# Feasibility of Optical Flow Field Measurements of the Coolant in a Grinding Machine
## 4 Measurement Results
### Impact of Optical Distortions on MWF Jet Flow Measurements  
To examine the influence of light refraction on both measuring methods for different inflow conditions qualitatively, raw images are captured with the PIV and the shadowgram imaging system for the volume flows 14.9 L/min, 26.6 L/min and 39.4 L/min, resulting in a laminar, transient and turbulent flow respectively. The resulting images are shown in Figure 5.  
For the lowest volume flow of 14.9 L/min (Figure 5a,b) a laminar free jet without temporally varying structures results. Considering the top view, a smooth centre area is visible and light refractions occur only at the cylindrical edges. In the range of 40 mm to 120 mm of the \(x\)-axis, the scattered light of the seeding particles is visible and in the side view, the originally round particles are observed significantly distorted due to the curved jet surface and appear as ellipses (see red circle). So in both views, the particles can be identified, which allows for the calculation of their velocities with PIV via cross-correlation algorithms. Since almost no characteristic flow structures can be observed, a SIV velocity measurement is not possible.  
At an increased volume flow of 26.6 L/min (Figure 5c,d), faint turbulences of the transient flow in form of waves are present on the free jet geometry. These structures follow the flow direction and are visible in the shadowgraphy, which principally enables a SIV velocity evaluation.  
Due to the surface structures, further light refractions occur in the illumination with the laser light section for PIV, visible at 90 mm of the \(x\)-axis. However, the occurring light refraction is not significant and the particles are still observable. An evaluation with PIV seems therefore still possible. The comparison of the velocity measurement with SIV (based on flow structures) and PIV (based on seeding) in Section 4.3 will reveal, if the characteristic flow structures reflect the flow velocity.  
For the flow rate of 39.4 L/min (Figure 5e,f), a fully turbulent flow with more intense surface structures on the MWF appear. Due to the dominant surface inhomogeneities, total reflections occur in parts of the wave structures. The influence of the total reflections is a major obstacle for the PIV measurement technique. If total reflections occur, the laser light does not enter the MWF and no particles are illuminated. In addition, the reflections are significantly brighter than the scattered light from the particles, which reduces their visibility. As a result, a velocity evaluation based on the motion of the particles without  
Figure 5: Images of the free jet region for the top view and the side view captured with PIV (**left**) and shadowgram imaging (**right**). The top view and the side view for each volume flow is displayed as a pair. (**a**) laminar flow visualised with PIV and red circle highlighting the impact of light refraction; (**b**) laminar flow visualised shadowgram; (**c**) laminar–transient flow visualised with PIV; (**d**) laminar–transient flow visualised shadowgram; (**e**) turbulent flow visualised with PIV; (**f**) turbulent flow visualised shadowgram.  
further adjustment is no longer feasible, since mainly the wave structures are visualised. The existing turbulent structures are distinguishable in the shadowgraph picture and their size increases with the distance to the nozzle orifice in \(x\)-direction. As a consequence, velocity measurements with PIV and SIV are feasible, but both are based on the motion of wave structures.