# Fluid Mechanics Interview Questions

Here are the top 50 commonly asked questions in Fluid Mechanics interviews. Whether you’re just starting your preparation or need a quick refresher, these questions and answers will help you tackle your interview with confidence.

## Basic Fluid Mechanics Interview Questions with Answers

### 1. How is a Fluid defined?

Fluid is defined as a substance that will continue to deform when subjected to a tangential shear stress.

### 2. What do you mean by Fluid Mechanics?

Fluid is a substance which can flow. In mechanics, we study the effect of force. Hence fluid mechanics is the science that studies the behavior of fluid subjected to force.

### 3. How is a Fluid classified?

A fluid is classified into Liquid and Gas based on the free surface. A liquid forms free surface when it is filled in a container. A gas cannot form a free surface due to weak inter-molecular forces.

### 4. What is the difference between a Fluid and a Solid?

A solid can resist the tangential shear stress applied to it while a fluid cannot. A fluid flows when the shear stress is applied to it.

### 5. What is the difference between an Ideal Fluid and a Real Fluid?

An ideal fluid is incompressible and has no viscosity. A real fluid is compressible and viscous in nature. Real fluids with low viscosity can be treated as ideal fluids.

### 6. What do we study in Fluid Mechanics?

We study the behavior of all liquids and incompressible gases either at rest or in motion in fluid mechanics.

### 7. What is the difference between Fluid Mechanics and Thermodynamics?

Fluid mechanics deals with the flow of fluid while thermodynamics deals with the flow of heat. Generally, the effect of temperature is not considered in fluid mechanics.

### 8. What are the limitations of Fluid Mechanics?

The basic equations of fluid mechanics are too difficult to apply to an arbitrary geometry. Hence the numerical approach is required. Also, the basic equations of fluid mechanics cannot be applied to real fluids.

### 9. Who is the father of modern Fluid Mechanics?

Ludwig Prandtl (1875-1953) is known as the father of modern fluid mechanics due to his contributions towards development of the boundary layer theory.

### 10. What are the characteristics of Fluids?

Fluids do not have fixed shape. Fluids take the shape of the container they are stored in. Fluids deform when shear stress is applied to it.

### 11. How is the Fluid Mechanics classified?

Fluid mechanics is classified into three parts – Fluid Statics, Fluid Kinematics, Fluid Dynamics. Fluid statics studies the fluid at rest. Fluid kinematics studies fluid motion without considering the force inducing it. Fluid dynamics studies the fluid motion along with the force inducing it.

### 12. What is the difference between Fluid Mechanics and Computational Fluid dynamics?

Fluid mechanics deals with the analytical solutions of fluid flow problems. Computational fluid dynamics is a branch of fluid mechanics. It approximates the governing equations of fluid mechanics into differential equations and provides solutions in the form of numbers.

### 13. Which fundamental principles are used to derive the equations of Fluid Mechanics?

The principle of conservation of mass, conservation of energy and Newton’s laws of motion are used to derive the governing equations of fluid mechanics.

### 14. What are the important properties of a Fluid?

The important properties of a fluid are – Pressure, Density, Viscosity, Compressibility, Surface Tension.

### 15. What are the applications of Fluid Mechanics?

Every system in the nature is either made of or surrounded by the fluid. Fluid mechanics is applicable to natural flows and weather, pipe systems, cars, aircrafts, human body, power plants, wind turbines, ships etc.

### 16. How is the molecular spacing in a Fluid compared to a Solid?

The inter-molecular force is lower in fluids. Due to this, the molecules are spaced wider in a fluid compared to a solid. Hence the molecular spacing is higher in a fluid.

### 17. When was the Fluid Mechanics developed as a science?

Fluid mechanics was developed as a science by Galileo Galilei in 17th century.

### 18. What are the types of Real Fluids?

Real fluids are classified as Newtonian Fluids, Non-Newtonian Fluids and Plastic Fluids based on the stress-strain rate relationship.

### 19. When can we consider the physical properties constant in Fluid Mechanics?

When the Knudsen Number (Kn) is less than 0.01 for a given fluid flow system, then the properties can be considered constant.

### 20. Why is Fluid Mechanics called Hydraulics?

‘Hydro’ means water and ‘Aulics’ means pipe in Greek language. Most engineering applications involve water as a working fluid flowing through the pipes. Since the working fluid is water, the fluid mechanics is called Hydraulics.

## Intermediate Fluid Mechanics Interview Questions with Answers

### 21. What are the Manometers?

Manometers are the devices used to measure the pressure at any point in a stationary fluid.

### 22. Why is mercury used as a barometer fluid?

The fluid column cannot exceed a certain height due to size limitation of barometer. Mercury is almost 13.6 times denser than water. Hence the height of the mercury column will be lower. Thus mercury is used as a barometer fluid.

### 23. How can we convert gauge pressure into absolute pressure?

Gauge pressure is measured by considering atmospheric pressure as datum. Hence adding atmospheric pressure into gauge pressure gives absolute pressure. Mathematically, Absolute Pressure = Gauge Pressure + Atmospheric Pressure.

### 24. What is the use of the Pascal’s law?

Pascal’s law states that the pressure applied at any point is transmitted equally in all the directions. Pascal’s law is used to convert the small magnitude of applied force into a large magnitude of output force. Hydraulic press, hydraulic brakes, hydraulic cranes, automobile lift etc. operate on the Pascal’s law.

### 25. What is the difference between a Manometer and a Mechanical gauge?

In a Manometer, pressure is measured by balancing the column of a known liquid with the column of a liquid whose pressure is to be found. A Mechanical Gauge uses an elastic element which deflects under the pressure. The pressure is calculated by measuring the deflection.

### 26. What are the types of equilibrium of a floating body?

Depending upon the metacentric height, there are three types of equilibrium of a floating body – Stable equilibrium, Unstable equilibrium and Neutral equilibrium.

### 27. What is the difference between centre of buoyancy and centre of gravity?

Centre of the buoyancy is the centroid of the immersed part of a floating body. The centre of gravity is the centroid of the whole floating body. Net upward buoyancy force acts at the centre of buoyancy while the weight of the body acts at the centre of gravity.

### 28. Why can a person float easily in the Dead Sea?

The net upward buoyant force is directly proportional to the density. In the Dead Sea, the density of the water is 24% higher due to the dissolved salts. Higher the density, higher will be the buoyancy. Hence a person can float easily in the dead sea.

### 29. Who discovered Buoyancy?

Greek mathematician Archimedes discovered the principle of buoyancy.

### 30. How can we reduce the time period of rolling for a passenger ship?

Time period of rolling is inversely proportional to square root of the metacentric height (T∝1/√GM). Hence to reduce the time period of rolling, we can increase the distance between the centre of gravity and the metacentre.

### 31. What is the difference between a scalar and a vector?

Scalar is a physical quantity which is specified only by the magnitude. Vector is a physical quantity which is specified by magnitude and direction. Typical scalar quantities are mass, density, temperature. Typical vector quantities are force, velocity.

### 32. Why cannot two streamlines intersect each other?

If two stream lines intersect at a point, then that point will have two directions of motions which is not physically possible. Hence two stream lines cannot intersect each other.

### 33. What is a Rankine Vortex?

Rankine vortex is a combination of forced and free vortex flow. A tornado is the best example of a Rankine vortex.

### 34. Is the flow net analysis applicable to rotational flow?

Flow net is a grid of stream lines and equipotential lines. The equipotential lines are defined for irrotational flows only. Hence the flow net analysis is not applicable to rotational flow.

### 35. What are the types of flow lines?

Flow lines are classified as Path lines, Stream lines and Streak lines.

### 36. Which forces are considered to derive the Bernoulli’s equation?

Only the pressure force and the gravity force are considered to derive the Bernoulli’s equation.

### 37. What are the limitations of a rotameter?

Accuracy of a rotameter is less compared to a venturi meter. Rotameter can be mounted only vertically. Rotameter can be used only for small pipes with low flow rates.

### 38. What are the applications of impulse-momentum equation?

Impulse-momentum equation is used to determine the resultant force acting on the pipe bends, reducers, moving vanes. Impulse-momentum equation is used to find sudden enlargement in pipe or hydraulic jump in a channel.

### 39. What is the limitation of a siphon?

If the pressure in a siphon reaches the vapor pressure of the liquid, then the liquid will start to evaporate. Due to this, the siphon will be vapor locked and the liquid flow will stop.

### 40. Why cannot we apply the Bernoulli’s equation to a turbulent flow?

The Bernoulli’s equation is derived assuming that the energy loss is negligible. In a turbulent flow, viscous effects are dominant and large amount of energy is dissipated. Hence, we cannot apply the Bernoulli’s equation to a turbulent flow.

### 41. What is the difference between a prototype and a model?

A prototype is a full scale or actual size system. A model is a laboratory scale system. A model can be smaller or larger than the prototype.

### 42. What are the limitations of dimensional analysis?

Dimensional analysis does not provide the value of proportionality constant. Dimensional analysis does not provide any information regarding selection of the variables.

### 43. How many types of hydraulic models are there?

Hydraulic models are of two types – distorted models and undistorted models.

### 44. What are the limitations of a distorted model?

In a distorted model, the magnitude and distribution of velocity and pressure may not be reproduced accurately. Another limitation is that the slope of riverbed and dikes cannot be reproduced accurately in the model.

### 45. What do you mean by Reynold’s model law?

Reynold’s model law is used when the inertia force and the viscous force are dominant. In Reynold’s model law, similarity of flow is obtained by equating the Reynold’s numbers of model and prototype. Mathematically, (Re)model = (Re)prototype.

### 46. Why does an aircraft fly at very high altitude?

An aircraft experiences drag force (air resistance) during the flight. The formula of drag force is given by, Drag force = 1/2 CD ρAv2, where ρ is the air density. Air density decreases with increase in the altitude. Hence to reduce the drag force (air resistance), aircrafts fly at very high altitude.

### 47. Is the boundary layer theory valid at the leading edge of a flat plate?

In the boundary layer theory, it is assumed that the boundary layer thickness is much smaller than the characteristic length. At the leading edge, the boundary layer thickness and the characteristic length are of similar size. Hence, the boundary layer theory is not valid at the leading edge of a flat plate.

### 48. What is the difference between form drag and friction drag?

Drag force arising due to pressure difference is called form drag. The shear stress acting on the surface of a body creates friction drag.

### 49. Can we apply the Bernoulli’s equation inside a boundary layer?

The flow is rotational inside a boundary layer. The Bernoulli’s equation can be applied to irrotational flows only. Hence, we cannot apply the Bernoulli’s equation inside a boundary layer.

### 50. What is the value of vorticity inside and outside a boundary layer?

The flow is rotational inside a boundary layer. Hence the vorticity is nonzero inside a boundary layer. Flow outside a boundary layer acts as a potential flow with zero vorticity.

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