The friction term is assumed to be proportional to the flow velocity with a phase difference.

粘性假设正比于流体速度，相位存在差异。

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Over the long term the SARS outbreak will add friction to the cost of doing business in Asia, much as impromptu shoe checks, cargo inspections and travel jitters followed Sept. 11 in the U.S. Hotel business in Asia fell 25% from February to March.

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