Fluid Mechanics — Part 1 MCQs with Explanations

Magnetic flowmeters work on the principle of electromagnetic induction . This means they rely on the liquid being measured to have some electrical conductivity . Here's a simplified explanation: Imagine the liquid flowing through a pipe surrounded by a magnetic field. If the liquid is electrically conductive , its moving through this field generates a small voltage. The flowmeter measures this voltage and relates it to the flow rate. Liquid hydrocarbon fuels (like gasoline or kerosene) are typically poor conductors of electricity. In other words, their electrical conductivity is very low. Therefore, if you try to use a magnetic flowmeter on a liquid hydrocarbon fuel, the signal generated will be too weak or nonexistent, making an accurate measurement impossible. Let's look at why the other options are less likely to be correct: Option A: Thermal conductivity: How well a substance conducts heat, while important in some applications, doesn't directly impact the operation of a magnetic flowmeter. Option C: Specific gravity: This is the ratio of the density of a substance to the density of water. While it relates to the fluid properties, it's not the primary factor affecting measurement in this type of flowmeter. Option D: Electrical resistivity: This is the opposite of electrical conductivity (how much a substance resists electrical current). While related, the more direct factor influencing the flowmeter's operation is the conductivity itself - whether it's high enough to generate a measurable signal.

The capillary rise of mercury in a small diameter tube is proportional to 1/d , where d is the diameter of the tube, and σ is the surface tension of mercury. Capillary rise occurs due to the balance between cohesive forces within the liquid and adhesive forces between the liquid and the tube wall. In a small diameter tube, the capillary rise is inversely proportional to the diameter of the tube. Mathematically, the capillary rise h is given by the capillary rise equation: h = (2σ cosθ) / (ρgr) Where: σ is the surface tension of the liquid (mercury). θ is the contact angle between the liquid and the tube wall (usually close to 0° for mercury in glass). ρ is the density of the liquid (mercury). g is the acceleration due to gravity. r is the radius of the tube (half of the diameter). Since the radius r is directly proportional to 1/d , and the capillary rise is proportional to the radius r , the capillary rise is inversely proportional to the diameter d , which is represented by option B.

The head developed by a centrifugal compressor is dependent on the kinetic energy imparted to the gas. The kinetic energy is directly related to the velocity of the gas molecules. With an increase in molecular weight of the gas, the average velocity of the gas molecules decreases due to higher mass. This results in a decrease in kinetic energy and consequently, a decrease in the head developed by the centrifugal compressor. Hence, the correct answer is Option A: Decrease .

The correct answer is Option B: Develop effective pressure rise by the pump . Here's why: Priming a centrifugal pump means filling the pump casing and suction pipe completely with the liquid to be pumped *before* starting the pump. Why is this important? If the pump casing is filled with air or gas, the impeller (the rotating part) will mostly just churn the air. Centrifugal pumps need to be filled with liquid to work correctly. Let's look at the other options: * Option A: Increase the mass flow rate of fluid: Priming doesn't directly increase the flow rate once the pump is running, but is a prerequisite for pumping to commence. * Option C: Avoid chances of separation inside the impeller: While cavitation (separation) *can* be worsened by improper priming, the *primary* reason for priming is to create an initial pressure head. * Option D: None of these: This is incorrect since priming does have a specific purpose. Therefore, the main reason for priming is to remove air and create a liquid-filled environment so the impeller can effectively develop pressure and begin pumping.

The terminal velocity of a small sphere settling in a viscous fluid is given by Stokes' law for low Reynolds numbers, which is expressed as: v = (2/9) * (r² * g * (ρ_s - ρ_f)) / μ Here: v = terminal velocity r = radius of the sphere g = acceleration due to gravity ρ_s = density of the sphere ρ_f = density of the fluid μ = dynamic viscosity of the fluid From the equation, it is evident that the terminal velocity is inversely proportional to the viscosity of the fluid (μ). A higher viscosity results in lower terminal velocity, while a lower viscosity increases the terminal velocity. Thus, the terminal velocity varies as the inverse of the fluid viscosity.

Here's a breakdown of how to solve this fluid mechanics problem, step-by-step: The question asks for the pressure drop in a fluidized bed, expressed in cm of water. The key idea is that in a fluidized bed, the pressure drop across the bed is approximately equal to the effective weight of the solids per unit area . Let's break down the formula: ΔP = (1 - ε) * ρ p * g * L Where: ΔP is the pressure drop ε is the bed porosity ρ p is the particle density g is the acceleration due to gravity (approximately 9.81 m/s 2 ) L is the bed depth Let's plug in the values given: ε = 0.5 ρ p = 2000 kg/m 3 (Note : given 200 kg/m3 is wrong,it is 2000 kg/m3 as per answer) L = 60 cm = 0.6 m g = 9.81 m/s 2 So, ΔP = (1 - 0.5) * 2000 kg/m 3 * 9.81 m/s 2 * 0.6 m ΔP = 0.5 * 2000 * 9.81 * 0.6 = 5886 Pa (Pascals) Now, we need to convert Pascals to cm of water. We know that: ΔP (in cm of water) = ΔP (in Pa) / (ρ water * g * 100) where ρ water is 1000 kg/m 3 . ΔP (in cm of water) = 5886 Pa / (1000 kg/m 3 * 9.81 m/s 2 ) and multiplying 100 to convert meter into centimeter. ΔP (in cm of water) = 5886 / (1000 * 9.81) * 100 = 60 cm of water Therefore, the bed pressure drop is approximately 60 cm of water. The correct answer is Option B: 60 .

Streamlines are lines that are tangent to the velocity vector of the flow at every point. Option A is correct – Streamlines are always parallel to the direction of fluid flow. Option B is correct – There is no flow across a streamline, making it similar to a rigid boundary in fluid analysis. Option C is incorrect – Streamlines never intersect. If they did, it would imply two different velocity directions at the same point, which is physically impossible. Option D is correct – The fluid between two streamlines behaves like an isolated streamtube, and the spacing between streamlines varies inversely with the velocity (closer spacing indicates higher velocity). Correct Answer: Option C: Streamlines intersect at isolated point of zero velocity and infinite velocity

Hydraulic mean depth (D m ) is defined as the cross-sectional area of flow divided by the wetted perimeter. For a circular pipe flowing full, D m = 0.25 D, where D is the diameter of the pipe. For a circular channel flowing partially, when D m = 0.3 D, it represents the condition for maximum efficiency of flow. At this condition, both the flow rate and the mean velocity are maximized, because the hydraulic radius is at its optimum value. Therefore, the correct answer is Option C: Both A & B

The correct answer is Option B: Reciprocating pump. Let's break down why: Pulsating discharge flow means the flow rate isn't constant; it goes up and down in cycles. Reciprocating pumps use pistons or diaphragms that move back and forth. This creates a distinct "push" of fluid, followed by a "refill" stroke, leading to a strong pulsation. Centrifugal pumps use a rotating impeller to add energy to the fluid, resulting in a smoother, more continuous flow. Gear pumps use rotating gears to displace fluid. While there can be some minor pulsations, they are significantly less pronounced than in reciprocating pumps. Axial flow pumps , similar to centrifugal pumps, use a rotating impeller to move fluid along an axis, resulting in a relatively smooth flow. In summary , the intermittent pumping action of a reciprocating pump makes it the most likely to produce a pulsating discharge.

Pascal's law states that the pressure applied to a confined fluid is transmitted undiminished in all directions throughout the fluid. This law is only valid when the fluid is at rest because any motion would introduce additional forces such as viscosity and turbulence. A frictionless fluid in motion would not follow Pascal's law due to the influence of dynamic pressure changes.

The correct answer is Option C: Remain unchanged Here's why: NRe stands for Reynolds Number , which is a dimensionless number that helps predict flow patterns in fluids. It's calculated as: NRe = (density * velocity * diameter) / viscosity At a Reynolds number of 2100, the flow is considered laminar flow . In laminar flow, the fluid flows in smooth layers, and the friction is mainly due to the viscosity of the fluid. The surface roughness has very little impact on friction in laminar flow because the layers of fluid slide past each other in an ordered fashion, and the irregularities of the wall are hidden under a smooth flowing layer. Since the flow is already laminar at NRe = 2100, further smoothening the pipe (reducing roughness) won't change the flow pattern or the friction factor. Therefore, the friction factor (f1) will remain unchanged.

The pressure coefficient is the ratio of pressure forces to inertial forces. In fluid mechanics, the pressure coefficient is a dimensionless quantity that relates the pressure at a certain point to the free stream dynamic pressure. Inertial forces are associated with the motion and acceleration of fluid particles. Therefore, in this context, the correct option is Option B: Inertial .

$$\frac{{\text{V}}}{{{{\text{V}}_{\max }}}} = {\left( {\frac{{\text{x}}}{{\text{r}}}} \right)^{\frac{1}{7}}}$$ is the correct answer to “For turbulent fluid flow in pipe, the expression for Prandtl one seventh power law is (where, r = pipe radius, x = distance).”. This question is standard in competitive examination syllabi for Fluid Mechanics.

$$\frac{{4\pi {\text{g}}}}{3}$$ is the correct answer to “What is the force required (in Newtons) to hold a spherical balloon stationary in water at a depth of H from the air-water iterface? The balloon is of radius 0.1 m and is filled with air.”. This question is standard in competitive examination syllabi for Fluid Mechanics.

V2 is the correct answer to “Pressure drop (Δp) for a fluid flowing in turbulent flow through a pipe is a function of velocity (V) as”. This question is standard in competitive examination syllabi for Fluid Mechanics.

Plunger is the correct answer to “__________ pump is the most suitable device for discharging a liquid against a pressure of ≥ 1500 kgf/cm 2 .”. This question is standard in competitive examination syllabi for Fluid Mechanics.

More than is the correct answer to “Pressure drop in a fluidised bed reactor is __________ that in a similar packed bed reactor.”. This question is standard in competitive examination syllabi for Fluid Mechanics.

Pseudo plastic fluid is the correct answer to “Curve III in the above diagram represents a/an”. This question is standard in competitive examination syllabi for Fluid Mechanics.

Impulse is the correct answer to “A hydraulic ram acts as a/an __________ pump.”. This question is standard in competitive examination syllabi for Fluid Mechanics.

Energy which it can store is the correct answer to “The capacity of an accumulator is the maximum”. This question is standard in competitive examination syllabi for Fluid Mechanics.

$$\frac{{{\text{dq}}}}{{{\text{dx}}}} = {\text{K}}$$ is the correct answer to “The inherent characteristic of an equal percentage valve relating flow rate 'q' with valve stem movement 'x' are described by the equation”. This question is standard in competitive examination syllabi for Fluid Mechanics.

0.003 is the correct answer to “What is the value of Fanning friction factor f ' for smooth pipe at N Re = 10 6 approximately”. This question is standard in competitive examination syllabi for Fluid Mechanics.

M is below G is the correct answer to “For an unstable equilibrium of a floating body (where, M = metacentre.)”. This question is standard in competitive examination syllabi for Fluid Mechanics.

Density of the body is the correct answer to “Drag force acting on a body does not depend upon the”. This question is standard in competitive examination syllabi for Fluid Mechanics.

$$\infty $$ N/m2 is the correct answer to “For an incompressible fluid, the bulk modulus of elasticity is”. This question is standard in competitive examination syllabi for Fluid Mechanics.

Inclined tube water is the correct answer to “Very small pressure difference (< 5 mm water coloumn) can be most conveniently measured by a/an __________ manometer.”. This question is standard in competitive examination syllabi for Fluid Mechanics.

Newtonion is the correct answer to “A fluid which has a linear relationship between the magnitude of applied shear-stress and the resulting rate of deformation is called a/an __________ fluid.”. This question is standard in competitive examination syllabi for Fluid Mechanics.

Transition is the correct answer to “The length of the tube necessary for the boundary layer to reach the centre of the tube and for fully developed flow to be established is called the __________ length.”. This question is standard in competitive examination syllabi for Fluid Mechanics.

Mach is the correct answer to “The ratio of inertial forces to elastic forces is called the __________ number.”. This question is standard in competitive examination syllabi for Fluid Mechanics.

Galileo is the correct answer to “$$\frac{{{\text{N}}_{{\text{Re}}}^2}}{{{{\text{N}}_{{\text{Fr}}}}}}$$ is called the __________ number.”. This question is standard in competitive examination syllabi for Fluid Mechanics.

Froude is the correct answer to “The ratio of inertial forces to gravity forces is called the __________ number.”. This question is standard in competitive examination syllabi for Fluid Mechanics.

Euler is the correct answer to “The ratio of pressure forces to inertial forces is called the __________ number.”. This question is standard in competitive examination syllabi for Fluid Mechanics.

Weber is the correct answer to “Ratio of inertial forces to surface tension forces is called the __________ number.”. This question is standard in competitive examination syllabi for Fluid Mechanics.

Reynold is the correct answer to “The ratio of inertial forces to viscous forces is called the __________ number.”. This question is standard in competitive examination syllabi for Fluid Mechanics.

Inlet pressure high is the correct answer to “Cavitation in a centrifugal pump can be avoided by keeping the”. This question is standard in competitive examination syllabi for Fluid Mechanics.

II, III, V is the correct answer to “Which of the following must be followed by the flow of a fluid (real or ideal)? (I) Newton's law of viscosity. (II) Newton's second law of motion. (III) The continuity equation. (IV) Velocity of boundary layer must be zero relative to boundary. (V) Fluid cannot penetrate a boundary.”. This question is standard in competitive examination syllabi for Fluid Mechanics.

Low barometric pressure is the correct answer to “Cavitation in a centrifugal pump results from”. This question is standard in competitive examination syllabi for Fluid Mechanics.

Are not subject to air binding is the correct answer to “Reciprocating pumps compared to centrifugal pumps”. This question is standard in competitive examination syllabi for Fluid Mechanics.

Both A & B is the correct answer to “Mass velocity in case of steady flow and through a constant cross-section conduit is independent of the”. This question is standard in competitive examination syllabi for Fluid Mechanics.

Union is the correct answer to “For pipes that must be broken at intervals for maintenance, the connector used should be a/an”. This question is standard in competitive examination syllabi for Fluid Mechanics.