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arXiv2018

Aerodynamics or Quantum Collisions: Drag coefficient revision by Schrodinger's equation

Despite the fact that the calculations of drag coefficient and pressure distribution for airfoils can be completed by using Navier-Stoke's equation with help of experimental parameters and advanced computer programming, a simple theoretical approach to these classical problems is still lacked. In this paper we show Schrodinger equation can in fact be a handy tool to describe the mechanics of fluids using rigid sphere in air as an example and further investigate the wave-like properties of fluids. We also provide computational results for simulations of drag coefficient, as well as a comparison to potential theory results of velocity distribution along the surface of a sphere. The final discussion will be focusing on potential generalization of the formulas to other geometrical objects (e.g. airfoils).

physics.flu-dyn↗

arXiv2018

Physical Modeling and Numerical Studies of Three-dimensional Non-equilibrium Multi-temperature Flows

For increasingly rarefied flowfields, the Navier-Stokes (NS) equations lose accuracy partially due to the single temperature approximation. To overcome this barrier, a continuum multi-temperature model based on the Bhatnagar-Gross-Krook (BGK) equation coupled with the Landau-Teller-Jeans relaxation model has been proposed for two-dimensional hypersonic non-equilibrium multi-temperature flow computation. In recent study, a two-stage fourth-order gas-kinetic scheme (GKS) has been developed for equilibrium flows, which achieves a fourth-order accuracy in space and time as well as high efficiency and robustness. In this paper, targeting for accurate and efficient simulation of multi-temperature non-equilibrium flows, a high-order three-dimensional multi-temperature GKS is implemented under the two-stage fourth-order framework, with the fourth-order Simpson interpolation rule for the newly emerged source term. Simulations on decaying homogeneous isotropic turbulence, low-density nozzle flow, rarefied hypersonic flow over a flat plate, and type IV shock-shock interaction are used to validate the multi-temperature model through the comparison with experimental measurements. The unified gas kinetic scheme (UGKS) results, and the Direct simulation Monte Carlo (DSMC) solutions will be used as well in some cases for validation. Computational results not only confirm the high-order accuracy and quite robustness of this scheme, but also show the significant improvement on computational efficiency compared with UGKS and DSMC, especially in the near continuum flow regime.

physics.flu-dyn↗

arXiv2018

Dynamic vortical flow profile quantitatively characterizes mitral valvular-left ventricular hemodynamic coupling: In vivo analysis by dynamic 3D enstrophy mapping from 4D Flow MRI

Vortical blood flow in the human left ventricular (LV) inflow initiates from the mitral valve (MV) and evolves within the LV during diastolic E-filling. Hence, vortical flow links MV and LV hemodynamics. This study sought to elucidate and quantitatively characterize the in vivo 3D dynamics of LV vortical flow over E-filling and relation to MV-LV hemodynamic coupling using 4D Flow MRI flow field. 34 healthy volunteers and 5 example patients underwent 4D Flow MRI. Vortical blood flow evolution was mapped in the LV over E-wave using enstrophy density. A new dimensionless profile PMV-LV was derived as a function of both MV vortex formation time (VFT) and LV volumetric enstrophy density. Results reveal that 3D vortical flow evolution in the healthy LV follows a bi-phasic behavior with a vortical growth phase followed by a vortical decay phase. In healthy LVs studied, the PMV-LV profile showed that the vortical growth and decay phases are characterized by a vortical growth time T_growth= 1.23+/-0.25, growth rate α=0.80+/-0.17, decay time T_decay= 0.96+/-0.39 and decay rate \b{eta}=-1.02+/-0.49. Distinctly altered parameters were found in the pilot patients studied. The derived PMV-LV profile quantitatively characterizes MV-LV hemodynamic coupling by vortical flow dynamics. Results herein unravel new insights into cardiac physiology and could enable a novel standardized methodology to study MV-LV hemodynamic coupling and association to cardiac function in health and disease.

physics.med-ph↗

arXiv2018

Turbulence, Gravity, and Multimessenger Asteroseismology

Part IA: We present numerical measurements of relativistic scaling relations in $(2+1)$-dimensional conformal fluid turbulence, which perform favourably over their non-relativistic versions. As seen with incompressible turbulence in past studies, we find that the energy spectrum exhibits $k^{-2}$ scaling rather than the Kolmogorov/Kraichnan expectation of $k^{-5/3}$. Part IB: We compute the fractal dimension $D$ of a turbulent anti-deSitter black brane reconstructed from boundary fluid data using the fluid-gravity duality. Our value of $D=2.584(1)$ is consistent with the upper bound $D\leq 3$, resolving a recent claim that $D=3+1/3$. We describe how to covariantly define the fractal dimension of spatial sections of the horizon, and we speculate on assigning a `bootstrapped' value to the entire horizon. Part II: We report progress implementing a fluid code with post-Newtonian (PN) gravity in spherical symmetry. The PN formalism couples a fluid, its self-gravity, and a black hole via elliptic equations. This eliminates radiative modes, allowing larger time steps, which is helpful for studying systems with very long time scales, eg. tidal disruption events. Part III: Asteroseismology of rotating core-collapse supernovae is possible with a multimessenger strategy. We show an $l=2$, $m=0$, $n\gtrsim 2$, $f\lesssim 280$ Hz mode of the core is responsible for emission in gravitational waves and neutrinos. The angular harmonics of the neutrino emission is consistent with the mode energy around the neutrinospheres, where $r\sim 70$ km. Thus, neutrinos carry information about the mode in the outer region of the core, whereas gravitational waves probe the deep inner core $r \lesssim 30$ km.

astro-ph.HE↗

arXiv2018

The Lamb-Oseen Vortex and Paint Marbling

The displacement pattern arising from the decay of a two-dimensional Lamb-Oseen vortex in a Newtonian fluid can be closely modeled by the closed-form expression presented here. This formula enables Lamb-Oseen vortex simulation orders of magnitude faster than can be accomplished using finite-element methods, and without the accumulation of errors. "French Curl" marbling patterns look as though they are created by a vortex. Analysis and simulation of a nineteenth century example of French Curl finds that the pattern was created without a vortex. True vortexes are rarely seen in paint marbling because, in order to reach Reynolds numbers larger than 90, viscosity of the fluid bath must be much lower than is customarily used.

cs.CG↗

arXiv2018

Sound of Interfacial Flows: Unraveling the Forces Shaping Fast Capillary Flows using their Acoustic Signature

Many familiar events feature a distinctive sound: paper crumpling or tearing, squeaking doors, drumming rain or boiling water. Such characteristic sounds actually carry a profusion of informations about the fleeting physical processes at the root of acoustic emission, which appears appealing especially in situations precluding direct or in-situ measurements, such as e.g. the rupture of micron-thick liquid sheet. Here we report on such a link between fast interfacial hydrodynamics and sound. The acoustic emission of a bursting soap bubble is captured by means of antennae and deciphered with the conceptual framework of aeroacoustics. This reveals that capillary forces, thin-film hydrodynamics, but also out-of-equilibrium surfactants dynamics all shape the capillary burst sound. Whereas ultra-fast imagery only captures the shapes of flows, the acoustic signature radiated by hydrodynamical forces offers a timely complement for it allows a direct experimental access to these dynamical quantities.

cond-mat.soft↗

arXiv2018

Mathematical analysis of unstable density fluctuations in the dissipative gravitational collapse

A detailed analysis of the dynamics of unstable modes present in the linearized Navier-Stokes-Fourier system in the presence of a gravitational field is carried out. The transition between the non-dissipative and dissipative regimes is explored from the mathematical point of view by considering the same equation of state in both cases, a procedure which sheds light on the discontinuity present in the critical parameter. It is also shown that two distinct behaviours for the propagation of unstable modes can be identified below the critical value for a gravitational instability to occur, depending on a parameter $r$ that quantifies the relevance of dissipation relative to gravitational effects.

physics.flu-dyn↗

arXiv2018

Uniform coating of self-assembled non-iridescent colloidal nanostructures using Marangoni effects and polymers

Colloidal crystals exhibit structural color without any color pigment due to the crystals' periodic nanostructure, which can interfere with visible light. This crystal structure is iridescent as the resulting color changes with the viewing or illumination angle, which limits its use for printing or displays. To eliminate the iridescent property, it is important to make the packing of the colloidal nanoparticles disordered. Here, we introduce a drop-casting method where a droplet of a water- ethanol mixture containing monodisperse polymer-coated silica nanoparticles creates a relatively uniform and non-iridescent deposit after the droplet evaporates completely on a heated substrate. The uniformity is caused by a thermal Marangoni flow and fast evaporation effects due to the heated substrate, whereas non-iridescence is the outcome of short-range-ordered packing of nanoparticles by depletion attraction and friction effects produced by polymer brushes. We show that the colors of the final deposits from individual droplets remain unchanged while the viewing angle is varied under ambient light. We expect that the coating method is compatible with ink-jet printing and the uniformly coated self-assembled non-iridescent nanostructures have potential for color displays using reflection mode and other optical devices.

cond-mat.soft↗

arXiv2018

Alternating Current Electrothermal Flow for Energy Efficient Thermal Management of Microprocessor Hot Spots

The paper presents numerical results on an innovative concept of an efficient, site-specific cooling technology for microprocessor hot spots using Alternating Current Electrothermal Flow (ACET). ACET deploys an electrokinetic transport mechanism without requiring any external prime mover and can be shown to be highly effective in reducing hot spot temperatures below their allowable limits. The proposed technique leverages the heat source(s) itself to drive the fluid in the ACET cooling mechanism, thereby making this a highly energy efficient active technique. Our parametric analyses present the optimal range of fluid properties, viz. electrical conductivity where the cooling efficiency is maximum. Further, the impact of geometrical parameters as well as input voltages has been characterized. Our results show a reduction of hot spot temperature by more than 30% for flow through a channel of 600 microns under appropriate conditions at an applied AC voltage of 10-12 V. Comparison with other micro-cooler technologies show the proposed technique to be more energy efficient in its range of operation. These results establish the potential of the ACET cooling and can open up a new avenue to be explored for thermal management of miniaturized devices and systems.

physics.app-ph↗

arXiv2018

An investigation into the interaction between waves and ice

This thesis is submitted in the partial fulfillment of the requirements for the degree of Doctor of Philosophy at the University of Oslo. It represents work that has been carried out between 2015 and 2018, under the supervision of Pr. Atle Jensen, Dr. Graig Sutherland, and Dr. Kai H. Christensen, in collaboration with Pr. Aleksey Marchenko and Pr. Brian Ward. The work presented was carried at the University of Oslo and the University Center in Svalbard. Financial support for the work was provided by the Norwegian Research Council under the Petromaks 2 scheme, through the project WOICE (Experiments on Waves in Oil and Ice), NFR Grant number 233901. The thesis consists of an introduction, and a selection of 7 publications. The introduction presents the scientific context in which the work was undertaken, the methodology used, the results obtained, as well as some personal thoughts about unsuccessful directions encountered during the project and possible future work. I certify that this dissertation is mine and that the results presented are the result of the work of our research group, to which I brought significant contribution.

physics.ao-ph↗

arXiv2019

Extended KdV equation for the case of uneven bottom

We derived consistently, according to the second order perturbation approach, the extended KdV equation for an uneven bottom for the case of $α=O(β)$ and $δ=O(β^2)$. This equation can be obtained only when the bottom is given by a piecewise linear function. For the case of $α=O(β)$ and $δ=O(β)$, a unidirectional wave equation is derived in the first order approach with the same limitation for the bottom profile.

physics.flu-dyn↗

arXiv2018

Assessment of deconvolution-based flamelet methods for progress variable rate modeling

A novel approach for modeling the progress variable reaction rate in Large Eddy Simulations of turbulent and reacting flows is proposed. This is done in the context of two popular flamelet models which require the progress variable variance as input. The approach is based on using a recently proposed deconvolution method for modeling the variance. The deconvolution-modeled variance, is used as an input in the flamelet models for modeling the filtered progress variable rate. The assessment of the proposed approach is conducted a priori using direct numerical simulation data of turbulent premixed flames. For the conditions tested in this study, deconvolution does not introduce a significant bias in the flamelet models' predictions, while a quantitatively good prediction of the progress variable rate is obtained for both flamelet models considered.

physics.flu-dyn↗

arXiv2020

The role of the asymmetric Ekman dissipation term on the energetics of the two-layer quasi-geostrophic model

In the two-layer quasi-geostrophic model, the friction between the flow at the lower layer and the surface boundary layer, placed beneath the lower layer, is modeled by the Ekman term, which is a linear dissipation term with respect to the horizontal velocity at the lower layer. The Ekman term appears in the governing equations asymmetrically; it is placed at the lower layer, but does not appear at the upper layer. A variation, proposed by Phillips and Salmon, uses extrapolation to place the Ekman term between the lower layer and the surface boundary layer, or at the surface boundary layer. We present theoretical results that show that in either the standard or the extrapolated configurations, the Ekman term dissipates energy at large scales, but does not dissipate potential enstrophy. It also creates an approximately symmetric stable distribution of potential enstrophy between the two layers. The behavior of the Ekman term changes fundamentally at small scales. Under the standard formulation, the Ekman term will unconditionally dissipate energy and also dissipate, under very minor conditions, potential enstrophy at small scales. However, under the extrapolated formulation, there exist small "negative regions", which are defined over a two-dimensional phase space, capturing the distribution of energy per wavenumber between baroclinic energy and barotropic energy, and the distribution of potential enstrophy per wavenumber between the upper layer and the lower layer, where the Ekman term may inject energy or potential enstrophy.

physics.ao-ph↗

arXiv2018

From Deep to Physics-Informed Learning of Turbulence: Diagnostics

We describe tests validating progress made toward acceleration and automation of hydrodynamic codes in the regime of developed turbulence by three Deep Learning (DL) Neural Network (NN) schemes trained on Direct Numerical Simulations of turbulence. Even the bare DL solutions, which do not take into account any physics of turbulence explicitly, are impressively good overall when it comes to qualitative description of important features of turbulence. However, the early tests have also uncovered some caveats of the DL approaches. We observe that the static DL scheme, implementing Convolutional GAN and trained on spatial snapshots of turbulence, fails to reproduce intermittency of turbulent fluctuations at small scales and details of the turbulence geometry at large scales. We show that the dynamic NN schemes, namely LAT-NET and Compressed Convolutional LSTM, trained on a temporal sequence of turbulence snapshots are capable to correct for the caveats of the static NN. We suggest a path forward towards improving reproducibility of the large-scale geometry of turbulence with NN.

physics.flu-dyn↗

arXiv2018

Modelling turbulent premixed flames using convolutional neural networks: application to sub-grid scale variance and filtered reaction rate

A purely data-driven approach using deep convolutional neural networks is discussed in the context of Large Eddy Simulation (LES) of turbulent premixed flames. The assessment of the method is conducted a priori using direct numerical simulation data. The network has been trained to perform deconvolution on the filtered density and the filtered density- progress variable product, and by doing so obtain estimates of the un-filtered progress variable field. Any non-linear function of the progress variable can then be approximated on the LES coarse mesh and explicitly filtered to advance the LES solution in time. This new strategy for tackling turbulent combustion modelling is demonstrated with success, for both the sub-grid scale progress variable variance and the flamelet filtered reaction rate, two fundamental ingredients of premixed turbulent combustion modelling.

physics.flu-dyn↗

arXiv2018

Performance Comparison on Parallel CPU and GPU Algorithms for Unified Gas-Kinetic Scheme

Parallel algorithms on CPU and GPU are implemented for the Unified Gas-Kinetic Scheme and their performances are investigated and compared by a two dimensional channel flow case. The parallel CPU algorithm has a one dimensional block partition that parallelizes only the spatial space. Due to the intrinsic feature of the UGKS, a compromised two-level parallelization is adopted for GPU algorithm. A series of meshes with different sizes are tested to reveal the performance evolution of the algorithms with respect to problem size. Then special attentions are paid to UGKS applications where the molecular velocity space range is large. The comparison confirms that GPU has relative elevated accelerations with the latest device having a speedup of 118.38x. Parallel CPU algorithm, on the contrary, might provide better performances when the grid point number in velocity space is large.

physics.comp-ph↗

arXiv2018

Free surface flows using inverse smoothed particle hydrodynamics

Free surface flow problems including mixing processes in dam break flows and wave breaking phenomena are characterized by large deformation of the free surface. As such, they cannot be easily investigated by analytical and numerical mesh based models. In this paper a new method called un-smoothed particle hydrodynamics (SPH-i) solver is tested to study how well it can capture wave breaking and mixing of water bodies in 2D dam-break flows over wet beds. The model captures the breaking and mixing processes relatively well when compared against experimental results. Therefore, it can be inferred that the capability of the proposed model to predict the development and evolution of breaking waves as well as the interface at the water-water interface in dam-break mixing processes has been demonstrated.

physics.flu-dyn↗

arXiv2018

Matched asymptotic analysis of self-similar blow-up profiles of the thin film equation

We consider asymptotically self-similar blow-up profiles of the thin film equation consisting of a stabilising fourth order and destabilising second order term. It has previously been shown that blow up is only possible when the exponent in the second order term is above a certain critical value (dependent on the exponent in the fourth order term). We show that in the limit that the critical value is approached from above, the primary branch of similarity profiles exhibits a well-defined structure consisting of a peak near the origin, and a thin, algebraically decaying tail, connected by an inner region equivalent (to leading order) to a generalised version of the Landau--Levich `drag-out' problem in lubrication flow. Matching between the regions ultimately gives the asymptotic relationship between a parameter representing the height of the peak and the distance from the criticality threshold. The asymptotic results are supported by numerical computations found using continuation.

physics.flu-dyn↗