Windtunnels: blowing air to vs sucking air from test section

[g-force_addict]

For open circuit wind tunnels
i.e. wind tunnels that don't "recirculate air".

What are the differences between placing the impeller fans before the test secton thus blowing fast moving air to the model
Like this ones but with axial propellers


http://www.aerolab.com/Blowdown.html

And placing the fans after the test section thus fans suck air from the test section

[g-force_addict]

A popular configuration for small tunnels is the blower type (Fig. 2), with the impeller (usually a centrifugal blower) at entry, and -- usually -- with no exit diffuser because power consumption is not very important. This allows any type of test section to be fitted without problems of matching to the diffuser. Centrifugal blowers are preferred to axial fans mainly because they will run efficiently, and generate acceptably steady flow, over a wider range of load -- i.e. a wider range of test-section configurations.
Open-circuit tunnels which take in air from the atmosphere or the laboratory are sensitive to draughts -- the NASA Ames 80 x 120 (Fig. 1) points into the prevailing northwest wind. Centrifugal blowers seem to attenuate most entry disturbances, but they are very sensitive to fluctuating swirl (axial vorticity) in the inlet, which changes the rate of rotation of the blades relative to the airflow. However, blower tunnels are often fitted with commercial air filter panels covering a large box connected to the blower intake, and the filter extracts airflow irregularities as well as dust. It is more difficult to fit a filter to a suckdown tunnel because the air entering the intake must have roughly uniform total pressure over the cross section or the "turbulence management" devices (screens and honeycombs) will not be able to deliver adequately uniform total pressure to the test section.

http://navier.stanford.edu/bradshaw/tun ... onfig.html

[riff_raff]

One limit with the centrifugal blowers used in open tunnel designs is the size of the blower itself. When the airflow requirements get to a certain point centrifugal blowers are no longer a practical option, and axial flow devices must be used. There can be quite a bit of swirl and turbulence produced by axial flow fans, so it would seem logical to locate an axial fan downstream of the test section.

Many years ago I worked for a race team that built its own 20% scale tunnel with a moving belt in the test section. It was constructed similar to the open design shown in the last image (from NASA). It had an axial fan driven by a 75hp electric motor, and it was installed in a closed room that had all sorts of aero devices attached to the walls and ceiling designed to redirect the discharged airflow back to the tunnel inlet with a minimum of turbulence.

Also, a few years ago I spent some time working for a company that designed and built aerospace wind tunnel models. I had to attend a design review at the NASA Langley facility for one of the projects I was working on, and while there I had some free time and a Langley engineer was kind enough to give me a tour of the facilities. It was quite impressive to walk inside the test section of their 14x22 foot subsonic tunnel. I was told the fan was driven by a 12,000hp motor.

[Tommy Cookers]

presumably the NASA Ames 80'x120' is the largest/most powerful open jet tunnel
but pressurised tunnels are the key thing for aircraft design (to raise the Re no)
so the power requirement is very much higher (though AFAIK none exceeds 100000 hp)

[gixxer_drew]

One issue I have with recirculating tunnels is they some times try to get rid of the pressure oscillation phenomenon by tuning the outlet and inadvertently create a pressure gradient somewhere in the test section. Seen a few tunnels get an accuracy problem like that... But open test section or open circuit have their own bag of problems to deal with as well.

[riff_raff]

presumably the NASA Ames 80'x120' is the largest/most powerful open jet tunnel
but pressurised tunnels are the key thing for aircraft design (to raise the Re no)
so the power requirement is very much higher (though AFAIK none exceeds 100000 hp)

The high-speed/transonic aircraft tunnels are mostly small scale blow-down designs. They have massive compressed gas receivers that are slowly pressurized, and then they are rapidly discharged into the tunnel test section. This provides a few seconds of high mach number flow conditions over the model. Some of these tunnels use compressed air, while others use different gas mixtures to give more accurate Re no. results as you noted.

[Tommy Cookers]

our world of mass takeup of Boeing and Airbus etc product relies greatly on large, pressurised (continuous flow) wind tunnels
they are physically much larger (but use lower pressurisation) than earlier pressurised continuous tunnels
it might appear that the USA (and former USSR) are underequipped, and some parts of Europe better requipped in this regard
(the US position was recognised by Clinton as justifying a $3 billion response)
Boeing and Airbus spend lavishly on such testing, to match design exactly to the mandated approach and climbout behaviour
eg various deployments of multiple slotted flaps (like an F1 season concentrated into 3 minutes of actual flight)
for this they need closer Re no similarity than for cruise flight, and accept the high tunnel fixed and running costs
(Mach no similarity is significant for cruise, this can be helped by cryogenic working)

but they never have such close similarity as F1 'enjoys'
it's touching to hear of the quality issues and engineering difficulties (eg hot plume modelling) in F1 tunnels