Flow Characteristics around an Inclined Circular Cylinder with Fin

Transcription

1 Lisbon, Portugal, 7- July, 28 Flow Characteristics around an Inclined Circular Cylinder with Fin Tsuneaki ISHIMA, Takeshi SASAKI 2, Yoshitsugu GOKAN 3 Yasushi TAKAHASHI 4, Tomio OBOKATA 5 : Department of Mechanical Engineering, Gunma University, Kiryu, Japan, ishima@me.gunma-u.ac.jp 2: Graduate School of Gunma University, Kiryu, Japan, s_kitkc@yahoo.co.jp 3: Honda R&D Co., Ltd., Asaka, Japan, Yoshitsugu.Gokan@mail.a.rd.honda.co.jp 4: Honda R&D Co., Ltd., Asaka, Japan, Yasushi.Takahashi@mail.a.rd.honda.co.jp 5: Graduate School of Tokyo Denki University, Tokyo, Japan, tommy@takauji.or.jp Abstract: The wake flow of the circular cylinder is one of the important phenomena for understanding the heat transfer and flow dynamics. In the present study, flow characteristics around an inclined circular cylinder are evaluated by using PIV (particle image velocimetry) and LDA (laser Doppler anemometer). The flow features are discussed with statistical data of mean velocity and fluctuation intensity of velocity and its results of frequency analysis. Two types of circular cylinder with or without fins are used in the experiment. The circular cylinder is inclined with, 5 and 3 degrees in the plane made by the vertical line and flow direction line. Mean velocity of incoming horizontal flow is 3.8 m/s and relative turbulence intensity is about.5%. From the results of flow around the no-fin (smooth) cylinder, the length of the recirculation zone was elongated at the 5 degree of inclined angle cylinder comparing with that of degree cylinder. However, the no-fin cylinder with maximum inclination angle of 3 degree has much smaller recirculation area. The downward flow along the back surface of the cylinder was observed in this condition. The vertical velocity component perpendicular to the incoming velocity has opposite tendency compared with the other inclined cases. The flow field around the 3 degree of the inclined angle with no-fin circular cylinder has the velocity along the cylinder axis. For circular cylinder with fins, the flow field becomes complicated, especially near the cylinder region. The flow around the inclined angle of degree cylinder with fins is almost the same as that of the circular cylinder without fins. When the circular cylinder with fins has inclined angle, the wake width is increasing. The downward flow along the cylinder is also formed for 3 degree of inclined angle of fin cylinder. In the results of the frequency analysis for the cylinder without fins, the peak value of the power becomes small with inclined angle. The distribution shape becomes broad when the inclined angle increases. For the cylinder with fins, the distributions have peak values in all inclined angle results. The peak value shifts to low frequency side with increasing in the inclined angle. The equivalent diameter calculated from the peak frequency is not sensitive for the cylinder without fins and is sensitive for the cylinder with fins.. Introduction Wake flows around a circular cylinder are one of the historical topics in the fluid engineering (Schlichiting 955). The historical works have successfully presented the observation of the Karman s vortex street, the relationship between Reynolds number and drag coefficient, separation point of boundary layer and so on. These works are performed with the cylinder which is set on the floor vertically. In the industrial fields, many variation angles between flow and cylinder can be observed. For example, the inclined circular cylinder is used in the reduction of the drag force, and it is related with aerodynamic sounds (Haramoto et al. 22). The wake flow is often observed in the flow field including the heat transfer problems. The authors group has investigated about the heat transfer problems related with air-cooled motorcycle based on numerical simulation (Takahashi and Gokan 26 and Gokan et al. 27). In the air-cooled motorcycle, the air flow around the engine is very important for cooling the engine. The numerical works are successfully performed, however there is no verification method due to lack of experimental data. In the present study, the flow fields around inclined cylinders are evaluated by using laser - -

2 Lisbon, Portugal, 7- July, 28 techniques. Two-types of cylinders with and without fins are tested with several inclined angles. The large area flow measurement is performed by using PIV (particle image velocimetry) and the frequency analysis of time-series velocity data is tested by using LDA. The purpose of the present study is to obtain knowledge of the flow field around the inclined cylinder and to obtain the experimental data for the verification of numerical simulation. 2. Experimental apparatus and condition Figure shows the experimental setup. A suction type horizontal wind tunnel is used and the test section has 4 mm in width, 2 mm in height, and mm in length. The wind tunnel has a nozzle, one honeycomb and four meshes for obtaining the uniform flow velocity and fluctuation at the inlet of the test section. Inlet flow condition is 3.8 m/s in mean velocity and about.5 % in velocity fluctuation. Under this condition, the Reynolds number based on circular cylinder diameter and inlet mean velocity is 5. x 3. BSA Photomultiplier CCD Camera Oscilloscope Computer y z x YAG Laser Processor Computer Argon gas laser Fig. Experimental set up Two-types of the cylinders are tested in the present study. One is normal cylinder which has 2 mm in diameter. The other is cylinder with fins, where the cylinder diameter is 2 mm and fin has circular shape with 4 mm in diameter and 3 mm in thickness as shown in Fig. 2. The cylinder is set with inclined angle θ of, 5 and 3 degrees. In all of the conditions, the cylinder center on the half height of it is set on 2 mm downstream from the inlet of test section. The origin is set on the center of cylinder and half height of the test section, which is the center of the cylinder. The coordinate system is set mainstream, horizontal and vertical directions as x, y and z axes, respectively as shown in Figs. and 3. 2 mm z 4 mm y= x 2 mm mm 3 mm Flow Fig. 2 Model of circular cylinder with fin θ Fig. 3 Definitions of inclined angle and coordinate system for circular cylinder with fin - 2 -

3 Lisbon, Portugal, 7- July, 28 The measurement instruments are a particle image velocimetry (PIV) and a laser Doppler anemometer (LDA). The PIV consists of CCD camera (Kodak ES-.), Nd:YAG laser (New wave Research SOLO-III) and PIV processor (Dantec flowmap 2). The measurement area is 8.2 mm x 8. mm. Since the number of pixel is 8 x 8, each pixel has 79.5 µm x 79.5 µm of measurement size. The interrogation area is 32 pix x 32 pix which is.59 mm x.59 mm in actual measurement area. 5 % x 5 % overlaps are applied. In the PIV measurement, oil mist of 3 µm in diameter is added for seeding particle. The error vectors are removed by using a function of the PIV processor, and after statistical data of mean velocity and fluctuation are calculated from 5 instantaneous velocity maps. The flow field around the cylinder has Karman s vortex shedding. In the field, high temporal velocity measurements are requested. In the present study, the LDA is applied for measurement of time-series velocity data. The LDA used in the present study has one velocity component optical fiber system. It consists of Argon ion laser, sending optics with optical fiber, receiving optics, signal processor (Dantec BSA 57N2) and PC. In the present study, only blue light of 488 nm is used. The double Bragg cells system is used to measure the reverse flow. The LDA measurement is performed at (x, y, z) = ( mm, 2 mm, mm) where the Karman vortex can be observed clearly. In the experiments, the water mist which is about 5 µm in mean diameter is added for seeding particle. The definitions of instantaneous velocity u(t), mean velocity U and fluctuation velocity u are follows: u t = U + u' t () ( ) ( ) U = N N n= u ( t) N 2 u' ( t) = { u( t) U} (3) N n= (2) 3. Results and discussions Figure 4 shows vector maps around the cylinder. In the figure, the color contour indicates velocity magnitude. The measurement plane is z = mm (x-y plane). When the incline angle with no-fin cylinder increases from to 5 degree (Fig. 4 (a) and (b)), the recirculation zone becomes large. The end of the recirculation zone on the x axis (y = mm) is x = 44 mm for degree of inclined angle and x = 57 mm for 5 degree. The large velocity area which is observed around x = 2 mm and y = 3 mm becomes small with the increasing the inclined angle. The largest inclined angle of 3 degree with no-fin cylinder as shown in Fig.4 (c) has no recirculation zone. In Fig. 4(c), the velocity vectors have positive value in x direction at very near region of the cylinder. The maximum velocity at the side of the cylinder becomes smaller than the other inclined angles cases. The cylinder with fins for degree of inclined angle has almost same recirculation zone in x direction as that of no-fin case. The width of the wakes becomes wider than that with no fin case. The inclined angle of 5 and 3 degrees of the cylinder with fins has nearly equal length of the recirculation zone. The length is much larger than that of degree inclined angle. The length of the recirculation zone of each angle is 45 mm, 78 mm and 76 mm for, 5 and 3 degree of inclined angle, respectively. The v velocity which denotes velocity component in y direction becomes remarkable at 3 degree of inclined angle with fin case around the region of x = 2 5 mm and y = 2 35 mm. The large velocity magnitude area is also observed at x = 4 mm and y = 5 4 mm. An increase in the inclined angle causes smaller maximum velocity magnitude. Since the width of the test section is much wider than the cylinder diameter, the difference in the tendencies seems to be resulted by the three-dimensional motion in the flow field

4 Lisbon, Portugal, 7- July, 28 y mm 4 2 Velocity magnitude m/s m/s -2-4 (a) degree, without fins (d) degree, with fins (b) 5 degree, without fins (e) 5 degree, with fins (c) 3 degree, without fins (f) 3 degree, with fins 5 5 x mm 5 5 Fig. 4 Velocity vector maps in x-y plane (z = mm) x mm Figure 5 shows the velocity vector map for all experimental conditions. The measurement plane is y = mm (x-z plane). The color contour is indicates velocity magnitude. For degree of inclined angle with no-fin cylinder as shown in Fig. 5 (a), the velocity has no distribution pattern in z direction. The flow with 5 degree of inclined angle as shown in Fig. 5 (b) has large downward velocity at x < 9 mm. The velocity distribution in z direction is not uniform. The influence of inclined cylinder on the velocity components appears in the direction. Near region of the cylinder also has downward flow in z < mm. The flow direction is along the set angle of cylinder. With 3 degree of inclined angle as shown in Fig. 5 (c), the flow has no negative u velocity in the present measurement area. The flow direction is along the cylinder inclined angle. Figure 5 (d) shows the result of cylinder with fins and degree of inclined angle. The flow seems to have small downward flow, however it will be caused by unsteady effects and measurement error. The flow between the fins cannot be measured with sufficient accuracy. The overall flow pattern is almost same as no-fin case as shown in Fig. 5 (a). Figure 5 (e) shows the flow vector map of 5 degree of inclined angle of fin-cylinder. The flow is similar to that of no-fin condition but large negative u velocity component is observed around x = 4 mm. The recirculation zone is elongated compared with no-fin cylinder case (see Fig. 5 (b)). In the figure, the air doesn t flow along the cylinder at the near cylinder region. The flow for 3 degree inclined angle of fin-cylinder has large downward velocity as shown in Fig. 5 (f). Since the flow near the cylinder becomes complicated, the flow vector includes measurement error. At the very near region of cylinder, the air cannot flow along the cylinder due to disturbing flow caused by the fins

5 Lisbon, Portugal, 7- July, 28 Velocity magnitude m/s z mm 4 2 m/s z mm (a) degree, without fins -2 (d) degree, with fins (b) 5 degree, without fins -2-4 (e) 5 degree, with fins (c) 3 degree, without fins (f) 3 degree, with fins x mm 5 5 x mm Fig. 5 Velocity vector maps in x-z plane (y = mm) The velocity distribution in both directions at x = 3 mm is indicated in Fig. 6. The results are normalized by using initial mean velocity U (= 3.8 m/s) and cylinder diameter d (= 2 mm). Figure 6 (a) shows the result of the cylinder without fins. The mean velocity U for degree of inclined angle has typical distribution shape of the wake of cylinder. The U velocity has maximum value around =. and almost velocity near the cylinder center. An increase in the inclined angle causes a decrease in the U velocity around =. and an increase in the U velocity around =. Figure 6 (b) shows the mean velocity in y direction for the cylinder without fins. The mean velocities in perpendicular direction for and 5 degree of inclined angles are very small. With these conditions, the flow goes toward the outer side direction from the cylinder center line. The V velocity for 3 degree of inclined angle has opposite tendency compared with and 5 degree of inclined angle. The result of 3 degree of inclined angle indicates the flow goes toward the wake region and the air flows along the cylinder at very near region of the cylinder. This is one of the reasons why the U is larger than that of the other inclined cases. The U velocity distributions of cylinder with fins are shown in Fig. 6 (c). The flow becomes complicated structure compared with no-fin cylinder case. For degree of inclined angle with finscylinder, outer side flow structure around =. is similar to that of no-fin case. The minimum velocity can be observed at cylinder center ( = ). At the cylinder center, there is negative velocity region. With inclined angle of 5 degree with fin-cylinder, the width of the wake region becomes wider than that of the case with degree. When the cylinder with fins has inclined angle, the projection area of the cylinder and fin becomes large. The large projection area results in the wide wake region. The distribution shape of U in the wake for 5 degree of the inclined angle is similar to the inclined angle of and 5 degrees with no-fin cases. With 3 degree of inclined angle, the wake becomes wider than the other cases. The U velocity gradient around =. becomes - 5 -

6 Lisbon, Portugal, 7- July, 28 small. The V velocity distributions for the cylinder with fins are shown in Fig. 6 (d). The V velocity for degree of inclined angle becomes remarkable compared with no-fin case. The flow toward the cylinder center direction is active at the outer edge of the wake region ( =.). With 5 degree of the inclined angle, the V becomes small compared with degree in inclined angle. The V velocity for 3 degree of inclined angle has flat shape near the cylinder center. Around =.3, the flow goes toward wake region deg. 3 deg. U/U.5 V/U -.5 (a) without fins, U deg. 3 deg (b) without fins, V deg. 3 deg. U/U.5 V/U -.5 (c) with fins, U deg. 3 deg (d) with fins, V Fig. 6 Two-components velocity profiles at x = 3 mm -2-2 Figure 7 shows the fluctuation velocity at the same position of Fig. 6. The fluctuation velocity is normalized by the inlet mean velocity U. The result of u fluctuation of the cylinder without fins is illustrated in Fig. 7 (a). The u fluctuation velocity for degree of inclined angle has typical wake fluctuation velocity distribution. The distribution shape of the u fluctuation velocity has very sharp peaks around =.8. For 5 degree of inclined angle, the u fluctuation velocity is similar to that of no inclined angle case, however the peak value becomes small. The result with the 3 degree inclined angle has far different from the other inclined angle cases. There are also two peaks in the u fluctuation velocity, however the u fluctuation velocity around cylinder center is also large compared with the other cases. The result of v fluctuation velocity for the cylinder without fins is shown in Fig. 7 (b). The v fluctuation velocity for degree of inclined angle has the peak value on the center line. The v fluctuation velocity distribution for 5 degree of inclined angle differs from that of no inclined case, and it becomes similar to that of the u fluctuation velocity. The result for 3 degree of inclined angle is much higher than the others. It has a peak on the center line. These results indicate the inclined angle causes active mixing in vertical direction for the cylinder without fins

7 Lisbon, Portugal, 7- July, 28 The results of the cylinder with fins are shown in Fig. 7 (c). The result of no inclined angle has the largest fluctuation velocity. When the circular cylinder with fins has inclined angle, the fluctuation velocity becomes small. The distribution shape of 5 degree of inclined angle is similar to that of degree of inclined angle for no-fin cylinder. This indicates that the wake is formed like as one large wake. In the condition, the flow between the fins becomes less significance. With 3 degree of inclined angle, the flow seems to become unstable and then there is no clear tendency in the fluctuation velocity distribution. The v components for the cylinder with fins are shown in Fig. 7 (d). The distribution of degree of inclined angle is similar to that of u fluctuation velocity (see Fig. 7(c)). The result seems to be caused by that the fluctuation motion in this condition is in three dimensional by the fins. The wakes are formed by both of the circular cylinder and fins. The wakes with two sizes related with circular cylinder and fins are mixing in the downstream of the cylinder. Then the fluctuation velocity becomes large and the values in both directions become similar to each other. For 5 and 3 degree of the inclined angles, the v fluctuation velocity becomes much smaller than that of the degree of the inclined angle around cylinder center line. This result also indicates the wake flow of the inclined cylinder with fins behaves like as the wake for one large cylinder..8.6 (a) without fins, u' deg. 3 deg..8.6 (b) without fins, v' deg. 3 deg. u'/u.4 v'/u (c) with fins, u' deg. 3 deg..8.6 (d) with fins, v' deg. 3 deg. u'/u.4 v'/u Fig. 7 Two-components fluctuation velocity profiles at x = 3 mm Figure 8 shows the mean velocity distributions at x = 5 mm. The results are normalized by the same way as Fig. 6. The results for the cylinder without fins are shown in Fig. 8 (a) for U velocity and Fig. 8 (b) for V velocity. The distribution shapes of U are similar to each other. The values around the cylinder center are different with each other. The difference in the value is related with the length of the recirculation area. About the V velocity, there is no significant difference in the results. At this position, the V has the profiles of common wake flow

8 Lisbon, Portugal, 7- July, 28 Figures 8 (c) and 8 (d) show the U and V mean velocity distributions, respectively for the cylinder with fins. The U velocity with degree of inclined angle has near distribution shape for no-fin case. The result with 3 degree of inclined angle has very small peak at =.. Except of it, both of the results with 5 and 3 degree of inclined angle is similar to each other. The widths of the wake region, which is represented by the region of U/U <., for 5 and 3 degree are much larger than that of degree of inclined angle. This is also an evidence of existence of a large substantial wake for inclined cylinder with fins deg. 3 deg. U/U.5 V/U -.5 (a) without fins, U deg. 3 deg (b) without fins, V deg. 3 deg. U/U.5 V/U -.5 (c) with fins, U deg. 3 deg (d) with fins, V Fig. 8 Two-components velocity profiles at x = 5 mm -2-2 Figure 9 shows the fluctuation velocities at the same position of Fig. 8. The normalization is carried out by the same way as Fig. 7. The result of u fluctuation velocity for the cylinder without fins as shown in Fig. 9 (a) shows no significant difference by the inclined angle conditions. The result of 3 degree of the inclined angle has slightly small values. This is caused by small wake region at this condition. The v fluctuation velocity for the cylinder without fins is shown in Fig. 9 (b). The largest v fluctuation velocity is observed for degree of inclined angle. The other inclined angle has downward flow as shown in Fig. 5. The downward flow seems to cause to decrease the v fluctuation velocity. Figure 9 (c) shows u fluctuation velocity for the cylinder with fins. An increase in the inclined angle causes a decrease in the u fluctuation velocity. The distribution shapes for inclined cylinder look to wider than that of no inclined case. As discussed in above, the flow field with inclined cylinder with fins has wide substantial wake. Fig. 9 (c) also indicates the wide wake for inclined cylinder with fins. Figure 9 (d) shows v fluctuation velocity. The result with degree of inclined angle has largest values. The distribution is almost same as no-fin cylinder case. Since the inclined cylinder has small mean velocity of V component, the fluctuation velocity becomes small

9 Lisbon, Portugal, 7- July, (a) without fins, u' deg. 3 deg..8.6 (b) without fins, v' deg. 3 deg. u'/u.4 v'/u (c) with fins, u' deg. 3 deg..8.6 (d) with fins, v' deg. 3 deg. u'/u.4 v'/u Fig. 9 Two-components velocity fluctuation profiles at x = 5 mm Figure shows the results of the frequency analysis by using time-series data of LDA. The results are obtained by using FFT analysis. Normally, there is Karman s vortex street in the wake of the cylinder. Under the condition, there is a sharp peak in the results of the frequency analysis. For the no-fin cylinder, the peak value is decreasing with increasing the inclined angle. The peak shape becomes broad with increasing inclined angle. For 3 degree of the inclined angle case, the sharp peak disappears. The results of the cylinder with fins have sharp peaks. The peak position shifts to the low frequency side with increasing the inclined angle. Table shows the peak frequency in the FFT results and equivalence diameter which is calculated by using the peak frequency and constant Strouhal number of.2. For the case without fins, the peak position in the frequency has small changes with inclined angle. The equivalent diameter with 3 degree of inclined angle is % larger than that of the case of degree of inclined angle. The results point out that the inclined angle cannot affect the vortex shedding frequency for no-fin cylinder and vortex structure becomes unclear with an increasing the inclined angle. For the cylinder with fins, the peak frequency changes with the inclined angle. The equivalent diameter changes 3 % larger at 3 degree of inclined angle compared with degree of inclined angle. The results indicate that the wake of the cylinder with fins has clear Karman s vortex street, and the shedding frequency is different with each other. When the cylinder with fins has inclined angle, the wake behaves like as that of one large diameter circular cylinder

10 Lisbon, Portugal, 7- July, 28 5 deg., without fins 5 4, without fins 3 deg., without fins 4 PSD 3 2 PSD 3 2 deg., with fins, with fins 3 deg., with fins frequency Hz frequency Hz Fig. Result of frequency analysis by FFT method at (x, y, z) = (, 2,) Table Peak frequency and equivalence diameter peak frequency Hz Equivalence diameter mm without fins, degree without fins, 5degree without fins, 3degree with fins, degree with fins, 5degree with fins, 3degree Conclusions The flow measurements around inclined circular cylinders have been carried out. Two-types of the cylinder with and without fins are tested under initial condition of 3.8 m/s in mean inlet velocity. The inclined angles for both cylinders are, 5 and 3 degrees. The overall flow field is measured by PIV. Point measurements for frequency analysis are also performed by using LDA. Main conclusions are indicated as follows:. The inclined angle affects length of the recirculation zone. The velocity component in perpendicular to the mainstream direction has large difference by the inclined angle. 2. With 3 degree of inclined angle, the flow has a component of downward direction. The flow is formed along the inclined cylinder. 3. The fins make flow complicated. Inclined cylinder with fins has larger equivalent circular cylinder diameter. References Gokan Y, Takahashi Y, Inayoshi M, Ishima T, Obokata T (27) Development of Air/Oil-Cooled Motorcycle Engine Using Thermal and Fluid Analyses. SAE paper Haramoto Y, Arita Y, Singai M, Ohba H (22) The Flow around an Inclined Circular Cylinder and Aerodynamic Sound. JSME annual meeting Vol.22, No.7(2292): 2-22 Schlichting H (955) Boundary-Layer Theory: Takahashi Y and Gokan Y (26) CFD Analysis of Air Flow of Air-Cooled Motorcycle Engines. SAE paper