Fluid Mechanics Flashcards
What is fluid mechanics?
it is science of pyhsical behaviour of fluids
In which parts can man divide fluids?
into Fluid statics and fluid dynamics
What’s the difference fluid statics and fluid dynamics?
Fluid statics is considers as resting fluid, while fluid dynamics is moving fluids
Why do we have to describe the flow processes of a fluid?
it helps to solve flow problems, designt of components through or around which flow occure and the flow monitoring
Which energy can be used to describe fluid movement?
convective energy, where impules and mass transport is related to the movement of matter (fluid)
What are properties of fluids?
- permanent change of the average position of the molecules to one another (Amorphous behaviour)
- it’s a continuum
- only compressive forces can be applied (no tensile nor shear forces)
- continuous deformation under the influence of shear force
why do we have continuous deformtion under the influence of shear force?
low shear stresses lead to a change in shape–> movement–>flow
why do we have just compressive forces?
because fluid doesn’t have any solid lattices
Learn to draw the figure on how to describe the forces between the static and moving plane
do you know how to draw it?
Which elements describe in the figure
Static , moving plane and water as the fluid
Can you explain when the moving plane moves?
the moving plane is push by a force, creating a velocity gradient, which is proportional to the force
How is the shear stress obtained?
know the equation
by diving the force by the area which is proportional to the velocity gradient
Defination of viscosity
it is the internal resistance against movement (resistance of growth)
what id the Newton’s law of viscosity?
it describe the relation between shear stress and rate of strain, which states that the shear stress is proportinal to the strain rate
Is the dynamic viscosity temperature dependent?
it depends on the state of the material.
what is the cause of viscosity?
it is the interchange of momentum between layers of fluid moving with different velocities due to the chaotic molecular motion
How does the viscosity changes in liquid and gases if temperature is increased?
For liquids, the viscosity descreases, while for gases it increases
Why does the viscosity in gases increase with increase of temperature?
in increase of temperature we have collision of molecules with different average velocities, impulse exchnage ad obtaining shear stress
Why is the viscosity of gases smaller than in liquid?
due to lower density, less molecular collisions, causes lower viscosity
Why is the vicsoity off liquids descreasing with increasing temperature?
with the increase of temperature, one increase the average distance between molecules, which decrease the intermolecular attractive forces.
Addtionally, the increase of T, leads to increase of the kinetic energy of molecues, which causes frequency of place changes
why is the vicsoity of gases increasing with increasing temperature?
increase in temperature , leads to an increase in molecular collisions, which increases the viscosity
Fluid statics, names the parts
Pressure in static fluid
communication vessels
Buoyance and sedimentation
draw the forces which arts in the static fluids
do you know it?
How can one describe the pressure forces in static fluids?
surface of volume elemnts is only affected by respective pressure forces of the neighbouring element
How can you describe the pressure force in the x-direction?
Know the equation
what other forces can affect the centr of the volume element?
Gravity, electrostatic, centrifugal or inertial froces
What is the basic low in static fluid?
the sum of all froces is zero
what is the general equation for the forces acting in x, y and z direction?
do you know it?
Pressure in static fluids is describe as? (know the eqaution
in one dimension (in the z-direction)
How does the following formual p(z)=p(o) + pgz results?
in a heavy fluid, one has a hydrostatic pressure distribution.
the density of the incompressibilie fluid is constant, leading to a coordinate system changed
for what can the hydrostatic pressure can be used?
for U-tube as measuring instrument
How is the pressures between two connected water columns?
the hydrostatic pressure is equal in all planes
what is the formula for the equilibrium of forces in a U-tube?
p1=P2 +pgh
if the pressure differences is to high, what can be done to still obtain a small U-tube?
at high differences, fluid with high density should be used and vice vicel
what is the Archimdes’s principle?
A body of any shape immersed in a liquid experiences a weight reduction equal to the weight of the displaced liquid
what is the buoyance force equation?
F(b)= m(water)*g
what do you have to do if you want to know if your tablet will float or sink?
Do a force equilibrium, where the sum of the fg and fb equa zero.
if the tablet density is low, it will float, otherwise it will sink
what does the kinematics describes?
it describe the movement of a fluid without considerung the forces that cause this movemnet
what are the two approcaches used to decriebe the movements?
La grange and the euler
the La Grange approach is?
- obersever moves with fluid element
- flow variables are each linked to one fluid element
- temporal change of the fluid element properties
the Euler approach is?
Stationary observer, decribes passing flow
Behavior of the constantly changing fluid particles, which pass the defined loctation
flo variables are each linked to several fluid elements
what is the correlation between La grange and Euler approache?
material (La grangian) change of peroperties (derviation) of a fluid element expressed in Eulerian reference system
what are the different lines?
Streamlines and Pathlines
what are Stremlines?
it show at a certain time the directional filed of the velocity vector
what is the methematical description of the streamlines?
tangents of velocity vectors (v*dx=0)
what are Pathlines?
Path on which a fluid elemnt moves with advancing time
what is the methematical description of the pathlines?
dx/dt=v(x,t) (like la grande)
what happend for a steady flow?
the speed field is indepnedent of time, therefore streamlines is equal to the pathlines
what is a stream filament theory?
- a one dimensional stream, where the steamlines form a closed surface flow tube
how are the velocity vectors in the stream filament theory?
streamlines are tangents
What does a streamlines tangents of velocity vectors means?
no penetration of fluid mass through the outer surface of tube
what are other characteritics of the stream filament theory?
change of flow variables small in d-direction in compaerison to chnage in s-direction
change of flow variables small in d-direction in compaerison to chnage in s-direction (what does this means)
one diemnsional changes (s-direction) along stream line calculable
Why is the stream filament theory interesting for industry?
Flows through pipes and in channels can be treated as a single stream filament using values of two variables averaged over the cross-section
what is the continuity equation?
A1w1=A2w2–>V1=V2
what is the Bernoulli equation?
Do you know it?
When is the Bernoulli equation valid?
Stationary flows
incompressible media
frictionless fluid
no field forces other than gravity
How can the Bernoulli equation be used to describe the nozzle for spray drying?
it is used to describe the pressure forces, where if the dynamic pressure increase, the static pressure has to decrease, for the pressure to be constant through out the drying process
When does the dynamic pressure ncreases?
when something is flowing, the faster the liquid flows the lower the static pressure, therefore z1 nd z2 i the bernoulli equation are the same
When do we used the hydrosataic, Bernoulli and continity equation?
contiunity equation for speed
Bernoulli equation for Pressure
hydrostatic for the High
What are stirred tanks used for?
to reducs complex experiment in the lab, by simulation of stirred systems via numerical fluid dynamic
how do you determined P in a stirred specific diagram?
Calculation of Re-number
Reading the Ne-number from diagram
Calculation of P
when is no baffles needed?
when the laminar flow infleunes of the viscosity forces is dominates
When is Turbulent reached?
mostly only reached when baffles are used
the transition are depends on?
reactor type, hwereby the bafffles reduce the transition area, leading to turbulent flow
