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Hello again, I´m translating a text regarding internal combustion engines and I came across with this term "main bulk velocity."

The context is as follows:

"As engine speed increases, flow rates increase, with a corresponding increase in swirl, squish, and turbulence. This increases the real-time rate of fuel evaporation, mixing of the fuel vapor and air, and combustion. When flow is turbulent, particles experience random fluctuations in motion superimposed on their main bulk velocity. These fluctuations occur in all directions, perpendicular to the flow and in the flow direction. This makes it impossible to predict the exact flow conditions at any given time and position."

Please help me

Thanks in advance