Fluid Dynamics Research

Journal Information
ISSN / EISSN : 0169-5983 / 1873-7005
Published by: IOP Publishing (10.1088)
Total articles ≅ 2,099
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Published: 13 September 2021
Fluid Dynamics Research, Volume 53; https://doi.org/10.1088/1873-7005/ac2620

Abstract:
The nonlinear motion of two interfaces in a three-layer fluid with density stratification is investigated theoretically and numerically. We consider the situation such that a uniform current is present in one of the three layers. The linear dispersion relation is calculated by the Newton's method, from which the initial conditions for numerical computations are determined. When the uniform current is present in the upper (lower) layer, strong vorticity is induced on the upper (lower) interface, and it rolls up involving the other interface at the late stage of computations. When the current is present in the middle layer, a varicose wave appears at the initial stage, and it evolves into an asymmetric heart-shaped vortex sheet at the last computed stage. These phenomena are presented using the vortex sheet model (VSM) with and without regularizations.
, , Eduardo M Germer, Admilson T Franco
Published: 7 September 2021
Fluid Dynamics Research, Volume 53; https://doi.org/10.1088/1873-7005/ac2481

Abstract:
The current work is devoted to analyzing viscoplastic fluid flow in a circular axisymmetric abrupt contraction with ratio D/d=1.85. The velocity fields and static pressure along a hydraulic loop were measured employing the Particle Image Velocimetry technique and a pressure transducers system, respectively. Four aqueous solutions of Carbopol® 940 with different rheological parameters were used as viscoplastic fluids. Laminar and turbulent flow conditions are evaluated and compared with the generalized Hagen-Poiseuille law and the Malin friction factor correlations supported by turbulent fluctuations measurements. The formation of a plug core region is observed around the centerline of the pipe. The relationships between the velocity profiles and the shear stress distribution with the plug core were investigated supported via shear rate calculations. The results show that the yield stress highly influences the shape of the velocity profiles and the appearance of unyielded regions at the contraction edges. The shear stress maps show a layered distribution across the pipe diameter with higher values located at the contraction entrance. Instead, a constant shear stress distribution is found in the unyielded regions. The size of these unyielded regions increases with the yield stress but is reduced at regions with high shear rates due to increased inertial forces. Pressure drop measurements contrasted with the shear stress and velocity gradients maps illustrated that the pressure losses along the pipe are affected by the yield stress and the Reynolds number. However, a non-dependence of the pressure loss coefficient at the contraction plane with the rheological parameters is evidenced, as presented in earlier experimental results. This differs from many numerical studies since the contraction's pressure loss coefficient for viscoplastic fluids is directly affected by the increase of inertial forces at this region.
, Ryozo Ooka, Hideki Kikumoto
Published: 23 July 2021
Fluid Dynamics Research, Volume 53; https://doi.org/10.1088/1873-7005/ac1782

Abstract:
In the lattice Boltzmann method (LBM), the widely utilized wall boundary is the bounce-back (BB) boundary, which corresponds to the no-slip boundary. The BB boundary prevents the LBM from capturing the accurate shear drag on the wall when addressing high Reynolds number flows using coarse-grid systems. In this study, we proposed the "wall-function bounce (WFB)" boundary, a general framework to incorporate wall functions into the LBM's boundary condition, independent of specific information of discrete velocity schemes and collision functions. The WFB boundary calculates the appropriate shear drag on the wall using a wall function model, and thereafter just modifies partial diagonal distribution functions to reflect the shear drag. The Spalding's law was utilized as the wall function in WFB. Simulations of turbulent channel flow at Reτ=640 and 2003 using the LBM-based large-eddy simulation (LBM-LES) were conducted to validate the effectiveness of the proposed boundary condition. The results indicate that the BB boundary underestimated the time-averaged velocity in the buffer layer at Reτ=640, and the averaged velocity in the entire domain at Reτ=2003, when using coarse-grid systems. However, WFB obtained the proper shear drag on the wall and thus, compensated for the underestimation and agreed better with the experimental or direct numerical simulation data, especially at the first-layer grid. In addition, WFB improved the Reynolds normal stress in the near-wall region to some extent. The distributions of shear stress on the wall by WFB were analogous to those by the wall model function in the finite volume method.
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