N811291 - Converging airflows

[mchristlieb]

I find this a difficult read.

The question asks what will happen over the wing where airflow converges.

The given answer is that as in all convergent situations the Pressure goes down and velocity increases.

But surely the air converges at the trailing edge where the two flows come together and there the air flow stagates - P up v down.

[Ortac]

I don't have access to the question bank, so don't know the exact wording of this question.

If I were asking a question on this topic I would express it in terms of convergence or divergence of the streamlines. Also we are assuming incompressible flow.

A streamline is defined such that the direction of the flow velocity is always a tangent to the streamline. Therefore the flow direction is always along the streamline and there is never any velocity component across a streamline. This means that if you draw two adjacent streamlines you can think of them as if they were the solid walls of a venturi (even though they are imaginary lines).

The volume (or mass) flow rate between two adjacent streamlines is constant. Where the streamlines are converging the continuity principle means that the velocity will increase and Bernoulli's principle means the static pressure will decrease. Where the streamlines are diverging the continuity principle means that the velocity will decrease and Bernoulli's principle means the static pressure will increase.

Trailing edge flow is affected by viscosity and I think beyond the kind of simple flow theory this kind of question is addressing.

[mchristlieb]

OK - I see what you mean. I don't think it would have occurred to me to read the question that way. But the given answer makes sense if read that way. Wish the examiners would be less cryptic and make better use of diagrams.

Thank you for your thoughts.

[Ortac]

Learning objective 081 01 02 01 says "Describe converging and diverging streamlines and their effect on static pressure and velocity"

Should not be too hard for the examiner to write an unambiguous question to test this knowledge