Fluid Mechanics - Interview Questions

Fluid mechanics is the study of behavior of fluids and the forces that they produce. Fluids include liquids, gases, blood and plasma. Fluid mechanics studies the behavior of fluids that are at rest and well in motion.

Fluid mechanics is applied in a wide range of disciplines, including civil engineering, mechanical engineering, chemical engineering, biomedical engineering, aeronautics, aerospace, geophysics, etc.

Following are the basic properties of fluids.

1. Density

2. Temperature

3. Pressure

4. Viscosity

5. Specific Volume

6. Specific Weight

7.Specific Gravity

8. Surface Tension

Fluids are classified as follows.

1. Ideal Fluid

2. Real Fluid

3. Newtonian Fluid

4. Non-Newtonian Fluid

5. Ideal Plastic Fluid

Surface tension is the tendency of liquid surfaces to shrink to the minimum surface area possible. Due to surface tension, liquid surfaces seem as a stretched elastic membrane.

Because of surface tension, objects having higher density than the liquid - such as coins, razor blades, some insects etc. can stay on top of the liquid surface.

Capillarity, or Capillary action, is the phenomenon in which a liquid spontaneously either raises or falls in a narrow passage like a tube with a small cross-sectional area.

Capillarity occurs due to the combination of surface tension - which is caused by cohesion of liquid molecules, and adhesive forces between the liquid and the container wall.

Newtons law of viscosity states that the shear stress between adjacent fluid layers is directly proportional to the velocity gradients (Rate of shear strain) between the two layers.

For a given temperature and pressure, the ratio of shear stress to rate of shear strain is a constant - which is defined as viscosity or coefficient of viscosity.

Compressibility is a measure of the change in volume of a fluid in response to a change in pressure. For a given mass of fluid, an increase in pressure will cause a decrease in volume of the fluid. Compressibility is defined as the ratio of change in volume to the change in pressure.

Bulk modulus is the inverse of compressibility. Bulk modulus is defined as the ratio of change in pressure to the change in volume.

Pascal's law states that in a fluid at rest in a closed container, a pressure change in one part is transmitted without loss to every portion of the fluid and to the walls of the container.

Isothermal process is a process in which a gas expands or compresses at constant temperature.

Adiabatic process, also known as isentropic process, is a process in which a gas expands or compresses with out giving out or taking in heat energy.

There are two methods of describing fluid motion - Lagrangian and Eulerian.

In Lagrangian method, fluid particles are followed and variations of physical quantities (pressure, temperature, density, velocity etc.) are described round each fluid particle along its trajectory.

In Eulerian method variations of physical quantities are described at fixed locations as a function of time. Different fluid particles pass the location at different times.

Fluid flow can be classified in six different ways based on the variation of fluid properties such as density, velocity etc..

1. Steady and Unsteady flow

2. Uniform and Non-uniform flow

3. Laminar and Turbulent flow

4. Compressible and Incompressible flow

5. Rotational and Irrotational flow

6. One, Two and Three dimensional flow

Velocity potential function and stream function are scalar functions that help determine if a fluid flow is rotational or irrotational.

Velocity potential function is a scalar function of time and space whose negative derivative in any direction gives the velocity component in that direction.

Stream function is a scalar function of time and space whose negative derivative in any direction gives the velocity component at right angles to that direction.

Following are the different types of forces that act on moving fluids.

Inertia force

Viscous force

Gravity force

Pressure force

Surface tension force

Elastic force

Dimensionless numbers in fluid mechanics are a set of dimensionless quantities that have an important role in analyzing the behavior of fluids. Following are some important dimensionless numbers.

Reynolds number

Froude number

Weber number

Mach number

Euler's number

fluid dynamics is a sub-discipline of fluid mechanics that describes the fluids — liquids and gases. Fluid dynamics has several sub-disciplines, including aerodynamics (the study of air and other gases in motion) and hydrodynamics (the study of liquids in motion).

Fluid dynamics is governed by the following conservation laws.

Conservation of mass

Conservation of linear momentum

Conservation of energy

Euler's equations, named after Leonhard Euler, are a set of equations governing adiabatic and Inviscid flow of fluids.

Euler's equations provide a relationship between velocity, pressure, and density of a moving fluid - represent the conservation of mass, momentum, and energy - and is based on Newton's second law of motion.

Euler's equations assume that the fluid is non-viscous, homogeneous and incompressible, and has a steady continuous flow along the streamline.

Bernoulli's principle states that an increase in velocity of a fluid occurs simultaneously with a decrease in static pressure or a decrease in the fluid's potential energy.

Bernoulli's principle assumes that the flow is steady or laminar, and the compressibility as well as viscosity of the fluid is negligible.

Darcy's equation, named after Henry Darcy, describes the flow of a fluid through a porous medium. Darcy's equation relates the loss in pressure due to friction, to the average velocity of the fluid flow.

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