# Fluid mechanics viva questions and answers

## Q: What is fluid mechanics?

A: Fluid mechanics is the study of fluids in motion, including both liquids and gases. It deals with the behavior of fluids under various conditions and the principles governing their flow.

## Q: What is a fluid?

A: A fluid is a substance that can flow and take the shape of its container. Liquids and gases are examples of fluids.

## Q: What is viscosity?

A: Viscosity is a measure of a fluid’s resistance to flow. It is a property that describes how easily a fluid can be deformed or sheared.

## Q: What is Bernoulli’s equation?

A: Bernoulli’s equation relates the pressure, velocity, and height of a fluid flowing through a pipe or over a surface. It is based on the principle of conservation of energy.

## Q: What is the Reynolds number?

A: The Reynolds number is a dimensionless quantity that is used to predict the flow regime of a fluid. It is based on the ratio of inertial forces to viscous forces.

## Q: What is laminar flow?

A: Laminar flow is a smooth, orderly flow of fluid in which the velocity at any point remains constant over time. It is characterized by the absence of turbulence.

## Q: What is turbulent flow?

A: Turbulent flow is a chaotic, unpredictable flow of fluid in which the velocity at any point varies over time. It is characterized by the presence of eddies and vortices.

## Q: What is a boundary layer?

A: A boundary layer is the thin layer of fluid that forms near a solid surface in a flowing fluid. It is characterized by a gradual transition from the velocity of the fluid at the surface to the velocity of the free stream.

## Q: What is a lift force?

A: A lift force is the force that acts perpendicular to the direction of motion of a fluid around an object. It is responsible for the ability of wings to generate lift and keep airplanes in the air.

## Q: What is a drag force?

A: A drag force is the force that acts parallel to the direction of motion of a fluid around an object. It is responsible for the resistance that objects experience when moving through a fluid.

## Q: What is a pump?

A: A pump is a device that is used to increase the pressure of a fluid and move it from one location to another. It works by converting mechanical energy into hydraulic energy.

## Q: What is a turbine?

A: A turbine is a device that is used to extract energy from a flowing fluid and convert it into mechanical energy. It works by using the force of the fluid to rotate a set of blades or a rotor.

## Q: What is a nozzle?

A: A nozzle is a device that is used to increase the velocity of a fluid by reducing its cross-sectional area. It is commonly used in engines and propulsion systems.

## Q: What is a diffuser?

A: A diffuser is a device that is used to decrease the velocity of a fluid by increasing its cross-sectional area. It is commonly used in fluid systems to reduce turbulence and increase efficiency.

## Q: What is the continuity equation?

A: The continuity equation states that the mass flow rate of a fluid in a pipe or channel must remain constant as the fluid flows. This equation is based on the principle of conservation of mass.

## Q: What is the Navier-Stokes equation?

A: The Navier-Stokes equation is a set of partial differential equations that describe the motion of fluid. It is used to calculate the velocity, pressure, and other properties of a fluid in motion.

A: Steady flow refers to a fluid in motion that has a constant velocity and flow rate at any given point. Unsteady flow, on the other hand, refers to a fluid in motion that has a velocity and flow rate that varies with time.

## Q: What is the difference between compressible and incompressible flow?

A: Compressible flow refers to a fluid that changes density when its pressure or temperature changes. Incompressible flow refers to a fluid that maintains a constant density regardless of changes in pressure or temperature.

## Q: What is the boundary condition at a solid wall?

A: At a solid wall, the velocity of the fluid is zero due to the no-slip condition. This means that the fluid adheres to the surface of the wall and does not slip past it.

## Q: What is the Bernoulli effect?

A: The Bernoulli effect is the phenomenon whereby an increase in the velocity of a fluid is accompanied by a decrease in pressure. This effect is commonly observed in the lift generated by wings and the flow of fluids through pipes.

## Q: What is the Coanda effect?

A: The Coanda effect is the phenomenon whereby a fluid flowing over a curved surface will tend to follow the contour of the surface. This effect is commonly observed in the behavior of fluids around airplane wings and in industrial applications such as spray coatings.

## Q: What is the Prandtl number?

A: The Prandtl number is a dimensionless quantity that describes the ratio of momentum diffusivity to thermal diffusivity in a fluid. It is used to predict the behavior of fluids in heat transfer applications.

## Q: What is the Mach number?

A: The Mach number is a dimensionless quantity that describes the ratio of the velocity of an object to the speed of sound in the surrounding fluid. It is used to predict the behavior of fluids in high-speed flow applications, such as supersonic aircraft and rocket engines.

## Q: What is a stagnation point?

A: A stagnation point is a point in a fluid flow where the velocity of the fluid is zero. This occurs when the fluid flow encounters a solid surface at a perpendicular angle, such as the nose of an airplane.

## Q: What is the difference between laminar and turbulent boundary layers?

A: Laminar boundary layers are smooth and predictable, with a gradual transition from the velocity of the fluid at the surface to the velocity of the free stream. Turbulent boundary layers are chaotic and unpredictable, with eddies and vortices that can cause increased drag and reduced efficiency.

## Q: What is the Euler equation?

A: The Euler equation is a set of partial differential equations that describe the motion of an inviscid fluid, meaning a fluid with zero viscosity. It is used to study fluid flow in situations where the effects of viscosity can be neglected.

## Q: What is the Reynolds transport theorem?

A: The Reynolds transport theorem is a mathematical equation that relates the time rate of change of a fluid property in a control volume to the flow of that property across the control volume boundaries. It is used to study the transport of mass, momentum, and energy in fluid systems.

## Q: What is the Venturi effect?

A: The Venturi effect is the phenomenon whereby the velocity of a fluid increases and the pressure decreases as the fluid flows through a constricted section of pipe. This effect is commonly used in fluid systems to create a pressure differential that can be used to power machinery or measure fluid flow rate.

## Q: What is a stream function?

A: A stream function is a mathematical function that describes the flow of a fluid in a two-dimensional plane. It is commonly used to visualize fluid flow patterns and calculate properties such as velocity and pressure.

## Q: What is a vortex?

A: A vortex is a swirling motion of fluid around a center of rotation. Vortices can be either stable or unstable and can be observed in many natural and industrial fluid systems.

## Q: What is a shock wave?

A: A shock wave is a type of pressure wave that is created when a fluid flow encounters a sudden change in geometry or speed. Shock waves can cause significant changes in fluid properties and can be observed in supersonic aircraft, explosions, and other high-speed fluid flows.

## Q: What is a hydraulic jump?

A: A hydraulic jump is a sudden transition from a fast-moving, low-depth flow to a slow-moving, high-depth flow that occurs when a fluid flow encounters a surface with a sudden change in geometry or elevation. Hydraulic jumps are commonly observed in open-channel flows such as rivers and spillways.

## Q: What is a cavitation?

A: Cavitation is the formation and subsequent collapse of vapor bubbles in a fluid due to changes in pressure. Cavitation can cause significant damage to machinery and is commonly observed in fluid systems such as pumps and propellers.

## Q: What is the Froude number?

A: The Froude number is a dimensionless quantity that describes the ratio of the inertia forces to the gravitational forces in a fluid. It is used to predict the behavior of fluids in open-channel flows and other situations where gravity plays a significant role.

## Q: What is the Weber number?

A: The Weber number is a dimensionless quantity that describes the ratio of the inertial forces to the surface tension forces in a fluid. It is used to predict the behavior of fluids in situations where surface tension plays a significant role, such as the breakup of liquid droplets.

## Q: What is the Reynolds number?

A: The Reynolds number is a dimensionless quantity that describes the ratio of the inertial forces to the viscous forces in a fluid. It is used to predict the transition from laminar to turbulent flow in fluid systems.

## Q: What is the difference between internal and external flow?

A: Internal flow refers to a fluid flow within a confined space, such as a pipe or duct. External flow refers to a fluid flow over a surface, such as the flow of air over an airplane wing or the flow of water over a dam.

## Q: What is the boundary layer?

A: The boundary layer is a thin layer of fluid adjacent to a solid surface where the velocity of the fluid changes from zero at the surface to the free stream velocity. The boundary layer is important in determining the drag and heat transfer characteristics of a solid surface.

## Q: What is the no-slip condition?

A: The no-slip condition is the principle that at a solid surface, the fluid velocity is zero due to the adhesion of the fluid molecules to the surface. This condition is important in determining the drag and shear stress at a solid surface.

## Q: What is the difference between laminar and turbulent flow?

A: Laminar flow refers to a smooth and predictable flow of fluid with no mixing between adjacent layers. Turbulent flow, on the other hand, is characterized by chaotic and unpredictable motion, with mixing between adjacent layers.

## Q: What is the difference between a Newtonian and a non-Newtonian fluid?

A: A Newtonian fluid is a fluid whose viscosity does not change with the applied shear stress or rate of deformation. A non-Newtonian fluid, on the other hand, is a fluid whose viscosity changes with the applied shear stress or rate of deformation.

## Q: What is the difference between a fluid and a gas?

A: A gas is a state of matter that expands to fill its container, with low density and compressibility. A fluid, on the other hand, is a substance that can flow and take the shape of its container, including both liquids and gases.

## Q: What is the difference between a steady and unsteady state flow?

A: Steady state flow refers to a fluid flow where the velocity and pressure at any point in the flow do not change with time. Unsteady state flow, on the other hand, refers to a fluid flow where the velocity and pressure at any point in the flow vary with time.

## Q: What is the principle of continuity?

A: The principle of continuity is a fundamental principle of fluid mechanics that states that the mass flow rate of a fluid is constant along a streamtube. In other words, the mass of fluid entering a given section of a pipe or channel per unit time must equal the mass of fluid leaving that section per unit time.

## Q: What is the difference between a centrifugal pump and a positive displacement pump?

A: A centrifugal pump is a type of pump that uses a rotating impeller to impart energy to the fluid, increasing its velocity and pressure. A positive displacement pump, on the other hand, uses a mechanism to displace a fixed volume of fluid with each cycle of operation.

## Q: What is the difference between a laminar and a turbulent boundary layer?

A: A laminar boundary layer is a thin layer of fluid adjacent to a solid surface where the fluid flows in smooth, parallel layers with minimal mixing. A turbulent boundary layer, on the other hand, is characterized by chaotic, three-dimensional flow with extensive mixing.

## Q: What is the difference between drag and lift?

A: Drag is the force that resists the motion of a solid object through a fluid in the direction parallel to the flow. Lift is the force that acts perpendicular to the direction of flow and is responsible for the upward force that allows airplanes and birds to fly.

## Q: What is the Venturi effect?

A: The Venturi effect is a phenomenon where the velocity of a fluid increases as it passes through a constriction in a pipe or channel, while the pressure decreases. It is named after Italian physicist Giovanni Venturi, who first observed the effect.

## Q: What is the difference between a laminar and a turbulent jet?

A: A laminar jet is a smooth, well-defined stream of fluid that maintains its shape over a significant distance from the point of exit. A turbulent jet, on the other hand, is characterized by chaotic, three-dimensional flow with extensive mixing and a rapidly decreasing velocity profile.

## Q: What is a boundary layer suction system?

A: A boundary layer suction system is a method of reducing drag on a solid surface by removing the boundary layer through suction. By removing the boundary layer, the skin friction drag is reduced, resulting in a net reduction in total drag.

## Q: What is the difference between an Eulerian and a Lagrangian approach to fluid mechanics?

A: An Eulerian approach to fluid mechanics involves studying the properties of a fluid at fixed points in space, while a Lagrangian approach involves studying the motion of individual fluid particles as they move through space. The Eulerian approach is more common in macroscopic fluid mechanics, while the Lagrangian approach is more common in microscopic fluid mechanics.

## Q: What is Reynolds number?

A: Reynolds number is a dimensionless parameter that describes the ratio of inertial forces to viscous forces in a fluid flow. It is named after Osborne Reynolds, who first proposed it in the 19th century. Reynolds number is important in determining the transition from laminar to turbulent flow in a fluid system.

## Q: What is the difference between a centrifugal and a centripetal force?

A: A centrifugal force is a force that appears to act outward on a rotating object, while a centripetal force is a force that acts inward toward the center of rotation. In fluid mechanics, centrifugal forces are often associated with rotating flows, while centripetal forces are associated with curved flows.

## Q: What is the Magnus effect?

A: The Magnus effect is a phenomenon where a spinning object experiences a sideways force perpendicular to the direction of motion. The effect is caused by the interaction between the spinning object and the fluid it is moving through.

## Q: What is the difference between an inviscid and a viscous flow?

A: An inviscid flow is a fluid flow where there is no internal friction or viscosity in the fluid. In contrast, a viscous flow is a fluid flow where there is significant internal friction or viscosity. In general, inviscid flows are simpler to analyze and model than viscous flows.

## Q: What is the difference between a laminar and a turbulent flow?

A: A laminar flow is a fluid flow where the fluid moves in smooth, parallel layers with minimal mixing. A turbulent flow, on the other hand, is characterized by chaotic, three-dimensional flow with extensive mixing and fluctuations in velocity, pressure, and other properties.

## Q: What is a boundary layer?

A: A boundary layer is a thin layer of fluid adjacent to a solid surface where the fluid experiences shear stresses due to the no-slip condition at the surface. In a laminar boundary layer, the fluid flows in smooth, parallel layers, while in a turbulent boundary layer, the flow is chaotic and mixed.

## Q: What is the difference between a fixed and a moving reference frame in fluid mechanics?

A: A fixed reference frame is a coordinate system that is stationary with respect to the fluid, while a moving reference frame is a coordinate system that moves with the fluid. In fluid mechanics, fixed reference frames are often used to analyze the motion of fluids relative to stationary objects, while moving reference frames are used to analyze the motion of fluids in rotating or accelerating systems.

## Q: What is the difference between a compressible and an incompressible flow?

A: A compressible flow is a fluid flow where there are significant changes in density due to changes in pressure or temperature. In contrast, an incompressible flow is a fluid flow where the density remains constant. In general, compressible flows are more complex and difficult to analyze than incompressible flows.