JSSC- JE Mechanical Engg. Question Bank (Chapter – 12) Fluid Statics:

Mechanical

What is pressure at a point in a fluid at rest?

• The force acting parallel to the surface per unit area
• The normal force acting per unit area
• The weight of fluid per unit volume
• The volume of fluid per unit mass

Explanation:

• Pressure is the normal force exerted by a fluid per unit area.
• Pressure = Force/Area.
• In a fluid at rest, shear stress is zero, so pressure acts normally on any surface.

What is the SI unit of pressure?

• N
• N/m
• N/m²
• m²/N

Explanation:

• The SI unit of pressure is pascal (Pa).
• 1 Pa = 1 N/m².

In a fluid at rest, pressure at a point acts:

• Only vertically downward
• Only horizontally
• Equally in all directions
• Only upward

Explanation:

• According to Pascal’s law, pressure at a point in a static fluid is transmitted equally in all directions.
• Therefore, pressure at a point has the same magnitude regardless of the orientation of the imaginary surface passing through that point.

In a static fluid, pressure generally increases with:

• Decrease in depth
• Increase in depth
• Decrease in density only
• Decrease in gravity

Explanation:

• For a fluid at rest, pressure increases with depth according to p = p₀ + ρgh.
• Thus, points deeper below the free surface experience greater pressure.

What is the hydrostatic pressure at a depth h below the free surface of a liquid having density ρ?

• ρ/hg
• h/ρg
• ρgh
• ρg/h

Explanation:

• The gauge pressure due to the liquid column is:
• p = ρgh
• Here, ρ is density, g is gravitational acceleration and h is depth below the free surface.

What is gauge pressure?

• Pressure measured relative to absolute vacuum
• Pressure measured relative to atmospheric pressure
• Pressure equal to vapour pressure
• Pressure due only to liquid weight

Explanation:

• Gauge pressure is the pressure above atmospheric pressure.
• p_gauge = p_absolute − p_atmospheric.
• It can be positive or negative.

What is the relationship between absolute pressure and gauge pressure?

• p_absolute = p_gauge − p_atm
• p_absolute = p_gauge + p_atm
• p_absolute = p_atm − p_gauge
• p_absolute = p_gauge

Explanation:

• Absolute pressure is measured from absolute vacuum.
• Therefore, p_absolute = p_atmospheric + p_gauge.

Which instrument is commonly used for measuring pressure difference between two points in a fluid system?

• Thermometer
• Barometer
• Differential manometer
• Hydrometer

Explanation:

• A differential manometer measures the pressure difference between two points.
• It commonly uses a manometric liquid of known density to determine the pressure difference from the difference in liquid levels.

What is the basic principle of a manometer?

• Conservation of momentum
• Conservation of mass
• Hydrostatic pressure balance
• Newton’s law of viscosity

Explanation:

• A manometer works by balancing pressure with the weight of liquid columns.
• The analysis is based on the hydrostatic relation p = ρgh.

Which type of manometer consists of a transparent tube bent into the shape of the letter U?

• Inclined manometer
• U-tube manometer
• Bourdon gauge
• Piezoelectric pressure gauge

Explanation:

• A U-tube manometer consists of a U-shaped tube partially filled with a manometric liquid.
• The difference in liquid levels is used to determine pressure or pressure difference.

A simple U-tube manometer is commonly used to measure:

• Temperature
• Flow velocity only
• Pressure or pressure difference
• Fluid viscosity directly

Explanation:

• Depending on its arrangement, a U-tube manometer can measure gauge pressure, vacuum pressure or pressure difference between two points.

Which liquid is commonly used as a manometric liquid for measuring relatively high pressure differences?

• Water only
• Mercury
• Mercury
• Petrol

Explanation:

• Mercury has a high density, approximately 13.6 times that of water.
• Therefore, a relatively small mercury column can balance a large pressure difference.
• Other liquids may also be used depending on the application.

Why is a liquid of high density such as mercury used in many manometers?

• It produces very large columns
• It has zero density
• It produces a smaller column height for a given pressure difference
• It cannot transmit pressure

Explanation:

• From p = ρgh, for a given pressure difference, increasing density decreases the required height difference.
• This makes high-density liquids useful for measuring relatively high pressures with compact instruments.

What is the main advantage of an inclined-tube manometer?

• It measures temperature directly
• It provides greater sensitivity for small pressure differences
• It eliminates atmospheric pressure
• It is suitable only for gases at very high pressure

Explanation:

• In an inclined manometer, a small vertical movement produces a longer movement along the inclined tube.
• Therefore, small pressure differences can be measured more accurately.

An inclined manometer is particularly suitable for measuring:

• Very high pressure only
• Small pressure differences
• Temperature differences
• Liquid density directly

Explanation:

• The inclined tube magnifies the movement of the manometric liquid along the tube.
• This increases sensitivity and makes it useful for small pressure differences.

Which statement about pressure in a continuous static liquid is correct?

• Pressure is always zero at every point
• Pressure decreases with depth
• Points at the same horizontal level have the same pressure
• Pressure depends only on container shape

Explanation:

• For the same continuous liquid at rest, pressure at the same elevation is equal.
• It does not depend on the shape of the container.
• This is associated with the hydrostatic principle and is often called the hydrostatic paradox when discussing different container shapes.

Which of the following does NOT directly affect hydrostatic pressure at a given depth below a free surface?

• Fluid density
• Depth
• Acceleration due to gravity
• Shape of the container

Explanation:

• Hydrostatic gauge pressure is p = ρgh.
• Thus, it depends on density, gravity and depth, but not on the shape of the container.

What is a streamline?

• A line of constant pressure
• A line of constant temperature
• A line whose tangent at every point gives the instantaneous direction of fluid velocity
• A line connecting points of equal density

Explanation:

• A streamline is an imaginary curve drawn in a flowing fluid such that the tangent at any point gives the direction of the velocity vector at that point.
• There is no flow across a streamline under the idealized instantaneous definition.

Two streamlines in a fluid flow cannot intersect because:

• Pressure becomes zero
• Density becomes infinite
• There would be two different velocity directions at the same point
• Temperature would become constant

Explanation:

• At a particular instant, a fluid particle at a point can have only one velocity direction.
• If two streamlines intersected, they would indicate two different velocity directions at the same point, which is physically impossible for ordinary flow.

What is laminar flow?

• Flow with random large-scale fluctuations
• Flow in which fluid particles move in relatively smooth and orderly layers
• Flow with maximum turbulence
• Flow with zero velocity

Explanation:

• In laminar flow, fluid particles move in smooth, relatively parallel layers with limited transverse mixing.
• For pipe flow, laminar flow generally occurs at Reynolds number below about 2300.

What is turbulent flow?

• Flow consisting of perfectly parallel layers
• Flow characterized by irregular fluctuations and strong mixing
• Flow with zero pressure
• Flow occurring only in stationary fluids

Explanation:

• Turbulent flow contains irregular velocity and pressure fluctuations and significant mixing.
• For flow through a circular pipe, turbulent flow is generally associated with Reynolds number above about 4000.

For flow through a circular pipe, laminar flow generally occurs when Reynolds number is:

• Greater than 10,000
• Between 4000 and 10,000
• Less than about 2300
• Exactly 4000

Explanation:

• For internal flow through a circular pipe:
• Re < 2300: generally laminar
• Re ≈ 2300 to 4000: transition region
• Re > 4000: generally turbulent

For flow through a circular pipe, turbulent flow is generally obtained when Reynolds number is:

• Less than 500
• Less than 1000
• Between 1000 and 2000
• Greater than about 4000

Explanation:

• For internal pipe flow, Reynolds number above approximately 4000 generally indicates turbulent flow.
• The exact transition behavior can depend on disturbances and inlet conditions.

Which dimensionless number is primarily used to distinguish laminar and turbulent flow?

• Mach number
• Froude number
• Reynolds number
• Prandtl number

Explanation:

• Reynolds number represents the ratio of inertial forces to viscous forces.
• For pipe flow, Re = ρVD/μ = VD/ν.

Which expression represents Reynolds number for flow through a circular pipe?

• ρμVD
• ρVD/μ
• μVD/ρ
• ρD/μV

Explanation:

• For circular pipe flow, Reynolds number is:
• Re = ρVD/μ = VD/ν
• Here, ρ is density, V is mean velocity, D is pipe diameter, μ is dynamic viscosity and ν is kinematic viscosity.

What is meant by internal flow?

• Flow completely outside a body
• Flow bounded by a solid surface, such as flow inside a pipe or duct
• Flow only in open oceans
• Flow without viscosity

Explanation:

• Internal flow occurs within a conduit such as a pipe, duct or channel with boundaries that constrain the flow.
• Pipe flow is a common example of internal flow.

What is meant by external flow?

• Flow only inside a closed pipe
• Flow over or around a solid body immersed in a fluid
• Flow through a capillary tube
• Flow in a completely enclosed tank at rest

Explanation:

• External flow occurs when fluid moves around an immersed body.
• Examples include airflow over an aircraft wing, car or cylinder.

Which of the following is an example of internal fluid flow?

• Air flowing around an automobile
• Water flowing around a submerged sphere
• Water flowing through a pipe
• Wind flowing over an aircraft wing

Explanation:

• Water flowing inside a pipe is an internal flow because the pipe walls completely constrain the fluid.
• Flow around a car, sphere or wing is external flow.

Which of the following is an example of external flow?

• Water flowing through a pipeline
• Oil flowing through a hydraulic cylinder passage
• Air flowing over an aircraft wing
• Water flowing through a nozzle

Explanation:

• Air flowing over an aircraft wing is external flow because the fluid moves around an immersed solid body.

What does the continuity equation represent for fluid flow?

• Conservation of energy
• Conservation of mass
• Conservation of temperature
• Conservation of viscosity

Explanation:

• The continuity equation is derived from conservation of mass.
• For steady flow, the mass flow rate entering a control volume equals the mass flow rate leaving it when there is no accumulation.

What is the continuity equation for steady incompressible flow through a pipe?

• pV = constant
• A₁V₁ = A₂V₂
• p₁A₁ = p₂A₂
• V₁/A₁ = V₂/A₂

Explanation:

• For steady incompressible flow, density is constant.
• Therefore, conservation of mass gives:
• A₁V₁ = A₂V₂
• The discharge is Q = AV.

For steady compressible flow, the general continuity relation between two sections is:

• A₁V₁ = A₂V₂ only
• ρ₁A₁V₁ = ρ₂A₂V₂
• ρ₁A₁ = ρ₂A₂V₁V₂
• A₁/V₁ = A₂/V₂

Explanation:

• For steady flow, conservation of mass requires constant mass flow rate.
• Therefore, ρAV = constant.
• For incompressible flow, density is constant and the relation reduces to AV = constant.

What is the discharge through a pipe carrying an incompressible fluid?

• Q = A/V
• Q = V/A
• Q = AV
• Q = ρAV

Explanation:

• Volumetric discharge is the volume of fluid flowing per unit time.
• For a uniform section, Q = AV.
• Its SI unit is m³/s.

If the cross-sectional area of a pipe is reduced to half for steady incompressible flow, the velocity will:

• Become half
• Remain unchanged
• Become twice
• Become four times

Explanation:

• From A₁V₁ = A₂V₂:
• If A₂ = A₁/2, then V₂ = 2V₁.
• Thus, velocity doubles.

A pipe has an area of 0.02 m² and carries water at 3 m/s. What is the discharge?

• 0.006 m³/s
• 0.06 m³/s
• 0.6 m³/s
• 6 m³/s

Explanation:

• Discharge Q = AV.
• Q = 0.02 × 3 = 0.06 m³/s.

In steady flow, which quantity remains constant along a stream tube when there is no mass accumulation?

• Pressure only
• Velocity only
• Mass flow rate
• Temperature only

Explanation:

• For steady flow without accumulation, the mass flow rate remains constant.
• Thus, ρAV = constant.
• Pressure and velocity can change from one section to another.

Which statement correctly describes a streamline in a steady flow?

• It always represents a constant-pressure line
• Its tangent gives the local fluid velocity direction
• It is always perpendicular to the velocity
• It represents a line of constant density

Explanation:

• A streamline is defined by the instantaneous velocity direction.
• In a steady flow, streamlines coincide with particle paths and streaklines under the usual ideal conditions of a steady velocity field.