2.5
Fluids in Motion
Mechanical Properties of Fluids · Maharashtra HSC
Theory
What the section covers
The branch of physics dealing with the properties of fluids in motion is called hydrodynamics. Because the motion of a real fluid is very complicated, the syllabus restricts itself to the motion of an ideal fluid — incompressible, non-viscous, irrotational and in steady flow.
Diagram to draw
A pipe whose direction and cross sectional area change arbitrarily, with the direction of flow marked. Draw several individual flow lines inside it, and bundle a few of them into a flow tube bounded by an imaginary wall.
Terms used to describe fluid flow
- Steady flow: a measurable property such as the pressure or the velocity of the fluid at a given point is constant over time.
- Flow line: the path of an individual particle in a moving fluid.
- Streamline: a curve whose tangent at any point in the flow is in the direction of the velocity of flow at that point. Streamlines and flow lines are identical for a steady flow.
- Flow tube: an imaginary bundle of flow lines bounded by an imaginary wall. For a steady flow the fluid cannot cross the walls of a flow tube, and fluids in adjacent flow tubes cannot mix.
- Laminar or streamline flow: a steady flow in which adjacent layers of a fluid move smoothly over each other. A steady flow of a river can be treated as laminar.
- Turbulent flow: a flow at a very high flow rate, so that there is no steady flow and the flow pattern changes continuously. A flooded river, or a tap running very fast, is turbulent.
Distinguishing streamline flow from turbulent flow
| Sr. No. | Streamline flow | Turbulent flow |
|---|---|---|
| i | The smooth flow of a fluid, with velocity smaller than a certain critical velocity (limiting value of velocity), is called streamline or laminar flow. | The irregular and unsteady flow of a fluid when its velocity increases beyond the critical velocity is called turbulent flow. |
| ii | In a streamline flow, the velocity of the fluid at a given point is always constant. | In a turbulent flow, the velocity of the fluid at any point does not remain constant. |
| iii | Two streamlines can never intersect, i.e. they are always parallel, and hence can never form eddies. | At some points the fluid may have rotational motion, which gives rise to eddies. |
| iv | Streamline flow over a plane surface can be assumed to be divided into a number of plane layers; in a pipe of uniform cross section all the streamlines are parallel to the axis of the tube. | A flow tube loses its order and the particles move in random directions. |
Two streamlines can never intersect, because at the point of intersection the fluid would have to have two different velocities at the same instant, which is impossible for a steady flow.
Standard results
- Streamlines and flow lines coincide only in a steady flow.
- Streamline flow occurs below the critical velocity, turbulent flow above it.
- Any two points of the comparison table above are enough for a two-mark "distinguish between" question.
Theory Questions Asked on This Subtopic
Real board questions. Answers are at the end of the page.
- 1
Distinguish between streamline flow and turbulent flow. (Any Two points).
Maharashtra HSC board question — asked in July 2023Answer 1 ↓
Answers & Solutions
Work the questions above first, then check yourself here. Each answer is set out the way it should be written in the paper.
Sr. No. Streamline flow Turbulent flow i. The smooth flow of a fluid, with velocity smaller than certain critical velocity (limiting value of velocity) is called streamline flow or laminar flow of a fluid The irregular and unsteady flow of a fluid when its velocity increases beyond critical velocity is called turbulent flow. ii. In a streamline flow, velocity of a fluid at a given point is always constant. In a turbulent flow, the velocity of a fluid at any point does not remain constant. iii. Two streamlines can never intersect, i.e., they are always parallel and hence can never form eddies. In a turbulent flow, at some points, the fluid may have rotational motion which gives rise to eddies. iv. Streamline flow over a plane surface can be assumed to be divided into a number of plane layers. In a flow of liquid through a pipe of uniform cross sectional area, all the streamlines will be parallel to the axis of the tube. A flow tube loses its order and particles move in random direction. (Any two points of difference)