scholarly journals Direct numerical simulation of pattern formation in subaqueous sediment

2014 ◽  
Vol 750 ◽  
Author(s):  
Aman G. Kidanemariam ◽  
Markus Uhlmann
Keyword(s):  
Numerical Simulation ◽  
Direct Numerical Simulation ◽  
Propagation Speed ◽  
Immersed Boundary ◽  
Flow Conditions ◽  
Sphere Model ◽  
Soft Sphere ◽  
Sediment Bed ◽  
Fluid Bed ◽  
Two Parameter

AbstractWe present results of direct numerical simulation of incompressible fluid flow over a thick bed of mobile spherically shaped particles. The algorithm is based upon the immersed-boundary technique for fluid–solid coupling and uses a soft-sphere model for the solid–solid contact. Two parameter points in the laminar flow regime are chosen, leading to the emergence of sediment patterns classified as ‘small dunes’, while one case under turbulent flow conditions leads to ‘vortex dunes’ with significant flow separation on the lee side. The wavelength, amplitude and propagation speed of the patterns extracted from the spanwise-averaged fluid–bed interface are found to be consistent with available experimental data. The particle transport rates are well represented by available empirical models for flow over a plane sediment bed in both the laminar and the turbulent regimes.

DIRECT NUMERICAL SIMULATION OF DETACHMENT OF SINGLE CAPTURED LEUKOCYTE UNDER DIFFERENT FLOW CONDITIONS

2011 ◽  
Vol 11 (02) ◽  
pp. 273-284 ◽  
Author(s):  
Z. Y. LUO ◽  
F. XU ◽  
T. J. LU ◽  
B. F. BAI
Keyword(s):  
Numerical Simulation ◽  
Global Health ◽  
Direct Numerical Simulation ◽  
Adhesion Force ◽  
Target Cells ◽  
Simple Liquid ◽  
Cell Capture ◽  
Flow Conditions ◽  
Rectangular Microchannel ◽  
Flow Acceleration

Antibody-based cell isolation using microfluidics finds widespread applications in disease diagnostics and treatment monitoring at point of care (POC) for global health. However, the lack of knowledge on underlying mechanisms of cell capture greatly limits their developments. To address this, in this study, we developed a mathematical model using a direct numerical simulation for the detachment of single leukocyte captured on a functionalized surface in a rectangular microchannel under different flow conditions. The captured leukocyte was modeled as a simple liquid drop and its deformation was tracked using a level set method. The kinetic adhesion model was used to calculate the adhesion force and analyze the detachment of single captured leukocyte. The results demonstrate that the detachment of single captured leukocyte was dependent on both the magnitude of flow rate and flow acceleration, while the latter provides more significant effects. Pressure gradient was found to represent as another critical factor promoting leukocyte detachment besides shear stress. Cytoplasmic viscosity plays a much more important role in the deformation and detachment of captured leukocyte than cortex tension. Besides, better deformability (represented as lower cytoplasmic viscosity) noteworthy accelerates leukocyte detachment. The model presented here provides an enabling tool to clarify the interaction of target cells with functional surface and could help for developing more effective POC devices for global health.

scholarly journals Channel flow over large cube roughness: a direct numerical simulation study

2010 ◽  
Vol 651 ◽  
pp. 519-539 ◽  
Author(s):  
STEFANO LEONARDI ◽  
IAN P. CASTRO
Keyword(s):  
Numerical Simulation ◽  
Direct Numerical Simulation ◽  
Channel Flow ◽  
Outer Part ◽  
Area Coverage ◽  
Immersed Boundary ◽  
Mean Flow ◽  
Surface Drag ◽  
Effective Roughness ◽  
Plane Displacement

Computations of channel flow with rough walls comprising staggered arrays of cubes having various plan area densities are presented and discussed. The cube height h is 12.5% of the channel half-depth and Reynolds numbers (uτh/ν) are typically around 700 – well into the fully rough regime. A direct numerical simulation technique, using an immersed boundary method for the obstacles, was employed with typically 35 million cells. It is shown that the surface drag is predominantly form drag, which is greatest at an area coverage around 15%. The height variation of the axial pressure force across the obstacles weakens significantly as the area coverage decreases, but is always largest near the top of the obstacles. Mean flow velocity and pressure data allow precise determination of the zero-plane displacement (defined as the height at which the axial surface drag force acts) and this leads to noticeably better fits to the log-law region than can be obtained by using the zero-plane displacement merely as a fitting parameter. There are consequent implications for the value of von Kármán's constant. As the effective roughness of the surface increases, it is also shown that there are significant changes to the structure of the turbulence field around the bottom boundary of the inertial sublayer. In distinct contrast to two-dimensional roughness (longitudinal or transverse bars), increasing the area density of this three-dimensional roughness leads to a monotonic decrease in normalized vertical stress around the top of the roughness elements. Normalized turbulence stresses in the outer part of the flows are nonetheless very similar to those in smooth-wall flows.

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