# Impacts of Micro-Deviations of Aperture on the Characteristics of Collision Atomization Field
## 3 Principle and Methods  
### Measurement Method of Nozzle Diameter  
To enhance measurement precision, the nozzle evaluation adopted in this investigation is an ultra-high-precision three-dimensional micro-displacement measurement system  
Figure 1: Status of cavitating flow. (**a**) Cavitation inception. (**b**) Cavitation growth. (**c**) Super cavitation. (**d**) Hydraulic flip.  
independently developed by an optical fiber probe based on orthogonal optical fiber collimation, which is made of FBG (fiber Bragg grating). The radial deflection or axial buckling deformation of the fiber probe is induced by the force of the fiber probe. The deformation displacement of the fiber rod at the detection point is measured by the optical fiber collimation optical path. The fiber probe converts the three-dimensional micro-displacement of the fiber probe into the centroid displacement of the collimating image, which can realize high-precision three-dimensional micro-displacement measurement.  
The principal diagram of the optical fiber probe and measurement system based on orthogonal optical fiber collimation is shown in Figure 2[5]. The optical fiber probe consists of an optical fiber rod and a spherical tip. The optical fiber rod is used as a MFLC-lens (micro focal-length cylindrical lens). The arrangement of two mutually orthogonal optical fiber collimating paths is identical, which are distributed along X and Y directions, respectively. The beam emitted by the laser diode (LD/Laser-X, LD/Laser-Y) through the reflective objective lens (ROL) converges into a point light source on the front focal plane of the cylindrical lens through the long working distance objective lens (Objective-X, Objective-Y). The beam emitted by the point light source is collimated through the cylindrical lens, and its collimating image is detected by the linear array charge-coupled devices (CCD-X, CCD-Y). The deformation and displacement of the optical fiber measuring rod are measured at the detection point OP by the orthogonal optical fiber collimating optical path. The optical power meters (PM1 and PM2) receive the power spectrum.  
The experimental device of the optical fiber probe based on orthogonal fiber collimation is exhibited in Figure 3. The length of the optical fiber probe is 15 mm, the diameter of the optical fiber rod (single-mode optical fiber without coating) is 125 \(\upmu\)m and the focal length is 93 \(\upmu\)m. The tip diameter is 131.52 \(\upmu\)m, the working distance of the long working distance objective is 20 mm, the resolution of the linear array camera is 2048 pixel \(\times\) 1 pixel and the pixel size is 10 \(\upmu\)m/pixel. The imaging distance between the detection surface of the linear array camera and the optical fiber probe is 125 mm, and the detection point is OP, whose height is about 6 mm.  
The system is allocated with a visual system to monitor the fiber probe in real time. The experimental instrument is established in a glass enclosure to decrease the influence of air flow and installed on the active vibration isolation platform to curtail the effect of vibration interference. Driven by the three-dimensional nanopositioning platform, the plane mirror engages with the tip of the optical fiber probe.  
The three-dimensional detection organization can accomplish 5 nm radial resolution and 8 nm axial resolution. The expanded uncertainty of optical fiber probe based on orthogonal optical fiber collimation is 0.21 \(\upmu\)m for k = 2 (k is coverage factor). There are more adequate descriptions of the optical fiber probe measurement arrangement in Refs. [5; 29; 30]. Recently, an advanced three-dimensional isotropy microprobe for the assessment of microstructures to estimate the diameter was proposed by [PERSON] et al. [31] to  
Figure 2: Schematic diagram of the 3D detection system [5].  
provide higher efficiency and accuracy of the appraisal. The measurement data of employed nozzles experimentally are detailed in Table 1.