ORIGINAL ARTICLE

The effect of textile air permeability on the drag of high-speedwinter sports apparel

Lars Morten Bardal • Robert Reid

� International Sports Engineering Association 2013

Abstract In a number of sport disciplines characterized by

high velocities, aerodynamic performance of sports apparel

is a concern. The goal is often to reduce the aerodynamic

drag force and thereby increase speed. In the design of

optimized competition apparel the fabric properties will be

very important. One fabric property which has traditionally

been considered an influencing parameter on aerodynamic

performance is the air permeability. In this paper the effect of

air-permeability, treated as an independent variable, upon

aerodynamic drag on a bluff body is investigated. Similar

multilayer textiles with internal membranes regulating air

permeability were tested on cylindrical models in wind

tunnel experiments in order to identify a possible relation

between air-permeability and drag force. A weak depen-

dence of flow transition on air-permeability could be found,

but this could be considered to have a limited effect on the

aerodynamic performance of sports garments.

Keywords Textiles � Aerodynamics � Air

permeability � Sports apparel

1 Introduction

In the design of winter sports performance apparel aero-

dynamics is often an important consideration. Common

winter sports in which air resistance plays an important role

on the total performance include, but are not limited to,

alpine skiing, speed skiing, speed skating, cross-country

skiing, ski cross and ski jumping. These sports all have

different requirements and restrictions for the design of

aerodynamically optimized apparel. Important consider-

ations in the design process should be velocity range and

typical ambient conditions, posture, rules and regulations,

insulation, physiological influence and functionality. Over

the past 20 years, wind tunnel testing of athletes, equip-

ment and suit materials has become more and more com-

mon and the awareness of the importance of aerodynamics

in such sports seems to be increasing.

In most high-speed winter sports, with the exception of

ski jumping, the main aerodynamic benefits are found in

drag reduction. As a consequence, the competition apparel

of these sports have developed in to advanced formfitting

body suits through an evolutionary process going back to

the dusk of modern sports. Today, the apparel of an elite

athlete is a result of a design process based on modern

technology, experience and thorough testing. The textiles

used will have to be considered by their surface roughness,

elasticity, thickness and other parameters. A review on

sports garment aerodynamics covering the most important

influencing parameters can be found in [15]. Both among

clothing manufacturers and in the sports themselves there

is a common conception that air permeability (AP) is also

an important influencing factor when it comes to aerody-

namic performance of textiles. Zippe and Graf [18] showed

that porosity of a rough surface could increase the friction

coefficient of a flat plate using plastic grains with a

diameter of 2,883 mm. However, little scientific effort has

been made to identify any direct correlation between AP

and drag for textile surfaces and no real evidence is

available to prove or disprove such a relation for sports

L. M. Bardal (&)

Department of Energy and Process Engineering, Norwegian

University of Science and Technology, Kolbjørn Hejes vei 2,

7491 Trondheim, Norway

e-mail: lars.m.bardal@ntnu.no

R. Reid

Norwegian Ski Federation, Oslo, Norway

e-mail: robert.reid@skiforbundet.no

Sports Eng

DOI 10.1007/s12283-013-0134-y

garment. The background for the conception may come

from studies and field tests using air-permeability as a

measured variable and correlating it to drag measurements

without isolating variables. In such cases, the results may

be influenced by other variables such as wear, stretch or

surface structure. Another reason for the focus on perme-

ability may be the introduction of the FIS permeability rule

for alpine skiing race suits which sets a minimum AP-limit

of 30 l/m2 s under 10 mm H2O differential pressure[10].

This rule was introduced as a safety measure with three

possible effects [11]:

• Athlete health To assure that the suit is breathable and

allows air and moisture transport.

• Athlete safety Increase friction between the suit and the

snow in case of a crash.

• Speed regulation Increase aerodynamic drag and hence

reduce maximum speed.

The 2010 FIS ISS report [16] based on interviews with

53 persons related to alpine skiing and 10 experts, com-

ments on the potential to reduce speed, and thereby risk of

injury, by altering race suit requirements. The report has

split opinions and is not conclusive. It is stated that the

effect of AP on race speed is minimal, but still it is sug-

gested that a higher AP limit should be introduced in order

to decrease race speeds. It is also stated that research is

needed in order to clarify the aerodynamic potential for

speed control by race suit regulations. FIS announced an

ongoing process regarding suit materials with research

focusing on air-permeability in a presentation on safety in

alpine skiing in 2011 [9].

In addition, some research papers mention the hypoth-

esis that air permeability increases drag, but with poor

evidence to support the statement [3, 7]. Brownlie et al. [6]

performed a wind tunnel test on alpine skiing speed suits

measuring drag and air permeability on a number of dif-

ferent suits while worn by athletes. The experiment failed

to indicate any reliable relation between the two variables,

possibly because the number of other variables was so

high.

In general the surface roughness of a bluff body does

not affect the drag force of the body in cross-flow as long

as the boundary layer is in a laminar subcritical state. The

roughness does, however, influence the critical Reynolds

number (at which turbulence is introduced in the bound-

ary layer) and the minimum and supercritical drag coef-

ficient. This is demonstrated over a wide range of

Reynolds numbers with emery paper roughness on cir-

cular cylinders by Achenbach [1]. By adding roughness to

a bluff body it is thereby possible to reduce the drag over

a certain range of speeds compared to a smooth surface.

This principle is commonly used in the design of sports

apparel [4, 13].

When evaluating fabrics for use in formfitting sports

apparel, one should also consider textile stretching. When

the race suit is worn by the athlete the textile is stretched,

mostly in the transversal direction, depending on the fitting

[12]. The amount of strain and the elasticity of the textile

will influence how well the apparel follows the contours of

the body, but it also influences the air permeability. Pre-

vious studies performed at NTNU have found no correla-

tion between air permeability and critical Reynolds number

for varying strain [2, 14]. However, a suit which is worn

repeatedly may lose some of the stretch which, in turn,

would lead to a poor fit.

This paper aims to evaluate the influence of air perme-

ability of textiles on the drag of a bluff body clad in

formfitting textiles. This paper will not evaluate such

effects on non-formfitting textiles.

2 Methods

2.1 Fabrics

In order to observe the effect of air permeability as an

isolated variable, double layer fabrics were tested in wind

tunnel experiments. The fabric samples were produced

with a perforated membrane between the inner and outer

layer which was adjusted in order to regulate the air per-

meability. The samples were otherwise identical. The

advantage of this method compared to previous experi-

ments is that influence of other variables such as wear,

strain and surface treatment are avoided. The type of fab-

rics tested is commonly used in alpine skiing race suits and

their construction can be seen in Fig. 1. The fabric samples

Fig. 1 Exploded view of tested fabrics showing basic fabric

construction

L. M. Bardal, R. Reid

tested were custom made by Plastotex SRL and were not

printed or coated.

The air permeability (AP) of the fabrics was measured

according to the ISO 9237 standard with an Akustron air-

permeability tester. The AP values are the average of four

measurements from different portions of the fabric sample.

Note that, due to limitations of the test instrument, the

fabrics were tested with a differential pressure (DP) of

200 Pa, which is almost twice the pressure difference of the

FIS standard test (10 mm water column). The average

permeability values for the different fabrics can be found in

Table 1.

2.2 Wind tunnel experiments

A common method for aerodynamic testing of fabrics,

based on drag measurements on circular cylinder models,

was used [4, 5, 8]. Each fabric sample was fitted over

120-cm-long PVC cylinder models with 25 % relative

stretch in the tangential direction and no relative stretch in

the axial direction. This strain is approximately the strain

of an alpine skiing speed suit around the limbs of an ath-

lete. Because of the low elasticity of the fabrics this strain

ensures a tight fit, not allowing any air pockets between the

fabric and the cylinder surface. The seam of the fabric

sample was placed in the leeward wake region of the cyl-

inder to avoid any influence on the boundary layer. Two

cylinder diameters (D) were used, D = 11 cm and D = 20

cm. Experiments were conducted in large-scale, low-speed

wind tunnels at the Norwegian University of Science and

technology (NTNU), Norway, and at Loughborough Uni-

versity, England. The NTNU wind tunnel is a closed loop

tunnel and has a cross-section measuring 2.7 (w) 9 1.8

(h) m while the Loughborough wind tunnel is a open loop

tunnel with a smaller cross-section measuring 1.92

(w) 9 1.32 (h) m. The blockage effects would hence be

different for the two experiments. The models used in the

two wind tunnels were similar but not identical. In the

NTNU experiments 18-cm dummy cylinders were mounted

between the model and the wind tunnel floor, while the

other end of the cylinder was a free end. In the Lough-

borough experiments both cylinder ends were free ends

with 32 and 90 mm gaps to the walls, respectively, leaving

parts of the model in the wind tunnel boundary layer. This

makes a comparison between experiments uncertain. The

drag force acting on the fabric clothed cylinder was mea-

sured using strain gauge balances at Reynolds numbers

(ReD) ranging from 9 9 104 to 1.6 9 105 and 1.6 9 105 to

2.9 9 105 for D = 11 cm and D = 20 cm, respectively, in

the NTNU wind tunnel and 0.75 9 104 to 3 9 105 for

D = 11 cm in the Loughborough wind tunnel. The corre-

sponding wind speeds were measured using pitot-static

probes. All data were acquired at a rate of 100 samples/s

and averaged over a period of 30 s. For measurements

performed in the NTNU wind tunnel the wind speed

standard deviation is \0.5 %, and the drag coefficient

standard deviation is \3.5 and \1.5 % for the small and

large diameter, respectively, except for in the transition

region where the flow is unstable. From the acquired data

the drag coefficient was calculated as

CD ¼D

12qU2A

; ð1Þ

where D, drag force; q, air density; U, wind speed and

A, projected frontal area. Wind speeds were normalized

using the non-dimensional Reynolds number ReD ¼ UDm

characterizing the flow regime. U, wind speed; D, cylinder

diameter and m, kinematic viscosity.

3 Results and discussion

Identifying the critical Reynolds number (Rec) as the

measurement point with the lowest drag coefficient,

marking the onset of the dual separation bubble regime

[17], we can quantify the difference between the transi-

tional behaviour of the fabrics. Due to the limited number

of measurement points this difference can unfortunately

only be determined with a limited accuracy as seen in the

figures. The drag coefficient for three fabrics with varying

air permeability mounted on a D = 11 cm cylinder is

shown in Fig. 2. The medium- and high-permeability fab-

rics show very similar behaviour in this range of Reynolds

numbers while in comparison the no-permeability fabric

shows a delayed boundary layer transition, indicating less

flow disturbance. In this range of Reynolds numbers the

critical Reynolds number as defined above was not

reached. Figure 3 shows the corresponding results from the

Loughborough wind tunnel experiments. Due to the dif-

ferent end and blockage conditions, we find slightly dif-

ferent values and gradients from this experiment. The trend

is, however, the same as for the NTNU experiment with the

high- and medium-permeability fabrics having the same

critical Reynolds number while the no-permeability fabric

has delayed transition with a difference in Rec of 33 9 103.

However, the non-permeable fabric does not have a lower

minimum drag coefficient than the permeable fabrics and

does not have a lower drag coefficient at higher speeds,

Table 1 Measured air-permeability of fabrics and standard deviation

[ lm2s

]

Fabric Low AP Zero AP High AP Underlayer

AP 36.8 0 54.5 104

SD 1.5 – 0.6 1.4

The effect of textile air permeability

which would be the benefits of a smoother fabric. In con-

trast, we notice that the low-permeability fabric shows a

lower supercritical CD. In the results for the larger cylinder

(D = 20 cm) seen in Fig. 4 this delay does not appear for

the non-permeable fabric, but again the density of the

measurement points is too low to accurately determine the

critical Reynolds number. A slightly lower super critical

CD for the low-AP fabric compared to the no-AP and high-

AP fabrics can also be noticed here, but not as pronounced

as in the Loughborough experiments. The reason for the

difference between cylinder dimension is not clear at this

point, but can be related to the relative roughness, aspect

ratio or end conditions of the model. More detailed mea-

surements should also be performed in order to determine

the critical Reynolds number more accurately.

In practical applications an athlete would sometimes

wear a layer of underwear or an undersuit between the

body and the race suit. In order to investigate a possible

influence of an additional fabric layer, the model was clad

with a thick permeable fabric under each of the three test

fabrics. The underlayer fabric used was a smooth two-layer

fabric with an air-permeability of 104 1/m2 s measured at

DP ¼ 200 Pa. The results for the dual fabric configuration

are presented in Fig. 5. It appears that the high-perme-

ability fabric triggers flow transition at a slightly lower

Reynolds number when placed over a highly permeable

fabric in contrast to the other fabrics which show no sig-

nificant change in behaviour compared to the single fabric

test. However, the low density of measurement points

makes it difficult to verify or quantify this difference. Nor

is it significant in an sport apparel application where the

Reynolds number varies rapidly over a large scale. The

80000 100000 120000 140000 1600000,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,0

Low APNo APHigh AP

CD

Reynolds number

Fig. 2 Cylinder drag coefficient from NTNU wind tunnel. D = 11

cm. Error bars indicate standard deviation

100000 150000 200000 250000 300000

1,1 Low APNo APHigh AP

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,0

CD

Reynolds number

Fig. 3 Cylinder drag coefficient from Loughborough wind tunnel.

D = 11 cm

160000 180000 200000 220000 240000 260000 280000 300000

Low APNo APHigh AP

Reynolds number

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,0

CD

Fig. 4 Cylinder drag coefficient from NTNU wind tunnel. D = 20

cm. Error bars indicate standard deviation

160000 180000 200000 220000 240000 260000 280000 3000000,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,0

CD

Reynolds number

Low APNo APHigh AP

Fig. 5 Cylinder drag coefficient of two fabric layers (including

underlayer) from NTNU wind tunnel. D = 20 cm. Error bars indicate

standard deviation

L. M. Bardal, R. Reid

supercritical drag coefficient was not affected by the

smooth and permeable fabric underlayer, except for the

non-permeable fabric which surprisingly shows a higher

CD when placed over a permeable underlayer.

An important point to consider when studying these

results is the fitting of the textile on the body. The exper-

iments were performed with a high relative strain of 25 %

over a convex surface allowing no air to be trapped under

the fabric. This might not always be comparable to a race

suit worn by an athlete where the body posture and shape

or poor fitting may lead to air pockets under the fabric.

Trapped air might cause flickering and will include stiff-

ness and possibly air-permeability as influencing parame-

ters on drag. This case is not considered in this study.

The results of these experiments indicate that there is a

weak trend between a fabric’s permeability and aerody-

namic properties. The high-permeability fabric induces

transition at slightly lower Reynolds number compared to

the low-permeability fabric in the three tests performed at

NTNU, while the completely non-permeable fabric shows

a small delay in flow transition in all cases. However, it can

be seen that the non-permeable fabric does not give the

benefit of a lower supercritical CD associated with a higher

critical Reynolds number. On the contrary, this fabric

results in a higher supercritical drag than the low AP fabric

in all cases. The differences found in critical Reynolds

numbers between the permeable fabrics are so small that

they can be considered irrelevant for sports garment

applications. A range of random selected performance

fabrics would likely show some correlation between air-

permeability and drag in a controlled test. This can be

explained by the fact that a smoother fabric may be asso-

ciated with a higher gauge and would likely have a higher

density and therefore leaving less space for air to pass

through. A surface with a rough macro-structure would

also likely have areas of smaller thickness allowing for

higher airflow, while a multilayer fabric would be less

permeable. This means that air-permeability is a result of

fabric construction, but should not be considered a concern

in the design of low-drag textiles.

It may also not be desirable to minimize the surface

roughness of all sections of the suit depending on the sports

discipline and velocity range. In almost all sports with the

exception of the extreme-speed sports, some amount of

roughness is required to trigger flow transition around

bluff-shaped segments of the body where pressure drag is

dominant.

4 Conclusion

The cylinder drag-measurements performed on formfitting

fabrics of varying air permeability in this study shows

limited influence of air permeability as a controlled vari-

able on flow transition and drag coefficient. A non-per-

meable fabric showed a delayed flow transition on a

D = 11 cm cylinder, and a smaller delay on a D = 20 cm

cylinder. Also a non-permeable fabric gave a higher

supercritical drag coefficient than a fabric with low per-

meability. Differences between fabrics of low and high

permeability could be considered less relevant in the design

of low-drag sports garment. The important aerodynamic

properties of the fabric appear rather to be the result of the

surface structure on both micro and macro-scales. It is also

shown that a smooth, permeable textile underlayer has little

influence on the aerodynamic behaviour of the outer

textile.

Considering the use of air permeability as a means of

reducing race speeds in alpine skiing as discussed by FIS,

this approach seems to have little or no effect on aerody-

namic drag. A minimum value limit for air permeability as

regulated today will ensure that race suits are made from

textiles, and not impermeable matter such as latex. This, in

turn, will ensure that suits will have some amount of sur-

face roughness, as woven or knitted textiles will never be

completely smooth. This was also claimed to be one of the

initial arguments for such a regulation. Constraining the

air-flow through a permeable formfitting textile by means

of an internal membrane does not seem to influence the

drag of the textile and thereby it can be assumed that

regulating the air-permeability is not an effective way to

reduce race speeds. Furthermore, a higher AP limit can be

met by using thinner textiles at the compromise of thermal

and protective properties.

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