# The Impact of Skinsuit Zigzag Tape Turbulators on Speed Skating Performance
## 3 Measurements  
### Cylinder Measurements  
To determine a frame of reference for a suitable location and thickness of the zigzag tape on the legs in relation to the Reynolds number, a number of wind-tunnel tests were dedicated to the flow over a cylinder.  
In a first series of tests, a polished plastic cylinder with circular cross-section was used with a diameter of 11 cm and a span of 60 cm, later on followed by a second series on a 90 cm span cylinder with the same diameter. The measurements were performed in the TU Delft LTT. The turbulence intensity in the tunnel ranges from 0.02% to 0.07% in a speed range from 10 to 75 m/s. Figure 3 shows the front view of the setup with the 90 cm span cylinder. The tests on the smaller span were performed in a similar manner, with a higher table. The cylinder was attached to a circular disc flush with the tunnel wall, and rotated with the mechanical 6-component external balance system. The forces were recorded with a frequency of 5 Hz and averaged over 10 s.  
In view of the strong dynamics in the flow, the sampling time of 5 Hz might seem very low; however, due to the inertia of the balances (rotating spindles with running weights), strong fluctuations will not be followed, which already averages out large and fast variations effectively. The gap between the cylinder lower end and the table was kept within 0.5 mm. All measurements were corrected for wind-tunnel blockage with the  
Figure 2: Fluorescent oil flow visualization of the effect of zigzag tape on the separation location on the lower leg. The flow comes from the right. Zigzag-tape thickness in right picture is 0.93 mm.  
method of [PERSON] described in Reference [11], using a blockage factor, \(\varepsilon\) = 0.96. The drag coefficient was calculated from the following:  
\[\mathrm{C_{D}}=\frac{\mathrm{D}}{\mathrm{qA}} \tag{2}\]  
where D is the measured drag force, q is the test section dynamic pressure following from a dedicated calibration and A is the model frontal area. Figure 4 presents the azimuth dependency of the drag coefficients for 0.93 mm thick zigzag tape. It shows that placing the tape too close to the leading edge at low Reynolds numbers does not lead to sufficient destabilization of the laminar boundary layer to generate turbulent flow at lower wind speeds. Tape with this thickness requires a higher Reynolds number to effectively trigger transition. When placed too far downstream, the laminar boundary layer is already too thick for the tape to add sufficient momentum to the lower part of the boundary layer in order to effectively suppress early separation.  
Figure 4: The impact of zigzag tape location on the drag coefficient of a smooth cylinder. Tape on the left and the right from the leading edge. Tape thickness is 0.93 mm; cylinder span is 60 cm.  
Figure 3: Setup of a 90 cm span circular cylinder in the Delft University low-speed, low-turbulence wind tunnel. In this case, the aerodynamic characteristics of a fabric with small rubber triangles were determined. The diameter of the cylinder is 11 cm.  
Figure 5 shows the effect of tape thickness at an azimuth angle of 30\({}^{\circ}\) on the same cylinder. For drag reduction at a lower Reynolds number, a higher tape thickness is required, as expected. Depending on the Reynolds number range the zigzag tape should do its work in, the tape location and thickness can be estimated.