STUDY OF MICROBUBBLES FOR SKIN-FRICTION DRAG REDUCTION. *

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1 STUDY OF MICROBUBBLES FOR SKIN-FRICTION DRAG REDUCTION Yan Yao 1 *, Jin-ling Luo 1,Kun Zhu 1, Hai-bo He 1, Rui Wu 2, Shi-jie Qin 3 1 Beijing Electromechanic Engineering Institute, Beijing, , China 2 Shanghai Ship and Shipping Research Institute, Shanghai , China 3 Institute of Process Equipment, Zhejiang University, Hangzhou , China * yaoyanyy@163.com ABSTRACT: The main objectives of the present study are to visualize a bubbly turbulent boundary layer and to investigate the role of the bubbles in frictional drag reduction. The turbulent boundary layer is formed on the surface of a 2-D flat plate. The behaviors of the microbubbles are visualized quantitatively by using the high speed camera. The skin-friction drag are measured by using balances. The influences of the drag reduction performance with the gas flow rate, free-stream velocity, buoyancy are analyzed. It shows that the microbubbles is effective in reducing the skin friction in high Reynolds number regime of It proves that microbubbles can significantly reduce skin-friction of the solid wall and the drag reduction radio up to 36%. The results showed that when the gas flow rate increases, the skin-friction reduction ratio increases. KEY WORDS: Microbubbles, Turbulent boundary layer, Skin friction, INTRODUCTION Skin-friction drag is an important component of resistance in nearly all transportation systems moving in a fluid.the interest in energy saving devices and technologies for use in marine vehicles is increasing. There have been many research attempts to reduce the energy consumption of marine vehicles and to increase their speed, for which decreasing the hydrodynamic drag is crucial. Skin friction is a hydrodynamic drag known as viscous drag because it is most influenced by the fluid viscosity and increases with the fluid flow velocity. Thus, skin-friction reduction techniques have been investigated for several decades. There are passive and active methods for reducing the skin friction occurring on the surface of a moving body. Both passive and active means to reduce the skin-friction of a fluid flowing near a solid surface are of interest in a variety of marine, hydraulic and aerospace applications. Fully passive friction-reduction methods that function without ongoing expenditure of energy include applying riblets to the surface, adding compliance to the surface, and shaping and polishing the surface to maintain laminar flow to the greatest possible downstream extent. Passive methods have attracted much attention because they do not require additional energy. Although active methods do require additional energy to perturb the turbulent flow structure or boundary layer, their drag reduction effects are not small compared to those of the passive method. Active methods such as injecting bubbles into the boundary layer have been tested with the aim of decreasing the fraction of the wetted area or of changing the effective viscosity and other properties of the boundary layer. Microbubbles are perhaps the cheapest and non-polluted drag reducer. Various studies have examined drag reduction via the microbubbles injection method. Many experimental results have shown that microbubbles can reduce the frictional drag of a turbulent boundary layer by 20% - 80% [1-3]. McCormick & Battacharyya showed promising results for skinfriction drag reduction. Net drag reduction approaching 40% was observed on a body 0.91m

2 in length at flow speeds to 2.6m/s [4]. In another early study, Bogdevich & Evseev observed that drag reduction peaks immediately downstream of the point of injection; however, further downstream, skin-friction returned to its usual level [5]. Since then, numerous experiments have been conducted that investigated the parameters influencing drag reduction. Kodama et al. reported that drag reduction measurements made in the spanwise direction showed poor uniformity, decreasing from a maximum at the model centerline to nearly zero at the outer edges. This observation agrees with the concept of gas escaping from underneath the test models [6]. Hassan and Gutierrez-Torres studied the drag reduction mechanism to elucidate the influence of microbubbles within the boundary layer, and reported that increases in the microbubble concentration contribute to decreases in the Reynolds stress and turbulence production in the boundary layer [7]. Jacob et al. also investigated the frictional drag reduction produced by microbubbles with diameters less than 100 m, which corresponded to the local Kolmogorov length scale within the turbulent boundary layer studied [8]. In their study, by measuring the concentration of microbubbles and characterizing their behavior with the particle image velocimetry technique, they found that microbubbles decrease the Reynolds stress and change the flow velocity gradient in the turbulent boundary layer. Hara et al. applied image analysis to the study of frictional-drag reduction by microbubbles in a turbulent channel flow and found that oscillatory motion in the vertical direction reduces the Reynolds stress near the wall, and that this effect is more relevant upstream than downstream [9]. Fukagata et al. reported the near-wall Reynolds stress was critical for the prediction and control of wall turbulence affecting skin friction by theoretical method [10]. Ferrante and Elghobashi studied the Reynolds number effect in a spatially developing turbulent boundary layer with the direct numerical simulation (DNS) method [11]. They reported that an increase in the momentum thickness Reynolds number squeezes the quasi-streamwise vortical structures toward the wall, whereas the microbubbles push them away from the wall. Previous many studies of these experiments were performed for Reynolds numbers (based on channel height) of 3000 to 5000, and the turbulent boundary layer has been simulated asymptotically at rather low flow velocities. In the present study, the experiments were conducted in a medium-sized cavitation tunnel to achieve high Reynolds numbers of approximately 10 6 (based on the free-stream velocity and the length of model). The skinfriction drag was measured by using the strain balance. Moreover, the high-speed photography technology are also utilized to obtain field-of-view in order to observe the flow structures. The microbubble concentration (gas flow rate) was found to be the dominant influence on frictional drag reduction in the turbulent boundary layer. EXPERIMENTAL APPARATUS AND METHOD The drag measurements for the 2-D flat plate and the flow visualization of the turbulent boundary layer were carried out in the cavitation tunnel at SSSRI (Shanghai Ship & Shipping Research Institute), which is a square water tunnel with maximum speed up to 12 m/s. Size of the test section is 0.6 m 0.6 m 2.6 m. And the non-uniformity of water velocity is less than 1%; instability of water velocity is less than 1%. The static pressure in the test section was kept at atmospheric pressure in our test. A flat plate model was designed and manufactured to investigate the effects on drag of injected microbubbles. The material of the flat plate was LY12 aluminum alloy. The plates were supported by two struts to maximize their flatness and to minimize twisting or bending effects on the fluid flow, as shown in Fig. 1. The dimensions of the flat plate were 1510mm 305mm 40mm. Four skin friction force balances were flush-mounted at the locations X/L = and X/L = (X is the distance from the leading edge, L = the length of the flat plate) to measure the local skin friction. The strain friction balance was the type of underwater

3 with 0.2% accuracy. Each balance was instrumented with a full Wheatstone bridge of semiconductor strain gauges. The sensor outputs were amplified and low-pass filtered at 10 Hz. The output signal was recorded at 50 Hz with a National Instruments data acquisition board and a LabView virtual instrument. Real time data acquisition system is shown in Fig.2. Figure 1 Photo of the experimental model in the water tunnel Figure 2 Schematic diagram of the flat plate model in cavitation tunnel Nine slots were aligned in flow direction on each of the upper and lower surfaces of plate. Every slot has 25 holes with 1mm diameter to discharge the microbubbles into the boundary layer. The diameter of bubbles generated by injecting air into a turbulent boundary layer through array of holes depends on the mean shear stress at the wall. Fig. 3 shows a close view of the setup of the experiment, and Fig. 4 shows set up of the air-jet system. Fixing Strut Air Inlet Tunnel Wall Fixing Strut Top view Flow Balance Balance Side view Figure 3 Schematic diagram of the flat plate model in cavitation tunnel To elucidate the mechanism of the drag reduction produced by the microbubbles, we

4 investigated the behaviors of the microbubbles. Thus, flow visualization techniques were utilized to determine the physical properties of the microbubbles within the boundary layer formed at the flat plate wall. A high speed camera with a high time resolution was employed to capture bubble images. The high-speed camera set-up for bubbly flow visualization was established as in Fig. 3. This set-up consisted of a high speed camera (NAC Memrecam HX- 3), a macro lens (60 mm, AFNIikkor), and a lamp. The bubble images were captured with a frame rate of 6000 fps (6000 frames per second) and a spatial resolution of 1280 x 480 pixels. Figure 4 Set up of the air-jet system Lamp High Speed Camera Figure 5 High speed camera set-up RESULTS AND DISCUSSION Drag of the flat plate As known, the flow will change from laminar to turbulence as the value of Re increases. The classical view of the critical Reynolds number is believed in the rang of 3*10 5 to 3*10 6. The flow can be considered to be laminar the value of Re below 3*10 5, while the flow is turbulent the value of Re above 3*10 6. The alternative flow mode can be identified for specified situations when the value of Re lies in the range of 3*10 5 to 3*10 6. Then the drag force of the flat plate can be predicted according to the boundary layer theory [12], which gives: for laminar flow (1) and for turbulent flow (2) Where ρ andν stand for the density and dynamic viscosity of water respectively.

5 The experimental data and laminar and turbulent theoretical value of surface shear stress measurements, without air injection, are presented in Fig.6, which shows that the experimental data is in good agreement with that of the turbulent theoretical values. On the one hand, the result indicates that the flow has already turned into turbulence. On the other hand, the method of measuring the drag is demonstrated to be valid in the experiment Cf Laminar flow theoretical value Turbulent flow theoretical value Experimental data U(m/s) Figure 6 Skin friction coefficient of flat plate versus the flow velocity Drag reduction with bubble injection The local skin friction on the wall was measured when the microbubble of different the air-jet flow rate were injected into the wall boundary layer at various flow speeds. Figure 7 shows a comparison of the local skin friction reduction measured with and without microbubble injection. The drag reduction ratio (DR) is defined as follows: 30% % (3) 25% DR 20% 15% U=4m/s U=5m/s U=6m/s U=8m/s U=10m/s 10% 5% 0% Q(m 3 /s) (a) The upper and lower surfaces both with microbubble injection

6 40% 35% 30% 25% DR 20% 15% 10% U=4m/s U=5m/s U=6m/s U=8m/s U=10m/s 5% 0% Q(m 3 /s) 25% (b) Only the lower surfaces with microbubble injection 20% 15% DR U=4m/s 10% U=8m/s 5% 0% Q(m 3 /s) (c) Only the upper surfaces with microbubble injection Figure 7 Comparison of drag reduction ratio with microbubbles Figure 7(a) shows the local skin friction reduction for the upper and lower surfaces both with microbubble injection when Q a =0.007, 0.01, and m 3 s 1, U=4, 5, 6, 8, 10ms 1, where Q a is the volumetric flow rate of gas (m 3 s 1 ), U is the flow speed. As the flow speed is increased further, the reduction ratio gradually first increases and then decreases. But the reduction ratio increases with the flow rate of gas. Figure 7(b) shows the same information for only the lower surfaces with microbubble injection when Q a =0.0035, 0.005, 0.007, , 0.01, and 0.014m 3 s 1, U=4, 5, 6, 8, 10ms 1, The maximum reduction ratio was found to be approximately 36% at a flow speed of 4m/s~5m/s and air-jet flow rate of 0.014m 3 s 1. As the flow speed is increased further, the reduction ratio gradually decreases. The amount of microbubble in the boundary layer was considered to be reduced because the boundary layer s thickness was known to be getting thinner with the increase of flow speed. It can be seen the similar phenomenon, the reduction ratio increases with the flow rate of air. Figure 7(c) gives the same information for only the upper surfaces with microbubble

7 injection when Q a =0.0035, 0.005, 0.007, , 0.01, and 0.014m 3 s 1, U=4, 8ms 1. For the 4ms 1 conditions, the reduction ratio nearly to zero. It is contrast to the lower surface. It is reason that the gravitational buoyancy forced bubbles toward the test lower surface and away from the upper surface. This phenomenon is showed in figure 8. Figure 8 shows typical images of a bubbly flow captured with conventional visualization techniques at flow speed of U= 4 m/s, and the flow rate Q a =0.014 m 3 s 1. It was impossible to measure the bubble diameter distribution accurately in the test section. It was also noted that the appearance of the bubble cloud was not dependent on the free-stream velocity. Figure 8 Typical Bubbly flow image ( U= 4 m/s, Q a =0.014 m 3 s 1.) Figure 9 ~10 shows typical bubble images captured with high speed video camera in the present study. Figure 9 gives bubble images for only the lower surfaces with microbubble injection when Q a =0.0035, 0.007, 0.01and 0.014m 3 s 1, U=4 ms 1. As the flow rate of air is increased further, the continuous buoyancy-induced air films formation on the lower surface at this flow speed. The sizes and shapes of the bubbles in the near-wall flow indicate that bubble splitting is not dominant and that bubble coalescence must be more prevalent as bubbles move downstream. Skin-friction drag reduction was still achieved when sufficient bubbles were close to the surface. Figure 10 gives the same images for only the upper surfaces with microbubble injection when Q a =0.0035, 0.007, 0.01and 0.014m 3 s 1, U=4 ms 1. It is observed that buoyancy acts to move bubbles away from the solid surface and the size of the bubbles increases with the flow rate of gas. (a) Q a =0.0035m 3 s 1 (b) Q a =0.007m 3 s 1

8 (c) Q a = 0.01m 3 s 1 (d) Q a = 0.014m 3 s 1, Figure 9 Typical bubble images (the lower surfaces U= 4 m/s) (a) Q a =0.0035m 3 s 1 (b) Q a =0.007m 3 s 1 (c) Q a = 0.01m 3 s 1 (d) Q a = 0.014m 3 s 1, Figure 10 Typical bubble images (the upper surfaces U= 4 m/s) CONCLUSIONS The skin friction drag reduction resulting from the introduction of air bubbles into a flatplate turbulent boundary layer has been investigated in controlled experiments at Reynolds numbers and length scales more than an order of magnitude larger than prior laboratory studies of this phenomenon. Surface shear stress was measured along with the near-wall bubble characteristics. Conclusions are drawn as follows:

9 (1) Reductions in the frictional drag were measured with friction sensor. Microbubbles is effective in reducing frictional drag at high Reynolds numbers O(10 6 ) (2) The maximum reduction ratio was found to be approximately 36% at a flow speed of 4m/s~5m/s and air-jet flow rate of 0.014m 3 s 1 for only the lower surfaces with microbubble injection. (3) As the flow speed is increased further, the reduction ratio gradually first increases and then decreases for the upper and lower surfaces both with microbubble injection, gradually decreases for only the lower surfaces with microbubble injection, increases for only the upper surfaces with microbubble injection. It is reason that the gravitational buoyancy forced bubbles toward the test lower surface and away from the upper surface. (4)For lower surface, as the flow rate of air is increased, the buoyancy-induced air films are formed. The sizes and shapes of the bubbles in the near-wall flow indicate that bubble splitting is not dominant and that bubble coalescence must be more prevalent as bubbles move downstream. (5) For upper surface, it is observed that buoyancy acts to move bubbles away from the solid surface and the size of the bubbles increases with the flow rate of gas. REFERENCES 1.Madavan, N.K., Deutsch, S., Merkle C.L.(1984) Reduction of turbulent skin friction by microbubbles. Phys Fluids 27, Guin, M. M., Kato, H., Yamaguchi, H. et al (1996) Reduction of skin friction by microbubbles and its relation with near-wall bubble concentration in a channel. J Mar Sci Technol1, Kato, H. Miura, K., Yamaguchi,H. et al (1998) Experimental study on the microbubble ejection method for frictional drag reduction. J Mar Sci Technol 3, McCormick, M. E. & Battacharyya, R. (1973) Drag reduction of a submersible hull by electrolysis. Naval Engrs J. 85, Bogdevich, V. G. & Evseev, A. R. (1976) The distribution on skin friction in a turbulent boundary layer of water beyond the location of gas injection. Investigations of Boundary Layer Control (in Russian), Thermophysics Institute Publishing House, Kodama, Y., Kakugawa, A., Takahashi, T., Nagaya, S. & Sugiyama, K. (2002) Microbubbles: drag reduction mechanism and applicability to ships. 24th Symp. Naval Hydrodyn Hassan Y. A., Gutierrez-Torres C. C.(2006) Investigation of drag reduction mechanism by microbubble injection within a channel boundary layer using particle tracking velocimetry. Nuclear Engineering and Technology, 38, Jacob, B., Olivieri, A., Campana, E.F., Piva, R.(2010) Drag reduction by microbubbles in a turbulent boundary layer. Physics of Fluids,.22, Hara, K.i, Suzuzi, T., and Yamamoto F.,( 2011) Image analysis applied to study on frictional-drag reduction by electrolytic microbubbles in a turbulent channel flow. Experiments in Fluids, 50, Fukagata, K., Iwamoto, K. and Kasagi, N.(2002) Contribution of Reynolds stress distribution to the skin friction in wall-bounded flows. Physics of Fluids, 14, Ferrante, A., and Elghobashi, S.(2005) Reynolds number effect on drag reduction in a microbubble laden spatially developing turbulent boundary layer. J. of Fluid Mechanics, 543, Schlichting H.,Gersten K. and Krause E. et al. (2000) Boundary-layer theory. 8 th Edition, New York: Springer,2000.