Zeinab Pouransari

Formation of coherent structures by fluid inertia in three-dimensional laminar flows

Mixing under laminar flow conditions is key to a wide variety of industrial fluid systems of size extending from micrometres to metres. Profound insight into threedimensional laminar mixing mechanisms is essential for better understanding of the behaviour of such systems and is in fact imperative for further advancement of (in particular, microscopic) mixing technology. This insight remains limited to date, however. The present study concentrates on a fundamental transport phenomenon relevant to laminar mixing: the formation and interaction of coherent structures in the web of three-dimensional paths of passive tracers due to fluid inertia. Such coherent structures geometrically determine the transport properties of the flow and thus their formation and topological structure are essential to three-dimensional mixing phenomena. The formation of coherent structures, its universal character and its impact upon three-dimensional transport properties is demonstrated by way of experimentally realizable time-periodic model flows. Key result is that fluid inertia induces partial disintegration of coherent structures of the non-inertial limit into chaotic regions and merger of surviving parts into intricate three-dimensional structures. This response to inertial perturbations, though exhibiting great diversity, follows a universal scenario and is therefore believed to reflect an essentially three dimensional route to chaos. Furthermore, a first outlook towards experimental validation and investigation of the observed dynamics is made.

Direct numerical simulation of an isothermal reacting turbulent wall-jet

In the present investigation, Direct Numerical Simulation (DNS) is used to study a binary irreversible and isothermal reaction in a plane turbulent wall-jet. The flow is compressible and a single-step global reaction between an oxidizer and a fuel species is solved. The inlet based Reynolds, Schmidt, and Mach numbers of the wall-jet are Re = 2000, Sc = 0.72, and M = 0.5, respectively, and a constant coflow velocity is applied above the jet. At the inlet, fuel and oxidizer enter the domain separately in a non-premixed manner. The turbulent structures of the velocity field show the common streaky patterns near the wall, while a somewhat patchy or spotty pattern is observed for the scalars and the reaction rate fluctuations in the near-wall region. The reaction mainly occurs in the upper shear layer in thin highly convoluted reaction zones, but it also takes place close to the wall. Analysis of turbulence and reaction statistics confirms the observations in the instantaneous snapshots, regarding the intermit...

Statistical analysis of the velocity and scalar fields in reacting turbulent wall-jets

The concept of local isotropy in a chemically reacting turbulent wall-jet flow is addressed using direct numerical simulation (DNS) data. Different DNS databases with isothermal and exothermic reactions are examined. The chemical reaction and heat release effects on the turbulent velocity, passive scalar and reactive species fields are studied using their probability density functions (PDF) and higher order moments for velocities and scalar fields, as well as their gradients. With the aid of the anisotropy invariant maps for the Reynolds stress tensor the heat release effects on the anisotropy level at different wall-normal locations are evaluated and found to be most accentuated in the near-wall region. It is observed that the small-scale anisotropies are persistent both in the near-wall region and inside the jet flame. Two exothermic cases with different Damkohler number are examined and the comparison revealed that the Damkohler number effects are most dominant in the near-wall region, where the wall cooling effects are influential. In addition, with the aid of PDFs conditioned on the mixture fraction, the significance of the reactive scalar characteristics in the reaction zone is illustrated. We argue that the combined effects of strong intermittency and strong persistency of anisotropy at the small scales in the entire domain can affect mixing and ultimately the combustion characteristics of the reacting flow.

Probability Density Functions of Reacting Species Concentrations in Turbulent Wall-Jet

Direct numerical simulation of a simple reaction between two scalars in a plane turbulent wall jet has been performed. In addition to mean and fluctuation intensities also higher order statistics and the probability density functions of the reacting scalars have been examined. The probability density functions shed light on the behavior of the passive and reacting scalars close to the wall and also to the near-wall characteristics of the reaction.

Assessment of subgrid-scale stress statistics in non-premixed turbulent wall-jet flames

ABSTRACT We investigate the heat-release effects on the characteristics of the subgrid-scale (SGS) stress tensor and SGS dissipation of kinetic energy and enstrophy. Direct numerical simulation data of a non-premixed reacting turbulent wall-jet flow with and without substantial heat release is employed for the analysis. This study comprises, among others, an analysis of the eigenvalues of the resolved strain rate and SGS stress tensors, to identify the heat-release effects on their topology. An assessment of the alignment between the eigenvectors corresponding to the largest eigenvalues of these two tensors is also given to provide further information for modelling of the SGS stress tensor. To find out the heat-release effects on the dynamics of the turbulent kinetic energy and enstrophy dissipation, probability density functions (PDFs) and mean values are analysed. The mean SGS shear stress and turbulent kinetic energy both slightly increase in the buffer layer and substantially decrease further away from the wall, due to the heat-release effects. Contrary to the kinetic energy, heat release decreases the mean SGS dissipation of enstrophy in the near-wall region. Moreover, differences in the shapes of the PDFs between the isothermal and exothermic cases indicate changes in the intermittency level of both SGS dissipations. Heat release also increases the SGS stress anisotropy in the near-wall region. Although, the structure of the mean resolved strain-rate tensor only marginally differs between the isothermal and exothermic cases in the near-wall region, substantial differences are observed in the jet area, where compressibility effects are important and heat-release effects are found to promote compression states. The differences in the relative alignment between the SGS stress and resolved strain-rate tensors in the isothermal and exothermic cases are discussed in connection with the differences in the SGS dissipation of kinetic energy.

Higher Order Moments of Velocity Fluctuations and Their Gradients in Turbulent Wall-Jets

The concept of local isotropy is addressed in a turbulent wall-jet. Direct numerical simulations (DNS) of a reacting turbulent wall-jet flow are used to evaluate the probability density functions (PDF) and higher order moments of the velocity and of the gradient in our set-up, in order to illustrate different aspects of the degree of isotropy at small scales.We observe a strong persistency of small-scale anisotropy up to y/y1/2 ≈ 1.5, where y1/2 is the half width of the jet.

Direct Numerical Simulation of a Turbulent Reacting Wall-Jet

The turbulent wall-jet includes a number of interesting fluid mechanics phenomena with close resemblance to many mixing and combustion applications. During the last decades, both DNS (Ahlman et al., 2007; Ahlman et al., 2009), and LES (Dejoan & Leschziner, 2005) have been used to study the turbulent wall-jet. Ahlman et al. (2009) performed DNS of nonisothermal turbulent wall jets. Earlier in 2007, Ahlman et al. investigated turbulent statistics and mixing of a passive scalar for an isothermal case by means of DNS. The first three-dimensional DNS of a reacting turbulent flow was performed by Riley et al. (1986) who simulated a single reaction of two scalars, without heat release, for a mixing layer. Recently, Knaus et al. (2009) studied the effect of heat release in non-premixed reacting shear layers (Knaus & Pantano, 2009).

Detached-Eddy Simulation of a Horizontal Axis Wind Turbine

Aerodynamic simulations of a small horizontal-axis wind turbine, suitable for integration of wind energy in urban and peri-urban areas, are performed using the improved delayed detached-eddy simulation method. Simulations are carried out for three rotation rates and inlet conditions. Aerodynamic characteristics of the wind turbine such as forces, power production, pressure distribution as well as flow topologies are presented. The effect of different rotation rates as well as the effect of free stream turbulence on the turbine aerodynamics are discussed.