**5. Setup**

The setup for the experiments is presented in this section. An image of the complete setup is shown in Figure 4. The measurement system followed the concept of multi-camera fringe projection profilometry and was based on the findings of Bräuer-Burchard et al. [16] and Reich et al. [17]. We explained the calibration and the main functions of the measurement system in a previous paper [15]. The forced air flow actuator consisted of three commercially available nozzles with a minimum inner diameter of *dn* = 0.8 mm and a custom-made aluminum connection block. The design study examining different flow directions and nozzle setups resulting in the presented setup is shown in Appendix A. The connection block placed the nozzles at equidistant points on a line with 100 mm in between them and an angle to the outside of 5°. A magnetic valve and a manually operated pressure regulator connected the nozzles to the pressurized air container in the laboratory. An image of the nozzles is part of Figure 4 (upper left). The nozzles were set up above the measurement object to use the occurring forces to hold the measurement object in place. To ensure that the measurement light and the forced air flow took similar paths, the FPS was also mounted above the object. Depending on the air pressure *pf f* set in the pressure regulator, the velocities of the air flow field were controlled. Figure 5 shows the designed and simulated velocity field in comparison to a measured field using an air pressure of *pf f* = 2.5 bar. The flow velocity measurements were conducted using a scale and a hand-held anemometer. The flow velocity field showed significant differences from the simulated velocity field in magnitude and in form. The main reasons for the different shapes lied in the lack of a concentric effect of screw threads, which were used to connect the nozzles and the connection block. The differences in velocity magnitude were mainly due to the low accuracy of the hand-held anemometer, as well as numerical dispersion and diffusion effects in the simulation.

**Figure 4.** Image of the complete measurement setup.

**Figure 5.** Comparison of the simulated and measured flow velocity fields of the nozzle setup. (**a**) Top view on the simulated air flow; (**b**) Section A-A through (**a**); (**c**) measurement of air flow velocities comparable to (**b**) in the implemented experimental setup. All given distances are in mm.

Even though there were velocity differences of up to 90 %, the overall design process was considered successful, since the aim of surrounding a cylinder (*dc* = 50 mm, *lc* = 250 mm) with an homogeneous air flow field with 1 m s<sup>−</sup><sup>1</sup> < *vf f* < 4ms−<sup>1</sup> was achieved.
