G'day Fluent Users,

I would just like to know the answer to a very simple question.

Within a windtunnel does the downstream wind tunnel dimensions effect the upstream flow? Let me explain.

At the university of Queensland we have a high speed (75 m/s) wind tunnel for pesticide application which we are upgrading. The working section is currently 1 m x 1 m but after the measurement of the droplets (1m after) it will expand to 1.75 m x 1.75m, will this effect the laminar flow within the measuring (working) section.

Thanks for your advice in this matter.

Sincerely,

Paul Kleinmeulman Australia

Well, I assume the flow is subsonic - so a downstream expansion will effect the flow upstream. If you have separation of the flow during the expansion, I will be even more noticable.

When we modeled our tunnels in Fluent, we started from the Test section inlet and went all the way to the end of the expansion. That way you capture the expansion effects as well as giving the wake of the model time to dissipate before it hits your specified Boundary condition.

Using pressure taps, we measured static pressures at the inlet and end of expansion, and used these values as BC for the Fluent run.

We did not measure forces, but the pressures on our model corresponded well to the pressures Fluent predicted. Hope this helps,

Regards Riaan

Riaan,

Thank you very much for your detailed answer I appreciate it.

Can you please explain in a non technical way why it actually disrupts the flow upstream?

Thank you,

Sincerely,

Paul Kleinmeulman

Since it is a subsonic flow (elliptic characteristic equation), any flow disturbance in the flow field will effect the whole flow field.

In your case - in the expanding section downstream, there is an adverse pressure gradient (static pressure increase, total pressure decrease) which could cause the boundary layer to grow and become unstable, and possibly transition to turbulent or even separate.