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 L32: On Turbulent Channels and Boundary Layers |
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Chair: Ronald Panton, University of Texas Room: Oregon Ballroom 201 |
Monday, November 21, 2016 4:30PM - 4:43PM |
L32.00001: Pressure Fluctuations in Turbulent Wall Layers Ronald Panton, MyoungKyu Lee, Robert Moser Pressure fluctuation profile data from the channel flow DNS of Lee and Moser [\textit{J. Fluid Mech.}, vol 774, 2015] extend to $Re_\tau \approx 5200$. In the outer region, with $Y=y/h$, the overlap layer pressure correlates very well by a log law; $\lim_{Y \rightarrow 0}\langle p^2 \rangle^+ \sim (1/\eta) \ln Y + D_o$. The constant $\eta = - 0.380$ is remarkable like the von K\'{a}rm\'{a}n value. In the inner region, the defect variable $\mathcal{P} (y^+) \equiv \langle p^2 \rangle^+ - \langle p^2 \rangle^+ \vert_{y=0}$ absorbs the $Re_\tau$ dependence. The inner overlap equation is; $\lim_{y^+ \rightarrow \infty}\mathcal{P} \sim (1/\eta) \ln y^+ + D_i$. Together, the overlap laws imply that the wall pressure relation is $\langle p^2 \rangle^+ \vert_{y=0} \sim (-1/\eta) \ln Re_\tau + D_i - D_o$. A completely equivalent expression, which is finite as $Re_\tau \rightarrow \infty$, is obtained by rescaling the pressure variable; $\langle p^2 \rangle^\# \vert_{y=0} \equiv (u_\tau / U_o)\langle p^2 \rangle^+ \vert_{y=0} \ = C_1 + C_2 (u_\tau/U_o)$. Here, the constants are related to $\eta, D_0$, and $D_i$ . Additionally, it was found that the wavenumber spectrum $E_{pp} \{k_x/h\}$ does not have a $k^{-1}$ region. However, the trends do not rule out this at higher Re. [Preview Abstract] |
Monday, November 21, 2016 4:43PM - 4:56PM |
L32.00002: Vortex statistics in turbulent channel flows José Hugo Elsas, Luca Roberto Augusto Moriconi In order to address the role of coherent structures in wall bounded turbulence, we study the statistics of morphological and kinematic properties of vortices, such as circulation, radius and height distributions. To accomplish that, we introduce a novel vortex identification method named as "vorticity curvature criterion" which is based on the local properties of the vorticity field. We furthermore employ a background subtraction procedure to remove shearing background effects expected to be present in the topology of the streamwise/wall-normal plane flow configurations. We discuss, through a comparative study of performance with the usual swirling strength criterion, and extending the previous analyses to the detection of coherent structures in the spanwise/wall normal planes, isotropization issues for the paradigmatic case of numerical turbulent channel flows. [Preview Abstract] |
Monday, November 21, 2016 4:56PM - 5:09PM |
L32.00003: Direct numerical simulation of the fully developed turbulent boundary layer Melissa Kozul, Daniel Chung The term `fully developed' is commonly applied to channel and pipe flows that are statistically stationary in time and no longer exhibit streamwise development. Following the temporal turbulent boundary layer simulation of Kozul, Chung \& Monty (\textit{J. Fluid Mech.}, vol. 796, 2016, pp. 437-472) where streamwise development was removed with periodic boundary conditions, we now remove the remaining development in time, giving a turbulent boundary layer that is `fully developed' at finite Reynolds numbers. This is achieved by rescaling in the wall-normal direction and assuming arrested boundary-layer growth, motivated by a large-eddy turnover time estimated to be much shorter than the growth time scale of the boundary-layer. Analysis of outer-layer similarity shows that this setup, with only one additional computational term, gives a dominant balance equivalent to the high-Reynolds number asymptotics for both the spatially and temporally developing turbulent boundary layers. Our idealised, but non-physical simulation thus allows us to enforce the infinite Reynolds number dominant balance assumptions commonly made at finite Reynolds numbers. This simple setup could be used to generate inflow conditions for spatial simulations, or as a test case for model development and analysis. [Preview Abstract] |
Monday, November 21, 2016 5:09PM - 5:22PM |
L32.00004: Turbulent Boundary Layers and Sediment Suspension Absent Mean Flow-Induced Shear: An Experimental Study Blair Johnson, Edwin Cowen We investigate turbulent boundary layers in the absence of mean shear at solid and sediment boundaries in an experimental facility designed to generate high Reynolds number (Re$_{\mathrm{\lambda }}$\textasciitilde 300)horizontally homogeneous isotropic turbulence via randomly actuated synthetic jet arrays (RASJA - Variano {\&} Cowen 2008). One array is an 8 x 8 grid of jets, while the other is a 16 x 16 array. We control the turbulence levels, including the integral length scale and dissipation rate, by changing the mean on-times in the jet algorithm. Particle image velocimetry (PIV) measurements are used to study the isotropic turbulence and the boundary layer. The flow is characterized by statistical metrics such as the mean flow and turbulent velocities, turbulent kinetic energy, spectra, and integral length scale. We consider the turbulent kinetic energy transport equation and examine the relationships between dissipation, production, and turbulent transport, in absence of mean flows. We compare an impermeable flat boundary, a flat permeable sediment boundary, and a rippled sediment boundary. We find that while an immobile sediment bed acts as a sink of turbulence, an active sediment boundary with frequent suspension increases turbulent velocity fluctuations. Because traditional viscous stresses due to mean velocity gradients suggest no bed friction, we develop a method for considering Reynolds stresses over short time periods as a surrogate for understanding bed stress in a zero mean shear environment. [Preview Abstract] |
Monday, November 21, 2016 5:22PM - 5:35PM |
L32.00005: Estimation of turbulent channel flow based on the wall measurement with a statistical approach Yosuke Hasegawa, Takao Suzuki A turbulent channel flow at $Re_tau$ = 100 with periodic boundary conditions is estimated with linear stochastic estimation only based on the wall measurement, i.e. the shear-stress in the streamwise and spanwise directions as well as the pressure over the entire wavenumbers. The results reveal that instantaneous measurement on the wall governs the success of the estimation in y+ < 20. Degrees of agreement are equivalent to those reported by Chevalier et al. (2006) using a data-assimilation approach. This suggests that the instantaneous wall information dictates the estimation rather than the estimator solving the dynamical system. We feed the velocity components from the linear stochastic estimation via the body-force term into the Navier–Stokes system; however, the estimation slightly improves in the log layer, indicating some benefit of involving a dynamical system but over-suppression of turbulent kinetic energy beyond the viscous sublayer by the linear stochastic estimation. Motions inaccurately estimated in the buffer layer prevent from further reconstruction toward the centerline even if we relax the feedback forcing and let the flow evolve nonlinearly through the estimator. We also argue the inherent limitation of turbulent flow estimation based on the wall measurement. [Preview Abstract] |
Monday, November 21, 2016 5:35PM - 5:48PM |
L32.00006: Characterization of linear-like Orr bursts in fully turbulent channel flows Miguel P. Encinar, Javier Jimenez The linearised Orr-Sommerfield equation predicts that initially small perturbations of the cross-shear velocity become transiently amplified when tilted by the effect of a mean shear. Such transient behaviour can also be found in the large-scale structures of fully developed nonlinear shear turbulence, although affected by the non linearity of the flow. We investigate the dynamics of the bursting structures in properly filtered large-box turbulent channels at $Re_\tau = 950$, and find that all velocity components play an important role in their formation. This implies that their underlying geometry is three dimensional. We explore the latter using spatio-temporal conditionally averaged structures that show the formation of tilted rollers at the moment of the burst, and reveal a relation between the Orr-like bursts and the vertical momentum transfer. [Preview Abstract] |
Monday, November 21, 2016 5:48PM - 6:01PM |
L32.00007: Experimental measurements of a non-equilibrium thermal boundary layer flow Drummond Biles, Alireza Ebadi, Chris Whie Data from a newly constructed non-equilibrium and thermal boundary layer wind tunnel is presented. The bottom wall of the tunnel is a sectioned-wall design composed of twelve aluminum 6061 plates with resistive heaters adhered to their underside. Each section is heated and controlled using independent feedback loop controllers. The freestream temperature is controlled by an upstream array of resistive heaters and a feedback controller. Experimental data with strong perturbations that produce non-equilibrium boundary layer flow behaviors is presented. Data for ZPG conditions are provided for validation purposes, and the effects of non-equilibrium behaviors on the transport of momentum and heat are discussed. [Preview Abstract] |
Monday, November 21, 2016 6:01PM - 6:14PM |
L32.00008: Transitional boundary layer in low-Prandtl-number convection at high Rayleigh number Joerg Schumacher, Vinodh Bandaru, Ambrish Pandey, Janet Scheel The boundary layer structure of the velocity and temperature fields in turbulent Rayleigh-B\'{e}nard flows in closed cylindrical cells of unit aspect ratio is revisited from a transitional and turbulent viscous boundary layer perspective. When the Rayleigh number is large enough the boundary layer dynamics at the bottom and top plates can be separated into an impact region of downwelling plumes, an ejection region of upwelling plumes and an interior region (away from side walls) that is dominated by a shear flow of varying orientation. This interior plate region is compared here to classical wall-bounded shear flows. The working fluid is liquid mercury or liquid gallium at a Prandtl number of $Pr=0.021$ for a range of Rayleigh numbers of $3\times 10^5\le Ra \le 4\times 10^8$. The momentum transfer response to these system parameters generates a fluid flow in the closed cell with a macroscopic flow Reynolds number that takes values in the range of $1.8\times 10^3 \le Re\le 4.6\times 10^4$. It is shown that particularly the viscous boundary layers for the largest $Ra$ are highly transitional and obey some properties that are directly comparable to transitional channel flows at friction Reynolds numbers below 100. [Preview Abstract] |
Monday, November 21, 2016 6:14PM - 6:27PM |
L32.00009: Wall-resolved LES of high Reynolds number airfoil flow near stall condition for wall modeling in LES: LESFOIL revisited Kengo Asada, Soshi Kawai Wall-resolved large-eddy simulation (LES) of an airfoil flow involving a turbulent transition and separations near stall condition at a high Reynolds number 2.1 x 10$^{\mathrm{6}}$ (based on the freestream velocity and the airfoil chord length) is conducted by using K computer. This study aims to provide the wall-resolved LES database including detailed turbulence statistics for near-wall modeling in LES and also to investigate the flow physics of the high Reynolds number airfoil flow near stall condition. The LES well predicts the laminar separation bubble, turbulent reattachment and turbulent separation. The LES also clarified unsteady flow features associated with shear-layer instabilities: high frequency unsteadiness at St $\simeq $130 at the laminar separation bubble near the leading edge and low frequency unsteadiness at St $\simeq $1.5 at the separated turbulent shear-layer near the trailing edge. Regarding the near-wall modeling in LES, the database indicates that the pressure term in the mean streamwise-momentum equation is not negligible at the laminar and turbulent separated regions. This fact suggests that widely used equilibrium wall model is not sufficient and the inclusion of the pressure term is necessary for wall modeling in LES of such flow. [Preview Abstract] |
Monday, November 21, 2016 6:27PM - 6:40PM |
L32.00010: Transitions to different kinds of turbulence in a channel with soft walls. Viswanathan Kumaran, Sagar Srinivas The flow in a soft-walled channel undergoes a transition to turbulence at a Reynolds number which is a fraction of the transition Reynolds number of 1200 for a rigid channel, due to a dynamical instability caused by a fluid-wall coupling. The turbulent flow after transition in a channel with walls made of polyacrylamide gel is experimentally characterised. There are two other types of turbulence observed in sequence as the Reynolds number is increased. The first is the soft-wall turbulence, which involves wall oscillations primarily tangential to the surface, coupled with large fluid velocity fluctuations. The fluid velocity fluctuations share many of the characteristics of those in the flow past a rigid surface, but there are significant differences; the velocity fluctuations do not seem to decay to zero at the wall, and the mechanism of turbulence production seems to be different. As the Reynolds number is increased, there is a second wall-flutter transition which involves solid displacement perpendicular to the wall, and takes place only if the wall is unrestrained. The two transitions take place in sequence from a laminar flow when the soft-wall transition Reynolds number is less than 1200, and from a turbulent flow if the soft-wall transition Reynolds number exceeds 1200. [Preview Abstract] |
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