Search results

Understanding the interaction of turbulence and cavitation is an essential step towards better controlling the cavitation phenomenon. The purpose of this paper is to bring out the efficacy of different modelling approaches to predict turbulence and cavitation-induced phase changes.

This paper compares the dynamic cavitation (DCM) and Schnerr–Sauer models. Also, the effects of different modelling methods for turbulence, unsteady Reynolds-averaged Navier–Stokes (URANS) and detached eddy simulations (DES) are also brought out. Numerical predictions of internal flow through a venturi are compared with experimental results from the literature.

The improved predictive capability of cavitating structures by DCM is brought out clearly. The temporal variation of the cavity size and velocity illustrates the involvement of re-entrant jet in cavity shedding. From the vapour fraction contours and the attached cavity length, it is found that the formation of the re-entrant jet is stronger in DES results compared with that by URANS. Variation of pressure, velocity, void fraction and the mass transfer rate at cavity shedding and collapse regions are presented. Wavelet analysis is used to capture the shedding frequency and also the corresponding occurrence of features of cavity collapse.

Based on the performance, computational time and resource requirements, this paper shows that the combination of DES and DCM is the most suitable option for predicting turbulent-cavitating flows.

To perform 3D unsteady Reynolds Averaged Navier‐Stokes (URANS) simulations to predict turbulent flow over bluff body.

Turbulence closure is achieved through a non‐linear k−ε model. This model is incorporated in commercial FLUENT software, through user defined functions (UDF).

The study shows that the present URANS with standard wall functions predicts all the major unsteady phenomena, with a good improvement over other URANS reported so far, which incorporate linear eddy viscosity models. The results are also comparable with those obtained by LES for the same test case.

When comparing the computational time required by the present model and by LES, the accuracy achieved is significant and can be used for simulating 3D unsteady complex engineering flows with reasonable success.

The purpose of this paper is to study the effects of Angle of Attack (AOA), Axis Ratio (AR) and Reynolds number (Re) on unsteady laminar flow over a stationary elliptic cylinder.

The governing equations of fluid flow over the elliptic cylinder are solved numerically on a Cartesian grid using Projection method based Immersed Boundary technique. This numerical method is validated with the results available in open literature. This scheme eliminates the requirement of generating a new computational mesh upon varying any geometrical parameter such as AR or AOA, and thus reduces the computational time and cost.

Different vortex shedding patterns behind the elliptic cylinder are identified and classified using time averaged centerline streamwise velocity profile, instantaneous vorticity contours and instantaneous streamline patterns. A parameter space graph is constructed in order to reveal the dependence of AR, AOA and Re on vortex shedding. Integral parameters of flow such as mean drag, mean lift coefficients and Strouhal number are calculated and the effect of AR, AOA and Re on them is studied using various pressure and streamline contours. Functional relationships of each of integral parameters with respect to AR, AOA and Re are proposed with minimum percentage error.

The results obtained can be used to explain the characteristics of flow patterns behind slender to bluff elliptical cylinders which found applications in insect flight modeling, heat exchangers and energy conservation systems. The proposed functional relationships may be very useful for the practicing engineers in those fields.

The results presented in this paper are important for the researchers in the area of bluff body flow. The dependence of AOA on vortex shedding and flow parameters was never reported in the literature. These results are original, new and important.

The heat transfer rates from the body to the working fluid can be improved through altering geometric configurations of the body and its arrangement in the flow system. One of the arrangements for this purpose is to locate the body at the channel inlet while the convection current opposes it. Since the flow field in the channel inlet influences the heat transfer rates, changing the aspect ratio and inclination of the body is expected to modify the flow field while enhancing the heat transfer rates. Consequently, investigation into the influence of the aspect ratios and tilting angles of the body on the heat transfer rates in the channel flow becomes essential. The paper aims to discuss these issues.

Numerical simulation of flow in a channel with the presence of solid block is carried out. The block aspect ratio is changed while keeping the area of the block constant for all aspect ratios. The tilting angle is also incorporated analysis to examine its effect on the Nusselt number.

The throttling effect of the block at channel inlet accelerates the flow between the channel wall and the block faces. This, in turn modifies the thermal boundary layer around the block. In this case, heat transfer rates increase considerably at the block faces where the flow acceleration suppresses the thermal boundary layer thickness. This is more pronounced for large block tilting angles. The Nusselt number attains low values for the block face opposing to the flow at the channel inlet and the back face of the block. This is attributed to the mixing of the thermal current emanating from the side faces of the block in the region close to the back surface. In this case, thermal boundary layer thickens and the heat transfer rates from the block reduce significantly. The Nusselt number improves with reducing the block aspect ratio, which is particularly true along the side faces of the block. In addition, the influence of the block tilting angle on the Nusselt number is considerable for the low block aspect ratios.

The model study is validated with the previous studies for the drag coefficient. The study covers all the aspects of the flow situations and discusses the resulting fluid field and the heat transfer rates from the block.

It is an interesting work for cooling applications. The block aspect ratio and its tilting angle in the channel influence considerably the flow field and the Nusselt number variation around the block faces.

The cooling technology may be improved through implementing the findings of the current work.

It is an original work and it has never been submitted to other journals.

In recent years, the partially averaged Navier–Stokes (PANS) methodology has earned acceptability as a viable scale-resolving bridging method of turbulence. To further enhance its capabilities, especially for simulating separated flows past bluff bodies, this paper aims to combine PANS with a non-linear eddy viscosity model (NLEVM).

The authors first extract a PANS closure model using the Shih’s quadratic eddy viscosity closure model [originally proposed for Reynolds-averaged Navier–Stokes (RANS) paradigm (Shih et al., 1993)]. Subsequently, they perform an extensive evaluation of the combination (PANS + NLEVM).

The NLEVM + PANS combination shows promising result in terms of reduction of the anisotropy tensor when the filter parameter (f_k) is reduced. Further, the influence of PANS filter parameter f on the magnitude and orientation of the non-linear part of the stress tensor is closely scrutinized. Evaluation of the NLEVM + PANS combination is subsequently performed for flow past a square cylinder at Reynolds number of 22,000. The results show that for the same level of reduction in f_k, the PANS + NLEVM methodology releases significantly more scales of motion and unsteadiness as compared to the traditional linear eddy viscosity model (LEVM) of Boussinesq (PANS + LEVM). The authors further demonstrate that with this enhanced ability the NLEVM + PANS combination shows much-improved predictions of almost all the mean quantities compared to those observed in simulations using LEVM + PANS.

Based on these results, the authors propose the NLEVM + PANS combination as a more potent methodology for reliable prediction of highly separated flow fields.

Combination of a quadratic eddy viscosity closure model with PANS framework for simulating flow past bluff bodies.

The purpose of this paper is to comprehensively evaluate the progress in the development of trapped vortex combustors (TVCs) in the past three decades. The review aims to identify the needs, predict the scope and discuss the challenges of numerical simulations in TVCs applied to gas turbines.

TVC is an emerging combustion technology for achieving low emissions in gas turbine combustors. The overall operation of such TVCs can be on very lean mixture ratio and hence it helps in achieving high combustion efficiency and low overall emission levels. This review introduces the TVC concept and the evolution of this technology in the past three decades. Various geometries that were explored in TVC research are listed and their operating principles are explained. The review then categorically arranges the progress in computational studies applied to TVCs.

Analyzing extensive literature on TVCs the review discusses results of numerical simulations of various TVC geometries. Numerical simulations that were used to optimize TVC geometry and to enhance mixing are discussed. Reactive flow studies to comprehend flame stability and emission characteristics are then listed for different TVC geometries.

To the best of the authors’ knowledge, this review is the first of its kind to discuss extensively the computational progress in TVC development specific to gas turbine engines. Earlier review on TVC covers a wide variety of applications including land-based gas turbines, supersonic Ramjets, incinerators and hence compromise on the depth of analysis given to gas turbine engine applications. This review also comprehensively group the numerical studies based on geometry, flow and operating conditions.

The purpose of this study is to numerically investigate the influence of corner radius on the flow around two square cylinders in tandem arrangements at a Reynolds number of 100.

Six models of square cylinders with corner radii R/D = 0.0, 0.1, 0.2, 0.3, 0.4 and 0.5 (where R denotes the corner radius and D denotes the characteristic dimension of the body) were studied using an immersed boundary-lattice Boltzmann method, and the results were compared with those obtained using a two-dimensional unsteady finite volume method. The cylinders were mounted in a tandem configuration (1.5 ≤ L/D ≤ 10 where L denotes the in-line separation between the cylinder centers). The simulated models were quantitatively compared to the aerodynamic force coefficients and Strouhal number. Furthermore, qualitative analysis is presented in the form of flow streamlines and vorticity contours.

The R/D and L/D values were varied to observe the variation in the flow characteristics in the gap and wake regions. The numerical results revealed two different regimes over the spacing range. The drag force on the downstream cylinder was negative for all corner radii values when the cylinders were placed at L/D = 3.0 (a single-body system). Subsequently, a sudden increase was observed in the aerodynamic forces (drag and lift) when L/D increased. A different gap value was identified in the transformation from a single-body to a two-body system for different corner radii. To verify the single-body system, a simulation was carried out with a single cylinder having a longitudinal geometric dimension equal to the tandem arrangement (L/D + D). Furthermore, in a single-body regime, the total drag of a tandem cylinder was less than that of a single cylinder, thus demonstrating the benefits of using tandem structures. A significant reduction in the aerodynamic forces and drag force was achieved by rounding the sharp corners and placing the cylinders in close proximity. An appropriate configuration of the tandem cylinders with a rounded corner of R/D = 0.4 and 0.5 at L/D = 3.0 and the range is enhanced to L/D = 4.0 for 0.0 ≤ R/D < 0.4 to achieve adequate drag reduction.

To the best of the author’s knowledge, there is a paucity of studies examining the effect of corner radius on bluff bodies arranged in a tandem configuration.

An Unsteady Reynolds‐Averaged Navier‐Stokes (URANS) equation method has been applied to compute the flow over two‐dimensional smooth topography and compared with conventional RANS and large‐eddy simulation (LES) results. The URANS calculation with sufficient grid resolution near solid surface and an appropriate near‐wall model has been shown to simulate much of the large‐scale unsteadiness and some of the turbulent motion for flows with and without separation. Although the results with unadjusted model constants do not show an overwhelming improvement over a standard two‐equation model, it is demonstrated that it may be improved and, more importantly, can be generalized to a new simulation technique by refining the model, considering such factors as grid‐dependent length scales and by making a three‐dimensional calculation.

The purpose of this paper is to numerically solve Eikonal and Hamilton‐Jacobi equations using the finite element method; to use both explicit Taylor Galerkin (TG) and implicit methods to obtain shortest wall distances; to demonstrate the implemented methods on some realistic problems; and to use iterative generalized minimal residual method (GMRES) method in the solution of the equations.

The finite element method along with both the explicit and implicit time discretisations is employed. Two different forms of governing equations are also employed in the solution. The Eikonal equation in its original form is used in the explicit Taylor Galerkin discretisation to save computational time. For implicit method, however, the convection‐diffusion form in its conservation form is used to maintain spatial stability.

The finite element solution obtained is both accurate and smooth. As expected the implicit method is much faster than the explicit method. Though the proposed finite element solution procedures in serial is slower than the standard search procedure, they are suitable to be used in a parallel environment.

The finite element procedure for Eikonal and Hamilton‐Jacobi equations are attempted for the first time. Though the finite volume and finite difference‐based computational fluid dynamics (CFD) solvers have started employing differential equations for wall distance calculations, it is not common for finite element solvers to use such wall distance calculations. The results presented here clearly show that the proposed methods are suitable for unstructured meshes and finite element solvers.

This paper aims to study the fluid flow and forced convection heat transfer from an isothermal circular cylinder with control rods in the laminar unsteady flow regime.

The overset grid method was used for accurate simulation of the unsteady flows around different arrangements of the cylinders. Grid generation for overset grids was performed using a general orthogonal boundary fitted coordinate system. The method of solution was based on a finite volume discretization of the Navier-Stokes equations. Simulations were carried out for the Prandtl numbers of 0.7 and 7.0 with the Reynolds numbers ranging from 60 to 300.

The results indicate that the performance of multiple control rods depends strongly on the spacing ratio. Furthermore, in a manner similar to the flow patterns, four different thermal regimes were recognized based on the variations of mean Nusselt number versus G/D, as the thermal regimes follow the categories of flow regimes at different diameter ratios. However, for different Prandtl numbers, no single trend of heat transfer variation versus the spacing ratio exists for same regime.

Few studies have been conducted to investigate the heat transfer characteristics from control rods. The results of this study provide a comprehensive knowledge on the dynamical and thermal behavior of the flow around multiple cylinders.