Danesh K Tafti

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Verified

Danesh verified their affiliation via an institutional email.

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• PhD
• Professor at Virginia Tech

This study performs particle resolved simulations (PRSs) to investigate the behavior of linear shear flow over a near-wall oblate spheroidal particle. Simulations are conducted for particles with aspect ratios ( λ) of 1.5, 2, and 4, covering Reynolds numbers ( Re) ranging from 1 to 200 and particle inclination angles ( θ) from 0° to 180°. The resul...

Flow across an array of solid obstructions is a common phenomenon observed in many applications such as multiphase flows, heat exchangers, and environmental flows. In this work, we aim to train deep learning models and to predict the time evolution of unsteady flow fields in a domain of randomly arranged 2D cylinders at Reynolds number 50. Two diff...

The phase field method provides a simple mass conserving method for solving two-phase immiscible-incompressible Navier–Stokes Equations. The relative ease in implementing this method compared to other interface reconstruction methods, coupled with its conservativeness and boundedness makes it an attractive alternative. We implement the method in a...

The dynamic subgrid-scale stress model (Germano et al., 1991) is used for the Large Eddy Simulations of the turbulent channel flow at Reτ = 1050. Finite difference approximations based on the Harlow-Welch scheme are used to discretize the governing flow equations. The test filter is based on a second-order trapezoidal rule. The effects of model pre...

Three reduced order models are evaluated in their capacity to predict the future state of an unsteady chaotic flow field. A spatially fully developed flow generated in a random packing of cylinders at a solid fraction of 0.1 and a nominal Reynolds number of 50 is investigated. For deep learning (DL), convolutional autoencoders are used to reduce th...

The collision-adhesion and deposition of micron-sized dust particles in the internal cooling duct with pin fin arrays are investigated, integrating the energy dissipation collision model of spherical particles and gas-solid two-phase theory. The effects of particle Stokes number (Stk = 0.12, 0.48, and 3), particle temperature (Tp = 1173, 1223, 1273...

Cooling of turbine blades is necessary for longer durability and better thermal efficiency of the turbines. Turbine blade cooling is fulfilled by passing a coolant through the serpentine channel roughened with turbulators. In this work, the combined effect of the centrifugal buoyancy and the Coriolis force on heat transfer of a square duct roughene...

The role of aerodynamics and wing inertia on the motion dynamics for the maneuvering flight of two bats from two species of roundleaf bats, H. armiger and H. pratti are investigated. Comparative studies among a straight flight, two ascending sweeping right turns, and a U-turn reveal that inertial forces play an essential and sometimes crucial role...

According to the United Nations, 55% of the world’s population currently lives in urban areas and which is projected to increase to 67% by 2050. Trees and green spaces are effective strategies for mitigating urban pollution. This study develops a framework to investigate the impact of trichome morphology and wind speed on the deposition of 0.3 μm a...

This study focuses on predicting the pollution dispersion in a street canyon using Large-Eddy Simulations (LES). The predictions are compared to wind tunnel experiments done at Karlsruhe Institute of Technology (KIT), Germany. The geometry consists of a long canyon with width-to-height, W/H = 1, placed in a turbulent boundary layer flow perpendicul...

Bats exhibit a high degree of agility and provide an excellent model system for bioinspired flight. The current study investigates an ascending right turn of a Hipposideros pratti bat and elucidates on the kinematic features and aerodynamic mechanisms used to effectuate the manoeuvre. The wing kinematics captured by a three-dimensional motion captu...

The paper derives correlations for lift force and fluid torques acting on stationary prolate ellipsoid suspensions of aspect ratio (AR) 2.5, 5, and 10 subjected to uniform flow. Particle Resolved Simulations (PRS) are conducted on a suspension of infinite extent in two directions for Reynolds number 10 ≤ Re ≤ 200 and solid fractions (φ) between 0.1...

Blood-borne bacteria disseminate in tissue through microvasculature or capillaries. Capillary size, presence of red blood cells (RBCs), and bacteria motility affect bacteria intracapillary transport, an important yet largely unexplored phenomenon. Computational description of the system comprising interactions between plasma, RBCs, and motile bacte...

Heat transfer characteristics of random suspensions of 0.25 aspect ratio (AR) cylinders are investigated for Reynolds numbers (Re) between 10 to 300 and solid fraction (f) ranging from 0.1 to 0.3 using Particle Resolved Simulations (PRS). The effect of particle inclination with respect to flow and particle clustering on heat transfer is investigate...

Two deep learning methods, Multi-Layer Perceptron (MLP) network and Convolution Neural Network (CNN) are evaluated to predict drag forces in dense suspensions of ellipsoidal particles using data from Particle Resolved Simulations (PRS). The MLP is trained on the mean flow Reynolds number, solid fraction of the suspension, the aspect ratio of the pa...

Particle Resolved Simulations (PRS) using the Immersed Boundary Method (IBM) are performed for flow through suspensions of ellipsoids with aspect ratios of 2.5, 5, and 10 for solid volume fractions from 0.1 to 0.3 in the Reynolds number range from 10 to 200. The mean Nusselt number increases as Re1/2but shows only a weak dependence on the aspect ra...

The cooling of the turbine blade plays a vital role in increasing the durability and thermal efficiency of the turbine. Blade cooling is achieved by passing coolant through internal channels roughened with rib turbulators. In this work, the effect of rotation on heat transfer of a square ribbed duct is studied using Large Eddy Simulation (LES) at a...

The flapping flight of many bat species is characterized by a high degree of maneuverability and provides fertile ground for biomimetic design. However, there has been little prior work toward understanding bat flight maneuvers, particularly using a coupled kinematic and aerodynamic framework. Here, wing kinematic data of a large insectivorous bat...

The study focuses on evaluating fully-coupled conjugate heat transfer simulation in a ribbed cooling passage with a fully developed flow assumption using LES with the immersed boundary method (IBM-LES-CHT). The IBM-LES and the IBM-CHT frameworks are validated by simulating purely convective heat transfer in the ribbed duct, and a laminar boundary l...

Bats possess wings comprised of a flexible membrane and a jointed skeletal structure allowing them to execute complex flight maneuvers such as rapid tight turns. The extent of a bat’s tight turning capability can be explored by analyzing a 180-degree U-turn. Prior studies have investigated more subtle flight maneuvers, but the kinematic and aerodyn...

Physics-based simulations are often used to model and understand complex physical systems in domains such as fluid dynamics. Such simulations, although used frequently, often suffer from inaccurate or incomplete representations either due to their high computational costs or due to lack of complete physical knowledge of the system. In such situatio...

Three-dimensional numerical simulations are conducted to calculate the surface heat transfer coefficient (HTC) of the water jet impinging on a TA2/Q235B clad plate. The accuracy of the simulation model is validated by experiment. The simulation results are basically in agreement with the experimental results. The effect of clad plate temperature an...

Particulate systems in practical applications have mostly been represented using spherical shapes, even though the majority of particles in archetypal fluid-solid systems are non-spherical. Modeling dense fluid-particulate systems using non-spherical particles involves increased complexity, with computational cost manifesting as the biggest bottlen...

Local scour around structures is a natural process that impacts the design of submerged hydraulic structures. The phenomenon of erosion and movement of sediment particles in the vicinity of a submerged object is a result of the interaction between the turbulent flow and a bed of sediment particles. In this work Computational Fluid Dynamics (CFD) co...

Oscillating plunging motion is one of the fundamental motions which comprise the kinematics used by many flying and swimming organism for locomotion. It is characterized by the frequency and amplitude of oscillation. Past studies have investigated this motion for relatively small amplitudes. In this paper we investigate reduced frequencies 0.25 ≤ k...

Fluid forces and torques in a random stationary suspension of oblate cylinder-like particles of AR0.25 are investigated in the Reynolds number range 10 ≤ Re ≤ 300 and solid fraction range 0.1 ≤ φ ≤ 0.3 using Particle Resolved Simulations with the Immersed Boundary Method (IBM). While existing correlations were found to grossly under predict the dra...

To model drag the current state-of-the-art is to use isolated non-spherical particle drag correlations modified by a solid fraction correlation that is based on experimental or simulation results of spherical particle suspensions. It is shown that this practice can lead to substantial inaccuracies when the particle geometry deviates significantly f...

Physics-based simulations are often used to model and understand complex physical systems and processes in domains like fluid dynamics. Such simulations, although used frequently, have many limitations which could arise either due to the inability to accurately model a physical process owing to incomplete knowledge about certain facets of the proce...

Heat transfer augmentation is of paramount importance in energy transfer and storage systems and the idea of using the inherent vibrations in a system to enhance heat transfer needs to be thoroughly researched upon. The current study numerically investigates an infinitesimally thin plate-fin undergoing forced oscillations over a range of amplitudes...

Bats possess unique flight capabilities enabled by their wing morphology. While the articulated bone structure and flexible membrane constituting the wing are known to play a critical role in aerodynamic performance, the relationship has never been robustly quantified. Characterization of the sensitivity between precise wing contour and aerodynamic...

Bats, with highly articulated wings, are some of the most agile flyers in nature. A novel three-dimensional geometric decomposition framework is developed to reduce the complex kinematics of a bat wing into physical movements used to describe flapping flight: namely flapping, stroke plane deviation and pitching, together with cambering and flexion....

The present study focuses on evaluating fully-coupled conjugate heat transfer simulation in a ribbed cooling passage with a fully developed flow assumption using LES with the immersed boundary method (IBM-LES-CHT). The IBM-LES and the IBM-CHT frameworks are validated prior to the main simulations by simulating purely convective heat transfer in the...

This paper presents a theoretical study of the heat transfer during particles colliding with a surface considering the material elastoplastic properties and adhesion forces of particles. The model divides the impact processes into three stages, the elastic stage, the elastic-plastic stage, and full plastic stage, and assumes that the recovery stage...

Particle deposits that form in gas turbine engines can change flow dynamics and heat transfer, leading to performance degradation. A computational framework that models the coupled behavior of sand deposits with flow and heat transfer is developed. The coupling is done by using a multiphase framework in which a physics based collision model is used...

CFD-DEM simulations have been used extensively to study dense fluid-particle systems. In the point mass representation of particles in DEM, the modeled drag force plays an important role in the dynamics. Current state-of-the-art methodologies use the mean drag correlations based on the superficial Reynolds number and void fraction. In this work, as...

The linear-slider-dashpot (LSD) soft sphere model is used extensively to model particle collisions in dense particle systems. The contact mechanics in the normal and tangential directions are modeled by a spring, damper, and dashpot system. The current work evaluates different existing tangential direction implementations of the LSD model. The exis...

Sand ingestion and deposition in gas turbine engine components can lead to several operational hazards. This paper discusses a physics-based model for modeling the impact, deposition, and sticking of sand particles to surfaces. The collision model includes both normal and tangential components of impact. The normal collision model divides the impac...

In this paper we present the application of a physics-based three dimensional hard-sphere impact model to many-particle systems. The hard sphere model is used with conventional time-advancement which is not event driven. A particle relocation technique based on using both pre- and post-collision velocities is developed and validated for dense syste...

The aerodynamic mechanisms of bat flight have been studied using a numerical approach. Kinematic data acquired using a high resolution motion capture system was employed to simulate the unsteady air flow around a bat’s wings. A flapping bat wing contains many degrees of freedom, which make 3D motion tracking challenging. In order to overcome this c...

Time series of force and power data from the bat flight simulation. (CSV)

Cooling of gas turbine blades is critical to long term durability. Accurate prediction of blade metal temperature is a key component in the design of the cooling system. In this design space, spatial distribution of heat transfer coefficients plays a significant role. Large-Eddy Simulation (LES) has been shown to be a robust method for predicting h...

Non-spherical objects exist in many multiphase applications but there is very limited data available on the fluid forces acting on these different shapes. This work derives correlations for drag, lift and torque coefficients for a single low aspect ratio 1:4 cylinder (tablet shaped object) placed at different incidence angles to the approach flow u...

Fluid equations in Computational Fluid Dynamics coupled with Discrete Element Method (CFD-DEM) simulations solve the volume-averaged Navier–Stokes equations. Full coupling between the dispersed phase and continuous phase is made by the exchange of source terms as well as the void fraction. The void fraction is calculated from the presence of the pa...

This paper introduces a dynamic model of platelet-rich thrombus growth in stenosed vessels using computational fluid dynamics methods. Platelet adhesion, aggregation and activation kinetics are modeled by solving mass transport equations for blood components involved in thrombosis. Arbitrary Lagrangian–Eulerian formulation is used to model the grow...

Previous research has mostly been focused on the drag force in fluid-particle assemblies. However, when the particle geometry is non-spherical, secondary forces and torque may no longer be negligible. In this study particle-resolved simulations are performed to study the drag force, other secondary forces, and torque in flow through a fixed random...

In this article, observed mechanisms of platelet activation, adhesion and aggregation at the molecular level are reviewed. The role of different blood constituents, platelet integrins, tissue factors, and extracellular matrix ligands in platelet adhesion and activation are also presented. In extreme flows, as found in heart assist devices or severe...

The application of Computational Fluid Mechanics (CFD) coupled to Discrete Element Method (DEM) to simulate industrial and natural scale system requires integrating large number of particles for long times which can make these simulations impractical. One method to reduce the computational cost of DEM simulations is by lumping particles in represen...

Fixed and dynamic assemblies of non-spherical particles participate in many heat and mass transfer industrial processes. However, most convective heat transfer coefficients are adapted from correlations available for spherical particles by using the concept of equal volume spheres or in some cases the sphericity, which is a measure of deviation fro...

In this work, a fully-coupled Computational Fluid Dynamics (CFD) model and Discrete Element Method (DEM) are used to simulate a unidirectional turbulent open-channel flow over the full range of sediment transport regimes. The fluid and particles are computed on separate grids using a dual-grid formulation to maintain consistency and avoid instabili...

The current work presents a sensitivity study of selected numerical parameters on the LES-DEM predictions of sediment transport in a unidirectional open turbulent channel. The sensitivity of the particle friction factor, restitution coefficient and spring stiffness used in the soft sphere collision model are tested in the three regimes of sediment...

The paper reviews the state‐of‐the‐art in computational modeling of thrombus formation and growth and related phenomena including platelet margination, activation, adhesion, and embolization. Presently, there is a high degree of empiricism in the modeling of thrombus formation. Based on the experimentally observed physics, the review gives useful s...

The paper investigates the transport of sand particles in the two-pass internal cooling duct of a turbine blade under rotation. The size range is representative of Fine Arizona Road Test Dust from Powder Technology Inc., corresponding to 50% of injected particles being less than 4 microns in diameter, with 90% being less than 50 microns, The geomet...

Sand ingestion and deposition in gas turbine engine components can lead to several operational hazards. This paper discusses a physics based model for modeling the impact and deposition of sand particles. The collision model divides the impact process into three stages, the elastic stage, the elastic-plastic stage, and full plastic stage. The recov...

High-performance, all-aromatic, insoluble, engineering thermoplastic polyimides, such as pyromellitic dianhydride and 4,4'-oxydianiline (PMDA-ODA) (Kapton), exhibit exceptional thermal stability (up to ≈600 °C) and mechanical properties (Young's modulus exceeding 2 GPa). However, their thermal resistance, which is a consequence of the all-aromatic...

http://onlinelibrary.wiley.com/journal/10.1002/(ISSN)1097-0290/homepage/media.htm https://www.youtube.com/watch?v=yoSKllC62Ow

Particle impact behavior description is significant for dense multiphase flow simulation and particle deposition prediction. The paper presents a novel impulse based particle collision model considering sliding and rolling. The collision model includes three components, normal, tangential and adhesion model. Normal collision model is established to...

Shear stresses play a major role in platelet-substrate interactions and thrombus formation and growth in blood flow, where under both pathological and physiological conditions platelet adhesion and accumulation occur. In this study, a shear-dependent continuum model for platelet activation, adhesion and aggregation is presented. The model was first...

Particle-resolved simulations are performed to study the momentum transfer in flow though fixed random assembly of non-spherical particles. Ellipsoidal particles with sphericity (ψ = 0.887) are investigated in a periodic cubic domain to simulate an infinite assembly. The incompressible Navier-Stokes equations are solved using the Immersed Boundary...

In this study, a shear-dependent continuum model for platelet activation, adhesion and aggregation is validated using computational fluid dynamics (CFD). To take the presence of red cells into account, a combination of excess-platelet boundary layer and enhanced mass diffusivity of platelets and large species is used to mimic this behavior. The mod...

The focus of this research is to predict the flow and heat transfer in a rotating two-pass duct geometry with staggered ribs using Large-Eddy Simulations (LES). The geometry consists of a U-Bend with 17 ribs in each pass. The ribs are staggered with an e/Dh = 0.1 and P/e = 10. LES is performed at a Reynolds number of 100,000, a rotation number of 0...

In this work, we present an approach for solving fluid structure interaction problems by combining a non-linear structure solver with an incompressible fluid solver using immersed boundary method. The implementation of the sharp-interface immersed boundary method with the fluid solver is described. A structure solver with the ability to handle geom...

In recent years there has been an increased effort to utilize hydrokinetic devices to harvest energy from both wind and tidal flows. Vertical axis turbines utilization is of particular interest in tidal flows. A major disadvantage in vertical axis turbines is that they are less efficient than horizontal axis turbines. To increase the efficiency of...

The present work lays out an accurate, three-dimensional computational fluid dynamics (CFD) model of a human blood capillary. This model is composed of red blood cells and blood plasma inside a cylindrical section of a capillary. The plasma flow is resolved using an incompressible Navier-Stokes solver. At the level of capillaries, red blood cells m...

Discrete Element Method (DEM) coupled to Computational Fluid Mechanics (CFD) is a powerful tool for simulating complex multiphase flows. The cost of this Eulerian-Lagrangian description, however, increases with the increase of the number of particles ∼O(Np) which limit its use in natural and industrial scale systems. Efforts to reduce the cost of C...

The history force model accounts for temporal development in fluid gradients in the viscous region surrounding a particle in point particle methods. The calculation of the history force typically requires storing and using relative velocity information during the life time of the particle. For a large number of particles integrated over large times...

Sand transport and deposition is investigated in a two-pass internal cooling ribbed geometry at near engine conditions. Large-eddy simulation (LES) calculations are performed for bulk Reynolds number of 25,000 to calculate flow field and heat transfer. Constant wall temperature boundary condition is used to investigate the effect of temperature on...

The paper is concerned with the prediction and analysis of dynamic stall of flow past a pitching NACA0012 airfoil at 1 million Reynolds number based on the chord length of the airfoil and at reduced frequency of 0.25 in a three dimensional flow field. The turbulence in the flow field is resolved using large eddy simulations with the dynamic Smagori...

The most popular iterative linear solvers in Computational Fluid Dynamics (CFD) calculations are restarted GMRES and BiCGStab. At the beginning of most incompressible flow calculations, the computation time and the number of iterations to converge for the pressure Poisson equation are quite high. In this case, the BiCGStab algorithm, with relativel...

GPU computation in recent years has seen extensive growth due to advancement in both hardware and software stack. This has led to increase in the use of GPUs as accelerators across a broad spectrum of applications. This work deals with the use of general purpose GPUs for performing CFD computations. The paper discusses strategies and findings on po...

In this paper, immersed boundary method (IBM) is evaluated in a ribbed duct geometry to show the potential of simulating a complex geometry with a simple structured grid. In this framework, instead of resolving the geometry with a body conforming grid, the geometry is immersed into a volume background grid, and the immersed boundary cuts through th...

Applications of reaction–diffusion systems in porous media pose a challenging problem for computational modeling approaches due to their multi-physics and multi-scale nature. The length scales usually span 3–4 orders of magnitude while physical phenomena involved include heat and mass transfer processes, and surface reactions. In this paper, a nove...

This study proposes an improved physical model to predict sand deposition at high temperature in gas turbine components. This model differs from its predecessor (Sreedharan and Tafti, 2011) by improving the sticking probability by accounting for the energy losses during particle-wall collision based on our previous work (Singh and Tafti, 2013). Thi...

Blade cooling technology will play a critical role in the next generation of propulsion and power generation gas turbines. Accurate prediction of blade metal temperature can avoid the use of excessive compressed bypass air and allow higher turbine inlet temperature, increasing fuel efficiency and decreasing emissions. Large eddy simulation (LES) ha...

The Discrete Element Method (DEM) coupled to Computational Fluid Dynamics (CFD) is used to predict dense fluid-particle system in a blind study using the in-house code GenIDLEST (Generalized Incompressible Direct and Large Eddy Simulation of Turbulence). The experimental measurements were performed at the Department of Energy's (DOE's) National Ene...

The paper is concerned with the prediction and analysis of dynamic stall of flow past pitching NACA-0012 airfoil at 105 Reynolds number based on the chord length of the airfoil and at reduced frequency of 0.188 in a three dimensional flow field. The turbulence in the flow field is resolved using large eddy simulations with dynamic Smagorinsky model...

Large Eddy Simulations (LES) of wall-bounded flows at high Reynolds numbers demand an extremely fine mesh resolution in the wall proximal inner layer. Accurate modeling of near wall turbulence is therefore crucial in reducing the computational cost of LES at practical Reynolds numbers. One approach is the implementation of a two-layer model that so...

The incidence of stent late restenosis is high (Zwart et al., 2010, "Coronary Stent Thrombosis in the Current Era: Challenges and Opportunities for Treatment," Curr. Treat. Options Cardiovasc. Med., 12(1), pp. 46-57) despite the extensive use of stents, and is most prevalent at the proximal and distal ends of the stent. Elastic modulus change in st...

Swirling flows for combustion stabilization, flame confinement, and proper fuel mixing and recirculation are prevalent in gas turbine combustor applications. Modern gas turbines use swirlers to induce strong rotating vortices and recirculation of the combustion gases to enhance combustion efficiency and stability. This study presents an experimenta...

Fluidized beds with multiple jets have widespread industrial applications. The objective of this paper is to investigate the jet interactions and hydrodynamics of a fluidized bed with multiple jets. Discrete element modeling coupled with in-house CFD code GenIDLEST has been used to simulate a bed with nine jets. The results are compared with publis...

In the paper, we discuss the use of general purpose GPUs for simulating bat wing flapping. The bat wing with prescribed motion is simulated using a CFD-IBM approach. These computations are optimized for GPU platform using CUDA Fortran to obtain about six times speed up over CPUs. Strong scaling study and code profiling is performed to understand co...

Use of immersed boundary method (IBM) based techniques have helped considerably in easing the grid generation process in flows involving complex geometries and/or large boundary movements. Body fitting grid based techniques still, however, are advantageous in terms of accuracy and efficiency. In this work, we have developed an IBM scheme applicable...

This paper supplements previous work done by Gopalakrishnan and Tafti [21] for the parallel implementation of the Discrete Element Method (DEM) in the open source code Multiphase Flow with Interphase exchange (MFIX) for distributed memory architectures using MPI. The current study extends the parallel implementation to shared memory architectures u...

This paper examines the augmentation of heat and mass transfer due to dual clearances on cylindrical pin fins, relative to a channel between parallel plates, in mini/micro-channel reactors at low Reynolds numbers. In this work, diffusion limitations to heat and mass transfer in smooth-walled mini/micro-channel reactors were minimized by the impleme...

Jet engines often operate under dirty conditions where large amounts of particulate matter can be ingested, especially, sand, ash and dirt. Particulate matter in different engine components can lead to degradation in performance. The focus of this study is to investigate the sand transport and deposition in the internal cooling passages of turbine...

Understanding flow through real porous media is of considerable importance given their significance in a wide range of applications. Direct numerical simulations of such flows are very useful in their fundamental understanding. Past works have focused mainly on ordered and disordered arrays of regular shaped structures such as cylinders or spheres...