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 H32: Turbulent Boundary Layers: Spectral Analysis and Scaling |
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Chair: Nicholas Hutchins, University of Melbourne Room: Oregon Ballroom 201 |
Monday, November 21, 2016 10:40AM - 10:53AM |
H32.00001: Comparison of spatial and temporal characteristics of a turbulent boundary layer in the presence of free-stream turbulence Eda Dogan, R. Jason Hearst, Ronald E. Hanson, Bharathram Ganapathisubramani Free-stream turbulence (FST) has previously been shown to enhance the scale interactions occurring within a turbulent boundary layer (TBL). This is investigated further by generating FST with an active grid over a zero-pressure gradient TBL that developed on a smooth flat plate. Simultaneous measurements were performed using four hot-wires mounted to a rake that traversed the boundary layer height. Planar PIV measurements were also performed. Hot-wire measurements indicate that on average large-scale structures occurring in the free-stream penetrate the boundary layer and increase the streamwise velocity fluctuations throughout. Two-point correlations of the streamwise velocity fluctuations from the hot-wires enable determination of the inclination angle of the wall-structures in the boundary layer using Taylor's hypothesis. This angle is observed to be invariant around 11-15 degrees in the near-wall region in agreement with the literature for canonical TBLs. This presentation will compare the planar PIV data to these hot-wire measurements to determine if these phenomena that appear in the statistics using Taylor's hypothesis can be tracked to instantaneous spatial features in the TBL subjected to FST. [Preview Abstract] |
Monday, November 21, 2016 10:53AM - 11:06AM |
H32.00002: Distance-from-the-wall scaling in turbulent boundary layers Rio Baidya, Jimmy Philip, Nicholas Hutchins, Jason Monty, Ivan Marusic An assessment of self-similarity in the inertial sublayer of turbulent boundary layers (TBL) is presented by simultaneously considering the streamwise and wall-normal velocities. Here, we utilise carefully conducted subminiature $\times$-probe experiments at high Reynolds number. Moreover, the turbulent stresses are compared against results from a synthetic flow where the distance-from-the wall ($z$-) scaling is strictly enforced, following the Attached Eddy Hypothesis. We show that not all stresses approach the asymptotic solution at an equal rate as the friction Reynolds number ($Re_\tau$) is increased. Specifically, the motions which contribute to the wall-normal variance and Reynolds shear stress are found to follow the asymptotic solution at a relatively lower $Re_\tau$ even when the streamwise variance does not, and this trend is attributed to the contribution from attached eddies. Based on these findings, the Reynolds shear stress cospectra, through its $z$-scaling, are used to assess the wall-normal limits where self-similarity applies within the TBL. The limits are found to be consistent with the recent observations that the self-similar region starts and ends at viscous scaled wall-distances of $\mathcal{O}$$(\sqrt{Re_\tau})$ and $\mathcal{O}$$(Re_\tau)$ respectively. [Preview Abstract] |
Monday, November 21, 2016 11:06AM - 11:19AM |
H32.00003: Spectral link for the mean velocity profile in the atmospheric boundary layer Dongrong Zhang, Gustavo Gioia, Pinaki Chakraborty Turbulent flow in the atmospheric boundary layer is sheared and stratified. For this flow, we consider the mean velocity profile (MVP), the vertical profile of the time-averaged horizontal wind velocity. We employ the theoretical framework of the spectral link, originally proposed for MVP in sheared flows (Gioia et al., 2010) and later extended to stratified flows (Katul et al., 2011). Accounting for the whole structure of the turbulent energy spectrum---the energetic range, the inertial range, and the dissipative range---we examine the scaling of the MVP in the "wall coordinates" and in the Monin--Obukhov similarity coordinates, for both stable and unstable stratification. Our results are in excellent accord with field measurements and numerical simulations. [Preview Abstract] |
Monday, November 21, 2016 11:19AM - 11:32AM |
H32.00004: Onset of spatio temporal disorder described by directed percolation Tom Wester, Dominik Traphan, Gerd Gülker, Joachim Peinke The energy transport and mixing behavior of a fluid strongly depends on the state of the flow. These properties change drastically if the flow changes from laminar to turbulent state. This transition is a very complex and highly unsteady phenomenon, which is not fully understood up to now. The biggest problem is the characterization of the onset of spatio temporal disorder. This means that turbulent spots in the flow field irregularly spread or decay on their way downstream. In this presentation we will show that this critical behavior of turbulent spreading in the flow can be described by the directed percolation model. This approach was already used for a transitive channel flow, pipe flows or different couette flows. The charm of this model is the complete characterization of the whole transition with only a few unique exponents. In contrast to the majority of previous studies, the underlying data base of this study is acquired experimentally by high-speed Particle Image Velocimetry. Thus the evolving flow can be captured in a highly resolved spatio-temporal manner. In this way it is easily possible to determine the critical exponents which describe the transient area between laminar and turbulent flow. The results will be presented and compared to theoretical expectations. [Preview Abstract] |
Monday, November 21, 2016 11:32AM - 11:45AM |
H32.00005: Spectral stochastic estimation of high-Reynolds-number wall-bounded turbulence for a refined inner-outer interaction model Woutijn J. Baars, Nicholas Hutchins, Ivan Marusic For wall-bounded flows, the model of Marusic, Mathis and Hutchins (2010) allows one to predict the statistics of the streamwise fluctuating velocity in the inner-region, from a measured input signal in the logarithmic region. Normally, a user-defined portion of the input forms the large-scale content in the prediction. Incoherent smaller scales are then fused to the prediction via universally expressed fluctuations that are subject to an amplitude modulation. Here we present a refined version of the model using spectral linear stochastic estimation, which eliminates a user-defined scale-separation of the input. An empirically-derived transfer kernel comprises an implicit filtering via a scale-dependent gain and phase---this kernel captures the coherent portion in the prediction. An additional refinement of the model embodies a relative shift between the stochastically estimated scales in the prediction and the modulation envelope of the universal small-scales. Predictions over a three-decade span of Reynolds numbers, $Re_{\tau} \sim O(10^3)$ to $O(10^6)$, highlight promising applications of the refined model to high-Reynolds-number flows, in which coherent scales become the primary contributor to the fluctuating energy. [Preview Abstract] |
Monday, November 21, 2016 11:45AM - 11:58AM |
H32.00006: Scaling properties of the mean wall-normal velocity in the zero pressure gradient boundary layer Tie Wei, Joseph Klewicki The scaling properties of the mean wall-normal velocity, $V(x,y)$, in zero-pressure-gradient laminar and turbulent boundary layer flows is investigated using numerical simulation data, physical experiment data, and integral analyses of governing equations. The maximum mean wall-normal velocity, $V_\infty$, and the boundary layer thickness, $\delta$, are evidenced to be the proper scaling for $V$ over most if not the entire boundary layer. This is different from the behavior of the mean streamwise velocity ($U$) or the turbulent shear stress ($T=-\rho\langle u v \rangle$), which depend on different characteristic length scales in the regions near to and away from the surface. Insights pertaining to this are further surmised from an analytical relationship for the ratio of the displacement to momentum thickness, i.e., shape factor, $H$. Integral analyses using the continuity and mean momentum equation show that $(U_\infty V_\infty)/u_{\tau}^2 = H$, where $u_{\tau}$ is the friction velocity. Both the laminar similarity solution and DNS data in post-transitional flows convincingly support this relation. Over the transitional regime, sufficiently high quality data is still lacking to check if this relation remains valid. [Preview Abstract] |
Monday, November 21, 2016 11:58AM - 12:11PM |
H32.00007: Dissipation scaling in constant-pressure turbulent boundary layers Jovan Nedic, Stavros Tavoularis, Ivan Marusic We use previous direct numerical simulations and experimental data to investigate the streamwise and wall-normal evolution of the dissipation parameter $C_{\varepsilon}$ (namely the dissipation rate scaled by appropriate powers of the local turbulent kinetic energy and integral length scale) in the outer region of spatially evolving turbulent boundary layers. For $Re_{\theta}\geq 10,000$, $C_{\varepsilon}$ is essentially constant in the streamwise direction, but varies measurably in the wall-normal direction. For lower $Re_{\theta}$, however, $C_{\varepsilon}$ changes in both directions. The constancy of $C_{\varepsilon}$ is a central assumption of turbulence models based on the eddy viscosity concept and so they would inadequately represent wall bounded flows as they evolve spatially, a scenario that is common in engineering and atmospheric science applications. Accounting for the dependence of $C_{\varepsilon}$ on the local $Re_{\lambda}$ provides a means for possibly improving such models. [Preview Abstract] |
Monday, November 21, 2016 12:11PM - 12:24PM |
H32.00008: New multi-scale causality analysis of streak-roll interactions in wall-bounded turbulence X. San Liang, Adrian Lozano-Duran An important observation in nonlinear dynamical systems in general, and turbulent flows in particular, is that, as time goes on, the correlation between two events may be lost and re-emerge. How the causality evolves between them is hence of particular interest. Using a newly developed rigorous causality analysis based on the information transfer, we have examined the causality evolution between the streaks and rolls within the wall-bounded turbulence of a channel flow model with doubly periodic boundaries. The streaks are represented with the low principal component modes of the streamwise velocity U, while the rolls are reflected in the spanwise velocity W and vertical velocity V. It is found that the causal relation mainly occurs between the lower U modes, i.e., the streak structures, and V and W. For U and V, the causality is almost one-way, i.e., from the streaks to V, and that from the first two U modes (domain modes hereafter) dominates. In contrast, no causal relation has been identified between the domain modes and W. The W-U causality is between W and the non-domain lower U modes, which are mutually causal, though the influence from the latter to the former dominates. [Preview Abstract] |
Monday, November 21, 2016 12:24PM - 12:37PM |
H32.00009: Statistics of passive scalar released from a point source in a turbulent boundary layer Kapil Chauhan, Murali Krishna Talluru, Jimmy Philip Measurements in a turbulent boundary layer are performed to document the statistics of a passive scalar when released from a point source in the logarithmic region. The nominal Reynolds number is $Re_\tau = \delta U_\tau/\nu\approx 8500$, where $\delta\approx 0.36$\,m is the boundary layer thicknes, $U_\tau$ is the skin-friciton velocity and $\nu$ is the kinemactic viscosity. Simultaneous single-point measurements are performed using a combination of hot-wire and photo-ionisation detector traversing in the wall-normal direction. The tracer gas (1.5\% iso-butylene) is released at a streamwise distance, $s_x/\delta$ = 1, upstream of the test location with the exit velocity matched to the local mean velocity at the source height. Preliminary results adhere to the known reflected Gaussian behaviour for the mean and variance profiles of scalar fluctuations. Also, we find support for the exponential probability distribution of scalar at $z=s_z$, which is extended to other wall-normal locations. Further, results on the interaction between large-scale velocity fluctuations and scalar fluctuations will be discussed. [Preview Abstract] |
Monday, November 21, 2016 12:37PM - 12:50PM |
H32.00010: Scaling analysis of the mean and variariance of temperature in a developing thermal boundary layer Clayton Byers, Marcus Hultmark A developing thermal boundary layer in a turbulent boundary layer is investigated both theoretically and experimentally. A scaling analysis of the mean temperature field and temperature variance, $\frac{1}{2}\overline{\theta^{2}}$ , is developed by utilizing the “Asymptotic Invariance Principle” developed by George and Castillo (1997), including the possible effects of the Reynolds and Prandtl number. The derived solution for the inner and outer scaling is then used to develop a “heat transfer law” for the wall heat flux, $q_{w}$. The condition of constant wall temperature is utilized, with an analysis of the temperature field treated as a passive scalar through ensuring the temperature differences remain small. Data collection is performed with a nanoscale temperature sensor, providing an improvement to performance over previous cold wire data acquisition. [Preview Abstract] |
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