## 5 Summarizing Discussion  
Overall, the flow and spray characteristics are obtained from several measurement approaches described above. As far as possible, the same diesel injector and the same injection parameters have been used. It was possible to reach injection pressures of 180 MPa with transparent nozzles. The realistic flow simulation inside the nozzle is realized by the detailed inner geometry data. The predicted asymmetric flow and cavitation inside the nozzle affect the breakup process near the nozzle exit. With the optical connectivity method (OCM), and with special effort with the ROCM, the liquid structure can be visualized after leaving the nozzle, despite the very small size and extremely dense spray structure due to the increased illumination from inside the nozzle. Even the length of the cylindrical smooth liquid jet in the first 100 \(\upmu\)m behind the injector can be seen with the OCM technique, and it fits perfectly in the smooth region measured with X-ray synchrotron radiation in the phase contrast imaging mode. The smooth surface does not disturb the X-ray signal as well as the OCM light. This confirms the measurability of this \"unperturbed length\". The breakup length (or dense core length) is found to be two-to-three times higher than the unperturbed length, being observed in the range of 300 \(\upmu\)m for the investigated three-hole injector, with 115 \(\upmu\)m nozzle holes. For increased injection pressure up to 150 MPa, both the unperturbed length and the dense core breakup length decrease. Both quantities are expected to depend on the amount of cavitation inside the nozzle, which is not yet investigated in detail. For velocity measurements, the structural image velocimetry (SIV) is invented, being an approach between the laser correlation velocimetry (LCV) and the evaluation of the particle image velocimetry (PIV). It is found to be a rather simple measurement technique, being now even capable for high speed application. The differentiation between the measurement plane and the moving objects in front and behind this plane is partly given, being lower than for the LCV approach, but being better than for X-ray velocimetry. SIV is applicable for diesel sprays from 2-3 mm on (partly from 4 mm on) downstream. It is a rather easy applicable measurement technique. Further downstream, from 5-10 mm on, the laser 2-focus (L2F) velocimetry is applicable, allowing not only the measurement of the spray velocity, but also of the droplet size. The different measurement approaches are supporting each other. They now allow one to confirm the theoretical ideas of spray breakup, even for high pressure injection. Additionally, they allow one to measure detailed data such that numerical models can be validated. It should be emphasized that the combined approach of the description of the processes inside the nozzle and outside in the spray has to be taken, and that the combination of experimental work with numerical models will be needed, in order to reach predictive models of the overall engine processes (e.g., [6]). [PERSON] and [PERSON] wrote the paper; [PERSON], [PERSON], [PERSON], and [PERSON] performed the measurements, simulations and the data processing; [PERSON] and [PERSON] supervised the related projects. All authors have read and agreed to the published version of the manuscript. The research work for the investigations on the flow and cavitation inside nozzles is financially supported by German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) within the project DI 591/29-1. The research activity for the spray measurements near the nozzles is financially supported by Research Association for Combustion Engines (Forschungsvereinigung Verbrennungskraftmaschinen, FVV) within the project Methods of Spray diagnostics II (FVV IGF 18958 N). The publication of this article was funded by the Open Access Fund of the Leibniz Universitat Hannover. The authors would like to express our gratitude to DFG and FVV for their financial support.