Flow community; a question on flow visualization: What is an accepted way of comparing flow patterns, lets say that I observe in a blender or a boat propeller, against that I calculate in a 3-D model of the same parts? In the calculations, if I say there are vortexes and secondary flows because I follow their pattern around the rotor and corners of the case and compare them qualitatively with photographs of the real pattern, how can I say that my calculations are valid and realistic without actually measuring values of velocity and fluctuations in the flow? Is this realistic or illusory?

(1). Very good question. As a matter of fact, currently, I am using the computed particle paths heavily in my 3-D analysis of turbomachinery flows. (2). For flow around the turbine blade, first of all, there are flow separations in the region around the leading edge of the blade (in the junction areas with the hub of the shroud). This complex vortex systems will carry the fluid downstream around the corner and across the blade passage. (3). The one moves across the blade passage will create the so-called secondary flow as the flow moves in the downstream direction. This secondary flow system will eventually climb up the suction side of the turbine blade and then interact with the wake of the blade in a rather complicated way. (4). As you can see that there is a history effect, that is you have to be able to trace back the secondary flow to its origin in the blade leading edge region. (5). Since the turbulence model with the use of the law of the wall treatment is not suitable for this complex leading edge separation region, low Reynolds number models must be used in the calculations. (6). So far, it is not practical to measure the surface skin friction for the separated flow regions. So, the only reliable way to check whether the 3-D calculation has captured the basic features of the complex flow separation systems is to see whether the computed flow field can reveal the same type of the fluid particle paths. (7). The fluid particle path represents the most accurate flow field information in both the measured (visualized) and the computed flow field. (8) Using this flow visualization technique in 3-D separated flow field, one can easily identify whether a particular turbulence model is giving you the correct answer or not. (9). Once you have captured the basic features of the flow field in the computation, you can move on to check the flow parameters of interest to you. These parameters sometimes are global flow parameters, such as the total pressure , loss and efficiency. (10). Now , you can see that flow visualization techniques are very important in measurement and computation of 3-D complex flow field with flow separations. One practical application of the fluid particle path is to check whether the mesh refinement is required as the next step in the study. If the mesh is too coarse, most of these complex flow features will not be captured at all.