1. IEEE TRANSACTIONS ON MAGNETICS, VOL. 48, NO. 8, AUGUST 2012 2395
Communication
Computational Fluid Dynamics and Experimental Visualization of
Time-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, Japan
Samsung Electronics, Suwon, Gyeonggi 443-742, Korea
Seagate 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 both
through simulation and experiment for the first time. Even though earlier studies reported time-invariant polygonal flow pattern inside
fully shrouded co-rotating disks, we found that the polygonal flow pattern varied its polygonal number repeatedly with time in not fully
shrouded 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 the
polygonal flow patterns. It was also found that each of these frequencies basically coincides with the frequency expected by the relation
between the polygonal number and the vortex frequency which has been reported by earlier studies using fully shrouded co-rotating
disks.
Index Terms—Computational fluid dynamics, flow visualization, fluid-induced vibration, hard disk drives.
I. INTRODUCTION
TO 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 by
the disk rotation excites mechanical vibrations of the head car-
riage arm and the disk [1]. Such flow-induced vibrations have
been dominant disturbance factors against the head positioning
accuracy and are the biggest concerns in the development of
drives with higher recording density. Therefore, it is crucial
to understand the aerodynamic phenomena inside the actual
drives to achieve higher recording density. Earlier studies
found a polygonal flow pattern within the co-rotating disk
domain for simplified cylindrical fully shrouded drives [2]–[8].
In these studies, a rotating time-invariant polygonal pattern was
reported for the cases using fully shrouded drives. However,
the actual structures of commercially available drives are
not fully shrouded and have a head carriage arm, filter, and
other complicated structures. Ikegawa et al. recently reported
several peaks in the spectrum of air pressure experimentally
measured at the tip of the arm [9]. This result indicates that
multiple polygonal flow patterns exist in the HDD. However,
the existence of these multiple polygonal flow patterns has still
not been directly confirmed through numerical simulation or
experimental flow visualization [10]–[13]. In this study, we
Manuscript received November 24, 2011; revised January 11, 2012 and
March 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 investigated
the existence and conformation of the polygonal flow pattern
inside the drives.
II. METHODS AND CONDITIONS
The air flow inside a 2.5-inch mobile HDD was analyzed
by using both computational fluid dynamics (CFD) and exper-
imental flow visualization. We analyzed the air flow behavior
under arm unloaded conditions, so that the basic characteristics
of air flow inside the HDD could be confirmed.
0018-9464/$31.00 © 2012 IEEE

2. 2396 IEEE TRANSACTIONS ON MAGNETICS, VOL. 48, NO. 8, AUGUST 2012
TABLE I
SIMULATION CONDITIONS
TABLE II
EXPERIMENTAL CONDITIONS
A. Numerical Simulation
The large eddy simulation (LES) with the dynamic
Smagorinsky model was employed as the turbulence model in
order to reproduce precise and accurate air flow behavior inside
the HDD. Fig. 1(a) shows the numerical simulation model. We
modeled most of the key components such as the disk, base,
arm, yoke, bracket, and filter in the CFD model. We imposed
periodic boundary conditions on both the upper and lower
boundaries of the fluid domain. The rotational velocity was
applied to the surfaces of the disks and the spacer between two
disks. Table I summarizes the simulation conditions.
B. Experimental Visualization
To clarify the simulation results and confirm the actual air
flow behavior inside the HDD, we conducted flow visualization
based on a particle scattering technique using a laser sheet. For
the flow visualization between the co-rotating disks, we build a
special drive, as illustrated in Fig. 1(b). This drive has a clear
side window, a clear cover, and clear glass disks. Oil tracer par-
ticles were seeded into the drive using an atomizer. A thin laser
sheet was injected into the drive through its clear side window,
and it illuminated the tracer particles in the mid-plane between
the co-rotating disks. The illuminated tracer particles made it
possible to visualize the air flow, and the flow images were cap-
tured by a charge-coupled device (CCD) through the clear disk
and 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 after
the 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 manner
of a laminar flow. Between the turbulent region at the OD and
the laminar region at the ID, a polygonal flow pattern rotating
with the disk rotation was observed. Furthermore, we found that
over time the polygonal number varied repeatedly between three
and six in the actual HDD. We observed triangular, tetragonal,
pentagonal, and hexagonal flow patterns in the actual HDD, as
illustrated in Fig. 2. Even though the triangular and tetragonal
flow patterns are relatively not clear compared to other patterns,
in Fig. 2 the polygonal numbers were determined based on the
spectrogram analysis. The flow pattern changed from one pat-
tern to another in a quasi random manner. Duration time of one
pattern typically corresponded to the rotation time for one to
four disk revolutions.
Fig. 3 plots the power spectrum density of the circumferen-
tial directional air velocity at three radius positions. In the ID the
flow was laminar, and the velocity fluctuation was the smallest.
In the OD the flow was turbulent, and the velocity fluctuation
was quite large compared to the ID. Furthermore, sharp peaks
were observed in the mid-diameter (MD) and OD positions. The
first four peak frequencies in the MD were 200, 310, 370, and

3. 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 the
velocity magnitude contours in Fig. 2.
Here, for the flow between two fully shrouded co-rotating
disks, the relation between the polygonal number and the peak
frequency 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 to
the 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, for
the fully shrouded cases. These frequencies basically coincide
with those appearing in the power spectrum density of Fig. 3.
The earlier studies found the time-invariant polygonal pattern
with a fixed polygonal number. However, we found that the
polygonal 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 HDD was experimentally measured
under the same conditions as in the numerical simulation. Fig. 4
illustrates the results of instantaneous flow visualization at the
mid-plane between two co-rotating disks. The laser sheet was
injected from the right side of the figures. Because the spindle
motor and spacers are opaque to the laser sheet, the left portions
of 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 shown
in Fig. 4. In the ID the air behaved as a central core body in
the manner of a laminar flow, resulting in nonuniform density
of the mist. Between the turbulence region in the OD and the
laminar 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 confirmed
inside the actual HDD. These experimental results revealed the
existence of polygonal flow patterns and the time-variation of
the polygonal pattern with disk rotation inside an actual hard
disk 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 in
the wider frequency range less than 1 kHz. A possible solution
for suppressing the polygon structure and its degradation of the
head positioning error might be to put the spoiler between the
disks, which might distort the polygonal structure and reduce
the flow induced arm vibration.
IV. CONCLUSION
The air flow inside an HDD was analyzed using CFD and
experiment. The existence of rotating polygonal flow patterns
inside the actual HDD was directly confirmed both by sim-
ulation and experimental flow visualization. Furthermore, we
found that over time the polygonal number varied repeatedly in
the 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 to
build 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.

4. 2398 IEEE TRANSACTIONS ON MAGNETICS, VOL. 48, NO. 8, AUGUST 2012
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