By consideration of the general problem of viscous liquid dynamics (item 32) it was noted that its effective decision is possible at movement of bodies (a liquid flow) with Reynolds numbers when viscosity of liquid is small, and its speed is rather great. In this case influence of viscosity forces as numerous experiments show considerably affects only in the relative thin layer of liquid adjacent to a surface of a body and called an interface, and also in rather small area behind a body, called a concomitant stream or a hydrodynamic trace. Out of an interface and a concomitant stream the influence of viscosity forces in comparison with inertia and pressure forces is so little that it is possible to neglect them and, considering liquid is nonviscous to determine such important characteristics of a stream, as pressure and speeds by formulas of nonviscous liquid dynamics.
However, the interface subtlety doesn’t at all mean that forces of viscosity operating from a layer on a body are very small. Speed of a current of liquid across an interface significantly changes: it is equal in points of a body surface to zero owing to a boundary condition of «sticking of particles», and on border of a layer is almost identical with a speed of an external stream of the nonviscous liquid which is flowing round a body. Therefore, the speed gradient on a normal to a body surface in each its point can be rather great. But then the tangent tensions operating on a surface of a body have to be rather great, and their resultant is friction force.
The liquid current in an interface can be both as laminar and turbulent. Crossing of the current from laminar to the turbulent happens as at liquid movement in a pipe by reaching Reynolds number to critical value. This value significantly depends on the sizes of a body and for ship hulls have an order 106. It can be reached at some distance from a front end of a body, then a liquid current in an interface located in fore part of a ship will be laminar; then rather narrow transitional zone with the stream transformed in turbulent follows. In back end of a body the current in an interface will be turbulent. Existence of various modes of a current and transitional zone complicate research and calculation of an interface and power impact of a viscous liquid on a body moving in it, in particular, the vessel hull.