Physics

Fluid Mechanics and Hydraulics

376 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

A steady flow occurs in an open channel with lateral inflow of q m3/s per unit width as shown in the figure. The mass conservation equation is
  1. $\frac{\partial q}{\partial x} = 0$
  2. $\frac{\partial Q}{\partial x} = 0$
  3. $\frac{\partial Q}{\partial x} -q = 0$
  4. $\frac{\partial Q}{\partial x} +q = 0$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

For steady flow with lateral inflow, the change in discharge along the channel is equal to the lateral inflow rate. The continuity equation reflects this balance.

Multiple choice

A steep wide rectangular channel takes off from a reservoir having an elevation of 101.2 m. At the entrance, the bottom elevation of the channel is 100 m. If the slope of the channel is increased by 4%, the discharge per unit length in the channel will be

  1. 2.24 m2/s

  2. higher than 2.24 m2/s by 4%

  3. higher than 2.24 m2/s by 2%

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

In a steep channel, flow is often governed by normal depth. Using Manning's equation, Q is proportional to the square root of the slope. A 4% increase in slope leads to a 2% increase in discharge.

Multiple choice

A thin flat plate 0.5 m x 0.7 m in size settles in a large tank of water with a terminal velocity of 0.12 m/s. The coefficient of drag$C_D = \frac{1.328}{\sqrt{R_L}}$for a laminar boundary layer and$C_D = \frac{0.072}{(R_L)^{1/5}}$for a turbulent boundary layer, where RL is the plate Reynolds number. Assume $\mu$= 10-3 N-s/m2 and $\rho$= 1000 kg/m3 The submerged weight of the plate is

  1. 0.0115 N

  2. 0.0118 N

  3. 0.0231 N

  4. 0.0376 N
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The submerged weight of the plate equals the drag force at terminal velocity. Calculate the Reynolds number, determine the drag coefficient, and solve for the drag force.

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 following data are given for a channel-type grit chamber of length 7.5 m.

  1. Flow-through velocity = 0.3 m/s
  2. The depth of wastewater at peak flow in the channel = 0.9 m
  3. Specific gravity of inorganic particles = 2.5
  4. g = 9.80 m/s2, m = 1.002 x 10-3 N-s/m2 at 20 oC, $\rho w$ = 1000 kg/m3 Assuming that the Stokes is valid, the largest diameter particle that would be removed with 100 percent efficiency is

  1. 0.04 mm

  2. 0.21 mm

  3. 1.92 mm

  4. 6.64 mm
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Using Stokes' law, settling velocity vs = g(G-1)d^2 / (18*nu). The removal efficiency is 100% if vs >= depth / (length / velocity). Solving for d gives approximately 0.21 mm.

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