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 A33: Turbulence: Coherent Structures and Taylors Hypothesis |
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Chair: Beverley McKeon, California Institute of Technology Room: Oregon Ballroom 202 |
(Author Not Attending)
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A33.00001: Roll/streak Structure Instability Induced by Free-stream Turbulence in Couette Flow Brian Farrell, Petros Ioannou, Marios Nikolaidis Statistical state dynamics (SSD) provides a new perspective for studying mechanisms underlying turbulence in shear flow including instabilities which arise intrinsically from interaction between coherent and incoherent components of the turbulence. Implementations of SSD in the form of a closure at second order is used in this work to analyze the instability emergent from the statistical interaction between coherent perturbations of roll/streak form and the incoherent free-stream turbulence in a minimal channel configuration of Couette flow. By perturbing the nonlinear SSD dynamics a new manifold of stable modes with roll/streak structure is shown to exist in the presence of small amplitude free-stream turbulence. With increase in a parameter controlling the free-stream turbulence energy, a member of this set of stable roll/streak structures is destabilized at a bifurcation and the associated roll/streak eigenmode is found to equilibrate at finite amplitude. The bifurcation structure predicted by the SSD roll/streak instability is reflected in both a closely related quasi-linear dynamical system, referred to as the restricted non-linear (RNL) system, and in DNS. This correspondence is further verified using ensemble implementations of the RNL and DNS systems. [Preview Abstract] |
Sunday, November 20, 2016 8:13AM - 8:26AM |
A33.00002: Structure and mechanism of turbulence under dynamical restriction in plane Poiseuille flow Navid Constantinou, Brian Farrell, Petros Ioannou, Javier Jimenez, Adrian Lozano-Duran, Marios-Andreas Nikolaidis The perspective of Statistical State Dynamics (SSD) is used to investigate plane Poiseuille turbulence at moderately high Reynolds numbers ($Re_\tau\approx940$). Simulations of a quasi-linear restricted nonlinear dynamics (RNL), which is an approximation to the full SSD, provide insight into the mechanism and structure of turbulent flow. RNL dynamics spontaneously limits the support of its turbulence to a small set of streamwise Fourier components giving rise to a natural minimal representation of its turbulence dynamics. Although greatly simplified, this RNL turbulence exhibits natural-looking structures and turbulent statistics. RNL turbulence at the Reynolds numbers studied is dominated by the roll/streak structure in the buffer layer and similar very-large-scale structure (VLSM) in the outer layer. Diagnostics of the structure, spectrum and energetics of RNL and DNS turbulence are used to demonstrate that the roll/streak dynamics supporting the turbulence in the buffer and logarithmic layer is essentially similar in RNL and DNS. This mechanism, which has analytical expression in the SSD, comprises a cooperative interaction between the coherent streamwise mean flow and the incoherent turbulent perturbations. [Preview Abstract] |
Sunday, November 20, 2016 8:26AM - 8:39AM |
A33.00003: On the structure of pressure fluctuations of self-sustaining attached eddies Minjeong Cho, Haecheon Choi, Yongyun Hwang A numerical experiment, which isolates the energy-containing motions at a prescribed spanwise length scale, was recently performed by Hwang (2015, J. Fluid Mech., 767:254-289). In his study, the velocity structures of these motions were shown to emerge in the form of Townsend’s attached eddies. In the present study, pressure fluctuations of the self-sustaining attached eddies are analyzed for turbulent channel flow at $Re_\tau=2000$. The second-order moment and the spectra of the pressure field of each attached eddy are found to be self-similar with respect to the given spanwise size, which is consistent with the velocity statistics. Depending on the nature of the source terms in the pressure Poisson equation, the pressure field of each attached eddy is also decomposed into the rapid and slow parts: the former describes linear interaction of the velocity fluctuation with mean shear, while the latter represents nonlinear interactions between the velocity fluctuations. In this talk, a detailed discussion will be provided with particular emphasis on the role of the rapid and slow parts of the pressure fluctuations in relation to its statistical and dynamical features. [Preview Abstract] |
Sunday, November 20, 2016 8:39AM - 8:52AM |
A33.00004: Large-scale structures in turbulent Couette flow Jung Hoon Kim, Jae Hwa Lee Direct numerical simulation of fully developed turbulent Couette flow is performed with a large computational domain in the streamwise and spanwise directions (40$\pi h$ and 6$\pi h)$ to investigate streamwise-scale growth mechanism of the streamwise velocity fluctuating structures in the core region, where $h$ is the channel half height. It is shown that long streamwise-scale structures ($>$3$h)$ are highly energetic and they contribute to more than $80\% $ of the turbulent kinetic energy and Reynolds shear stress, compared to previous studies in canonical Poiseuille flows. Instantaneous and statistical analysis show that negative-$u' $ structures on the bottom wall in the Couette flow continuously grow in the streamwise direction due to mean shear, and they penetrate to the opposite moving wall. The geometric center of the log layer is observed in the centerline with a dominant outer peak in streamwise spectrum, and the maximum streamwise extent for structure is found in the centerline, similar to previous observation in turbulent Poiseuille flows at high Reynolds number. Further inspection of time-evolving instantaneous fields clearly exhibits that adjacent long structures combine to form a longer structure in the centerline. [Preview Abstract] |
Sunday, November 20, 2016 8:52AM - 9:05AM |
A33.00005: An alternative to Reynolds stresses in turbulent channels Javier Jimenez It is remarked that fluxes in conservation laws, such as the Reynolds stresses in the momentum equation of turbulent shear flows, or the spectral energy flux in isotropic turbulence, are only defined up to an arbitrary solenoidal field. While this is not usually significant for long-time averages, it becomes important when fluxes are modelled locally in large-eddy simulations, or in the analysis of intermittency and cascades. As an example, a numerical procedure is introduced to compute fluxes in scalar conservation equations in such a way that their total integrated magnitude is minimised. The result is an irrotational vector field that derives from a potential, thus minimising sterile flux `circuits'. The algorithm is generalised to tensor fluxes and applied to the transfer of momentum in a turbulent channel. The resulting instantaneous Reynolds stresses are compared with their traditional expressions, and found to be substantially different. [Preview Abstract] |
Sunday, November 20, 2016 9:05AM - 9:18AM |
A33.00006: Effects of Taylor-Görtler vortices on turbulent flows in a spanwise-rotating channel Yijun Dai, Weixi Huang, Chunxiao Xu Fully developed turbulent channel flow with spanwise rotation has been studied by direct numerical simulation at \textit{Re}$_{m}=$2800, 7000 and 20000 with rotation number 0$\le $\textit{Ro}$_{m}\le $0.5. The width of the computational box is adjusted for each case to contain two pairs of Taylor-Görtler (TG) vortices. Under a low rotation rate, the turbulent vortical structures are strongly affected by the TG vortices. A conditional average method is employed to investigate the effects. In the upwash region where the fluid is pumped away from the pressure wall by the TG vortices, turbulence is enhanced, while the reverse is the case in the downwash region. Through budget analysis of the transport equation of vorticity fluctuation, it is revealed that the stretching along the wall-normal direction caused by the TG vortices plays an important role in initiating the difference of turbulence intensity between the two regions, which is further augmented by the Coriolis force in the streamwise direction. The effects of TG vortices is weakened at higher Reynolds number. Meanwhile, the shear stress on the suction wall is observed to fluctuate in a quasi-periodic manner at \textit{Re}$_{m}=$7000 and \textit{Ro}$_{m}=$0.3, which is induced by the TG vortices. [Preview Abstract] |
Sunday, November 20, 2016 9:18AM - 9:31AM |
A33.00007: A theory for coupled uniform momentum zones and vortical fissures in turbulent wall flows Brandon Montemuro, Joe Klewicki, Chris White, Greg Chini Both field observations and laboratory experiments suggest that at high Reynolds numbers $Re$ the outer region of incompressible turbulent wall flows self-organizes into uniform momentum zones (UMZs) separated by internal shear layers called `vortical fissures' (VFs). In this investigation, a candidate flow configuration is identified that has the potential to generate a self-sustaining interaction between a single VF and adjacent UMZs. Large-$Re$ asymptotic analysis is used to derive coupled, reduced sets of equations that elucidate the dominant physical processes operative in the different regions of the flow. The results indicate that large-scale, streamwise roll modes can act as a homogenizing agent that leads to the formation of the UMZs while simultaneously producing a concentrated region of spanwise vorticity that comprises the VF. The analysis also highlights possible feedback mechanisms between the VF and UMZs that may enable their self-sustenance. [Preview Abstract] |
Sunday, November 20, 2016 9:31AM - 9:44AM |
A33.00008: A Scale-by-Scale Linear Analysis of Convective Velocities and Taylor's Hypothesis in Turbulent Channel Flows Ismail Hameduddin, Dennice Gayme We examine convective velocities in turbulent channel flows using a linear, stochastically forced model approximation of the Navier-Stokes equations. The resulting system is analytically tractable and includes the terms that are typically associated with the break-down of Taylor's hypothesis. We show that this approach leads to convective velocity predictions that are consistent with DNS. We demonstrate that our observed differences between the local mean and convective velocities can be attributed to the dependence of the phase velocities on both the streamwise and spanwise wavelengths. The convective velocity in the viscous sublayer is roughly constant and distinct from the mean velocity. Previous work suggests that this viscous sublayer convective velocity arises due to (a) buffer layer rolls and (b) large-scale outer layer structures that influence the near-wall region. We show that there is also a series of structures, self-similar in the cross-stream plane, that modify the convective velocity in the sublayer. The streamwise extent of these structures scales as the square of the cross-stream dimensions, which is similar to previously proposed scalings of near-wall spectra based on DNS. This work is supported by a Johns Hopkins University Catalyst Award. [Preview Abstract] |
Sunday, November 20, 2016 9:44AM - 9:57AM |
A33.00009: Assessing the role of spanwise roughness heterogeneity and establishing conditions for application of Townsend's hypothesis of outer-layer similarity. Jianzhi Yang, William Anderson In the present work, turbulent secondary flows in turbulent channel flows are forced by virtue of passive-actuator-like topographic features. The topographies are constructed with streamwise-repeating pyramidal topographic elements which create ``rows'' with infinite streamwise extent; then, the spacing between such rows is systematically varied. The spanwise spacing is varied from 0.2 to 6.4. For spanwise spacing exceeding double the channel half-height, free domain-scale secondary flows are observed. The domain-scale secondary flows exist in the Reynolds-averaged statistics, and remain permanently positioned such that downwelling and upwelling occurs above the elements and flat region, respectively (i.e., we have introduced low- and high-momentum pathways; Ken Christensen et al.). At the element scale, counter-rotating vortices flank the roughness elements, with rotational sign opposite to the larger-scale circulations. When spanwise spacing is less than double the channel half-height, the domain-scale secondary flows attenuate, while the element-scale secondary flows remain for all spacings. Since the domain-scale circulations alter the outer-layer turbulence statistics, we are working to assess spacing as a control on the efficacy of Townsend's hypothesis of outer-layer similarity in turbulent channel flow. [Preview Abstract] |
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