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All current Rolling Hills Research Corporation water tunnels (Model 0710, Model 1520, and Model 2436) feature the new flow conditioner design that allow bubbles to be cleared easily.  The photos below demonstrate the technique of rotating the honeycomb to allow air bubbles to float out and clear the passageways. Please note that the water has been removed from the tunnel for this demonstration... normally this is done with the tunnel full.


Flow conditioners in the running position


Honeycomb rotated to clear bubbles


Why is it important to clear the bubbles from the honeycomb?  The idea behind the flow conditioners is to reduce turbulence and straighten the flow before it accelerates to the test section.  When the tunnel is initially filled with water, air often is trapped in the honeycomb passageways.  If the tunnel is run with these bubbles in place, the air acts as an obstruction and blocks flow through that individual passage.  The result is that instead of a uniform velocity at the exit, there are varying areas of higher and lower velocity.  This distortion will cause uneven flow over the model in the test section, and can often be seen as uneven pulsing or wavering of the flow visualization dye.  It can also distort the forces measured in the test section.  All of these results are extremely undesirable.


One of the things that sets the RHRC water tunnels apart is extremely high flow quality.  However, this can only be achieved if the problems described above are corrected.  Unlike traditional designs, the RHRC tunnels make it easy to maintain high flow quality. The plot below shows an LDV measurement of the velocity in the test section of a Model 0710. It can be seen that the velocity is extremely constant from the centerline of the tunnel (0) until the boundary layer encounter at about 1/2" from the tunnel wall.


Another unique feature of RHRC tunnels is the careful attention to the use of non-corrosive materials.


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