Home | Engineering | Fluids in Motion

Civil - Mechanics Of Fluids - Fluid Properties And Fluid Statics

Fluids in Motion

   Posted On :  26.07.2016 12:32 pm
Fluids in Motion

At the surface there will be little movement of the fluid (it will ‘stick’ to the surface), whilst furtheraway from the surface the fluid flows faster (has greater velocity):

Fluids in Motion

 

For example, consider the fluid shown flowing along a fixed surface. At the surface there will be little movement of the fluid (it will �stick� to the surface), whilst furtheraway from the surface the fluid flows faster (has greater velocity):

 


 

If one layer of is moving faster than another layer of fluid, there must be shear forcesacting between them. For example, if we have fluid in contact with a conveyor beltthat is moving we will get the behaviour shown:


When fluid is in motion, any difference in velocity between adjacent layers has the same effect as the conveyor belt does.

 

Therefore, to represent real fluids in motion we must consider the action of shear forces

 

Consider the small element of fluid shown, which is subject to shear force and has a dimension sinto the page. The force F acts over an area A = BC� s. Hence we have a shear stress applied:


Any stress causes a deformation, or strain, and a shear stress causes a shear strain. This shear strain is measured by the angle ? . Remember that a fluid continuously deforms when under the action of shear. This is different to a solid: a solid has a single value of ? for each value of ? . So the longer a shear stress is applied to a fluid, the more shear strain occurs. However, what is known from experiments is that the rate of shear strain (shear strain per unit time) is related to the shear stress:


We need to know the rate of shear strain. From the diagram, the shear strain is:


If we suppose that the particle of fluid at E moves a distance x in time t, then, using = R? for small angles, the rate of shear strain is:


Where u is the velocity of the fluid. This term is also the change in velocity with height. When we consider infinitesimally small changes in height we can write this in differential form, du/ dy . Therefore we have:


Newton�s Law of Viscosity:


Generalized Laws of Viscosity

We have derived a law for the behaviour of fluids  that of Newtonian fluids. However, experiments show that there are non-Newtonian fluids that follow a generalized law of viscosity:


Where AB and n are constants found experimentally. When plotted these fluids show much different behaviour to a Newtonian fluid:

Behaviour of Fluids and Solids

 

In this graph the Newtonian fluid is represent by a straight line, the slope of which is ? . Some of

 

the other fluids are:

Plastic: Shear stress must reach a certain minimum before flow commences.

 

Pseudo-plastic: No minimum shear stress necessary and the viscosity decreases with rate of shear, e.g. substances like clay, milk and cement.

 

Dilatant substances; Viscosity increases with rate of shear, e.g. quicksand.

 

Viscoelastic materials: Similar to Newtonian but if there is a sudden large change in shear they behave like plastic.

 

Solids: Real solids do have a slight change of shear strain with time, whereas ideal solids (those we idealise for our theories) do not. Lastly, we also consider the ideal

 

fluid. This is a fluid which is assumed to have no viscosity and is very useful for developing theoretical solutions. It helps achieve some practically useful solutions.

 

 

 

Properties

 

Further Reading

Here we consider only the relevant properties of fluids for our purposes. Find out about

surface tension and capillary action elsewhere. Note that capillary action only features in

pipes of

 

? 10 mm diameter.

 

 

Tags : Civil - Mechanics Of Fluids - Fluid Properties And Fluid Statics
Last 30 days 48 views
Related words :

What is Fluids in Motion Define Fluids in Motion Definition of Fluids in Motion where how meaning of Fluids in Motion lecturing notes for Fluids in Motion lecture notes question and answer for Fluids in Motion answer Fluids in Motion study material Fluids in Motion assignment Fluids in Motion reference description of Fluids in Motion explanation of Fluids in Motion brief detail of Fluids in Motion easy explanation solution Fluids in Motion wiki Fluids in Motion wikipedia how why is who were when is when did where did Fluids in Motion list of Fluids in Motion school assignment college assignment Fluids in Motion college notes school notes kids with diagram or figure or image difference between Fluids in Motion www.readorrefer.in - Read Or Refer

​TOP MENU

OTHER SUGEST TOPIC