i see sometimes the term "aerodynamic blockage" used in technical papers;
for example, in his last technical analysis , Gary Anderson says "influencing the airflow direction needs to be done with the minimum cross-sectional increase which is why a turning vane will work much more efficiently than the volume change, which creates blockage."
The blockage phenomenon in aerodynamics usually refers to sonic blockage: as the flow reaches the speed of sound, there is a point where it cannot manage to go any faster unless the right conditions (in surrounding geometry, temperature and flow differential) are there. So essentially, the flow stops accelerating at the speed of sound, and this is referred to as sonic blockage.
In the sentence you quoted, the term "blockage" is misused as it refers more to the debit of air in a duct being mostly dictated by pressure differential between entry and exit, so you can't "add more air" without changing the pressure in the duct.
For thin and long ducts, there is also the fact that the skin friction on the walls of the duct locally slows down the air and creates something called a "boundary layer" in which the air progressively accelerates as you get further away from the wall. When the duct is very thin compared to its length, the boundary layer from one wall and from the opposite wall join up, so the whole mass of air going through the duct is slowed down, not just on the edges. This phenomenon is sometimes also referred to as blockage, but it's not the correct term for it.
Do take what I'm saying here with a small pinch of salt as most of this is translated from my knowledge of aerodynamics in French, and the technical terminology in English might vary a bit.
More specifically the phenomenon you are referring to is termed solid blockage, this is an important parameter for wind tunnel testing. Usually it is used with reference to the test model; here it is the ratio of the models projected frontal area to the wind tunnel test section area (taken normal to the free stream) and is usually presented in a percentage.
An acceptable value is usually below 5% for aerospace applications but for automotive purposes a blockage of up to 7.5% is acceptable. However I have read reports that state reasonable results can be obtained with blockages exceeding 11%.
Keeping this parameter in acceptable limits is very important for the validity of wind tunnel data; placing walls around a test model (as in a wind tunnel) works to compress the streamlines and cause an over speed of fluid close to the model. As this local speed is higher than the reference speed (measured away from the model) which is used to calculate aerodynamic coefficients, we will perceive a higher drag and lift than that of a model in a free stream.
Basically larger blockage leads to data that does not correlate to the real world.
A lot of research work conducted has focused on correcting this problem and there exists several blockage correction techniques. However these are usually empirical in nature and do not correct for time dependant phenomena and thus are not relied upon for detailed worked.
There is also several other phenomena in wind tunnel testing that must be considered; interference, wake blockage and horizontal buoyancy.
This post is already long enough so I shall leave it there.
Bot6 and Burchenal: First, thank you very much for your responses! your explanations are very clear. Now i would like to do apologize, because i realize my question was too vague and i have to give you some more details; here follows the complete analysis of Gary Anderson with 2 illustrations (i hope it could help):
"The original, walrus-like nose aimed to help the expansion of the airflow on the centre-section of the wing. force india has now followed mclaren's lead with more of a horizontal ski ramp in this area to try to achieve the same effect of speeding up the airflow on the underside of the wing section. Influencing the airflow direction needs to be done with the minimum cross-sectional increase which is why a turning vane will work much more efficiently than the volume change, which creates blockage."
