52 research outputs found
A model for transport of interface-confined scalars and insoluble surfactants in two-phase flows
In this work, we propose a novel scalar-transport model for the simulation of
scalar quantities that are confined to the interface in two-phase flows. In a
two-phase flow, the scalar quantities, such as salts and surfactants, can
reside at the interface and can modify the properties of the interface, in the
time scales of interest. This confinement of the scalars leads to the formation
of sharp gradients of the scalar concentration values at the interface,
presenting a serious challenge for its numerical simulations.
To overcome this challenge, we propose a computational model for the
transport of scalars that maintains the confinement condition for these
quantities. The model is discretized using a central-difference scheme, which
leads to a non-dissipative implementation that is crucial for the simulation of
turbulent flows. The model is used with the ACDI diffuse-interface method
(Jain, J. Comput. Phys., 2022), but can also be used with other algebraic-based
interface-capturing methods. Furthermore, the provable strengths of the
proposed model are: (a) the model maintains the positivity property of the
scalar concentration field, a physical realizability requirement for the
simulation of scalars, when the proposed criterion is satisfied, (b) the
proposed model is such that the transport of the scalar concentration field is
consistent with the transport of the volume fraction field, which results in
effective discrete confinement of the scalar at the interface; and therefore,
prevents the artificial numerical diffusion of the scalar into the bulk region
of the two phases.
Finally, we present numerical simulations using the proposed model for both
one-dimensional and multidimensional cases and assess: the accuracy and
robustness of the model, the validity of the positivity property of the scalar
concentration field, and the confinement of the scalar at the interface.Comment: 18 pages, 8 figure
Stable, entropy-consistent, and localized artificial-diffusivity method for capturing discontinuities
In this work, a localized artificial-viscosity/diffusivity method is proposed
for accurately capturing discontinuities in compressible flows. There have been
numerous efforts to improve the artificial diffusivity formulation in the last
two decades, through appropriate localization of the artificial bulk viscosity
for capturing shocks. However, for capturing contact discontinuities, either a
density or internal energy variable is used as a detector. An issue with this
sensor is that it not only detects contact discontinuities, but also falsely
detects the regions of shocks and vortical motions. Using this detector to add
artificial mass/thermal diffusivity for capturing contact discontinuities is
hence unnecessarily dissipative. To overcome this issue, we propose a sensor
similar to the Ducros sensor (for shocks) to detect contact discontinuities,
and further localize artificial mass/thermal diffusivity for capturing contact
discontinuities.
The proposed method contains coefficients that are less sensitive to the
choice of the flow problem. This is achieved by improved localization of the
artificial diffusivity in the present method. A discretely consistent
dissipative flux formulation is presented and is coupled with a robust
low-dissipative scheme, which eliminates the need for filtering the solution
variables. The proposed method also does not require filtering for the
discontinuity detector/sensor functions, which is typically done to smear out
the artificial fluid properties and obtain stable solutions. Hence, the
challenges associated with extending the filtering procedure for unstructured
grids is eliminated, thereby, making the proposed method easily applicable for
unstructured grids. Finally, a straightforward extension of the proposed method
to two-phase flows is also presented.Comment: 24 pages, 11 figures, Under review in the Physical Review Fluids
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Large-eddy simulations of the NACA23012 airfoil with laser-scanned ice shapes
In this study, five ice shapes generated at NASA Glenn's Icing Research
Tunnel (IRT) are simulated at multiple angles of attack (Broeren et al., J. of
Aircraft, 2018). These geometries target different icing environments, both
early-time and longer-duration glaze and rime ice exposure events, including a
geometry that results from using a thermal ice-protection system. Using the
laser-scanned geometries, detailed representations of the three-dimensional ice
geometries are resolved on the grid and simulated using wall-modeled LES.
Integrated loads (lift, drag, and moment coefficients) and pressure
distributions are compared against experimental measurements in both clean and
iced conditions for several angles of attack in both pre-and post-stall
regions. The relevant comparisons to the experimental results show that
qualitative and acceptable quantitative agreement with the data is observed
across all geometries.
Glaze ice formations exhibit larger and highly nonuniform ice features, such
as `horns', in contrast to rime ice formations characterized by smaller,
uniformly distributed roughness elements. In wall-modeled LES, it was observed
that larger roughness scales in the glaze ice that trigger transition can be
accurately resolved. Therefore, it is possible for WMLES to accurately capture
the aerodynamics of glaze ice shapes without the need for additional modeling.
In contrast, rime ice geometries required additional resolution to accurately
represent the aerodynamic loads. This study demonstrates the effectiveness of
the wall-modeled LES technique in simulating the complex aerodynamic effects of
iced airfoils, providing valuable insights for aircraft design in icing
environments and highlighting the importance of accurately representing ice
geometries and roughness scales in simulations
Effect of interpolation kernels and grid refinement on two way-coupled point-particle simulations
The predictive capability of two way--coupled point-particle Euler-Lagrange
model in accurately capturing particle-flow interactions under grid refinement,
wherein the particle size can be comparable to the grid size, is systematically
evaluated. Two situations are considered, (i) uniform flow over a stationary
particle, and (ii) decaying isotropic turbulence laden with Kolmogorov-scale
particles. Particle-fluid interactions are modeled using only the standard drag
law, typical of large density-ratio systems. A zonal,
advection-diffusion-reaction (Zonal-ADR) model is used to obtain the
undisturbed fluid velocity needed in the drag closure. Two main types of
interpolation kernels, grid-based and particle size--based, are employed. The
effect of interpolation kernels on capturing the particle-fluid interactions,
kinetic energy, dissipation rate, and particle acceleration statistics are
evaluated in detail. It is shown that the interpolation kernels whose width
scales with the particle size perform significantly better under grid
refinement than kernels whose width scales with the grid size. Convergence with
respect to spatial resolution is obtained with the particle size--based kernels
with and without correcting for the self-disturbance effect. While the use of
particle size--based interpolation kernels provide spatial convergence and
perform better than kernels that scale based on grid size, small differences
can still be seen in the converged results with and without correcting for the
particle self-disturbance. Such differences indicate the need for
self-disturbance correction to obtain the best results, especially when the
particles are larger than the grid size.Comment: Submitted to International Journal of Multiphase Flo
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