IEEE TRANSACTIONS ON MAGNETICS, VOL. 48, NO. 8, AUGUST 2012 2395

Communication

Computational Fluid Dynamics and Experimental Visualization ofTime-Variable Air Flow Pattern Inside Hard Disk Drives

Hiroyuki Kubotera , Dae-Wee Kong , YongHan Song , Takahiro Tokumiya , and Cheol-Soon Kim

Samsung Yokohama Research Institute, Yokohama, Kanagawa 230-0027, JapanSamsung Electronics, Suwon, Gyeonggi 443-742, KoreaSeagate Korea, Suwon, Gyeonggi 443-742, Korea

The time-variable air flow pattern inside hard disk drives (HDDs) was analyzed using computational fluid dynamics (CFD) and ex-perimental flow visualization. The existence of a rotating polygonal flow pattern inside an actual HDD was directly confirmed boththrough simulation and experiment for the first time. Even though earlier studies reported time-invariant polygonal flow pattern insidefully shrouded co-rotating disks, we found that the polygonal flow pattern varied its polygonal number repeatedly with time in not fullyshrouded actual HDD. Triangular, tetragonal, pentagonal, and hexagonal flow patterns were observed both in simulation and experi-mental visualization. We also observed peaks in the power spectrum density of the simulated air velocity and these corresponded to thepolygonal flow patterns. It was also found that each of these frequencies basically coincides with the frequency expected by the relationbetween the polygonal number and the vortex frequency which has been reported by earlier studies using fully shrouded co-rotatingdisks.

Index Terms—Computational fluid dynamics, flow visualization, fluid-induced vibration, hard disk drives.

I. INTRODUCTION

T O ACHIEVE higher recording density of hard disk drives(HDDs), nanometer-order positioning accuracy of the

magnetic read/write head is required. The air flow caused bythe disk rotation excites mechanical vibrations of the head car-riage arm and the disk [1]. Such flow-induced vibrations havebeen dominant disturbance factors against the head positioningaccuracy and are the biggest concerns in the development ofdrives with higher recording density. Therefore, it is crucialto understand the aerodynamic phenomena inside the actualdrives to achieve higher recording density. Earlier studiesfound a polygonal flow pattern within the co-rotating diskdomain for simplified cylindrical fully shrouded drives [2]–[8].In these studies, a rotating time-invariant polygonal pattern wasreported for the cases using fully shrouded drives. However,the actual structures of commercially available drives arenot fully shrouded and have a head carriage arm, filter, andother complicated structures. Ikegawa et al. recently reportedseveral peaks in the spectrum of air pressure experimentallymeasured at the tip of the arm [9]. This result indicates thatmultiple polygonal flow patterns exist in the HDD. However,the existence of these multiple polygonal flow patterns has stillnot been directly confirmed through numerical simulation orexperimental flow visualization [10]–[13]. In this study, we

Manuscript received November 24, 2011; revised January 11, 2012 andMarch 08, 2012; accepted March 20, 2012. Date of publication April 09, 2012;date of current version July 20, 2012. Corresponding author: H. Kubotera(e-mail: h.kubotera@samsung.com).Digital Object Identifier 10.1109/TMAG.2012.2194299

Fig. 1. Computational model and air flow visualization drive: (a) computa-tional model, and (b) special drive for air flow visualization.

analyzed the air flow behavior in actual drives and investigatedthe existence and conformation of the polygonal flow patterninside the drives.

II. METHODS AND CONDITIONS

The air flow inside a 2.5-inch mobile HDD was analyzedby using both computational fluid dynamics (CFD) and exper-imental flow visualization. We analyzed the air flow behaviorunder arm unloaded conditions, so that the basic characteristicsof air flow inside the HDD could be confirmed.

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2396 IEEE TRANSACTIONS ON MAGNETICS, VOL. 48, NO. 8, AUGUST 2012

TABLE ISIMULATION CONDITIONS

TABLE IIEXPERIMENTAL CONDITIONS

A. Numerical Simulation

The large eddy simulation (LES) with the dynamicSmagorinsky model was employed as the turbulence model inorder to reproduce precise and accurate air flow behavior insidethe HDD. Fig. 1(a) shows the numerical simulation model. Wemodeled most of the key components such as the disk, base,arm, yoke, bracket, and filter in the CFD model. We imposedperiodic boundary conditions on both the upper and lowerboundaries of the fluid domain. The rotational velocity wasapplied to the surfaces of the disks and the spacer between twodisks. Table I summarizes the simulation conditions.

B. Experimental Visualization

To clarify the simulation results and confirm the actual airflow behavior inside the HDD, we conducted flow visualizationbased on a particle scattering technique using a laser sheet. Forthe flow visualization between the co-rotating disks, we build aspecial drive, as illustrated in Fig. 1(b). This drive has a clearside window, a clear cover, and clear glass disks. Oil tracer par-ticles were seeded into the drive using an atomizer. A thin lasersheet was injected into the drive through its clear side window,and it illuminated the tracer particles in the mid-plane betweenthe co-rotating disks. The illuminated tracer particles made itpossible to visualize the air flow, and the flow images were cap-tured by a charge-coupled device (CCD) through the clear diskand clear cover. The air was used as the working fluid to mea-sure the actual air flow behavior inside the HDD. Table II sum-marizes the experimental visualization conditions.

Fig. 2. Simulated instantaneous velocity magnitude distributions: (a) trian-gular, (b) tetragonal, (c) pentagonal, and (d) hexagonal flow patterns.

III. RESULTS AND DISCUSSIONS

A. Numerical Simulation

Fig. 2 illustrates the instantaneous in-plane directional ve-locity magnitudes at the mid-plane of two co-rotating disks afterthe flow was fully developed. Turbulent flow behavior was ob-served in the outer diameter (OD) region. In the inner diameter(ID) region, the air behaved as a central core body in the mannerof a laminar flow. Between the turbulent region at the OD andthe laminar region at the ID, a polygonal flow pattern rotatingwith the disk rotation was observed. Furthermore, we found thatover time the polygonal number varied repeatedly between threeand six in the actual HDD. We observed triangular, tetragonal,pentagonal, and hexagonal flow patterns in the actual HDD, asillustrated in Fig. 2. Even though the triangular and tetragonalflow patterns are relatively not clear compared to other patterns,in Fig. 2 the polygonal numbers were determined based on thespectrogram analysis. The flow pattern changed from one pat-tern to another in a quasi random manner. Duration time of onepattern typically corresponded to the rotation time for one tofour disk revolutions.Fig. 3 plots the power spectrum density of the circumferen-

tial directional air velocity at three radius positions. In the ID theflow was laminar, and the velocity fluctuation was the smallest.In the OD the flow was turbulent, and the velocity fluctuationwas quite large compared to the ID. Furthermore, sharp peakswere observed in the mid-diameter (MD) and OD positions. Thefirst four peak frequencies in the MD were 200, 310, 370, and

KUBOTERA et al.: COMPUTATIONAL FLUID DYNAMICS AND EXPERIMENTAL VISUALIZATION OF TIME-VARIABLE AIR FLOW PATTERN 2397

Fig. 3. Simulated power spectrum density.

450 Hz. These frequencies correspond to the triangular, tetrag-onal, pentagonal, and hexagonal flow patterns observed in thevelocity magnitude contours in Fig. 2.Here, for the flow between two fully shrouded co-rotating

disks, the relation between the polygonal number and the peakfrequency has been experimentally well studied [7] and is ex-pressed by

(1)

where is the vortex frequency, is the polygonal number,and is the disk rotation frequency (Reynolds numbers

, and the ratios of the gap between two disks tothe disk radius ). If we apply the param-eters used in the simulation into (1), we can expect vortex fre-quencies of 180, 270, 360, and 450 Hz for the triangular, tetrag-onal, pentagonal, and hexagonal flow patterns, respectively, forthe fully shrouded cases. These frequencies basically coincidewith those appearing in the power spectrum density of Fig. 3.The earlier studies found the time-invariant polygonal patternwith a fixed polygonal number. However, we found that thepolygonal number of the polygonal flow pattern varied period-ically inside the actual drive structure.

B. Experimental Visualization

To clarify the existence of the time-varying polygonal pattern,the actual air flow inside the HDDwas experimentally measuredunder the same conditions as in the numerical simulation. Fig. 4illustrates the results of instantaneous flow visualization at themid-plane between two co-rotating disks. The laser sheet wasinjected from the right side of the figures. Because the spindlemotor and spacers are opaque to the laser sheet, the left portionsof the figures are in shadow. Similar to the simulation results,the flow in the OD was turbulent, and the air was well stirred,resulting in a uniform and highly dense tracer mist, as shownin Fig. 4. In the ID the air behaved as a central core body inthe manner of a laminar flow, resulting in nonuniform densityof the mist. Between the turbulence region in the OD and thelaminar region in the ID, the polygonal patterns were observed.

Fig. 4. Experimental instantaneous images of air flow visualization: (a) trian-gular, (b) tetragonal, (c) pentagonal, and (d) hexagonal flow patterns.

Furthermore, as in the simulation results, the triangular, tetrag-onal, hexagonal, and pentagonal flow patterns were confirmedinside the actual HDD. These experimental results revealed theexistence of polygonal flow patterns and the time-variation ofthe polygonal pattern with disk rotation inside an actual harddisk drive.

C. Discussion

The time-variable polygonal flow structure acts as a distur-bance torque against the arm. It excites the rigid mode vibra-tion of the arm and degrades the head positioning accuracy inthe wider frequency range less than 1 kHz. A possible solutionfor suppressing the polygon structure and its degradation of thehead positioning error might be to put the spoiler between thedisks, which might distort the polygonal structure and reducethe flow induced arm vibration.

IV. CONCLUSION

The air flow inside an HDD was analyzed using CFD andexperiment. The existence of rotating polygonal flow patternsinside the actual HDD was directly confirmed both by sim-ulation and experimental flow visualization. Furthermore, wefound that over time the polygonal number varied repeatedly inthe actual HDD and we observed triangular, tetragonal, pentag-onal, and hexagonal flow pattern both in simulation and exper-imental visualization. The outcomes of this study contribute tobuild basis of understanding of the actual air flow behavior in-side the HDD to accomplish the higher areal density HDDs.

ACKNOWLEDGMENT

The authors would like to thank Prof. S. Obi and Mr. T.Washizu from Keio University, Japan, for their useful sugges-tions and discussions.

2398 IEEE TRANSACTIONS ON MAGNETICS, VOL. 48, NO. 8, AUGUST 2012

REFERENCES[1] K. Aruga, “3.5-inch high-perfomance disk drives for enterprise appli-

cations: AL-7 series,” Fujitsu Sci. Tech. J., vol. 37, no. 2, pp. 126–139,Dec. 2001.

[2] E. Lennemann, “Aerodynamic aspects of disk files,” IBM J. Res. De-velop., pp. 480–488, 1974.

[3] R. Kaneko, S. Oguchi, and K. Hoshiya, “Hydrodynamic characteristicsin disk packs for magnetic storage,”Rev. Elect. Commun. Labs, NipponTelegraph and Telephone Public Corp., Japan, vol. 25, pp. 1325–1336,1977.

[4] S. D. Abrahamson, J. Eaton, and D. J. Koga, “The flow betweenshrouded co-rotaing disks,” Phys. Fluids, vol. 1, no. 2, pp. 241–251,1989.

[5] J. A. C. Humphrey, C. A. Schuler, and D. R. Webster, “Unsteady lam-inar flow between a pair of disks corotating in a fixed cylindrical en-closure,” Phys. Fluids, vol. 7, no. 6, pp. 1225–1240, 1995.

[6] M. Tatewaki, N. Tsuda, and T. Maruyama, “An analysis of disk flutterin hard disk drives in aerodynamic simulations,” IEEE Trans. Magn.,vol. 37, no. 2, pp. 842–846, Mar. 2001.

[7] D.-W. Kong, “Behavior of Periodic Flow Disturbances in ShroudedCo-Rotating Disks,” Ph.D. dissertation, Yonsei University, Korea,2008.

[8] J. Mizushima, G. Sugihara, and T. Miura, “Two modes of oscillatoryinstability in the flow between a pair of corotating disks,” Phys. Fluids,vol. 21, p. 014101, 2009.

[9] M. Ikegawa, H. Mukai, T. Sugii, and M. Watanabe, “Fluid simulationand measurement of polygonal airflow pattern in HDD,” in APMRC2010 Dig.

[10] K. Sundaravadivelu, Q. D. Zhang, N. Y. Liu, E. H. Ong, T. H. Yip, G.L. Chin, and J. Q. Mou, “Flow-induced slider vibration in a functionalhard disk drive: Influence of air shroud,” IEEE Trans. Magn., vol. 45,no. 11, pp. 4923–4928, Nov. 2009.

[11] M. Kazemi, “Investigation of fluid structure interaction of a head stackassembly in a hard disk drive,” IEEE Trans. Magn., vol. 45, no. 12, pp.5344–5351, Dec. 2009.

[12] M. Ikegawa, H. Mukai, and M. Watanabe, “Airflow-simulation byvoxel mesh method for complete hard disk drive structure,” IEEETrans. Magn., vol. 45, no. 11, pp. 4918–4922, Nov. 2009.

[13] H. Min, X. Huang, and Q. Zhang, “Aerodynamic pressure fluctua-tions associated with flow-induced vibration of the head gimbals as-sembly inside a hard disk drive,” IEEE Trans. Magn., vol. 48, no. 1,pp. 101–106, Jan. 2012.