MCQ on Fluid Dynamics

Explanation:
When a liquid flows through a narrow capillary tube, the speed of different liquid layers does not remain the same throughout the tube’s cross-section. The question asks about the geometric pattern formed by this variation in velocity from the center toward the walls of the tube.

In Fluid mechanics, liquid particles touching the inner wall experience strong friction due to adhesion, causing their speed to become extremely small. As we move toward the central region of the tube, the frictional effect decreases and the liquid layers move progressively faster. This creates a smooth and symmetrical velocity pattern across the radius of the tube.

The mathematical treatment of streamlined viscous flow through narrow tubes was developed while studying Poiseuille flow. According to this concept, the velocity depends on the square of the radial distance from the center, producing a curved profile instead of a straight-line variation. The maximum speed occurs along the central axis, while the minimum speed occurs near the walls.

A useful comparison is traffic movement on a crowded road. Vehicles near the edges slow down because of obstacles, while those in the middle lane move faster, creating a gradual speed variation across the road width.

The flow pattern formed inside the capillary is therefore smooth, symmetric, and governed by viscous effects acting differently at different radial positions.

Explanation:
The term “deca-poise” belongs to the system of units commonly used in Fluid mechanics and material science. The question focuses on identifying the physical property associated with this unit and understanding why such units are important in practical measurements.

In Physics, certain properties describe how substances respond when forces are applied. Some materials resist stretching, some oppose compression, while fluids exhibit resistance when one layer tries to slide over another. This internal resistance determines how easily a liquid can flow under applied pressure.

The poise is a traditional CGS unit connected with the study of Fluid motion. Larger or smaller forms of this unit are created using prefixes such as deca-, centi-, or milli-. In laboratories and industries, these units help compare the thickness or flow behavior of liquids like oil, honey, glycerin, paints, and lubricants.

Consider pouring water and honey from separate containers. Water spreads quickly because its internal resistance is low, whereas honey moves slowly due to stronger intermolecular friction between adjacent layers. Units like poise quantify this difference scientifically.

Thus, the unit mentioned in the question is associated with a flow-related property that determines how strongly a Fluid resists relative motion between its layers during movement.

Explanation:
This question involves determining the viscosity of a liquid using observations obtained from Poiseuille’s apparatus. The setup is commonly used to study the flow of liquids through narrow capillary tubes under controlled pressure conditions.

Poiseuille’s law relates the rate of flow of a liquid to factors such as pressure difference, radius of the tube, length of the tube, and viscosity. According to this law, the flow rate increases strongly with tube radius and pressure but decreases when viscosity or tube length becomes larger. The mathematical relation contains terms like r⁴, showing how sensitive the flow is to small changes in tube radius.

To solve such numerical problems, all measured values are substituted into the Poiseuille equation after converting them into consistent units. The pressure difference is usually determined from the liquid head, while the discharge rate represents the volume flowing per second. Rearranging the formula helps isolate the unknown quantity being calculated.

An everyday example can be seen while drinking juice through straws of different thicknesses. A wider straw allows liquid to move more easily, while a narrower straw restricts the flow significantly.

The calculation therefore depends on understanding how Fluid resistance, pressure, and capillary dimensions together influence the movement of liquids through narrow passages.

Explanation:
Devices such as paint sprayers and perfume atomizers work by converting liquids into fine droplets that spread efficiently into the surrounding air. This question asks about the physical principle responsible for that spraying action.

When air moves rapidly through a narrow region, its pressure decreases compared to the surrounding stationary air. Because of this pressure difference, liquid from a connected container is pushed upward and breaks into tiny droplets. The process demonstrates the relationship between Fluid speed and pressure in moving streams.

Fluid dynamics explains that faster-moving fluids exert lower pressure in certain conditions. This principle is widely applied in engineering systems such as carburetors, airplane wings, burners, and medical inhalers. In atomizers, compressed air passing through a nozzle creates a low-pressure zone that pulls the liquid upward through a thin tube.

A simple analogy is blowing air across the top of a straw dipped in water. The moving air reduces pressure near the straw opening, causing the liquid to rise and spray outward.

Thus, the working mechanism depends on the interaction between fluid velocity and pressure variation, allowing liquids to disperse as a mist without direct mechanical pumping.

Explanation:
Bernoulli’s theorem is one of the fundamental ideas in fluid dynamics and describes how different forms of energy behave while a fluid flows through a pipe or open channel. The question asks which physical quantity remains conserved according to this theorem.

A moving fluid possesses several forms of energy simultaneously. It may have pressure energy because of the force exerted by the fluid, kinetic energy because of its motion, and potential energy because of its height above a reference level. Bernoulli’s theorem states that under ideal conditions, the total of these energies remains constant along a streamline.

When fluid speed increases, another form of energy must decrease to maintain balance. This is why fast-moving fluid regions often show lower pressure. The theorem assumes the fluid is incompressible, non-viscous, and flowing steadily without turbulence.

An airplane wing demonstrates this principle effectively. Air moving faster over the curved upper surface develops lower pressure compared to the lower surface, producing an upward lifting effect.

Therefore, the theorem explains how different energy forms continuously transform into one another during fluid motion while maintaining an overall constant total throughout the flow system.