arXiv subjects

Himani Garg

Publications and source records attributed to Himani Garg.

9 recordsLinked to original sources

arXiv2024

Large Eddy Simulations of Flow over Additively Manufactured Surfaces: Impact of Roughness and Skewness on Turbulent Heat Transfer

Additive manufacturing creates surfaces with random roughness, impacting heat transfer and pressure loss differently than traditional sand-grain roughness. We conducted high-fidelity heat transfer simulations over three-dimensional additive manufactured surfaces with varying roughness heights and skewness. Based on an additive manufactured Inconel 939 sample from Siemens Energy AB, we created six surfaces with different normalized roughness heights, $R_a/D = 0.001, 0.006, 0.012, 0.015, 0.020,$ and $0.028$, and a fixed skewness, ${s_k} = 0.424$. Each surface was also flipped to obtain negatively skewed counterparts (${s_k} = -0.424)$. Simulations were conducted at a constant Reynolds number of 8000 and with temperature treated as a passive scalar. We analyzed temperature, velocity profiles and heat fluxes to understand the impact of roughness height and skewness on heat and momentum transfer. The inner-scaled mean temperature profiles are of larger magnitude than the mean velocity profiles both inside and outside the roughness layer. This means the temperature wall roughness function differs from the momentum wall roughness function. Surfaces with positive and negative skewness yielded different estimates of equivalent sand-grain roughness for the same $R_a/D$ values, suggesting a strong influence of slope and skewness on the relationship between roughness function and equivalent sand-grain roughness. Analysis of the heat and momentum transfer mechanisms indicated an increased effective Prandtl number within the rough surface in which the momentum diffusivity is larger than the corresponding thermal diffusivity due to the combined effects of turbulence and dispersion. Results consistently indicated improved heat transfer with increasing roughness height and positively skewed surfaces performing better beyond a certain roughness threshold than negatively skewed ones.

physics.flu-dyn↗

arXiv2024

Large Eddy Simulations of Turbulent Pipe Flows At Moderate-To-High Reynolds Numbers

Wall-bounded turbulence is relevant for many engineering and natural science applications, yet there are still aspects of its underlying physics that are not fully understood, particularly at high Reynolds numbers. In this study, we investigate fully-developed turbulent pipe flows at moderate-to-high friction velocity Reynolds numbers ($361 \leq Re_τ \leq 2,000$), corresponding to bulk velocity-based Reynolds numbers of $11,700 \leq Re_{b} \leq 82,500$, using wall-modeled Large Eddy Simulations (LES) in OpenFOAM. A grid convergence study is performed for $Re_τ = 361$, followed by an investigation of the accuracy of various subgrid-scale stress models for the same Reynolds number. Results show that the Wall-Adapting Local Eddy (WALE) model performs well compared to experiments and Direct Numerical Simulations (DNS), while One-Equation Eddy-Viscosity Model (OEEVM) and Smagorinsky (SMG) are too dissipative. LES utilizing WALE are then performed for four different Reynolds numbers with gradually refined grids, revealing excellent agreement with DNS data in the outer region. However, a significant deviation from DNS data is observed in the sub-viscous layer region, indicating the need for further mesh refinement in the wall-normal direction to accurately capture the smallest-scale motions' behavior. Additional mesh sensitivity analysis uncovered that, as the $Re_τ$ value rises, it becomes crucial for a grid to adhere to the condition of $Δx^{+} \leq 20 - 25$ and $Δz^{+} \leq 10$ in order to precisely capture substantial large and small scale fluctuations. Overall, the WALE model enables accurate numerical simulations of high-Reynolds-number, wall-bounded flows at a fraction of the computational cost required for temporal and spatial resolution of the inner layer.

physics.flu-dyn↗

arXiv2023

Effect of fiber curvature on gas diffusion layer two-phase dynamics of a proton exchange membrane fuel cell

The dynamics of two-phase flow within the cathode of a proton exchange membrane fuel cell, particularly in Gas Diffusion Layers (GDLs) with varying fiber curvatures, remain underexplored. Using a periodic surface model, we stochastically reconstruct three GDL types with different fiber curvatures, incorporating vital parameters derived from a physical GDL. Considering the randomness in reconstruction, the structure generation process is iterated four times for each GDL type, enabling an ensemble average analysis. Pore network models are adopted to reveal disparities in these GDL porous structures. The subsequent two-phase simulations are conducted to explore liquid transport through these GDLs and interfaces to assembled gas channels. Time-varying GDL total, local water saturation, and capillary pressure are investigated. Results show stochastic reconstructions exhibit similar frequency peak ranges in pore and throat diameters, and coordination numbers, but diverge from the physical GDL. Bigger fiber curvature tends to enhance pore network connectivity by increasing smaller pores, leading to heightened water saturation and capillary pressure. Straight-fiber GDLs, compared to curved-fiber GDLs, show greater potential proximity to the physical GDL in terms of overall water saturation and capillary pressure but are also accompanied by increased uncertainty. Despite similar layer porosity, water saturation in the same layer of all samples differs increasingly from the inlet to the outlet. Water breakthrough and detachment near the GDL can induce significant water saturation instability at the GDL and gas channel interface. Detached droplets in gas channels connected with straight-fiber GDLs exhibit larger sizes and slower movement than those in channels assembled with curved-fiber GDLs. These findings can be utilized in future GDL design and optimization.

physics.flu-dyn↗

arXiv2023

Enhanced heat transfer in a 2D serpentine micro-channel using elastic polymers

In the presence of elastic forces, even dilute polymer suspensions can exhibit erratic flow fluctuations even when the viscous forces dominate over the inertial forces, which occur at vanishing-low Reynolds numbers (Re). This phenomenon is called Elastic Turbulence (ET). ET can be generated in small-scale laboratory settings and is relevant to enhancing mixing efficiency and heat transfer in microfluidic devices. In this study, we investigate the hydraulic and thermal properties of a dilute polymer solution under ET conditions characterized by inflow conditions of vanishing Re and high Weissenberg numbers (Wi). We carry out extensive direct numerical simulations of the 2D curvilinear channel flow of an Oldroyd-B viscoelastic fluid using Rheotool. We analyze the variations of friction factor and Nusselt numbers along the serpentine channel to reveal the global and local characteristics of ET. Based on Wi, we identify three regimes. First, for 0 < Wi < 3, we observe roughly 10% heat transfer enhancement accompanied by roughly 5% reduction of friction factor compared to laminar flow, known as polymer-induced thermal conductivity enhancement. Second, for 3 < Wi < 5, we observe a sharp linear increase of heat transfer (roughly 30%) at the cost of up to 15% enhanced friction factor. Finally, in the fully developed elastic turbulence regime (Wi > 5), we observe up to 60% heat transfer enhancement accompanied by reduced friction factor. The substantial enhancement of heat transfer with increasing Wi is mainly attributed to the increasing intensity of the elastic instability resulting from the balance between normal stresses and streamlined curvatures.

physics.flu-dyn↗

arXiv2023

Large Eddy Simulations of Fully-Developed Turbulent Flows Over Additively Manufactured Rough Surfaces

In the last decade, progresses in additive manufacturing (AM) have paved the way for optimized heat exchangers, whose disruptive design will depend on predictive numerical simulations. Typical AM rough surfaces show limited resemblance to the artificially constructed rough surfaces that have been the basis of most prior fundamental research on turbulent flow over rough walls. Therefore, a high-fidelity LES database is built to develop and assess novel wall models for AM. This article investigates the flow in rough pipes built from the surfaces created using AM techniques at Siemens based on Nickel Alloy IN939 material. We developed a code to generate the desired rough pipes from scanned planar surfaces and performed high-fidelity LES of turbulent rough pipe flows at Re = 11,700 to reveal the influence of roughness on turbulence, mainly the average roughness height and the Effective Slope. The equivalent sand-grain roughnesses, ks, of the present AM rough surfaces are predicted using the Colebrook correlation. In the present study, the existence of a logarithmic layer is marked even for high values of ks. The mean flow, the velocity fluctuations, and the Reynolds stresses show turbulence's strong dependence on the roughness topography. Profiles of turbulence statistics are compared by introducing an effective wall-normal distance. The effective distance collapses the shear stresses and the velocity fluctuations outside the roughness sublayer; thus, Townsend's similarity of the streamwise mean velocity is marked for the present roughnesses. Furthermore, a mixed scaling is introduced to improve the collapse of turbulence statistics in the roughness sublayer.

physics.flu-dyn↗

arXiv2023

Effect of temperature-dependent thermophysical properties on turbulent forced convection under constant heat flux boundary condition

In this study, we performed highly resolved large-eddy simulations (LES) to investigate the influence of variable properties on the forced turbulent convection in a channel. The constant heat flux boundary condition permits wall temperature fluctuations and thus induces variations of fluid properties. Since the effect of viscosity on the flow exhibits $Re_τ^{-1}$ scaling, we only considered $Re_τ= 180$ in the present study. Compared to the flow with constant properties, results indicate that the variable properties have trivial effects on the mean velocity and temperature profiles, Reynolds shear stress, wall-normal heat flux, as well as the small-scale turbulence characteristics. However, we also observed that the turbulence intensities, low-speed streaks, burst motions, and budgets for temperature variance and wall-normal heat flux are modified by the variable properties in a perceptible way. In addition, we showed that the classic wall scaling is a good choice for flow with small and moderate variations of fluid properties.

physics.flu-dyn↗

arXiv2022

Numerical simulation of two-phase flow in gas diffusion layer and gas channel of proton exchange membrane fuel cells

Liquid water within the cathode Gas Diffusion Layer (GDL) and Gas Channel (GC) of Proton Exchange Membrane Fuel Cells (PEMFCs) is strongly coupled to gas transport properties, thereby affecting the electrochemical conversion rates. In this study, the GDL and GC regions are utilized as the simulation domain, which differs from previous studies that only focused on any one of them. A volume-of-fluid method is adopted to numerically investigate the two-phase flow (gas and liquid) behavior, e.g., water transport pattern evolution, water coverage ratio as well as local and total water saturation. To obtain GDL geometries, an in-house geometry-based method is developed for GDL reconstruction. Furthermore, to study the effect of GDL carbon fiber diameter, the same procedure is used to reconstruct three GDL structures by varying the carbon fiber diameter but keeping the porosity and geometric dimensions constant. The wall wettability is introduced with static contact angles at carbon fiber surfaces and channel walls. The results show that the GDL fiber microstructure has a significant impact on the two-phase flow patterns in the cathode field. Different stages of two-phase flow pattern evolution in both cathode domains are observed. Due to the difference in wettability, the water coverage of the GDL/GC interface is smaller than that of the channel side and top walls. It is also found that the water saturation inside the GDLs stabilizes after the water breakthrough, while local water saturation at the interface keeps irregular oscillations. Last but not the least, a water saturation balance requirement between the GDL and GC is observed. In terms of varying fiber diameter, a larger fiber diameter would result in less water saturation in the GDL but more water in the GC, in addition to faster water movement throughout the total domain.

physics.flu-dyn↗

arXiv2021

Statistical properties of two-dimensional elastic turbulence

We numerically investigate the spatial and temporal statistical properties of a dilute polymer solution in the elastic turbulence regime, i.e., in the chaotic flow state occurring at vanishing Reynolds and high Weissenberg numbers. We aim at elucidating the relations between measurements of flow properties performed in the spatial domain with the ones taken in the temporal domain, which is a key point for the interpretation of experimental results on elastic turbulence and to discuss the validity of Taylor's hypothesis. To this end, we carry out extensive direct numerical simulations of the two-dimensional Kolmogorov flow of an Oldroyd-B viscoelastic fluid. Static point-like numerical probes are placed at different locations in the flow, particularly at the extrema of mean flow amplitude. The results in the fully developed elastic turbulence regime reveal large velocity fluctuations, as compared to the mean flow, leading to a partial breakdown of Taylor's frozen-field hypothesis. While second-order statistics, probed by spectra and structure functions, display consistent scaling behaviors in the spatial and temporal domains, the third-order statistics highlight robust differences. In particular the temporal analysis fails to capture the skewness of streamwise longitudinal velocity increments. Finally, we assess both the degree of statistical inhomogeneity and isotropy of the flow turbulent fluctuations as a function of scale. While the system is only weakly non-homogenous in the cross-stream direction, it is found to be highly anisotropic at all scales.

physics.flu-dyn↗

arXiv2018

Particle-laden two-dimensional elastic turbulence

The aggregation properties of heavy inertial particles in the elastic turbulence regime of an Oldroyd-B fluid with periodic Kolmogorov mean flow are investigated by means of extensive numerical simulations in two dimensions. Both the small and large scale features of the resulting inhomogeneous particle distribution are examined, focusing on their connection with the properties of the advecting viscoelastic flow. We find that particles preferentially accumulate on thin highly elastic propagating waves and that this effect is largest for intermediate values of particle inertia. We provide a quantitative characterization of this phenomenon that allows to relate it to the accumulation of particles in filamentary highly strained flow regions producing clusters of correlation dimension close to 1. At larger scales, particles are found to undergo turbophoretic-like segregation. Indeed, our results indicate a close relationship between the profiles of particle density and fluid velocity fluctuations. The large-scale inhomogeneity of the particle distribution is interpreted in the framework of a model derived in the limit of small, but finite, particle inertia. The qualitative characteristics of different observables are, to a good extent, independent of the flow elasticity. When increased, the latter is found, however, to slightly reduce the globally averaged degree of turbophoretic unmixing.

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