## 3 Spray Development from Nozzle Orifices
### Near Nozzle Velocity Measurement with the Structural Image Velocimetry  
Velocity measurements are commonly based on the determination of the movement of droplets or tracer particles, or at least structures between two or more instants of time. For the very near region of the spray, the phase surface between liquid and gas could eventually give the required velocity information. Limitations are the very small scales, the high velocity, multiple scattering in the high dense spray region and eventually missing visible structures in the very near field of the spray. One successful approach is based on time dependent double point measurement of the structure, assuming that it moves downstream without changing the structural elements between the two measurement points. With two focus points in the main spray direction, the laser correlation velocimetry (LCV) method ([PERSON] et al. [33]) allows one to determine spray velocities, even very near the exit [34; 35; 36; 37], with the help of a cross correlation evaluation of the two time-dependent signals. In our group, the structural image velocimetry (SIV) method was proposed in 2013, being based on direct double imaging with high resolution spray shadowgraphy [28]. With a double-shutter camera and a long distance microscope, image pairs can be obtained with time difference in the order of 1 \(\upmu\)s. The working principle can be seen in Figure 14 for a 50 MPa injection process. The method is based on the observation that structural elements and gradations inside the spray are moving, as is highlighted on the right side, in this case with approximately 250 m/s. Figure 13: Length scales of the primary breakup with the non-perturbed length (L\({}_{\text{P}}\)) and the breakup length (intact core length, L\({}_{\text{b}}\)). Determination of both quantities for different injection pressures for the steady injection state with two methods. Three-hole diesel injector with a nozzle diameter of 115 \(\upmu\)m, gas pressure 0.1 MPa. Reproduced with permission from [30], Elsevier, 2019. Similar to the LCV technique, the SIV approach can evaluate the velocity of such movements, as long as sufficient structural elements are detectable. For the automatic evaluation of velocity fields, the structural image velocimetry (SIV) follows the approach from particle image velocimetry (PIV), which is based on the evaluation of the spatial cross-correlation between two consecutive images [28, 38, 39]. The approach has some similarities to the optical flow method from [PERSON] et al. [40], where a similar evaluation approach is applied for the velocity determination of the tip and rim of large scale sprays during the early spray injection process. For the SIV approach, the velocity distribution inside the spray is evaluated from the moving structures inside the measured spray. According to the working principle, the velocity is especially evaluated from the object plane of the lens and camera system (Figure 15). However, the structural movements behind and in front of the image plane are also giving contributions to the autocorrelation signal in the camera plane. This can be reduced, if the focal length of the lens system is as small as possible. Figure 14: Double shots shadow graph imaging: (**a**) Entire spray; (**b**) Close-up views. Injection with 50 MPa injection pressure. Figure 15: Working principle of the Structural Image velocimetry (SIV). Adapted with permission from [28], Esytec, 2013. Figure 16a shows the evaluated velocity field of a fuel spray measured at 150 MPa injection pressure by high speed SIV for the similar injector, where the OCM and the X-ray measurements have been reported; see before. The color indicates the magnitude of the velocity. The vertical axis indicates the distance from the injector nozzle. The SIV method evaluates relatively high velocities (red) between 2 and 6 mm downstream from the nozzle around the spray center. Low velocities (blue) appear on the edges of the spray.