Flow Wall Interaction Research Articles

Computational fluid dynamic simulations of granular flows: Insights on the flow-wall interaction dynamics

• Topography controls the dynamics and propagation of dry volcanic granular flows • MFIX two-fluid model was used to study the granular flow – wall interaction • Experimentally-generated volcanic granular flows were simulated with MFIX • The solid phase wall boundary condition affects the flow conditions Dry volcanic granular flows are gravity-driven currents composed of solid particles where particle-particle interactions dominate the motion. The interaction with topography is a relevant factor controlling the propagation of such flows. In this paper we investigate the dynamics of channelised volcanic granular flows by comparing large-scale experiments with multiphase computational fluid dynamic simulations using the Two-Fluid Model approach, with an emphasis on the dynamics regulating the flow-wall interactions. We use the software MFIX to carry out sensitivity analysis of the boundary conditions for the solid phase implemented in the numerical code. The sensitivity analysis shows how the choice of the boundary condition and of the relevant parameters controlling the boundary conditions highly affect the dynamics of the whole flow. Finally, a preliminary benchmark of the MFIX boundary conditions with one large-scale experiment is presented, showing good agreement between the simulated and experimental flow front velocities.

Large eddy simulation of the flow energy dissipation in the blade channels of a stalled pump impeller

Pump impellers usually work with the rotating stall phenomenon. The performance will be strongly affected by the undesirable flow. Understanding the features of flow energy dissipation is very important. In this study, large eddy simulation is conducted for the turbulent flow field based on a typical 6-blade pump impeller with “alternating stall” phenomenon. The stalled channels and well-behaved channels alternatively and stably distribute. Compared with the experimental data, the numerical simulation is proved accurate. In the well-behaved channel, flow pattern is relatively smooth. In the stalled channel, flow is completely blocked by several stall cells. Vortex shedding is well predicted mainly in three regions including the leading-edge region of stalled channel, the outlet region of stalled channel, and the blade suction side region of well-behaved channel. Turbulence kinetic energy is high in these regions. High flow energy dissipation is found among shedding vortexes due to flow-flow interaction and in the near-wall region because of flow-wall interaction. In the core region of vortexes, flow energy dissipation is not that high. The flow-flow interaction and the flow-wall interaction are two main sources of flow energy dissipation. In the well-behaved impeller blade channel with smooth flow pattern, the flow energy dissipation is mainly caused by flow-wall interaction. On the contrary, in the stalled impeller blade channel with vortical flow pattern, the flow energy dissipation is mainly caused by flow–flow interaction. The large eddy simulation-based study will be very helpful in enhancing the performance of pump impellers especially with the rotating stall phenomenon.

Dry granular avalanche impact force on a rigid wall: Analytic shock solution versus discrete element simulations.

The present paper investigates the mean impact force exerted by a granular mass flowing down an incline and impacting a rigid wall of semi-infinite height. First, this granular flow-wall interaction problem is modeled by numerical simulations based on the discrete element method (DEM). These DEM simulations allow computing the depth-averaged quantities-thickness, velocity, and density-of the incoming flow and the resulting mean force on the rigid wall. Second, that problem is described by a simple analytic solution based on a depth-averaged approach for a traveling compressible shock wave, whose volume is assumed to shrink into a singular surface, and which coexists with a dead zone. It is shown that the dead-zone dynamics and the mean force on the wall computed from DEM can be reproduced reasonably well by the analytic solution proposed over a wide range of slope angle of the incline. These results are obtained by feeding the analytic solution with the thickness, the depth-averaged velocity, and the density averaged over a certain distance along the incline rather than flow quantities taken at a singular section before the jump, thus showing that the assumption of a shock wave volume shrinking into a singular surface is questionable. The finite length of the traveling wave upstream of the grains piling against the wall must be considered. The sensitivity of the model prediction to that sampling length remains complicated, however, which highlights the need of further investigation about the properties and the internal structure of the propagating granular wave.

Dry granular avalanche impact force on a rigid wall of semi-infinite height

The present paper tackles the problem of the impact of a dry granular avalanche-flow on a rigid wall of semi-infinite height. An analytic force model based on depth-averaged shock theory is proposed to describe the flow-wall interaction and the resulting impact force on the wall. Provided that the analytic force model is fed with the incoming flow conditions regarding thickness, velocity and density, all averaged over a certain distance downstream of the undisturbed incoming flow, it reproduces very well the time history of the impact force actually measured by detailed discrete element simulations, for a wide range of slope angles.

DEM study of granular flow around blocks attached to inclined walls

Damage due to intense particle-wall contact in industrial applications can cause severe problems in industries such as mineral processing, mining and metallurgy. Studying the flow dynamics and forces on containing walls can provide valuable feedback for equipment design and optimising operations to prolong the equipment lifetime. Therefore, solids flow-wall interaction phenomena, i.e. induced wall stress and particle flow patterns should be well understood. In this work, discrete element method (DEM) is used to study steady state granular flow in a gravity-fed hopper like geometry with blocks attached to an inclined wall. The effects of different geometries, e.g. different wall angles and spacing between blocks are studied by means of a 3D DEM slot model with periodic boundary conditions. The findings of this work include (i) flow analysis in terms of flow patterns and particle velocities, (ii) force distributions within the model geometry, and (iii) wall stress vs. model height diagrams. The model enables easy transfer of the key findings to other industrial applications handling granular materials.

Depth-averaged analytic solutions for free-surface granular flows impacting rigid walls down inclines.

In the present paper, flows of granular materials impacting wall-like obstacles down inclines are described by depth-averaged analytic solutions. Particular attention is paid to extending the existing depth-averaged equations initially developed for frictionless and incompressible fluids down a horizontal plane. The effects of the gravitational acceleration along the slope, and of the retarding acceleration caused by friction as well, are systematically taken into account. The analytic solutions are then used to revisit existing data on rigid walls impacted by granular flows. This approach allows establishing a complete phase diagram for granular flow-wall interaction.

Numerical simulation and mathematical analysis of flow-wall interaction in the large deformation application to the dynamics of the aneurysms

A numerical method is derived to take account of full flowwall interaction in la large deformation domain. To this end, a simplified Lagrangian and nonlinear model is derived to describe the wall motion. the flow is described by two dimensional Naiver stokes equation. The projection method is used to solve for the flow and fourth Rung-Kutta method is used to solve wall equation. The formulation of the problem allows full flow and wall interaction via the boundary conditions at the interface flow-wall. Some numerical simulation will be presented with periodic inlet flow. The method is applied to study the dynamics of aneurysms in arteries and veins. The flow inside the aneurysm is examined under the effects of a steady inlet flow as well as a pulsatile inlet flow for different aneurysm sizes. The wall model is analyzed when the wall is subjected to a constant transmural pressure and a quasi uniform inviscid flow. For a steady constant transmural pressure, a formal solution of the non linear integral-partial differential equation governing the wall motion is derived. For a steady and a quasi uniform inviscid flow, a first integral of the wall equation is obtained, then the solution is found to satisfy an integral non linear equation which is solved by numerical iteration

On the origin of spin-up processes in decaying two-dimensional turbulence

A remarkable feature of two-dimensional turbulence in a square container with no-slip walls is the spontaneous production of angular momentum due to flow-wall interactions, also known as spontaneous spin-up of the flow. In this paper we address the statistics of spin-up and discuss its likely origin. A signature of the spontaneous production of angular momentum is the development of a large-scale circulation cell. It is found that the global turnover time of the flow guides the spin-up process, which can be considered as a relaxation process of the macroscopic flow to an angular momentum containing state. The high turnover rate of the small-scale vortical structures emerging from the no-slip walls apparently has no significant effect on the spin-up rate. The presented data on the spin-up process strongly suggest that spin-up is not the net result of isolated vortex-wall interactions, with its associated pressure fluctuations on the domain boundaries, alone. The rapid spin-up of the flow clearly suggests the attraction to an angular momentum containing state.

A Fourier Spectral Solver for Confined Navier-Stokes Flow

No-slip boundaries have an important effect on forced and decaying two-dimensional turbulence due to their role as vorticity source. During intensive vortex-wall interactions high-amplitude vorticity filaments are produced. Most of these filaments roll up and form small-scale vortices that are advected into the interior by larger-scale vortices. From a computational point of view, it is a challenge to resolve the multiple temporal and spatial scales. Another challenge is to solve 2D turbulence in different geometries, e.g. square, triangle, circle, or ellipse. In this study we use a fast Fourier spectral technique to simulate the Navier-Stokes equations with no-slip boundary conditions. This is enforced by an immersed boundary technique called volume penalization. The approach has been justified by analytical proofs of the convergence with respect to the penalization parameter. However, the solution of the penalized Navier-Stokes equations is not smooth on the surface of the penalized volume. Therefore, it is not a priori known whether it is possible to actually perform accurate fast Fourier spectral computations. Convergence checks are reported using a recently revived, and unexpectedly difficult, dipole-wall collision as a test case. It is found that Gibbs oscillations have a negligible effect on the flow evolution, also for 2D flows without the presence of reflection symmetry. Convergence results are reported of the angular momentum production by intensive flow-wall interaction.

Transient blood flow—wall interaction in abdominal aortic aneurysms

Transient blood flow—wall interaction in abdominal aortic aneurysms