Page 93 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

a transition region where the flow is turbulent and the layer increases in thickness. To obtain high vehicle speed, the design must be toward retaining laminar flow as long as possible, for the drag in the laminar layer is much less than that within the turbulent layer.
An important factor determining the condition of flow about a body and the relative effect of fluid viscosity is the “Reynolds number.” This number was evolved from the work of Englishman Osborne Reynolds in the 1880s. Reynolds observed laminar flow become abruptly turbulent when a particular value of the product of the distance along a tube and the velocity, divided by the vis- cosity, was reached. The Reynolds number expresses in nondimensional form a ratio between inertia forces and viscous forces on a particle, and the transition from the laminar to the turbu- lent area occurs at a certain critical Reynolds number value. This critical Reynolds number value is lowered by the effects of surface imperfections and regions of increasing pressure. In some cir- cumstances, sufficient kinetic energy of the flow may be lost from the boundary layer such that the flow separates from the body and produces large pressure or form drag.
The Reynolds number effect of a fluid flowing around a cylinder (such as a tether being pulled against an oncoming current) is depicted at Figure 3.19 and can be calculated by the following formula:
3.5 Design theory 81
Re<<1
Re~10
Re>~90
45 Re 10 –10
Re>105
FIGURE 3.19
Water flow around a cylinder by Reynolds number as the flow increases.