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Nilanjan Chakraborty - One of the best experts on this subject based on the ideXlab platform.

  • modelling of the tangential strain rate term in the flame surface density transport equation in the context of reynolds averaged navier stokes simulations a direct numerical simulation analysis
    Mathematical Problems in Engineering, 2014
    Co-Authors: Mohit Katragadda, Sean P Malkeson, Nilanjan Chakraborty
    Abstract:

    A direct numerical simulation (DNS) database of freely propagating statistically planar turbulent premixed flames with a range of different values of Karlovitz number Ka, turbulent Reynolds number , heat release parameter , and global Lewis number Le has been used to assess the models of the tangential strain rate term in the generalised flame surface density (FSD) transport equation in the context of Reynolds averaged Navier Stokes (RANS) simulations. The tangential strain rate term has been split into contributions arising due to dilatation rate and flame normal strain rate (). Subsequently, and () were split into their resolved (i.e., and ()) and unresolved ( and ()) components. Detailed physical explanations have been provided for the observed behaviours of the components of the tangential strain rate term. This analysis gave way to the modelling of the unresolved dilatation rate and flame normal strain rate contributions. Models have been identified for and () for RANS simulations, which are shown to perform satisfactorily in all cases considered, accounting for the variations in Ka, , and Le. The performance of the newly proposed models for the FSD strain rate term have been found to be either comparable to or better than the existing models.

  • conditional velocity statistics for high and low damkohler number turbulent premixed combustion in the context of reynolds averaged navier stokes simulations
    Proceedings of the Combustion Institute, 2013
    Co-Authors: Nilanjan Chakraborty, Andrei Lipatnikov
    Abstract:

    The statistics of fluid velocity components conditional in unburned reactants and fully burned products in the context of Reynolds Averaged Navier Stokes (RANS) simulations have been analysed using a Direct Numerical Simulation (DNS) database of statistically planar turbulent premixed flames for both high and low values of Damkohler number for different values of heat release parameter. It has been found that the contributions arising from chemical reaction to the conditional mean velocities and the conditional Reynolds stresses remain strong under high values of Damkohler number. The expressions for conditional mean velocity components and conditional Reynolds stresses, which are derived based on bi-modal probability density function of reaction progress variable for unity Lewis number flames, are modified in this study in such a manner that the new expressions can be used for low Damkohler number flames where bi-modal distribution is not realised. Suitable models for conditional surface-averaged velocity components and the Reynolds stresses have been identified, which are shown to work satisfactorily for all values of Damkohler number and heat release parameter considered in this analysed.

  • modelling of the tangential strain rate term of the flame surface density transport equation in the context of reynolds averaged navier stokes simulation
    Proceedings of the Combustion Institute, 2011
    Co-Authors: Mohit Katragadda, Sean P Malkeson, Nilanjan Chakraborty
    Abstract:

    Abstract The modelling of the tangential strain rate term in the Flame Surface Density (FSD) transport equation in the context of Reynolds Averaged Navier–Stokes (RANS) simulations of turbulent premixed combustion has been addressed by a priori analysis of a Direct Numerical Simulation (DNS) database of statistically planar freely propagating flames with wide variations of Damkohler number Da , heat release parameter τ and global Lewis number Le . It has been found that the dilatation rate contribution to the FSD transport strengthens with increasing value of τ and decreasing value of Le . The behaviour of the normal strain rate term shows significant differences in response to Da and Le . It has been found that the normal strain rate contribution to the FSD transport remains a sink term for the flames with high and small values of Da and Le , respectively, where the effects of strain rate induced by heat release a chem overcome the effects of turbulent straining a turb . By contrast, the effects of a turb overcomes the effects of a chem for low Da flames with Le ⩾ 1 , which leads to a positive value of the normal strain rate term towards the unburned gas side, but this term becomes negative towards the burned gas side due to strong a chem overcoming a turb in the regions of intense heat release. The strengthening of the dilatation rate and a chem at small and large values of Le and Da , respectively, is explicitly taken into account to propose new models for the strain rate contributions to the FSD transport. The new model is shown to satisfactorily capture the effects of Damkohler number Da , heat release parameter τ , and global Lewis number Le , on the tangential strain rate term of the FSD transport equation for all the cases considered in this study.

  • modelling of the curvature term of the flame surface density transport equation for reynolds averaged navier stokes simulations
    7th Mediterranean Combustion Symposium, 2011
    Co-Authors: Mohit Katragadda, Sean P Malkeson, Nilanjan Chakraborty
    Abstract:

    Three-dimensional statistically planar simplified chemistry based Direct Numerical Simulations (DNS) of turbulent premixed flames with wide variations of Karlovitz number Ka , heat release parameter τ and global Lewis number Le have been used for the a-priori modeling of the curvature term of the generalised Flame Surface Density (FSD) transport equation in the context of Reynolds Averaged Navier Stokes (RANS) simulations. The simulation parameters used in the current study have been chosen in such a manner that both the corrugated flamelets and thin reaction zones regime of premixed turbulent combustion have been considered. The curvature term has been split into the contributions arising due to the reaction and normal diffusion components (i.e. 1 T ) and the term arising due to the

Catherine Gorle - One of the best experts on this subject based on the ideXlab platform.

  • quantifying turbulence model uncertainty in reynolds averaged navier stokes simulations of a pin fin array part 2 scalar transport
    Computers & Fluids, 2020
    Co-Authors: Zengrong Hao, Catherine Gorle
    Abstract:

    Abstract The accuracy of Reynolds-Averaged Navier-Stokes results for turbulent scalar transport are affected by epistemic uncertainty in the Reynolds stress model in two ways: by altering the mean velocity field that advects the scalar, and by altering the inputs required for scalar flux models. We investigate these effects by propagating uncertainty in the Reynolds stress model to the prediction of scalar quantities in simulations of a pin-fin heat exchanger. The Reynolds stress model uncertainty is quantified by perturbing the anisotropy of the predicted stress tensor towards the three limiting realizable states. This uncertainty is then propagated to the scalar turbulence transport via the conservation law for the mean scalar and the turbulent scalar flux model. We consider three different scalar flux models that explicitly take the Reynolds stresses as input, and evaluate the performance of the models by verifying if high-fidelity large eddy simulation results are encompassed by the model predictions. The results indicate that the predicted uncertainty depends on both the general level of momentum transport and the most-relevant stress component, which is affected by the anisotropy perturbations. The predictions provide a similar bounding behavior for the overall temperature field as for the momentum field, but fail in bounding the local heat transfer rates in several locations. The bounding behaviors are further analyzed in terms of the predicted uncertainty in the flux magnitude, direction, and divergence to identify opportunities for further improvement of the proposed methods.

  • quantifying turbulence model uncertainty in reynolds averaged navier stokes simulations of a pin fin array part 1 flow field
    Computers & Fluids, 2020
    Co-Authors: Zengrong Hao, Catherine Gorle
    Abstract:

    Abstract The assumptions that form the basis for Reynolds stress closure models have been formulated by considering canonical flows. As a result, the accuracy of Reynolds-Averaged Navier–Stokes simulations can deteriorate significantly when modeling complex flows, and engineering applications would benefit from methods that can quantify the corresponding uncertainty in the predictions. This paper analyzes the performance of a previously proposed turbulence model uncertainty quantification (UQ) framework for simulations of flow through a pin-fin array. The method is a physics-based, data-free, interval approach that perturbs the Reynolds stress tensor shape towards the three limiting realizable states of anisotropy. The performance of the method is evaluated by determining whether large-eddy simulation results for the quantities of interest are encompassed by the intervals predicted by the UQ method. The results demonstrate that perturbing the stress shapes towards the one-component or two-component limit generally enhances the momentum transport between the bulk flow and wake regions, whereas perturbations towards the three-component limit suppress this transport. For quantities of interest that depend on the mean velocity and pressure field this results in predictions for uncertainty intervals that encompass the reference solution. For the turbulence kinetic energy the method fails to predict an adequate upper bound in regions with larger-scale turbulent structures, and it predicts overly conservative bounds in the pin stagnation regions. Based on these results several suggestions for improving the framework are made.

  • epistemic uncertainty quantification for reynolds averaged navier stokes modeling of separated flows over streamlined surfaces
    Physics of Fluids, 2019
    Co-Authors: Catherine Gorle, Stephanie Zeoli, Michael Emory, Johan Larsson, Gianluca Iaccarino
    Abstract:

    It is well known that linear eddy-viscosity turbulence models can introduce uncertainty in predictions for complex flow features such as separation and reattachment. The goal of this paper is to advance our understanding of a physics-based approach to quantify this turbulence model form uncertainty in Reynolds-Averaged Navier-Stokes simulations of separated flows over streamlined surfaces. The methodology is based on perturbing the modeled Reynolds stresses in the momentum equations; perturbations are defined in terms of a decomposition of the Reynolds stress tensor, i.e., based on the tensor magnitude and the eigenvalues and eigenvectors of the normalized anisotropy tensor. We demonstrate that the accuracy of the predicted Reynolds stress magnitude is strongly influenced by the turbulence production term and subsequently explore the anisotropy tensor eigenvalue and eigenvector perturbations that maximize or minimize turbulence production; these could be expected to provide bounds on the prediction of separation and reattachment locations. The method uses two user-defined parameters to identify the spatial extent of the perturbed region and the magnitude of the eigenvalue perturbations. Results for the flow over a periodic wavy wall and over a three-dimensional hill indicate that the perturbations that increase turbulence production decrease the extent of the separation region, while perturbations that decrease production increase the region of separated flow. The predicted bounds can successfully encompass the reference data, provided the extent of the perturbed region and the eigenvalue perturbation magnitudes are sufficiently large. Importantly, we observed a monotonic behavior of the magnitude of the predicted bounds as a function of the two user-defined parameters.

  • a framework for epistemic uncertainty quantification of turbulent scalar flux models for reynolds averaged navier stokes simulations
    Physics of Fluids, 2013
    Co-Authors: Catherine Gorle, Gianluca Iaccarino
    Abstract:

    Reynolds-Averaged Navier-Stokes (RANS) simulations are a practical approach for solving complex multi-physics turbulent flows, but the underlying assumptions of the turbulence models introduce errors and uncertainties in the simulation outcome. The flow in scramjet combustors is an example of such a complex flow and the accurate characterization of safety and operability limits of these engines using RANS simulations requires an assessment of the model uncertainty. The objective of this paper is to present a framework for the epistemic uncertainty quantification of turbulence and mixing models in RANS simulations. The capabilities of the methodology will be demonstrated by performing simulations of the mixing of an underexpanded jet in a supersonic cross flow, which involves many flow features observed in scramjet engines. The fundamental sources of uncertainty in the RANS simulations are the models used for the Reynolds stresses in the momentum equations and the turbulent scalar fluxes in the scalar transport equations. The methodology consists in directly perturbing the modeled quantities in the equations, thereby establishing a method that is completely independent of the initial model form to overcome the limitations of traditional sensitivity studies. The perturbations are defined in terms of the decomposed Reynolds stress tensor, i.e., the tensor magnitude and the eigenvalues and eigenvectors of the normalized anisotropy tensor. The turbulent scalar fluxes are perturbed by using the perturbed Reynolds stresses in a generalized gradient diffusion model formulation and by changing the model constant. The perturbations were parameterized based on a comparison between the Reynolds stresses obtained from a baseline RANS simulation and those obtained from a large-eddy simulation database. Subsequently an optimization problem was solved, varying the parameters in the perturbation functions to maximize a quantity of interest that quantifies the downstream mixing. The result encompasses the value for the quantity of interest obtained from the LES database. It is shown that a traditional sensitivity study, in which the turbulent Schmidt number is varied, cannot capture this uncertainty, which further demonstrates the effectiveness of the proposed approach.

Jack R. Edwards - One of the best experts on this subject based on the ideXlab platform.

  • hybrid large eddy reynolds averaged navier stokes simulations of flow through a model scramjet
    AIAA Journal, 2014
    Co-Authors: Amarnatha Sarma Potturi, Jack R. Edwards
    Abstract:

    The reactive flow through a model scramjet combustor is simulated using a hybrid large-eddy/Reynolds-Averaged Navier–Stokes technique. The scramjet configuration considered is similar to the one investigated at the Institute for Chemical Propulsion of the DLR, German Aerospace Center. The model scramjet consists of 15 fuel-injecting holes, located on the base of a wedge-shaped fuel injector, through which hydrogen is injected at sonic conditions. In the present study, only five of the 15 fuel-injecting holes are considered, and periodicity is assumed in the spanwise direction. Several parametric studies are conducted with a view toward determining the sensitivities of the predictions to modeling and algorithmic variations. Different grids (two different topologies), flux reconstruction methods (total variation diminishing and piecewise parabolic method), reaction mechanisms, and inflow boundary conditions (uniform and nonuniform) are used. To enhance fuel–air mixing, a synthetic eddy method is used to gen...

  • compressible flow simulations using a new large eddy simulation reynolds averaged navier stokes model
    AIAA Journal, 2011
    Co-Authors: Daniel A Gieseking, Jack R. Edwards, Jungil Choi, H. A. Hassan
    Abstract:

    A new hybrid large-eddy simulation/Reynolds-Averaged Navier–Stokes simulation (LES/RANS) method is presented in this work. In this approach, the resolved turbulence kinetic energy, ensemble-averaged modeled turbulence kinetic energy and turbulence frequency, and time-resolved turbulence frequency are used to form an estimate of an outer-layer turbulence length scale that is nearly Reynolds-number-independent. The ratio of this outer-layer scale with an inner-layer length scale (proportional to the wall distance) is used to construct a blending function that facilitates the shift between an unsteady RANS formulation near solid surfaces and a LES formulation away from the wall. The new model is tested through simulations of compressible flat-plate boundary layers over a widerangeofReynoldsnumbersandMach2.86 flowoverasmoothcompressionramp.Theresultsshowthatthenew modelpredicts mean andsecond-moment statistics that arein goodagreement withexperiment andare comparable with those obtained using an earlier model (Edwards, J. R., Choi, J-I., and Boles, J. A., “Hybrid Large-Eddy/ Reynolds-Averaged Navier–Stokes Simulation of a Mach-5 Compression Corner Interaction,” AIAA Journal, Vol. 464, 2008, pp. 977–991.) which required a case-by-case calibration of a model constant.

  • large eddy reynolds averaged navier stokes simulations of sonic injection into mach 2 crossflow
    AIAA Journal, 2010
    Co-Authors: Joh A Oles, Jack R. Edwards, Robe A Aurle
    Abstract:

    Computational predictions of transverse injection of air, helium, and ethylene into a Mach 1.98 crossflow of air are presented. A hybrid large-eddy simulation/Reynolds-Averaged Navier―Stokes turbulence model is used. A blending function, dependent on modeled turbulence variables, is used to shift the turbulence closure from the Menter t-ω model near solid surfaces to a Smagorinsky subgrid model in the outer part of the incoming boundary layer and in the jet mixing zone. The results show reasonably good agreement with time-averaged Mie-scattering images of the plume structure for both helium and air injection and with experimental surface pressure distributions, even though the penetration of the jet into the crossflow is slightly overpredicted. Predictions of ethylene mole fraction at several transverse stations within the plume are in good agreement with time-averaged Raman-scattering mole-fraction data. The model results are used to examine the validity of the commonly used assumption of the constant turbulent Schmidt number in the intense mixing zone downstream of the injection location. The assumption of a constant turbulent Schmidt is shown to be inadequate for jet mixing dominated by large-scale entrainment.

  • compressible boundary layer predictions at high reynolds number using hybrid les rans methods
    AIAA Journal, 2009
    Co-Authors: Jungil Choi, Jack R. Edwards, Robert A. Baurle
    Abstract:

    Simulations of compressible boundary-layer flow at three different Reynolds numbers (Re δ = 5.59 × 10 4 , 1.78 × 10 5 , and 1.58 x 10 6 ) are performed using a hybrid large-eddy simulation/Reynolds-Averaged Navier-Stokes method. Variations in the recycling/rescaling method, the higher order extension, the choice of primitive variables, the Reynolds-Averaged Navier-Stokes to large eddy simulation transition parameters, and the mesh resolution are considered in order to assess the model. The results indicate that the present model can provide good predictions of the mean-flow properties, second-moment statistics, and structural features of the boundary layers considered. Normalized turbulent statistics in the outer layer are found to be independent of Reynolds number, similar to incompressible turbulent boundary layers.

  • large eddy reynolds averaged navier stokes simulation of a mach 5 compression corner interaction
    AIAA Journal, 2008
    Co-Authors: Jack R. Edwards, Jungil Choi, John A. Boles
    Abstract:

    Simulations of Mach 5 turbulent flow over a 28-deg compression corner are performed using a hybrid large-eddy/ Reynolds-Averaged Navier-Stokes method. The model captures the mean-flow structure of the interaction reasonably well, with observed deficiencies relating to an underprediction of the displacement effects of the shock-induced separation region. The computational results provide some support for a recent theory concerning the underlying causes of low-frequency shock-wave oscillation. In the simulations, the sustained presence of a collection of streaks of fluid with lower/higher momentum than the average induces a low-frequency undulation of the separation front. Power spectra obtained at different streamwise stations are in good agreement with experimental results. Downstream of reattachment, the simulations capture a three-dimensional mean-flow structure, dominated by counter-rotating vortices that produce wide variations in the surface skin friction. Predictions of the structure of the reattaching boundary layer agree well with experimental pitot pressure measurements. In comparison with Reynolds-Averaged model predictions, the hybrid large-eddy/Reynolds-Averaged Navier-Stokes model predicts more amplification of the Reynolds stresses and a broadening of the Reynolds stress distribution within the boundary layer that is probably due to reattachment-shock motion.

Elise Jennings - One of the best experts on this subject based on the ideXlab platform.

  • a turbulent eddy viscosity surrogate modeling framework for reynolds averaged navier stokes simulations
    Computers & Fluids, 2021
    Co-Authors: Romit Maulik, Himanshu Sharma, Saumil Patel, Bethany Lusch, Elise Jennings
    Abstract:

    Abstract The Reynolds-Averaged Navier-Stokes (RANS) equations for steady-state assessment of incompressible turbulent flows remain the workhorse for practical computational fluid dynamics (CFD) applications. Consequently, improvements in speed or accuracy have the potential to affect a diverse range of applications. We introduce a machine learning framework for the surrogate modeling of steady-state turbulent eddy viscosities for RANS simulations, given the initial conditions. This modeling strategy is assessed for parametric interpolation, while numerically solving for the pressure and velocity equations to steady state, thus representing a framework that is hybridized with machine learning. We achieve competitive steady-state results with a significant reduction in solution time when compared to those obtained by the Spalart–Allmaras one-equation model. This is because the proposed methodology allows for considerably larger relaxation factors for the steady-state velocity and pressure solvers. Our assessments are made for a backward-facing step with considerable mesh anisotropy and separation to represent a practical CFD application. For test experiments with either varying inlet velocity conditions or step heights we see time-to-solution reductions around a factor of 5. The results represent an opportunity for the rapid exploration of parameter spaces that prove prohibitive when utilizing turbulence closure models with multiple coupled partial differential equations.

  • a turbulent eddy viscosity surrogate modeling framework for reynolds averaged navier stokes simulations
    arXiv: Fluid Dynamics, 2019
    Co-Authors: Romit Maulik, Himanshu Sharma, Saumil Patel, Bethany Lusch, Elise Jennings
    Abstract:

    The Reynolds-Averaged Navier-Stokes (RANS) equations for steady-state assessment of incompressible turbulent flows remain the workhorse for practical computational fluid dynamics (CFD) applications. Consequently, improvements in speed or accuracy have the potential to affect a diverse range of applications. We introduce a machine learning framework for the {surrogate modeling of steady-state turbulent eddy viscosities for RANS simulations, given the initial conditions. This modeling strategy} is assessed for parametric interpolation, while numerically solving for the pressure and velocity equations to steady state, thus representing a framework that is hybridized with machine learning. We achieve {competitive} steady-state results with a significant reduction in solution time when compared to those obtained by the Spalart-Allmaras one-equation model. This is because the proposed methodology allows for considerably larger relaxation factors for the steady-state velocity and pressure solvers. Our assessments are made for a backward-facing step with considerable mesh anisotropy and separation to represent a practical CFD application. For test experiments with \textcolor{black}{either} varying inlet velocity conditions or step heights we see time-to-solution reductions around a factor of 5. The results represent an opportunity for the rapid exploration of parameter spaces that prove prohibitive when utilizing turbulence closure models with multiple coupled partial differential equations. \blfootnote{Code available publicly at \texttt{this https URL}}.

Alfredo Pinelli - One of the best experts on this subject based on the ideXlab platform.

  • comparison between large eddy simulation and reynolds averaged navier stokes computations for the must field experiment part ii effects of incident wind angle deviation on the mean flow and plume dispersion
    Boundary-Layer Meteorology, 2010
    Co-Authors: A Dejoan, Alberto Martilli, Jose Luis Santiago, Fernando Martin, Alfredo Pinelli
    Abstract:

    Large-eddy simulations (LES) and Reynolds-Averaged Navier-Stokes (RANS) computations of pollutant dispersion are reported for the Mock Urban Setting Test (MUST) field experiment flow. In particular we address the effects of incident wind angle deviation on the mean velocity and on the mean concentration fields. Both computational fluid dynamical methods are assessed by comparing the simulation results with experimental field data. The comparative analysis proposes to relate the plume deflection with the flow channelling effects. The results show that the plume deflection angle varies with the altitude. As the ground is approached the plume is shown to be almost aligned with the street canyon direction and independent of the incident wind directions considered. At higher altitudes well above the obstacles, the plume direction is aligned with the mean wind direction as in dispersion over flat terrain. The near-ground plume deflection is the consequence of a strong channelling effect in the region near the ground. The mean concentration profiles predicted by LES and RANS are both in good qualitative agreement with experimental data but exhibit discrepancies that can be partly explained by the influence of small incident wind angle deviation effects. Compared to RANS, LES predicts a higher channelling and thus a higher deflection of the plume. Results on the fluctuating intensity of the concentration obtained from LES show a satisfactory agreement with experiments. This information is not available from RANS for which only the mean concentration modelling is considered.

  • comparison between large eddy simulation and reynolds averaged navier stokes computations for the must field experiment part i study of the flow for an incident wind directed perpendicularly to the front array of containers
    Boundary-Layer Meteorology, 2010
    Co-Authors: Jose Luis Santiago, Alberto Martilli, A Dejoan, Fernando Martin, Alfredo Pinelli
    Abstract:

    The large-eddy simulation (LES) and Reynolds-Averaged Navier–Stokes (RANS) methodologies are used to simulate the air flow inside the container’s array geometry of the Mock Urban Setting Test (MUST) field experiment. Both tools are assessed and compared in a configuration for which the incident wind direction is perpendicular to the front array. The assessment is carried out against available wind-tunnel data. Effects of including small geometrical irregularities present in the experiments are analysed by considering LES and RANS calculations on two geometries: an idealized one with a perfect alignment and an identical shape of the containers, and a second one including the small irregularities considered in the experiment. These effects are assessed in terms of the local time-mean average and as well in terms of spatial average properties (relevant in atmospheric modelling) given for the velocity and turbulent fields. The structural flow properties obtained using LES and RANS are also compared. The inclusion of geometrical irregularities is found significant on the local time-mean flow properties, in particular the repeated flow patterns encountered in a perfect regular geometry is broken. LES and RANS provide close results for the local mean streamwise velocity profiles and shear-stress profiles, however the LES predictions are closer to the experimental values for the local vertical mean velocity. When considering the spatial average flow properties, the effects of geometrical irregularities are found insignificant and LES and RANS provide similar results.