请将下列句中翻译成汉语: (1)In the description of a flow field, it is convenient to think of individual particles each of which is considered to be a small mass of fluid, consisting of a large number of molecules, that occupies a small volume that moves with the flow. (2)As for developed flows, the velocity profiles do not vary with respect to the space coordinate in the direction of flow. (3)In a turbulent flow fluid motions vary irregularly so that quantities such as velocity and pressure show a random variation with time and space coordinates.