Mechanical
Which of the following best defines a fluid?
- A substance that cannot change its shape
- A substance that resists all deformation permanently
- A substance that continuously deforms under an applied shear stress
- A substance having zero density
Explanation:
- A fluid cannot sustain a shear stress at rest and continuously deforms when subjected to shear stress.
- Liquids and gases are classified as fluids.
Which of the following is an ideal fluid?
- A fluid having very high viscosity
- A hypothetical fluid having zero viscosity and generally treated as incompressible
- A fluid having infinite density
- A fluid having maximum friction
Explanation:
- An ideal fluid is a hypothetical fluid with zero viscosity.
- In basic fluid mechanics, it is also generally assumed to be incompressible.
- Therefore, an ideal fluid has no viscous shear resistance.
Which property is neglected for an ideal fluid?
- Density
- Pressure
- Viscosity
- Gravity
Explanation:
- An ideal fluid is assumed to have zero viscosity.
- Therefore, no energy is lost due to viscous friction in ideal-fluid analysis.
Which of the following is a characteristic of a real fluid?
- Zero viscosity
- No internal friction
- Non-zero viscosity
- Zero density
Explanation:
- All actual fluids possess viscosity to some extent.
- Viscosity represents the internal resistance of a fluid to relative motion between adjacent fluid layers.
Which statement about an ideal fluid is correct?
- It has infinite viscosity
- It has zero shear stress for any finite rate of deformation
- It always has zero pressure
- It cannot flow
Explanation:
- For an ideal fluid, viscosity μ = 0.
- From Newton’s law of viscosity, τ = μ(du/dy), so shear stress due to viscosity is zero.
Which of the following is a property of a real fluid that causes resistance to flow?
- Specific weight
- Surface tension only
- Viscosity
- Vapour pressure only
Explanation:
- Viscosity is the property of a fluid that offers resistance to relative motion between its layers.
- Higher viscosity generally means greater resistance to flow under comparable conditions.
Newton’s law of viscosity relates shear stress to:
- Pressure only
- Density only
- Velocity gradient
- Temperature only
Explanation:
- Newton’s law of viscosity is:
- τ = μ(du/dy)
- Thus, shear stress is proportional to the velocity gradient normal to the direction of flow.
Which equation represents Newton’s law of viscosity?
- τ = ρgh
- τ = pA
- τ = μ(du/dy)
- τ = ρV²
Explanation:
- Newton’s law of viscosity is expressed as τ = μ(du/dy).
- Here, τ is shear stress, μ is dynamic viscosity and du/dy is the velocity gradient.
In Newton’s law of viscosity, μ represents:
- Kinematic viscosity
- Dynamic viscosity
- Density
- Specific gravity
Explanation:
- In τ = μ(du/dy), μ is the coefficient of dynamic or absolute viscosity.
- Its SI unit is Pa·s or N·s/m².
What is the SI unit of dynamic viscosity?
- m²/s
- N/m²
- Pa·s
- kg/m³
Explanation:
- The SI unit of dynamic viscosity is Pa·s.
- It is also equivalent to N·s/m² or kg/(m·s).
What is the SI unit of kinematic viscosity?
- Pa·s
- N·s/m²
- m²/s
- kg/m³
Explanation:
- Kinematic viscosity is defined as:
- ν = μ/ρ
- Its SI unit is m²/s.
What is the relationship between dynamic and kinematic viscosity?
- ν = μρ
- ν = μ/ρ
- μ = ν/ρ
- ν = ρ/μ
Explanation:
- Kinematic viscosity is dynamic viscosity divided by density.
- ν = μ/ρ
A fluid that obeys Newton’s law of viscosity is called:
- Ideal fluid
- Newtonian fluid
- Perfect gas
- Compressible fluid
Explanation:
- A Newtonian fluid obeys the linear relationship τ = μ(du/dy).
- At a given temperature and pressure, its viscosity remains essentially independent of the velocity gradient.
Which of the following is a Newtonian fluid?
- Toothpaste
- Paint
- Water
- Blood
Explanation:
- Water behaves approximately as a Newtonian fluid under ordinary conditions.
- Other common Newtonian fluids include air and many simple oils.
For a Newtonian fluid, the relationship between shear stress and velocity gradient is:
- Nonlinear
- Exponential
- Linear
- Independent of velocity gradient
Explanation:
- For a Newtonian fluid:
- τ = μ(du/dy)
- Therefore, shear stress varies linearly with velocity gradient when μ is constant.
Which of the following is generally classified as a non-Newtonian fluid?
- Water
- Air
- Toothpaste
- Simple mineral oil under suitable conditions
Explanation:
- Toothpaste exhibits non-Newtonian behavior.
- Its apparent viscosity changes with shear rate and it may require a finite stress before significant flow occurs.
Which statement is correct for a non-Newtonian fluid?
- Shear stress is always directly proportional to velocity gradient
- Viscosity is always constant under all conditions
- The relationship between shear stress and velocity gradient is not necessarily linear
- It must have zero viscosity
Explanation:
- Non-Newtonian fluids do not obey the simple linear Newtonian relationship.
- Their apparent viscosity may depend on shear rate, time or other factors.
Which of the following fluids generally exhibits shear-thinning behavior?
- Water
- Paint
- Ideal fluid
- Air
Explanation:
- Shear-thinning fluids show decreasing apparent viscosity as shear rate increases.
- Many paints, polymer solutions and some biological fluids exhibit shear-thinning behavior.
A fluid whose apparent viscosity decreases as the rate of shear increases is called:
- Dilatant fluid
- Pseudoplastic fluid
- Newtonian fluid
- Ideal fluid
Explanation:
- Pseudoplastic fluids are shear-thinning fluids.
- Their apparent viscosity decreases with increasing shear rate.
A fluid whose apparent viscosity increases with increasing shear rate is called:
- Pseudoplastic fluid
- Newtonian fluid
- Dilatant fluid
- Ideal fluid
Explanation:
- Dilatant fluids are shear-thickening fluids.
- Their apparent viscosity increases as the shear rate increases.
Which type of non-Newtonian fluid requires a finite yield stress before it begins to flow?
- Newtonian fluid
- Pseudoplastic fluid without yield stress
- Plastic or Bingham plastic fluid
- Ideal fluid
Explanation:
- A Bingham plastic requires a yield stress before appreciable continuous flow occurs.
- Toothpaste and some slurries are common examples of materials that can show yield-stress behavior.
Which graph correctly represents a Newtonian fluid when shear stress is plotted against velocity gradient?
- A horizontal curve
- A vertical curve
- A straight line passing through the origin
- A random curve
Explanation:
- For a Newtonian fluid, τ = μ(du/dy).
- Therefore, a plot of shear stress against velocity gradient is a straight line through the origin, with slope equal to μ.
In a Newtonian fluid, the slope of the shear stress versus velocity gradient graph represents:
- Density
- Pressure
- Dynamic viscosity
- Specific gravity
Explanation:
- From τ = μ(du/dy), the slope of the τ versus du/dy graph is μ.
- Thus, the slope represents dynamic viscosity.
If the dynamic viscosity of a Newtonian fluid is doubled while the velocity gradient remains constant, the shear stress will:
- Become half
- Remain unchanged
- Become double
- Become four times
Explanation:
- Newton’s law gives τ = μ(du/dy).
- If μ is doubled and du/dy remains constant, τ also doubles.
If the velocity gradient in a Newtonian fluid is doubled while viscosity remains constant, the shear stress will:
- Become half
- Remain unchanged
- Become double
- Become four times
Explanation:
- Since τ = μ(du/dy), shear stress is directly proportional to velocity gradient for a Newtonian fluid.
- Therefore, doubling the velocity gradient doubles the shear stress.
Which statement correctly distinguishes an ideal fluid from a real fluid?
- Ideal fluid has density, while real fluid does not
- Ideal fluid has zero viscosity, while real fluids have viscosity
- Ideal fluid cannot flow, while real fluid can flow
- Ideal fluid always has higher density
Explanation:
- An ideal fluid is a theoretical model with zero viscosity.
- Real fluids have finite viscosity and therefore experience viscous resistance.
What is a compressible fluid?
- A fluid whose pressure is always zero
- A fluid whose density changes appreciably with pressure or other flow conditions
- A fluid having zero viscosity
- A fluid that cannot flow
Explanation:
- In compressible flow, changes in pressure can produce significant changes in density.
- Gases are generally treated as compressible fluids, especially when pressure or velocity changes are significant.
Which fluid is generally treated as compressible in fluid mechanics?
- Water
- Mercury
- Air
- Hydraulic oil
Explanation:
- Gases such as air have density changes that can become significant with changes in pressure and temperature.
- Therefore, air is generally treated as a compressible fluid.
Which fluid is generally treated as incompressible under ordinary engineering conditions?
- Air
- Natural gas
- Water
- Steam
Explanation:
- Liquids have relatively small changes in density with pressure.
- Therefore, water is commonly treated as incompressible in many engineering calculations.
What is an incompressible fluid?
- A fluid having zero pressure
- A fluid having zero viscosity
- A fluid whose density remains essentially constant
- A fluid having infinite velocity
Explanation:
- For an incompressible fluid, density is treated as constant during the flow.
- This assumption greatly simplifies the continuity and energy equations.
Which of the following statements about liquids is generally correct?
- Liquids are highly compressible
- Liquids are generally treated as incompressible
- Liquids always have zero viscosity
- Liquids cannot exert pressure
Explanation:
- Liquids do have finite compressibility, but their density changes are usually small compared with gases.
- Hence, they are commonly approximated as incompressible in engineering applications.
For an incompressible fluid, the density during a flow is:
- Always zero
- Always increasing
- Approximately constant
- Always decreasing
Explanation:
- The defining approximation for an incompressible fluid is constant density.
- Therefore, ρ1 = ρ2 in the simplified continuity equation.
For a steady incompressible flow, the continuity equation is:
- ρAV = constant with changing density
- AV = constant
- pV = constant
- μAV = constant
Explanation:
- The general steady-flow mass conservation equation is ρAV = constant.
- For incompressible flow, ρ is constant, so it reduces to AV = constant.
Which of the following is most likely to require compressible-flow analysis?
- Slow flow of water through a large pipe
- Static water in a tank
- High-speed flow of air through a nozzle
- Slow flow of hydraulic oil
Explanation:
- At sufficiently high gas velocities, density changes can become significant.
- High-speed gas flow through nozzles is therefore commonly analyzed as compressible flow.
Which classification is based on whether density changes significantly during flow?
- Ideal and real fluids
- Newtonian and non-Newtonian fluids
- Compressible and incompressible fluids
- Laminar and turbulent fluids
Explanation:
- Compressible/incompressible classification concerns the variation of fluid density during flow.
- Newtonian/non-Newtonian classification concerns the relationship between shear stress and deformation rate.
- Ideal/real classification is primarily related to viscosity and viscous effects.
Which classification is based on the relationship between shear stress and velocity gradient?
- Compressible and incompressible
- Ideal and real
- Newtonian and non-Newtonian
- Laminar and turbulent
Explanation:
- Newtonian fluids obey τ = μ(du/dy), giving a linear relationship.
- Non-Newtonian fluids do not necessarily follow this linear relationship.
Which statement is correct about viscosity of gases and liquids with increasing temperature?
- Viscosity of liquids generally increases and gases decrease
- Viscosity of liquids generally decreases, while viscosity of gases generally increases
- Both always increase
- Both always decrease
Explanation:
- For most liquids, increasing temperature reduces intermolecular cohesive effects and decreases viscosity.
- For gases, increasing temperature generally increases molecular momentum transfer and hence dynamic viscosity.
Which of the following has the highest viscosity among the listed fluids under ordinary conditions?
- Air
- Water
- Heavy lubricating oil
- Steam
Explanation:
- Heavy lubricating oils have much greater resistance to flow than air or water.
- Viscosity is a measure of internal resistance to fluid motion.
Which of the following statements is correct?
- All real fluids are ideal fluids
- All gases are incompressible under every condition
- Ideal fluid is a theoretical model, whereas real fluids have viscosity
- Newtonian fluids always have zero viscosity
Explanation:
- Ideal fluid is an approximation used to simplify fluid-flow analysis.
- Real fluids possess viscosity.
- A Newtonian fluid can have finite viscosity; Newtonian behavior means the shear stress–velocity-gradient relationship is linear.