Fluid Systems Technical Interview Prep

What are the assumptions behind Bernoulli’s equation?

Steady flow, incompressible fluid, inviscid, and is applicable along a streamline

What is the difference between static pressure, dynamic pressure, and stagnation pressure?

Static pressure is the pressure acting outward on a fluid that is at rest, in all directions. Dynamic pressure is the pressure that is caused by fluid that is in motion. The stagnation pressure is the sum of the static and dynamic pressures of a fluid.

How would you calculate pressure drop through a pipe?

When the assumptions for Bernoullis aren’t met, the we can use the darcy weisbach formula, which would be the darcy friction factor multiplied by the length of the pipe divided by the internal diameter, then multiplied by the density and the average fluid velocity squared over two. I would also make sure to take into account fittings and valves, elevation changes, temperature changes, and any bends.

What is the darcy friction factor?

It describes friction losses in pipe flow and open channel flows

What is open channel flow?

Gravity driven movement of a liquid

How do you find darcy friction factor for a pipe in laminar flow?

It would be a constant based on the geometry of the pipe divided by the reynolds number. For a circular pipe that number is 64.

How do you find the darcy friction factor for turbulent flow?

It depends on both the reynolds number and the relative roughness of the pipe, which can be found using the moody diagram.

How do bends in pipes affect pressure?

They would reduce the overall downstream fluid pressure because changing the direction of a moving fluid creates resistance

What is laminar flow?

Particles follow a smooth, straight, parallel path that never cross

What is turbulent flow?

Particles move in more erratic, crossing, and three dimensional paths

How do the velocity profiles for laminar and turbulent flow differ?

Laminar flow forms sharp, parabolic shapes in a pipe while turbulent flow has a more uniform velocity profile because it redistributes momentum

What causes laminar flow?

High viscosity and low velocity

What causes turbulent flow?

Low viscosity and high speed

What is Reynolds number?

The ratio of forces of fluid pushing forward to the viscous forces resisting fluid flow

When does laminar flow occur?

When the Reynold’s number <2300

When does turbulent flow occur?

When the Reynold’s number >4000

If you reduce a pipe’s diameter, what happens to velocity and pressure?

The fluid velocity increases using the continuity equation. Pressure would decrease because there is more surface area relative to the volume that is affected by friction against the pipe walls.

What is cavitation and why is it problematic for pumps?

Cavitation is when bubbles rapidly form and burst when the static pressure of a liquid drops below the liquids vapor pressure. This creates high-pressure shockwaves which can end up eroding metal or violent vibrations which increase wear on a system.

What is the difference between mass flow rate and volumetric flow rate?

Volumetric flow rate measures the physical amount of volume moving past a point in time, whereas mass flow rate measures the amount of mass moving past a point in time.

How would you troubleshoot a fluid system that has lower than expected flow?

I would check instrument accuracy and operating conditions first, before examining the pump performance, and checking for any leaks. I’d use evidence I gathered to narrow down a cause.

What is gauge pressure vs absolute pressure?

Gauge pressure is measured relative to local atmospheric pressure while absolute pressure is measured relative to a perfect vacuum.

What information would you need to choose a pump?

Its flow rate capacity, power requirements, system geometry,

How do elbows, valves, and fittings affect pressure loss?

So the pressure drop due to these components would be equal to the velocity squared, times the density over two, multiplied by a resistance coefficient

What determines the maximum allowable pressure in a pipe?

What is the difference between design pressure and operating pressure?

Operating pressure is pressure under routine operations of the pipe, while design pressure is the maximum pressure a system is built to handle

What happens if a valve closes suddenly?

Fluid hits a barrier at a potentially high speed, potentially causing a shockwave

What is a shockwave?

Its a moving disturbance where the fluid properties like pressure and temperature jump abruptly.

What is the difference between thrust and mass flow rate?

Thrust is the pushing force generated by accelerating a mass, while mass flow rate is amount of mass passing through the engine per unit time

How does a rocket engine generate thrust?

By burning fuel and an oxidizer in the combustion chamber, and then expelling the resulting high-pressure gas out the back through a nozzle.

What is an RDRE?

It is a engine that relies on supersonic detonation waves. Fuel and oxidizers are continuously injected into the chamber, and an ignitor sets of a detonation wave which travels around an annulus, while also consuming propellants. The pressure spike from the detonation wave forces the gas out the back of the nozzle, creating continuous thrust

Why are RDREs efficient?

The pressure jump caused by the detonation wave improves specific impulse

What is specific impulse?

Its a measure of how efficiently a rocket generates thrust from its propellant, and its equal to the exhaust velocity over the gravitational acceleration constant.

How would you optimize a design with pressure, flow, and packaging constraints?

I would look to meet the hard constraints of the system

What is the formula for flow rate for laminar flow?

Pi x radius^4 / (8 x dynamic viscosity x length)

What is dynamic viscosity?

A fluids internal resistance to flow when external shear force is applied

When would you use a hand calculation versus MATLAB, CFD, or FEA?

For sanity checks or initial conceptual sizing, or working with more standard components like straight pipes or beams

How would you size a pipe for a required flow rate?

I would use the continuity equation, where flow rate = area * velocity, but then also incorporate safety standards and operating conditions as well

What causes vibrations in mechanical systems?

Cyclic oscillations from forces on the structure, or components that are out of alignment, loose fittings or mounts, or pulsating pressures

What is the first law of thermodynamics?

Energy cannot be created or destroyed, heat supplied = internal energy + work done

What is pump head vs head loss?

Pump head is the energy a pump gives to lift or push a fluid through a system (ft), and head loss is the energy lost as a fluid rubs against walls or flows through bends

What would you look for when choosing a pump?

The total head, to see if it can produce the energy to overcome the elevation between the source and the discharge point and head losses in the system

Major vs minor losses

Major losses are due to friction in straight pipes, minor losses are due to bends or fittings