Abstract Symmetrical room geometry is not a sufficient condition for the design of a ventilated room where two-dimensional airflows are to be generated. Three-dimensional effects were observed in a symmetrically designed 3 m high by 5 m wide by 8·5 m long test room having a 0·019 m high slot inlet opening under the ceiling. The attached jet velocity profile measured at the ceiling in the symmetrical centre plane agreed well with the theoretical calculations based on two-dimensional flow, but large differences were found away from the symmetrical centre plane. The velocities in the jet 4·5 m downstream from the inlet wall were up to twice as high on one side compared to the other side. During the measurement period the side with high velocities occasionally changed without any obvious disturbance in the room. Smoke tests showed that the jet for some periods turned towards the right downstream corner, and then changed and turned to the left, thus showing a semi-stable flow behaviour. The measured velocities in the symmetrical centre plane were only slightly affected by the switch-over and remained at the same level throughout the experiment. In both semi-stable conditions, the return air direction diverged 30° from the symmetrical plane. Modification of the room with four 0·5 m high vertical guiding plates attached to the ceiling resulted in an acceptably uniform two-dimensional flow. The guiding plates changed the ceiling jet profile so it did not agree with the theoretical jet profile for an attached, two-dimensional flow. The maximum velocity became lower. At floor level, the direction of the return air was parallel to the symmetrical centre plane, without changes in the mean velocity. Systematic validation of flow behaviour is thus necessary before the assumption of two-dimensional flow could be accepted and a study is carried out in the symmetrical plane only.
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