Physics · Mechanical Properties Of Fluids · NEET
Almost, yes, for NEET they are treated as the same idea. A streamline is the path a particle takes in steady flow. When the fluid moves in smooth parallel layers that slide over one another (like the pages of a book pushed sideways), that flow is called laminar. So laminar flow is streamline flow where the fluid moves in orderly layers. Both mean the flow is steady and the particle path stays fixed with time.
Critical velocity is the fixed speed above which streamline (laminar) flow breaks down and becomes turbulent. Below this speed the flow is smooth and orderly. Above it, eddies form and the flow becomes chaotic. The critical velocity depends on the fluid's viscosity, its density, and the tube radius. Higher viscosity or a smaller tube raises the critical velocity, so the flow stays laminar up to a higher speed.
A streamline shows the direction of fluid velocity at each point. At any point the fluid can move in only one direction at a time. If two streamlines crossed, the particle at the crossing point would have two possible directions, which is impossible for a steady flow. So in steady (streamline) flow, streamlines never intersect.
Reynolds number Re is a pure number: Re = (density x velocity x diameter) / viscosity. It compares inertia forces to viscous forces. Roughly, Re below about 1000 means laminar flow, Re above about 2000 means turbulent flow, and in between is unsteady. High speed or low viscosity gives a large Re and turbulent flow; thick, slow fluids give a small Re and laminar flow.
In laminar flow the layers slide smoothly, so friction (viscous loss) is small. In turbulent flow the eddies and mixing make fluid layers collide and drag on each other much more, so more kinetic energy is converted into heat. That is why fast, turbulent flow loses energy quickly, while slow streamline flow keeps energy better.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The fluid speed compared to the critical velocity. Below critical velocity the flow is streamline (laminar); above it, the flow becomes turbulent. The Reynolds number combines speed, density, tube size, and viscosity to predict this.
Smoke rising straight from a still candle is streamline flow near the bottom. Higher up it breaks into curls, which is turbulent. Water flowing slowly from a tap is streamline; open it fully and it becomes turbulent and noisy.
Higher viscosity favours laminar (streamline) flow because thick fluids resist the formation of eddies. Low viscosity fluids like water turn turbulent more easily at high speed.
Reynolds number is a pure number with no units and no direction. It is just a ratio of inertia force to viscous force, so all units cancel out.
