a wind tunnel study on coat fabrics drag - iftomm 2015 · a wind tunnel study on coat fabrics drag...
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The 14th IFToMM World Congress, Taipei, Taiwan, October 25-30, 2015 DOI Number: 10.6567/IFToMM.14TH.WC.PS20.001
A wind tunnel study on coat fabrics drag
A.JEANG*,F.LIANG*¥ , W. P. Chiu¥ and Liao Jian ping¥ *Department of Industrial Engineering and System Management,Feng Chia University, PO Box 25‐150, Taichung, Taiwan , ROC
¥Department of Research and Development, Cycling &Health Technology Industry R&D Center, Taichung 40724, Taiwan ROC
Summary:
This research studies the critical Reynolds number of different fabric textures and weaves via wind
tunnel experiments under a finite cylinder. Experimental conditions include a smooth cylindrical body
and a variety of fabrics with different textures and weaves. Experiments using three‐axis load cell scan
simultaneously measure instantaneous forces in three different directions. Furthermore, with the
proper dimensions of the model, Reynolds number can reach 1x105 ~ 3.8x105, i.e. the range of the
boundary layer transition region TrBL(Transition in boundary layer) [1] (2×105<Re<106).The appropriate
ranges for different fabrics can be obtained from experimental results. The results of this study will help
operators understand the effects of flow fields on fabric textures and weaves and provide a reference
for relevant engineering designs.
Keywords: Critical Reynolds number, Fabrics
1 Introduction
As the aerodynamic bicycle market becomes increasingly popular, more products that boast lightweight,
high rigidity and low wind resistance characteristics are appearing more and more on the racing bike
market, such that the racing bike industry and consumers pay considerable attention to these three
items. Due to the demand of races or to save energy of the body, the air resistance of the bike is very
important, especially because resistance accumulated over a long time will become a heavy load and
negatively affect strength or speed. Generally, the biggest source of air resistance for bikes comes from
cyclists themselves. According to statistics, air resistances between bicycles and cyclists are about 3:7 to
2:8. Therefore, in addition to the riding posture of cyclists, what cyclists wear, especially shirts and pants,
is vital.
The purpose of this study is to investigate aerodynamic flow field characteristics of fabrics and define
the critical Reynolds number range of fabrics. Air flow field behavior is one of the controllable methods
with regards to resistance, and the bike flow field Reynolds number is about 105, within the range of the
critical Reynolds number. The literature research shows that the flow fields are complex and roughness
will affect the drag coefficient much more. Obtaining research data related to fabric to air resistance
from wind tunnel experiments will be a great help to those designing cyclist attire.
2 Literature review
2.1 Critical Reynolds number of flow around circular cylinders
In the early 20th century, Wieselsberg[2] used circular cylinders with different ODs to discuss the
changes in drag coefficients vs. Reynolds number. It was the first study to record that drag coefficients
increased and declined with Reynolds number; furthermore, when the Reynolds number rose to
2x105,the drag coefficient dropped from 1.2 to 0.3. Therefore, this Reynolds number was called the
critical Reynolds number and was used to explain the roughness of a cylindrical surface’s major impact
on flow.
Zdravkovich [1,3] combined a variety of different research describing circular cylinder flow in the
boundary layer transition region (TrBL) and classified them as follows:
2.1.1 TrBL0 (Pre‐critical regime): (105~2x105) < Re < (3x105~3.4x105)
This is characterized by a discontinuous drop of the drag coefficient accompanied by intense Vortex
Sheet frequencies.
2.1.2 TrBL1 (One‐bubble regime): (3x105~3.4x105) < Re < (3.8x105~4x105)
After flow separation at one side of the circular cylinder, the cylindrical surface was contacted, forming a
unilateral separation bubble. Early in the formation of the single separation bubble, due to unsteady
properties, its location can be random and not necessarily fixed to one side of the cylindrical surface;
however, along with the Reynolds number increasing, the structure of the single separation bubble
tends to be stable, and its position does not change. At this point, the circular cylinder flow becomes
asymmetrical.
2.1.3 TrBL2 Two‐bubble regime: (3.8x105~4x105) < Re < (5x105~106)
As Reynolds number continue to increase, separation bubbles appeared on two sides of the circular
cylinder. After flow separation on two sides of the circular cylinder, the cylindrical surface is contacted
again to form a symmetrical separation bubble. Because the turbulent flow separation cylindrical at two
sides of the circular cylinder made the wake regime shrink, the drag coefficient Cd decreased to its
lowest value. [4][5]
2.2 Effect of cylindrical surface roughness
Güvenand others [6,7,8] used different cylindrical surface roughness in their experiments with
experimental Reynolds number in the rangebetween104~106.Their conclusions determined that the
increase of surface roughness stimulated circular cylinder flow into the pre‐critical regime earlier with a
lower Reynolds number, which is regarded as local interference caused by cylindrical surface roughness.
3. Laboratory equipment and models
3.1 Wind tunnel
The environment wind tunnel located on Kuei‐Jen Campus, NCKU is a low‐speed, loop‐type atmospheric
border layer wind tunnel with a cross section area of 4m×2.6m, a length of 36.5m, and an experimental
speed up to 20m/s (Figure 1).This experiment discusses the aerodynamic characteristics of a circular
cylinder in critical regime. The Reynolds number was able to reach 3.72×105 in this experiment to match
the appropriate model size.
Figure 1 NCKU Wind Tunnel
3.2 Circular cylinder model
This experiment uses a finite circular cylinder model with a diameter of 300mm and a height of 500mm,
onto which the experimental cloth can be set. Sensors are tied under the circular cylinder to measure
force signals simultaneously from the cylinder’s x‐ and y‐directions, which a rewind ward and lateral,
respectively(Figure 2), as shown in the following schematic diagram:
Figure 2 Cylinder Model
3.3 Experimental cloth materials
The experimental cloth materials used in this experiment are coded below:
1 SHELL 2 DRYSLIK 3 COOLMAXDRY 4 TAVOTARA180 5 COMPRESS 6 Power Lycra 7 Active Skin 8 Power Skin
4. Laboratory procedures and methods
4.1 Parameter analysis
In fluid dynamics, in order to simplify complex physical quantities, choosing dimensionless parameters
as the main parameters governing the flow problems is common, and one of the most important
dimensionless parameters is the Reynolds number.
Re Reynolds number:
μ
XY
Where ρ
and μ i
the expe
kept wit
Cd drag c
Fd is cyli
the cylin
4.2 Finit
Our exp
1)to me
cylindric
respecti
aerodyn
(Figure 3
V(km/hr)
Re
ρ is air dens
s the dynam
erimental Re
thin the critic
coefficient:
ndrical drag
ndrical projec
te cylindrical
eriment was
asure the cy
cal flow is in
vely, and dra
namic charac
3):
20 1.0510E+05
sity, V is the
mic viscosity o
eynolds num
cal regime.
force, ρ is a
ction area.
resistance m
s controlled a
lindrical resi
a pre‐critica
ag coefficien
cteristics of d
30 1.586E+05
velocity of th
of air. During
ber is contro
air density, V
measuremen
at a speed w
stance of the
l regime, a s
nt scan be ob
different fabr
40 2.1230E+05
he wind tunn
g this experim
olled betwee
V is the veloc
nt
within V=20~7
e different fa
ingle bubble
btained from
rics on the cy
45 2.3791E+05
Table 1
nel contracti
ment, to mea
en 1.05x105~
city of the wi
70km/hr und
abrics. Unde
e regime, and
the experim
ylinder mode
50 2.6523E+05
on exit, D is
asure the res
3.71x105whi
ind tunnel co
der specific R
er different R
d a double bu
mental result
el as shown
55 2.9193E+05
the cylinder
sistance of th
ile the cylind
ontraction ex
Reynolds num
Reynolds num
ubble regime
ts to compar
in the follow
60 3.1835E+05
r’s diameter,
he cylinder,
drical flow is
xit, and A is
mber(Table
mber, the
e,
e the
wing diagram
70 3.7148E+05
Figure 3 Fabrics setup
5. Results and discussion
5.1 Pre‐critical regime
The classification of critical regime in this experiment can be defined by y‐direction resistance varied
with Re. The data diagram for the cloth material (shell) is shown in Figure 4 as a relation diagram in
which the x‐axis is time and they‐axis is drag force Fd, varying with the Reynolds number from 1.05x105
to 3.71x 105 in the experiments. Figure 5 is the relation diagram of the corresponding Cd drag
coefficients varying with the Reynolds number.
Using the cloth Shell as an example, first observe the drag force distribution shown by the red lines
within the green box (the y‐direction of the cylinder) in Figure 4; when the Reynolds number is 2.38x105,
the linear disturbance of drag force in they‐direction is found to become larger, which suggests that the
Vortex Sheet appears at this time. Figure 5 shows that when the Reynolds number are
between2.38x105~2.65x105, Cd begins to decline significantly. According to the above literature that has
explored the relationship of flow behaviors in critical regimes, and based on the definition of critical
regimes, we can judge that the circular cylinder surface flow enters the pre‐critical regime starting from
a Reynolds number of 2.38x105.This Reynolds number is called the critical Reynolds number.
Figure 4 “Shell” Pre‐critical regime
Figure 5 “Shell” Re VS Cd ( Pre‐critical regime)
5.2 Single bubble regime
To distinguish the single bubble regime by using the cloth Shell, Figure 6 shows that when the Reynolds
number is 2.65x105, Fd, as shown by the red line as drag force in the y‐axis within the black box (y‐
direction of the cylinder),increases significantly, showing that pressure distribution on two sides of the
circular cylinder is asymmetrical while drag force is biased to one side. Please refer tothe original signal
(Figure 6) under this Reynolds number. According to the above literature that has explored the
relationship of flow behaviors in single bubble regimes, and based on the definition of single bubble
regimes, we can judge that the circular cylinder surface flow enters the single bubble regime starting
from a Reynolds number of 2.65x105.
Figure 6 Single bubble regime
5.3 Double bubble regime
As the Reynolds number continue to increase to 2.92x105, Figure 7 shows that Fd, as shown by the red
line as drag force in the y‐axis within the orange box (y‐direction of the cylinder),returns back to baseline
values, showing that the pressure distribution on two sides of the circular cylinder returns to being
symmetrical and that drag coefficient Cd after this Reynolds number appears is relatively flat, as also
shown in Figure7.According to the above literature that has explored the relationship of flow behaviors
in double bubble regimes, and based on the definition of double bubble regimes, we can judge that the
circular cylinder surface flow enters the double bubble regime starting from the Reynolds number of
2.92x105.
Figure 7 Double bubble regime
Figure 8 “Shell” Re VS Cd (Double bubble regime)
5.4 Experimental results of cloth
The experimental results from the eight kinds of cloth are shown as follows (Figure 9):
Figure 9 eight kinds of cloth (critical Reynolds Number)
6 Conclusions and recommendations
This study has discussed the aerodynamic characteristics of cloth cylinder flow of the Reynolds number
between 1.05x105~3.71x105 by using experimental methods. With different Reynolds number, their flow
behaviors differ greatly. Then we define the experimental material flow in the pre‐critical regime, the
single bubble regime and the double bubble regime for the experimental cloth. The purpose of this
study is for cyclists and the bicycle industry to choose the appropriate cloth according to different riding
postures and speeds based on the experimental results in order to decrease resistance and increase
performance.
In this research, we primarily understood the flow critical Reynolds number of the experimental cloth.
However, as for the fabric composition and the weaving method, we have no further statistics to
analyze, and their weighted effects related to flow need to be discussed in‐depth later.
7 References
[1] Zdravkovich, M. M., Flow Around Cylinders, Vol.1: Fundamentals, University Press, 1997.
[2] Wieselsberger, C., "New Data on the Law of Hydro and Aerodynamics Resistance ",Physikalsche
Zeitschrift, Vol.22, pp.321‐328, 1922.
[3] Zdravkovich, M. M., "Conceptual Overview of Laminar and Turbulent Flows past Smooth and Rough
Circular Cylinders", Journal of Wind Engineering and Industrial Aerodynamics, Vol.33, pp.53‐62, 1990.
[4] Roshko, A., "Perspectives on Bluff Body Aerodynamics", Journal of Wind Engineering and Industrial
Aerodynamics, Vol.49, pp.70‐100, 1993.
[5] Higuchi, H., Kim, H. J. and Farell, C., "On Flow Separation and Reattachment around a Circular
Cylinder at Critical Reynolds number ",J. Fluid Mech., Vol.200, pp.149‐171, 1989.
[6] Schewe, G., "Sensitivity of Transition Phenomena to Small Perturbations in Flow around a Circular
Cylinder", J. Fluid Mech., Vol.172, pp.33‐46,1986.
[7] Güven, O., Farell, C. and Patel, V. C., "Influence of Surface Roughness on the Cross‐Flow around a
Circular Cylinder ", J. Fluid Mech., Vol.98,pp.673‐701, 1980.
[8] Nakamura, Y. and Tomonari, Y.," The Effect of Surface Roughness on the Flow past Circular Cylinders
at High Reynolds number", J. Fluid Mech.,Vol.123, pp.363‐378, 1982.