INSTALLATION, CALIBRATION AND TESTING OF LOW SPEED WIND TUNNEL

INDEPENDENT PROJECT BASED COURSE IN791

Under Supervision of

DR. VINOD NARAYANAN

MECHANICAL ENGINEERING

Submitted By

RAVI KANT (13310029)

&

RAVI PATEL (13310028)

MECHANICAL ENGINEERING

TABLE OF CONTENT

1) INTRODUCTION …………………………………………………………………………………….3

2) COMPONENTS AND SETUP OF WIND TUNNEL………………………………….......4

3) EXPERIMENTAL MEASUREMENTS AND ANALYSIS ……...………………...........9

4) CFD SIMULATION OF WIND TUNNEL…………………………………………………….17

5) RESULTS AND DISCUSSIONS……………………………...…….............................20

6) CONCLUSION…………………………….........................…….............................22

1. INTRODUCTION

Wind tunnel is an apparatus used to study the flow behaviour and effects of air over the test solid object. In the tunnel, one can control the flow conditions which affect forces on the test object. Through the measurements of the forces on the model, one can predict the forces on the full scale test object. In early days the wind tunnels were used to understand and improve the performance of an aircraft only, but later several things such as, car body, buildings, bridges etc. are also being tested in the wind tunnel.

The following classification of a typical wind tunnel is done on the basis of working speed:

(a) Subsonic wind tunnel (M < 0.4)(b) High subsonic wind tunnels (0.4 < M < 0.75)(c) Transonic wind tunnel (0.75 < M < 1.2)(d) Supersonic wind tunnel (1.2 < M < 5)(e) Hypersonic wind tunnel (5 < M < 15)

FIG-1: TYPICAL BLOWER DIRVEN LOW SPEED WIND TUNNEL

2. COMPONENTS

(a) BLOWER

FIG-2: CAD REPRESENTATION OF BLOWER SECTION OF WIND TUNNEL

(b) BLOWER DIFFUSER

FIG-3: CAD REPRESENTATION OF BLOWER DIFFUSER OF WIND TUNNEL

(c) MESH SECTION

FIG-4: CAD REPRESENTATION OF MESH SECTION OF WIND TUNNEL

(d) BLOWER DIFFUSER

FIG-5: CAD REPRESENTATION OF REDUCER OF WIND TUNNEL

(e) TEST SECTION

FIG-6: CAD REPRESENTATION OF TEST SECTION OF WIND TUNNEL

(f) EXIT DIFFUSER

FIG-7: CAD REPRESENTATION OF EXIT DIFFUSER OF WIND TUNNEL

Assembly setup :

Fig-8: Assembly setup of wind tunnel in our campus IIT Gandhinagar

(a) FULL ASSYMBLY

FIG-9: CAD REPRESENTATION FULL ASSYMBLY OF WIND TUNNEL

FIG-10: 2D DRAFTING OF FULL ASSYMBLY OF WIND TUNNEL

3. EXPERIMENTAL MEASUREMENTS AND ANALYSIS

Objective of experiments were as follows:

(a) To find the δ ¿for given dimension of the flat plate inside the wind tunnel test section.(b) To check the uniformity of the flow at the inlet of the test section.(c) To calculate the velocity profiles at particular section of the test sections.(d) To calculate the variation of Cf with x for the test section.(e) To calculate the variation of θ (momentum thickness) with x for the test section.

(a) To find the δ ¿for given dimension of the flat plate inside the wind tunnel test section.

Given data specifications:

Sr.No Parameters symbol Value

1. Volume flow rate of blower Q 13500 m3/Hr

2. Cross section Area of the test section A 330mm X 330mm

3. Length of the flat plate L 1.5 m

4. Density of air at room temp ρ 1.205 Kg/m3

5. Kinematic Viscosity of air at room temp ϑ 15.68 x10-6 m2/s

Velocity calculation:

U avg=QA

= 13500m3 /Hr330×330mm2=34.43m / s

We know that, we have laminar region till ℜ=3.5×105. The location of x for region to be laminar is,

x= ℜ×ϑU

=3.5×105×15.68×10−6

34.43=0.159m

δ ¿ at this point would be calculated as (laminar),

δ ¿=1.72 x

(ℜx)12

= 1.72×0.159(3.5×105)1 /2

=4.63×10−4m

Initial δ ¿ from this point would be calculated as (turbulent), (with below 1/7th power law formula)

δ ¿=0.020 x

(ℜx)17

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.60

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

0.004

f(x) = − 0.00133301 x⁶ + 0.00738819 x⁵ − 0.0162979 x⁴ + 0.018249 x³ − 0.0110982 x² + 0.00581025 x − 0.000232834

turbulent ( )𝛿∗

turbulent boundry layer

x location (in m)

𝛿∗(in

m)

Fig -11: variation of δ ¿ with location x

Above graph is extrapolated back, till it cuts the x axis (at 0.0369m) (to get the new reference point x’ to be chosen),

x’ = 0.1221m (form origin)

Now δ ¿ is calculated from this new reference point.

The displacement thickness at the end of the plate.

δ ¿=0.020 x

(ℜx)17

=0.020×1.3779(3.29×106)1/7

=0.00323m

So, in order to compensate the flow,

We need to increase the height of 4 x 3.23 = 12.92 mm at the end of the section as shown in the figure below:

Fig 12: In order to compensate the flow, the given δ ¿ at respective location is shown over the flat plate.

(b) To check the uniformity of the flow at the inlet of the test section.- To chech uniformity at various speed, velocity at each point should be measured

at different speeds.- To track down exact position each time for velocity measurement travese

mechanism would be advisable. But it is costly and complex in installation.- A simple wooden frame has been suggested to accomplish same task. Easy in

installation and working.- Schematic of wodden frame is shown below. It consists of 9 rows and 9 columns,

results into 81 velocity measurement points.

Fig-13: wooden frame structure to calculate the velocities at each grid points at the inlet cross section of the test section

- Velocity is measured with U tube menometer with water as fluid. Pitot tube was fitted at the centre of cross to match with each position. Other end of manometer was held alingned with cross and stationary at the bottom of frame.

Fig 14– Placing of U tube manometer at cross centre

- Readings as a height of water column is converted to velocity by

h ρwater g=12ρairV

2

- Below table and plot shows velocity variation at each grid point.

Velocity Variation @ 15 HzGrid Point

s 1 2 3 4 5 6 7 8 9

1 11.10 11.78 11.78 11.4

6 11.10 11.80 11.12 10.3

9 11.12

2 11.10 11.10 11.78 11.1

2 11.10 11.12 11.12 11.1

2 11.12

3 11.10 11.10 11.10 11.1

2 11.10 10.39 11.12 11.1

2 11.12

4 11.10 11.10 11.10 11.1

2 11.10 11.12 11.12 11.1

2 11.12

5 11.10 11.10 11.10 11.1

2 11.10 11.12 11.12 11.1

2 11.12

6 11.10 11.10 11.10 11.1

2 11.10 11.12 11.12 11.1

2 11.12

7 11.10 11.10 11.10 11.1

2 11.10 11.12 11.12 11.1

2 11.12

8 11.10 11.10 11.10 11.1

2 11.10 11.12 11.12 11.1

2 11.12

9 10.74 10.74 11.10 11.1

2 11.10 11.12 10.39 11.4

6 11.12

Table No- 1: Variation of velocities over various grid point at the inlet section of test section

1 2 3 4 5 6 7 8 9

9.510

10.511

11.512

12

34

56

78

910

Velocity at various grid points

12345678910

grid point

Velo

city

(m/s

)

grid

poi

nt

Fig 15 – Velocity variation as a function of grid point for 15 Hz rotating speed.

- Above shown data represents average of 11.12 m/s with 0.22 standard deviation, which is 2% of average value.

- Similarly velocity readings were taken for 22.5 Hz and 30 Hz as well. Average velocity at 22.5 Hz was 16.29 m/s with 1.9% of standard deviation and at 30 Hz was 20.81 m/s with 1% of standard deviation. Percentage standard deviation decreases as expected at higher velocity.

(c) Velocity, δ*, Cf and θ measurement at different section of test section.- To have diferent velocity at test section three different operating frequencies

has beed selescted as operating frequency in Variable Frequency Drive. 15 Hz, 22.5 Hz and 30 Hz has been selected.

- To measure velocity, U tube manometer filled with water has used. One linb of manometer is connected to bottom of the required section and other linb is pitot tube.

- Pitot tube position is varied from plate (y=0) to upwards till constant reading (free stream).

Fig 16– Manometer and pitot tube connected to the test section

Fig 17– Static pressure taps at various locations

Fig 18 – Stagnation pressure taps at various locations (along with inserted pitot tube)

- Manometer setup at test section is shown in below schematic.

Fig 19– Schematic of u-tube manometer connected to test section

- As shown in figure, one limb of manometer is connected to pitot tube and other is connected to bottom hole. Thus resultant water column difference can be measured and eventually velocity can be found at any location.

- Measurement will be staty at bottom of section (while touching to plate). Increase pitot tube by 1mm and take measurement again.

- After some measurements, readings will attain a constant value. This indicates that probe has moved out of boundary layer and measuring free stream. Now one can move to next location.

- In initial region location for probes are pitched 5 cm. After 5 locations pitch changes to 10 cm. As boundary layer has changed from laminar to turbulent, 10 cm pitch will be sufficient.

- After measuring h from all location with height variation, velocity can be

computed by h ρwater g=12ρairu

2 .

- At every section, where u becomes constant, will be consider as free stream velocity and denoted as U. And that y location will be taken as δ, boundary layer thickness.

- Thus velocity can be converted to non dimensionalize form, u/U. By plotting u/U on x axis vs Vertical distance on y axis, we can see boundary layer profile.

- With the use of δ ¿=∫0

δ

(1− uU

)dy , one can compute δ* at each location.

- With the use of θ=∫0

δ uU

(1− uU

)dy , one can compute θ at each location.

- With the use of C f=2θx , one can compute Cf at each location.

4. CFD SIMULATION

CFD simulation is done in ANSYS to check the flow behaviour for the wind tunnel.Initial and boundary conditions for CFD Simulation are as follows:

Fig: Boundary condition definition

At x =0, Velocity inlet = 4.87 m/s

At x = 6.252 m , p = patm

At all exterior boundaries has no slip wall condition.

Fluid = Air

T ref = 300 K

μ = 1.789 x 10-5 (dynamic viscosity of air)

ρ = 1.2 kg/m3 (density of air)

Fig: Geometry (Fluid domain) along with the vanes

Fig: Mesh element type – TetrahedronNo of elements – 0.3 million

Fig: Velocity streamlines

Fig: Velocity vector plot at 3 locations in test sectionFor uniformity check (@ x= 3.5, 4, 4.5 m from origin

Fig: Pressure contour at the XY plane section of the wind tunnel

Fig: Velocity contour at the XY plane section of the wind tunnel

Fig: Velocity variation at the mid line(y=z=0 line) of the wind tunnel

5. RESULTS AND DISCUSSIONS

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

10

20

30

40

50

60

Variation of Cf with x

15 Hz 22.5 Hz 30 Hz

x (in m )

Cf co

-effi

ciet o

f ski

n fr

iction

Figure-20 : Cf variation with x location

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

Variation of θ with x

15 θ 22.5 θ 30 θ

x (in m)

θ (m

omen

tum

thick

ness

)

Figure- 21: θ variation with x location

Fig : 22-Boundary Layer Development along Test Section Length

Figure-23 : u/U variation with y

6. CONCLUSION

To compensate the flow reduction, We need to increase the height δ*= 12.92 mm at the end of the section.

Previously stated data represents average velocity at the inlet of the test section is 11.12 m/s with 0.22 standard deviation, which is 2% of average value. Similarly the standard deviation for 22.5 Hz (vel 16.29 m/s) was 1.9% and at 30 Hz (vel 20.81 m/s) was 1% .(as expected)

As depicted in above plots, we were getting the proper velocity profile for u/U at all the stations.

As show previously the variation of Cf is decreasing first, then increasing due to transition and again its decreasing as expected in downstream.