## 2 Materials and Methods
### Measurement of the Force Components Acting on the Model and Corresponding Wind Speed  
The schema of the applied measuring system is presented in Figure 6. The model of the scissor lifts 1 was installed on the aerodynamic balance 2. A, B--special segments with strain gauges. It is the device that enables the determination of components \(F_{x}\) and \(F_{y}\) of the aerodynamic forces as well as the moment \(M_{z}\) with respect to the coordinate system shown in Figure 3. The aerodynamic balance relates to the electronic system 3 controlling the stepper motor of the turntable and with the strain gauge bridge 4 measuring the aerodynamic forces. The wind speed was evaluated via two thermo-anemometer probes 5 and 6, which were connected to the thermo-anemometer ANT 2000 7. Aerodynamic forces and wind speed measurements were recorded via a PCI 1710 card 8 on a PC 9.  
Figure 5: Model of the scissor lift at medium working height (conf. 3): (a) During the aerodynamic tests, (b) the CFD simulation.  
Figure 6: The schema of the measuring system (1—the scissor lift, 2—the aerodynamic balance, 3—the electronic system, 4—the stepper motor, 5 and 6—the thermo-anemometer probes, 7—the thermo-anemometer, 8—PCI 1710 card, 9-PC, and A, B—the segments with strain gauges).  
The sampling time of the wind speed is equal to 5000 ms. The exemplary graph of the wind speed variations measured via sensors 5 and 6 is presented in Figure 7. Here, it should be noted that the average value of the wind speed measured by sensor 6 is about 15% lower in comparison with the value indicated by sensor 5. Moreover, based on the graph in Figure 7, the average intensity of turbulence \(I_{v}\) can also be evaluated according to the following formula:  
\[I_{v}=\frac{\sigma_{v}}{V_{ref}}, \tag{1}\]  
where \(\sigma_{v}\) is the standard deviation of the measuring wind speed and \(V_{ref}\) is the reference speed, which is equal:  
\[p_{ref}=\sqrt{\frac{2\cdot p_{ref}}{\rho}}, \tag{2}\]  
where \(p_{ref}\) is the reference pressure. The standard deviation can be computed based on the dynamic component \(v_{dyn}(t)\) of the instantaneous velocity \(V(t)\), where \(V(t)=V_{ref}+\sigma_{dyn}(t)\).  
The wind speed, which was measured at the point localized 500 mm over the floor of the aerodynamic tunnel, was generally lower in comparison with the wind speed measured at 920 mm over the floor. It was caused by the set of \"spires\", which were installed at the beginning of the tunnel in order to induce the initial turbulence. The spires were much wider at the basis in comparison with their summits. Therefore, the wind speed was much more disturbed at the level of 500 mm and, in consequence, the average wind speed was lower.  
The formulas determining the coefficient of aerodynamic drag \(C_{x}\), the coefficient of the lateral aerodynamic force \(C_{y}\), the aerodynamic coefficient of torque \(C_{\text{Mz}}\), and the aerodynamic coefficient of the moment, which caused the tip-over \(M_{C}\) are as follows:  
\[C_{x}=\frac{2F_{x}}{\rho V^{2}\cdot A_{ref}},\ C_{y}=\frac{2F_{y}}{\rho V^{2} \cdot A_{ref}},\ C_{Mz}=\frac{2M_{z}}{\rho V^{2}\cdot A_{ref}\cdot r_{ref}},\ C_{Mc}=\frac{2M_{C}}{\rho V^{2} \cdot A_{ref}\cdot h_{ref}} \tag{3}\]  
where: \(F_{x}\)--aerodynamic drag N, \(F_{y}\)--lateral force respectively N, \(V\)--the average wind speed m/s, \(\rho\)--the mass air density, \(A_{ref}\)--effective area of one of the supporting structures of the model, i.e., the area of the shadow normal projected by its members on a plane parallel to the wall equal 0.051 m\({}^{2}\), \(r_{ref}\)--the reference dimension adopted by convention, the width of the structure of the model equal 0.116 m and \(h_{ref}\) is half of the total height of the model equal to 0.446 m.  
Figure 7: Wind speed measured with two anemometers placed at different heights.