Question 1

The Reynolds number increases when&#10;A) Water velocity decreases&#10;B) Size of the object increases&#10;C) Water velocity increases&#10;D) Both b and c

Accepted Answer

The Reynolds number is directly proportional to both the velocity of the fluid and the characteristic length (size of the object). Therefore, increasing either the water velocity or the size of the object will increase the Reynolds number.

Question 2

Which of the following is true?&#10;A) Dynamic viscosity of air is greater than water&#10;B) Dynamic viscosity increases with increasing temperature&#10;C) Dynamic viscosity increases as the size of a particle in a fluid decreases&#10;D) None of the above

Accepted Answer

A) is incorrect because the dynamic viscosity of water is greater than that of air. B) is only true for gases, where dynamic viscosity increases with temperature, but for liquids, it typically decreases with temperature. C) is incorrect because dynamic viscosity is a property of the fluid itself and is not directly affected by the size of particles suspended in it.

Question 3

A bacterium likely lives&#10;A) Under high Reynolds number conditions&#10;B) Dominated by inertia&#10;C) Dominated by viscosity&#10;D) Under conditions only of very low dynamic viscosity

Accepted Answer

Bacteria are small and operate in environments where viscous forces are much more significant than inertial forces, meaning their movement is dominated by viscosity.

Question 4

Within a boundary layer&#10;A) Flow is always turbulent&#10;B) Water velocity is less than in the mainstream&#10;C) Water velocity is zero&#10;D) Reynolds number is always higher than in the mainstream

Accepted Answer

Within a boundary layer, especially close to the surface, the water velocity is less than in the mainstream due to frictional forces and viscosity effects.

Question 5

Bernoulli's principle predicts&#10;A) That pressure varies inversely with flow velocity&#10;B) That velocity varies with cross-sectional area&#10;C) That pressure near an object increases as water velocity increases&#10;D) That small objects sink at constant velocity

Accepted Answer

According to Bernoulli's principle, in a streamline flow, an increase in the velocity of the fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy. Thus, pressure varies inversely with flow velocity.

Question 6

In a system of pipes, if water enters the system at a given velocity, what happens if the cross-sectional area triples?&#10;A) Velocity triples&#10;B) No effect on velocity&#10;C) Velocity decreases to the third power of initial velocity&#10;D) Velocity decreases to one-third the initial velocity

Accepted Answer

According to the principle of continuity for fluids, the product of the cross-sectional area of the pipe and the velocity of the fluid flow through it remains constant. Therefore, if the cross-sectional area triples, the velocity of the water must decrease to one-third of its original value to conserve the mass flow rate.

Question 7

You can reduce drag on a fixed object in a fluid by&#10;A) Increasing the cross-sectional area exposed to the flow&#10;B) Decreasing the cross-sectional area exposed to the flow&#10;C) Adding projections out from the side to break up the flow&#10;D) Changing its density

Accepted Answer

Decreasing the cross-sectional area exposed to the flow reduces the surface area in contact with the fluid, thereby reducing drag.

Question 8

If pressure drag is acting on an object in moving water&#10;A) The velocity downstream of the object is greater than upstream&#10;B) The velocity downstream of the object is always laminar&#10;C) The velocity downstream of the object is less than upstream&#10;D) None of the above

Accepted Answer

Pressure drag causes a wake behind the object, reducing the velocity downstream compared to upstream.

Question 9

Skin friction at the surface of a fish can be reduced by&#10;A) Swimming faster&#10;B) Swimming slower&#10;C) Having an irregular surface with small bumps&#10;D) None of the above

Accepted Answer

Having an irregular surface with small bumps, known as denticles in sharks, can reduce skin friction by disrupting the flow of water close to the skin, leading to a reduction in drag.

Question 10

Inertial forces dominate strongly when A) Re < 1 B) Re < 200 C) Re > 1000 D) Re > 10

Accepted Answer

Inertial forces dominate strongly when the Reynolds number (Re) is less than 1.

Deck 6: Life in a Fluid Medium