International Journal of Informatics Society, VOL.15, NO.2 (2023) 55-65

55

Regular Paper

Air Flow Channel Planning for Droplet-Spray-Type Olfactory Displays
Using a Small Wind Tunnel
Yohei Seta*,***, Mitsunori Makino**, Yuichi Bannai ***, and Motofumi Hattori****

Graduate School of Science and Engineering, Chuo University, Japan**
Faculty of Science and Engineering, Chuo University, Japan***
Department of Information Media, Kanagawa Institute of Technology, Japan{a15.s845, makino.fme7}@g.chuo-u.ac.jp{bannai, hattori}@ic.kanagawa-it.ac.jp
Abstract - To develop an olfactory display with highfragrance presentation performance and ease of maintenancefor long-term use, a small wind tunnel was constructed toobserve the inside of the flow channel of a prototype with apiezoelectric droplet spray mechanism for solving the
problem of poor volatilization of the fragrance solution in theflow channel. The cross-sectional dimensions of the flow
channel minimize the time for visible droplets to exist in thechannel after purified water is sprayed in a small wind tunnel,
where the channel height and width can be changed.
Spraying was performed on the obtained cross-sectional
dimensions in five fragrance solutions, and no volatilization
defects were observed in all but one of them. The
piezoelectric olfactory display with the cross-sectional
dimensions of the flow path obtained in this study is expected
to contribute to the development of olfactory displays that can
precisely synchronize the aroma with video and audio
information.
Keywords: olfactory display, pulse ejection, droplet
atomization，visualization

1

INTRODUCTION

Among the devices that present stimuli to people's fivesenses, a device that presents visual stimuli is called a display,whereas a device that presents olfactory stimuli is called an
Olfactory Display. In 1962, the Sensorama [1] was the first
known example of an olfactory display. Since the 1990s,
multimodal interfaces have been actively researched in thefield of human interfaces, and interest in the five senses ofpeople has increased [2]. Various types of olfactory displays
have been developed since the 2000s [3]-[4], and theirapplications include a system that presents smells
synchronized with TV images and movies, with smells shownin movie theaters [5]. In virtual reality, by presenting sensory
stimuli in addition to visual and auditory stimuli, users canexperience smells in a VR environment [6].
Olfactory displays can be broadly classified into two types:
those that control the injection of air mixed with fragrancecomponents in advance and those that atomize solutions
containing fragrance components as tiny droplets andvaporize them in air at room temperature. In either format, theuser inhales air-containing fragrance components to allow

them to reach the olfactory cells in the nasal cavity, thereby
enabling the presentation of fragrance information [7].
When presenting an odor synchronized with video or VR
content, if a high concentration of odor is continuously giventhat everyone can recognize, the odor will remain in the space,which is a residual problem [8]. If another odor is present ina residual state, the odor will be mixed up. Another problem
is olfactory adaptation, which is the inability to perceive
odors when the odor stimuli are administered on a sustained
basis.
To solve these problems, Okada et al. developed an inkjet
olfactory display [9] that used a thermal inkjet printer head tospray fragrance droplets. This is characterized by the fact thatseveral hundred microscopic holes in the printer head ejectseveral picoliters of droplets at a frequency of several KHz,
enabling accurate control of the ejection volume and density
over a wide dynamic range.
To confirm the basic performance of this olfactory display,experiments were conducted on the sensory characteristics ofsmell through subjective evaluation, and the following
findings were obtained: Kadowaki et al. showed that humans
could adequately perceive fragrances when pulsed fragranceswere presented by injection for a very short period(approximately 0.1 s). They noted that this allowed theexperiment to be conducted with a very small amount ofaroma injection, thereby reducing the residual aroma in theair [10]. In addition, although the human sense of smell adapts
to aroma stimuli in a short period of time, it is possible toreduce the onset of olfactory adaptation by presenting minuteamounts of aroma in a pulsed (intermittent) form [11].
Noguchi et al. showed that there is a time threshold for the
fragrance injection interval at which two pulsed fragrances
can be perceived separately during one breath (two-point
fragrance separation threshold) [12]. Based on the results ofolfactory sensitivity measurements over time within one
breath, they proposed an injection method that maximizes theperceived fragrance intensity during one breath. Aruga et al.
proposed a method to measure scent discriminationthresholds using a pairwise comparison of two scent pulses asa measure of olfactory sensitivity, and applied it to an
experiment to investigate the effect of self-motion sensation
on olfactory sensitivity [13].
These findings indicate that pulse injection of scents is more
effective than continuous presentation of scents in

ISSN1883-4566 © 2023 - Informatics Society and the authors. All rights reserved.

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Y. Seta et al. / Air Flow Channel Planning for Droplet-Spray-Type Olfactory Displays Using a Small Wind Tunnel

synchronization with other information media [9]. Thus, theinkjet olfactory display is superior in terms of granularity andprecision of injection volume and injection time, and can besaid to have a sufficient level of performance for olfactory
displays. Although ink cartridges are commercially available,
they are black boxes and cannot be disassembled and cleaned
to deal with head clogging and odors over long periods of use,which is a maintenance problem [8]. This problem may cause
inkjet olfactory displays that have been used for a long periodof time to receive extra fragrance components, even whenfragrances are not present, leading to a decline in fragrance
switching performance and experimental accuracy. In
addition, in thermal inkjet printers, the heat from the heater
creates bubbles in the liquid, which push out droplets throughpressure changes, and there are concerns regarding the effecton heat-sensitive fragrances. Printer manufacturers holdpatents for inkjet technology, including technologies forpreventing head clogging and removing clogging, and there
are significant obstacles to the development of olfactory
displays using new inkjet engines in the future.
Therefore, for the practical application of olfactory displays
based on open technology, this research group is conducting
research and development of a new olfactory display focusing
on spraying droplets using the ultrasonic vibration of apiezoelectric element with microscopic holes [14], aiming at
precise droplet spraying based on a simpler principle. In thisstudy, this is referred to as a piezoelectric olfactory display.
The piezoelectric olfactory display is intended to be applied
not only to VR content but also to a wide range of fields, suchas psychophysical experiments, by achieving both a pulse
injection performance of fragrance equivalent to that of theinkjet type and maintainability that allows long-term use.
In this process, a prototype was made following the bodychannel dimensions used for the inkjet-type [15]. When the
prototype was sprayed with droplets for a short period toevaluate its performance, droplet adhesion on the bottom
surface of the channel was confirmed [16]. The droplets wereconfirmed to vaporize afterwards, but some dropletsremained for a few seconds after the end of droplet spraying.
This indicates the possibility of continuous scent presentation
for more than ten times the time of droplet atomization. This
problem must be solved to achieve a pulse injection
performance of a fragrance equivalent to that of an inkjet-type.
Possible solutions to this problem include changing to amore volatile fragrance solution, reducing the diameter of thesprayed droplets to improve their surface area, increasing thevolatility, and expanding the flow path shape such that thesprayed droplets do not contact each other until they fully
evaporate. In the case of a change in the fragrance of asolution, a change in its physical properties may interfere withthe atomization of the piezoelectric element. In addition, toreduce droplet diameter, it is necessary to perforate thepiezoelectric element mold with micropores of different sizes.
When changing the shape of the flow path, it is necessary tofind a flow path dimension that is sufficiently volatile, even
if the atomizing object comes into contact with it; however,this will lead to a larger body.
In this study, we employed a change in channel geometry,
which has the lowest change cost among the above
approaches, to obtain the necessary and sufficient channel

cross-sectional dimensions without droplet adhesion during
droplet spraying. An experimental wind tunnel with variablechannel walls was constructed to allow direct observation ofdroplet volatilization failure in the channel. In theexperimental wind tunnel, the structure was created using
fluid dynamics to achieve a stable and unbiased airflow inside
the tunnel [17]-[18]. Based on the hypothesis that if there isno droplet adhesion in purified water, there will be no droplet
adhesion in fragrance solutions containing a portion of
ethanol. We will conduct a purified water spray experiment
while moving along the channel wall and confirm whether
there is droplet adhesion by visually observing and extracting
the sprayed object from the video captured using image
processing technology. When the flow dimensions without
droplet adhesion of water were obtained, the effectivenesswas verified by injecting a fragrance solution with the same
dimensions. By creating a piezoelectric olfactory display withthe obtained channel dimensions, we expect to make progressin the development of an olfactory display with a droplet
spray mechanism that is easy to maintain and has the same
pulse injection performance as an inkjet-type fragrance
solution and easy removal of fragrance.

2

METHODOLOGY
2.1 Piezoelectric Olfactory Display

Our research group developed a piezoelectric olfactory
display that adopts a piezoelectric element as a droplet
atomization mechanism [14] to circumvent the disadvantagesof inkjet olfactory displays such as thermal injection,difficulty in downsizing the atomization mechanism, andcomplicated cleaning and maintenance [9].
Figure 1 illustrates the operation of the piezoelectricolfactory display [15]. This device uses a fan to blow a liquidfragrance that is atomized by a piezoelectric device built intoa liquid fragrance tank to present the scent to the user. The
piezoelectric device had a diameter of approximately 1 cm,
and 100 micropores of 9 μm diameter were drilled in its center.
Although the liquid fragrance in the liquid fragrance tankdoes not pass through the micropores owing to surface
tension when no voltage is applied, it is atomized through themicropores by vibrations when a voltage is applied. The piezo
element oscillated once every 10 μs, and approximately 167
pL of pure water atomization was observed with each
oscillation.
A fan was employed to blow air over the atomized liquidfragrance, promote its volatilization of the liquid fragrance,and present the scent to the user.
A 3 mL capacity liquid fragrance tank was placed above thepiezoelectric device and the fragrance in the tank wascontinuously ejected from the piezoelectric device. The tank,piezoelectric device, and the piezoelectric device holder at thebottom of the tank were removable to facilitate cleaning andmaintenance. An experiment to measure the mass of purified
water atomized 10 times for 10 s verified that the average
injection mass was 0.17 g (standard deviation: 0.0077) andremained constant regardless of the number of atomization.
Different types of fragrances have different viscosities,

International Journal of Informatics Society, VOL.15, NO.2 (2023) 55-65

57

Figure 1: Cross-sectional image of piezoelectric olfactory display (side).
surface tensions, and other characteristics; therefore,
fragrance solutions to be atomized in olfactory displays must
be adjusted according to their composition.

2.2 Limitations of the Piezoelectric Device
Olfactory Display
Our prototype's air flow channel has a cross section 20 mm
high and 60 mm wide, which follows the structure of theinkjet olfactory display [9], [15]. To install the liquidfragrance tank, a 10 mm rise section was provided at the topof the air flow channel, and a cylindrical hole with a diameterand height of 15 mm and 15 mm, respectively, was created in
the rise. The surface of the liquid fragrance tank was attachedto the top of the raised section and atomization was directedvertically toward the bottom of the airflow channel. To
prevent the piezoelectric device from being directly touchedby hand, a 5 mm high dimple exists at the bottom of the liquidfragrance tank as a frame to protect the surface of thepiezoelectric device, and a space for fixing the piezoelectric
device exists above the frame. Therefore, the distance fromthe atomization position of the piezoelectric device to thebottom of the channel is 40 mm, which is equivalent to theatomized height of an inkjet olfactory display, and previous
experiments with inkjet in olfactory displays, atomized
substances are sufficiently vaporized in airflow channels [10].
Therefore, it was assumed that the atomized substance usedin the piezoelectric olfactory display experiment vaporized
immediately after ejection and did not remain as droplets on
the wall surface.
However, the atomization mechanisms of inkjet andpiezoelectric olfactory displays differ, including thecomposition of the liquid fragrance solution; hence, thevaporization of the atomized substance is also expected todiffer.
We created an acrylic plate airflow channel with a height of
20 mm and a width of 60 mm to confirm the conditions in the
airflow channel immediately after the atomization of dropletsin the piezoelectric device olfactory display of our prototype[16]. We connected the liquid fragrance tank of our prototype
method to the acrylic plate airflow channel and observed theinside of the airflow channel by capturing video images at thetime of atomization. In the 0.3 s pulse injection experiment,
purified water clearly left droplets over a wide area at thebottom of the channel, and it took approximately 10 s before

the droplets were no longer visible. The droplets of thebanana fragrance solution were not visible earlier than thoseof the purified water, with an average of 4.8 s. The timerequired for the droplets of the banana fragrance solution todisappear was approximately 10 s, whereas that of purified
water was approximately 10 s. The results showed that theatomized substance adhered to the bottom surface of theconventional structure as droplets at the time of atomizationof the banana liquid fragrance and purified water andevaporated later than after the system had finished atomizingthem.
To accurately determine a person’s scent detection
threshold using a droplet-atomizing olfactory display, it isnecessary to switch between odorless and attached states witha high degree of accuracy [9]. If the presentation state by thesystem is odorless but the subject is presented with a scentbecause of the above problem, the olfactory characteristics ofthe subject may not be measured correctly.

3

METHODTo improve the volatilization of droplets observed in thepiezoelectric olfactory display prototype, an experimental
apparatus that allowed visual inspection of the inside of thedisplay was constructed to confirm the conditions at the timeof spraying. Based on the hypothesis that purified water isless likely to volatilize than fragrance solutions based on theresidual time of droplets observed in the prototype describedin Chapter 2, we created an experimental apparatus with afixed channel width and changeable channel height inExperiment 1 to find a channel height at which droplet
adhesion of purified water on the bottom surface is notobserved. The channel height was determined to prevent
droplets of purified water from adhering to the channelbottom. Because droplet adhesion was observed in the width
direction in Experiment 1, Experiment 2 was conducted todetermine a channel width that could be changed while
keeping the channel height fixed at the height obtained inExperiment 1, so that no droplet adhesion could be observedon the sides. The presence or absence of droplets isdetermined by image processing, which highlights thedroplets when visual observation is impossible. The
effectiveness of the method was verified by spraying fivefragrance solutions of different compositions on the final
channel dimensions.

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Y. Seta et al. / Air Flow Channel Planning for Droplet-Spray-Type Olfactory Displays Using a Small Wind Tunnel

3.1 Experiment 1: Confirmation of theRequired Air Flow Channel Bottom
Distance During Purified Water
Atomization
An experimental airflow channel was constructed using
acrylic plates to determine the minimum distance at which a
droplet was atomized by the piezoelectric device without
contact with the bottom of the airflow channel. Figure 2
shows the configuration and dimensions of the experimental
airflow channel.
In the experiment, the distance from the piezoelectric
device to the bottom of the airflow channel was varied, and avideo of the droplet atomization was recorded. Video imagesof the atomized droplets were captured and checked for thepresence or absence of droplet adhesion at the bottom of the
airflow channel. The threshold value of the bottom height forthe presence or absence of deposits at the bottom of the
airflow channel was defined as the minimum distance without
contact with the bottom of the airflow channel whenatomizing the droplets.
Previous studies have demonstrated that liquid fragrances
containing ethanol or other solvents vaporize faster thanpurified water [16]. By determining the threshold of thebottom height relative to the purified water, liquid fragrancesthat could be used in the future can be considered. Therefore,
only purified water was used in this experiment.
The experimental airflow channel employed a bottom
surface whose height could be modified as desired; this isreferred to as a movable floor in this study. The movable floor
was an acrylic plate with the same dimensions as the bottomof the airflow channel, wrapped with vinyl tape at both ends
and fixed in position by friction with the acrylic plate on thechannel side. Therefore, the movable floor could be easily
removed by changing the bottom height. To maintain
constant conditions on the movable floor surface duringcontinuous injection, each time the bottom height waschanged, the surface was wiped with a paper rag moistenedwith purified water, and then dried again with a dry paper rag.
While fixing the movable floor, the height of the installedsurface was checked using a digital caliper, and a level wasused to check the level at the part protruding from the channel
prior to the experiment.
The air velocity in the airflow channel during the droplet
atomization experiment was 1.8 m/s, which was adopted inconventional experiments on inkjet olfactory displays [10].
A PWM-controllable PC fan was utilized to provide aconstant airflow in the experimental airflow channel. The
selection of the fan was based on the maximum airflow rate,
and the fan was required to sufficiently satisfy the target air
velocity over the entire experimental airflow channel crosssection. Blowing air using rotating fans generated low
velocity near the center of the air velocity distribution,
asymmetrical bias, and turbulence. The airflow in theexperimental airflow channel was stabilized to improve thereproducibility of the experimental results.
In this experimental airflow channel, a grid-like structure[17]-[18] used for rectification in the experimental windtunnel was installed in the cross-section of the airflow
channel, which flowed into the airflow channel from the fan
mounting area. Previous experiments have demonstrated thatthe grid shape is effective in stabilizing the airflow in the
airflow channels of piezoelectric olfactory displays when thepitch width, plate thickness, and open area ratio are 5 mm, 1mm, and 60%, respectively [19]; in addition, the same
parameters will be adopted in this study. The fan was
mounted in the direction of the intake from the top of theexperimental apparatus and perpendicular to the cross-sectionof the airflow channel to reduce the influence of the lowspeed portion at the center of the fan. During the experiment,
an anemometer was placed at the center of the cross-section,
which was at the end of the experimental airflow channel, andthe power of the fan was adjusted to achieve the target windvelocity while checking the wind velocity. The experiment
was conducted only at wind speeds of 1.8 ± 0.03 m/s.

3.1.1

Figure 2: Structure of experimental channel for confirmationof the required airflow channel height (unit: mm).

Experiment 1: Pre-Confirm ofWind Velocity Distribution in theExperimental Air Flow Channel

To verify the maximum air velocity and rectification effectin the experimental airflow channel, and to check whether
the use of the movable floor significantly disturbed the air
velocity distribution in the airflow channel cross-section, theair velocity distribution was measured at the vertical andhorizontal centers of the cross-section at the airflow channel
end when the experimental airflow channel height was h =200 mm and h = 100 mm, with the movable floor at the output
under the maximum airflow rate of the fan.
The experimental channel was created, and a 180 mm PC
case fan “AP184i PRO” from Silver Stone was used for thefan, with PWM control using an Arduino uno 3. The
experiment was conducted at a voltage of 12 V, which wasInternational Journal of Informatics Society, VOL.15, NO.2 (2023) 55-65

3.2 Experiment 2: Confirmation of theRequired Air Flow Channel Width
During Purified Water Atomization

3.5

100

Measurement position (mm)59Wind velocity (m/s)

3.0
75

50

25

0

2.5
2.0
1.5
1.0
0.5
0.0

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

0Wind velocity (m/s)

5

10

15

20

25

30

Measurement position (mm)Figure 3: Wind speed distribution (h = 100 mm).
3.5
3.0
150

Wind velocity (m/s)

Measurement position (mm)

200

100

50

0

2.5
2.0
1.5
1.0
0.5
0.0

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

0Wind velocity (m/s)

5

10

15

20

25

30

Measurement position (mm)Figure 4: Wind speed distribution (h = 200 mm).
Table 1: Average wind velocity of channel cross-section
Horizontalh (mm)
Mean (m/s)
SD (m/s)

100
2.61
0.21

Vertical
2.7
0.12

Horizontal
200
2.8
0.21

Vertical
2.7
0.06

the rated output of the fan, to determine the average windspeed at which a stable output was possible. The anemometerused was a KANOMAX Thermal Anemometer (Anemo
Master Light Model 6006). The center of the anemometer
element was captured for 10 s at each sampling point of theflow cross section, and the average
The value recorded per second was the measured value ateach point. The anemometer element has a length ofapproximately 3 mm. During the measurement, the windanemometer was oriented perpendicular to the direction inwhich the sampling point moved along the longitudinal axis
such that the wind speed values were recorded as points.
In both cases, the vertical wind velocity distribution wasalmost flat, with an average of approximately 2.7 m/s, thehorizontal wind velocity distribution was almost symmetrical,
with the fastest near the center and gradually slowing as it
approached the wall (Figs. 3-4). Table 1 presents the averagewind velocity and standard deviation of the cross-section of
the experimental airflow channel.
The obtained results indicate that the experimental airflow
channel can be utilized for this experiment because the target
air velocity of 1.8 m/s is sufficiently satisfied, and there is
negligible turbulence in the air velocity distribution owing to
the asymmetric flow caused by fan rotation and the influenceof the movable floor.

From the results of Experiment 1, no adhesion of purified
water to the bottom of the channel was observed when the
distance from the bottom of the channel to the piezoelectricelement was at least 70 mm. In addition, droplet adhesion to
the channel surface occurred regardless of the channel height
when the width of the experimental channel was 30 mm.
Therefore, to obtain a channel shape that does not allow
droplet adhesion, this study describes a new experimental
channel whose side width can be changed to obtain a channel
width that does not allow droplet adhesion.
Based on the results of Experiment 1, the experimental
channel shape was fixed at a channel height of 70 mm, andthe total distance from the piezoelectric element to the bottomof the channel was 76 mm, including a 6 mm depression in
the liquid fragrance tank. The channel width can be varied
within the range 30-70 mm. Because the flow beyond achannel length of 200 mm could not be observed at theExperiment 1, a channel length of 300 mm was used inExperiment 2.
Using the PC fan connection module used in Experiment 1,
we created a new acrylic plate flow channel and a rectifying
flow structure. Two acrylic panels were placed parallel toeach other in the channel and the distance between the panels
was manually adjusted from the outside to change the channel
width, which was used as a movable sidewall. When aprototype of the movable sidewall was fabricated using a 2mm-thick acrylic plate, it was confirmed that the acrylic plate
was warped, making it difficult to install it parallel to the wall.
Because warpage was reduced using a 3-mm-thick acrylic
plate, a 3-mm-thick acrylic plate was used for the movablesidewall in this method.
In a channel using only a rectifying grid with an apertureto-area ratio of 60% and a grid spacing of 5 mm as therectifying structure, the distribution was biased in thehorizontal direction. This biased distribution can be attributed
to the sudden change in the cross-sectional area of the flow
path from the cross-section of a 200 mm high, 200 mm wide
acrylic box connected to a fan to an experimental flow
channel with a height of 70 mm and width of 80 mm. To
weaken this effect, a bell-mouth shape was adopted, whichenabled stable fluid intake by smoothly decreasing the crosssectional area of the flow channel while connecting it from awide space to a narrow duct. To prevent sudden contractionof the cross-sectional area of the experimental flow channel
after it is narrowed by the movable sidewall at the connection
between the bell mouth and the experimental flow channel, ahole is provided where the handle of the movable sidewall
overlaps the cross-sectional area of the flow channel to allow
air to pass through.
In this experimental channel, the wind velocity distributions
at w = 70 mm and w = 35 mm were analyzed in the same way
as described in Experiment 1 Both of mean wind speeds inhorizontal directions were 2.51 m/s, the maximum windspeed differences at the horizontally symmetrical positions
were 0.18 m/s and 0.14 m/s, and the standard deviations were0.08 m/s and 0.17 m/s. The mean wind speeds in vertical

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Y. Seta et al. / Air Flow Channel Planning for Droplet-Spray-Type Olfactory Displays Using a Small Wind Tunnel

Figure 5: Structure of experimental channel for confirmationof the required air flow channel width (unit: mm).
directions were 2.57 m/s and 2.65 m/s, and the maximum
wind speed difference at the vertical target position was 0.32
m/s and 0.30 m/s, with standard deviations of 0.08 m/s and0.09 m/s. These wind velocity measurements did not reveal
any extreme fluctuations in the wind velocity distribution due
to changes in the channel width. The maximum wind velocity
difference at the symmetrical horizontal position was 0.33
m/s at a channel width of 70 mm before the bell mouth was
installed, whereas it was approximately half that after the bellmouth was installed. The wind velocity bias of each channel
was considered acceptable and the experimental channels
were used for the experiment.
Figure 5 shows an image of the experimental apparatus.
Using this experimental channel, atomization experiments ofpure water were conducted at 5 mm intervals at a channel
width ranging from 30 mm to 70 mm. When the width of theflow channel was changed, the movable sidewalls wereoperated. Moreover, the width of the channel was measured
with a digital caliper to confirm that the movable sidewalls
were parallel to each other and that the experimental channel
was horizontal using a leveler before conducting theexperiment. In each channel-width experiment, the droplets
were first atomized for more than 10 s and observed with the
naked eye at a close distance of approximately 100 mm fromthe channel wall to check for abnormalities in theexperimental equipment, droplet atomization, and droplet
adhesion on the channel wall.
After confirmation, four sets of videos were captured fromthe side surface of the channel, with one set consisting of
three consecutive droplet atomization for 1 s at 5 s intervals.
An anemometer was used to check the wind speed at thecenter of the cross-section at the channel tip. Similar to thesetup in Experiment 1, the camera was ready to start capturing

the wind speed when it was 1.8 ± 0.03 m/s. Each set useddifferent camera settings. In this experiment, full HD
resolution (240 fps), 4 K resolution high dynamic range
setting (30 fps), 4 K resolution normal capture (60 fps)
brightness adjustment based on bright points, and 4 K
resolution normal capture (60 fps) brightness adjustment
based on dark points were used. After the experiment, thefollowing were confirmed based on the results for eachchannel width: No obvious droplet adhesion was observed by
naked-eye inspection. In each of the 12 videos of droplet
atomization, the target frame was captured 0.17 s after the endof atomization when an object moving at 1.8 m/s had traveled
the full distance of 300 mm from the end of ejection.
When a channel width that simultaneously satisfied thesecriteria was obtained in the atomization experiment withpurified water, the atomization experiments of solutions withfive different water-soluble fragrances (banana, lemon, rose,
vanilla, and blueberry) were conducted at the minimum
channel width in the same manner as the method used for
purified water.
The atomizing solution for these experiments was preparedusing commercially available water-soluble flavoring agentsfor food additives from Marugo Corporation, as described in
our prototype. For the fragrance solution, dilute the watersoluble fragrance of each flavor with purified water, obtain afragrance component to be 5% by mass. The substances andamounts of the flavor components of each flavor used in theexperiment are not disclosed. Therefore, based on the mass of
the indicated organic solvent, the difference between theoverall mass and the mass of the organic solvent was assumed
to be the mass of the flavor components, and the ratio of thefragrance solutions mixed with purified water for each flavor
was determined. Table 2 lists the ratios of the solutions usedin the experiments. Of the five fragrance solutions selected
for the experiment, two (banana and lemon) have been used
in a previous study. Among the newly prepared fragrance
solutions, we selected three (rose, vanilla, and blueberry) thatdiffered significantly from those used in the previous study interms of the physical properties (surface tension andviscosity) and composition of the fragrance solutions. If thisexperiment shows similar to those obtained for purified water,
the experimental channel width and channel height can beconsidered appropriate shape parameters for the design of arectangular channel to be used in novel piezoelectricolfactory displays with a central air velocity of 1.8 m/s at the
tip channel cross-section.
Table 2: Component ratios of atomization solutions (%).
VanillaRose

BananaBlueberry

Lemon

Fragrance
Components

5.00

5.00

5.00

5.00

5.00

Ethanol

2.24

4.00

5.11

5.00

7.92

Glycerol

0.22

1.00

1.14

-

0.97

Propylene
Glycol

-

-

0.11

2.50

-

92.54

90.00

88.64

87.50

86.11

Water

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61

3.3 Image Processing for Improved Visibilityof Droplets
To confirm the presence or absence of droplet adhesionon the atomized target and channel wall, a backgroundsubtraction method was used to visualize the atomized
target, using the frame before the start of droplet
atomization as a background image. The contrast-enhanced
image was then observed using histogram flattening toconfirm the absence of droplet atomization-caused droplet
adhesion to the channel. For image processing, Python3
and Open CV 4.6.0 library were used.
The “cvtColor method” with the COLOR_BGR2GRAY
parameter was used to convert the target frame andbackground images to grayscale and the RGB values of the
pixels to luminance values. The difference between theluminance values of the gray scaled target frame and thebackground image is obtained using the “absdiff method.”This method only visualizes areas in which the luminancevalues change between the target and background images.
Histogram averaging was performed on the images
obtained using the background subtraction method toenhance the contrast and improve the visibility of theobserved object. The method uses the “clahe method” toperform adaptive histogram averaging on an image
obtained by subtracting the background image from thetarget frame.

4

Droplets adhering to a wide area at the channelbottomFigure 7: Experimental results (h = 34 mm, Color difference
from 0 s and after contrast adjustment, Clipping
only in air flow channel).

RESULTS

4.1 Experiment 1: Confirmation of theRequired Air Flow Channel Bottom DistanceDuring Purified Water Atomization
An experimental airflow channel was fabricated using a 3D
printer and an acrylic sheet.
Figure 6 shows the atomization at h = 34 mm. A digital
camera was positioned at the center of the screen at an angle
that captured the entire movable floor surface, and any droplet
adhesion on the bottom of the channel during purified water
spraying was illuminated by white LED light adopted asillumination and visualized in white.
Figures 7-9 show present the experimental results for h =34, 63, and 64 mm, respectively, during 1 s of purified water

Figure 6: Illustration of experimental results (h = 34 mm).

Droplets adhering like a thin line on
the center of the bottom

Figure 8: Experimental results (h = 63 mm, Color difference
from 0 s and after contrast adjustment, Clipping onlyin air flow channel).
atomization, where; the time series changed when theatomization start frame was set to 0 s.
These images were clipped only inside the airflow channel,
and the difference in color values between the atomizing startframe and each frame was determined using the method ofsection 3.3. The color values were output only for the areasthat changed from the initial frame; and the areas that did notchange were output in black. At h = 34 mm (in Fig. 7), droplet
adhesion was clearly observed over a wide area of the
movable floor surface. The droplets remained at the bottomof the air flow channel for 1.1 s after atomization wascompleted, and the atomized substance flowed out of the
airflow channel. As shown in Fig. 8, at h = 63 mm, linear
droplet adhesion was observed at the center of the movable

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Y. Seta et al. / Air Flow Channel Planning for Droplet-Spray-Type Olfactory Displays Using a Small Wind Tunnel

Droplets adhering to the side wall

Figure 9: Experimental results (h = 64 mm, Color difference
from 0 s and after contrast adjustment, Clipping onlyin air flow channel).
floor in front of the airflow channel in the form of a thin line.
In Fig. 9, h = 64 mm, the atomized substance flowing in thechannel was confirmed; however, no bottom adhesion wasobserved, and the same result was obtained in six subsequent
trials conducted for confirmation.
In all cases, however, droplet adhesion was observed on asurface on the channel side 1.1 s after the end of injection,
when the 1.8 m/s wind velocity should have sufficiently
ejected substances out of the channel.
This means that the 30 mm channel width of theexperimental channel was insufficient to prevent droplet
adhesion to the channel sides.

Figure 10: Side view of experimental results of purified
water atomization (images obtained by Contrast
Limited Adaptive Histogram Equalization forbackground subtraction before and after
atomization).

4.2 Experiment 2: Confirmation of theRequired Air Flow Channel Width
During Purified Water Atomization
An experimental channel with a variable width was created,and purified water atomization experiments were conducted.
Similar to the setup of Experiment 1, a shape model of theexperimental channel was created using Autodesk’s“Fusion360” software. A bell mouth was made from theshape model using a 3D printer (“Raise 3D E2”) with a PLA
filament as the material. The experimental flow channel,
rectifying grating, and movable sidewalls were fabricated by
cutting acrylic sheets using a laser cutting machine based on
the shape data. The parts were fixed with an acrylic adhesiveand sealed with vinyl tape to prevent air leakage from thejoints.
In previous experiments, droplet atomization in liquidfragrance tanks was found to fluctuate owing to thedeterioration of the piezoelectric element, the presence of dirt,
and poor contact between the connection terminals. The
droplet-atomizing performances of the four liquid fragrance
tanks were compared. The liquid fragrance tank with the

Figure 11: Side view of experimental results of four
fragrance solution atomization (w = 60 mm,
h = 70 mm).
highest average atomizing mass and the lowest coefficient ofvariation was selected for use in the experiment. The average
atomizing mass of purified water droplets in the liquidInternational Journal of Informatics Society, VOL.15, NO.2 (2023) 55-65

Droplets adhering on the bottom of channel

63

atomization by applying image processing to a video captured
at w = 60 mm. Figure 10 shows the results after image
processing just before the end of 1 s of purified water
atomization and 0.17 s after the atomization.
Next, experiments were conducted at w = 60 mm byatomizing the solutions of five water-soluble fragrances(banana, lemon, rose, vanilla, and blueberry).
No droplet adhesion to the channel sidewall was observed
for any of the liquid fragrance solutions, either with the naked
eye or in the target frames of the video after image processing.
However, droplet adhesion to the bottom of the channel wasobserved in the 11 atomization of the lemon fragrance
solution. White streaky adherent droplets were observed in
the center of the bottom of the channel from 150 mm to 200
mm from the atomizing start position to the end of the channel
after ≥ 0.17 s. Figures 11-12 show the results after image
processing just before the end of 1.00 s of liquid fragrance
solutions atomization and 0.17 s after the atomization.

5

Figure 12: Side view of experimental results of the lemon
fragrance solution atomization.
fragrance tank used in the experiment was 2.08 mg/s, with astandard deviation of 0.08 mg/s and a coefficient of variationof 3.60%. The following results were obtained using one of
the liquid fragrance tanks. A piezoelectric element and aliquid fragrance tank containing purified water were placedin the experimental channel such that the piezoelectricelement surface was horizontal and fixed with vinyl tape toprevent displacement during the experiment.
During the filming, the lights were turned off, the areaaround the experimental apparatus was covered with ablackout curtain to prevent the influence of flickering naturalor fluorescent light, and a small LED light source was usedfor video recording. Purified water atomization experiments
were conducted using the experimental setup described above.
A DJI “OSMO ACTION” camera was used to capture imagesof the experimental flow channel while switching the camera
settings for each experiment. The temperature and humidity
at the start of the experiment were 21.6°C and 20.7%,
respectively.
The results of the purified water atomization experiments
are presented below. Naked-eye inspection of the channel
wall during 10 s of purified water atomization revealed
numerous small droplets on the channel wall and top surface
at the center of the channel at w = 30-35 mm. The small
droplets that adhered to the wall volatilized immediately after
contact. At w = 40-55 mm, droplet adhesion to the top surfaceof the channel was no longer observed. Small droplets wereobserved to contact the channel-side surface in the range of100-150 mm from just below the liquid fragrance tank toward
the channel tip and volatilized immediately after contact. The
frequency of the droplet contacts with the channel-side
surface decreased as the channel width increased. At w = 6070 mm, almost no droplet contact was observed by naked-eye
inspection. The target frame was captured 0.17 s after droplet

ANALYSIS
5.1 Experiment 1: Confirmation of theRequired Air Flow Channel Height at theTime of Purified Water Atomization

The airflow channel height of the conventionalpiezoelectric olfactory display (h = 34 mm) was insufficientbecause of the large number of droplets adhering to thebottom surface. Bottom adhesions were observed at h ≤ 63
mm; however, no bottom adhesions were observed at h > 64
mm even after repeated atomization. The required air flow
channel bottom distance at the time of purified water
atomization at a flow velocity of 1.8 m/s, when vibrating at
100 kHz using a piezoelectric device with 100 micro-holes of
9 μm, was verified to be 70 mm, including the dimple in theliquid fragrance tank.
In all cases, contact was observed on the sides duringatomization at an 30 mm airflow channel width.

5.2 Experiment 2: Confirmation of theRequired Air Flow Channel Width at theTime of Purified Water Atomization
Experiments using purified water and four kinds of except
lemon fragrance solution showed that no droplet residue wasobserved on the wall surface when droplets were atomized byusing a channel with geometry parameters of w ≥ 60 mm andh ≥ 70 mm. This confirms that for water-soluble liquid
flavorings other than lemon flavorings used in the experiment,
there are no visible droplets in the channel at 0.17 s after theend of spraying, due to the obtained channel shape parameters,
rather than the conventional parameters of w = 60 mm and h= 34 mm. Compared to the 4.8 s required for the volatilizationof banana flavor droplets observed in the prototype, theresidual time of the droplets in the channel was reduced toapproximately 3.5%. The ambient fragrance during all theatomization experiments using water-soluble liquidfragrances qualitatively indicated that each fragrance could
be represented by the piezoelectric olfactory display.

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Y. Seta et al. / Air Flow Channel Planning for Droplet-Spray-Type Olfactory Displays Using a Small Wind Tunnel

However, the lemon fragrance solution used in thisexperiment in a channel with a flow channel more than 200
mm from the atomizing point may cause adhesion problems
at the bottom of the channel.
In this experiment, we considered why bottom adhesion
occurred only with the lemon fragrance solution. Considering
the volatility of the liquid from the boiling points understandard atmospheric pressure of the components of thelemon fragrance solution, glycerin (boiling point 290°C),
which has the highest boiling point among the known
components, was present at approximately 1%. However,under the same conditions, no adhesion to the bottom surface
was observed for rose and banana fragrance solutions
containing similar amounts of glycerin. The lemon fragrance
solution contained the highest amount of ethanol (boilingpoint 78.37°C) and the lowest amount of water (boiling point
100°C) among the tested solutions. The unique flavor
components of the lemons used in this study are not disclosed.
Therefore, it is difficult to explain the reason for droplet
adhesion to the bottom surface because of the volatility of theingredients. In the future, it will be necessary to conduct
experiments using lemon fragrance solutions with known
ingredients and to measure the actual volatilization time of
the fragrance solutions.
The effect of specific gravity of the fragrance solution was
also considered. The different specific gravities of thedroplets may cause differences in the diffusion range owing
to the effect of gravity. All video frames for a certain periodof time during atomization were superimposed and added
together to visualize the diffusion range of the atomized
droplets. This may explain the difference in the diffusion
range of each fragrance solution, and the relationship between
the specific gravity of the fragrance solution and the diffusion
range of the droplet, which may explain the lower adhesionof the lemon fragrance.
In the movie taken from the experiment using purified waterwith a channel width of w = 30-55 mm, where naked-eye
inspection confirmed droplet adhesion, droplet adhesion was
not as clearly identified as in Experiment 1. First, there was adifference in the attachment method of the liquid fragrance
tank between Experiment 2 and Experiment 1. In Experiment2, the liquid fragrance tank was fixed to the experimental
channel using vinyl tape to maintain the direction of
atomization of the liquid fragrance tank perpendicular to thebottom of the channel. Therefore, in the results of Experiment
1, where droplet adhesion was observed on the side surfaceof the flow channel, it is possible that a tilt existed in theatomizing direction of the liquid fragrance tank, and droplets
were sprayed on one side of the channel. Second, the effectof illumination on the visualization of droplets may differ
from that of the experimental channel used in Experiment 1
because one more acrylic plate passes between the channel
wall with attached droplets and the video equipment owing to
the movable sidewalls. It indicates the need for improved
lighting and experimental filming conditions.

6

CONCLUSION

To improve the performance of the prototype piezoelectricolfactory display, experiments were conducted to determine

the channel height and width at which sprayed droplets wereno longer visible in the channel. To correct the bias in thewind velocity distribution identified during the process ofdeveloping the experimental channel, data from wind tunnel
production were used to obtain a stable wind velocity
distribution with reduced bias using a rectifying grid andupon shrinkage of the channel cross-section. Using anexperimental flow whose height could be changed, we
conducted a purified water atomization experiment at a
central wind velocity of 1.8 m/s at the channel tip, similar tothe conditions of the conventional method. The results of thenaked-eye inspection and experimental filming indicated no
droplet adhesion to the channel bottom when the channelheight was 64 mm or when the distance from the channelbottom to the piezoelectric element was 70 mm. An
experimental channel with a variable width was used to
conduct the purified water atomization experiment.
No droplet adhesion was observed on the channel-side
surface when the channel width was 60 mm through video
and visual observations. In the experimental channel withgeometric parameters of w = 60 mm and h = 70 mm, where
no droplet adhesion was observed in purified water, solutionsof five liquid fragrances (banana, lemon, rose, vanilla, andblueberry) were atomized, and no droplet adhesion wasobserved on the channel sidewall, similar to the results for
purified water.
However, in the lemon-flavored solution used in this study,
droplet adhesion to the bottom of the channel was observedin all results. This indicates that in an experiment to determine
the appropriate channel height for an olfactory display, only
atomizing purified water and checking for droplet adhesion
to the bottom are not sufficient to draw a conclusion. In the
future, it will be effective to use this experimental flow
channel to set the fragrances to be used in advance, determine
the appropriate shape sufficient to use them, and check theeffects of changing the mixture components of the fragrance
solution to adapt to the set channel shape.
In addition, the piezoelectric olfactory display qualitatively
presented the liquid fragrances used in the experiment. Theseexperimental results indicate that for applications involving
the liquid fragrance used in the experiment, other than thelemon solution, the residual droplets on the channel wall canbe reduced using a channel width of ≥ 60 mm and channelheight of ≥ 70 mm when designing a new appropriate channel
structure. Future experiments using new fragrances will
confirm whether the shape parameters obtained in this study
are applicable to a wide variety of liquid fragrances. Based on
the shape parameters determined in this study, we will createa prototype of a new piezoelectric olfactory display andcompare with an inkjet olfactory display using a test subject.

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Yohei Seta He is a Doctor Course Studentof Graduate School of Science andEngineering at Chuo University. He
received a Bachelor of Media Science anda Master of Media Science from Tokyo
University of Technology. in 2009 and2011, respectively. His research interests
include real-time computer graphics,
virtual reality, computational fluid
dynamics.
Mitsunori Makino received his B.E, M.E
and Dr. Eng in Electronic Communication
Engineering from Waseda University in
1987,1989 and 1992, respectively. He wasa research assistant at Waseda University
from 1991 to 1992. He joined Chuo
University in 1992, and has been a full
professor of the Faculty of Science andEngineering, Chuo University since 2004.
His research interests are mainly intechnologies and their applications in CG, XR and visualization. His
activities also include engineering education. He is currently the
director and the committee chairperson of the Criteria in Japan
Accreditation Board of Engineering Education.
Yuichi Bannai He is a professor of
Department of Information Media at
Kanagawa Institute of Technology, Japan.
He received a BE and a ME from Waseda
Univ. in 1978 and 1980, respectively, andjoined Canon Inc. in 1980. He also
received MS from Michigan State
University in 1988, and Ph D from Keio
University in 2007. His research interests
include human five senses interaction,
artificial intelligence, and virtual/augmented reality. He is amember of ACM, IEEE CS, IPSJ, Japanese Society for AI, andVirtual Reality Society of Japan (VRSJ).
Motofumi Hattori He is a professor of
Department of Information Media at
Kanagawa Institute of Technology, Japan.
He received a Master of Science from
Kobe University in 1991. He received a
Master of Engineering from Kobe
University in 1993. He received a Doctor
of Engineering from Kobe University in
2000. His research interests include Game
Amusement,
VirtualReality,Computational Fluid Dynamics, and Statistical Physics. He is amember of Game Amusement Society of Japan, Virtual Reality
Society of Japan, IPSJ, The Japan Society for Computational
Engineering and Science, and The Japan Society of Fluid
Mechanics.