Abstract
The purpose of this study was to investigate the effects of wind speed and wearing methods on cold protection performance. Three levels of wind speeds and two typical wearing methods were determined based on the winter meteorological data and a dress habits questionnaire, respectively. Thermal insulation values were measured using an instrumented manikin for three down jackets with the same amount of down content per square meter and different lengths. With wind blowing, a significantly decreased tendency of the total thermal insulations was observed (p = 0.005). With an increase in wind speed, the total thermal insulation was more sensitive to the change of wind speed when the placket was opened. The longer the length of down jackets, the greater influence of wind speed upon thermal insulation values. A wind speed above 0.4 m·s−1 led to a reduced rate of total insulation ranging from 27% to 47%. Opening the placket resulted in a 10–26% decrease in the total thermal insulation. The thermal insulation values measured locally at different segments of the thermal manikin pointed to greater regional variations in the obtained insulation values, especially during exposures to greater wind speed.
Introduction
Maintenance of a stable range of body temperature relies on keeping a dynamic balance between metabolic heat and dissipation to the environment of the human body. In diverse and even extreme environments, wearing suitable clothing is an effective approach to help maintain a balance. 1 The temperature difference between the wearer’s skin surface and the ambient atmosphere divided by the resulting dry heat flow per unit area in the direction of the temperature gradient is defined as the thermal insulation, 2 which can be used to determine the physiological effect of clothing on wearers in specific climates or activity scenarios. The direction of heat transfer is determined by the temperature difference between the outer and inner surfaces of the clothing, in the form of conduction, convection, and radiation.3,4 When the temperature of the environment is lower than that of the human body, clothing provides effective cold protection by reducing the heat dissipation from the human body to the environment.5,6
Ventilation openings of clothing provide channels for airflow to facilitate the exchange of heat between the air within clothing and the external environment, 7 and the extent of air exchange dependes on body posture, movement8,9 environmental conditions10,11 and their combination. 12 Wear trials and instrumented manikin tests are two common approaches to measure thermal insulation.
The effects of body posture, movement, and wind on thermal insulation have been widely studied.1,13,14 Nielsen et al. 3 simulated the wind by a windbox with three fans to study the impacts of 1.1 m·s−1 wind and different activity levels. The results showed that the wind speed of 1.1 m·s−1 had no effects on intrinsic thermal insulation in a standing position, whereas a decrease of 34% in total thermal insulation was found, which indicated the impact of wind in reducing the thermal insulation of the surface air layer. Havenith et al. 4 expanded the wind speed to 4.1 m·s−1, which demonstrated that the air thermal insulation was mainly affected by the wind. The authors 13 also proposed prediction equations for thermal insulation considering the influences of walking and wind and their interaction for both the workwear and the cold-protective clothing. Total and local thermal insulation impacted by body movement and wind were investigated by Lu et al., 14 and an empirical formula for estimating the resulting thermal insulation was developed.
In addition, studies have also explored the effects of clothing fitness, length, thickness, and air permeability on thermal insulation.4,7,15,16 Havenith et al. 4 found that loose clothing more easily showed the influences of the wind and movement on thermal insulation than the tight clothing. The authors also pointed out that clothing ensembles with higher measured insulation values probably trap more still air inside the microclimate. Špelić et al. 16 indicated that the impacts of the wind on thermal insulation were closely related to the permeability of clothing. Wigö and Nilsson 10 proposed a regression equation to predict the thermal insulation of winter clothing, in which the introduction of air permeability improved the accuracy significantly. A one-layer, air-impermeable ensemble and a three-layer, air-permeable ensemble were tested using a thermal manikin to quantify the relationship between ventilation and thermal insulation properties. 17 The air exchange between the microclimate of clothing and the surrounding environment through openings in clothing was also considered. Experiments involving the impact of clothing openings on the thermal insulation value pointed to a decrease in the value of intrinsic clothing insulation due to increased ventilation in the microclimatic area as a result of the placket opening. Morrissey and Rossi’s study 15 testing the three-layered ensemble configurations, by combining base-layer underwear, a zipped hoodie, and an outerwear sport zipped jacket, pointed to larger ventilation-induced insulation decreases with clothing unsealed. By testing the outerwear garment sealed, the outerwear garment fully unzipped and both mid- and outer-layer garments fully unzipped, the study demonstrated that opening the placket will impact the total thermal insulation due to ventilation, but will also be affected by fabric air permeability.
Manikin technology has become an essential tool for evaluating the impact of garments and microclimate conditions on thermal comfort, finding extensive applications across various research domains. This technology enables us to simulate and predict human thermal responses under different environmental conditions, such as extreme cold and heat stress. For instance, Li et al. 18 used warm-bodied mannequins to test the efficacy of personal heating measures in cold environments; El Akili et al. 19 applied this technology to assess thermal comfort among vulnerable populations in non-uniform thermal environments. Additionally, Wang et al. 20 highlighted the influence of backpack design on user thermal comfort, while Gao et al. 21 further explored methods for measuring garment insulation, providing precise parameters for thermal regulation models. Research by Del Ferraro et al. 22 demonstrated that cooling garments have significant potential in reducing heat stress for workers in extremely hot environments. These studies underscore the importance of warm-bodied mannequin technology in understanding thermal comfort, optimizing the design of personal protective equipment, and enhancing work efficiency under various climatic conditions.
The thermal insulation of the cold protective clothing is vital 23 due to the special climatic conditions of usage. Adequate protection can be ensured by selecting and using cold weather protective clothing appropriately. 24 The ability of cold weather protective clothing to trap still air with low thermal conductivity determines its thermal insulation. Down is one kind of animal protein fiber, consisting of down branches and down fibrils. Many branches are available for trapping still air, and the triangle and crotch nodes on fibrils also cross each other, which enables the down to recover quickly from compression and retain the entrapped air, thus producing superior thermal insulation. 25 As a typical cold-weather protective item, down jackets tend to protect against cold, but the heat transfer mechanism provided by them should be further investigated. Considering the specific environmental parameters and clothing wearing habits accommodated to different physical activities and environmental conditions, the effects of wind speed and clothing wearing method on total thermal insulation of the down jackets were investigated in this study. Three wind speed levels and two wearing methods were selected for the experiments. A thermal manikin was used to measure the total and local thermal insulation, to provide useful guidelines for clothing selection and wear.
Methodology
Clothing Ensembles
Three models of down jackets (175/92A) with similar patterns and different lengths were selected as testing samples (Figure 1). They were named DJ-S (Down jacket-Short length), DJ-M (Down jacket-Medium length), and DJ-L (Down jacket-Long length) based on clothing lengths of 70.5, 87, and 105 cm, respectively. The amount of down content per square meter for each jacket was 150 g·m−2, while the total quantities of down content for DJ-S, DJ-M, and DJ-L were 150, 182, and 211 g, respectively. All three models shared the same shell fabric (87.2% polyester fiber and 12.8% polyurethane fiber), lining (100% polyester), and bladder (100% polyester), and they were all stuffed with duck down.
Images of testing samples.
Wind and Clothing Wearing Method
Wind
Three levels of wind speed, 0.4 ± 0.1, 2 ± 0.1, and 4 ± 0.1 m·s−1, were set to explore the effects on clothing thermal insulation in this study. According to the data from CMDC (China Meteorological Data Service Center), the average meteorological wind speed of representative Chinese cities in winter was 3 m·s−1, and the highest average speed could reach 6 m·s−1 for local areas. Considering the meteorological wind speed was about 1.5 times the ground wind speed according to ISO 11079, 26 2 and 4 m·s−1 were selected as the experimental wind speeds. The air velocity of the climate chamber ranged from 0 to 1.8 m·s−1, which could not meet the requirements of higher speeds. Uniformity of wind speed is a crucial factor in the experiment. In this experiment, three fans were utilized as the designed apparatus to meet specific wind speed requirements and achieve an even distribution of airflow, as shown in Figure 2. The adjustable frequency of fans ranged from 0 to 50 Hz to achieve the wind speeds of 0.3–8 m·s−1. The condition of 0.4 m·s−1 wind speed was set by the climate chamber, and the wind speeds of 2 and 4 m·s−1 were controlled using the windbox by setting its frequency. After testing with an anemometer, 19.4 Hz was used to control the wind speed of 2 m·s−1 and 39.7 Hz for 4 m·s−1.
Photographs of the windbox and anemometer.
Clothing Wearing Method
A dress habits survey was conducted to collect information of factors influencing the choices of the down jacket and wearing methods. A total of 200 valid questionnaires were backed and the results were used to perform the experimental design. Based on the questionnaires, the age distribution of participants was mainly between 18 and 25. Most of the participants would like to select down jackets in cold conditions (87%), especially when the ambient temperature dropped below −5°C. Regarding the wearing methods, half-open placket and fully closed placket were two common ways that users wore down jackets of different lengths (short length, medium length, and long length). The total percentage of participants choosing any of the three half-open options exceeded 70.6%, while the percentage of those opting for the fully open method exceeded 54.6%. However, the participants reported they would sweat when they entered environments with higher temperatures or performed physical activities. The perspiration instance of all participants by body region has been calculated and shown as a percentage of sweating incidence in brackets. The body regions of the back (72.55%) and armpits (62.75%) were prone to sweating with the highest frequency, followed by the neck (27.45%) and chest (19.61%). Placket fully opened was chosen as an effective method for heat dissipation to meet the thermal comfort requirements of the wearers. The placket fully open accounted for 54.9% of the observed dress habits by the wearers conducting trials. Considering both types of clothing conditions, two common wearing methods, placket closed (zipper was pulled up to the neckline and hat was worn) and placket open, of the down jackets were investigated in this study.
Insulation Measurement on a Thermal Manikin
A thermal manikin was used for the thermal resistance study. A thermal manikin can be used to repeat the experiment as many times as possible, and the more extreme experimental conditions are set up as permitted by the environmental chamber, so that the experimental results are stable, and the measurements are accurate. 27 The experiments were conducted following the method for measuring thermal insulation of clothing using a thermal manikin addressed in ASTM F2732. 28 The Newton manikin system (Measurement Technology Northwest, Seattle, WA, USA) was applied for the tests, as shown in Figure 3.
Photographs of the thermal manikin located in a climate chamber.
Insulation Measurement of the Air Layer
The nude manikin test was conducted to determine the thermal insulation of the boundary air layer with the thermal manikin resting in a standing position without the extremities moving. According to ASTM F2732, the ambient temperature should be at least 12°C lower than the average surface temperature of the thermal manikin. A few pretests were conducted to determine the ambient temperature for the nude tests, which should be within the range of 20–23°C. The surface temperature of the manikin was set at 35°C, and the mean value was not allowed to drift more than ±0.2°C. The same manikin settings were used in thermal insulation measurement of the basic clothing and down jackets, but the ambient temperature was set lower to 0°C. After stabilization, each test lasted 30 min and three repeated tests were conducted. The mean value of the thermal insulation of the boundary air layer, determined by the nude manikin test before each manikin testing, was set to calculate the total thermal insulation value of the dressed manikin.
Insulation Measurement of Basic Clothing
The basic clothing worn under the down jacket was an ensemble. Referred to ASTM F2732, a set of basic clothing with an intrinsic thermal insulation of 0.82 clo was measured and selected as shown in Table 1. Note that the front edge of the hat was worn above the eyes, the underwear was tucked into the outer pants, the shirt hem covered the waist of the pants, the gloves were tucked into the cuffs, all the buttons were be fastened, and the zipper was pulled up.
Characteristics of the basic clothing.
Insulation Measurement of Down Jackets
To meet the standard requirements 27 that the minimum heat flux of the manikin section should be up to 20 W·m−2 when testing the cold weather ensemble, the ambient temperature and relative humidity were then determined as 0°C and 50%. The manikin experiments of the down jackets under three wind speeds and two wearing methods were completed. Three repeated tests were performed for each condition and the total and local thermal insulation values of the clothing ensembles were obtained.
Thermal Insulation Calculation
Total Thermal Insulation
The thermal insulation of each ensemble was calculated by the parallel method as in equation (1):
where It is the total thermal insulation of clothing, clo; Tsk and Ta are the skin surface temperature of the manikin and the air temperature in the chamber, respectively, °C; Hi is the heat of heat flux of manikin segment I, W·m−2; Ai is the surface area of segment i, m2; and A is the total surface area of the manikin, m2.
Local Thermal Insulation
The Newton manikin adopted in the tests has 34 segments, two of which are designed for seating purposes, so these two parts need not be set. As illustrated in Figure 4, the remaining 32 segments were reclassified into 20 main regions. Considering the potential discrepancy between the front and back body affected by the open placket, the front and back regions were calculated separately. The labels 1–20 represent face, head, front upper arm, back upper arm, front forearm, back forearm, hand, upper chest, upper back, lower chest, lower back, abdomen, waist, pelvis, hip, front thigh, back thigh, front calf, back calf and foot, respectively. The local thermal insulation of each segment was calculated by the parallel method. The surface temperature and heating flux of each segment of the manikin can be controlled separately, and the real-time data were recorded. Equation (2) shows the calculation method of the thermal insulation of each main part.
where, It,i and It,j are the local thermal insulation of manikin segment i and main part j, clo; Ai and Aj are the surface area of segment i and main part j, m2; Tmanikin, i and Ta are the surface temperature of the manikin in segment i and air temperature, respectively, °C; Hi is the heat flux of segment i, W·m−2.
Diagram of the reclassification for body segments of the thermal manikin.
Statistical Analysis
The Statistical Package for the Social Sciences (SPSS) 24.0 was applied for statistical analysis. One-way ANOVA analysis was used to clarify the effects of wind speed on the total insulation of the down jackets at a significant level of 0.05. The Levene test was employed to differentiate the total insulation under different wind speed levels. A paired-samples t-test was applied to differentiate the total insulation of two wearing methods. To analyze the relationship between total down content and the total thermal insulation, a linear regression model as presented in equation (3) was used:
where It is the total thermal insulation of clothing, clo;
Results
Total Thermal Insulation
Total thermal insulations under different wind speeds or air velocities are presented in Table 2. Both the wind speed and clothing wearing method affected the total thermal insulation of the down jackets significantly. A downward tendency was found for total thermal insulation with increasing wind speed. Opening the placket also resulted in a decline in the insulation. In addition, increased thermal insulation was apparent with an increase in the down jacket length.
Total thermal insulation under different test conditions (mean ± SD).
Effects of Wind Speed
The effects of wind speed on the total thermal insulation of the down jackets under two different wearing methods are shown in Figure 5. A declining tendency of the total thermal insulation was observed with the increase of wind speed. According to Neilson et al., 3 the air volume within clothing is optimal in a standing posture and without the effect of environmental factors. Any influence which decreases the air volume, or causes convection therein, will reduce the clothing insulation. Therefore, the reduction percentage of the insulation was calculated based on the data while the manikin was standing in a wind speed of 0.4 m·s−1.
Total thermal insulation and percentage of reduction under different wind speeds and wearing methods: (a) placket closed, (b) placket open.
As shown in Figure 5(a), under the condition of placket closed, thermal insulation declined sharply when the wind speed increased from 0.4 to 2 m·s−1 (27%, 31%, and 34% for DJ-S, DJ-M, and DJ-L). However, the slope of the thermal insulation decreased when the wind speed increased from 2 to 4 m·s−1. In addition, the average values of total thermal insulation between 0.4 and 2 m·s−1 were significantly different (p = 0.005); however, no significant difference was found between the wind speed of 2 and 4 m·s−1 (p = 0.102).
The reduction percentage of total thermal insulation at 4 m·s−1 wind was higher than that at 2 m·s−1. The reduction rate increased with the length of down jackets and the value for the DJ-L was the largest (34%). Moreover, the difference of reduction rate between each length was smaller under 2 m·s−1 wind speed than 4 m·s−1.
The condition of the placket open is shown in Figure 5(b). Unlike with a closed placket, the average values of total thermal insulation among 0.4, 2, and 4 m·s−1 all showed significant differences (p < 0.005). However, the difference in thermal insulation values among all three jacket models affected by different wind speed values, decreased with the placket open as seen in Figure 5(a). The DJ-L displayed the largest reduction, followed by DJ-S, and DJ-M under the condition of 2 m·s−1 wind. When the velocity increased to 4 m·s−1, the reduction rate increased with the jacket length. Compared with the reference condition, total thermal insulation reduction ranged from 29% to 32% for the wind speed of 2 m·s−1, and from 44% to 47% for 4 m·s−1 wind.
Effects of the Clothing Wearing Method
The influences of closed and open plackets on thermal insulation are also depicted in Figure 5. The data of the closed placket was selected as the reference to calculate the reduction rate of total thermal insulation of each ensemble when the placket was open. It was evident that in any case the total thermal insulation decreased after opening the placket and the longer clothes were more susceptible to wind by comparing the reduction rate.
Under the wind speed of 0.4 m·s−1, the greatest reduction of thermal insulation value of 22% was observed for the down jacket with the longest length, followed by DJ-M and DJ-S. However, the thermal insulation of the DJ-L was still the highest. When the wind speed increased to 2 m·s−1, the decrease rate of the thermal insulation value was elevated to 14% for DJ-S, whereas it declined to 17% and 20% for DJ-M and DJ-L respectively. Regarding the wind speed of 4 m·s−1, the total insulation of each ensemble was further reduced and the reduction rates of thermal insulation value for DJ-M and DJ-L were 19% and 26%, respectively. With the jacket unzipped and the placket open, the wind will cause a greater decrease in the thermal insulation value. The insulation decrease will be more evident with an increase in the wind velocity. The decrease in the thermal insulation values measured locally at the front torso area rose due to an increase in the wind velocity. The reduction of thermal insulation increased for the DJ-S at 2 m·s−1 wind which proved the effects of the blowing of the garment and the enhanced air convection at the anterior zones. DJ-M and DJ-L with greater clothing length and weight were more difficult to blow, therefore the variation in the reduction of the thermal insulation value for these two ensembles was minor. However, when the wind increased to 4 m·s−1, a larger area of the front placket was blown away, resulting in a dramatic drop in thermal insulation. Therefore, clothing with longer length was more easily affected by the stronger wind after opening the placket.
In addition, there was a significant difference in the average values of total thermal insulation of each ensemble before and after opening the placket, which occurred during higher wind speed exposures of 2 m·s−1 (p = 0.019) and 4 m·s−1 (p = 0.041). Comparisons of the average value of total thermal insulation among the three wind speeds and each ensemble indicated that opening the placket enhanced the wind effect, while weakening the effect of clothing length on total thermal insulation.
Effects of the Total Down Content
The total down content is an index to represent the cold protective performance of a jacket in the market. In the current study, the relationship between total down content and the total thermal insulation under three different wind speeds and two wearing methods was investigated, as shown in Figure 6. Note the down content per square meter was the same for tested jackets.
Effects of total down content on total thermal insulation: (a) placket closed, (b) placket open.
The coefficients a and b, and the correlation factor R2 in equation (3) are displayed in Table 3. The regression results exhibited a good correction equation with the value of R2 higher than 0.9, except for the condition of placket open with 4 m·s−1 wind. In any case of the two wearing methods, the slope of the regression model for 0.4 m·s−1 wind was larger than that at the wind speeds of 2 and 4 m·s−1, which indicated that the effects of total down content on total thermal insulation were weakened with the increase of wind speed. The slopes of the regression model for the opening placket also declined compared with that of the closed placket, indicating the negative effect of the open placket on thermal insulation of total down content increased.
Coefficients of the linear regression models.
Local Thermal Insulation
Effects of the Wind Speed
The reduction percentages of local thermal insulation at 20 body regions were calculated based on the data while the manikin was standing at the wind speed of 0.4 m·s−1, which are illustrated in Figure 7. Differences were observed at two wind speeds, and the data for 4 m·s−1 was less than that for 2 m·s−1. At the same wind speed level, the changes of insulation were also different among the three ensembles. In addition, the insulation reduction of down jackets with open plackets was significantly higher than that for jackets with closed plackets.
Reduction rate of the local thermal insulation under three wind speeds: (a) placket closed, (b) placket open.
As seen in Figure 7(a), the highest reduction rate of local thermal insulation was observed at the abdomen with a value of 77.73% when the wind speed was 2 m·s−1. On the contrary, increased local thermal insulation was detected at some body segments, such as the front forearm. The abdominal thermal insulation decreased the most and the lower back value increased by 3.27% for DJ-S. Regarding DJ-M, the local thermal insulation of the abdomen and pelvis decreased significantly and the increase occurred at the back forearm, lower back, back upper arm and front forearm, with a maximum value of 15.56%. For DJ-L, the reduction rate of hand was the highest. The local thermal insulation of the waist and hip increased, with a maximum value of 11.54%. Generally, the thermal insulation of these three ensembles dramatically dropped at the abdomen, pelvis, and hands. Increased values were observed at the front forearm, hip, back upper arm, lower back, waist, and back forearm. When the wind speed increased to 4 m·s−1, the maximum reduction rate was elevated to 90.81%, which appeared at the abdomen as well. The thermal insulation increased only at the front forearm when wearing DJ-S, with a value of 8.21%. The greater reduction rate occurred at the abdomen, lower chest, and pelvis for DJ-S, the abdomen, pelvis, lower chest, and hand for DJ-M, and the hands and face for DJ-L. All local thermal insulation values of DJ-M and DJ-L decreased at the wind speed of 4 m·s−1, and the former was larger than the latter.
As shown in Figure 7(b), the local thermal insulation decreased obviously at the abdomen and lower chest for all down jackets with wind speed increased and placket open. The maximum reduction rate occurred at the abdomen with a value of 71.71% when the wind speed was 2 m·s−1. The value increased by 7.22% at the back forearm for DJ-S, 3.11% at the back thigh and hip for DJ-M, and no increase was observed for DJ-L. Therefore, the local thermal insulation of DJ-L manifested the most obvious downward trend, which was different from the case when the placket was closed. When the wind speed increased to 4.0 m·s−1, the maximum reduction rate of the local thermal insulation was 86.16%, and no increase occurred. There were six segments where the local thermal insulations showed a large degree of decline for DJ-S, respectively at abdomen, lower chest, upper chest, waist, pelvis and lower back, that was, the front torso and lower back. The thermal insulation of the front upper arm decreased as well. Regarding DJ-L, the values of seven segments including hip dropped sharply as well. The thermal insulation decrease trend of the three down jackets increased with the clothing length.
Effects of the Clothing Wearing Method
The effects of wearing methods on local thermal insulation of the three ensembles are shown in Figure 8. The reduction rate of the open placket was calculated based on the ensembles with closed plackets. The local thermal insulation presented different changes at each wind speed. Differences were also observed among the three ensembles under the same test conditions.
Reduction rate of the local thermal insulation under different wearing methods: (a) wind 0.4 m/s, (b) wind speed 2 m/s, (c) wind speed 4 m/s.
Figure 8(a) displays the insulation change after opening the placket at 0.4 m·s−1 wind speed. The maximum reduction rate observed at the back thigh was 61.18% and the maximum rise rate was 18.5% at the front upper arm. No evident drop was found for DJ-S. On the contrary, the local thermal insulation increased at six segments, including upper back, front thigh, waist, front calf, front forearm, and front upper arm, with a maximum increase rate of 18.5%. Regarding DJ-M, the highest reduction rate was detected at the pelvis, while the largest rising rate of 3.01% was found at the front calf. As for DJ-L, the greatest reduction rate occurred at the back thigh, while increases were observed at the front calf, forearm, waist, and upper arm, with a maximum of 17.72%. In addition, DJ-S showed the smallest decrease in local thermal insulation among all the samples.
At the wind speed of 2 m·s−1, the thermal insulation of five segments increased for DJ-S, and the maximum rising rate was 17.39% at the hand. The greater drop of local insulation for DJ-M was at the head and lower chest, while an increase was found at four segments. The reduction rates for DJ-L were larger at the head, abdomen, pelvis, and lower chest, and the increase was also observed at three segments. DJ-L showed the greatest decrease in local thermal insulation at the head with a percentage of 71.26%. When the wind speed increased to 4 m·s−1, the maximum reduction rate of local thermal insulation appeared at the lower chest (80.3%), and the highest increase rate occurred at the hand (106%). The local thermal insulation of DJ-S decreased greatly at the head and upper chest and increased at four segments. The lower back was added to the segments where the local thermal insulation decreased for DJ-M, while two other segments improved the warmth protection. Regarding DJ-L, the lower chest had the largest reduction rate, while the value of the five segments increased. The decreasing trend of local thermal insulation among the three ensembles increased with the length, which was the same as the consequence under a wind speed of 2 m·s−1.
Discussion
The effects of wind and clothing wearing methods on the total thermal insulation of the down jackets with three different lengths were investigated in this study. The total thermal insulation of down jackets was found to decrease with the increasing wind speed and open placket. It was generally believed that the thermal insulation of the long down jackets had better cold protective properties than the shorter ones. Analyzed results showed that thermal insulation of the longest DJ-L model was more easily affected by the increasing wind speed and openings. Further analysis of the local thermal insulation indicated that the difference was caused by the combined effects of the open placket and wind directly blowing to the front part of the thermal manikin from the windbox. The wind blowing straight to the manikin caused the jacket to slip to the back, especially in the longest jacket model. The longer the length, the more the garment turned back under the action of wind speed.
The total and local thermal insulation values decreased with the increase of wind speed and the opening of the placket. The downward tendency of the total thermal insulation values with the increase of the wind speed corresponded to prior studies found in the literature.4,9 The sharp reduction of the total thermal insulation was observed when the wind speed increased from 0.4 to 2 m·s−1. Presumably, the surface air layer was stripped off by the increase in the wind velocity, resulting in a decrease in the thermal insulation of the surface air layer. Besides, the airflow penetrating the fabric also reduced the volume of still air stored in fabric layers and induced air convection. When the wind speed increased to 4 m·s−1, the decline rate of the total thermal insulation slowed down, while the values themselves declined continuously. The difference in reduction rate between each length was smaller under the wind velocity of 2 m·s−1 than at 4 m·s−1, which was also attributed to the significant impacts of the surface air insulation. As indicated by Havenith et al., the surface air insulation ranged from 62% to 70% at a wind speed of 4.1 m·s−1. The longer down jacket has embedded a relatively larger surface air layer, which is stripped off by wind and leads to greater insulation decrease. Bouskill et al. 17 observed a reduction of 26% and 20.9% for the one- and three-layer ensemble respectively at a wind speed of 1 m·s−1. The decrease rate indicated by Lu et al. 9 was 12.8% at a wind speed of 1.55 m·s−1 for cold weather protective clothing filled duck down with a clo value higher than 3.2. In our study, the reduction rate was from 27% to 34% at 2 m·s−1 wind. Havenith et al. 4 observed a decline in total thermal insulation value from 34% to 40% of the original value when standing in a wind speed of 4.1 m·s−1. Jussila et al. 23 indicated a total thermal insulation reduction of the cold protective clothing in Arctic open-pit mining from 27.4% to 35.7% by a serial method when the wind speed increased from 0.3 to 4 m·s−1. Lu et al. 9 reported the reduction of total thermal insulation of 31.7% at 4 m·s−1 wind for the down jackets. The present study showed a reduction in the thermal insulation from 42% to 46% at 4 m·s−1 wind speed, which is in line with the previous studies. The results showed that the thermal insulation of clothing decreased with the increase of wind speed, and it can be inferred that the declining trend was affected by the increased wind speed.
In addition, the effects of wind speed on total thermal insulation showed differences with the different wearing methods. As plackets were opened, the total thermal insulation decreased significantly, and the average values of the thermal insulation of the three wind speeds were observed to be different. Nielsen et al. 3 found a reduction in intrinsic clothing insulation of 18% and a decrease in surface air insulation of 51% when standing with the jacket open. The reduction percentage of the total thermal insulation varied with the increase of wind speed. Opening the plackets will promote convective effects caused by the wind between the microclimate of the clothing and the external environment.
The local thermal insulation further revealed the complex relationship between wind speed and wearing methods. When the placket was opened, the wind reduced the thermal insulation at the front segments of the torso significantly, which was also proved by Morrissey and Rossi 15 due to the wind blowing directly on the front of the manikin. On the contrary, the 2 m·s−1 wind blowing on the front of the manikin and flowing to the back of the torso expanded the volume of still air in the microclimate and clothing, which increased the measured thermal insulation in the back. The lower insulation at the front torso area when compared to the rear areas caused the human subjects to feel wind chill probably due to a large sensation difference at various body segments, hence reducing the thermal comfort of wearers. The thermal insulation values remained fairly unchanged with the further wind speed increasing due to the enhanced convection. Opening the placket was not recommended for wearing the longer down jackets under the higher wind speeds due to the dramatic decrease of thermal insulation affected by the enhanced convection within the microclimate.
Conclusions
The effects of wind speed and the wearing method on the thermal insulation of down jackets with different lengths under three levels of wind speeds and two wearing methods were discussed in the current study. A thermal manikin system was utilized for measuring the total and local thermal insulations. The change of insulation caused by variable conditions was concluded based on data analysis. Several suggestions were proposed for the choice of down jackets and the ways of wearing them under different wind speed values. Considering the significant effects of wind speed on clothing thermal insulation, all jacket models despite their length were acceptable for low wind speed. Adjusting the clothing openings according to human thermal sensation was an effective approach to reduce heat accumulation. While adjusting the placket opening is an effective approach to reduce the heat accumulation under lower wind speed, the higher wind speed would cause the jacket to be blown to the back of the body if the placket remained opened, thus causing the wearer to feel discomfort. The findings presented by the current study point to necessary design considerations, which should be considered while designing wind-protective clothing. There are still some limitations of this study. When considering the total thermal insulation of the down jackets, sweating conditions and varying degrees of opening are also important influencing factors, which are not considered in this experiment. The influence of these on the thermal resistance of down jackets will also be explored in future research. We believe that these additional data will provide us with a more comprehensive understanding and aid in optimizing clothing choices and wearing recommendations for cold weather conditions.
Footnotes
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: We would like to acknowledge the support of the Fundamental Research Funds for the Central Universities (Grant no. 2232023D-06/2232024G-08).
