Experimental and Numerical Study of Swirling ... - Solid Mechanics

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Experi imental and Numerical N Stud dy of Swirling g Flow in Scaveenging Processs for 2-Stroke Marin ne Diesel Engin nes Fig gure 4.13: Mea an Normalized Axial A Vort ticity @ z5 L3 3=4D Chapter 4 The irro otational regio on diminishes downstream with the gradual decay of thhe swirl an nd the conseq quent decreasse in the vortticity of the vvortex core annd transfer r of vorticity y to larger raadial distancees. However, this decay in potential flow region n, as a functionn of downstreaam axial distannce, seems to bbe faster at high Re wit th existence oof a comparatiively less vortical vortex coore and larg ge core size (Figure 4.13). 62 Swirling Flow in a Pipe
Experimental and Numerical Study of Swirling Flow in Scavenging Process for 2-Stroke Marine Diesel Engines 4.2.5 Reynolds Normal Stresses Chapter 4 The Reynolds stresses are expressed in Cartesian coordinates rather than cylindrical coordinates. The major reasons are (i) the particle image Velocimetry (PIV) measurement equipment gives data in Cartesian coordinate system and (ii) transforming the velocity components to the corresponding components in cylindrical coordinate is very difficult in instantaneous velocity filed due to the lack of a clear and single defined vortex core. In the current experiment only at z1, there exist some instantaneous snapshots of the flow, among all snap shots for that particular measurement, where a single vortex core can reasonably be observed. (iii) to the knowledge of author, most of the major CFD codes are written for Cartesian coordinates and the model validation requires the experimental data to be in the same coordinate system. The contour plots of velocity components in Cartesian coordinates can still provide some good information about the corresponding components in polar (cylindrical) coordinates. The only difference is to understand the relation of u and v components to V and Vr because the w and Vz are the same in both the coordinate system. In case of swirling flow, the interpretation of Cartesian components in to polar components is easily understood. Along X-axis, the u component represents Vr and v component represents V . In case of Y-axis, it is vice versa. Figure 4.14 show contour plots of u and v components and can thus be used as an example of understanding the distribution of tangential and radial velocities from contour plots of u and v components (signs denote the direction of a given vector component). The contours of u along Y-axis indicate that the tangential velocity increases from cylinder axis to its peak value at a region r/R=0.17 (approximately) and then decreases again towards large radial positions. Whereas along X-axis, the value of u shows that radial velocity has a very low magnitude. The contour of v gives similar information about the distribution of tangential and radial velocity components but along opposite axis lines i.e. X-axis, as discussed before. The distribution of fluctuating/ turbulent part of the individual velocity components, for polar coordinates, can also be understood in the same manner. 63 Swirling Flow in a Pipe
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DTU Mechanical Engineering Section
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