The Effect of Scaling of Owl s Flight Feather on Aerodynamic Noise at Inter-coach Space of High Speed Trains based on Biomimetic Analogy

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1 IJR Internatinal Jurnal f Railway Vl. 4, N. 4 / December 2011, pp The Krean Sciety fr Railway The Effect f Scaling f Owl s Flight Feather n Aerdynamic Nise at Inter-cach Space f High Speed Trains based n Bimimetic Analgy Jae-Hyun Han, Tae-Min Kim* and Jung-S Kim** Abstract An analysis and design methd fr reducing aerdynamic nise in high-speed trains based n bimimetics f niseless flight f wl is prpsed. Five factrs related t the mrphlgy f the flight feather have been selected, and the candidate ptimal shape f the flight feather is determined. The turbulent flw field analysis demnstrates that the ptimal shape leads t diminished vrtex frmatin by causing separatin f the flw as well as allwing the fluid t climb up alng the surface f the flight feather. T determine the effect f scaling f the wl s flight feather n the nise reductin, a tw-fld and a fur-fld scaled up mdel f the feather are cnstructed, and the numerical simulatins are carried ut t btain the aerdynamic nise levels fr each scale. Original mdel is fund t reduce the nise level by 10 dba, while tw-fld increase in length dimensins reduces the nise by 12 dba. Validatin f numerical slutin using wind tunnel experimental measurements is presented as well. Key wrds : Aerdynamic nise, Bimimetics, Cmputatinal fluid dynamics, High speed train, Inter-cach space, Owl s feather 1. Intrductin In recent years, high-speed trains have cme t enjy wide acceptance as swift, cnvenient, and envirnmentfriendly means f transprtatin. At the same time, increase in the perating speed pses additinal challenges. Fr the train speed in excess f 300 km/h, aerdynamic nise level rapidly increases due t separatin f turbulent flw and vrtex shedding. The parts f the train that significantly cntribute tward aerdynamic nise are the pantgraph and inter-cach space. The pantgraph which supplies electric pwer t the train during peratin is lcated n tp f the train and acts as a surce f aerdynamic nise due t vrtex shedding. The inter-cach space between the adjacent cars disturbs the air flw, leading t flw separatinwhich causes vrtex shedding [1]. The vrtex then impinges n the exterir surface f the * ** Crrespnding authr: Department f Mechanical Engineering, Hngik University, Seul , Krea wd2007@naver.cm Department f Mechanical Engineering, Hngik University, Seul , Krea Department f Mechanical and System Design Engineering, Hngik University, Seul , Krea train car which in turn transmits nise int the cabin. Research wrks n reducing inter-cach space nise by adjusting the gap at the inter-cach space, fr instance, have been reprted [2,3]. Bimimetic refers t an engineering design methdlgy that draws inspiratin frm, and mimics structures, materials r prcesses existing in nature r life frms. Bimimetics has allwed varied engineering designs and prcesses including rbts, envirnmentally sustainable and energy-saving materials and devices, as well as innvative prductin prcesses. An example f bimimetics is niseless flight f wl where the nise reductin is effected by micr-scale cmblike structure in the wing feather that disrupts the frmatin f large-scale turbulence. A structure that mimics wl s feather has been installed n pantgraph f Shinkansen Line in Japan where appreciable reductin in aerdynamic nise has been achieved [4]. The present study extends the previus wrk by the authrs n turbulent flw analysis and aerdynamic nise [5] by seeking t further elucidate nise reductin mechanism f wl s feather installed at inter-cach space f the Krean High-speed Train. A full-scale mck-up f inter- Vl. 4, N. 4 / December

2 Jae-Hyun Han, Tae-Min Kim and Jung-S Kim / IJR, 4(4), , 2011 cach space is used t btain aerdynamic nise level measurement data via wind tunnel testing. The numerical simulatins are used t elucidate the mechanism whereby the wl s wing feather affects the turbulent flw during wl s flight, which in turn influences the aerdynamic nise generatin. By cnstructing simulatin test cases with different shape factr levels using Taguchi-based design f experiment technique, steady state flw analysis is perfrmed t btain ptimal feather shape that minimizes the turbulence level. The turbulent flw analysis that incrprates unsteady flw is then perfrmed t gain understanding f the mechanism whereby the vrtex frmatin that leads t aerdynamic nise can be abated. Fr characterizing aerdynamic nise at inter-cach space in Krean high-speed trains, a full-scale mck-up f inter-cach space is cnstructed and tested in the wind tunnel. The same set f cnditins as the experimental testing is numerically mdeled and turbulent flw simulatin perfrmed by applying apprpriate cmputatinal fluid dynamics techniques. The nise level is numerically cmputed by applying the acustic analgy methdlgy. The numerically btained nise level is cmpared with the experimental results as a way t validate numerical cmputatin prcess emplyed in the present study. The validated cmputatin is then applied t determine the effect f scaling f the wl s flight feather n the nise reductin. A tw-fld and a fur-fld scaled up mdel f the feather are cnstructed and the resulting aerdynamic nise levels are cmpared. It is hped that the ptimal scaling factr can be btained fr maximum reductin in the aerdynamic nise f the inter-cach space f the Krean High-speed Train. 2. Nise Reductin f Owl s Feather 2.1 Cntrl factrs fr nise reductin Five factrs describing the feather shape that culd play majr rle in the frmatin f vrtices have been extracted and simulatin cases based n Taguchi-based design f experiment methd have been set up by assigning three Table 1. Cntrl Factrs and Levels Cntrl factr Level Unit Length (A) mm Y-axis angle (B) Z-axis angle (C) Width (D) mm Gap distance (E) mm Fig. 1 Cntrl factrs in flight feather Fig. 2 Swirling strength fr different cases levels fr each f the five shape factrs dented the cntrl factrs. Table 1 lists five cntrl factrs each with three levels. Fig. 1 illustrates hw the cntrl factrs define the shape f the flight feather. 2.2 Numerical predictin f vrtices Applying Taguchi-based design f experiment methd yields 27 rthgnal arrays which serve as test cases fr numerical experimentatin. Fr each simulatin case, swirling strength has been cmputed as a measure f the magnitude f vrtex frmatin. Fr the base mdel f the inter-cach space with n plumage, i.e., fr the case befre any nise reductin design based n wl s feather have been implemented, the swirling strength is cmputed t be S -1. Fr Case24 which shws the minimum vrtex frmatin, swirling strength f S -1 is btained. Fig. 2 shws swirling strength fr 27 simulatin test cases. It shuld be nted that the swirling strength increases fr many f the cases, which in turn culd adversely affect the aerdynamic nise level. 2.3 Determinatin f majr factrs Since the swirling strength directly measures the magni- 110

3 The Effect f Scaling f Owl s Flight Feather n Aerdynamic Nise at Inter-cach Space f High Speed Trains based n Bimimetic Analgy Table 2. Optimal Feather Shape Cntrl factr Selected value Unit Length 14 mm Y-axis angle 15 Z-axis angle 0 Width 3 mm Gap distance 3 mm Fig. 4 Inter-cach space test mdel Fig. 3 Vrtices distributin (side & tp view) tude f the vrtex frmatin, smaller-the-better characteristics is applied with S/N rati calculated as in Eq. (1). 1 SN i n y 2 = lg ( (1) n j= 1 ij) It has been shwn that the length f the feather and Z- axis angle are the factrs that have majr influence n S/N rati and hence can be cnsidered t be majr design factrs. By cmparing the magnitude f the vrtex frmatin calculated by the steady flw analysis, the flight feather shape that minimizes the vrtex frmatin has been prpsed as shwn in Table 2 [5]. Figs. 3-a and 3-b shw the side view and tp view cnturs f the vrtices distributins fr the base mdel and Case 24, respectively. The white parts represent the rigid bdies crrespnding t mdel utlines. Fr the base mdel, Figures 3-a shw the generatin f peridic vrtices that lead t large vrtices generated at the tip. Fr Case 24, vrtices are still generated at the tip but they are much smaller and the climbing-up f the fluid alng the surface f the flight feather is als bserved. 3. Predictin f Aerdynamic Nise 3.1 Wind tunnel experiment Fr characterizing aerdynamic nise at inter-cach space in Krean high-speed train, a full-scale mck-up f inter-cach space is cnstructed and tested in the wind tunnel. The mck-up is made f acryl, and the dimensin f the inter-cach space is set at 300 mm. The experimental set up f the inter-cach space mck-up is shwn in Fig. 4. A suctin-type wind tunnel with medium sizesubsnic speedpen-lp cntrl has been utilized fr sund measurements. The inlet wind speed is varied frm 185 km/h t 235 km/h in five speed stages with measurements taken at each stage. The inlet velcity is equivalent t the train velcity.fr nise measurement, a DAT recrder and a sund-level meter NL-20, a calibratr and a tripd are used. As shwn in Fig. 4, the sund level meter is placed at a lcatin 300 mm frm the end f the inter-cach space in the dwnstream directin [6]. 3.2 Numerical predictin f aerdynamic nise Flw field analysis The same set f cnditins as the wind tunnel experimentis duplicated in a numerical mdel and turbulent flw simulatin perfrmed by applying apprpriate cmputatinal fluid dynamics techniques. Fr minimizing the number f meshes and ptimizing the cmputatinal efficiency, nly 1/2 f the dmain is mdeled by utilizing the symmetry. Fr the flw analysis t be subsequent used in the aerdynamic nise analysis, the mesh size shuld be determined by cnsidering the smallest wavelength f the sund wave f interest, and als requires apprpriately small time step. Fr the sund frequency in the range f 50 Hz t 2,000 Hz, the mesh size has been set t be 1/10 f the wavelength crrespnding t the upper limit frequency f 2,000 Hz. Apprximately six hundred thusand tetrahedral meshes with the size range f 1~34 mm have been generated in cnstructing a CFD mdel as shwn in Fig. 5. The fluid flw assciated with the aerdynamic nise is dminated by unsteady-state flw, and the surce f nise exhibits nise levels fr different frequency cmpnents. In the present study, unsteady fluid mtin analysis f 111

4 Jae-Hyun Han, Tae-Min Kim and Jung-S Kim / IJR, 4(4), , 2011 Fig. 5 Mesh mdeling fr CFD Table 3. Mdeling and Simulatin Run Parameters Frequency range Element size N. f Elements Timestep Flw duratin Fig. 6 Sund pressure level at peak frequencies 50 ~ 2000 Hz 1 ~ 34 mm s 0~3s tin is expressed in terms f sund surces, which includetw surface surces (mnple and diple) and ne vlume surce (quadruple), as shwn belw: three dimensinal incmpressible flw has been carried ut using cmmercially available fluid analysis sftware FLUENT. Fr cmputatinal efficiency, steady-state fluid mtin analysis based n K ω 2 equatin turbulent flw mdel is first carried ut and the result subsequently fed as the initial cnditin fr unsteady-state fluid mtin analysis. Fr btaining turbulent flw mtin in the unsteadystate, Large-Eddy Simulatin (LES) methd is emplyed, and Smagrinski-Lilly mdel is selected fr cmputing subgrid scale viscus mdel [7]. Fr cmparisn with wind tunnel testing results, the inlet wind speed has been set at 185, 200, 235 km/h, respectively. Since the acustic radiatin energy is quite small, highprecisin fluid mtin analysis is needed. A 2nd rder implicit finite difference methd is used in cnjunctin with duble decisin which entails tw calculatins at each time step. T ensure sufficient accuracy, the time step f 0.05 ms which is less than 1/10th the perid crrespnding t 2,000 Hz has been selected. The flw duratin f 0 ~ 3 s is chsen s that the time duratin exceeds 10 perids f 50 Hz. (see Table 3 fr summary f parameters selected) Acustic analgy Acustic analgy methd based n FW-H (Ffwcs-Williams Hawkings) equatinis applied t estimate aerdynamic nise frm the fluid flw. The equatin is cast in the frm f a wave equatin develped frm the cntinuity equatin and the Navier-Stkes equatin, and has prven quite useful in prblems invlving fluid flw. FW-H equa- p' ( x, t ) = p't ( x, t ) + p L, ( x, t ) + p Q ( x, t ) (2) Equatin (2) cntains the thickness nise dependent n the shape and kinematic cnditins (mnple: p't ), the lading nise engendered frm frictin between the bdy and the surrunding fluid (diple: p'l ), and quadruplenise ( p'q ) surce due t nnlinear wave prpagatin, shck waves, and turbulence in the fluid flw field. Owing t separate terms fr each surce type, the surce f nise can be readily identified. The thickness nise and the lading nise can be evaluated by cmputing surface integrals, while the quadruple nise requires vlume integrals t be evaluated. Fr lw Ma fluid mtin, the effect f the quadruple nise surce is negligibly small. In the present study, the analysis emplys FW-H mdule in FLUENT that includes the thickness nise and the lading nise surces nly [8]. Just like in the wind tunnel experiment, the sund measurement pint is placed at a lcatin 300 mm frm the end f the inter-cach space in the dwnstream directin.the acustic pressure variatin at the sund measurement pint is cmputed and the initial time interval shwing appreciable fluctuatin in acustic pressure is excluded frm the analysis Cmparisn f aerdynamic nise T validate numerical simulatin results, cmparisn with wind tunnel experiment under identical bundary cnditins and at the identical inlet wind speed f 200 km/h is perfrmed. T arrive at the wind tunnel experiment result, the effect f the blwer which is related t the num- 112

5 The Effect f Scaling f Owl s Flight Feather n Aerdynamic Nise at Inter-cach Space f High Speed Trains based n Bimimetic Analgy 1st Peak 2nd Peak 3rd Peak Table 4. Sund Pressure Level at Peak Frequencies 185 km/h 200 km/h 235 km/h Freq. SPL Freq. SPL Freq. SPL Simul Thery Exp Simul Thery Exp Simul Thery Exp Fig. 7 Numerical and experimental cmparisn based n Rssiter s equatin ber f the turbine blades in the blwer has been calculated and deleted frm the measured data. Fig. 6 cmpares the numerical sund pressure level with the experimental level in the frequency dmain, which is characterized by the presence f peak frequencies. Table 4 lists peak frequency cmpnentsbtained frm numerical simulatin, thery based n the applicatin f Rssiter s equatin, and wind tunnel experiment. The table shws that except fr the first frequency peak, the magnitudes f the ther peaks are cmparable but frequency at which the peak is bserved is smewhat different. This discrepancy is thught t be due t differences in the cnditins inputted int the numerical simulatin and the actual cnditins existing in the wind tunnel experiment. The effect f the cavity flw arises frm the flw feedback phenmenn that starts with the frmatin f the vrtices due t the desquamatin f the flw as it passes ver the frnt edge f the mud-flap and the cavity. The flw then crashes int the rear edge and generates sund wave. The sund wave is in turn transmitted t the frnt edge and engenders yet additinal vrtices. During wind tunnel testing, severe vibratin f the mck-up was bserved due t this phenmenn. The frequency f the vibratin within the cavity engendered by acustic pressure feedback mechanism can be predicted by empirically based Rssiter s equatin derived by cnsidering the cavity flw and acustic pressure feedback phenmena, as given by Eq. (3): L L = n β, n = 123,, (3) U c c f n where L dentes the length f the cavity, U c dentes thecnvectin velcity, c is the sund velcity, and β is the phase lag set t 0.25.The cnvectin velcity used in Rssiter's equatin is apprximately 60% f the flw velcity. Fig. 7 shws the frequency cmpnents fr different Fig. 8 Cmparisn fllwing the fluid flw speed adjustment fluid flw speeds based n Rssiter s equatin. The figure shws that the wind tunnel results are similar t the theretical results but the numerical simulatin results shw that the peaks ccur at slightly higher frequencies than what the thery predicts. This discrepancy is thught t be due t differences in the cnditins inputted int the numerical simulatin and the actual physical envirnment. As a way f crrecting this discrepancy, the numerical analysis is repeated by apprpriately shifting the fluid flw speed. Fig. 8 depicts the case where the numerical simulatin is repeated but with the fluid flw speed input increased by 10 m/s. A much imprved crrelatin between the numerical and experimental values is bserved. 4. The Effect f Scaling f Flight Feathers n Aerdynamic Nise Level 4.1 Scale mdels f flight feathers The dimensins f the flight feather mdel thus far have been based n the actual measured dimensins f wl s feather. T determine the effect f scaling f the wl s flight feather n the nise reductin, a tw-fld and a fur- 113

6 Jae-Hyun Han, Tae-Min Kim and Jung-S Kim / IJR, 4(4), , 2011 Length Table 5. Scale Mdel Specificatins Y-axis angle Z-axis angle width Gap distance N. f Feather Scale 1 14 mm mm 3mm 41 Scale 2 28 mm mm 6mm 20 Scale 4 56 mm mm 12 mm 10 Fig. 10 Nise level fr different scale mdels mdel reduces the nise level by 10 db Acmpared with the base mdel with n flight feathers installed. Fr Scale 2 mdel, a further reductin f 2 dba is achieved, fr the ttal reductin f 12 dba. Fr Scale 4 mdel, hwever, a substantial deteriratin in the nise reductin is bserved. In fact, it is wrse than Scale 1 and nly slightly better than the base mdel. 5. Cnclusin Fig. 9 Vrtex distributin fr different scale mdels fld scale mdel f the feather are cnstructed, and the numerical simulatins are carried ut t btain the aerdynamic nise levels fr each scale mdel. Table 5 lists the riginal mdel fr Case 24 f Sectin 2.2, dented Scale 1, as well as a tw-fld and a fur-fld scale mdels, dented Scale 2 and Scale 4, respectively. Figs. 9-a, 9-b and 9-c depict vrtex distributins fr varius scale mdels f flight feather. The discrepancy between the mdels is quite apparent. The average magnitudes f vrtices at inter-cach spacing are 1,190 S -1, 1,185 S -1 and 1,272 S -1 fr Scale 1, Scale 2 and Scale 4, respectively. While Scale 1 and Scale 2 exhibit similar magnitude, Scale 4 exhibits appreciably larger magnitude f vrtices. 4.2 Effect f scaling n aerdynamic nise The nise levels fr varius scale mdels f Case 24 are cmputed by applying acustic analgy methdlgy described in Sectin Fig. 10 shws that Scale 1 The present study prpses an analysis and design methd fr reducing aerdynamic nise in high-speed trains based n bimimetics f niseless flight f wl.five factrs related t the mrphlgy f the flight feather have been selected, and the candidate ptimal shape f the flight feather is determined. The turbulent flw field analysis incrprating unsteady state fluid flw is then carried ut fr the base mdel as well as with the ptimally-shaped mdel f the flight feather. The ptimal shape is shwn t lead t diminished vrtex frmatin by causing separatin f the flw as well as allwing the fluid t climb up alng the surface f the flight feather. T determine the effect f the scaling f the wl s flight feather n the nise reductin, a tw-fld and a fur-fld scale mdel f the feather are cnstructed, and the numerical simulatins are carried ut t btain the aerdynamic nise levels fr each scale mdel. The riginal mdelis fund t reduce the nise level by 10 dba, while tw-fld increase in length dimensins further reduces the nise by 12 dba. Additinally, prpsed numerical simulatin methd is validated by cmparing the simulatin results with the wind tunnel experiment results under identical bundary cnditins and at the identical inlet wind speed. While the magnitudes f aerdynamic nise are different by a certain amunt, clse crrelatin in the verall aerdynamic nise characteristics are bserved. 114

7 The Effect f Scaling f Owl s Flight Feather n Aerdynamic Nise at Inter-cach Space f High Speed Trains based n Bimimetic Analgy Acknwledgement This research was supprted by a grant (Title: Develpment f Bimimetic Design Methdlgy) frm the Natinal Research Fundatin f Krea. 1. D. Rcchi and A, Zass (2002). Vrtex shedding frm a circular cylinder in a smth and wired cnfiguratin: cmparisn between 3D LES simulatin and experimental analysis, Jurnal f Wind Engineering and Industrial Aerdynamics 90, pp Chi, S. H., Park, C. S., Park, J. H. and Kim, S. S. (2007). Experimental investigatin f nise generatin frm the inter-cach spacing f a high-speed train, Transactins f the Krean Sciety fr Railway. Vl. 10, N. 6, pp Se, S. I., Chi, S. H. and Chung, I. S. (2006). A study n the effect f mud-flap n the cabin nise in KTX, Transactins f the Krean Sciety fr Railway, Vl. 9, N. 5, pp Chi, H. C. Park, H. M. and SAGONG, W. (2010). Bimimetic flw cntrl, physics & high technlgy, pp Han, J. H., Kim, J. S. (2011). An aerdynamic nise reductin design at inter-cach space f high speed trains based n bimimetic analgy, Internatinal Jurnal f Railway, Vl. 4, N. 3, pp Park, K. Y., Park, J. H., Sng, S. M., Kim, T. H., Lee, T. J. and Chi, S. H. (2006). Wind tunnel tests fr analyzing nise generatin frm the inter-cach spacing f a highspeed train, Prceedings f the KSNVE Annual Fall Cnference, pp Kang, S. M. (2000). On subgrid-scale mdels fr largeeddy simulatin f turbulent flws, Transactins f the KSME B, Vl. 24, N. 11, pp Y, S. W., Min, O. K., Park, J. S. and Chung, H. C. (2000). A study n aerdynamic and aeracustic characteritics arund pantgraph, Prceedings f the KSR Annual Spring Cnference, pp Received(December 20, 2011), Accepted(December 22, 2011) 115