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Shahab Mirjalili, Ph.D.
Shahab Mirjalili, Ph.D.
msey@kth.se
Assistant Professor in Fluid Mechanics
KTH Royal Institute of Technology, Sweden
Chan, W. H. R.*, Mirjalili, S.*, Jain, S. S., Urzay, J., Mani, A., & Moin, P. (2019). Birth of microbubbles in turbulent breaking waves. Physical Review Fluids, 4(10), 100508. Link
In this video and its associated paper, with aid from numerical simulations at various physical scales, we outlined a pathway for the formation of microbubbles in turbulent breaking waves. The video was a recipient of the Gallery of Fluid Motion award in APS DFD, 2018. (Video)
Cundy, C. J.*, Mirjalili, S.*, Laurent, C., Ermon, S., Iaccarino, G., Mani, A. (2024), A physics-informed machine learning model for the prediction of drop breakup in two-phase flows. International Journal of Multiphase Flow, 180, 104934. Link
A machine learning model is developed to predict the statistics of daughter drops during the breakup of under-resolved drops. In a proof-of-concept study, by training on two canonical flows (3D Taylor-Green and homogeneous isotropic turbulence), we find a suitable combination of neural networks, quantities of interest, and loss functions. The physics of breakup is incorporated by encoding time-dependent quantities as mixtures of sigmoid functions and by conserving mass via a penalty term in the loss function. The model outperforms baseline models and paves the path for surrogate models for drop breakup in large-scale simulations involving drop/bubble breakup.
Mirjalili, S., & Mani, A. (2024). A conservative second order phase field model for simulation of N-phase flows. Journal of Computational Physics, 498, 112657. Link
We introduce a novel second order PDE for modeling immiscible N-phase flows. The model has many desirable properties, including conservation, boundedness, reduction-consistency, symmetry, mass-momentum consistency. This is all achieved without resorting to any Lagrange multipliers, by taking advantage of computing pairwise volume fractions, normal vectors, and curvature fields. A novel surface tension model is proposed for modeling surface tension forces in N-phase flows. The phase field model allows for variable interface thicknesses between different phase pairs. This endows the model with the property of attaining correct equilibirum configurations at triple junction points.
new_R3.mp4new_R1.mp4
Mirjalili, S., & Mani, A. (2020). Transitional stages of thin air film entrapment in drop-pool impact events. Journal of Fluid Mechanics, 901, A14. Link
In this work, we used numerical simulations of water drop-pool impacts along with theoretical analyses to discover a capillary transition that prevents early contact in drop-pool impact events. This transition allows the drop to penetrate further into the pool and provides a pathway for the formation of elongated films. Since Mesler entrainment is only possible if early contact is prevented, we use the occurrence of transition as a criterion to provide an upper boundary for the Mesler entrainment regime. We observe from low We simulations that after transition, the drop spreads on the pool surface, during which the minimum film thickness increases and the film regularizes. Interestingly, we observe the formation of kinks between the center of the film and the spreading fronts, and find asymptotic scaling laws governing the film thickness. Lastly, by examining the role of liquid viscosity, we shed light on transition dynamics for different liquids.
Mirjalili, S., & Mani, A. (2021). Consistent, energy-conserving momentum transport for simulations of two-phase flows using the phase field equations. Journal of Computational Physics, 426, 109918. Link
We present a consistent, kinetic energy conserving momentum transport scheme in the context of a second order conservative phase field method. This is achieved by (1) accounting for the mass flux associated with the right-hand-side of the phase field equation in the convective flux of the conservative form of the momentum transport equation---a correction absent in previous phase field simulations (2) utilization of non-dissipative spatial discretization. We demonstrate accuracy and robustness improvements from our proposed scheme using numerical tests, including a realistic jet in cross-flow attaining turbulent conditions. Our proposed modifications to the momentum transport equation can be readily extended to other conservative phase field methods.
Mirjalili, S., Chan, W. H. R., & Mani, A. (2018). High Fidelity Simulations of Micro-Bubble Shedding from Retracting Thin Gas Films in the Context of Liquid-Liquid Impact. Symposium on Naval Hydrodynamics, Hamburg, Germany, 2018. Link
In this paper we studied the dynamics of thin air films retracting in background water. While focusing on films with thickness of few microns, we observed a power law retraction speed in 2D simulations. Through 3D simulations we observed how transverse instabilities can lead to the shedding of microbubbles from the retracting film edge.
Mirjalili, S.*, Taverniers, S.*, Collis, H., Behandish, M., & Mani, A. (2023). Inverse asymptotic treatment for modeling discontinuities in fluid flows via equation modification. Journal of Computational Science, 73, 102141. Link
In this work, we introduce the inverse asymptotic treatment (IAT) for modeling sharp discontinuities in various fluid dynamics settings. We introduce the approach in the context of phase field models for two-phase flows. Afterward, in the context of single-phase compressible flows, we explain how localized artificial diffusivity (LAD) schemes can be considered as an instance of IAT. We finally propose an LAD scheme that reverse-engineers the TVD-endowing flux limiters, as an option that can result in robust and accurate simulation results without any parameter tuning.
Kananian, S.*, Rho, J.*, Chen, C., Mirjalili, S., Daus, A., Kim, M., Niu, S., Pop, E., Wong, H. S. P., Bao, Z., Mani, A., and Poon, A. S. Y. (2023). A Disposable Reader-Sensor Solution for Wireless Temperature Logging. Device, 1, 100183. Link
A battery-free, disposable, passive, wireless temperature sensor with memory functionality is proposed that works by using the irreversible phase change of a Gallium structure. This sticker-like temperature reader can be used for clinical diagnostics or to record temperature abuse history for food. I performed two-phase flow simulations of the molten Gallium structure to demonstrate how the circuit is disconnected once phase change (solid to liquid) takes place.
Mirjalili, S., Jain, S. S., & Mani, A. (2022). A computational model for interfacial heat and mass transfer in two-phase flows using a phase field method., International Journal of Heat and Mass Transfer, 197, 123326. Link
Assuming a geometric microstructure for a diffuse interface, we derive a two-scalar model for interfacial heat/mass transfer that is consistent with phase field (diffuse interface) models. By assuming thermodynamic equilibrium, a one-scalar model is derived as well. The two-scalar model is shown to be especially advantageous compared to one-scalar models in realistic problems where the diffusivity ratio between the two phases is large.
Mirjalili, S., & Chan, W. H. R. (2021). Linear stability of a thin fluid film interacting with its surrounding bulk fluid. Physics of Fluids 33, 072104 (Featured Article). Link
Thin films can become unstable when attractive van der Waals forces overcome the stabilizing effects of surface tension and viscous forces. In this work, we examine the two limits of potential flow and Stokes flow in the surrounding bulks to derive dispersion relations in each limit. We show that the effect of the surrounding bulks cannot be ignored for many film--bulk fluid pairings and film thicknesses, and present conditions for the validity of each regime. In particular, the potential-flow regime exists when van der Waals forces are sufficiently strong, while the Stokes-flow regime exists when the bulk dynamic viscosity is sufficiently large.
Mirjalili, S., Khanwale, M. A., & Mani, A. (2023). Assessment of an energy-based surface tension model for simulation of tw-phase flows using second-order phase field methods., Journal of Computational Physics, 474, 111795. Link
In this short note, we justify the usage of an energy-based surface tension model for second order phase field models. We demonstrate the equivalence of this model to a localized continuum surface force at equilibrium. Through numerical tests, we show that the energy-based model significantly reduces spurious currents compared to the CSF models. Moreover, it captures capillary-inertial-viscous interplay more accurately than the CSF models.
Mirjalili, S., Ivey, C. B., & Mani, A. (2020). A conservative diffuse interface method for two-phase flows with provable boundedness properties. Journal of Computational Physics, 401, 109006. Link
We introduced a non-dissipative spatio-temporal discretization for a second order conservative phase field equation. We proved that with the appropriate choice of free parameters, the phase field variable remains bounded. This conservative and bounded diffuse interface method is easy to implement and allows for valuable properties that cannot be achieved with alternative two-phase flow methods.
Mirjalili, S., Ivey, C. B., & Mani, A. (2019). Comparison between the diffuse interface and volume of fluid methods for simulating two-phase flows. International Journal of Multiphase Flow, 116, 221-238. Link
Note: Featured in the list of most cited articles from IJMF published since 2019.
In this work, we compared a two-phase solver based on a diffuse interface method against a solver employing VOF. Using standard test cases at various resolutions, we focused on comparing the methods in terms of cost versus accuracy. The overall finding was that diffuse interface methods can compete with state-of-the-art VOF schemes if one considers accuracy versus computational cost and parallel scalability.
Mirjalili, S., Jain, S. S., & Dodd, M. (2017). Interface-capturing methods for two-phase flows: An overview and recent developments. Center for Turbulence Research Annual Research Briefs, 117-135. Link
In this review article, we provided an overview of various interface-capturing numerical methods with a focus on recent advancements. Overall, we observed that phase field and VOF seem to be the most promising classes of methods.
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