Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session L32: Turbulence: Internal Flows |
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Chair: Roberto Verzicco, Universita degli Studi di Roma Tor Vergata Room: 2020 |
Monday, November 24, 2014 3:35PM - 3:48PM |
L32.00001: Direct numerical simulations of Taylor-Couette up to Re=400,000 Rodolfo Ostilla Monico, Roberto Verzicco, Siegfried Grossmann, Detlef Lohse Taylor-Couette, the flow between two coaxial, independently rotating cylinders, is simulated up to shear Reynolds numbers of $Re_s\sim4\cdot 10^{5}$, corresponding to frictional Reynolds numbers of $Re_\tau\sim 4000$. The radius ratio is set to $\eta=r_i/r_o=0.909$, to reduce curvature effects and axially periodic boundary conditions are used. One-point statistics and spectra are calculated. Using these, the flow is divided into boundary layer and bulk. The boundary layer, containing a logarithmic sublayer is found to extend up to a tenth of the gap width from the wall. This log-layer shows comparable one-point statistics and spectra to log-layers in other DNS of channels, i.e. $\kappa\approx 0.4$ and a $k^{-1}$ energy spectra in the streamwise (azimuthal) velocity. Further away from the wall, the flow is modified by the large scale Taylor rolls, and a bulk region appears, with very different behaviour. Finally, the effect of the computational domain size on the flow is quantified, and larger computational boxes are compared to the ones used at high $Re$. [Preview Abstract] |
Monday, November 24, 2014 3:48PM - 4:01PM |
L32.00002: Direct numerical simulation of Taylor-Couette flows between two co-rotating cylinders with radial heating Hao Teng, Nansheng Liu, Xiyun Lu In the present work, Taylor-Couette (TC) flows subjected to radial heating have been investigated by using of direct numerical simulation (DNS). For our simulations, the base flow is driven between two co-rotating concentric cylinders; radial heating is modeled by a radial temperature gradient between the hot inner and cold outer cylinders, which introduces a coupling effect characterized by a ratio ($\sigma $ Gr/Re$^{2})$ between the buoyancy force driving fluid elements moving axially and the centrifugal force driving a radial fluid motion. Here, the Grashof (Gr) and Reynolds (Re) numbers represent the non-dimensionalized buoyancy and centrifugal forces, respectively. It is demonstrated that increasing $\sigma $ from 0 to 0.4 leads to a flow state transition from the wavy Taylor vortex (WTV) TC flows driven by the centrifugal force to the distorted Taylor vortex (DTV) TC flows arising as a result of the buoyancy and centrifugal force competition, and eventually to the buoyancy dominated turbulent (BDT) TC flows identified as the vanishing of Taylor vortices that are replaced by turbulent vertical structures of small scales. These three flow states are distinguished by their flow structures, dynamical properties, and energy spectra. [Preview Abstract] |
Monday, November 24, 2014 4:01PM - 4:14PM |
L32.00003: The phase space of turbulent Taylor-Couette flow Detlef Lohse, Rodolfo Ostilla Monico, Erwin van der Poel, Roberto Verzicco, Siegfried Grossmann Direct numerical simulations of Taylor-Couette flow, i.e. the flow between two coaxial and independently rotating cylinders were performed. Shear Reynolds numbers of up to $3\cdot10^5$, corresponding to $Ta=4.6\cdot10^{10}$, were reached. The transition to the ultimate regime, in which asymptotic scaling laws for the torque are expected to hold up to arbitrarily high driving, is analysed for different radius ratios, different aspect ratios and different rotation ratios. We also calculate the local angular velocity profiles and visualize different flow regimes that depend both on the shearing of the flow and the Coriolis force originating from the outer cylinder rotation. Two main regimes are distinguished, based on the magnitude of the Coriolis force, namely the co-rotating and weakly counter-rotating regime dominated by Rayleigh-unstable regions, and the strongly counter-rotating regime where a mixture of Rayleigh-stable and Rayleigh-unstable regions exist. The work culminates in phase spaces in the inner vs outer Reynolds number parameter space and in the Taylor vs inverse Rossby number parameter space, which can be seen as the extension of the Andereck \emph{et al.} (J. Fluid Mech. 164, 155-183, 1986) phase space towards the ultimate regime. [Preview Abstract] |
Monday, November 24, 2014 4:14PM - 4:27PM |
L32.00004: Direct numerical simulation of turbulent plane Couette flow at Rew=6000 Jie Gai, Zhenhua Xia, Qingdong Cai The large-scale counter-rotating streamwise vortices (secondary vortices) in fully-developed plane Couette flow have been reported by both experimental and numerical communities. However, the number of vortex pairs does not increase linearly with the spanwise width of the domain, which is the same as the reported results of the turbulent Taylor-Couette flow. In present work, a series of direct numerical simulations at Reynolds number of 6000 (based on the relative wall speed and half the channel height $\delta$) with different streamwise and spanwise lengths were conducted to investigate the effect of the box size on the large-scale structures. Our results showed that the correlation of secondary structures in the box with smaller streamwise length is much stronger than that with larger streamwise length and the rate of the turbulent kinetic energy contributed by the secondary structures is influenced by the mean spanwise scale of the secondary eddies and the streamwise length of the computational domain. In addition, the spanwise length scale of each secondary vortex pairs was found in a domain of $4(1\pm0.3)\delta$. [Preview Abstract] |
Monday, November 24, 2014 4:27PM - 4:40PM |
L32.00005: Coherent structures in turbulent rectangular duct flows Hassan Nagib, Ricardo Vinuesa, Cezary Prus, Philipp Schlatter Turbulent duct flows computed by means of DNS with spectral element code Nek5000 are analyzed to characterize coherent structures present in this flow. A number of aspect ratio (defined as duct width over height) cases ranging from 1 to 18 at two different Reynolds numbers ($Re_{\tau} \simeq 180$ and 330) constitute the data set under study. Common methods for coherent vortex identification ($\lambda_{2}$, $Q$, $\lambda_{ci}$ and $\Delta$), together with a less widely used approach by Kida and Miura, are used to characterize the structures in the various duct cases. All methods yield very similar results, and identify the occurrence of buffer layer vortices along the horizontal and side walls, with the well-documented spacing between streaks of $\simeq 100^{+}$. Secondary vortices in the duct corners are only found from two-dimensional fields when averaged over time and streamwise computational domain. The results indicate that the corner vortices may exhibit much slower time-scales than buffer layer vortices. The concept of turbulent net force is also applied to the 2D fields to assess impact of the corner vortices on the flow. The main features of these structures are compared with the ones found in spanwise-periodic turbulent channel flows at the same Reynolds numbers. [Preview Abstract] |
Monday, November 24, 2014 4:40PM - 4:53PM |
L32.00006: The effect of large property fluctuations on turbulent heat transfer to supercritical pressure fluids in pipes Rene Pecnik, Hassan Nemati, Ashish Patel, Bendiks Jan Boersma When a fluid slightly above the thermodynamic critical pressure is heated, such that the fluid's state crosses the pseudo-critical line, no distinct liquid to gas phase transition occur. However, the fluid properties change abruptly. If these property variations occur in a turbulent flow the conventional behavior of turbulence is strongly altered. We study the influence of these large property fluctuations in forced convection heat transfer to supercritical carbon dioxide in a pipe, with DNS at a Karman number of Re$=$180 (based on the pipe inlet conditions). At the inlet the temperature is slightly below the pseudo-critical point, such that during the heating process the developing thermal boundary layer crosses the pseudo-critical line. We show that the occurring property fluctuations have a strong effect on the averaged wall enthalpy if a constant wall heat flux boundary condition (infinite thermal effusivity ratio of fluid to solid) is used. By changing the boundary conditions to constant wall temperature (vanishing thermal effusivity ratio) these fluctuations are eliminated at the wall and the heat transfer coefficient is decreased. [Preview Abstract] |
Monday, November 24, 2014 4:53PM - 5:06PM |
L32.00007: Sparse energetically dominant frequencies in direct numerical simulation of turbulent pipe flow: origin and application to reduced-order models Francisco Gomez, Hugh M. Blackburn, Murray Rudman, Beverley J. McKeon, Mitul Luhar, Rashad Moarref, Ati S. Sharma The idea of constructing reduced-order models for canonical wall-bounded turbulent flows based on exploiting the sparse energetically dominant frequencies observed in direct numerical simulation of pipe flow by Bourguignon {\em et al.} (2013, {\em Phys. Fluids}) is examined. The resolvent analysis of a pipe flow is extended in order to consider the influence of finite domain length on the flow dynamics, which restricts the possible wavespeeds in the flow. This analysis shows that large sparse amplifications take place when one of the allowable wavespeeds is equal to the local wavespeed via the critical layer mechanism. A connection between amplification and energy is presented through the similar features displayed by the most energetically relevant flow structures, emerging from a dynamic mode decomposition of direct numerical simulation data, and the resolvent modes associated with the most amplified sparse frequencies. These findings support the viability of reduced-order models based on the selection of the most amplified modes arising from the resolvent model, with the potential to drastically decrease the computational costs required to represent turbulent flows. [Preview Abstract] |
Monday, November 24, 2014 5:06PM - 5:19PM |
L32.00008: Experimental study of a turbulent flow under transient conditions Shuisheng He, Sam Gorji, Mehdi Seddighi, Tom O'Donoghue, Dubravka Pokrajac, Alan Vardy Particle Image Velocimetry (PIV) is applied to investigate the behaviour of transient turbulent channel flows. During the experiments, the flowrate is accelerated from a lower Re turbulent flow to one at a higher Re. The investigations reveal novel insights into turbulence behaviour in the transient process. It is shown that the unsteady flows behave strikingly similar to the so-called boundary layer bypass transition due to free-stream-turbulence. Consistent with the DNS of He and Seddighi (J. Fluid Mech., 715: 60-102), the process begins with the elongation of streaks much similar to the Klebanoff modes in the buffeted laminar boundary layer in a bypass transition. During the second stage, the formation and propagation of isolated turbulent spots eventually lead to a complete breakdown of the organised streaky structures resulting in a new turbulent flow corresponding to the final Reynolds number. The present investigation covers a range of initial and final Reynolds numbers to elucidate the underlying mechanisms involved in transient flows. [Preview Abstract] |
Monday, November 24, 2014 5:19PM - 5:32PM |
L32.00009: Transition to Turbulence in curved pipe Amirreza Hashemi, Francis Loth Studies have shown that transitional turbulence in a curved pipe is delayed significantly compared with straight pipes. These analytical, numerical and experimental studies employed a helical geometry that is infinitely long such that the effect of the inlet and outlet can be neglected. The present study examined transition to turbulence in a finite curved pipe with a straight inlet/outlet and a 180 degrees curved pipe with a constant radius of curvature and diameter (D). We have employed the large scale direct numerical simulation (DNS) by using the spectral element method, nek5000, to simulate the flow field within curved pipe geometry with different curvature radii and Reynolds numbers to determine the point of the transition to turbulence. Long extensions for the inlet (5D) and outlet (20D) were used to diminish the effect of the boundary conditions. Our numerical results for radius of curvatures of 1.5D and 5D show transition turbulence is near Re$=$3000. This is delayed compared with a straight pipe (Re$=$2200) but still less that observed for helical geometries (Reynolds number less than 5000). Our research aims to describe the critical Reynolds number for transition to turbulence for a finite curved pipe at various curvature radii. [Preview Abstract] |
Monday, November 24, 2014 5:32PM - 5:45PM |
L32.00010: Direct numerical simulation of turbulence and heat transfer in a hexagonal shaped duct Oana Marin, Aleks Obabko, Philipp Schlatter Flows in hexagonal shapes frequently occur in nuclear reactor applications, and are also present in honeycomb-shaped settling chambers for e.g. wind tunnels. Whereas wall-bounded turbulence has been studied comprehensively in two-dimensional channels, and to a lesser degree also in square and rectangular ducts and triangles, only very limited data for hexagonal ducts is available, including resistance correlations and mean profiles. Here, we use resolved spectral-element simulations to compute velocity and temperature in fully-developed (periodic) hexagonal duct flow. The Reynolds number, based on the fixed flow rate and the hydraulic diameter, ranges between 2000 and 20000. The temperature assumes constant wall flux or constant wall temperature. First DNS results are focused on the mean characteristics such a head loss, Nusselt number, and critical Reynolds number for sustained turbulence. Profiles, both for mean and fluctuating quantities, are extracted and discussed in the context of square ducts and pipes. Comparisons to existing experiments, RANS and empirical correlations are supplied as well. The results show a complicated and fine-scale pattern of the in-plane secondary flow, which clearly affects the momentum and temperature distribution throughout the cross section. [Preview Abstract] |
Monday, November 24, 2014 5:45PM - 5:58PM |
L32.00011: Osborne Reynolds pipe flow: direct numerical simulation from laminar to fully-developed turbulence R.J. Adrian, X. Wu, P. Moin, J.R. Baltzer Osborne Reynolds' pipe experiment marked the onset of modern viscous flow research, yet the detailed mechanism carrying the laminar state to fully-developed turbulence has been quite elusive, despite notable progress related to dynamic edge-state theory. Here, we continue our direct numerical simulation study on this problem using a 250R long, spatially-developing pipe configuration with various Reynolds numbers, inflow disturbances, and inlet base flow states. For the inlet base flow, both fully-developed laminar profile and the uniform plug profile are considered. Inlet disturbances consist of rings of turbulence of different width and radial location. In all the six cases examined so far, energy norms show exponential growth with axial distance until transition after an initial decay near the inlet. Skin-friction overshoots the Moody's correlation in most, but not all, the cases. Another common theme is that lambda vortices amplified out of susceptible elements in the inlet disturbances trigger rapidly growing hairpin packets at random locations and times, after which infant turbulent spots appear. Mature turbulent spots in the pipe transition are actually tight concentrations of hairpin packets looking like a hairpin forest. The plug flow inlet profile requires much stronger disturbances to transition than the parabolic profile. [Preview Abstract] |
Monday, November 24, 2014 5:58PM - 6:11PM |
L32.00012: A Visualization Study of Wall Layer of Swirling Turbulent Pipe Flow Meriam Malek, Rachael Hager, Omer Savas The streaky vortical structure of the viscous sublayer of a turbulent boundary layer is well known. Turbulent flows in pipes also exhibit similar structures. The effect of swirl on that structure is the subject matter of this study. The experiments are conducted in water in a 5-cm diameter clear cast-acrylic pipe at Reynolds numbers up to 80,000. Initial geometric swirl angles up to 60$^\circ$ at the wall are generated by placing 3D printed inserts at the inlet of the pipe. Flows are visualized using reflective flakes of size distribution 10-80 $\mu$m under diffuse illumination. Flows are recorded at high framing rates. After preprocessing, the streaky structure is quantified by using autocorrelation of the images. Lateral spacing and longitudinal length scales are extracted. Also studied is the decay of the swirl angle and its influence of the wall structure. [Preview Abstract] |
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