Eastern Mediterranean University
Department of Mechanical Engineering
Laboratory Handout
COURSE: Fluid Mechanics (MENG353)
Semester: Fall (2014-2015)
Name of Experiment: FLOW OVER A CIRCULAR CYLINDER
Instructor: Assoc. Prof. Dr. Hasan Hacışevki
Assistant: Amir Teimourian
Submitted by:
Student No:
Group No:
Date of experiment:
Date of submission:
------------------------------------------------------------------------------------------------------------
EVALUATION
Activity During Experiment & Procedure
30 %
Data , Results & Graphs
35 %
Discussion, Conclusion & Answer to Questions
30 %
Neat and tidy report writing
5%
Overall Mark
Honor Pledge:
By electronically submitting this report I pledge that I have neither given nor received
unauthorized assistance on this assignment.
__________
Date
______________
Signature
1. Introduction
This experiment involve the study of flow past a circular cylinder in a uniform stream. The
flow past a two-dimensional cylinder is one of the most studied of aerodynamics. It is relevant
to many engineering applications. The flow pattern and the drag on a cylinder are functions of
the Reynolds number ReD = U∞D/µ, based on the cylinder diameter D and the undisturbed
free-stream velocity U∞. Recall that the Reynolds number represents the ratio of inertial to
viscous forces in the flow.
At the leading edge of the cylinder a stagnation point is formed where the oncoming flow is
brought to rest. The pressure here is equal to the stagnation pressure. The pressure
coefficient Cp = (p - p∞)/(½ρU∞2) is therefore equal to unity ( 1 ) by Bernoulli's equation. To
either side of the stagnation point the flow accelerates around the forward surface of the
cylinder producing a drop in the pressure.
2. Apparatus, Instrumentation and Methods
A. Instrumentation for measuring the properties of the air
The wind tunnel used in this experiment uses the laboratory atmosphere as the working fluid.
The properties of the air in the lab vary depending on the weather so it is important that you
measure them. From the point of view of the dynamics of the air, the important properties are
its density and viscosity (think of Bernoulli's equation and the Reynolds number). Rather than
measuring density directly, it is best obtained by measuring pressure and temperature and then
using the equation of state for a perfect gas. The gas constant R in the equation of state for a
perfect gas (p =ρRT) is 287 J/kg/K.
Free stream velocity can be calculated with the below equation which is derived from
Bernoulli equation.
𝑉= √
2(𝑝𝑠𝑡𝑔 − 𝑝∞ )
𝜌
B. Wind-tunnel and circular cylinder model
The experiment will be performed in the subsonic wind tunnel. the cylinder model is built
from Plexiglas. It has a diameter D of 50mm and a span of 75mm. Wind tunnel test section is
75 mm x 300 mm x 455 mm (width x height x length ). The cylinder model is mounted
spanwise across the test section. The mount allows the cylinder to be rotated about its axis by
a measured angle (indicated by the attached protractor).
C. Instrumentation for measuring the pressure distribution on the cylinder surface
you will need to measure the surface pressure distribution on the circular cylinder or, more
specifically, the distribution of surface pressure coefficient. The pressure coefficient is
defined as
Cp = (p - p∞)/(½ρU∞2)
where p representing the pressure at the cylinder surface. The cylinder is instrumented with
one one-millimeter diameter pressure taps at at mid span. These sense the surface
pressure p and transmit it through a tube to the outside world. The pressure p can be obtained
using the hydrostatic equation
p=ρwatergh
To form the numerator of the pressure coefficient it is then necessary to measure p∞ and
subtract it from these readings.
3. Ideal flow model of flow past a circular cylinder
Of particular interest here is the pressure coefficient distribution round cylinder. The
distribution is predicted by the theory, given by the expression Cp=1-4sin2θ, where θ is angle
measured from the back of the cylinder as shown in following figures and also will be ploted
as a result of this experiment. the theoritical pressure distribution is unrealistic in a number of
ways. However, in this experiment you will have an opportunity to make comparison between
the theoritical pressure distribution and experimental pressure distribution.
4. Technical Report
Introduction
The introduction should contain a brief discussion of the objectives and motivation for the
experiment. A concise description of pertinent background information, such as cylinder flow
patterns. In addition, the key assumptions, equations, and variables used in the analyses
should be described briefly, without in-depth derivation or excessive detail.
Methods
A methods section must be included with an overview of the procedure and at least one
schematic of the experimental apparatus. An important part of engineering is being able to
describe a process with a picture or drawing. Do not scan the figures from the manual for this
section.
Experimental Results and Discussion
This is the most important section of the report. At a minimum, the results should include the
following plots, charts, and tables:
a. Determine the surface pressure coefficient, Cp as a function of θ
b. Plot Cp vs. θ, as well as the inviscid theory result. In ONE plot, you should show two sets
of data and make comparison
c. Graphically compare results for positive and negative θ
d. Investigate the separation region and separation angle
Figure.1 Definition of symbols
Air temperature: ...........................................
Barometric pressure: ....................................
Air density (p =ρRT): ...................................
Stagnation pressure ( pstg ):............................
Static pressure ( p∞ ):......................................
Table. Pressure distribution round the cylinder
θ
[degree]
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
100
110
120
130
140
150
160
170
180
p - p∞
[N/m2]
Cp = [p - p∞)/(½ρU∞2]
θ
[degree]
0
-5
-10
-15
-20
-25
-30
-35
-40
-45
-50
-55
-60
-65
-70
-75
-80
-85
-90
-100
-110
-120
-130
-140
-150
-160
-170
-180
p - p∞
[N/m2]
Cp = (p - p∞)/(½ρU∞2)
Conclusion: