Effects of compressibility and heat release on entrainment processes in mixing layers

Mathew, Joseph; Mahle, Inga; Friedrich, Rainer

doi:10.1080/14685240802004924

Cited by 11 publications

(5 citation statements)

References 19 publications

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“…At the trailing edge the resolution is ∆x + = 2.8, ∆z + = 7.0 and the first point is at y + = 0.8. Taking the streamwise location x = 50 as representative of the mixing layer, the local Reynolds number based on the vorticity thickness δ ω = (U 1 − U 2 )/(d u /dy) max and velocity difference ∆U = U 1 − U 2 is Re ω = 15900, which is similar to the simulations of Mathew et al [5] for a similar number of points in the spanwise and lateral directions.…”

Section: Methodssupporting

confidence: 73%

“…The subject has re-emerged recently since the latest direct numerical simulations of subsonic temporally-developing mixing layers are rather notable for the absence of Brown-Roshko structures. Neither the inert low Mach number simulation of Mathew et al [5] nor the incompressible simulations of Tanahashi et al [6] show clear organised structure. Such simulations might appear to support the stability argument that, although two-dimensional waves are most unstable, there is a band of unstable oblique waves that are also strongly unstable and that will break down nonlinearly into turbulence without spanwise coherence.…”

Section: Introductionmentioning

confidence: 93%

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Direct numerical simulation of the early development of a turbulent mixing layer downstream of a splitter plate

Sandham

Sandberg

2009

Journal of Turbulence

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A direct numerical simulation is carried out of the initial stages of development of a mixing layer with a velocity ratio of ten, a fast stream Mach number of 0.6 and equal free-stream temperatures. The fast stream is a fully-developed turbulent boundary layer with a trailingedge displacement thickness Reynolds number of 2300, while the slow stream is laminar. The computations include a splitter plate with zero thickness. The initial flow development is dominated by the rapid spreading of an internal shear layer formed as the viscous sublayer of the upstream turbulent boundary layer crosses the trailing edge. A tendency towards spanwise-coherent structures is observed very early in the shear layer development, within five displacement thicknesses of the trailing edge, despite such structures not being present in the upstream boundary layer. A numerical search for a global mode in the vicinity of the splitter plate trailing edge found only convective growth of disturbances. Instead, a convective mechanism is examined and found to be a plausible explanation for the rapid change of observed flow structure near the trailing edge. The same mechanism indicates a trend towards more two-dimensional structures in the fully-developed mixing layer.

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Section: Methodssupporting

confidence: 73%

Section: Introductionmentioning

confidence: 93%

Direct numerical simulation of the early development of a turbulent mixing layer downstream of a splitter plate

Sandham

Sandberg

2009

Journal of Turbulence

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“…In their study of inert and reacting temporal compressible mixing layers based on DNS, Mathew, Mahle & Friedrich (2008), calculated the change to fractal dimension of the T/NT interface and found it to fall with increasing convective Mach number, consistent with a reduction in interface surface area.…”

Section: Introductionmentioning

confidence: 99%

Invariants of the velocity gradient tensor in a spatially developing compressible round jet

Thaker,

Mathew,

Ghosh

2023

J. Fluid Mech.

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Direct numerical simulation of a compressible round jet is carried out at Mach number of 0.9 and Reynolds number of 3600 and the data are used to perform velocity gradient tensor (VGT) analysis for different regions of the spatially developing jet. For the developed portion of the jet, the classical teardrop shape is observed for the joint probability density function (p.d.f.) of Q and R (second and third invariants of the VGT). In the region just after the potential core, between $X = 10$ and 15 $r_0$ ( $r_0$ is the jet inlet radius), an inclination towards the third quadrant is observed in the Q–R joint p.d.f. which represents the presence of tube-like structures. It is also shown that this inclination in the turbulent/non-turbulent (T/NT) boundary and interface towards the third quadrant is mainly a contribution of points that lie in regions with negative dilatation. Regions with weak expansion also show this third quadrant inclination to some extent. Points that lie in regions with relatively higher positive dilatation show no such inclination towards the third quadrant but are inclined towards the fourth quadrant which indicates the presence of sheet-like structures. Similarly for the domain segment $X = 15$ to 20 $r_0$ , it is observed that points that lie in the regions with positive dilatation have a joint p.d.f. with an inclination towards fourth quadrant, which suggests the presence of sheet-like structures at the T/NT boundary and interface. Points that lie in regions with negative dilatation show the appearance of a third quadrant lobe.

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“…Most of the previous works, which have studied the entrainment process in reacting turbulent shear flows, have focused on the effect of heat release on the overall growth rate of the shear layer (Hermanson & Dimotakis 1989;McMurtry et al 1989;Sekar & Mukunda 1991;Miller et al 1995;Livescu et al 2002;Pantano et al 2003;Mahle et al 2007;Mathew et al 2008;OBrien et al 2014). While all of these works have reported a reduction in growth rate due to the effects of heat release, none have quantified the mass that enters the shear layer.…”

Section: Introductionmentioning

confidence: 99%

The effect of heat release on the entrainment in a turbulent mixing layer

Jahanbakhshi

Madnia

2018

J. Fluid Mech.

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Direct numerical simulations of a 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 close 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.

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Effects of compressibility and heat release on entrainment processes in mixing layers

Cited by 11 publications

References 19 publications

Direct numerical simulation of the early development of a turbulent mixing layer downstream of a splitter plate

Direct numerical simulation of the early development of a turbulent mixing layer downstream of a splitter plate

Invariants of the velocity gradient tensor in a spatially developing compressible round jet

The effect of heat release on the entrainment in a turbulent mixing layer

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