A single stream, no bifurcations etc, not multiple streams added up.

Imagine taking a cross sectional view of a fluid, if there are no solid islands in the section that block the flow of the liquid then the flow is a single stream, otherwise multiple streams.

金张掖农业信息网- [ Translate this page ]进水口通常设计成单口或双口。由于进水应从鱼池上部流入，所以鱼池最好设置 ... 成鱼期水要保证一定的流速，一般最大不超过每秒30厘米，池水的交换率应大于或者等于 ...
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影响苗后除草剂药效的原因分析 近几年大豆- [ Translate this page ]f:喷头单口喷液量（升/分）×喷头个数车速6公里/小时以内为好，最高不要超过8公里/小时， ... 喷洒作业中应注意风速、风向、风速每秒超过4米时应停止作业。
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As a side remark, if there are no islands in the cross-section, any closed curve on the section can be shrunk to a point and the cross-section is called simply-connected (单连通). If there are islands, multiply-connected (复连通).