Acceleration, pressure and related quantities in the proximity of the turbulent/non-turbulent interface

Holzner, Markus; Lüthi, Beat; Tsinober, A.; Kinzelbach, Wolfgang

doi:10.1017/s0022112009991522

Cited by 28 publications

(22 citation statements)

References 23 publications

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“…Velocities beyond the plume edge cannot be induced by viscous effects at the plume edge and must be induced by relatively long-range pressure gradients (consistent with findings for other entraining flows, e.g. Taveira & da Silva 2013;Holzner et al 2009;Terashima et al 2016). The vertical transport outside the plume, i.e.…”

Section: Discussionsupporting

confidence: 78%

Conditional sampling of a high Péclet number turbulent plume and the implications for entrainment

et al. 2017

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We present simultaneous two-dimensional velocity and scalar measurements on a central vertical plane in a pure turbulent plume. We use an edge-detection algorithm to determine the edge of the plume, and compare the data obtained in both a fixed Eulerian frame and a frame relative to local coordinates defined in terms of the instantaneous plume edge. In an Eulerian frame we observe that the time-averaged distributions of vertical and horizontal velocity are self-similar, the vertical velocity being well represented by a Gaussian distribution. We condition these measurements on whether fluid is inside or outside of the plume, and whether fluid inside is mixed plume fluid or engulfed ambient fluid. We find that, on average, 5% of the total vertical volume transport occurs outside the plume and this figure rises to nearly 14% at heights between largescale coherent structures. We show that the fluxes of engulfed fluid within the plume envelope are slightly larger than the vertical transport outside the plume -indicating that ambient fluid is engulfed into the plume envelope before being nibbled across the turbulent/non-turbulent interface (TNTI) and then ultimately irreversibly mixed. Our new measurements in the plume coordinate (following the meandering fluctuating plume)show the flow within the plume and in the nearby ambient fluid is strongly influenced by whether an eddy is present locally within the plume, or absent. When an eddy is present and the plume is wide, the vertical velocities near the plume edge are small and hence all vertical transport is inside the plume. In regions where the plume is narrow † Email address for correspondence: h.burridge@imperial.ac.uk 2 H. C. Burridge, D. A. Parker, E. S. Kruger, J. L. Partridge and P. F. Linden and there is no eddy, large vertical velocities and hence transport are observed outside the plume suggesting that pressure forces associated with the eddies accelerate ambient fluid which is then engulfed into the plume. Finally, we show that observing significant vertical velocities beyond the scalar edge of the plume does not suggest that the characteristic width of the velocity distribution is greater than that of the scalar field; on the contrary, we show our observations to be consistent with a buoyancy distribution that is up to 20% wider than that of the velocity. Measurements in the plume coordinates show that the mixing of momentum across the plume results in a distribution for which the differential entropy is close to maximal and the mixing of momentum is uninhibited (i.e. not bounded) by the TNTI of the plume. Furthermore, our measurements suggest that the scalar mixing across the plume may also result in a distribution for which the differential entropy is close to maximal but, in contrast to the momentum, the scalar mixing is strictly bounded by the plume edge.

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Section: Discussionsupporting

confidence: 78%

Conditional sampling of a high Péclet number turbulent plume and the implications for entrainment

et al. 2017

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“…It was shown that the random error in the determination of velocity and acceleration derivatives is ∼10 %, which lies in the range of other PTV and particle image velocimetry (PIV) studies investigating small-scale aspects of turbulent flows (see e.g. Mullin & Dahm 2006;Ganapathisubramani, Lakshminarasimhan & Clemens 2007;Holzner et al 2009;Liberzon et al 2012). Random errors for the cases of Re 2000 and Re 8000 are comparable to the ones obtained for Re 5000.…”

Section: Experimental Jet Measurementsmentioning

confidence: 55%

“…Previous studies conducted by some of the authors have already employed the same numerical data set to examine small-scale features of turbulent entrainment (Holzner et al 2007(Holzner et al , 2008(Holzner et al , 2009Holzner & Lüthi 2011). In the current study we compare the simulations to the jet experiments described above in order to investigate certain aspects of the influence of mean shear on local entrainment.…”

Section: Direct Numerical Simulation Of a Flow With Zmsmentioning

confidence: 99%

“…jet or ZMS flow) and irrotational ambient flow by using a threshold on enstrophy, ω 2 = ω · ω, where ω is the vorticity vector. Similar to Holzner et al (2009) and Wolf et al (2012), the threshold is set to ∼2 % of the mean enstrophy value in the turbulent region. The value is used to determine an enstrophy iso-surface situated in the outer section of the interfacial region between turbulent and irrotational flows.…”

Section: Interface Detection and Conditional Samplingmentioning

confidence: 99%

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Effects of mean shear on the local turbulent entrainment process

et al. 2013

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We report on effects of mean shear on the turbulent entrainment process, focusing in particular on their relation to small-scale processes in the proximity of the turbulent/non-turbulent interface (TNTI). Three-dimensional particle tracking velocimetry (3D-PTV) measurements of an axisymmetric jet are compared to data from a direct numerical simulation (DNS) of a zero-mean-shear (ZMS) flow. First, conditional statistics relative to the interface position are investigated in a pseudoEulerian view (i.e. in a fixed frame relative to the interface position) and in a Lagrangian view. We find that in a pseudo-Eulerian frame of reference, both vorticity fluctuations and mean shear contribute to the vorticity jump at the boundary between irrotational and turbulent regions. In contrast, the Lagrangian evolution of enstrophy along trajectories crossing the entrainment interface is almost exclusively dominated by vorticity fluctuations, at least during the first Kolmogorov time scales after passing the interface. A mapping between distance to the instantaneous interface versus conditional time along the trajectory shows that entraining particles remain initially close to the TNTI and therefore attain lower average enstrophy values. The ratio between the rate of change of enstrophy in the two frames of references defines the local entrainment velocity v n = −(Dω 2 /Dt)/(∂ω 2 /∂x n ), where ω 2 is enstrophy andx n is the coordinate normal to the TNTI. The quantity v n is decomposed into mean and fluctuating components and it is found that mean shear enhances the local entrainment velocity via inviscid and viscous effects. Further, the analysis substantiates that for all investigated flow configurations the local entrainment velocity depends considerably on the geometrical shape of the interface. Depending on the surface shape, different small-scale mechanisms are dominant for the local entrainment process, i.e. viscous effects for convex shapes and vortex stretching for concave shapes, looking from the turbulent region towards the convoluted boundary. Moreover, turbulent fluctuations display a stronger dependence on the shape of the interface than mean shear effects.

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“…More detailed spatial analyses of this region have recently been made experimentally (e.g. Westerweel et al 2002Westerweel et al , 2009Holzner et al 2007b) and numerically (e.g. da Silva & Pereira 2008da Silva & Taveira 2010).…”

Section: )mentioning

confidence: 99%

Conditional statistics of the turbulent/non-turbulent interface in a jet flow

et al. 2013

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Using two-dimensional high-speed measurements of the mixture fraction Z in a turbulent round jet with nozzle-based Reynolds numbers Re 0 between 3000 and 18 440, we investigate the scalar turbulent/non-turbulent (T/NT) interface of the flow. The mixture fraction steeply changes from Z = 0 to a final value which is typically larger than 0.1. Since combustion occurs in the vicinity of the stoichiometric mixture fraction, which is around Z = 0.06 for typical fuel/air mixtures, it is expected to take place largely within the turbulent/non-turbulent interface. Therefore, deep understanding of this part of the flow is essential for an accurate modelling of turbulent non-premixed combustion. To this end, we use a composite model developed by Effelsberg & Peters (Combust. Flame, vol. 50, 1983, pp. 351-360) for the probability density function (p.d.f.) P(Z) which takes into account the different contributions from the fully turbulent as well as the turbulent/non-turbulent interface part of the flow. A very good agreement between the measurements and the model is observed over a wide range of axial and radial locations as well as at varying intermittency factor γ and shear. Furthermore, we observe a constant mean mixture fraction value in the fully turbulent region. The p.d.f. of this region is thus of non-marching character, which is attributed physically to the meandering nature of the fully turbulent core of the jet flow. Finally, the location and in particular the scaling of the thickness δ of the scalar turbulent/non-turbulent interface are investigated. We provide the first experimental results for the thickness of the interface over the above-mentioned Reynolds number range and observe δ/L ∼ Re −1 λ , where L is an integral length scale and Re λ the local Reynolds number based on the Taylor scale λ, meaning that δ ∼ λ. This result also supports the assumption often made in modelling of the stoichiometric scalar dissipation rate χ st being a Reynolds-number-independent quantity.

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Acceleration, pressure and related quantities in the proximity of the turbulent/non-turbulent interface

Cited by 28 publications

References 23 publications

Conditional sampling of a high Péclet number turbulent plume and the implications for entrainment

Conditional sampling of a high Péclet number turbulent plume and the implications for entrainment

Effects of mean shear on the local turbulent entrainment process

Conditional statistics of the turbulent/non-turbulent interface in a jet flow

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