What is blockage ratio?
 

Hi,

I am relatively new when it comes to CFD analysis and I have stumbled across this thing called blockage ration. Since I haven't found any useful information about it I was wondering if someone could give me an explanation.

What I know so far is that it is the ratio between the body/geometry and the height of the domain.

Now, let us say that we have a circular cylinder. If we have the same height for the circle but increase the heigh of the domain (thus getting a lower ratio), what impact does it have on the flow? Less pressure maybe?

Thanks!

It is usually defined as the projected area of the structure in flow direction to the cross sectional area of the domain around the structure.

Usually you wanna go below 3% of blockage ratio. Otherwise you might introduce an artificial acceleration of the flow - remember the continuity equation.

I am guessing that if the velocity is increased before the separation at the cylinder, the pressure will be smaller (more suction). It is a common problem in wind tunnel experiments, where the domain is fixed. Here, they use mathematical approaches to correct the pressure if the blockage is too high.

This. With a small addendum: in some experiments the blockage ratio is fixed parameter and should not be minimized. Like for example the flow over an obstacle confined by walls with a fixed position.

Thanks
 

Thanks for the help!

Hi Bazinga,

I have a follow up question to your statement. Will the articifial acceleration of the flow not only occur if the boundary condition on the walls parallel to the flow direction are (semi)impermeable? I mean if you e.g. have a symmetryPlane on the front/back patches (i.e. not bottom and top patches), the flow wont "feel" the presence of walls, and therefore not contract. Hence, blockage effect wont be a problem in this case. I guess that is the point of symmetryPlanes, right?

Anyway, thank you for your answer to the question. It still helps people some years later :)

/Rasmus

Dear Rasmus,

boundary layer effects and blockage effects are two separate topics.

If the blockage ratio is too high, acceleration of the flow might appear. This can happen even with symmetry conditions at top/bot/front/back. Imagine a very small cross-section compared to your object. Even with symmetry, the velocity needs to accelerate to compensate for the small area (continuity).

However, symmetry might help to prevent effects due to boundary layers. For example, let's say you have a wing through the whole span of your domain (or wind tunnel). When you want to predict the drag coefficient you should probably consider that the velocity close to the wall is reduced. Hence, if you estimate the drag coefficient by normalization using the undisturbed inflow velocity in the center you introduce an error by not accounting for the decelerated flow at the wall. Symmetry conditions in CFD help to overcome the issue.

Best regards

Hi again,

Thank you for your quick response! Didn't see that coming. I really appreciate it!

I get the continuity reference, although i think I am confused as to what the symmetryPlane condition then is. Perhaps i mix it with the "empty" boundary condition? Will blockage be the same issue if "empty" boundary conditions apply? Hence the 100 % permeable boundary condition.
My line of thought is that in reality blockage is practically =0, as there is no "domain width" in the ocean for example. In the model however, there will always be some sort of blockage since the numerical domain is restricted to a certain width. I am looking for a boundary condition that allow the flow to pass through a cross section without streamlines being contracted as consequence of that exact boundary.

Just to be sure, what you say is that if i apply symmetry conditions to the boundary in this instance, it helps by not having a reduced velocity at that exact boundary (the values at the boundary are rather equal to the values in the cells near to the wall), leading to a uniform free stream velocity? Hence, the presence of boundaries does not affect the solution. Correct?

The starting point is either a symmetry BC or slip wall as a starting point. If the artificial boundary is far enough away then the blockage ratio is small and the influence of the boundary can be negligible. This is fundamental and should not be overlooked.


What you're looking for is part of the family of so-called freestream boundary condition or farfield boundary conditions. These characteristic BC types allow semi-arbitrary constraints on the flow variables at the boundary as long as they relax to the far field conditions (pressure, velocity, Mach number, etc.). Briefly, they solve a characteristic value problem (i.e. Riemann invariants) to determine what the flow variables should be applied to the BC in order to approach the far field or freestream condition.


But this approach is only a bandaid. It is not the be all end all. There still will be artificial blockage. You still need to make sure the BC is far enough away.

Hello,

I see. I think I've got a different picture of the symmetryPlane boundary condition and when it is applied now. Consequently i think i get the reason for why it too poses a boundary condition disturbance to the flow and forces.
Thank you both for taking you time to answer my question.

Have a great day!

/Rasmus