Stream Tube

in hydromechanics, a tube composed of the stream lines that pass through the points of a small closed contour within a moving fluid. The tangents to the stream lines coincide with the direction in which the fluid particles located in the stream lines are moving. During unsteady motion of the fluid, the stream lines change from moment to moment, and therefore the stream tube also changes its shape. During steady motion of the fluid, the stream lines coincide with the particle trajectories and remain invariant; in this case the stream tube is similar to a tube with rigid walls in which the fluid flows at a constant rate through the tube’s cross section. If the density is constant, the stream tube will constrict or expand, depending on whether the flow speed increases or decreases. Such behavior of a stream tube also occurs when the density is variable—that is, in the case of a gas—but only until the speed of the steady gas flow exceeds the local speed of sound. After the local speed of sound is exceeded, a further increase in the speed of the gas flow is accompanied by expansion, and not constriction, of the stream tube.