ENT 257/4FLUID MECHANICS

Prof. Madya Dr. Ghulam Abdul QuadirSchool of Mechatronic Engineering

Email: gaquadir@unimap.edu.myH/P: 0174789452

Fluid Kinematics

• Fluid kinematics refers to features of a fluid in motion.

• Consideration of velocity, acceleration, flow rate, nature of flow and flow visualisation are taken up in fluid

kinematics.

There are two ways to analyse fluid motion:1) Lagrangian method, and 2) Eulerian methodIn the Lagrangian method, a particle or an element of fluid is identified

and followed during the course of motion with time, as shown in Fig. 1.

• The identified lump of fluid may change in shape, size and state as it moves.

• The laws of Mechanics must be applied to it at all times.

• Difficulty in tracing the lump of fluid rules out the possibility of applying the Lagrangian approach

• In the Eulerian method, the fluid is observed by setting up fixed stations or observatories in the flow field. Motion of the fluid is specified by velocity components expressed as functions of space and time,

),,,(),,,();,,,(

tzyxzztzyxvvtzyxuu

• In the Eulerian approach, the fluid motion at all points in the flow field is determined by applying the laws of mechanics at all fixed stations.

• This is considerably easier than the Lagrangian approach and is followed in the study of Fluid Mechanics.

Dimensions of FlowA fluid flow is said to be one, two, or three dimensional depending upon the number of independent space coordinates, i.e. one, two, or three respectively, required to describe the flow.

One dimensional Flow: When the dependent variables are functions of only one space coordinate, say x, it is one dimensional flow.Examples: Flow through pipes, channels and variable area ducts if the velocity distribution is considered constant at each cross section.Dependent variables such as velocity, pressure, density, temperature vary only with x, the only independent variable in one dimensional flow as shown in Fig. 2.The axis of the passage does not have to be a straight line for a flow to be one dimensional. For example, the flow shown in Fig. 2(b) is one dimensional if s, the streamwise direction is chosen as the independent coordinate.

The axis of the passage does not have to be a straight line for a flow to be one dimensional. For example, the flow shown in Fig. 2(b) is one dimensional if s, the streamwise direction is chosen as the independent coordinate.

Two dimensional Flow:

When the dependent variables in a fluid flow vary with only two space coordinates, the flow is said to be two dimensional. The flow does not vary along the third coordinate directionExample: The flow around a circular cylinder of infinite length (as shown in Fig. 2c) is two dimensional in the x-y plane.

Axisymmetric Flow: A flow is said to be axisymmetric if the velocity profile is symmetrical about the axis of symmetry. In other words, the velocity profile is the same at different diametral planes drawn through the passage.Example: Velocity profiles at two locations for an axisymmetric flow through a conical passage are shown in Fig. 2(d).

Steady Flow: If the dependent fluid variables at any point in the flow do not change with time, the flow is said to be steady flow. Thus

It follows that

Unsteady Flow: If the dependent fluid variables change with the passage of time at a position in the flow, the flow is called unsteady flow. Thus

0)var( iablesfluiddependentt

etctt

ptw

tv

tu ,0

0)var( iablesfluiddependentt

• Steadiness of flow means that the flow-pattern does not change with time whereas unsteadiness refers to changing flow-pattern with the passage of time at the same point in space.

Uniform Flow: If the velocity, in magnitude, direction and sense is identical throughout the flow field, the flow is said to be uniform flow. This requires the velocity components to be the same at different positions in the flow. In other words, the space rate of change of velocity components at that time must vanish.It follows that

for uniform flow.

0)var( iablesfluiddependents

etcyw

yv

xv

zu

yu

xu ,0

It is conventional to define the term “uniform flow” only in terms of the velocity components rather than in terms of other dependent fluid variables.Further, a flow may be considered uniform over the cross sections although it may not be uniform longitudinally.

Non-uniform Flow: If the velocity components at different locations are different at the same instant of time, the flow is said to be non-uniform.

Note: Steadiness refers to ‘no change with time’ and uniformity refers to ‘no change in space’. Therefore, a flow can be steady or unsteady quite independent of its being uniform or non-uniform. All the four combinations are possible.

Acceleration in Fluid Flow: The velocity components in a fluid flow are, in general, functions of space and time

From differential calculus, an infinitesimal change in u is given by

And the acceleration components are given by

),,,(),,,,(),,,,(

tzyxwwtzyxvvtzyxuu

dttudz

zudy

yudx

xudu

tw

zww

ywv

xwu

DtDw

dtdwa

tv

zvw

yvv

xvu

DtDv

dtdva

tu

zuw

yuv

xuu

DtDu

dtdua

z

y

x

Total acceleration = Convective acceleration + Local acceleration

Total acceleration is also called as substantial, material or particle derivative.

• For steady flow: Only Local acceleration components are zero.• For uniform flow: Only Convective acceleration components

vanish.• For steady uniform flow: Total acceleration becomes zero and

the flow is non-accelerating in space and time.

Expressions for the components of acceleration in cylindrical coordinates for an axisymmetric flow are given below:

tu

zuu

ruua

tu

ruu

zuu

ruua

tu

ru

zuu

ruua

zzz

zrz

rzr

rrz

rrr

2

Streamlines, Pathlines and StreaklinesA streamline in a fluid flow is a line tangent to which at any point is in the direction of velocity at that point at that instant.

• Streamlines are, therefore, equivalent to an instantaneous snap-shot indicating the directions of velocity in the entire flow field as shown in Fig. 4(a) and (b).

• There can be no flow across a streamline since the component of velocity normal to a streamline is zero.

• A streamline cannot intersect itself nor can any streamline intersect another streamline.

• In steady flow, the same streamline-pattern should hold at all times, since the velocity vector cannot change with time.Consider, first a streamline in a plane flow in the x-y plane as shown in Fig. 3. By definition, the velocity vector U at a point P must be tangential to the streamline at that point. It follows that

dy/dx = tan θ = v/u or u dy – v dx = 0where u and v are the velocity components along x and y directions respectively.

It may also be noticed that the velocity vector is expressed as U = U (s, t)

which shows that the velocity may vary along a streamline direction s as well as with the passage of time.Consider next an elementary displaced element δs along a general streamline where the velocity is U such that

U = u i+ v j + w kand δs = δx i+ δy j + δz k

By the definition of a streamline, U must be directed along δs. For collinear vectors, the cross product must vanish.Hence, U x δs = 0

Or, (vδz – wδy) i– (uδz – wδx) j + (uδy – vδx) k = 0whence δx/u = δy/v = δz/wwhich is the equation of a streamline in a three dimensional flow, steady or unsteady, uniform or non-uniform, viscous 0r inviscid, compressible or incompressible.

0zyxwvukji

A strteam surface is generated by a large number of closely spaced streamlines which pass through an arbitrary curve AB as in Fig. 4(c).

• A stream surface can be plane, axisymmetric or spatial.• If the arbitrary line constitutes a closed curve, e.g.,

ABCD as in Fig. 4(d), the closely spaced streamlines passing through the curve constitute a tubular stream surface called a stream tube.

• No flow can penetrate a stream surface or a stream tube.

A pathline in a fluid flow is the trajectory of a fluid particle, say P1 as it advances with the passage of time, say from ti to final time tf as in Fig. 4(e).

• Pathlines are, therefore, history lines of individual fluid particles over a period of time.

• Tangent at a point on a pathline must be in the direction of velocity at that point at the time when the particle passes that point.

• A pathline, therefore, intersect itself at different times and in general pathlines may constitute a pattern of intersecting zig-zag lines.

• In steady flow, the streamlines and pathlines coincide.

A streakline is the locus of locations, Fig. 4(f), at an instant of time, of all the fluid particles that have passed through a fixed point in a flow field.A little consideration will show that the streamlines, pathlines and streaklines are coincident in a steady flow.

Example:Given the velocity field

U = 5 x3 i– 15 x2 y jObtain the equation of the streamlines.

Law of Conservation of mass: Already explained in ENT 255/4. However, for completeness, I am giving you the derivation of the continuity equation again for ready reference.

Conservation of mass or Continuity Equation• Consider a control volume within the boundary layer as shown

in Fig. 7(b) and assume that steady state conditions prevail.• There are no gradients in the z direction and the fluid is

incompressible.

7

Rate of mass flow into the control

volume in the x-direction

Rate of mass flow out of the control

volume in the x-direction

Rate of mass flow into the C.V. in the y-

direction

Rate of mass flow out of the C.V.

in the y-direc.ion

7

From Fig. 7, the net mass flow into the element in the x direction is,

Similarly, the net mass flow into the control volume in the y direction is,

Since the net mass flow rate out of the control volume must be zero, we obtain

From which it follows that in two-dimensional steady flow, conservation of mass requires that

(13)

dxdyxu

dydxyv

0)(

dxdyyv

xu

0

yv

xu

Example:Does a velocity field given by

U = 5 x3 i – 15 x2 y j +t kRepresent a possible fluid motion of an incompressible fluid ?

Example:Two components of velocity in an incompressible fluid flow are given

byu = x3 – y3 and v = z3 – y3

Determine the third component, assuming that the origin is a stagnation point.

Example:Do the following velocity components represent a possible flow ?

Discuss the possibilities.u = 2 x + 3 y and v = 3 y – 4 x.

Stream Function for Two-Dimensional Incompressible Flow• Since no fluid can cross a streamline, the flow occurring

between two streamlines must remain unchanged.• For example, in Fig. 6, the mass flow between streamlines 1

and 2 taken across A1 A2 must be the same as across B1B2 or A1B2.

• The volumetric flow rate between them must also be constant for ρ = Constant.

• A value of stream function ψ is assigned to each streamline such that the volumetric flow rate is given by ψ2 – ψ1.

Mathematically, the streamfunction for a flow in the x-y plane is defined as a function of x and y such that, the velocity components are given by:

xvand

yu

Let us examine the continuity equation for two dimensional incompressible flow in the light of the definition of ψ, the stream function, defined in terms of the velocity components. We get

• Which shows that the continuity equation is identically satisfied. • Hence, existence of ψ for a flow implies a possible flow.• Conversely, for any possible flow, a function ψ must exist.

02222

yxyxxyyx

In the cylindrical polar coordinates, the stream function ψ is a function of r and θ such that

Here again, it may be seen that the velocity components identically satisfy the continuity equation.

Example;a) Does the stream function ψ = x y represent a physically possible flow ?

If so, determine the velocity at a point (2,3).

b) Does a stream function exist for a flow field described by u = 2 y and v = - 2 x ? If so, determine it and plot a set of streamlines.

c) The velocity in a flow field is given by u = 3 m/s and v = 6 m/s. Determine the equation of the streamline passing through the origin and the one passing through a point (2m, 3m).

ruand

rur

1

Kinematics of a Fluid elementA fluid element may move in a flow and undergo(a) Pure or irrotational translation(b) Pure rotation or rotational translation(c) Pure distortion or deformation; angular or linear.

These are illustrated in Fig. 9 (a), (b) and (c) respectively by taking an upright cubical fluid element to start with in each case.

Irrotational Flow• For irrotational flow, each component of rotation or of vorticity

(rotation = ½ vorticity) must vanish, i.e., ωx = 0 = ωy = ωz. • In particular, for plane flow in x-y plane,• δu/δy - δv/δx = 0

is the condition of irrotationality.

• A function Φ = Φ(x,y) can be defined with an attempt to satisfy the condition of irrotationality identically.

• For a two-dimensional flow in the x-y plane,

satisfy the condition of irrotationality identically

vy

andux

,

0;022

yxxyxv

yu

• In general, the space derivatives of Φ give the velocity components along the corresponding space coordinates.

• The function Φ is called the velocity potential function.• It may be stressed that if a flow is irrotational, a velocity potential

function Φ must exist.• Conversely, if a function Φ is specified then the flow represented by

the velocity components obtained from Φ must be irrotational.

Exampled) In a two dimensional incompressible flow the fluid velocity

components are given by u = x - 4y and v = - y – 4x.Show that the flow satisfies the continuity equation.Obtain the expression for the stream function.If the flow is potential, obtain also the expression for the velocity potential.

Soln. • Verify the continuity equation.• To obtain the stream function, use its definition in terms of the

velocity components; integrate u component definition of it w.r.t. y and find the constant of integration from the boundary condition (say, ψ = 0 at x = 0 and y = 0). To find the functional constant, differentiate the stream function w.r.t. x and equate it to v component of velocity. Simplify and you will get ψ = 2 x2 + xy – 2y2.

• To check whether the flow is potential, there are two possible approaches:(a) From the vorticity component in x direction(b) Laplace’s equation must be satisfied

.

• To obtain the velocity potential, use the its definition in terms of the velocity component u; integrate it w.r.t. x. You will get an expression for velocity potential with functional constant f(y).

• Differentiate the expression w.r.t. y and equate to v.• The expression for velocity potential will be Φ = x2/2 – 4yx - y2/2.

e) The velocity components in a two dimensional flow-field for an incompressible fluid are expressed as:

U = y3/3 + 2x – x2y and v = xy2 – 2y – x3/3i) Show that these functions represent a possible case of an

irrotational flowii) Obtain an expression for the stream function andiii) Obtain an expression for the velocity potential

• EULER’S EQUATION OF MOTION ALONG A STREAMLINETo derive a relation between velocity, pressure, elevation and density along a streamline.

• Fig. shows a short section of a streamtube surrounding the streamline and having a small cross sectional area so that velocity be considered constant over the cross section.

• AB and CD are two cross sections separated by a short distance δs.

• At AB, the area is A, velocity v, pressure p and elevation z, while at CD, the corresponding values are, A+ δA, v+ δv, p+ δp and z+ δz.

• The surrounding fluid will exert a pressure pside on the sides of the element.

• The fluid is assumed to be inviscid. Thus, there will be no shear stresses on the sides of the element and pside will act normally.

• The weight of the element mg will act vertically downward at an angle θ to the streamline.

Mass per unit time flowing = ρAvRate of increase of momentum from AB to CD = ρAv[(v+ δv) – v)] =

ρAv δv

The forces acting to produce this increase of momentum in the direction of motion are:

Force due to p in direction of motion = pAForce due to p+ δp opposing motion = (p+ δp)(A+ δA)Force due to pside producing a component

In the direction of motion = pside δA

Force due to mg producing a componentOpposing motion = mgcosθResultant force in the direction of motion pA – (P+ δp)(A+ δA)+pside

δA-mgcosθThe value of pside will vary from p at AB to p+ δp at CD and can be

taken up as p+k δp, where k is a fraction.Weight of element, mg = ρg*Volume = ρg(A+½ δA) δs,

cosθ = δz/ δsResultant force in the direction of motion = -p δA – A δ p - δp δA + p

δA + k δp. δA – ρg(A+½ δA) δs.(δz/ δs)

Neglecting products of small quantities,Resultant force in the direction of motion = -A δp - ρgA δzApplying Newton’s second law of motion, we get,

ρAv δv = -A δp - ρgA δzDividing by ρAδs, we get

Or in the limit as δs 0,

(A)

• This is known as Euler’s Equation, giving, in differential form, the relationship between pressure p, velocity v, density ρ and elevation z along a streamline for steady flow.

01

szg

svv

sp

01

dsdzg

dsdvv

dsdp

constgzvp

2

2

• It cannot be integrated until the relationship between density ρ and pressure p is known.

• For an incompressible flow, for which ρ is constant, integration of the above equation (A) along the streamline, w.r.t. to s, gives

• In which the terms represent the energy per unit weight. The above equation is known as Bernoulli’s Equation and states the relationship between pressure, velocity and elevation for steady flow of a frictionless fluid of constant density

constzgv

gp

2

2

• An alternative form is:

In which the terms represent the energy per unit volume.• These equations apply to a single streamline. The sum of the three

terms is constant along any streamline, but the value of the constant may be different for different streamlines in a given stream.

• If the earlier integration of Equation (A) is carried along the streamline between any two points indicated by suffixes 1 and 2, then

• For a compressible fluid, the integration of Equation (A) can only be partially completed, to give

constgzvp

2

2

2

222

1

211

22z

gv

gpz

gv

gp

Hzgv

gdp

2

2