Physics

Fluid Mechanics and Hydraulics

361 Questions

Fluid mechanics and hydraulics questions address the principles of fluid flow, pipe resistance, and open channel dynamics. The topics include Bernoulli equation, Navier-Stokes equation, and hydrograph calculations. These concepts are crucial for civil and mechanical engineering competitive examinations.

Fluid flow equationsOpen channel flowPipe frictionHydraulic jumpHydrograph analysis

Fluid Mechanics and Hydraulics Questions

Multiple choice

The average rainfall for a 3 hour duration storm is 2.7 cm and the loss rate is 0.3 cm/h. The flood hydrograph has a base flow of 20 m3/s and produces a peak flow of 210 m3/s. The peak of a 3-h unit hydrograph is

  1. 125.50 m3/s

  2. 105.50 m3/s

  3. 77.77 m3/s

  4. 70.37 m3/s
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The effective rainfall is calculated as (2.7 - 0.3 * 3) = 1.8 cm. The peak flow of the unit hydrograph is (Peak Flow - Base Flow) / Effective Rainfall = (210 - 20) / 1.8 = 190 / 1.8 = 105.55 m^3/s.

Multiple choice

The design parameter for flocculation is given by a dimensionless number Gt, where G is the velocity gradient and t is the detention time. Values of Gt ranging from 104 to 105 are commonly used, with t ranging from 10 to 30 mm. The most preferred combination of G and t to produce smaller and denser floccus is

  1. large G values with short t

  2. large G values with long t

  3. small G values with long t

  4. small G values with short t
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

To produce dense flocs, a lower velocity gradient (G) is required to prevent floc breakage, while a longer detention time (t) allows for sufficient collision and growth.

Multiple choice

The allowable net positive suction head (NPSH) for a pump provided by the manufacturer for a flow of 0.05 m3/s is 3.3 m. The temperature of water is 30oC (vapour pressure head absolute = 0.44 m), atmospheric pressure is 100 kPa absolute and the head loss from the reservoir to pump is 0.3 N-m/N. The maximum height of the pump above the suction reservoir is

  1. 10.19 m

  2. 6.89 m

  3. 6.15 m

  4. 2.86 m
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

NPSH_available = (Patm - Pvap)/gamma - Hs - Hf. 3.3 = (100 - 0.44*9.81)/9.81 - Hs - 0.3. Solving for Hs gives 6.15 m.

Multiple choice

The laminar flow takes place between closely spaced parallel plates as shown in figure below. The velocity profile is given by$u = V \frac{y}{h}$. The gap height h is 5 mm and the space is filled with oil (specific gravity = 0.86, viscosity m = 2 × 10-4 N-s/m2). The bottom plate is stationary and the top plate moves with a steady velocity of V=5 cm/s. The area of the plate is 0.25 m2.

The rate of rotation of fluid particle is given by
  1. $\omega_y = 0; \omega_z = \frac{y}{2h}$
  2. $\omega_y = 0; \omega_z = - \frac{y}{h}$
  3. $\omega_y = 0; \omega_z = \frac{y}{h}$
  4. $\omega_y = \frac{y}{h}; \omega_z = 0$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Rotation rate omega_z = 0.5 * (dv/dx - du/dy). Given u = V*y/h, du/dy = V/h. omega_z = -0.5 * V/h. The options seem to have a sign convention difference or typo, but A is the standard form.

Multiple choice

The laminar flow takes place between closely spaced parallel plates as shown in figure below. The velocity profile is given by $u = V \frac{y}{h}$. The gap height h is 5 mm and the space is filled with oil (specific gravity = 0.86, viscosity m = 2 × 10-4 N-s/m2). The bottom plate is stationary and the top plate moves with a steady velocity of V=5 cm/s. The area of the plate is 0.25 m2.

The power required to keep the plate in steady motion is

  1. 5 × 10-4 watts

  2. 10-5 watts

  3. 2.5 × 10-5

  4. 5 × 10-5 watts
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Shear stress tau = mu * du/dy = mu * V/h. Force F = tau * Area. Power P = F * V. P = (mu * V/h) * Area * V = (2e-4 * 0.05 / 0.005) * 0.25 * 0.05 = 2.5e-5 Watts.

Multiple choice

A flat plate is kept in an infinite fluid medium. The fluid has a uniform free stream velocity parallel to the plate. For the laminar boundary layer formed on the plate, pick the correct option matching:

 
Column I Column II
P. Boundary layer thickness 1. decreases in the flow direction
Q. Shear stress at the plate 2. increases in the flow direction
R. Pressure gradient along the plate 3. remains unchanged

  1. P - 1, Q - 2, R - 3

  2. P - 2, Q - 2, R - 2

  3. P - 1, Q - 1, R - 1

  4. P - 2, Q - 1, R - 3
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

 

Multiple choice

An ideal horizontal flow setting basin is 3 m deep having surface area 900 m2. Water flows at the rate of 8000 m3/d, at water temperature 20oC (m = 10-3 kg/m.s) and p = 1000 kg/m3). Assuming Stokes law to be valid, the proportion (percentage) of spherical sand particles (0.01 mm in diameter with specific gravity 2.65), that will be removed, is

  1. 32.5

  2. 67

  3. 87.5

  4. 95.5
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The removal efficiency is calculated using the ratio of settling velocity to surface overflow rate. The settling velocity is determined by Stokes Law, and the surface overflow rate is the flow rate divided by the surface area. Comparing these values yields the percentage of particles removed.

Multiple choice

A very wide rectangular channel carries a discharge of 8m3/s per m width. The channel has a bed slope of 0.004 and Manning’s roughness coefficient, n = 0.015. At a certain section of the channel, the flow depth is 1 m.

What Gradually Varied Flow profile exists at this section?

  1. M2

  2. M3

  3. S2

  4. S3
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

First, calculate normal depth (yn) using Manning's equation and critical depth (yc). Since the bed slope is steep (S0 > Sc) and the actual depth is less than normal depth, the flow profile is S3.

Multiple choice

A very wide rectangular channel carries a discharge of 8 m3/s per m width. The channel has a bed slope of 0.004 and Manning’s roughness coefficient, n = 0.015. At a certain section of the channel, the flow depth is 1 m.

At what distance from this section the flow depth will be 0.9 m? (Use the direct step method employing a single step)

  1. 65 m downstream

  2. 50 m downstream

  3. 55 m downstream

  4. 60 m downstream
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Using the direct step method for GVF, the distance is calculated by delta_x = (E2 - E1) / (S0 - Sf_avg). Calculating the specific energy at depths 1 m and 0.9 m and the average friction slope leads to 50 m.

Multiple choice

A pipeline (diameter 0.3 m, length 3 km) carries water from point P to point R (see figure). The piezometric heads at P and R are to be maintained at 100 m and 80 m, respectively. To increase the discharge, a second pipe is added in parallel to the existing pipe from Q to R. The length of the additional pipe is also 2 km. Assume the friction factor, f = 0.04 for all pipes and ignore minor losses.

What is the increase in discharge if the additional pipe has same diameter (0.3 m)?

  1. 0%

  2. 33%

  3. 41%

  4. 67%
Reveal answer Fill a bubble to check yourself
C Correct answer
Multiple choice

A pipeline (diameter 0.3 m, length 3 km) carries water from point P to point R (see figure). The piezometric heads at P and R are to be maintained at 100 m and 80 m, respectively. To increase the discharge, a second pipe is added in parallel to the existing pipe from Q to R. The length of the additional pipe is also 2 km. Assume the friction factor, f = 0.04 for all pipes and ignore minor losses.

If there is no restriction on the diameter of the additional pipe, what would be the maximum increase in discharge theoretically possible from this arrangement?

  1. 0%

  2. 50%

  3. 67%

  4. 73%
Reveal answer Fill a bubble to check yourself
C Correct answer