## 4 Comprehensive Discussion on the Spray Processes near the Nozzle
[PERSON] [27] also showed that the spray cone angle near the nozzle is significantly increased during this opening and closing process, being directly correlated to the appearance of the OCM signal. From that, the theoretical idea that the cavitation induced increased turbulence which, by itself, increases the spray breakup, can be confirmed. The OCM technique was also applied in comparison with X-ray synchrotron measurements for the same injector and injection conditions [30]. With highly resolved OCM, not only the break-up length of the liquid dense core, but also an even more detailed length of the cylindrical smooth liquid jet in the first 100 \(\upmu\)m behind the injector can be seen, as here, only a small amount of light comes out of the liquid core. With that, also the \"non-perturbed length\" can be measured. This quantity refers to earlier measurements of [PERSON] et al. [31], and it corresponds well with the smooth surface of a fuel jet being visible with X-ray phase contrast method [32]. Even the details of the asymmetric behavior of the smooth jet surface is found to be comparable between OCM and X-ray radiation [30]. The \"non-perturbed length\" decreases with increasing pressure, similar to the break-up length. It can be noted that the breakup length (dense core length) is not visible from an X-ray phase contrast method, as the wrinkling of the surface gives the dominant signal here. As the OCM technique requires an optical fiber to be introduced into the injector tip, this technique is limited to spray injection experiments without ignition and combustion. In order to overcome this limitation, the remote optical connectivity method (ROCM) was developed, where the light is delivered with a focused laser beam to a transparent orifice to be guided into the interior of the injector. Moreover, with the ROCM technique, it was possible to visualize the dense core of the injected spray in the near field of the nozzle and its influence with increasing injection pressure. A challenge for the application of the ROCM is stray light near the injector. As a third approach, further techniques have been applied to measure the spray velocity as near as possible to the exit of the injector. Although not investigated in our group, it turns out that the laser correlation velocimetry (LCV) technique allows the nearest determination of the high velocity spray near the exit of the injector, as being developed by [PERSON] et al. [33; 34], and being further developed and applied by several groups for velocity measurement very near to the spray exit. Here, the temporal cross correlation evaluation of two focused points with short focal length on the observation side allows the detection of moving structural elements (moving boundaries between liquid and gaseous phase), in some cases already very near to the spray exit in the sub-mm range. An extension of this measurement principle to the movement of spatially measured structures has been proposed and applied with the structural image velocimetry (SIV) [28]. Here, the velocity of structures is determined from double imaging, with the evaluation similar to the particle image velocimetry (PIV). The measurement plane is defined from the short focal length of the imaging optics, but not from a laser sheet in PIV, as this is not possible in the near field of the spray. With that, the discrimination between structures moving in the measurement plane is better than for X-ray velocimetry approaches, as they integrate along their optical path (not discussed in this paper). However, it is lower than for the LCV approach, as for the SIV approach, only on the detection side is the differentiation between moving structures in the intended measurement plane and before or behind possible. However, from the spatially resolved measurements, it is easily possible to determine the usable measurement range from the condition that the axial profile decreases in injection direction.