# Fluid Mechanics Concepts

Lead Author(s): **Saylor Academy**

Source: **Saylor**

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A question pack on fluid mechanics.

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A flow is considered to be compressible if its Mach number is greater than ______________.

0.03

0.3

5

1

Transition from a laminar pipe flow to a turbulent flow typically occurs with a Reynolds number in the range of ______________.

2,300‑4,000

500‑1,000

10$^5$ ‑10$^6$

50,000‑100,000

For a laminar pipe flow, how does the friction factor change when the Reynolds number increases?

The friction factor decreases.

The friction factor is kept constant.

The friction factor increases.

None of the above

What are the dimensions of surface tension?

M L$^{‑1}$

M L$^{‑1}$T$^{‑2}$

M$^{2}$L$^{‑1}$T

M$^{‑1}$T$^{‑2}$

What does a barometer measure?

Thermal conductivity

Wind velocity

Atmospheric pressure

Relative humidity

What does the Froude number measure?

The ratio of inertial forces to viscous forces.

The ratio of surface tension to inertial forces.

The ratio of inertial forces to gravitational forces.

The ratio of momentum diffusivity to thermal diffusivity.

What does the Reynolds number measure?

The ratio of inertial forces ρV$^2$/L to viscous forces μV/L$^2$

The ratio of surface tension to inertial forces

The ratio of gravitational force to fluid’s inertia

The ratio of momentum diffusivity to thermal diffusivity

What is a laminar pipe flow?

Movement of fluid slowly in layers in a pipe, without much mixing among the layers

A flow whose Reynolds number exceeds 2300

Movements of fluid are chaotic with significant mixing

None of the above

What is the pressure on a given object?

Pressure is the force divided by the area over which the force is applied.

Pressure is equal to surface tension.

Pressure is the force times the area over which the force is applied.

Pressure is equal to the object’s acceleration times its mass.

Which of the following dimensionless numbers is used to compare surface tension against inertial forces?

Reynolds number

Weber number

Prandtl number

Froude number

Which of the following is a streamline?

The locus of points of all the fluid particles that have passed through a given point

A family of curves, which are normal to the velocity vector of the flow

A family of curves that track the trajectories of fluid particles

A family of curves that are instantaneously tangent to the velocity vector of the flow

Which of the following is NOT a dimensionless number?

Pressure coefficient

Mach number M

Gas constant R

Friction factor f

Which of the following is the definition of Froude number?

V$^2$/Lg

μ VL/ρ

VL/σ

None of the above

Which of the following statement about streamlines is false?

Streamlines cannot cross each other.

Streamlines provide a snapshot of the entire flow field.

Streamlines track trajectories of fluid particles.

The tangent at each point on a streamline is the direction of the velocity vector at that point.

Which of the following statements about fully developed pipe flows is false?

The streamwise velocity component satisfies u$_z$ = 0.

“Boundary layers” from opposite sides of the pipe are separated and continue growing.

The radial component of the velocity is zero.

The entrance length depends on the Reynolds number.

Which of the following statements about surface tension is true?

Surface tension is responsible for the weight of liquid droplets.

Surface tension is measured in forces per unit length or of energy per unit area.

Surface tension is responsible for buoyancy

None of the above

Which of the following statements about the Moody chart is false?

In the laminar zone, friction factor decreases as the Reynolds number increases.

In the turbulent zone, friction factor increases as the relative roughness (D/ε) increases.

Within the zone of complete turbulence, friction factor is independent of Reynolds number.

As relative roughness increases, the boundary of the complete turbulence zone shifts to the right.

Which of the following statements about viscosity is true?

Viscosity of a fluid depends strongly on its temperature.

Viscosity measures the friction between the fluid and the wall.

Shear stress is independent of viscosity.

For a given rate of angular deformation of a fluid, shear stress is inversely proportional to viscosity.

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Course Designer: Steve Gibbs