The model and the prototype must be the same shape and all linear dimensions of the model must be related to the corresponding dimensions of the prototype by a constant scale factor (2) Dynamic similarity of the flow fields: The ratio of flow velovities of corresponding fluid particles is the same in model and prototype as well as the ratio of all forces acting on corresponding fluid particles. When dynamic similarity of two flow fields with geometrically similar flow patterns The first requirement is obvious and easy to accomplish but the second is difficult to understand and also to accomplish and needs explanation. For example, if force that may act on a fluid element in a fermenter during agitation are the viscosity force Fv, drag force on impeller D, and gravity force FG each can be expressed withThe model and the prototype must be the same shape, and all linear dimensions of the model must be related to the corresponding dimensions of the prototype by a constant scale factor. (2) Dynamic similarity of the flow fields: The ratio of flow velovities of corresponding fluid particles is the same in model and prototype as well as the ratio of all forces acting on corresponding fluid particles. When dynamic similarity of two flow fields with geometrically similar flow patterns. The first requirement is obvious and easy to accomplish, but the second is difficult to understand and also to accomplish and needs explanation. For example, if force that may act on a fluid element in a fermenter during agitation are the viscosity force FV, drag force on impeller FD, and gravity force FG,each can be expressed with