understand that I need to improve the mesh and add inflate boundary layer in the mesh settings.
-The block is to imitate a car hence the floor was moving, but did I do it right?
-Also wanted to clarify why can’t I see turbulent flow at the back end of this block I made because in theory the air at the back would break/create a reverse flow region as well?

I will look into the master lesson, thank you once again!

Hy @hmattoo

The wind tunnel test work by reverse velocity, in reality if it is quit nice sunny day with no winds only our car is moving when the atmospheric air is stationery and the road acts as a wall, when we model this in wind tunnel we reverse the reality making the car stationary and moving the air with the speed of the car. So by using inlet of 30 m/s air you already model the speed of the car so you don’t need the road to move in your case, just wall condition will do the work.
To see the vortices you should increase your mesh quality and run your simulation till it converge, I think your simulation run out of memory on a way of 50%.


1 Like

Hi @Anware & @hmattoo!

Accidentally stumbled upon this post and want to mention that there is a significant difference between using a moving wall boundary condition and ignoring this boundary condition as this is similar with a Poiseuille flow/Couette flow below the car which are two completely different approaches as @Anware mentioned. Maybe @pfernandez can give you more information on that one.




In this case the block is on the floor without any gap, like if the geometry was sliding across the ground. If you want to simulate a block that resembles a car, you could have a look at the Ahmed body.

As per the ground’s boundary condition, it mostly depends on what you want to correlate. If it’s an open road simulation, then you’ll definitely want to have a moving wall with a velocity equal to that of the vehicle but in the opposite direction —you take the vehicle as the reference frame—. On the other hand, if it is a wind tunnel you want to correlate to, then it will depend on the particularities of that wind tunnel. The more general wind tunnels have a fixed floor, while the ones built with automotive applications in mind will have a rolling belt that will go under the car between the tyres (like in the video), with the most advanced ones having a full rolling floor. So depending on the case, you should define your floor accordingly.

Regarding boundary layers, in the past I experienced that STL files with only one patch will generate no boundary layers. You’ll need to split the geometry into at least two patches. Also, I’m not sure whether it’s possible to get boundary layers or not with the automatic meshers; but in the parametric mesher I would have more cells in all directions. With the box dimensions and the number of cells you can know the base size. Then, knowing that each refinement level will halve the cell size, you can use that to define your refinements accordingly.

Kind regards,


Hi @Anware

I’m still facing the vorticity problem in my Vortex generator project :

-The mesh quality doesn’t seem to be improving
-I ran a trial simulation and still cannot see any vortices clearly or even by using the Slice option to see the rotational impact on the Slice plane.

Please help!

Thank you.

Any updates on that one @hmattoo although my response is way too late which I am sorry for by the way :wink: