Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session D23: Flow Instability: Transition to TurbulenceInstabilities Turbulence
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Chair: Fernando Grinstein, Los Alamos National Laboratory Room: 710 |
Sunday, November 19, 2017 2:15PM - 2:28PM |
D23.00001: Simulations and Modeling for Shock Driven Turbulence Fernando Grinstein, Juan Saenz, Rick Rauenzahn Transition can be captured by a large eddy simulation (LES) strategy, but not by a Reynolds-Averaged Navier-Stokes (RANS) approach based on equilibrium turbulence assumptions and single-point-closure modeling. However, with suitable initialization around each transition -- e.g., reshock, RANS can be used to approximately predict subsequent near-equilibrium flow statistics. We demonstrate state-of-the-art 3D RANS performance in one such flow regime [1]. We simulate the CEA planar shock-tube experiments by Poggi et al. (1998) with an implicit LES (ILES) strategy. CEA turbulence mixing and velocity data are used for benchmarking ILES; in turn, ILES generated data is used to initialize and as reference to assess state-of-the-art 3D RANS. ILES is based on the xRAGE code run on the 'clean' mode, whereas RANS uses xRAGE with activated BHR3. We find that by prescribing physics-based (ILES generated) 3D initial conditions and allowing for 3D flow convection with just enough resolution, the additionally computed dissipation in 3D RANS effectively blends with the modeled dissipation -- rather than multiple-counting effects -- to yield significantly improved statistical predictions. \textit{[1] F.F.Grinstein, Computers and Fluids 151 (2017) 58--72}. [Preview Abstract] |
Sunday, November 19, 2017 2:28PM - 2:41PM |
D23.00002: Direct Numerical Simulation of a Plane Transitional Wall Jet. O Ramesh, Joel Varghese A transitional plane wall jet is studied using direct numerical simulation. The presence of an inflectional point leads to the outer layer rolling up into vortices that interacts with the inner region resulting in a double array of counter rotating vortices before breakdown into turbulence. Past studies have focused on forced wall jet which results in shorter transition region and prominent vortical structures. In the present work, natural transition will be discussed by analysing the coherent structures and scaled frequency spectra. Clear hairpin like structures leaning downstream in the inner region(as in a boundary layer) and leaning upstream in the outerstream (as in a jet) are evident.. [Preview Abstract] |
Sunday, November 19, 2017 2:41PM - 2:54PM |
D23.00003: Transition to turbulence in reciprocating channel flow Christopher White, Alireza Ebadi, Ian Pond, Yves Dubief Direct numerical simulation of reciprocating channel flow is used to study transition to turbulence in periodic flows. The simulations are performed at two Stokes Reynolds numbers: $Re_s = 648$ and $1019$, representing type III (self-sustaining transition) and type IV (intermittently turbulent) flow regimes, respectively. It is found that the underlying mechanism of transition to turbulence is the emergence of an internal layer that first develops during the phases just prior to the onset of turbulence. In the absence of this internal layer (i.e., at $Re_s=648$), the flow remains transitional over the entire cycle. An analysis of instantaneous spanwise vorticity contours suggests that the internal layer is likely formed from the concatenation of strong opposite sign (relative to the mean) vorticity concentrated in the near-wall region. This concentrated region of near-wall opposite sign vorticity leads to the rapid production of Reynolds stress and Reynolds stress divergence that underlie transition to turbulence. The flow moves towards relaminarization during the late phases in the decelerating portion of the cycle when the concentrated region of opposite sign vorticity moves toward the center of the channel and the near-wall production of Reynolds stress is diminished. [Preview Abstract] |
Sunday, November 19, 2017 2:54PM - 3:07PM |
D23.00004: Self-sustained turbulence at the Kolmogorov Microscale. Ashley Willis, Qiang Yang, Yongyun Hwang Invariant solutions have been identified in shear flows that exist to as small as the Kolmogorov microscale, e.g. Deguchi (2015). For these solutions, which are steady in a moving frame, energy production and dissipation is in perfect balance. For developed turbulent flow, however, eddies at this scale are expected to be driven mainly by the energy cascade from larger scales. In this work we show that an energy production mechanism indeed exists at the Kolmogorov scale in simulations of turbulence. A uniform shear flow is generated in the Couette geometry by artificially limiting the spanwise dimension, where attached eddies are limited to this dimension. In a narrow periodic box of minimal spanwise wavelength, no scale separation exists between production and dissipation, as in transitional flow, and the self-sustaining mechanism is found to be consistent with the invariant solutions. [Preview Abstract] |
Sunday, November 19, 2017 3:07PM - 3:20PM |
D23.00005: The shape of turbulent spots in plane Couette flow Marie Couliou, Romain Monchaux We numerically investigate the temporal aspects of turbulent spots spreading in a plane Couette flow for transitional Reynolds numbers between 300 and 450. We focus on the spreading along the streamwise direction and on the shape of turbulent spots. Studying the topology of turbulent spots and the associated large-scale flows velocity, we suggest a decomposition of the streamwise growth rate. On one hand, the quadripolar large-scale flow steers inside the spot along the streamwise direction and slows down the growth. The associated growth rate is negative. On the other hand, we can also define positive growth rate associated to inside large-scale flow which enables the convection of the streaks. The total of these two growth rates is compared to the spot streamwise growth rate and shows good agreement. The resulting shape of the spot is then discussed. A scenario that gathers all these elements is providing a better understanding of the growth dynamics and the shape of turbulent spots in plane Couette flow that should possibly apply to other extended shear flows. [Preview Abstract] |
Sunday, November 19, 2017 3:20PM - 3:33PM |
D23.00006: Nonlinear optimal perturbations in a curved pipe Enrico Rinaldi, Jacopo Canton, Oana Marin, Michel Schanen, Philipp Schlatter We investigate the effect of curvature on transition to turbulence in pipes by comparing optimal perturbations of finite amplitude that maximise their energy growth in a toroidal geometry to the ones calculated in the absence of curvature. Our interest is motivated by the fact that even small curvatures, of the order of $d=R_{pipe}/R_{torus}<10^{-10}$, are sufficient to induce substantial changes to the laminar velocity profile and its linear stability, thus suggesting that peculiarities may also be observed in the nonlinear regime. In our study, we consider flows at several subcritical Reynolds numbers in a torus with $d=0.01$. We use state-of-the-art numerical algorithms, capable of tackling the optimisation problem on large computational domains, coupled to a high-order spectral-element code, which is used to perform direct numerical simulations (DNS) of the full Navier-Stokes and their adjoint equations. Results are compared to the corresponding states in straight pipes and differences in their structure and evolution are discussed. Furthermore, the newly calculated initial conditions are used to identify coherent flow structures that are compared to the ones observed in recent DNS of weakly turbulent and relaminarising flows in the same toroidal geometry. [Preview Abstract] |
Sunday, November 19, 2017 3:33PM - 3:46PM |
D23.00007: Universality of the transition to turbulence in Couette flow Grégoire Lemoult, Björn Hof Turbulence is one of the most frequently encountered non-equilibrium phenomena in nature, yet characterizing the transition that gives rise to turbulence in basic shear flows has remained an elusive task. Although, in recent studies, critical points marking the onset of sustained turbulence have been determined for several such flows, the physical nature of the transition could not be fully explained. More recently, in extensive experimental and computational studies, Lemoult \textit{et al.} show for the example of Couette flow that the onset of turbulence is a second-order phase transition and falls into the directed percolation universality class. Consequently, the complex laminar--turbulent patterns result from short-range interactions of turbulent domains and are characterized by universal critical exponents. In the present contribution, we present new experimental results, for the transition to turbulence in Taylor-Couette flow. We measured two new quantities, namely the correlation length and the correlation time, exhibiting universal scaling in agreement with the universality class of directed percolation. [Preview Abstract] |
Sunday, November 19, 2017 3:46PM - 3:59PM |
D23.00008: Interactions and "puff clustering" close to the critical point in pipe flow Mukund Vasudevan, Bj{\"o}rn Hof The first turbulent structures to arise in pipe flow are puffs. Albeit transient in nature, their spreading determines if eventually turbulence becomes sustained. Due to the extremely long time scales involved in these processes it is virtually impossible to directly observe the transition and the flow patterns that are eventually assumed in the long time limit. We present a new experimental approach where, based on the memoryless nature of turbulent puffs, we continuously recreate the flow pattern exiting the pipe. These periodic boundary conditions enable us to show that the flow pattern eventually settles to a statistically steady state. While our study confirms the value of the critical point of $Re_{c} \approx 2040$, the flow fields show that puffs interact over longer ranges than previously suspected. As a consequence puffs tend to cluster and these regions of large puff densities travel across the puff pattern in a wave like fashion. While transition in Couette flow has been shown to fall into the "directed percolation", pipe flow may be more complicated since long range interactions are prohibited for the percolation transition type. Extensive measurements at the critical point will be presented to clarify the nature of the transition. [Preview Abstract] |
Sunday, November 19, 2017 3:59PM - 4:12PM |
D23.00009: The laws of resistance in transitional pipe flows Rory Cerbus, Chien-chia Liu, Gustavo Gioia, Pinaki Chakraborty As everyone knows who has opened a kitchen faucet, pipe flow is laminar at low flow velocities and turbulent at high flow velocities. At intermediate velocities there is a transition wherein plugs of laminar flow alternate along the pipe with ``flashes" of a type of fluctuating, non-laminar flow which remains poorly understood. In the 19th century, Osborne Reynolds, who first reported flashes, sought to connect these states of flow with quantitative ``laws of resistance" whereby the fluid friction is determined as a function of the Reynolds number. While he succeeded for laminar and turbulent flows, the laws for transitional flows eluded him and remain unknown to this day. By properly distinguishing between flashes and laminar plugs in the transitional regime, we show experimentally and numerically that the law of resistance for laminar plugs corresponds to the laminar law and the law of resistance for flashes is identical to that of turbulence. [Preview Abstract] |
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