COMPARISON OF FLOW STRUCTURE BEHAVIORS AROUND A SPHERE AND ITS PASSIVE CONTROL IN A BOUNDARY LAYER FLOW

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1 International Journal of Arts & Sciences, CD-ROM. ISSN: :: 5(7):11 22 (2012) COMPARISON OF FLOW STRUCTURE BEHAVIORS AROUND A SPHERE AND ITS PASSIVE CONTROL IN A BOUNDARY LAYER FLOW Muammer Ozgoren, Abdulkerim Okbaz and Sercan Dogan Selcuk University Besir Sahin and Huseyin Akilli Cukurova University Comparison of the experimental results of fluid-structure interaction between a smooth sphere and a sphere with a vent hole, roughened, and an o-ring is presented when the sphere is placed in a developed turbulent boundary layer flow over a flat plate. Dye visualization and Particle Image Velocimetry (PIV) techniques were performed to examine effects of passive control methods on the sphere wake in open water channel for Reynolds number value of Re=5000 based on the sphere diameter with a 42.5mm. The sphere was embedded in a turbulent boundary layer with a thickness of 63mm which was larger than the sphere diameter of D=42.5mm. Wake region of the sphere were examined from point of flow physics for the different sphere locations in the ranged of 0 G/D 1.5 where G was the space between the bottom point of the sphere and the flat plate surface. The flow characteristics such as instantaneous velocity vectors, vorticity contours and time-averaged streamline patterns for both the smooth and passively controlled spheres were compared and discussed. The obtained flow patterns show that the effects of flow interference caused by the boundary layer are strong when the gap ratios are small. The rate of entrainment due to the high rate of circulatory flow motion of vortical flow structure in the boundary layer region is extremely high which results in causing unstable wake flow structures. For all spheres, the non-uniform velocity profile of the boundary layer flow causes a difference in the strength of the separated shear layers from the periphery of the sphere forming unsymmetry in the wake region. It is demonstrated that passive control methods singicantly affect the flow structure and delay the flow separation. The best control results are obtained when the o-ring is located in the front surface of the sphere. Keywords: Boundary layer, Passive control, PIV, Sphere, Turbulence, Wake. Introduction Flow around sphere has many engineering applications in single and two phase flows for various industries, gas tanks, artistic structures and some types of vehicles, sport ball and so on. Therefore, in designing such applications, it is necessary to obtain experimental data concerning the flow field. The presence of boundary layer flow over the plane wall introduces significant complications into the wake of the sphere due to non-uniform velocity profile, gap flow between the bottom section of the sphere and plate surface and also occurrence of the vorticity field in the 11

2 12 Muammer Ozgoren et al. boundary layer region. Active and passive flow control methods of boundary layer and wake region have been studied for decades. However, there have specially been only a few studies of flow structure and its control methods in the downstream of a sphere using the particle image velocimetry (PIV) technique. Details of these studies can be found in the studies as given below. Flow structures around a sphere in the boundary layer was studied by Tsutsiu (2008), Okamoto (1980), Ozgoren et al.(2011b), Ozgoren et al.(2012), Rashidi et al. (1990), Seban and Caldwell (1968), Hetsroni et al. (2001) and further investigations cited therein. When the sphere is exposed to uniform incoming flow condition, there are some studies performed by Ozgoren et al. (2011a), Hassanzadeh et al.(2011), Jang and Lee (2008), Yun etal. (2006), Leweke et al. (1999), Taneda (1978), Sakamoto and Haniu (1990), Achenbach (1974), Wu and Faeth (1993) and further investigations cited therein. Control of flow structure around a sphere for various active and passive methods was studied by Suryanarayana and Prabhu (2000), Ozgoren et al.(2011b, 2011c), Suryanarayana & Meier (1995), Kim and Durbin (1988), Kiya et al. (2011) and Sakamoto and Haniu (1990), Maxworthy (1969), Jeon et al. (2004), Alammar (2004) Bakic (2008), Choi et al. (2006), Kumar et al.(2008), Achenbach (1974) and Bearman & Harvey [28] and further investigations cited therein. The aim of this study is to evaluate quantitative and qualitative flow around smooth and passively controlled sphere at subcritical Reynolds number, Re=5000, in boundary layer flow effective region in the range of 0 G/D 1.5 by using PIV and dye methods. Experimental Setup and Instrumentation Experiments were performed in a large-scale open water channel with a test section length of 8000 mm and a width of 1000 mm at the Department of Mechanical Engineering at Cukurova University, Turkey. To perform the present experimental study, the test section made from 15 mm thick transparent Plexiglas sheet, which had a total height of 750 mm, was filled with water to a level of only 450 mm. Before reaching the test chamber, the water was pumped into a settling chamber and passed through a honeycomb section and a two-to-one channel contraction. An overview of the experimental system of the sphere is shown in Fig 1. The free-stream turbulence intensity of the flow is less than 0.5% in the range of the present Reynolds numbers, Re = ( U D)v, based on the sphere diameter. Here, ν and D are kinematics viscosity and the diameter of the sphere, respectively. U is the free-stream velocity taken as 118 mm/s. The sphere with a diameter of 42.5 mm was made of Plexiglas so that the laser light easily propagates through the sphere. The sphere surface was highly polished to avoid the effects of surface roughness. To fix the sphere in the water channel, a circular bar with a 5 mm diameter was connected to the sphere from the back surface of the sphere at the measurement plane in order to avoid support s effects while images were taken at the equator cross-section of the sphere. The disturbing effect of the support bar on the laser sheet location of the measurement plane that was observed by dye injection was negligible in the consideration of support diameter with respect to the sphere diameter. The solid blockage ratio of the sphere including support was 1.3 %. The Froude number based on the water depth h w was Fr = U gh =0.06 depending on w the free-stream velocity, which was subcritical flow region owing to the Froude number less than 1.0. A flat plate with dimensions of 2000mmx980mmx10mm having sharp leading edge is located over the bottom surface of the water channel. The hydrodynamically developed boundary layer was obtained by a tripping wire of diameter 5mm placed 80 mm downstream of the leading edge of the flat plate. The gap between the lower point of the sphere and the surface

3 Comparison of Flow Structure Behaviors Around a Sphere of the flat plate was changed from 0 to mm and normalized with the sphere diameter designated as G/D. The sphere was positioned at a distance of 1400 mm from the leading edge of the plate in order to allow development of a fully turbulent boundary layer. It was determined in the absence of the sphere that the resulting hydrodynamically developed boundary layer thickness is δ=63 mm at the center of the sphere location for Re=5000. The nominal thickness of the boundary layer δ was estimated from the velocity profile using the definition of one percent defect of the free-stream velocity. The ratios of the displacement δ * and momentum θ thicknesses relative to the boundary layer thickness were found to be δ * /δ = 0.18 and θ/δ = 0.13, respectively. The boundary layer shape factor H= δ * /θ is 1.42 which is very close to the well known range of 1.2<H<1.4 for fully developed turbulent flow. The Reynolds number ranges from 2500 to 10000, the results show that the ensemble-averaged velocity profiles exhibited shape factor variation of H= The ratio of ventilation hole to sphere diameter was 0.15, o- ring was located at 55 o with a 2mm from front stagnation point of the sphere and roughened surface was formed by means of totally 410 circular holes with a 3 mm diameter and around 2 mm depth in an equilateral triangle arrangement. Nd:YAG laser was used to generate a laser sheet that was perpendicular to the axis of the sphere and the symmetry axis (i.e. equator of the sphere) was passed through them. A CCD camera having a resolution of 1,600 x 1,186 pixels was used to record the images. The laser sheet was generated from a dual pulsed Nd:YAG system, having the maximum output of 120 mj per pulse, which had time delays t = ms for the present experiments. The suspended seeding particles with a diameter of 10 µm in the flow were silver metallic coated hollow spheres. The illuminating laser sheet thickness in the flow field was approximately 1.5 mm. As shown in Fig 1, the laser was mounted in a fixed position beneath the water tank while the camera was the right angle to the laser sheet. The high-image-density criterion was satisfied by ensuring that a minimum of approximately particles was contained within the interrogation area. Dantec Flow Grabber digital PIV software employing the cross-correlation algorithm was used to compute the raw displacement vector field from the particle image data. An interrogation window of 32x32 pixels in the image was selected and converted to grid size approximately 1.44x1.44 mm 2 for the sphere (0.034Dx0.034D). The overall fields of physical view had 7,227 (99x73) velocity vectors for whole taken images. During the interrogation process, an overlap of 50% was employed in order to satisfy the Nyquist criterion. Patterns of instantaneous particle images with a total of 700 images consisting of two separate 350 images in a continuous series were taken at the rate of 15 Hz to calculate the time-averaged patterns of the flow structure. The vorticity value at each grid point was calculated from the circulation around the eight neighboring points. Experimental setup

4 14 Muammer Ozgoren et al. (ii) Sphere models Figure 1. Schematic view of the experimental setup of PIV system, laser illumination for a sphere located in a boundary layer for smooth sphere, a sphere with a vent hole, roughened, and sphere with 2mm o-ring. Resuts and Discussion Figure 2 shows comparison of flow visualization images of instantaneous flow fields with laser illumination of Rhodamine dye injection technique around the smooth sphere (column I), roughened sphere (column II), sphere with 2 mm o-ring (column III) and vented sphere (column IV) located over a flat plate for different gap ratios at Re=5000. In the dye visualization representative images the small scale vortices are designated by A to G to show evolution and progress of them. The separated and recirculating flow in the near-wake region with the help of visualizing are clearly seen for the sphere with laser illumination using the Rhodamine dye injection technique in the near-wake region. Comparison of instantaneous velocity field V and corresponding instantaneous vorticity ω* for a smooth sphere (column I), roughened sphere (column II), a sphere with 2mm o-ring (column III) and a sphere with a vent hole (Column IV) for 0 G/D 1.5 at Re=5000 is shown in Figures 3 and 4. Instantaneous vorticity contours of the wake structure is normalized as ω* (i.e., ω*=ωd/u ). All figure dimensions are normalized with the appearance diameter of the sphere as x/d, y/d and G/D. Layers of positive and negative vortices are displayed with solid and dashed lines, respectively. For all cases of touching to the plate surface, the flow separates from only upper surface of the sphere in the visualization plane and small scale vortices in the wake region are formed around larger vortices with a wavy appearance due to Kelvin Helmholtz instability in the boundary layer. As the flow travels in the downstream direction, the dimensions of the vortices increase then these vortices are shed from the upper section of the sphere directly to the inward wake region and the separated flow reattaches on the flat plate. In the case of G/D=0.1, the gap flow between the bottom of the sphere and the flat plate highly affects the wakes of smooth and passively controlled spheres. The wake regions of all spheres move upward direction from the flat plate wall. For the gap ratio G/D=0.25, the wakes of smooth, roughened and vented spheres deflect upward whereas the wake of the sphere with 2 mm o-ring deflects downward due to the delayed flow separation from upper section of the sphere with 2 mm o-ring. As the gap ratio increases (G/D 0.50) the effect of gap flow between lower point of the sphere and the flat plate

5 Comparison of Flow Structure Behaviors Around a Sphere surface disappears in all sphere cases. However, the wake regions of the spheres still deflect upward direction. This situation occurs due to non-uniform velocity profile of the boundary layer. For the gap ratio range of 0.50<G/D 1.5, flow structures progress to become the similar conditions of the uniform incoming flow. The instantaneous velocity vector fields V and corresponding vorticity fields * observed in the measuring plane display the small and largescale waviness and rotate slowly around its axis while traveling in the downstream flow direction. The instantaneous velocity vector distributions, V, indicate that a flow with high magnitude of velocity vectors occurs along the shear layer. The velocity gradients appearing in the shear layer separating from the sphere surface increase with decreasing gap ratio. Welldefined small and large-scale swirling patterns of velocity vectors are evident in the wake. Shear layers emanating from the sphere create a complex flow field consisting of a number of vortices that move randomly in time and space. The location of reattachment point is not clearly defined by the instantaneous velocity vectors V, because the point of the reattachment moves forward and backward randomly due to the instability of the vortical flow structure. Therefore, timeaveraged streamline patterns <ψ> are calculated and shown in Figure 5. For G/D=0.25, as clearly seen in Figure 5, two recirculation regions with foci designated with F 1 and F 2 take place in the wake region of the smooth and roughened sphere cases. There is a tendency of the wake to move upward and the two recirculation regions become asymmetric about the sphere centerline. The lower recirculation region is wider in length and shorter in weight than the upper one due to the gap flow and the non-uniform velocity profile in the boundary layer. For the sphere with ventilation hole case two large-scale and one small scale recirculation regions form in the wake region with a deflection to upward. In contrast to other sphere cases, there is a deflection in the wake region of the controlled sphere with 2 mm o-ring through to downward direction. Due to the strong coupling between the boundary layer and the lower shedding shear layer of the sphere, the boundary layer deflects away from the wall forming a small downstream separation bubble with a size 0.5D in length (designated with F 3 for the smooth and roughened sphere cases and designated with F 4 for the ventilated sphere case). The concentrated secondary vortices travelling on the plate wall and lower pressure in the wake are the other reasons of the downstream separation bubble to form. The downstream separation bubble disappears when the sphere with 2 mm o-ring is used to control the flow. The flow structure in wake region of the sphere with 2 mm o-ring at 55 is significantly modified by passive control application and hence the distance between the sphere center and saddle point decreases as seen from streamline patterns <ψ> in Figures 4, as well as in Table 1. Therefore, it can be stated that the base pressure increases and this may be the reason of why the separation bubble does not form. For the higher gap ratios G/D 0.25, the two foci F 1 and F 2 in the recirculation wake region happen and they are asymmetric about the centerline of the sphere for the smooth sphere and roughened sphere cases. For larger gap ratios of G/D=0.5, 1.0 and 1.5, the effect of gap flow losses dramatically but the effect of the non-uniform velocity profile of the boundary layer still modifies the flow structure of the sphere for the smooth sphere and roughened sphere cases. For the sphere with 2 mm o-ring case,the streamline patterns form two recirculation regions which are reasonably symmetric about the centreline of the sphere similar to the case of uniform approaching flow conditions for the gap ratios G/D=1 and 1.5. For G/D=1.5, the streamline patterns form nearly symmetric about the centerline of the sphere, but only one additional small scale recirculation region forms that is above the centerline of the sphere. This may be because of non-uniform velocity profile of the boundary layer flow. As well known, separation on the surface of the plate occurs due to the streamwise adverse pressure gradient. This is the reason of

6 16 Muammer Ozgoren et al. occurring another small separation bubble on the plate surface in the wake of the sphere at G/D=0.25 for smooth, roughened and ventiled sphere cases. In other words, due to the strong coupling between the boundary layer and the lower shedding shear layer of the sphere, the boundary layer deflects away from the wall forming a small downstream separation bubble, F 3, at Re=5000 as shown in the third row of Fig 5. Thus, a small-sized focus takes place in close region of the sphere wake on the plate surface due to the occurrence of concentrated secondary vortices on the wall and lower pressure in the wake. For G/D>0.25, two recirculation regions occur in the wake region of the sphere for all Reynolds numbers. But there is a tendency to move upward of the wake of the sphere and the two recirculation regions are asymmetric about the sphere centerline. The upper recirculation region is slightly longer than the lower one in length whereas lower recirculation region is larger than the upper one in circulation shape and size. Figure 2. Comparison of flow visualization of flow structure with a laser illumination of Rhodamine dye injection technique for a smooth sphere (column I), roughened sphere (column II), a sphere with 2mm o-ring (column III) and a sphere with a vent hole (Column IV) for 0 G/D 1.5 at Re=5000.

7 Comparison of Flow Structure Behaviors Around a Sphere Figure 3. Comparison of instantaneous velocity field V for a smooth sphere (column I), roughened sphere (column II), a sphere with 2mm o-ring (column III) and a sphere with a vent hole (Column IV) for 0 G/D 1.5 at Re=5000.

8 18 Muammer Ozgoren et al. Figure 4. Comparison of instantaneous vorticity ω* for a smooth sphere (column I), roughened sphere (column II), a sphere with 2mm o-ring (column III) and a sphere with a vent hole (Column IV) for 0 G/D 1.5 at Re=5000. The minimum and incremental levels of the vorticity ω* are taken as ω* min =< ω* > min = ±2.

9 Comparison of Flow Structure Behaviors Around a Sphere Figure 5. Comparison of the variations of the time averaged streamline patterns <Ψ> for a smooth sphere (column I), roughened sphere (column II), a sphere with 2mm o-ring (column III) and a sphere with a vent hole(column IV) for 0 G/D 1.5 at Re=5000.

10 20 Muammer Ozgoren et al. Table 1. Comparison of the locations of retachment points and saddle points that are displayed in Figs 3 for Re=5000. Here, x is the distance between the sphere center and locations displayed in Fig 5. Gap Ratio Smooth Sphere <Ψ> ( x/d) Roughened Sphere <Ψ> ( x/d) Sphere with an oring,d o =2mm <Ψ> ( x/d) Sphere With a ventilation d h /D=0.15 <Ψ> ( x/d) G/D=0 R 1 (2.01) R 1 (1.96) R 1 (1.53) R 1 (2.0) G/D=0.1 S 2 (1.69) S 2 (1.67) (1.07) S 2 (1.69) G/D=0.25 (1.25) G/D=0.50 (1.16) (1.0) (1.08) (1.05) S1 (0.79) (1.22) S1 (1.27) G/D=1 (1.35) (1.32) (0.86) (1.36) G/D=1.50 (1.45) (1.52) (0.90) (1.46) Conclusions Flow characteristics around four different spheres have been investigated and the following results are obtained. It is demonstrated that the presence of boundary layer flow over the plane wall introduces significant complications into the wake of the sphere due to non-uniform velocity profile, gap flow between the bottom section of the sphere and plate surface and also occurrence of the vorticity field in the boundary layer region. The effects of flow interference caused by the boundary layer are powerfull when the gap ratios are small. The rate of entrainment due to the high rate of circulatory flow motion of vortical flow structure in the boundary layer region is extremely high which results in causing unstable wake flow structures. For G/D=0.1 and 0.25, the direction of the wake region of the sphere is deflected to upward from the flat plate while surrounding the rear surface of the sphere. It is demonstrated that the gap flow occurring between the sphere bottom section and flat plate surface has very high scouring effect until G/D=0.50 and then a distinguishable unsymmetrical flow structure of the wake region keeps up to G/D=1.0. It is observed that the reverse flow region in the wake is significantly reduced and the motion in that region also becomes weak owing to the roughened surface or o-ring. Surface roughness or o-ring induces a turbulent boundary layer on the sphere surface because turbulent boundary layer flow has a larger momentum than laminar boundary layer flow and thus delays separation. The saddle point locations for the sphere with 2mm o-ring become closer to the rear surface of the sphere in a smaller size which is clear evidence of passive control of the flow structure. The modified flow structure of the near wake of the vented sphere causes the occurrence of a pair of counter-rotating ring vortices, which have the effect of aerodynamically streamlining the sphere.

11 Comparison of Flow Structure Behaviors Around a Sphere From the point of flow physics, vortex formation lengths of large scale Karman Vortex Streets and Kelvin Helmholtz vortices in the wake decrease slightly due to the roughness effect although the Reynolds number is in the subcritical region. The obtained results can be helpful for developing and validating numerical predictions as well as designing. Acknowledgments The authors would like to acknowledge the funding of The Scientific and Technological Research Council of Turkey (TUBITAK) under contract No:109R028, Scientific Research Projects Office of Cukurova University Contract No: AAP20025, Selcuk University s Scientific Research Project Office Contract No: and DPT Project Contract No: 2009K References 1. Achenbach, E., Vortex shedding from spheres, Journal of Fluid Mechanics, 62:2, , Alammar, K.N., Turbulent flow over a sphere with suction and blowing holes and a golf ball: a numerical study, 22nd Applied Aerodynamics Conference and Exhibit, Providence, Rhode Island, Aug Choi, J., Jeon, W. and Choi, H., Mechanism of drag reduction by dimples on a sphere, Physics of Fluids, 18, , Hassanzadeh, R., Sahin, B., Ozgoren, M., Numerical Investigation of flow structures around a sphere. International Journal of Computational Fluid Dynamics Volume 25, Iss.10, , Hetsroni, G., Li, C.-F., Mosyak, A., Tiselj, I., Heat Transfer and thermal pattern around a sphere in a turbulent boundary layer, International Journal of Multiphase Flows, 27, , Jang, Y.II J., Lee, S.J., PIV analysis of near-wake behind a sphere at a subcritical Reynolds number, Experimental in Fluids. 44: Issue 6, , Jeon, S., Choi, J., Jeon,W., Choi, H. and Park, J., Active control of flow over a sphere for drag reduction at a subcritical Reynolds number, J. Fluid mech. vol. 517, p: , Kim, H. J. & Durbin, P. A., Observations of the frequencies in a sphere wake and of drag increase by acoustic excitation, Phys. Fluids 31, p: , Kiya, M., Ishikawa, H., Sakamoto, H., Near-wake instabilities and vortex structures of three-dimensional bluff bodies: a review, J. Wind Eng Ind Aerodyn, Vol. 89, p: , Kumar, R.A., Sohn, C.H. and Gowda, B.H.L., Passive Control of Vortex-Induced Vibrations: An Overview, Recent Patents on Mechanical Engineering, 1, 1-11, Leweke,T., Provansal, M., D. Ormie`res, Lebescond R Vortex dynamics in the wake of a sphere. Physics of Fluids Vol.11: Issue 9, 12, Maxworthy, T., Experiments on the flow around a sphere at high Reynolds numbers, J. Appl. Mech. 36, p:598, Okamoto, S., Turbulent shear flow behind a sphere placed on a plane boundary. Turbulent Shear Flow 2, , Ozgoren, M., Dogan, S., Okbaz, A., Sahin, B., Akilli, H., Comparison of Flow-Structure Around A Smooth and Roughened Sphere In A Turbulent Boundary Layer, International Scientific Conference, Gabrovo, Bulgaria, November, , 2011c. 15. Ozgoren, M., Okbaz, A., Dogan, S., Sahin, B., Akilli, H., Investigation of Flow Characteristics around a Sphere Placed in a Boundary Layer over a Flat Plate. Experimental Thermal and Fluid Science, accepted in press, 2012.

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