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http://hdl.handle.net/10477/21541
2018-02-06T21:00:40ZThe Effect of Heat Release on the Entrainment in a Turbulent Mixing Layer
http://hdl.handle.net/10477/75809
The Effect of Heat Release on the Entrainment in a Turbulent Mixing Layer
Jahanbakhshi, Reza; Madnia, Cyrus K.
Direct numerical simulations of temporally evolving compressible reacting mixing layer have been performed to study the entrainment of the irrotational flow into the turbulent region across the turbulent/non-turbulent interface (TNTI). In order to study the effects of heat release and interaction of the flame with the TNTI on turbulence several cases with different heat release levels, Q, and stoichiometric mixture fractions are chosen for the simulations with the highest opted value for Q corresponding to hydrogen combustion in air. The combustion is mimicked by a one-step irreversible global reaction, and infinitely fast chemistry approximation is used to compute the species mass fractions. Entrainment is studied via two mechanisms; nibbling, considered as the vorticity transport across the TNTI, and engulfment, the drawing of the pockets of the outside irrotational fluid into the turbulent region. As the level of heat release increases, the total entrained mass flow rate into the mixing layer decreases. In a reacting mixing layer by increasing the heat release rate, the mass flow rate due to nibbling is shown to decrease mostly due to a reduction of the local entrainment velocity, while the surface area of the TNTI does not change significantly. It is also observed that nibbling is a viscous dominated mechanism in non-reacting flows, whereas it is mostly carried out by inviscid terms in reacting flows with high level of heat release. The contribution of the engulfment to entrainment is small for the non-reacting mixing layers, while mass flow rate due to engulfment can constitute up to 40\% of the total entrainment in reacting cases. This increase is primarily related to a decrease of entrained mass flow rate due to nibbling, while the entrained mass flow rate due to engulfment does not change significantly in reacting cases. It is shown that the total entrained mass flow rate in reacting and non-reacting compressible mixing layers can be estimated from an expression containing the convective Mach number and the density change due to heat release.
2017-01-01T00:00:00ZDNS of Compressible Reacting Turbulent Shear Layer
http://hdl.handle.net/10477/64158
DNS of Compressible Reacting Turbulent Shear Layer
Jahanbakhshi, Reza
Direct numerical simulations (DNS) of temporally evolving shear layers have been performed to investigate the mechanisms associated with the entrainment process and to study the dynamics and the kinematics of turbulent/non-turbulent interface (TNTI). In order to study the effects of compressibility, heat release, and interaction of the flame with the TNTI on turbulence several cases with different convective Mach numbers, heat release levels, and stoichiometric mixture fractions are chosen for the simulations. The chosen values for convective Mach number cover a wide range of compressibility levels from subsonic and nearly incompressible to supersonic and highly compressible flows. Moreover, several heat release levels are opted for the reacting cases with the highest level of heat release corresponding to hydrogen combustion in air. Infinitely fast chemistry approximation is used to model a one-step irreversible global reaction that involves fuel, oxidizer, and product.
Since entrainment is intimately related to the TNTI, the characteristics of this interface are studied and their effects on the entrainment examined. As the compressibility level increases, the average size of the structures that form the local shape of the TNTI increases, however, as the heat release level increases, the average size of the structures that form this interface decreases. The geometrical shape of the TNTI looking from the turbulent region is examined. It is observed that in non-reacting cases this interface is dominated by the concave shaped surfaces. As the level of compressibility increases, the probability of finding highly curved concave shaped surfaces on the TNTI decreases, while the probability of finding flatter concave and convex shaped surfaces increases. In reacting flows with high heat release level, the TNTI is dominated by the convex shaped surfaces. As the heat release level increases the probability of finding highly curved convex shaped surfaces on the TNTI increases, whereas the probability of finding flatter concave and convex shaped surfaces decreases.
In order to gain a better understanding of features of the transport mechanisms across the TNTI, the budgets of enstrophy transport equation are studied. In a low compressible non-reacting flow, in addition to vortex stretching and viscous dissipation terms, which are dominant inside the fully turbulent region, the viscous diffusion term also becomes important in contributing to the total change of enstrophy inside the interface layer. However, in compressible or reacting cases there are additional terms contributing to the total change of enstrophy. In reacting and non-reacting flows, a viscous superlayer (VSL) is observed to be present at the outer edge of the interface layer. It is shown that compressibility seems to have little effect on the thickness of VSL, however, as the level of heat release increases the thickness of this layer decreases.
Examining the intense vorticity structures (IVSs) in the non-reacting cases, reveals that some of the important parameters that can affect the entrainment process, such as the thickness of VSL and the distance from the TNTI at which the average values of some of the generation terms in the enstrophy transport equation reaches a maximum, is approximately equal to the average radius of the IVSs. In the configuration studied here, it is observed that as the IVSs interact with the TNTI, the pressure gradient vectors become misaligned with the density gradient vectors, which are aligned with the direction normal to the TNTI, and generate a baroclinic torque. It is also shown that compressibility has a small effect on the structural features (size, orientation, and strength) of the IVSs in the shear layer.
Entrainment is studied via two mechanisms; nibbling, considered as the vorticity diffusion across the TNTI, and engulfment, the drawing of the pockets of the outside irrotational fluid into the turbulent region. As the levels of compressibility or heat release increases, the total entrained mass flow rate into the shear layer decreases. It is observed that nibbling is a viscous dominated mechanism in non-reacting cases, whereas it is mostly carried out by inviscid terms in reacting flows with high heat release level. It is shown that the contribution of the engulfment to entrainment is small for the non-reacting flows, while mass flow rate due to engulfment can constitute up to forty percent of total entrainment in reacting cases. This increase is primarily related to a decrease of entrained mass flow rate due to nibbling, while the entrained mass flow rate due to engulfment does not change significantly in reacting cases. In a low compressible reacting case, the decrease of entrained mass flow rate due to nibbling is shown to be mostly due to a reduction of the local entrainment velocity, while the surface area of the TNTI does not change significantly. In a high compressible case, both local entrainment velocity and surface area of the TNTI decrease resulting in a reduction of entrained mass flow rate due to nibbling.
2016-08-19T00:00:00ZEntrainment in a compressible turbulent shear layer
http://hdl.handle.net/10477/57966
Entrainment in a compressible turbulent shear layer
Jahanbakhshi, Reza; Madnia, Cyrus K.
Direct numerical simulations (DNS) of temporally evolving shear layers have been performed to study the entrainment of irrotational flow into the turbulent region across the turbulent/non-turbulent interface (TNTI). Four cases with convective Mach
number from 0.2 to 1.8 are used. Entrainment is studied via two mechanisms; nibbling, considered as vorticity diffusion across the TNTI, and engulfment, the drawing of the pockets of the outside irrotational fluid into the turbulent region. The mass flow rate due to nibbling is calculated as the product of the entrained mass flux with the surface area of the TNTI. It is found that increasing the convective Mach number results in a decrease of the average entrained mass flux and the surface area of the TNTI. For the incompressible shear layer the local entrained mass flux is shown to be highly correlated with the viscous terms. However, as the convective Mach number increases, the mass fluxes due to the baroclinic and the dilatation terms play a more important role in the local entrainment process. It is observed that the entrained mass flux is dependent on the local dilatation and geometrical shape of the TNTI. For a compressible shear layer, most of the entrainment of the irrotational flow into the turbulent region due to nibbling is associated with the compressed regions on the TNTI. As the convective Mach number increases, the percentage of the compressed regions on the TNTI decreases, resulting in a reduction of the average entrained mass flux. It is also shown that the local shape of the interface, looking from the turbulent region, is dominated by concave shaped surfaces with radii of curvature of the order of the Taylor length scale. The average entrained mass flux is found to be larger on highly curved concave shaped surfaces regardless of the level of dilatation. The mass fluxes due to vortex stretching, baroclinic torque and the shear stress/density gradient terms are weak functions of the local curvatures on the TNTI, whereas the mass fluxes due to dilatation and viscous diffusion plus the viscous dissipation terms have a stronger dependency on the local curvatures. As the convective Mach number increases, the probability of finding highly curved concave shaped surfaces on the TNTI decreases, whereas the probability of finding flatter concave and convex shaped surfaces increases. This results in a decrease of the average entrained mass flux across the TNTI. Similar to the previous works on jets, the results show that the contribution of the engulfment to the total entrainment is small for both incompressible and compressible mixing layers. It is also shown that increasing the convective Mach number decreases the velocities associated with the entrainment, i.e. induced velocity, boundary entrainment velocity and local entrainment velocity.
2016-05-24T00:00:00ZLocal flow topology and velocity gradient invariants in compressible turbulent mixing layer
http://hdl.handle.net/10477/38619
Local flow topology and velocity gradient invariants in compressible turbulent mixing layer
Vaghefi, N.S.; Madnia, C. K.
The local flow topology is studied using the invariants of the velocity gradient tensor in compressible turbulent mixing layer via direct numerical simulation (DNS) data. The topological and dissipating behaviours of the flow are analysed in two different regions: in proximity of the turbulent/non-turbulent interface (TNTI), and inside the turbulent region. It is found that the distribution of various flow topologies in regions close to the TNTI differs from inside the turbulent region, and in these regions the most probable topologies are non-focal. In order to better understand the behaviour of different flow topologies, the probability distributions of vorticity norm, dissipation and rate of stretching are analysed in incompressible, compressed and expanded regions. It is found that the structures undergoing compression–expansion in axial–radial directions have the highest contraction rate in locally compressed regions, and in locally expanded regions the structures undergoing expansion–compression in axial–radial directions have the highest stretching rate. The occurrence probability of different flow topologies conditioned by the dilatation level is presented and it is shown that the structures in the locally compressed regions tend to have stable topologies while in locally expanded regions the unstable topologies are prevalent.
2015-06-04T00:00:00Z