Proc. V European Conference on Computational Fluid Dynamics ECCOMAS CFD, 2010
Abstract. The filtered fluid dynamic equations are discretized in space by a high-order spectral ... more Abstract. The filtered fluid dynamic equations are discretized in space by a high-order spectral difference (SD) method coupled with large eddy simulation (LES) approach. The subgrid-scale stress tensor is modelled by the wall-adapting local eddy-viscosity model (WALE). We solve the unsteady equations by advancing in time using a second-order backward difference formula (BDF2). The nonlinear algebraic system arising from the time discretization is solved with the nonlinear lower-upper symmetric Gauss-Seidel (LU-SGS) ...
The computation of the Beta PDF is discussed and algorithms are described which permit calculatio... more The computation of the Beta PDF is discussed and algorithms are described which permit calculation of the PDF over the full range of its parameters without recourse to approximations or polynomial expansions. This is achieved by giving simple rules for the treatment of the trivial cases and a new algorithm for the general cases, particularly for the cases where the PDF becomes infinity at the extremes of the range. The problems of underflow and overflow when performing the calculations on a digital computer are also addressed, and a general prescaling procedure is presented which overcomes these problems. It is shown that the new algorithms perform more efficiently than commercial mathematical packages.
Journal of aerosol medicine and pulmonary drug delivery, Jan 23, 2016
While it is generally accepted that inertial impaction will lead to particle loss as aerosol is b... more While it is generally accepted that inertial impaction will lead to particle loss as aerosol is being carried into the pulmonary airways, most predictive aerosol deposition models adopt the hypothesis that the inhaled particles that remain airborne will distribute according to the gas flow distribution between airways downstream. Using a 3D printed cast of human airways, we quantified particle deposition and distribution and visualized their inhaled trajectory in the human lung. The human airway cast was exposed to 6 μm monodisperse, radiolabeled aerosol particles at distinct inhaled flow rates and imaged by scintigraphy in two perpendicular planes. In addition, we also imaged the distribution of aerosol beyond the airways into the five lung lobes. The experimental aerosol deposition patterns could be mimicked by computational fluid dynamic (CFD) simulation in the same 3D airway geometry. It was shown that for particles with a diameter of 6 μm inhaled at flows up to 60 L/min, the ae...
In this article, the wave propagation properties of the 1D spectral volume method are studied thr... more In this article, the wave propagation properties of the 1D spectral volume method are studied through analysis of the Fourier footprint of the schemes. A p-multigrid algorithm for the spectral volume method is implemented. Restriction and prolongation operators are discussed and the efficiency of the smoothing operators is analyzed. The results are verified for simple 1D advection problems and for a quasi-1D Euler flow. It is shown that a significant decrease in computational effort is possible with the p-multigrid algorithm.
Http Dx Doi Org 10 1080 10618560310001654319, Jan 25, 2007
In the present paper an implicit time accurate approach combined with multigrid and preconditioni... more In the present paper an implicit time accurate approach combined with multigrid and preconditioning is used for the large-eddy simulation of low Mach number flows. It will be shown that the present approach allows an efficiency gain of a factor 4 to 7 compared to the use of a purely explicit approach. The efficiency varies according to the test case, grid clustering, physical time stop and requested residual drop.
ABSTRACT Large Eddy Simulation (LES) is used to investigate the 3D-transport of sediment suspensi... more ABSTRACT Large Eddy Simulation (LES) is used to investigate the 3D-transport of sediment suspension in an open channel flow at a Reynolds number based on the frictional velocity.An Eulerian model was employed to represent the fluid and the sediment phases. The subgrid-scale fluid stresses were modeled with the Smagorinsky model, whereas subgrid-scale sediment fluxes were based on a gradient approach. To avoid tuning constants, a dynamic procedure was applied. A two-way coupling between sediment and fluid improved the computational description of sediment transport. Sediment concentration profiles and the statistics of sediment fluxes are presented and compared with the Rouse profile and the RANS solution. The results obtained agree well with the theory and with measurements. The LES results also indicate that the assumption of a constant Schmidt number, as is mostly assumed in RANS simulations is erroneous.
... Chang, YS and Scotti, A. 2003. Entrainment and suspension of sediments into a turbulent flow ... more ... Chang, YS and Scotti, A. 2003. Entrainment and suspension of sediments into a turbulent flow over ripples. ... [44. Nakagawa, S., Na, Y. and Hanratty, TJ 2003. Influence of a wavy boundary on turbulence. ... Direct numerical simulation of turbulent flow over a wavy wall. Phys. ...
The present paper focuses on a dynamic version of the variational multiscale model and investigat... more The present paper focuses on a dynamic version of the variational multiscale model and investigates its performance in large eddy simulations (LES) of turbulent channel flow at Reynolds numbers (based on friction velocity) of 180, 395, and 590. The dynamic procedure was implemented for the variational multiscale model on the basis of the classical Smagorinsky model and the wall-adapting local
ABSTRACT This paper deals with the acoustic velocity field simulation generated by interaction of... more ABSTRACT This paper deals with the acoustic velocity field simulation generated by interaction of flow with moving bodies. Starting from the Ffowcs Williams and Hawkings equation, an analytical formulation of the acoustic velocity is derived for sources in arbitrary motion. This makes the imposition of the boundary condition on a (rigid) scattering surface much more straightforward, as, if the traditional pressure formulation is used, then the pressure gradient must be calculated. Computational results for a pulsating sphere, dipole source, and a propeller case with subsonic tips verify this formulation.
Four cyclone separators with different dust outlet geometries (without dustbin, with dustbin, wit... more Four cyclone separators with different dust outlet geometries (without dustbin, with dustbin, with dipleg and with dustbin plus dipleg) have been computationally investigated. There are two purposes of this study. First, to study the effect of changing the dust outlet geometry on the flow field pattern and performance of air cyclones. Second, to demonstrate whether simulation of the flow without accounting for the dust outlet geometry (i.e., simulate only the cylinder on cone region) is sufficient for performance estimation. Results show that the maximum tangential velocity is almost the same for the four tested cyclones. No radial acceleration occurs in the cyclone space (the maximum tangential velocity is nearly constant throughout the cyclone). Three cyclones (without dustbin, with dipleg and with dustbin plus dipleg) exhibit the inverted W axial velocity profile. An inverted V axial velocity profile is observed for the cyclone with dustbin. Although the absence of the dust outlet geometry from the simulation domain will result in large savings in the computational effort, the present study indicated it can affect the estimated pressure drop and cut-off diameter significantly. If the cyclone performance parameters are estimated without the dust outlet geometry, an error margin of around 10% in the calculations of the Euler number and 35% in the calculation of the cut-off diameter should be taken into account.
Notes on Numerical Fluid Mechanics and Multidisciplinary Design, 2014
ABSTRACT We carried out large-eddy simulation to study turbulent air flow and local deposition of... more ABSTRACT We carried out large-eddy simulation to study turbulent air flow and local deposition of inhaled nanoparticles with a 10nm diameter in an idealized human upper airway model for a steady inspiratory flow rate of 30 l . We described the min particles in an Eulerian framework and accounted for their Brownian motion by a diffusion coefficient calculated from the Stokes-Einstein relation. We calculated the turbulence subgrid-scale stresses and the subgrid-scale particle mass flux by a localized dynamic Smagorinsky subgrid-scale model. Predicted mean velocity and turbulent kinetic energy profiles were in good agreement with particle image ve- locimetry measurements and slightly superior to Reynolds-averaged Navier-Stokes simulations in the same model. We further showed that the most pronounced par- ticle deposition hot-spots are located in the vicinity of intense turbulent structures and that their shapes are strongly correlated.
Proc. V European Conference on Computational Fluid Dynamics ECCOMAS CFD, 2010
Abstract. The filtered fluid dynamic equations are discretized in space by a high-order spectral ... more Abstract. The filtered fluid dynamic equations are discretized in space by a high-order spectral difference (SD) method coupled with large eddy simulation (LES) approach. The subgrid-scale stress tensor is modelled by the wall-adapting local eddy-viscosity model (WALE). We solve the unsteady equations by advancing in time using a second-order backward difference formula (BDF2). The nonlinear algebraic system arising from the time discretization is solved with the nonlinear lower-upper symmetric Gauss-Seidel (LU-SGS) ...
The computation of the Beta PDF is discussed and algorithms are described which permit calculatio... more The computation of the Beta PDF is discussed and algorithms are described which permit calculation of the PDF over the full range of its parameters without recourse to approximations or polynomial expansions. This is achieved by giving simple rules for the treatment of the trivial cases and a new algorithm for the general cases, particularly for the cases where the PDF becomes infinity at the extremes of the range. The problems of underflow and overflow when performing the calculations on a digital computer are also addressed, and a general prescaling procedure is presented which overcomes these problems. It is shown that the new algorithms perform more efficiently than commercial mathematical packages.
Journal of aerosol medicine and pulmonary drug delivery, Jan 23, 2016
While it is generally accepted that inertial impaction will lead to particle loss as aerosol is b... more While it is generally accepted that inertial impaction will lead to particle loss as aerosol is being carried into the pulmonary airways, most predictive aerosol deposition models adopt the hypothesis that the inhaled particles that remain airborne will distribute according to the gas flow distribution between airways downstream. Using a 3D printed cast of human airways, we quantified particle deposition and distribution and visualized their inhaled trajectory in the human lung. The human airway cast was exposed to 6 μm monodisperse, radiolabeled aerosol particles at distinct inhaled flow rates and imaged by scintigraphy in two perpendicular planes. In addition, we also imaged the distribution of aerosol beyond the airways into the five lung lobes. The experimental aerosol deposition patterns could be mimicked by computational fluid dynamic (CFD) simulation in the same 3D airway geometry. It was shown that for particles with a diameter of 6 μm inhaled at flows up to 60 L/min, the ae...
In this article, the wave propagation properties of the 1D spectral volume method are studied thr... more In this article, the wave propagation properties of the 1D spectral volume method are studied through analysis of the Fourier footprint of the schemes. A p-multigrid algorithm for the spectral volume method is implemented. Restriction and prolongation operators are discussed and the efficiency of the smoothing operators is analyzed. The results are verified for simple 1D advection problems and for a quasi-1D Euler flow. It is shown that a significant decrease in computational effort is possible with the p-multigrid algorithm.
Http Dx Doi Org 10 1080 10618560310001654319, Jan 25, 2007
In the present paper an implicit time accurate approach combined with multigrid and preconditioni... more In the present paper an implicit time accurate approach combined with multigrid and preconditioning is used for the large-eddy simulation of low Mach number flows. It will be shown that the present approach allows an efficiency gain of a factor 4 to 7 compared to the use of a purely explicit approach. The efficiency varies according to the test case, grid clustering, physical time stop and requested residual drop.
ABSTRACT Large Eddy Simulation (LES) is used to investigate the 3D-transport of sediment suspensi... more ABSTRACT Large Eddy Simulation (LES) is used to investigate the 3D-transport of sediment suspension in an open channel flow at a Reynolds number based on the frictional velocity.An Eulerian model was employed to represent the fluid and the sediment phases. The subgrid-scale fluid stresses were modeled with the Smagorinsky model, whereas subgrid-scale sediment fluxes were based on a gradient approach. To avoid tuning constants, a dynamic procedure was applied. A two-way coupling between sediment and fluid improved the computational description of sediment transport. Sediment concentration profiles and the statistics of sediment fluxes are presented and compared with the Rouse profile and the RANS solution. The results obtained agree well with the theory and with measurements. The LES results also indicate that the assumption of a constant Schmidt number, as is mostly assumed in RANS simulations is erroneous.
... Chang, YS and Scotti, A. 2003. Entrainment and suspension of sediments into a turbulent flow ... more ... Chang, YS and Scotti, A. 2003. Entrainment and suspension of sediments into a turbulent flow over ripples. ... [44. Nakagawa, S., Na, Y. and Hanratty, TJ 2003. Influence of a wavy boundary on turbulence. ... Direct numerical simulation of turbulent flow over a wavy wall. Phys. ...
The present paper focuses on a dynamic version of the variational multiscale model and investigat... more The present paper focuses on a dynamic version of the variational multiscale model and investigates its performance in large eddy simulations (LES) of turbulent channel flow at Reynolds numbers (based on friction velocity) of 180, 395, and 590. The dynamic procedure was implemented for the variational multiscale model on the basis of the classical Smagorinsky model and the wall-adapting local
ABSTRACT This paper deals with the acoustic velocity field simulation generated by interaction of... more ABSTRACT This paper deals with the acoustic velocity field simulation generated by interaction of flow with moving bodies. Starting from the Ffowcs Williams and Hawkings equation, an analytical formulation of the acoustic velocity is derived for sources in arbitrary motion. This makes the imposition of the boundary condition on a (rigid) scattering surface much more straightforward, as, if the traditional pressure formulation is used, then the pressure gradient must be calculated. Computational results for a pulsating sphere, dipole source, and a propeller case with subsonic tips verify this formulation.
Four cyclone separators with different dust outlet geometries (without dustbin, with dustbin, wit... more Four cyclone separators with different dust outlet geometries (without dustbin, with dustbin, with dipleg and with dustbin plus dipleg) have been computationally investigated. There are two purposes of this study. First, to study the effect of changing the dust outlet geometry on the flow field pattern and performance of air cyclones. Second, to demonstrate whether simulation of the flow without accounting for the dust outlet geometry (i.e., simulate only the cylinder on cone region) is sufficient for performance estimation. Results show that the maximum tangential velocity is almost the same for the four tested cyclones. No radial acceleration occurs in the cyclone space (the maximum tangential velocity is nearly constant throughout the cyclone). Three cyclones (without dustbin, with dipleg and with dustbin plus dipleg) exhibit the inverted W axial velocity profile. An inverted V axial velocity profile is observed for the cyclone with dustbin. Although the absence of the dust outlet geometry from the simulation domain will result in large savings in the computational effort, the present study indicated it can affect the estimated pressure drop and cut-off diameter significantly. If the cyclone performance parameters are estimated without the dust outlet geometry, an error margin of around 10% in the calculations of the Euler number and 35% in the calculation of the cut-off diameter should be taken into account.
Notes on Numerical Fluid Mechanics and Multidisciplinary Design, 2014
ABSTRACT We carried out large-eddy simulation to study turbulent air flow and local deposition of... more ABSTRACT We carried out large-eddy simulation to study turbulent air flow and local deposition of inhaled nanoparticles with a 10nm diameter in an idealized human upper airway model for a steady inspiratory flow rate of 30 l . We described the min particles in an Eulerian framework and accounted for their Brownian motion by a diffusion coefficient calculated from the Stokes-Einstein relation. We calculated the turbulence subgrid-scale stresses and the subgrid-scale particle mass flux by a localized dynamic Smagorinsky subgrid-scale model. Predicted mean velocity and turbulent kinetic energy profiles were in good agreement with particle image ve- locimetry measurements and slightly superior to Reynolds-averaged Navier-Stokes simulations in the same model. We further showed that the most pronounced par- ticle deposition hot-spots are located in the vicinity of intense turbulent structures and that their shapes are strongly correlated.
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Papers by Chris Lacor