Self Test 1 - Prerequisite knowledge from 3B2 Flashcards
What is a Newtonian fluid? What is it Mathematically?
A Newtonian Fluid’s viscosity remains constant, no matter the amount of sheer stress applied, at a constant temperature. They have a linear relationship between viscosity and sheer stress. E.g. Water, Air, Alcohol, Gasoline and Glycerine.
Describe the difference between solids, liquids and gases in terms of molecular forces and motions?
Solids have very strong inter-molecular forces, thus the movement of
Molecules is very limited and they have a rigid shape. Fluids also have :strong inter-molecular forces but the molecules are able to move about and slide past each other. Gases have the weakest intermolecular forces and thus the molecules can move freely around.
What is the difference between a gas and a liquid in terms of density and viscosity?
Gases have weak intermolecular strength and large spaces between their molecules. They thus have very low density and are easily compressible. Gas viscosity increases with increase in temperature. Liquids have strong intermolecular forces and thus very small gaps between molecules. Thus they have higher density and are very hard to compress. Liquid viscosity decreases with increase in temperature
How are the stresses in a fluid different from the stresses within an elastic solid?
In an elastic solid stresses depend on the strain, in a liquid they depend on the strain rate.
For a fluid undergoing steady flow, can a particle of fluid experience acceleration? Explain
The fluid particle can accelerate. E.g in the Lagrangian velocity field (u,v,w) the steady condition means the velocity doesn’t vary with time, i.e du/dx = 0, however it is not homogeneous. Velocity can still vary in space.
State why the presence of turbulence in a flow may be undesirable.
Turbulence in fluid flow can result in increased energy consumption. Turbulent flows have higher frictional losses compared to laminar flows, requiring more energy to maintain the same flow rate.
Turbulent flows can cause increased wear and tear on surfaces in contact with the fluid. The random and chaotic nature of turbulence can lead to erosion and abrasion of pipes, pumps, and other components in a system, reducing their lifespan.
For streamlined bodies, turbulence increases surface shear stress and so increases the drag.
State why turbulence in a flow can be desirable.
Turbulent flows have enhanced mixing compared to laminar flows. Thus, turbulence will enhance heat transfer and mixing of one fluid in another (e.g. dispersion of smoke) increasing homogeneity in a fluid.
What is the difference between laminar and turbulent flow?
Laminar flow: randomness and mixing takes place on the molecular scale and results in small heat, mass, and momentum transfer rates
Turbulent flow: the eddy-type motions induce mixing at a macroscopic scale, leading to very effective transport of energy and mass throughout the flow field.
