Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session E35: Turbulence: Turbulent/Non-turbulent Interface |
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Chair: Ivan Marusic, University of Melbourne Room: Oregon Ballroom 204 |
Sunday, November 20, 2016 5:37PM - 5:50PM |
E35.00001: Scale-dependent entrainment velocity and scale-independent net entrainment in a turbulent axisymmetric jet Jimmy Philip, Dhiren Mistry, James Dawson, Ivan Marusic The net entrainment in a jet is the product of the mean surface area ($\overline {S}$) and the mean entrainment velocity, $\overline {V} ~\overline{S}$, where, $\overline {V}=\alpha U_c$ with $\alpha$ the entrainment coefficient and $U_c$ the mean centreline velocity. Instantaneously, however, entrainment velocity ($v$) at a point on the interface is the difference between the interface and the fluid velocities, and the total entrainment $\int v ~{\rm d}s=V~ S$, where $S$ is the corrugated interface surface area and $V$ the area averaged entrainment velocity. Using time-resolved multi-scale PIV/PLIF measurements of velocity and scalar in an axisymmetric jet at $Re=25000$, we evaluate $V$ and $S$ directly at the smallest resolved scales, and by filtering the data at different scales ($\Delta$) we find their multi-scales counterparts, $V_\Delta$ and $S_\Delta$. We show that $\overline {V} ~\overline{S} = V_\Delta ~S_\Delta = V ~S$, independent of the scale. Furthermore, $S$ is found to have a fractal dimension $D_3 \approx 2.32\pm0.1$. Independently, we find that $V_\Delta\sim \Delta^{0.31}$, indicating increasing entrainment velocity with increasing length scale. This is consistent with a constant net entrainment across scales, and suggests $\alpha$ as a scale-dependent quantity. [Preview Abstract] |
Sunday, November 20, 2016 5:50PM - 6:03PM |
E35.00002: Scalar transport across the turbulent/non-turbulent interface in jets: Schmidt number effects Tiago S. Silva, Carlos B. da Silva The dynamics of a passive scalar field near a turbulent/non-turbulent interface (TNTI) is analysed through direct numerical simulations (DNS) of turbulent planar jets, with Reynolds numbers ranging from $142 \leq Re_\lambda \leq 246$, and Schmidt numbers from $0.07 \leq Sc \leq 7$. The steepness of the scalar gradient, as observed from conditional profiles near the TNTI, increases with the Schmidt number. Conditional scalar gradient budgets show that for low and moderate Schmidt numbers a {\it diffusive superlayer} emerges at the TNTI, where the scalar gradient diffusion dominates, while the production is negligible. For low Schmidt numbers the growth of the turbulent front is commanded by the molecular diffusion, whereas the scalar gradient convection is negligible. [Preview Abstract] |
Sunday, November 20, 2016 6:03PM - 6:16PM |
E35.00003: Evolution of the turbulent/non-turbulent interface in the near field of an axisymmetric jet James Dawson, Dhiren Mistry We characterise the near-field evolution of an axisymmetric jet by considering the multi-scale topology of the turbulent/non-turbulent interface (TNTI). Using planar laser-induced fluorescence data from a high Reynolds number jet we implement a multi-scale methodology to evaluate the fractal dimension of the TNTI as a function of streamwise distance. We show that the streamwise evolution of the fractal dimension, $D_f$, of the TNTI reaches a plateau just beyond the potential core which was measured to be $x/d \approx 4.5$ in the current experiment. Downstream of the potential core we show that $D_f \approx 0.33$, which agrees with recently reported values of $D_f$ measured in fully-developed turbulent flows, such as the far-field of a jet and in turbulent boundary layers. The onset of this fractal behaviour also coincides with evidence of flow homogeneity based on the radial auto-correlation functions of axial and radial velocity fluctuations. These results indicate that the flow-field about the TNTI beyond the potential core exhibits a hierarchy of scales (turbulent cascade) that is characteristic of fully-developed turbulence. [Preview Abstract] |
Sunday, November 20, 2016 6:16PM - 6:29PM |
E35.00004: The turbulent/non-turbulent interface in viscoelastic fluids João Melo, Carlos B. da Silva The dynamics of the enstrophy in shear free turbulent/non-turbulent interfaces (TNTI) is analysed through direct numerical simulations (DNS) using the Finitely Extensible Nonlinear Elastic constitutive equations closed with the Peterlin approximation (FENE-P). The Reynolds number and the Deborah number of the DNS range between $116 \leq Re_\lambda \leq 182$ and $0.11 \leq De \leq 1.23$, respectively. A new term emerges in the enstrophy transport equation for viscoelastic fluids - the {\it viscoelastic production} - which competes with the enstrophy diffusion and enstrophy production for the build up of enstrophy near the TNTI, particularly for high Deborah numbers. While for low Deborah numbers the viscoelastic production contributes to a depletion of vorticity inside the turbulent region, this effect is reversed at the higher Deborah number configurations. [Preview Abstract] |
Sunday, November 20, 2016 6:29PM - 6:42PM |
E35.00005: The role of the scalar and enstrophy flux in entrainment processes Dhiren Mistry, James R. Dawson Turbulent entrainment is a multi-stage, multi-scale process that describes the growth of a turbulent region of flow. Ultimately, turbulent entrainment is achieved through viscous diffusion of vorticity, and molecular diffusion in the presence of scalars, with irrotational and unmixed regions of the flow at the smallest scales. We do not fully understand how these small-scale processes are coupled to or modulated by the large-scales of turbulence. This is partly because the mean entrainment rates in turbulent shear flows can be determined by considering large-scales quantities only. We present experimental evidence that the large-scale flux of enstrophy and scalar towards the turbulent/non-turbulent interface (TNTI) coincides with a local increase in the entrainment velocity along the TNTI. This is achieved using a passive scalar ($Sc\gg1$) to identify the TNTI, and a time-resolved interface-tracking method to measure the local entrainment velocity. Our results indicate that the both scalar and enstrophy fluxes towards the TNTI increase the vorticity and scalar gradients increasing the local rates of diffusion. These results show how local processes of small-scale diffusion are modulated by the large-scale turbulence. [Preview Abstract] |
Sunday, November 20, 2016 6:42PM - 6:55PM |
E35.00006: High Reynolds numbers scaling of the turbulent/non-turbulent interface Carlos Bettencourt Da Silva, Tiago S. Silva The scaling of the turbulent/non-turbulent interface (TNTI) at high Reynolds numbers is assessed using new direct numerical simulations (DNS) of turbulent planar jets (PJET) and shear free turbulence (SFT) with Reynolds numbers ranging from $142 \leq Re_{\lambda} \leq 300$. The thickness of the turbulent sublayer (TSL), where the enstrophy production dominates over enstrophy diffusion, is of the order of the Taylor micro-scale, and is roughly one order of magnitude larger than the Kolmogorov micro-scale for these Reynolds numbers, however it clearly scales with the Kolmogorov micro-scale, at sufficiently high Reynolds numbers. It is argued the same scaling should be observed in TNTI from mixing layers, wakes and boundary layers, provided the Reynolds number is sufficiently high. [Preview Abstract] |
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