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৪৯তম বিসিএস ⎯ ফলিত রসায়ন [৫৪১]

পরীক্ষা৪৯তম বিসিএস ⎯ ফলিত রসায়ন [৫৪১]তারিখতারিখ অনির্ধারিতসময়15 minutes
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Exam - 03 Topics: Fluid Mechanics & Corrosion 1. Types of fluid. 2. General properties of fluid. 3. Fluid statics & Fluid dynamics. 4. Euler’s equation. 5. Bernoulli’s equation. 6. Fluid flow measurement. [Source: Class - 02 and Relevant Books]
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৪৯তম বিসিএস ⎯ ফলিত রসায়ন [৫৪১]

৪৯তম বিসিএস ⎯ ফলিত রসায়ন [৫৪১] · তারিখ অনির্ধারিত · ৩০ প্রশ্ন

১.
Which of the following fluids exhibits a constant viscosity regardless of the applied shear rate (i.e., behaves as a Newtonian fluid)? 
  1. ক
    Human Blood
  2. খ
    Pure Water at standard conditions
  3. গ
    Toothpaste
  4. ঘ
    Tomato Ketchup
ব্যাখ্যা
A Newtonian fluid has a constant viscosity that does not change with the applied shear rate or stress. The relationship between shear stress  and shear rate  is linear, expressed by Newton’s law of viscosity. Water at normal conditions (around 20°C and atmospheric pressure) is a Newtonian fluid because its viscosity remains constant regardless of flow conditions (until extreme pressures or temperatures are applied).

ক) Human blood → Non-Newtonian (Shear-thinning or pseudoplastic; viscosity decreases with increasing shear rate because of red blood cell aggregation and deformation).

গ) Toothpaste → Non-Newtonian, Bingham Plastic (It needs a certain yield stress to start flowing, then behaves like a viscous fluid).

ঘ) Tomato ketchup → Non-Newtonian, Shear-Thinning (Viscosity decreases significantly when shaken or stirred).
২.
Which of the following fluids is generally considered compressible under standard engineering conditions? 
  1. ক
    Liquid water at 25°C
  2. খ
    Air at atmospheric pressure
  3. গ
    Steam at 1 atm
  4. ঘ
    Supercritical CO2
ব্যাখ্যা

ক) Liquid water at 25°C → Incompressible for practical purposes. Its density changes less than 0.1% even under large pressure variations, so it is treated as incompressible in engineering.

খ) Air at atmospheric pressure → Compressible. All gases exhibit significant density changes when pressure or temperature varies, making compressibility important in fluid dynamics calculations.

গ) Steam at 1 atm → Compressible because it is in the gaseous phase (water vapor). Its properties follow the ideal gas approximation, so density changes with pressure and temperature.

ঘ) Supercritical CO2 → Compressible because its density is highly sensitive to pressure and temperature changes, especially near the critical point.

Air at atmospheric pressure is the correct answer, because it is the most common example of a compressible fluid.

৩.
Which of the following best represents an ideal fluid as assumed in fluid mechanics theory? 
  1. ক
    Air at standard conditions
  2. খ
    Superheated steam at 1 atm
  3. গ
    Water under very high pressure
  4. ঘ
    A hypothetical fluid that is incompressible and has zero viscosity
ব্যাখ্যা

An ideal fluid is a theoretical construct in fluid mechanics used to simplify analysis.
It has the following properties:

No viscosity (zero internal friction, so no energy loss due to shear).

Incompressible (constant density, regardless of pressure changes).

Non-conductive for heat (no thermal energy transfer internally).

This type of fluid does not exist in nature, but it is used in models like Bernoulli’s equation and potential flow theory for simplicity.

৪.
Which of the following statements about fluid types is TRUE? 
  1. ক
    A Bingham plastic fluid behaves like a Newtonian fluid after exceeding a yield stress.
  2. খ
    Pseudoplastic fluids exhibit viscosity that increases with increasing shear rate
  3. গ
    Dilatant fluids are a type of Non-Newtonian fluid that become less viscous when shear rate increases.
  4. ঘ
    All Non-Newtonian fluids have a constant viscosity under varying shear conditions.
ব্যাখ্যা
ক) TRUE → Bingham plastic fluids (e.g., toothpaste) require a minimum yield stress to start flowing. Once that stress is exceeded, they behave approximately like Newtonian fluids.

খ) FALSE → Pseudoplastic fluids (shear-thinning fluids like ketchup) have viscosity that decreases with increasing shear rate, not increases.

গ) FALSE → Dilatant fluids (shear-thickening fluids like cornstarch in water) become more viscous with increasing shear rate, not less.

ঘ) FALSE → Non-Newtonian fluids, by definition, do not have constant viscosity; their viscosity varies with shear rate (and sometimes time).
৫.
Which of the following best describes a real fluid?
  1. ক
    Has no viscosity
  2. খ
    Has viscosity and may be compressible
  3. গ
    Is incompressible and inviscid
  4. ঘ
    Exists only in theory
ব্যাখ্যা
A real fluid is an actual fluid found in nature or engineering applications. It has:

**Some viscosity (internal friction)

**May be compressible (like gases) or nearly incompressible (like most liquids)

**Exhibits energy losses due to friction and turbulence

Other options:

ক) Has no viscosity → That’s the definition of an ideal fluid, which is hypothetical.

গ) Is incompressible and inviscid → Again, this describes an ideal fluid, not real.

ঘ) Exists only in theory → Real fluids exist in nature; ideal fluids exist only in theory.
৬.
Which fluid property differentiates compressible from incompressible fluids?
  1. ক
    Viscosity
  2. খ
    Temperature dependence
  3. গ
    Density variation with pressure
  4. ঘ
    Surface tension
ব্যাখ্যা
The main criterion that differentiates compressible and incompressible fluids is how much their density changes when pressure changes:

**Compressible fluid → Density varies significantly with pressure (typical for gases).

**Incompressible fluid → Density is assumed constant (typical for liquids in most engineering calculations).

Other options:

ক) Viscosity → Determines internal resistance to flow, not compressibility.

খ) Temperature dependence → Both compressible and incompressible fluids can have temperature-dependent properties.

ঘ) Surface tension → Relates to the liquid’s surface behavior, not compressibility.
৭.
Which physical property of a fluid quantifies its internal resistance to deformation or shear flow? 
  1. ক
    Density
  2. খ
    Viscosity
  3. গ
    Surface tension
  4. ঘ
    Compressibilty
ব্যাখ্যা
Viscosity is the measure of a fluid's internal resistance to deformation or shear flow. It reflects how strongly the fluid resists motion between adjacent layers.

In Newtonian fluids, this is expressed by: 
τ = μ (du/dy)

​Where,
τ= Shear stress
​μ= Dynamic viscosity
du/dy= Velocity gradient
৮.
What is the dimensionless quantity defined as the ratio of the density of a fluid to the density of water at standard conditions called?
  1. ক
    Specific weight
  2. খ
    Specific gravity
  3. গ
    Relative viscosity
  4. ঘ
    Kinematic viscosity
ব্যাখ্যা
Specific gravity = ρfluid/ρwater

Here,
ρfluid= Density of the fluid
ρwater= Density of water at standard conditions (usually 4°C, where water density ≈ 1000 kg/m³).

It is dimensionless and indicates how heavy a fluid is compared to water.

ক) Specific weight → Weight per unit volume (γ=ρg); has units (N/m³), not a ratio.

গ) Relative viscosity → Ratio of viscosity of a fluid to that of a reference fluid (usually water), not related to density.

ঘ) Kinematic viscosity → Ratio of dynamic viscosity to density (ν=μ/ρ),measured in m²/s.

Extra Info:
Specific gravity > 1 → Fluid is denser than water (e.g., mercury ~13.6).
Specific gravity < 1 → Fluid is lighter than water (e.g., gasoline ~0.7).

৯.
Which physical property primarily governs the phenomenon of capillary rise in liquids within narrow tubes? 
  1. ক
    Density of the liquid
  2. খ
    Cohesive force between liquid molecules
  3. গ
    Surface tension at the liquid-air interface
  4. ঘ
    Bulk compressibility of the liquid
ব্যাখ্যা
Capillary rise is caused by surface tension, which acts along the liquid-air interface and pulls the liquid up in the tube when the adhesive force between the liquid and tube wall is stronger than the cohesive force within the liquid.

Other Options:

ক) Density of the liquid → Affects the height of the rise (in Jurin’s law) but does not cause the phenomenon.

খ) Cohesive force between liquid molecules → Influences contact angle, but the primary property responsible for rise is surface tension.

ঘ) Bulk compressibility of the liquid → Related to volume change under pressure, completely unrelated to capillary action.
১০.
What is the SI unit of dynamic viscosity?
  1. ক
    N.s/m2
  2. খ
    Pa.s
  3. গ
    m2/s
  4. ঘ
    Both A and B
ব্যাখ্যা
১১.
When the velocity of a fluid increases along a streamline, what happens to the static pressure according to Bernoulli’s principle (assuming incompressible, inviscid flow)? 
  1. ক
    Static pressure increases
  2. খ
    Static pressure decreases
  3. গ
    Static pressure remains constant
  4. ঘ
    Static pressure becomes zero
ব্যাখ্যা
Bernoulli’s equation:
P+1/2ρv2+ρgh=constant

If velocity v increases, the dynamic pressure term 1/2ρv 2  increases, so to keep the total constant, static pressure P must decrease.

ক) is wrong because it contradicts Bernoulli.

গ) is wrong unless velocity stays constant.

ঘ) is wrong; static pressure never becomes zero unless in extreme vacuum.
১২.
Which equation gives the pressure variation in a static fluid?
  1. ক
    Bernoulli’s equation
  2. খ
    Hydrostatic equation
  3. গ
    Euler’s equation
  4. ঘ
    Continuity equation
ব্যাখ্যা
The equation that describes pressure variation in a static fluid is dp/dy = -ρg, where: 

• dp/dy: represents the change in pressure (p) with respect to a change in depth (y). 
• ρ: is the density of the fluid. 
• g: is the acceleration due to gravity.
​
This equation indicates that pressure increases linearly with depth in a static fluid. The negative sign signifies that pressure increases as you move downwards (increasing depth). This is called Hydrostatic equation.
১৩.
In fluid dynamics, which fundamental principle does the continuity equation mathematically represent?
  1. ক
    Conservation of mass in a control volume
  2. খ
    Conservation of mechanical energy in a streamline
  3. গ
    Conservation of linear momentum in a fluid element
  4. ঘ
    Hydrostatic equilibrium in a stationary fluid
ব্যাখ্যা
Continuity equation A1V1=A2V2  for incompressible fluids expresses the conservation of mass in steady flow.
​
​
১৪.
In a steady fluid flow, what does a streamline indicate at any given instant?
  1. ক
    The actual trajectory of a specific fluid particle over time
  2. খ
    The locus of points having identical velocity magnitudes
  3. গ
    The instantaneous tangent direction of the local velocity vector
  4. ঘ
    The path connecting regions of constant pressure
ব্যাখ্যা
A streamline is an imaginary line in the fluid such that the tangent to the line at any point gives the direction of the velocity vector at that point at that instant.
It represents the instantaneous flow pattern, not the actual trajectory of a particle over time.

About other options:

ক) Path of a specific particle over time → That’s a pathline, not a streamline.

খ) Locus of identical velocity magnitude → That’s an isovel, not a streamline.

ঘ) Path connecting regions of constant pressure → That’s an isobar, not a streamline.
১৫.
In which type of flow does the velocity vector at a fixed spatial point remain constant with respect to time? 
  1. ক
    Steady flow
  2. খ
    Unsteady flow
  3. গ
    Turbulent flow
  4. ঘ
    Rotational flow
ব্যাখ্যা
Steady flow: The velocity vector at any fixed point in space does not change with time. It can vary from point to point, but remains constant in time at each location.

Unsteady flow: Velocity changes with time at a given point.

Turbulent flow: Highly irregular and almost always unsteady, involving random fluctuations.

Rotational flow: Fluid elements experience rotation (vorticity), but this does not determine time dependence.
১৬.
Euler’s equation of motion is derived by applying:
  1. ক
    Conservation of mass
  2. খ
    Conservation of momentum
  3. গ
    Conservation of energy
  4. ঘ
    Hydrostatic law
ব্যাখ্যা

** Euler’s Equation of Motion describes the relationship between pressure, velocity, and elevation in an inviscid (non-viscous) fluid under steady flow.

It is derived from Newton’s Second Law (F = ma) applied to a fluid element, which is essentially the principle of conservation of linear momentum.

** Conservation of mass gives the continuity equation, not Euler’s equation.

** Conservation of energy leads to Bernoulli’s equation, which is obtained by integrating Euler’s equation under specific assumptions.

** Hydrostatic law applies to fluids at rest, not to moving fluids.

১৭.
In the derivation of Euler’s equation for an ideal fluid, which type of force is assumed to be negligible?
  1. ক
    Gravitational body force
  2. খ
    Viscous shear force
  3. গ
    Pressure force
  4. ঘ
    Inertial force
ব্যাখ্যা

Euler’s equation of motion applies to an ideal fluid, which is assumed to be:
✔ Inviscid (no viscosity → viscous forces are zero)
✔ Non-conductive (no heat transfer)
✔ Continuous and non-rotational in some cases

The equation is derived from Newton’s Second Law (Conservation of Momentum), but during the derivation, viscous shear forces are neglected because the fluid is assumed to have zero viscosity.

Other forces considered in Euler’s equation:

Pressure forces → included

Gravity (body forces) → can be included as a term

Inertial forces → included

১৮.
When integrated along a streamline for steady, incompressible, inviscid flow, Euler’s equation yields which fundamental relation? 
  1. ক
    Continuity equation
  2. খ
    Navier–Stokes equation
  3. গ
    Bernoulli’s equation
  4. ঘ
    Hydrostatic pressure equation
ব্যাখ্যা
Euler’s equation of motion represents the application of Newton’s second law to a fluid element in motion. For an ideal fluid (inviscid, non-conductive, and incompressible) and under the assumptions of:
Steady flow (properties do not change with time at a point)
No viscosity (inviscid)
No heat transfer
Along a streamline

​Euler’s equation can be written as:
dp/ρ+g dz+v dv=0

Integrating this equation along a streamline gives:
p/ρ+v2+gz=constant

This is Bernoulli’s equation, which expresses the conservation of mechanical energy (pressure energy, kinetic energy, and potential energy) for an ideal fluid.
১৯.
In Euler’s equation, the acceleration term is due to:
  1. ক
    Pressure gradient only
  2. খ
    Body forces only
  3. গ
    Surface tension
  4. ঘ
    Pressure and body forces
ব্যাখ্যা
Acceleration in Euler’s equation comes from both pressure gradients (surface forces) and body forces like gravity.
২০.
What is Bernoulli’s equation based on ?
  1. ক
    Conservation of energy
  2. খ
    Conservation of mass
  3. গ
    Hydrostatic law
  4. ঘ
    Conservation of momentum
ব্যাখ্যা
The Bernoulli equation states that

The total mechanical energy of the moving fluid comprising the gravitational potential energy of elevation, the energy associated with the fluid pressure and the kinetic energy of the fluid motion, remains constant.
২১.
Which of the following is not an assumption of Bernoulli’s equation?
  1. ক
    Incompressible flow
  2. খ
    Viscous flow
  3. গ
    Steady flow
  4. ঘ
    Along a streamline
ব্যাখ্যা

Bernoulli’s equation is derived under the following assumptions:

Incompressible fluid – density remains constant.

Inviscid flow – viscosity is negligible.

Steady flow – fluid properties at any point do not change with time.

Along a streamline – applies to a single streamline unless extended for an irrotational field.

The option “Viscous flow” contradicts the assumption because Bernoulli neglects viscous effects

২২.
Which of the following scenarios would invalidate the direct application of Bernoulli’s equation without correction terms? 
  1. ক
    Water flowing through a horizontal frictionless pipe with varying cross-section
  2. খ
    Flow through a nozzle connected to a large reservoir under steady conditions
  3. গ
    Air flowing at high speed in a converging-diverging nozzle where Mach number exceeds 1
  4. ঘ
    Flow along a streamline in an inviscid, incompressible, and irrotational fluid
ব্যাখ্যা

Bernoulli’s equation assumes:
✔ Incompressible fluid
✔ Inviscid flow
✔ Steady state
✔ Along a streamline

(ক): Horizontal pipe with no friction → satisfies assumptions.
(খ): Nozzle with large reservoir under steady flow → valid (low-speed, incompressible).
(গ): High-speed air (Mach > 1) → compressibility becomes significant, so Bernoulli’s equation in its simple form does not apply.
(ঘ): Fits all assumptions → valid.

২৩.
In Bernoulli’s equation, the term p/ρg​​​ represents:
  1. ক
    Velocity head
  2. খ
    Pressure head
  3. গ
    Elevation head
  4. ঘ
    Total head
ব্যাখ্যা
Bernoulli’s equation in head form is:
p/ρg+v2/2g + z = constant

​Here, 
p/ρg → Pressure head (height equivalent of pressure energy)
v2/2g​ → Velocity head (height equivalent of kinetic energy)
z → Elevation head (potential energy per unit weight)

Sum of all three = Total head.
২৪.
To account for real fluid effects such as friction, Bernoulli’s equation is modified by including which additional term?
  1. ক
    Velocity head
  2. খ
    Elevation head
  3. গ
    Head loss term hL
  4. ঘ
    Surface tension term
ব্যাখ্যা

The original Bernoulli’s equation assumes ideal fluid (inviscid, no friction), but in real systems, there are energy losses due to:
Pipe friction
Fittings and bends
Valves

To include these losses, Bernoulli’s equation is modified as:
p1/ρg + v12/2g + z1 = p2/ρg + v22/2g + z2 + hL

Here, hL​ = head loss (energy loss per unit weight of fluid).

২৫.
Which of the following flow-measuring devices operates based on Bernoulli’s principle?
  1. ক
    Pitot tube
  2. খ
    Orifice meter
  3. গ
    Venturi meter
  4. ঘ
    All of the above
ব্যাখ্যা
Bernoulli’s principle states that an increase in the velocity of a fluid results in a decrease in its static pressure, assuming incompressible, non-viscous, and steady flow along a streamline.

Pitot tube: Measures fluid velocity by comparing stagnation and static pressures.
Orifice meter: Introduces a sudden contraction to create a pressure difference that relates to flow rate.
Venturi meter: Uses a converging-diverging section to measure flow rate from the pressure difference between throat and inlet.

All three exploit the pressure-velocity relationship from Bernoulli’s principle.
২৬.
In flow measurement devices such as Venturi meters and orifice meters, the discharge coefficient (Cd) is introduced primarily to account for
  1. ক
    Viscosity
  2. খ
    Energy losses
  3. গ
    Non-ideal behavior
  4. ঘ
    All of the above
ব্যাখ্যা

Discharge coefficient (Cd) is a correction factor applied to theoretical discharge calculations to match actual discharge in real-world conditions.
Ideal Bernoulli-based formulas assume incompressible, non-viscous, no-energy-loss flow, which is rarely true in practice.

Cd accounts for:
✔ Viscosity effects → causing friction losses.
✔ Energy losses → due to turbulence, friction, and separation at constrictions.
✔ Non-ideal behavior → deviations from theoretical assumptions like perfectly streamlined flow.

২৭.
Among the following flow measurement devices, which one is most suitable when minimal permanent head loss is desired?
  1. ক
    Orifice meter
  2. খ
    Venturi meter
  3. গ
    Pitot tube
  4. ঘ
    Rotameter
ব্যাখ্যা

Orifice meter → Causes a large permanent pressure drop because of sharp-edged restriction and flow separation.

Venturi meter → Designed with a smooth converging and diverging section, which minimizes energy losses due to reduced turbulence and gradual expansion → lowest head loss among differential pressure meters.

Pitot tube → Measures velocity head locally but does not typically measure total discharge directly.

Rotameter → Works on variable area principle; head loss is relatively small, but Venturi is superior for high accuracy with minimal loss in pipelines.

Head loss comparison:
Venturi meter < Rotameter < Pitot tube < Orifice meter (highest).

২৮.
What does a Pitot tube fundamentally measure in a fluid flow system?
  1. ক
    Static pressure only
  2. খ
    Velocity head only
  3. গ
    Difference between stagnation and static pressure
  4. ঘ
    Discharge coefficient
ব্যাখ্যা

Pitot tube principle is based on Bernoulli’s equation, when a moving fluid is brought to rest at the stagnation point in the tube, the pressure rises to stagnation pressure.
The Pitot tube compares this stagnation pressure with the static pressure of the flowing fluid using a differential manometer.

২৯.
Which flow-measuring device determines the discharge by balancing the upward fluid drag and the weight of a float in a vertically tapered tube?
  1. ক
    Venturi meter
  2. খ
    Rotameter
  3. গ
    Orifice meter
  4. ঘ
    Pitot tube
ব্যাখ্যা

Venturi meter: Works on Bernoulli’s principle (pressure difference).

Rotameter: Correct; free-floating float in a tapered tube.

Orifice meter: Works on differential pressure through an orifice plate.

Pitot tube: Measures stagnation and static pressure difference.

৩০.
Which of the following terms is NOT present in Bernoulli’s equation for incompressible, inviscid, steady flow along a streamline?
  1. ক
    Pressure head
  2. খ
    Velocity head
  3. গ
    Elevation head
  4. ঘ
    Viscous loss term
ব্যাখ্যা

Bernoulli’s equation for an ideal fluid is:



It contains pressure head, velocity head, and elevation head. In the ideal form, viscous losses are neglected. For real fluids, viscous losses are accounted for by adding a head loss term hL