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Control of Rough Wall Turbulent Boundary Layer

Info: 5284 words (21 pages) Dissertation
Published: 12th Oct 2021

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Abstract

The turbulent boundary layer is an active area of research considering its scope in the aerospace industry. Extensive research has already been performed to investigate the turbulent boundary layer over a smooth wall, wall suction is a technique developed to control the turbulent boundary layer over a smooth wall. In this experiment, the mentioned technique is applied to the rough wall to control the turbulent boundary layer.

The wind tunnel setup used in this experiment is previously used for projects related to boundary layer research. Here the roughness in the form of the cylindrical rods is introduced over a wall, the wall suction is performed through the porous strip and the measurements are performed using the hot-wire anemometry.

The objective here is to use the wall suction method to delay the formation of the turbulent boundary layer and relaminarize the initial region of the turbulent boundary layer.

Three sets of readings were taken, first set; without using wall suction and another two sets with the different rates of wall suction. The difference between the patterns of the behaviour of the boundary layer for each case is discussed in the results section.

Keywords: Boundary layer, Turbulent boundary layer, Rough wall, Hot-wire anemometry, Wall suction.

Table of Contents

1. Introduction........................................................................................................ 1

2. Experimental Method......................................................................................... 2

2.1. Wind Tunnel....................................................................................................... 2

2.2. Roughness Strip................................................................................................. 4

2.3. Porous Strip........................................................................................................ 4

2.4. Suction Blower..................................................................................................... 4

2.5. Rough Wall.......................................................................................................... 4

2.6. Hot Wire Anemometry.......................................................................................... 5

3. Results and Discussion:...................................................................................... 6

3.1. No Suction:.......................................................................................................... 6

3.2. Suction rate: 25 Hz.............................................................................................. 7

3.3. Suction rate: 50 Hz............................................................................................ 8

3.4. Comparison:....................................................................................................... 9

4. Conclusion:.......................................................................................................... 10

5. Future Work:......................................................................................................... 11

6. Appendices........................................................................................................... 12

6.1. Appendix A:......................................................................................................... 12

6.2. Appendix B:......................................................................................................... 13

7. References............................................................................................................ 16

Table of Figures

1. Schematic of the wind tunnel and Cartesian coordinate system Corresponding to the grid and working section............................................ 3

2. Sketch of the tunnel working section with the suction strip and the plenum chamber mounted on the tunnel floor......................................... 4

3. Schematic diagram of cylindrical Roughness elements mounted on a wall (Side View)...................................................................................... 5

4. Schematic diagram of cylindrical Roughness elements mounted on a wall (top View)....................................................................................... 5

5. Y vs U with no suction…………………………………………………………….... 6

6. Y vs U with suction rate 25Hz……………………………………………………… 7

7. Y vs U with suction rate 50Hz……………………………………….……………... 8

8. Y vs U Comparison………………………………………………………………..... 9

9. Nomenclature of the BL……………………………………………………………… 12

1. Introduction

Controlling the turbulent boundary layer (TBL) is very important to reduce the occurrence of turbulence, so that drag can be minimized to increase efficiency. The external wall-bounded flows with zero pressure gradient are considered for this experiment. The wall suction method developed for the control of the TBL over a smooth wall is applied to the rough wall with the objective of the relaminarization of the TBL.

Here in this experiment, the objective is to perform the wall suction operation on the rough wall, considering the cylindrical roughness elements. Here, with the help of the hot wire probe method, the measurements of the free stream velocity are performed. The velocity profile of the free stream velocity (U) is plotted against the distance(Y). The wall suction is used with the high-pressure suction blower. The first set of measurements was taken without any wall suction. The second set of measurements was performed with the wall suction rate of 25 Hz, and the next set was achieved using the wall suction rate of 50Hz.

The major part of the experimental setup in this report is adopted from the "Effects of wall suction on a 2D rough wall turbulent boundary layer" by Djenidi, Kamruzzaman and Dostal (2019) and "On The Effects of Non-Homogeneity On Small Scale Turbulence" by Kamruzzaman (2016).

2. Experimental Method

2.1. Wind Tunnel

The wind tunnel setup used for this experiment was previously used to analyze the effect of wall suction on the smooth wall TBL by Oyewola et al. (2003) and also to investigate the effects of wall suction on a two – dimensional rough wall TBL by Djenidi, Kamruzzaman, and Dostal (2019).

The fan with a power requirement of 15-kW is used, the fan has the capability of providing free stream velocity (Uꝏ) up to 40 m/s. the size of the settling chamber is defined to be 1.6 × 0.9 m2, which is subjected to the air from the fan, which flows through two – dimensional diffuser before entering the settling chamber. The settling chamber consists of six wire mesh screens and also an aluminum honeycomb with a thickness of up to 5mm. The air reaches the contraction section before the working section, the 9.5: 1 contraction section is defined two be two – dimensional. The working section measurements are 0.16m in height and 0.825m in width. Considering the future work scope with the variations of the pressure gradient the roof of this tunnel is kept adjustable, for this experiment we are working with zero pressure gradient (Djenidi, Kamruzzaman and Dostal 2019).

Figure 1. Schematic of the wind tunnel and Cartesian coordinate system Corresponding to the grid and working section (Kamruzzaman 2016).

Figure 2. Sketch of the tunnel working section with the suction strip and the plenum chamber mounted on the tunnel floor (Djenidi, Kamruzzaman and Dostal 2019).

2.2. Roughness Strip

The coarse sand grain paper is used here as the roughness strip, at the inlet, the flow is tripped using this 100 mm strip. Fig. 2 shows the location of roughness strip.

2.3. Porous Strip

The 35 mm porous strip with pore sizes of 40 to 80 μm is placed at the 1.2m downstream of the roughness strip mentioned above. This strip is the section of the sintered bronze- mounted flush with the tunnel floor and spans the entire width of the tunnel floor. (See fig. 2). This strip is fitted just above the blower.

2.4. Suction Blower

7.5 kW high-pressure DC suction blower, this blower is controllable and provides various suction rates as per the requirements. The control box of the motor allows the frequencies from 0 to 50 Hz.

2.5. Rough Wall

The circular rods are used to form a rough wall, which is placed at an equal distance of 24mm from each other, over the entire length of the working section. The diameter of the rods is denoted as 'k' which is defined to be 1.6mm. The λ is defined as the wavelength between the two consecutive rods. The rods are placed in such a way that the ratio of λ / k is 15. The location of the first rod is at 120mm from the trailing edge. The λ / k is 15 to ensure the fully rough development of the TBL (Djenidi, Kamruzzaman and Dostal 2019). Fig. 3 shows the arrangement of the cylindrical rough rods.

Figure 3. Schematic diagram of cylindrical Roughness elements mounted on a wall (Side View).

Figure 4. Schematic diagram of cylindrical Roughness elements mounted on a wall (Top View).

2.6. Hot Wire Anemometry

In this experiment, Hot-Wire anemometry is used for the measurements. The hot wire anemometry is consisting of a fine electrically heated wire called a Wollaston wire, which is composed of internal wire in platinum or platinum 10 A% Rhodium heated above the ambient temperature (generally 20°) (Kamruzzaman 2016).

The modified single wire probe is used, Dantec 55P15 sensor and has a spacing of 1.5mm with a 2.5 μm diameter wire soldered between prongs. The hot wire is calibrated in situ against the pitot-static tube positioned in the undisturbed free stream flow at 16 different speeds which are in the range of 0 to 22 m/s. this is repeated before and after every experiment. (Djenidi, Kamruzzaman and Dostal 2019).

3. Results and Discussion:

This is the initial stage of the experiment, after understanding the experimental setup, to understand the effect of wall suction over a rough wall. Measurements are performed with using the hot-wire anemometry. Three sets of measurements were performed.

  • Without using wall suction.
  • With using wall suction (25Hz).
  • With using wall suction (50Hz).

The graphs plotted are, distance(Y) in mm (along y-axis) against the free stream velocity (U) in m/s (along x-axis).

(Note: for better understanding of the graphs please refer to the nomenclature of the BL.)

3.1. No Suction

Figure 5. Y vs U with no suction.

Fig. 5. Shows the formation of the graph of velocity 'u0' as a change in the distance along the y-axis, away from the wall. Here the measurements are performed without using the wall suction. The nominal limit of the BL is drawn from the topmost data point of the 'u0' graph. The velocity at the wall is 0 m/s, this is called as 'No-slip condition'. As the distance along the y-axis has increased the velocity is also increased. The maximum velocity, in this case, is 5 m/s. Since these measurements were performed without using the wall suction this is the highest the nominal limit of BL can get at this particular set of free stream velocity.

3.2. Suction rate: 25 Hz

Figure 6. Y vs U with suction rate 25Hz.

Fig. 6. Shows the formation of the graph of the velocity 'u0' as a change in the distance along the y-axis away from the wall. Here, the measurements are performed with the use of wall suction with the intensity of 25Hz. The effect of the suction can be seen as the data points are more inclined to the horizontal axis as compared to fig. 5.

3.3. Suction rate: 50 Hz

Figure 7. Y vs U with suction rate 50Hz.

Fig. 7. Shows the formation of the graph of the velocity 'u0' as a change in the distance along the y-axis away from the wall. Here, the measurements are performed with the use of wall suction with the intensity of 50Hz. The effect of the suction can be seen as the data points are more inclined to the horizontal axis as compared to fig. 6. And the bigger difference can be found as compared to fig. 5. The effect can be observed where the data points are getting closer to the x-axis.

3.4. Comparison

Figure 8. Y vs U Comparison.

When the three graphs are put together, the effect of the wall suction operation can be observed. The blue data points show the 'u0' velocity graph where there is no suction. The original BL can be drawn from the topmost data point. As for where the 'u0' graph with the suction rate of 25 Hz clearly shows the effect of suction, as the data point pattern is more inclined to the horizontal axis, it delays the formation of the TBL and provides suitable time for the laminar boundary layer. This means as the initial region of the TBL is relaminarized with the use of suction. The next measurement is performed with the suction rate of 50 Hz, and more inclination towards the horizontal axis is found, here it can be said that more region is relaminarized than the last result.

4. Conclusion

The hot wire anemometry is used to take the measurements in the rough wall TBL. All the measurements were performed with a free stream velocity of 5 m/s. The first set of readings was taken with no suction at all, and we get the original BL structure. The second set of readings was taken with a suction rate of 25 Hz, where the turbulent region of the BL is reduced by a significant area. The third set of readings was taken with the suction rate of 50 Hz, where the turbulent region of the BL is reduced by larger area, the relaminarization using the wall suction method is successful, and in the future, it will be tested on larger Reynolds numbers.

  • The wall suction has successfully lowered the 'u0' velocity graph.
  • The initial part of TBL was successfully relaminarized.
  • Needs to be tested on large Reynolds Numbers.

5. Future Work

In the second part of the experiment, more measurements with an increase in the free stream velocity, suction rate, and Reynold's number will be performed.

6. Appendices

6.1. Appendix A

Figure 9. Nomenclature of the BL (Boundary Layer Basic 2019).

[Note : for this case 'u' is 'u0'.]

6.2. Appendix B

This includes the readings, for all three cases.

No Suction

No suction

y (mm)

Velocity (m/s)

0.2

0.7917

0.3

0.8494

0.4

0.9181

0.5

0.9803

0.6

1.0420

0.8

1.1360

1

1.2449

1.2

1.3402

1.5

1.4837

1.8

1.6181

2.2

1.7967

2.6

1.9553

3.1

2.1140

3.8

2.2796

4.5

2.4069

5.5

2.5502

6.5

2.6632

7.7

2.8030

9.2

2.9389

11

3.1104

13.1

3.2890

15.7

3.4862

18.7

3.7159

22.2

3.9550

26.8

4.2454

31.5

4.5275

37.7

4.8062

44.7

4.9749

53.1

5.0202

63.5

5.0116

75.4

4.9920

89.4

5.0053

106.1

5.0095

Suction rate: 25 Hz

25 Hz Suction Frequency

y (mm)

Velocity (m/s)

0.2

1.0526

0.3

1.1179

0.4

1.2020

0.5

1.2789

0.6

1.3376

0.8

1.4645

1

1.5940

1.2

1.7167

1.5

1.9195

1.8

2.1109

2.2

2.3406

2.6

2.5525

3.1

2.7437

3.8

2.9259

4.5

3.0638

5.5

3.2117

6.5

3.3268

7.7

3.4226

9.2

3.5221

11

3.6300

13.1

3.7479

15.7

3.9021

18.7

4.0704

22.2

4.2302

26.8

4.4304

31.5

4.5595

37.7

4.6443

44.7

4.6659

53.1

4.6821

63.5

4.7061

75.4

4.7253

89.4

4.7540

Suction rate: 50 Hz

50 Hz Suction Frequency

y (mm)

Velocity (m/s)

0.2

0.9103

0.3

1.0074

0.4

1.1033

0.5

1.1886

0.6

1.2712

0.8

1.4144

1

1.5835

1.2

1.7421

1.5

1.9998

1.8

2.2449

2.2

2.5599

2.6

2.8084

3.1

3.0368

3.8

3.2395

4.5

3.3922

5.5

3.5392

6.5

3.6517

7.7

3.7284

9.2

3.8226

11

3.9090

13.1

4.0285

15.7

4.1614

18.7

4.3122

22.2

4.4129

26.8

4.5179

31.5

4.5786

37.7

4.6164

44.7

4.6313

53.1

4.6618

63.5

4.6805

75.4

4.6765

89.4

4.7136

106.1

4.7383

7. References

Boundary Layer Basic 2019, Viewed 30 October 2019, http://www.examhill.com/boundary-layer-basic/

Djenidi, L., Kamruzzaman, M. and Dostal, L., 2019. Effects of wall suction on a 2D rough wall turbulent boundary layer. Experiments in Fluids, 60(3), p.43.

Kamruzzaman, Md., 2016. On The Effects of Non- Homogeneity On Small Scale Turbulence. The University of Newcastle NSW, Australia.

Oyewola, O., Djenidi, L. and Antonia, R.A., 2003. Combined influence of the Reynolds number and localised wall suction on a turbulent boundary layer. Experiments in fluids, 35(2), pp.199-206.

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