Momentum Fluida

8/13/2019 Momentum Fluida

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Engineering H191 - Drafting / CAD

The Ohio State University

Gateway Engineering Education Coalition

Lab 4 P. 1Autumn Quarter

Transport Phenomena

Lab 4


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Topics to be covered

Transport Phenomena

Energy

Mass Momentum (fluid)

Viscosity and rheology

Falling ball viscometers examine the effect of viscosity on object falling through

the fluid.


3/17





Transportation applications

Energy

Fighter jet cooling

Radiators

Air conditioners Mass

Intracellular transfer

Momentum (i.e. fluid)

Pumps

Airplane flight

Water flow

Applications all over

engineering:

Mechanical

Chemical

Aeronautical

Biomedical

Civil

Industrial Systems

Materials Science


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Energy and Mass Transport Mechanisms

Energy Transport

Mass Transport

Flow Direction

N2 O2valve

Difference in temperature is

the driving force for heat

transfer.

Difference in concentrationis the driving force for mass

transfer.
http://www.gifart.com/cgi-bin/affiliate/clickthru.cgi/school


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Momentum (Fluid) Transport

Flow types

Turbulent flow

Laminar flow

Velocity Gradient

Viscosity

Reynolds Number


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Momentum transport Mechanisms

If pressure drop is small across the object, fluidmotion is smooth and transfer is molecular. Thatis, momentum transfers from molecule tomolecule through the fluid. Fluid flows in layers

Difference in pressure is the driving force,causing fluid to flow

Laminar flow ( Molecular)


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8/17





Coefficient of Viscosity ()

Under conditions of laminar flow, the force (F) required tomove a plate at constant speed against the resistance of afluid is proportional to the area of the plate (A) and to thevelocity gradient (dVx/dy) perpendicular to the plate.

F = A (dVx/dy) (or)

= (dVx/dy) where, is shear stress per unit area

Newtons Law of Viscosity

Unit (SI): kg m-1 s-1 (preferred) or Pa-s


9/17





Reynolds Number (Re)

Re is a dimensionlessparameter that describes flowand is defined as

Re = DV /

D: Characteristic length scale (such as diameter of a

pipe, diameter or length of a body) (m)

V: Characteristic Velocity (m/s)

: Density of fluid (kg/m3)

: Viscosity of fluid (kg/ms)

Ratio /is called Kinematic Viscosityof fluid,

usually expressed in (m2/s)


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Re and Critical Velocity

At a critical value of Re, flow will change from

laminar to turbulent - the flow velocity at which

this occurs is called the critical velocity.

Critical Re changes based on application

there are no analytical methods for predicting

critical Re available due to complex origins of

turbulence.


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Re and Critical Velocity

For fluid flow throughpipes, critical Re 2000

Re < 2000 for

laminar Re >> 2000 for

turbulent

2000 < Re < 4000 istransition region

laminar or turbulent

Critical Re changes fordifferent flow types:

1 for object movingin a fluid (this lab)

1000 for flowbetween parallel walls

500 for flow in awide open channel


12/17





Falling Sphere Viscometer

Requires a transparent vertical

tube filled with test fluid and the

object (a sphere).

When object starts to drop (free

fall), it accelerates downward till itreaches a maximum velocity

called terminal velocity (Vt).

Terminal velocity affected by

Density, viscosity of the fluid Shape, size, density of object

Measure terminal velocity.

Vt

Assume:Sphereattainsterminal

velocityhere


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Sphere at terminalvelocity (Vt)

Fd = Fg Fb

Fg

Fd Fb

When body attains terminal velocity,

body experiences no acceleration

forces acting on the body are in

equilibrium.

Magnitude of terminal velocity should

result in a low Recritical Re is

about 1.

Gravitational Force (Fg) depends on:

Density of sphere

Radius of sphere

Acceleration due to gravity


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Force due to buoyancy (Fb) depends on:

Density of fluid

Radius of sphere

Acceleration due to gravity Drag force (Fd) is the resistance of the fluid to

motion of body given by Stokes law, depends on:

Absolute viscosity of fluid

Terminal Velocity (Vt)

Radius of sphere

Fg

Fd Fb

Fd = Fg Fb


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Vt

Design should consider:

Wall effects

Ratio of diameter of sphere todiameter of cylinder should beas small as possible.

Bottom effectsTo ensure minimal error, westop recording before aspecific height from thebottom of cylinder.

Terminal velocity of object

through fluidShould yield Re


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Lab Report Requirements - in pairs

Analysis and discussion of the two fluids at your tableplus a third fluid from the lab website

Position/time plots with trendlines

Analysis and discussion of the velocities from eachgroup in the class

Comparison of group data against class

Determination of Reynolds number and viscosity foreach fluid


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Todays Goals

Collect data using the LabVIEW application

Save at least 6 .csv files 3 per fluid using the two

fluids at your table

Collect 6 sample Vt(3 per fluid) and report to the

front, as described at end of procedure:

Open your .csv files and determine Vtby fitting

trendlines and calculating total velocity

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