# Feasibility of Optical Flow Field Measurements of the Coolant in a Grinding Machine
## 2 Measurement Approach
However, since the surface structures depend on the flow conditions, their usability as flow tracers must be analysed for Reynolds numbers covering laminar, laminar to turbulent transitional and fully turbulent jet regimes regarding possible differences in the results of the SIV flow field measurements. In order to realise PIV and SIV measurements in a grinding machine, the proposed measurement arrangement is schematically depicted in Figure 2. For SIV, a diffuse background light source is used to illuminate the fluid, and a high-speed camera on the opposite side records the resulting shadowgram over time. Note that with this arrangement no depth information in \(z\)-direction is acquired and a superposition of flow structures from different depths of the fluid cannot be separated and disturbs the detectability. The MWF supply flow is generated by a nozzle with a rectangular orifice shape, resulting in a liquid jet shape based on the orifice geometry as shown in Figure 3. The liquid free jet has the basic shape of a sheet with a thickness and width in the same scale as the orifice shape. However, due to the surface tension the liquid is tapering by time which leads to an increased rounded cross-section at the outer edges. According to the liquid jet geometry, the visualisation result depends on the perspective. For the top view, an observation of the jet over the width \(w\) through its thickness \(h\) is made. With the side view, an observation is made through the entire jet width \(w\). Comparing the measurement results from top and side view, the perturbations caused by the volumetric illumination of the shadowgraphy can be discussed qualitatively with respect to the geometry of the free jet. Measurements with top view result in observations through a thin jet volume. Due to the small volume, a 2d flow geometry can be assumed. Accordingly, the surface structures represent practically the entire flow of the fluid. In addition, almost no superposition of the characteristic flow structures from different depths occurs. However, for flow measurements in interaction with the grinding wheel, images must be taken from the side of the grinding wheel in \(z\)-direction (as shown in Figure 2). A superimposed observation of the characteristic flow structures from different depths is then expected, which results in possible misallocation and a reduced detectability of the flow structures. Therefore, it is necessary to determine the impact of the volumetric observation on the measurement. For this purpose, observations of the MWF jet's top and side view are performed and the measurements are compared with each other. Finally, the movement of the flow characteristics must be examined in relation to the flow velocity. To accomplish this, the velocity of the free jet is determined with a standard PIV system, i.e., with a pulsed laser and a camera in both perspectives, see Figure 2. Difficulties in illuminating and observing the flow tracer particles due to the effect of light refraction are expected, which increase with number and intensity of the flow structures. To ensure that PIV measurements of the MWF free jet are feasible, measurements are carried out for different jet flow conditions. First, a laminar free jet flow without unsteady surface structures is used to proof the general PIV measurement feasibility. Afterwards, measurements are performed on a laminar-turbulent transient flow, with faint flow structures present, which also allows SIV flow measurements. A comparison of the absolute measurement uncertainties for the laminar and transient case will reveal the impact of the flow-induced uneven surface. Figure 3: Free jet and nozzle orifice geometry. (**a**) Technical sketch of the Rouse-nozzle; (**b**) Free jet geometry generated by rectangular nozzle orifice. Figure 2: Schematic representation of the flow generation and the velocity measurement in a grinding machine with the laser light sheet illumination for PIV and the diffuse LED backlight illumination for the shadowgram imaging.