Abstract
The nature of the underlying structures in Taylor-Couette (TC) flow, the flow between two co-axial and independently rotating cylinders is investigated by two methods. First, the quadrant analysis technique for identifying structures with intense radial-azimuthal stresses (also referred to as ‘Q’s) of Lozano-Durán et al., (J. Fluid Mech. 694, 100-130) is used to identify the main structures responsible for the transport of angular velocity. Second, the vortex clusters are identified based on the analysis by del Álamo et al., (J. Fluid. Mech., 561, 329-358). In order to test these criteria, two different radius ratios η = ri/ro are considered, where ri and ro are the radii of inner and outer cylinder, respectively: (i) η = 0.5 and (ii) η = 0.909, which correspond to high and low curvature geometries, respectively and have different underlying structures. The Taylor rolls, i.e. the large-scale coherent structures, are effectively captured as ‘Q’s for the low curvature setup and it is observed that curvature plays a dominant role in influencing the size and volumes of these ‘Q’s. On the other hand, the vortex clusters are smaller in size when compared to the ‘Q’ structures. These vortex clusters are found to be taller in the case of η = 0.909, while the distribution of the lengths of these clusters is almost homogenous for both radius ratios.
Highlights
The ubiquitous nature of wall bounded turbulent flows both in nature and in process technology makes them a subject of interest for fundamental scientific research as well as for industrial applications
For wall-bounded turbulent flows, the existence of various types of coherent structures such as hairpins, horse-shoe vortices, herringbone-like streaks, and many others has been the subject of research in several pioneering studies [3, 4, 5]
A number of studies exist in the literature which elucidate the various techniques that can be used to identify organized ‘structures’ or ‘clusters’ in a turbulent flow field; in particular here we focus on the ‘quadrant’ analysis technique [16, 17, 18] and the identification of vortex clusters [19]. del Alamo et al [20] adopted the approach of Chong et al [19] based on the velocity gradient discriminant to identify clusters of vortical structures in a turbulent channel flow
Summary
The ubiquitous nature of wall bounded turbulent flows both in nature and in process technology makes them a subject of interest for fundamental scientific research as well as for industrial applications. In TC flow in many areas of the parameter space large-scale structures appear These are known as Taylor rolls, and are fixed in space, persistent in time, and are responsible for majority of the angular momentum transport [7]. This is in contrast to channel flows, where analysis based on cospectra show that the large-scale wall-attached structures are inactive [6]. The analysis of Lozano-Duran et al is interesting for TC because it comes to different conclusions about the large-scale wall-attached structures in channels, namely that they actively transport Reynolds stresses, similar to the Taylor-rolls.
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