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Vortex intensification plays an important role in a wide range of flows of engineering interest. One scenario of interest is when a streamwise vortex passes through the contracting streamtube of an aircraft intake. There is, however, limited experimental data of flows of this type to reveal the dominant flow physics and to guide the development of vortex models. To this end, the evolution of wing-tip vortices inside a range of streamtube contractions has been measured using stereoscopic particle image velocimetry. A semi-empirical model has been applied to provide new insight on the role of vorticity diffusion during the intensification process. The analysis demonstrates that for mild flow contractions, vorticity diffusion has a negligible influence due to the low rates of diffusion in the vortex flow prior to intensification and the short convective times associated with the streamtube contraction. As the contraction levels increase, there is a substantial increase in the rates of diffusion which is driven by the greater levels of vorticity in the vortex core. A new semi-empirical relationship, as a function of the local streamtube contraction levels and vortex Reynolds number, has been developed. The model comprises a simple correction to vortex filament theory and provides a significant improvement in the estimation of vortex characteristics in contracting flows. For the range of contractions investigated, errors in the estimation of vortex core radius, peak tangential velocity and vorticity are reduced by an order of magnitude. The model can be applied to estimate the change in vortex characteristics for a range of flows with intense axial strain, such as contracting intake streamtubes and swirling flows in turbomachinery. [ABSTRACT FROM AUTHOR]

In the present study, the performance of the vorticity confinement method has been improved by combining it with the vortex feature detection methods. In the conventional vorticity confinement method, the only parameter to apply or not to apply vorticity confinement is the non-zero value of vorticity. On the other hand, the presence of vorticity in some cases, like the boundary layer and the shear layer flows, does not imply the presence of vortices. Applying the vorticity confinement at these points can lead to errors, in addition to loss of solution time. In order to solve this problem, using the combination of vorticity confinement method and four methods of vortex feature detection (non-dimensional Q, non-dimensional λ2, non-dimensional modified Δ, and the S − Ω correlation) the vorticity confinement term is applied only in vortex regions. In order to investigate the effects of this combination, the compressible Euler equation has been investigated for the problem of two-dimensional stationary single vortex at Mach number 0.5. The results indicate significant positive effects in reducing the solving time, decreasing the sensitivity of the solution to the amount of confinement parameter and significant elimination of the oscillation. [ABSTRACT FROM AUTHOR]

This paper focuses on the multi-scale wake structures of a new asymmetric side mirror of a race car, which can generate the aerodynamic downforce. The turbulent wake generated by symmetric bluff bodies of triangle and semi-cylinder and an asymmetric bluff body (called CT-cylinder) were experimentally investigated and were compared. Firstly, the instantaneous velocity and vorticity were quantitatively measured by high-speed PIV at a Reynolds number of 14,440 in the circulating water channel. Then a one- and two-dimensional wavelet multi-resolution techniques were used to analyze the instantaneous velocity and vorticity in order to decompose turbulent structures into a number of subsets based on their central frequencies. It is found that the CT-cylinder wake generates the asymmetric flow structures and separation region that is smaller than that of the triangle wake. The contribution to vorticity concentration comes from the large-scale structure of CT-cylinder wake in the ( x , y )-plane, and the intermediate-scale structure of the CT-cylinder wake dominates the flow structure in the ( x , z )-plane. There is no evident difference among the three wakes in the relatively small-scale structures. The large-scale structures of three wakes exhibit the main contributions to the Reynolds shear stress, accounting for almost 82–92%. However, the small-scale structures make less contribution to Reynolds shear stresses. The phase-averaging and spectral analyses indicate that the CT-cylinder wake generates the relatively smaller vortices and smaller separation region and the CT-cylinder wake exhibits lower fluctuation in the shear layer. [ABSTRACT FROM AUTHOR]

Vortex merging is a basic fluid phenomenon which has been much studied for two-dimensional flows. Here we extend such a study to a specific class of three-dimensional flows, namely to vortices possessing a helical symmetry. In addition to the standard Reynolds number, this introduces another dimensionless control number, the pitch, which quantifies the periodicity length along the helix axis. Helical vortices with large pitches merge very much as in a two-dimensional setting. However, their rotation speed is reduced and the merging period is delayed. These effects, caused by the presence of a self-induced velocity in curved three-dimensional vortices, are understood by computing the streamfunction in the frame of reference rotating with the two vortices, and by inspecting the locations of its hyperbolic points. At intermediate pitch values, only viscous diffusion acts, resulting in a slow viscous type of merging. Finally for small pitches, the system is unstable resulting, at the nonlinear stage, in a different type of merging which breaks the initial central symmetry. [ABSTRACT FROM AUTHOR]

• Four micromixers with double V-shaped baffles are designed. • All mixers achieve high mixing efficiency at the short distance of 3500 μm. • The visualization of vortex formation is conducted to analyze the mixing mechanism. • Different vortex patterns form in the microchannels as inlet velocity varies. Achieving rapid mixing of different liquids in a short distance is important for biochemical applications. Here, a new type of micromixers consisting of double symmetrical V-shaped baffles (MMA and MMB mixers) and double asymmetrical V-shaped baffles (MMC and MMD mixers) is proposed. Numerical simulations are performed to evaluate their mixing performance when the Reynolds number (Re) ranges from 0.1 to 50. Meanwhile, the visualization of vortex formation is conducted to analyze the mixing mechanism. The results show that the mixing efficiency of all mixers approaches 0.8 for all Re at the short distance of 3500 μm. At relatively low Re =0.5, the MMA and MMB perform better than MMC and MMD due to more adequate molecular diffusion; whereas the unbalanced lateral flows and multidirectional vortices in the MMC and MMD induce stronger chaotic convection at high Re =50, leading to a higher performance. Moreover, the disturbance created by the vortex on the mass transfer surface is not only relying on its vorticity but also correlates with the position of vortex/vortex leg. Lastly, all micromixers are fabricated, tested and compared with the simulation results and both of them show a good agreement. [Display omitted] [ABSTRACT FROM AUTHOR]