Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session R31: Focus Session: Structure of Turbulent/Non-Turbulent Interface |
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Chair: Carlos da Silva, IST-Technical University of Lisbon Room: 402 |
Tuesday, November 26, 2013 1:05PM - 1:18PM |
R31.00001: Multiscale geometry, scaling and fluxes at the turbulent/non-turbulent interface in high Reynolds number boundary layers Charles Meneveau, Charitha M. de Silva, Jimmy Philip, Kapil Chauhan, Ivan Marusic The scaling and surface area properties of the wrinkled surface separating turbulent from non-turbulent regions in open shear flows are important to our understanding of entrainment mechanisms at the boundaries of turbulent flows. PIV data from high Reynolds number turbulent boundary layers covering three decades in scale are used to resolve the turbulent/non-turbulent interface experimentally and to determine unambiguously that such surfaces exhibit fractal scaling with box-counting exponent between -1.3 and -1.4. A complementary analysis based on spatial filtering of the velocity fields also shows power-law behavior of the coarse-grained interface length as a function of filter width, with an exponent between -0.3 and -0.4. These results establish that the interface is fractal-like with a multiscale geometry and fractal dimension of D $\sim$ 2.3-2.4. Measurements of viscous, subgrid-scale and turbulent fluxes across the interface at various scales confirm the complementary nature of viscous nibbling at small scales while turbulent and then large-scale engulfment dominate when viewed at large scales. [Preview Abstract] |
Tuesday, November 26, 2013 1:18PM - 1:31PM |
R31.00002: Geometrical properties and scaling of the turbulent-nonturbulent interface in boundary layers Guillem Borrell, Javier Jim\'enez The turbulent-nonturbulent interface of a boundary layer at $Re_\theta=1500-6500$ is analyzed by thresholding the vorticity magnitude field. The value of the threshold, whithin the range spanning the topological transition from smooth to turbulent, is considered a parameter, and the resulting surface is processed. Its geometrical properties like the relative position to the wall, the fractal dimension or the genus change significantly within the range spanning the topological transition, but the width of the transition scales well with $Re_\tau$ when outer units for the vorticity magnitude ($|\omega|^+/\sqrt{\delta_{99}^{+}} $) are used. The properties of the flow relative to the position of the surface are analyzed within the same range of thresholds, using as a definition of distance the radius of the smallest sphere centerd at the point and touching the surface. That definition works for arbitrarily complex surfaces. The properties of the flow at a given distance to the surface also depend on the threshold, but the average vorticity jump close to the surface scales with the Kolmogorov length, while the average vorticity jump away from the surface scales with the boundary layer thickness. [Preview Abstract] |
Tuesday, November 26, 2013 1:31PM - 1:44PM |
R31.00003: Structure of the turbulent/non-turbulent interface of turbulent boundary layers - DNS results Takashi Ishihara, Hiroki Ogasawara, Julian C. R. Hunt Direct numerical simulations (DNS) of turbulent boundary layers (TBL) along a flat plate are used to study the properties of turbulent/non-turbulent (T/NT) interface of the TBL. The values of the momentum-thickness-based Reynolds numbers, $Re_\theta$, used for this study, are $500-2200$. Analysis of the conditional statistics near the interface of the TBL shows that there is a small peak in the span-wise vorticity, and an associated small jump in stream-wise velocity. It is shown that the interfacial layer has a double structure which consists of a turbulent sub-layer with thickness of the order of the Taylor micro scale and its outer boundary (super layer) with thickness of the order of the Kolmogorov length scale. An approximate profile of the conditional average of span-wise vorticity near the interface fits well to the DNS data. The velocity jump near the T/NT interface of the TBL is of the order of the rms value of velocity fluctuations near the interface. Conditional cross correlations of the stream-wise or the wall-normal velocity fluctuations change sharply across the interface, which are consistent with the blocking mechanism of the interface (Hunt and Durbin 1999). [Preview Abstract] |
Tuesday, November 26, 2013 1:44PM - 1:57PM |
R31.00004: A Comparison of the Scalar and Vorticity Criterion defining the T/NT Interface Jonas Boschung, Fabian Hennig, Norbert Peters Free shear flows are characterized by a turbulent core region, a non-turbulent outer flow and a turbulent/non-turbulent interface separating the two zones. While there exist different approaches to identify this transitional region, the interface position is mostly defined to coincide with the isoscalar surfaces of either a passive scalar or the magnitude of the vorticity. Both criteria are examined and compared using a shear layer DNS. [Preview Abstract] |
Tuesday, November 26, 2013 1:57PM - 2:10PM |
R31.00005: The strain field across the turbulent/non-turbulent interface Gerrit Elsinga, Rodrigo Taveira, Carlos Da Silva The average flow velocity field associated to the local strain at the turbulent/non-turbulent (T/NT) interface is evaluated using existing Direct Numerical Simulations (DNS) of turbulent planar jets and shear free turbulence. The strain field is of interest as it sets the size of the smallest eddies at the interface (through the dissipation) and governs vorticity stretching, which contributes to the entrainment velocity. Moreover, a similar strain field analysis of the internal turbulence yielded shear layer structures, which appeared universal across different flows and representative of the small-scale features of turbulence. Hence, the principal strain axes provide a meaningful basis for comparing different flows and internal versus interface structure. The results for the T/NT interfaces reveal a shear layer structure separating larger scale motions on either side. The non-turbulent side is characterized by a saddle topology without vorticity, whereas the turbulent side shows a nearly uniform flow parallel to the layer. Thus the larger-scales in the flow not just only determine the interface surface area, but they directly affect the strain and vorticity stretching at the interface and thereby entrainment. [Preview Abstract] |
Tuesday, November 26, 2013 2:10PM - 2:23PM |
R31.00006: Lagrangian evolution of fluid particles in the vicinity of the turbulent non-turbulent interface of a turbulent boundary layer Callum Atkinson, Paul Stegeman, Jason Hackl, Guillem Borrell, Julio Soria Lagrangian evolution of a fluid in the immediate vicinity of the turbulent non-turbulent interface (TNT) of a turbulent boundary layer (Re$_\tau \approx 800$) is investigated via direct numerical simulation (DNS) and particle tracking. The TNT interface in the DNS is seeded with particles whose evolution in position, velocity and velocity gradient tensor (VGT) are calculated. The velocity and velocity gradients at each particle are determined using a third order Hermite spline interpolation. Probability density functions (PDFs) associated with the change in position, velocity, enstrophy and dissipation are calculated and the evolution of the local flow topology of the fluid near the TNT is examined in terms of the 2nd and 3rd invariants of the VGT (Q and R). Evolution in the QR-plane is compared with conditional mean evolution trajectories and the entrainment and expulsion of particles across the interface are discussed. [Preview Abstract] |
Tuesday, November 26, 2013 2:23PM - 2:36PM |
R31.00007: The turbulence boundary of a temporal jet Markus Holzner, Maarten van Reeuwijk We study the turbulence boundary of temporal plane jet at Reynolds number Re$=$5000 obtained from a direct numerical simulation. The analysis is based on statistics conditioned on the enstrophy spanning 24 orders of magnitude and identifying essentially irrotational fluid outside the jet to fully turbulent fluid in the jet core. At the jet boundary we find a viscous superlayer (VSL) that envelopes the turbulence. We further identify a turbulent core region (TC) and a buffer region connecting the VSL and the TC. The BR shows many similarities with the turbulent-nonturbulent interface (TNTI), although the TNTI seems to extend into the TC. The BR thickness is about 10 Kolmogorov length scales or half a Taylor length scale, which implies that intense turbulence and viscosity-dominated regions are in close proximity to each other. [Preview Abstract] |
Tuesday, November 26, 2013 2:36PM - 2:49PM |
R31.00008: Scaling of the viscous superlayer in zero pressure gradient turbulent boundary layers Kapil Chauhan, Jimmy Philip, Ivan Marusic Scaling of the viscous superlayer (a thin region that exists at the interface of a turbulent boundary layer and a non-turbulent free-stream) is sought using theoretical reasoning and experimental evidence. A kinetic energy criteria is successfully utilised to identify the turbulent/non-turbulent interface over two-dimensional velocity fields in the streamwise/wall-normal plane. The data-analysis utilises particle image velocimetry measurements at four different Reynolds numbers ($\delta^+=\delta u_\tau/\nu$=1200-14500). The presence of a viscous superlayer is illustrated in all four data sets. It is found that the mean normal velocity across the interface and the tangential velocity jump scales with the skin-friction velocity $u_\tau$. The width of the superlayer is characterised by the local vorticity thickness $\delta_\omega$ and scales with the viscous length scale $\nu/u_\tau$. An order of magnitude analysis of the tangential momentum balance within the superlayer indicates that the turbulent motions also scale with inner scaling, i.e. $u_\tau$ and $\nu/u_\tau$ are the velocity and length scales, respectively. [Preview Abstract] |
Tuesday, November 26, 2013 2:49PM - 3:02PM |
R31.00009: The viscous superlayer in turbulent planar jets Rodrigo Taveira, Carlos Silva Direct numerical simulations of turbulent planar jets are used to study the characteristics of the turbulent/non-turbulent interface (TNTI) separating the turbulent from the irrotational regions of the jet, and to define and visualize the viscous super-layer. Conditional statistics near the TNTI show the existence of a region of a region of ``dominating enstrophy diffusion and negligible enstrophy production,'' outside the TNTI. This is the much debated viscous super-layer. The super layer is not continuos and its thickness is of the order of the Kolmogorov micro-scale. [Preview Abstract] |
Tuesday, November 26, 2013 3:02PM - 3:15PM |
R31.00010: Effects of mean shear on the local turbulent entrainment process Marc Wolf, Markus Holzner, Beat L\"uthi, Dominik Krug, Wolfgang Kinzelbach, Arkady Tsinober 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 measurements of an axisymmetric jet are compared to data from a direct numerical simulation of a zero-mean-shear flow. Conditional statistics relative to the interface position are investigated in a pseudo-Eulerian (i.e. in a fixed frame relative to the interface position) and in a Lagrangian view. A mapping between distance to the instantaneous interface versus conditional time along the trajectory shows that entraining particles remain initially close to the TNTI. Furthermore, decomposing the local entrainment velocity $v_n$ into mean and fluctuating components, we find that mean shear enhances the local entrainment velocity via inviscid and viscous effects. [Preview Abstract] |
Tuesday, November 26, 2013 3:15PM - 3:28PM |
R31.00011: Characteristics of Turbulent/non-turbulent Interface in a Turbulent Planar Jet with a Chemical Reaction Tomoaki Watanabe, Yasuhiko Sakai, Kouji Nagata, Osamu Terashima, Yasumasa Ito, Toshiyuki Hayase Characteristics of chemical reaction (A + B $\to$ P) near the turbulent/non-turbulent (T/NT) interface are investigated by using the direct numerical simulation of reactive planar jet. The reactants A and B are separately premixed into the jet and ambient flows, respectively. DNS is performed at three different Damk\"{o}hler numbers. The conditional statistics conditioned on the distance from the T/NT interface is used to investigate the chemical reaction near the T/NT interface. The conditional mean concentration of product P shows a sharp jump near the T/NT interface, and the product P hardly exists in the non-turbulent region. This implies that the chemical reaction takes place in the turbulent region after the reactant B in the ambient flow is entrained into the turbulent region. The conditional mean scalar dissipation rate of mixture fraction has a large peak value slightly inside the T/NT interface. At the same point, the chemical reaction rate also has a peak value in the case of large Damk\"{o}hler number. On the other hand, when the Damk\"{o}hler number is small, the chemical reaction rate near the T/NT interface is smaller than that in the turbulent region. [Preview Abstract] |
Tuesday, November 26, 2013 3:28PM - 3:41PM |
R31.00012: Experimental study of entrainment in a gravity current via a combined Scanning PTV/LIF-technique Dominik Krug, Markus Holzner, Beat L\"uthi, Marc Wolf, Wolfgang Kinzelbach, Arkady Tsinober In this study, we report experimental results on small-scale entrainment characteristics in an inclined dense gravity current. The measurements were performed at $Re=3700$ and a bulk Richardson number of $Ri=0.26$. In an investigation domain located at the average interface position between turbulent and non-turbulent flow the instantaneous velocity and density field were obtained through simultaneous Scanning Particle Tracking Velocimetry (PTV) and Scanning Laser Induced Fluorescence (LIF). The data allow us to study the influence of buoyancy on small scale physics governing the turbulent/non-turbulent interface and entrainment, e.g. the contribution of the baroclinic torque to the local entrainment velocity. [Preview Abstract] |
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