Describes a novel modification of stroboscopic illumination used to give an unambiguous record of the direction of the flow in an otherwise conventional visualisation of a complex three dimensional air flow. The flow was seeded with small soap bubbles which were viewed by scattered light. Each flash of light was split into two periods of unequal length, separated by a short extinction, thereby marking the direction of flow on the photographs. The technique allows a more complete determination of the velocity field than can be obtained by the usual stroboscopic method.
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