Assignment – 2Taskone Explainthe term viscosity making reference to the following · Attraction – 1· Layers within the fluid – layers when moving-1· The nature of shear stress and strain -· Boundary layer Viscosity is the term used to describe the thickness of a liquid, i.e.
afluid that flows easily has what’s known as a low resistance to deformation andis therefore a low viscosity fluid, if a fluid has a high resistance than itwould be a viscous fluid. This is a very loose description of what is acomplicated term. In technical terms viscosity is the measurement of a fluidsinternal flow resistance and the best way to describe this is by looking at thelayers within the fluid. Santner Foundation.
2017.Fluids as opposed to gases or solids have a degree of attraction betweenmolecules that is strong enough to keep them together but not strong enough tokeep them rigid hence why it can ‘flow’. If you imagine a fluid travelling overflat ground and the fluid to be made up into tiny layers than it would flow asthe diagram shows above. This is known as laminar flow and when testing afluids viscosity it is crucial that the conditions are suitable to createlaminar flow. It allows the flow to be in a orderly structure where themolecules of the fluid do not change between layers, if the fluid is turbulentit will not create laminar flow and will normally mean the viscosity readingwill be higher than reality.
These twotypes of flow can be see below; Santner Foundation. 2017.When measuring Viscosity one of the factors is the shear rate thisrelates well to the layer diagram, when the layer breaks away from below it is’shearing’ the behaviour of the fluid when it does this defines whether it is aNewtonian or Non-Newtonian fluid. As with any movement there are forces acting upon fluids when in motion,the shear stress is the measure of the force of friction which is acting upon abody in the path of it. i.e.
if you take a river for an example the force wouldbe acting upon the bed of the river. When the shear stress occurs on a body itthen causes deformation this is measured by the size of the angle it isdeformed by (?) this is known as the shear strain.When a fluid is in motion as shown above in the diagrams with thelaminar flow, the outer or bottom are subject to a large amount of shear stressthis greatly reduces the velocity at this point of the fluid as you movefurther away from this point the shear stress reduces due to the lack ofdeformation and thus the velocity increases. The point between the outer layeras discussed and the point of 99% of maximum velocity in the fluid is classedas the boundary layer simply because it occurs on the boundary of the fluid. · B .
Explain the effect of shearrate, temperature and pressure on viscose fluids.Withfluctuations in the environment the properties of fluids change. For examplewith increase in temperature the fluid may become more viscous or in turn itsfluidity increase.Thereare various models for the relationship between the shear viscosity andtemperature in fluids.
One of the more commonly used is the Exponential modelcreated by Reynolds in 1886 the equation he derived is ?(T) = ?0 exp (-?T) T= Temperature ?0= The same can be said for pressure whether thisis the pressure in the atmosphere the fluid is being used or whether the fluiditself is ‘under’ pressure. As with the temperature this can result in thefluids viscosity changing this is normally due to an increase in pressure. Inmost liquids the viscosity changes because the molecular structure of the liquidcompresses this cause a reduction in free volume which causes a resistance tothe flow. · C. Explain the difference between dynamic and kinematic viscosity andstate the relationship between them.
You should include the SI units in whicheach are measured. Explain equation There is one big difference between dynamic and kinematic viscosity andthis is whether an external force is acting upon the fluid or not. Dynamicviscosity measures a fluids resistance to flow when an external force isapplied whereas kinematic viscosity measures the resistance to flow when noexternal force is applied apart from gravity. Density plays a key part inkinematic viscosity whereas with dynamic viscosity it is not a part of it thismeans you can have a fluid with the same dynamic but different kinematic viscosity.The equations is ;Kinematic Viscosity = Dynamic viscosity/density Dynamic viscosity is measured in Centipoise (cP)Kinematic viscosityis measured in Centistokes (cSt) 2 – Describe the falling sphere method formeasuring viscosity.
You need to include the equations used for thecalculations. MohsinSiddique, Assistant Professor 2017This method for measuring viscosity is called the falling sphere method.Utilising a spherical ball, a tube and the sample liquid. This type of viscositymeasuring is normally used for Newtonian fluids. essentially in order tomeasure the viscosity you place a sphere of a known size at the top of the tubeand time it falling through the liquid using known markings on the cylinder tomeasure distance and this allows us to calculate the viscosity, this isrepeated three times to prove. This type of test can be controlled such as withcooling or heating jackets to maintain set temperatures, one of the obviouslimitations of this type of test is it is only viable on transparent liquids soyou can see the sphere moving.
This test can also be called the rolling ball ifthe cylinder is angled at less than 80 degrees. Using stokes law we measure the time taken for the sphere to fall fromone defined mark to the next. The ball will fall at a constant velocity balancedby the resistance of the viscous fluid. The equations are; – we then re-arrange this equation to make thedynamic viscosity the subject – this is to calculate dynamic viscosity.
– This is the equation for all falling sphereviscometers – This is the equation used to calculate thedrag force on the sphere