Pressure distribution around a circular cylinder bodies | Fluid Laboratory

SAIF AL-DIN ALI MADI
Department of Mechanical Engineering/ College of Engineering/ University of Baghdad
26/11/2018 1 | P a g e
[Fluid Laboratory II]
University of Baghdad
Name: - Saif Al-din Ali -B-

26/11/2018 2 | P a g e
TABLE OF CONTENTS
ABSTRACT.........................................................................I
OBJECTIVE........................................................................II
INTRODUCTION..............................................................V
THEORY..........................................................................VI
APPARATUS...................................................................VII
Calculations and results................................................VIII
DISCUSSION ...............................................................VIIII

26/11/2018 3 | P a g e
Experiment Name:-
Pressure distribution around a circular cylinder bodies
1. Abstract
A cylinder in a closed circuit wind tunnel will be experimented upon
to gather the pressure distribution acting on it
Laminar flow is defined when a fluid flows in parallel layers, with no
disruption between the layers. In comparison to this Turbulent flow
has a much more disorganized pattern, it is characterized by
mixing of the fluid by eddies of varying size within the flow.
The Reynolds number (Re), gives the measure for laminar and
turbulent flows. Laminar flow takes place when Reynolds number
is lower than 104, and for Turbulent flow the Re must be greater
than 3Ã-105.
The pressure is measured using the manometer, and then
therefore the pressure at the tapping must be the same as the
pressure head.
The cylinder being experimented on is placed in the wind tunnel.
The pressure upstream of the cylinder is sensed by a taping on the
tunnel wall and is connected to one of the tubes.
2. OBJECTIVE
Calculate pressure coefficient (Cp) for a horizontal circular
cylinder in a uniform air duct and compares it with the
theoretical value.
The aims of the investigation is to measure the pressure
distribution on the surface of a smooth cylinder placed with
its axis perpendicular to the flow and to compare it with the
distribution predicted for frictionless flow, and to calculate
the drag coefficient of the cylinder.

26/11/2018 4 | P a g e
3. Introduction:-
External flows past objects have been studied extensively because
of their many practical applications. For example, airfoils are made
into streamline shapes in order to increase the lifts, and at the
same time, reducing the aerodynamic drags exerted on the wings.
On the other hand, flow past a blunt body, such as a circular
cylinder, usually experiences boundary layer separation and very
strong flow oscillation in the wake region behind the body. In
certain Reynolds number range, a periodic flow motion will develop
in the wake as a result of boundary layer vortices being shed
alternatively from either side of the cylinder. This regular pattern of
vortices in the wake is called a Karman vortex street. It creates an
oscillating flow at a discrete frequency that is correlated to the
Reynolds number of the flow. The periodic nature of the vortex
shedding phenomenon can sometimes lead to unwanted structural
vibrations, especial when the shedding frequency matches one of
the resonant frequencies of the structure.
Flow Separation:-
The presence of the fluid viscosity slows down the fluid particles
very close to the solid surface and forms a thin slow-moving fluid
layer called a boundary layer The flow velocity is zero at the
surface to satisfy the no-slip boundary condition. Inside the
boundary layer, flow momentum is quite low since it experiences a
strong viscous flow resistance. Therefore, the boundary layer flow
is sensitive to the external pressure gradient (as the form of a
pressure force acting upon fluid particles. If the pressure
decreases in the direction of the flow, the pressure gradient is said
to be favorable. In this case, the pressure force can assist the fluid
movement and there is no flow retardation. However, if the
pressure is increasing in the direction of the flow, an adverse
pressure gradient condition as so it is called exist. In addition to
the presence of a strong viscous force, the fluid particles now have
to move against the increasing pressure force. Therefore, the fluid
particles could be stopped or revered, causing the neighboring
particles to move away from the surface. This phenomenon is
called the boundary layer separation

26/11/2018 5 | P a g e
Boundary Layer
Boundary layer, in fluid mechanics, thin layer of a flowing gas or
liquid in contact with a surface such as that of an airplane wing or
of the inside of a pipe. The fluid in the boundary layer is subjected
to shearing forces. A range of velocities exists across the
boundary layer from maximum A boundary layer is a thin layer of
viscous fluid close to the solid surface of a wall in contact with a
moving stream in which (within its thickness δ) the flow velocity
varies from zero at the wall (where the flow “sticks” to the wall
because of its viscosity) up to Ue at the boundary, which
approximately (within 1% error) corresponds to the free stream
velocity. Strictly speaking, the value of δ is an arbitrary value
because the friction force, depending on the molecular interaction
between fluid and the solid body, decreases with the distance from
the wall and becomes equal to zero at infinity.
Pressure Coefficient Cp
The pressure coefficient is a dimensionless number which
describes the relative pressures throughout a flow field in fluid
dynamics. The pressure coefficient is used in aerodynamics and
hydrodynamics. Every point in a fluid flow field has its own unique
pressure coefficient, pressure coefficients can be determined at
critical locations around the model
Pressure coefficients can be used with confidence to predict the fluid
pressure at those critical locations around a full-size aircraft or boat.

26/11/2018 6 | P a g e
4. Theory
For inviscid flow, there is no friction to cause boundary layer
separation, vortices or a subsequent wake. However, inviscid flow
over a cylinder will generate areas of different pressure gradients.
Two stagnation points result one on the middle of the cylinder in the
fluid flow direction and one behind the cylinder. At these points, Cp
will be one. Since the cylinder is a symmetric body, there will be
symmetric pressure regions around the body. In the direction
perpendicular to the fluid flow, a suction force exists. Again, since
the body is symmetric, so are the forces and the force negate each
other. In this inviscid scenario, no aerodynamic forces result
because of the symmetry. The simplification that the flow is inviscid
fails because bluff bodies experience aerodynamic force. Theoretical
equation (1) can merely be used as a benchmark for validating data :
(Cp)theo=1-(𝟒𝒔𝒊𝒏𝜽) 𝟐
The Experimental Procedure
the purpose of this experimental, the fluid flow was analyzed as
viscous . Viscous flow over a circular cylinder does not separate,
but viscous flow separate, causing wake vortices and measurable
wake pressure/velocity date. the pressure coefficient is governed
as follow:
Pa+
𝟏
𝟐
pa 𝒗𝒂 𝟐
= P+
𝟏
𝟐
p 𝒗𝒂 𝟐
Cp=
𝑷−𝑷𝒂
𝟏
𝟐
𝐩 𝒗𝒂 𝟐
Po-Pa =
𝟏
𝟐
p 𝒗𝒂 𝟐
Sub.(5) into (4)
𝟏
𝟐
p 𝒗𝒂 𝟐
=pg(ho-ha)
Po-pa = pg(ho-ha)
(Cp)exp =
𝒉−𝒉𝒂
𝒉𝒐−𝒉𝒂
Where;
ha = static head from wind tunnel
h = static head at each angle
ho = static head at θ

26/11/2018 7 | P a g e
5. APPARATUS
The test device consists of a horizontal circular cylinder in a low
velocity wind tunnel. The diameter of the cylinder is (8 cm) and it
contain at (12) holes equally distributed around it. The angle
between one hole and other is (30°) and each one connected with
multitude manometer to measure the pressure (P) of each point
around the cylinder. In the same time, there is a hole before the
cylinder connected with a manometer to measure the static
pressure of the air (Pa) inside the duct. Wind tunnel is a device
consists of a wooden fan pulls air from the laboratory passing air
filtration stages, where the tunnel walls converge and diverge until
we get a uniform and steady flow.

26/11/2018 8 | P a g e
6. Calculations and results
𝒉𝒐 = 29 𝑐𝑚 ( 𝒉 − 𝒉𝒐)
1. Ө=0 h=29 = 0 cm
2. Ө=30 h=29.2 = 0.2 cm
3. Ө=60 h=31.4 =2.4 cm
4. Ө=90 h=31 = 2 cm
5. Ө=120 h=31.4 =2.4 cm
6. Ө=150 h=31.2 =2.2 cm
7. Ө=180 h=31.2 = 2.2 cm
8. Ө=210 h=31.3 =2.3 cm
9. Ө=240 h=31.4 =2.4 cm
10.Ө=270 h=31 =2 cm
11.Ө=300 h=31.3 =2.3 cm
12.Ө=330 h=30 =1 cm
13.Ө=260 h=29 =0 cm
Experimental;-
𝒉𝒐 = 29 𝑐𝑚 𝒉𝒂 = 31 𝑐𝑚
𝒄 𝒑 𝒆𝒙𝒑
=
𝒉−𝒉𝒂
𝒉𝒐−𝒉𝒂
1) Ө=0 h=29
= 𝟏
2) Ө=30 h=29.2
= 𝟎. 𝟗
3) Ө=60 h=31.4
= −𝟎. 𝟐
4) Ө=90 h=31
= 𝟎
5) Ө=120 h=31.4
= −𝟎. 𝟐
6) Ө=150 h=31.2
= −𝟎. 𝟏
7) Ө=180 h=31.2
= −𝟎. 𝟏
8) Ө=210 h=31.3
= −𝟎. 𝟏𝟓
9) Ө=240 h=31.4
= −𝟎. 𝟐

26/11/2018 9 | P a g e
10) Ө=270 h=31
= 𝟎
11) Ө=300 h=31.3
= −𝟎. 𝟏𝟓
12) Ө=330 h=30
= 𝟎. 𝟓
13) Ө=260 h=29
= 𝟏
Theoretically:-
𝒄 𝒑 𝐭𝐡𝐞𝐨
=1-𝟒𝒔𝒊𝒏𝜽 𝟐
1. Ө=0 h=29
= 𝟏
2. Ө=30 h=29.2
= 𝟎
3. Ө=60 h=31.4
= −𝟐
4. Ө=90 h=31
= −𝟑
5. Ө=120 h=31.4
= −𝟐
6. Ө=150 h=31.2
= 𝟎
7. Ө=180 h=31.2
= 𝟏
8. Ө=210 h=31.3
= 𝟎
9. Ө=240 h=31.4
= −𝟐
10. Ө=270 h=31
= −𝟑
11. Ө=300 h=31.3
= −𝟐
12. Ө=330 h=30
= −𝟎
13. Ө=360 h=29
= 𝟏

26/11/2018 10 | P a g e
Results:-
N0 θ(deg.) h-ho(mm) 𝑐 𝑝 𝑒𝑥𝑝
𝑐 𝑝theo
cosθ
1 0 0 1 1 1
2 30 2 0.9 0 0
3 60 24 -0.2 -2 -1
4 90 20 0 -3 0
5 120 24 -0.2 -2 1
6 150 22 -0.1 0 0
7 180 22 -0.1 1 -1
8 210 23 -0.15 0 0
9 240 24 -0.2 -2 1
10 270 20 0 -3 0
11 300 23 -0.15 -2 -1
12 330 10 0.5 0 0
13 360 0 1 1 1
0
50
100
150
200
250
300
350
-3.5 -3 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5
θ
𝑐p
exp
theor

26/11/2018 11 | P a g e
7. DISCUSSION
The difference in experience between practical and theoretical?
1. The location of the holes and the way the cylinder is installed
are defective
2. Accuracy of taking readings and unstable rounding
3. Calibration of the measuring device and the age of the device
also have an effect in accuracy
4. Failure to deliver the device to the state of stability when
taking the device
5. Parking people when the air corridor has an impact on the
values

Pressure distribution around a circular cylinder bodies | Fluid Laboratory

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to Pressure distribution around a circular cylinder bodies | Fluid Laboratory

Similar to Pressure distribution around a circular cylinder bodies | Fluid Laboratory (20)

More from Saif al-din ali

More from Saif al-din ali (20)

Recently uploaded

Recently uploaded (20)

Pressure distribution around a circular cylinder bodies | Fluid Laboratory