Building and calibrating an open circuit wind tunnel with

visualization and measuring turbulent characteristic of flow and

their dependence on screens and honey comb

KAMYAR MANSOUR, ALI KARGAR

Department of aerospace engineering

Amirkabir University of Technology (Tehran polytechnic)

Tehran , Iran, 15875-4413

mansour@aut.ac.ir, tameazadi@aut.ac.ir

Abstract: -This paper is focus on design, building and calibrating a small open circuit wind tunnel and

measuring turbulent characteristic of flow such as kurtosis, skewness , standard deviation, turbulence

intensity and variance of flow by hot wire anemometer and dependence of them on velocity and screen

meshes and honey comb in the wind tunnel. This wind tunnel specially built for stability and transition

studying around rotating bodies, so we built a smoke generator in order to show the transition region on

the boundary layer of models. This paper includes some pictures that were made by flow visualization on

rotating bodies. In this paper we also measure pressure coefficient around a cylinder and drag coefficient

around a sphere and compare our result by some another same experiments, also we visualize flow around

sphere in Reynolds 11500.

Key-Words: - open circuit wind tunnel, turbulent characteristics, flow visualization, calibrating, pressure

coefficient, drag coefficient, cylinder, sphere, turbulence intensity.

1 Introduction

When the first real scientific investigations

into the fledgling field of aeronautics,

scientists hoping to achieve heavier than air

flight soon realized that they would need to

understand airflow dynamics about an airfoil

in order to design a practical wing. In order to

do this, they would need to reliably measure

forces acting on a body passing quickly

through the air. Until the early 1700s, natural

wind sources such as high ridges and the

mouths of caves were used for early testing,

but these proved to be inadequate, and so a

mechanical means of creating airflow was

invented, the so-called whirling arm

apparatus. An English mathematician named

Benjamin Robins is credited as being the first

to use a whirling arm for aeronautical study.

The apparatus was driven by falling weights

attached via a system of ropes and pulleys to a

long arm, which rotated horizontally about a

spindle. Test bodies were attached to the tip of

the arm and could be positioned so as to

obtain varying angles of attack. Robins’ first

whirling arm was four feet in length, and the

tip reached speeds of only a few feet per

second, but longer arms could obtain speeds of

up to 20 feet per second.in designing we use

Bradshaw&Mehta “design rules for small low

speed wind tunnels”[ref. 1],Anatol roshko in

his paper [ref.3] has a good and certain figure

about pressure coeff. Around cylinder,

Giancarlo Bruschi &Tomoko Nishioka&Kevin

Recent Researches in Mechanical Engineering

ISBN: 978-1-61804-153-1 138

Tsang&Rick Wang in their paer [ref.4] are

measured pressure coeff. Around cylinder and

we validate our result by them,also we validate

our drag measurement around sphere by

Frederick W. Roos & William W. Willmarth,s

paper[ref.2].

2 Design and manufacturing

we design the wind tunnel basically to

studying instability problems so we must have

a low turbulence wind and we must could

rotate the model in the test section and we

need visualization and hot wire anemometer to

studying instability problems, so we design

the wind tunnel suction base

with e long settling chamber, to building the

body we use stainless steel sheets with 1.5 mm

thickness, we use iron to building work bench,

we use no rigid connection between the parts,

all of the parts are connected together with

dampers, also the part that include the fan

completely is apart from another parts to

propagate no vibration from the fan into the

body of wind tunnel, at the finish we painting

the body of wind tunnel with the black epoxy

color, so we reach a bright and smooth surface

in the wind tunnel.

Picture 4 Picture 3

Picture 2 Picture 1

Recent Researches in Mechanical Engineering

ISBN: 978-1-61804-153-1 139

Screens are typically located just downstream

of the honeycomb and sometime at the inlet of

the test section. Screens create a static

pressure drop and serve to reduce boundary

layer size and increase flow uniformity. At

least one screen in the settling chamber

(ideally the last) should have an open-area

ratio of β<0.57, as screens with lower ratios

are known to produced nonuniformities in the

flow. This is presumable due to the formation

of small vortices created by the random

coalescence of tiny jets emitted from the

screen. The pressure drop across a screen

depends upon the open-area ratio of the screen

and the density, kinematic viscosity, and mean

velocity of the fluid.in this paper we use 5

conditions (2 screen with β=0.32 and honey

comb and 2 screens with β=2.5 , 1 screen with

β=0.37 and honey comb and 2 screens with

β=2.5 , 2 screen with β=2.5 and honey comb ,

honey comb only , nothing in settling

chamber) and we measure some turbulent

characteristics of flow and compare them with

together, design of this is suction base and the

dimensions are :

Overall length = 4 m

Test section length = 80 cm

Test section area = 40 * 40 cm

Settling chamber area = 50*50 cm

Honey comb : 6*6 mm cells ,40 mm depth

Fan : variable speed 70 cm diameters and 1.5

kw power

3 Measurements and results : we

measure the skewness, kurtosis and turbulence

intensity for 5 different stages, first for nothing

in settling chamber, stage 2 is only using honey

comb in settling chamber, stage 3 is 1 honey

comb with 2 screens with β=2.5,stage 4 is honey

comb with 1 screen with β=0.38 and 2 screens

with β=2.5 and the stage 5 is honey comb with 2

screens with β=0.38 and 2 screens with β=2.5,

measurement with hotwire anemometer are

shown that the energy loss of screens with

β=0.38 is very high ,because the maximum

speed when we have nothing in settling chamber

is about 19 m/s and this value when we use 1

screen with β=0.38 is 6.5 m/s, and their effect on

turbulence intensity is negligible, so using very

low open area ratio screens in our wind tunnel is

not economic.

4.1 Visualization and measuring drag : For visualization of flow a metal tank with

electric heating elements are used. Outlet of

the tank using a hose, into the hose we implanted two screens, Smoke flow regulator tuned such that the exhaust flow wind speed got equal the mean wind speed in the test re 7

Turb

ule

nce

inte

nsi

ty (

%)

Turbulence intensity-

velocity diagram

Figure 1: Turbulence intensity-velocity diagram

Recent Researches in Mechanical Engineering

ISBN: 978-1-61804-153-1 140

Picture 5 shows the flow visualization on a cone with apex angle 30 degrees that rotating with 1386 RMP and 1.5 m/s free stream velocity .It shows that boundary layer is fully turbulence on the cone surface., to study flow around sphere we used a billiard ball, its surface had been before polishing. Then we put it on the rod holder, rod holder is a rod with 6mm diameter and made of titanium, its end through a hinged connected to a digital load cell. Picture 7 shows the image of visualization of flow around it at Re=11500. The detection of the separation point in nearly 100 degrees shows. For detection of vortices behind the cylinder ,a cylinder with 6 mm in diameter were used, in

1.2 m/s mean speed velocity the von Karman vortex were well represented. But due to the fluctuation of the vortex need to install a high speed camera in our test section. Succeeded we could not getting a clear picture of the vortex, the drag coefficient in the range of our wind tunnel is measured, figure 9 is shown the drag history of our experiment and drag history of sphere that was measured by Frederick w. roos [ref.2], picture 8 shows flow visualization around a cone with 30 degrees apex angle rotating 50 RPM and free stream velocity 2.1 m/s, this shows that boundary layer in this case is laminar.

Picture 5

Picture 6

Picture 7

Picture 8

Recent Researches in Mechanical Engineering

ISBN: 978-1-61804-153-1 141

4.2 Measuring pressure coefficient :

For measurement of pressure coefficient around

the cylinder, a plastic cylinder with 28mm

diameter and 40 cm long were used, A pressure

transducer used for measurement of the

difference between the value of static pressure of

free stream and value of static pressure in some

points on the surface of cylinder, These

measurements are shown in Figure 5.we

compare our result by result of kevin tsang&rick

wang’s measurement [ref.4],Comparison of

pressure coefficient and drag history in these

chart shows our wind tunnel is consistent with

the standard wind tunnel.in measuring pressure

and drag coefficients we correct the values by

wall interface correction by allen & Vincent

[ref.6] equation :

(17)

(18)

Figure 3: drag history of sphere

(19)

(Present work)

Recent Researches in Mechanical Engineering

ISBN: 978-1-61804-153-1 142

To varying Wind speed, an inverter device used

to continuously change the input frequency of

motor ,Figure 4 shows the test section velocity

versus frequency.

5 Conclusion: our goal in this paper was

to measure the turbulent characteristics and their

dependence to honey combs and screens, also

we did build a wind tunnel and in order to show

that how is its performance. We did some

experiments in our own wind tunnel and validate

them with other experiments which were

previously published. We did also visualize the

flow around sphere and cone in empty settling

chamber. Some of calibrating and visualizing

figures are shown in this paper.

References:

[1] R.D.MEHTA & P.BRADSHAW, designe

rules for small low speed wind tunnels,

Aeronautical journal of the royal aeronautical

society, November 1979.

[2] FREDERICK W. Roos & WILLIAM W.

WILLMARTH , Some Experimental Results on

Sphere and Disk Drag, AIAA journal, VOL. 9,

NO. 2, FEBRUARY 1971

[3] ANATOL ROSHKO , Experiments on the

flow past a circular cylinder at very high

Reynolds number,J.fluid mechanics, 15

November 1960

[4] Giancarlo Bruschi &Tomoko

Nishioka&Kevin Tsang&Rick Wang, ,A

comparison of analytical methods drag

coefficient of a cylinder, march 21, 2003

[5] Nathan Tatman, Wind Tunnel Design and

Operation, thesis ,his mentor : Dr,Rhett.Herman

[6] ALLENH, . J. & VINCENT, Wall interference in

a two-dimensional-flow wind tunnel, with

consideration of the effect of compressibility,

Nut. Adv. Comm. Aero,1944

0

1

2

3

4

5

6

7

8

9

10

0 20 40 60 80

Figure 4: velocity versus frequency input of motor

Recent Researches in Mechanical Engineering

ISBN: 978-1-61804-153-1 143