Kaustav Sengupta

Large-eddy simulation using a discontinuous Galerkin spectral element method

In this paper we discuss the development of a robust, high-order discontinuous Galerkin (DG) spectral element method for large-eddy simulation (LES) of compressible ∞ows. The method secures geometrical ∞exibility through a fully unstructured grid (triangles in 2D and tetrahedral elements in 3D), allows for arbitrary order of accuracy and has excellent stability properties. An element based flltering technique is used in conjunction with the dynamic procedure to model the efiect of sub-grid scales. We aim to use the LES methodology for large-scale simulation in geometrically complex dump combustors. As a flrst step towards these simulations, we perform validation simulations of compressible, turbulent ∞ow in a plane channel with isothermal walls.

Effect of subsonic microjets on cold flow in dump combustors

Fluidic control of shear layers in air-breathing liquid-fuel combustors is considered as a viable means for improving performance. The increasing demand for compact combustion accompanied by low drag, high turndown ratio, and a reliable flame anchor calls for concurrent application of advanced control strategies such as counter-current shear and microjets. Traditional flame holders, such as the backward-facing step dump combustor, provide a protective environment for the flame to reside. However, these systems also carry a significant drag penalty. The focus of this research is to explore the role of microjets as a shear layer control strategy in dump combustors via numerical simulation. The simulations have been conducted within the Reynolds-averaged Navier-Stokes (RANS) framework. Results for cold flow have shown that increasing the momentum of the microjets leads to greater non-uniformity in the turbulent flow field. Microjets can also affect the recirculation zone, thus influencing flame stabilization. Many parameters such as the location, size, mass flow rate ratio, and momentum ratio of the microjets determine the overall performance. A detailed parametric study is conducted to investigate the effect of various parameters.

Direct Numerical Simulation of Turbulent Flows Using Spectral Methods

Direct numerical simulation (DNS) is the most accurate method of solving turbulence in ∞uids. In DNS the Navier-Stokes equations are solved on a flne mesh to resolve all the spatial and temporal scales present in the ∞ow. In order to ensure high accuracy of the discrete solution, schemes with low numerical errors are necessary. Spectral methods with their low dissipation and dispersion errors are very attractive in this regard. Since their inception in early 1970’s, spectral methods have been routinely applied towards direct simulation of turbulent ∞ows. Some of the earliest applications were in isotropic turbulence using Fourier series based methods. As the community turned its attention to solving practical ∞ows, the need for schemes that are e‐cient in handling complex geometries arose. This led to the development of spectral element methods. In this paper, we describe the application of a semi-structured spectral element method for direct simulation of compressible, wall bounded turbulent ∞ows. A brief outline of the method is flrst presented. Then we review some of the results for turbulent channel ∞ow and ∞ow over a backward-facing step. The accuracy of the technique is established by comparing our simulation results with experiment and a previous DNS study.

Step Geometry and Countercurrent Effects in Dump Combustor, Part 1: Cold Flow

Nonreacting flow in a backward-facing step combustor is studied while employing a novel countercurrent shear (or counterflow) concept. Counterflow is used to manipulate the turbulent shear layer created by the step to increase turbulent burning velocities, and thereby, reduce ignition delay time. Unfortunately, this concept also leads to a smaller residence time because of a shorter recirculation vortex. These competing challenges of achieving higher burning velocities and longer residence time demand modification of the step geometry. Changes in the step geometry will alter the size and characteristics of the recirculation vortex and the shear layer dynamics within the combustor. These issues are addressed in this paper via a numerical study. For the simulations, Reynolds-averaged Navier–Stokes equations are solved in the framework of the realizable k–� turbulence model. A two-layer approach is used for the near-wall modeling. The paper includes a detailed account of the benefits of countercurrent shear technology, a parametric study based on step-geometry modifications, and an aerodynamic performance evaluation.

Step Geometry and Countercurrent Effects in Dump Combustor: Reacting Flow

In this article we report a numerical investigation of reacting flow in dump combustors with counterflow and modified step geometry. The flow is simulated by solving Reynolds-averaged Navier-Stokes equations in the framework of the realizable κ–ε turbulence model. A two-layer approach is used for the near-wall treatment, whereas a one-equation model is employed for the viscous sublayer. The closure for the reaction source term is based on the eddy dissipation concept. The article includes a study of the effect of reaction on the base flow, the effect of countercurrent on the flame, and finally, the role of step geometry. Application of countercurrent results in a thicker averaged flame surface, which leads to an overall increase in the heat release within the combustor. The effect of alteration of the step geometry is found to be much more complicated. At low levels of counterflow there is an increase in turbulence intensity with an increase of the step angle but little change in heat release.

Comparison of LES studies in backward-facing step using Chebyshev multidomain and Legendre spectral element methods

Large-eddy simulation (LES) has been increasingly applied to complex shear flows encountered in practical engineering devices. Here, we present the comparison of LES studies performed using the explicit Chebyshev multi-domain method and the semi-implicit Legendre spectral element method. Two different backward-facing step configurations are simulated at Reynolds numbers 5000 and 28; 000. A grid resolution study at Re = 28; 000 is carried out to compare the number of spectral elements required by the two spectral methods to resolve the flow field. Effect of synthetic turbulence at inflow is investigated using two techniques of generating inflow turbulence at Re = 28; 000. In the LES study at Re = 5000, it is observed that a semi-implicit method requires small time step size to correctly predict transition to turbulence.

Large-eddy simulation of compressible flow over backward-facing step using Chebyshev multidomain method

During the last two decades, large-eddy simulation (LES) has been increasingly applied to complex shear ∞ows encountered in practical engineering devices. The ability of LES to resolve the large-scale unsteady ∞ow physics directly on the computational grid, makes them attractive over Reynolds-averaged Navier-Stokes (RANS) modeling, despite the higher computational cost. Modeling of only the small scale ∞ow-physics as opposed to the entire spectrum of turbulence (as done in RANS), reduces the complexity of modeling in LES. At the same time, application of LES outside the well-validated regime requires careful assessment and interpretation of the results, consideration of the efiect of numerical method and grid resolution on the solution, and speciflcation of accurate boundary conditions. A Chebyshev spectral multi-domain method is used to simulate the ∞ow over an open backward-facing step conflguration at Reynolds number of 5000 based on the in∞ow velocity and step height.

Large eddy simulation using a high-order nodal discontinuous galerkin method on unstructured grids

In this paper we discuss the development of a robust, high-order Discontinuous Galerkin (DG) spectral element method for LES of compressible flows. The method secures geometrical flexibility through a fully unstructured grid (triangles in 2D and tetrahedral elements in 3D), allows for arbitrary order of accuracy and has excellent stability properties. An element based filtering technique is used in conjunction with the dynamic procedure to model the effect of sub-grid scales.Copyright