The suitability of materials for sports applications, including textiles for sports garments and like materials for all applications, must meet a range of performance parameters depending on the specific requirements imposed by any one application. Strangwood 1 points out that the close interplay of design and sports materials brought about by engineering modeling is only as good as the data on which it is based. Technological innovation, in both design and materials, has played a significant role in sport, achieving its current standing in both absolute performance and its aesthetics. In sports garments, two examples that clearly exemplify design and materials are graduated compression garments and swimming bodysuits.
Aerodynamic properties play a significant role in the garments across a wide range of sports, including cycling, skiing, bobsleigh, and speed skating. Considerations in this aerodynamic performance include the textile weave or knit, seam and fastener placement and air permeability. Elite competition usually involves very short winning time margins in events that often have much longer timescales, making aerodynamic resistance and its associated energy loss during the event significant in the outcome. In fact, a twofold increase in athlete velocity results in a fourfold increase in the drag force needing to be overcome 2. A series of research studies over the last two decades have identified the reduction of aerodynamic drag in such sports garments 3–6.
Kyle and Brownlie 4, 5, 7, carrying out systematic wind tunnel studies of drag and flow transitions utilizing both mannequins with athletic apparel and cloth-covered cylinders, showed that cylinders with three types of cloth experienced significantly less drag than bare cylinders and earlier onsets of the flow transition, which could have been attributed to surface roughness, although neither the cylinders nor the mannequin with non-porous fabric underwent such a flow transition.
The surface texture and the corresponding air permeability of sports fabrics can potentially exhibit subtle, yet significant influences on drag and flow transitions. Surface roughness is an important parameter for lift and drag due to the transitional properties at the boundary layer. Sports textiles represent a wide spectrum of surface topologies and wide boundary layer behaviors. Here, we report an experimental wind tunnel arrangement that provides precise information on both the drag and aerodynamic lift characteristics of textiles covering standard cylindrical geometries formed from rigid plastic materials, as well as ballistic test gel materials matched to muscle tissue density (1.3 g/cc). In the aerodynamic evaluation of surface-coating materials, such as textiles, a cylinder provides a readily available generic standard geometry if established within the correct experimental arrangement. Here, we show a standard cylinder arrangement for this textile testing consisting of three segments of the cylinder: non-active top and bottom sections and an active middle section, where the top and bottom sections are used to minimize the aerodynamic influence on the active middle section. This cylindrical arrangement has been designed to operate with its principle axis between perpendicular and horizontal to the force transducer to allow data on both aerodynamic drag and lift of textile samples to be determined. However, the effect of the non-active top and bottom sections of the cylinder on the aerodynamic properties of the active central section is not immediately apparent. Therefore, the primary objective of this paper is to design and develop standard cylinder testing protocols and also evaluate the overall effects of top and bottom sections on the middle section of the cylinder. This evaluation would then be used to optimize the arrangement for the simultaneous measurement of aerodynamic drag and lift.
2. EXPERIMENTAL PROCEDURE
2.1 Experimental Facility
To experimentally measure the aerodynamic properties (such as drag, lift, side force and their corresponding moments) of fabrics, the RMIT wind tunnel was used. The tunnel is a closed return circuit wind tunnel with a maximum air speed of approximately 150 km/h. The rectangular test section dimension is 3 m (wide)-2 m (high)×9 m (long) with a turntable to yaw suitably sized objects. A plan view of the tunnel is shown in Figure 1. The tunnel was calibrated before conducting experiments with air speeds measured via a modified National Physical Laboratory (NPL) ellipsoidal head pitot-static tube (located at the entry of the test section) connected to a MKS Baratron pressure sensor (MKS Instruments, Andover, MA, USA) through flexible tubing. Purpose-made computer software was used to compute all six forces and moments (drag, side, lift forces and yaw, pitch and roll moments) and their non-dimensional coefficients.
2.2 Description of Experimental Arrangements
The standard cylindrical geometry consisted of an active central section connected to the load cell (force sensor) and passive upper and lower sections assembled to minimize end-effects, as shown in Figure 2. The active cylinder has a diameter and length of 110 mm and 400 mm, respectively, while the six-axis force sensor (type JR-3) had a sensitivity of 0.05% over a range of 0–400 N. Textile sleeves were fabricated for the cylinders so that each fabric had similar tensions when installed on the cylinders.
The various experimental configurations used in the wind tunnel to determine the respective end-effects when using the textile sleeves are given in Figure 2. The cylinder was made of PVC tubing and was made solid by using fillers for structural rigidity. The middle section is vertically supported on a six-component force sensor using a threaded strut. The non-active bottom section was secured to the wind tunnel floor, while the top section was secured with the floor using an L-shaped bracket (Figure 2) so that the top and bottom sections had no physical contact with the active middle section; 5 mm gaps were maintained between the sections. In order to quantify the effects of the top and bottom sections on the aerodynamic properties of the active middle section, the active section was tested in following configurations:
Active section with top and bottom sections.
Active section with a non-active bottom section only (no top section).
Active section with a non-active top section only (no bottom section).
Active section only (no non-active top and bottom sections).
Figure 2 shows the complete testing configuration with the active middle section and the non-active top and bottom sections, as illustrative of the arrangements evaluated.
The standard cylinder configuration shown in Figure 2 is only used for aerodynamic drag measurements. Since body positions do not remain in an upright position during sports activity, deviations from an upright position can generate both aerodynamic drag and lift or down force. Therefore, the standard cylinder, with the active central section and the non-active bottom section only, was modified maintaining the same diameter of 110 and 300 mm of length using a specifically designed rotating mechanism to fix the cylinder at any angle from 30° to 150° relative to wind direction, as shown in Figure 3. The inclined cylinder allows simultaneous measurement of both aerodynamic drag and lift for various textiles in the experimental wind tunnel program at different angles of attack.
3.1 Textile Characterization
A textile sleeve was fabricated to examine the effects of surface texture and finish. The sleeve was knitted fabric and made from rotor spun yams using 50% cotton and 50% microfiber polyester material. As noted, it was produced so that it provided a constant fabric tension when placed on the cylinders. Figure 4 shows a scanning electron microscope image of the textile examined, where it can be seen that the surface topology consists of ∼60 µm linear arrays of regular yarn bundles of approximately 250 µm diameter and weft fibers straggling this regular warp array. The insert shows the bare cylinder surface for comparison of feature sizes.
3.2 Aerodynamic Characterization of Experimental Configurations
Figure 5 shows the response of the bare cylinder in terms of the drag force (FD) and drag coefficient with respect to wind velocity (V) and Reynolds number , respectively, characterizing the different experimental arrangements of the cylinder components shown in Figure 2. The aerodynamic tests were carried out in a range of wind speeds from 20 to 120 km/h, corresponding to Reynolds number ranges from 4.1×104 to 2.5×105.
Figure 5(a) indicates that the drag forces (FD) resulting from the four configurations follow similar trend lines with only small differences in magnitude. Figure 5(b) shows that the coefficient of drag (CD) is highest for the cylinder without the non-active top section at low velocities and that it undergoes the greatest laminar-to-turbulent flow transition. It was also found that at higher wind velocities, vibration of the top section caused the gap distance between the active cylinder and the top section to vary, influencing the force measurements. Thus, the standard configuration adopted consisted of the active central cylinder and the non-active bottom section, which was applied to the following drag and lift force measurements. It also indicates that the top section has minimal effect at high speeds (high Reynolds number), particularly over 80 km/h.
Figure 6 compares the bare cylinder arrangement with and without the textile sleeve (shown in Figure 4) in terms of the drag and lift forces (FD and FL), with the arrangement set at five angles (30°, 45°, 60°, 75°, 90°) of inclination to the wind direction. The corresponding drag and lift coefficients (CD and CL) of these systems are given in Figure 7. Inclination angles below 90° produce greater drag forces than the vertical position, with 75° dominating with the bare cylinder and 60° dominating with the textile sleeve. Very low angles, for example, 30°, yield low drag forces.
The corresponding drag coefficients clearly show progressive increases in the flow transitions with the textile sleeve where the final transition velocities increase from ∼40 to 50 km/h and finally 55 km/h for 60°, 75°, and 90°. Figure 6(a) shows that the bare cylinder does not exhibit these transitions. The textile surface at 90° produces both the largest reduction in drag coefficient with the transition, as well as the lowest CD in the high velocity region, for example, 70–120 km/h. The respective lift forces in Figure 5(c) and (d) show the increases in lift with lowering the angle of inclination to the wind with textile topology, clearly demonstrating both a larger effect and more systematic changes. Again, the textile surface provides a progressive increase in CL with greater inclination and 45° and 30° having similar values at wind speeds beyond ∼90 km/h.
The observed variations in surface texture, including roughness and air permeability, as observed by features shown in microscopic images, can potentially exhibit subtle, yet significant influences on the transitional properties at the boundary layer. It is well recognized that aerodynamic properties play a significant role in many sports garments. Here, we show an instrumental arrangement assembled to study both the aerodynamic drag and lift of sports textiles in a controlled manner that can be correlated to key material parameters, such as surface texture, deformation and air permeability. These studies led to the optimization of the standard cylindrical geometry configurations and methodology for the aerodynamic characterization of textiles at various testing speeds.
When wind speeds are 70 km/h and above, the complete configuration with non-active top and bottom sections is subjected to less end-effects, as shown by the constant CD values across this velocity range; the configuration without the top section gives the least interference to force measurements for the simultaneous study of aerodynamic drag and lift. The presence of a textile sleeve, with a surface topology characterized by weft fibers randomly straggling the regular array of warp bundles, produced a systematic shift in the laminar-to-turbulent flow transition, as seen in the CD plots, as inclination is varied, while aerodynamic lift increased linearly with the inclination angle. This behavior is not seen with the bare cylinder surface. Such a standard cylinder arrangement will be used in further studies evaluating lift and drag characteristics of high-performance sports garment and textile design and construction. Sports engineering studies and materials science thus deepens the understanding of the aerodynamic load contribution (drag and lift) of clothing on competitive athletic performance, and thus on optimal garment design.