Now showing 1 - 3 of 3
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    Publication
    The structure of turbulence in rotating rough-channel flows
    (01-06-2022)
    Jagadeesan, Karthikeyan
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    Andersson, Helge I.
    Direct numerical simulation (DNS) of rib-roughened turbulent channel flow rotating about its spanwise axis, by Narasimhamurthy and Andersson [Turbulence statistics in a rotating ribbed channel. International Journal of Heat and Fluid Flow 51, 29–41. (2015)], is revisited to seek complementary insights into the combined effects of roughness and Coriolis force on the turbulence. Flow in the channel was maintained at a friction Reynolds number, Reτ=400 and the non-inertial reference frame was rotated at different speeds quantified by the dimensionless rotation number, Ro=0, 2 and 6. Both the aforementioned parameters are based on the friction velocity uτ and half-height of the channel h. The channel walls were symmetrically mounted with transverse square ribs with cross-section of side k=0.1h and pitch λ=8k. Rotation causes preferential aligning of the near-wall vortical structures. The Taylor-Görtler–like roll-cells similar to those found in the rotating smooth-channel flows, survive the presence of the transverse ribs, but exhibit transient behavior. Increased transport of turbulent kinetic energy from the pressure side at higher Ro is evident from the variation of the vertical transport velocity Vk. The rotational production rates assume increasingly significant roles in distributing the kinetic energy in different directions. Anisotropy invariant maps and Taylor microscales show that the structure of turbulence is affected by rotation in a significant manner.
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    Publication
    LES and DNS of symmetrically roughened turbulent channel flows
    (01-12-2021)
    Varma, Harish
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    Jagadeesan, Karthikeyan
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    Kesarkar, Amit P.
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    Andersson, Helge I.
    A fully developed turbulent channel flow with symmetrically roughened walls is investigated, where the channel walls are roughened with square ribs, elongated along the span of the channel and are spaced uniformly in the streamwise direction at a constant pitch. The effects of Reynolds number variation on the statistical quantities, the near-wall dynamical structures and on the anisotropic nature of turbulence are studied at two Reynolds numbers Reτ= 180 and 400, where Reτ is based on the channel half-height h and the wall friction velocity uτ. Near-wall resolving large eddy simulations (LES) with different grid resolutions are carried out and validated with in-house direct numerical simulation (DNS) data. Turbulence anisotropy at both small and large scales of motion is investigated using anisotropic invariant maps. A variation in the anisotropic behavior of the flow in the near-wall region is noticed, where the flow is found to be more anisotropic at Reτ=180 than at Reτ=400. Also, the anisotropic behavior of the small-scale motions varies from the large-scale motions at Reτ=400. Two-point correlation and phase analysis using Hilbert transform reveals that the flow within the cavity is independent of the flow outside the cavity. The relatedness of the ‘worm-like’ vortical structures with the positive enstrophy production rate (ωiSijωj> 0) is investigated. The regions of positive enstrophy production rate are observed to be topologically ‘sheet-like’ predominantly at a height just above the rib. The regions of negative enstrophy production rate (ωiSijωj< 0) are less dominant, with a topology combination of weakly ‘sheet-forming’ and ‘tube-forming’. The statistical features could be captured by LES with a grid consisting of only one-fifth of the total number of grid points as that in the DNS mesh.
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    Publication
    Direct numerical simulation of coflowing rough and smooth turbulent channel flows
    (01-06-2023)
    Varma, Harish
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    Jagadeesan, Karthikeyan
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    ;
    Kesarkar, Amit P.
    Fully developed turbulent flows through ribbed channels have been simulated using direct numerical simulation (DNS). Square ribs were arranged transversely to the flow on one of the channel walls, and based on their spanwise extent, resulted in two configurations - two-dimensional (2D) configuration resulting from full-span-width ribs and a three-dimensional (3D) configuration, where the ribs extend only up to half the span-width of the channel, leaving the other half smooth. The 3D configuration thus produced a unique problem of coflowing rough and smooth turbulent channel flows. A striking phenomenon has been observed of the secondary roll cells, exhibiting a strong updraft in the smooth half of the channel. The mean velocity profile from the smooth half surprisingly possesses a linear region of constant slope at the channel core. Comparisons were also drawn with DNS of a smooth channel at the same friction Reynolds number 400 and it was found that the roll cells on the smooth half not only affect the bulk flow negatively but also attenuate the turbulence significantly. In spite of having a higher bulk velocity, the rough half of the 3D configuration is more turbulent than the 2D configuration; this has been attributed to the momentum transfer from the smooth half to the rough half. Statistical turbulence quantities, and the production rates of turbulent kinetic energy and enstrophy have been used to arrive at the inferences. In essence, the roll cells buffer the variations between the rough and the smooth halves in the 3D configuration. The instantaneous streamwise velocity fluctuations in the 3D configuration displayed a wavelike form.