Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session P40: Turbulent Boundary Layers: Control and Perturbations I |
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Chair: Jonathan Morrison, Imperial College London Room: 6b |
Monday, November 25, 2019 5:16PM - 5:29PM |
P40.00001: Experimental Studies of Streamwise Response of the Turbulent Boundary Layer to a Periodic Actuation Mitchell Lozier, Flint O. Thomas, Stanislav Gordeyev It has been established that the dynamics of large-scale structures (LSS) in turbulent boundary layers (TBL) and near-wall small-scale turbulence are correlated. In these studies, a plasma based active flow control device was placed at sixty percent of the boundary layer thickness to introduce periodic disturbances into the wake region of the turbulent boundary layer. The boundary layer Reynolds number was low enough, Re$\tau =$700, so no natural large-scale structure was present. Via actuation, a synthetic large-scale periodic shear-layer-like structure was introduced into the boundary layer, and the TBL response to this synthetic structure at various wall-normal and streamwise locations downstream of the actuator was studied using a single hot-wire. Due to the periodic nature of the forcing, a phase-locked triple Reynolds decomposition of velocity was used to analyze the data. The modal component of velocity corresponding to the actuation frequency and the residual turbulence levels are the parameters of interest in this study. The dynamics of the LSS and small-scale structures were quantified using several modulation coefficients that correlate changes in modal velocity and residual turbulence with respect to phase. These modulation coefficients show a strong positive correlation in the inner and log region of the boundary layer. By measuring these quantities at several streamwise locations, the evolution of the synthetic large-scale structure and its modulating effect on the near-wall turbulence can be described. [Preview Abstract] |
Monday, November 25, 2019 5:29PM - 5:42PM |
P40.00002: Turbulent boundary layer drag recovery downstream of spanwise wall oscillation Christopher Bryson, Fazle Hussain Spanwise wall oscillation (SWO) has been well researched for producing significant drag reduction. While the dynamics within the control region has been studied, the flow recovery downstream of the control region is poorly understood. Direct numerical simulation of a TBL with SWO is performed for a range of control velocity oscillation amplitudes $A^+=\{10,20,30\}$ and oscillation periods $T^+=\{50,100,200\}$. It has been found that stronger drag reduction within the control region correlates with faster recovery to higher saturation levels of drag, which are also higher than an uncontrolled TBL. Compared with the uncontrolled case, visualization of control cases reveal that streak spacing is higher in the saturation region with increased wall normal velocity gradients in between, thus increased drag. The flow near the wall accelerates during the recovery phase producing a mean wall normal velocity that pulls streak transient growth (STG) vortices closer to the wall. The vortices then strengthen the streaks which spawn additional streamwise $\lambda_2$ vortices by STG, contributing to the increased drag. Further details in terms of the connection between vortical structures and wall shear stress will be discussed. [Preview Abstract] |
Monday, November 25, 2019 5:42PM - 5:55PM |
P40.00003: On the structure of compliant wall deformation forced by a turbulent boundary layer Jin Wang, Subhra Koley, Joseph Katz Our previous (Zhang et al 2017) study examined the pressure-deformation correllations in a compliant wall turbulent channel flow with a stief wall and submicron deformations. Aiming to extend the scope to two-way coupling, where the deformation size is several wall units ($\delta_{\mathrm{\nu }})$, theoretical analysis is used for selecting a compliant material (PDMS $+$ silicone gel) with Young's modulus (0.158 MPa), thickness (5mm), and shear speed (7.85 m/s) comparable to the freestream velocity ($U=$1.2-6 m/s). Time-resolved (2 kHz) Mach-Zehnder Interferometry is used for mapping the deformation, and 2D PIV for measuring the flow. The deformations increase from submicron at $U=$1.2 m/s to well above 20 $\mu $m (4$\delta _{\mathrm{\nu }})$ at 6 m/s. The primary mode is advected at 0.66$U$ for all wavenumbers, but the peak wavenumber in both directions remains nearly constant. In addition, high-frequency low wavenumber lateral waves appearing at broad streaks dominate at low $U$, but persist at high-speed. Comparisons of the measured frequency spectra to 1-D linear models (Chase 1991, Benschop et al. 2019) show a good agreement for advected modes, but not for the lateral ones. At high $U_{,\thinspace }$the compliant wall causes a sharp decrease in mean velocity at y\textless 10$\delta_{\mathrm{\nu }}$, consistent with DNS results (Rosti and Brandt 2017). [Preview Abstract] |
Monday, November 25, 2019 5:55PM - 6:08PM |
P40.00004: Input-output analysis of a turbulent separation bubble with a time-periodic base flow Alberto Padovan, Clarence Rowley, Wen Wu, Charles Meneveau, Rajat Mittal Direct numerical simulations are performed for three-dimensional turbulent boundary layer flow at $Re_{\theta_0}=490$, in which a suction velocity profile is imposed at the top of the computational domain to induce separation at the bottom wall. We study the input-output characteristics of perturbations about two different base flows: a spanwise-averaged, time-averaged base flow and a spanwise-averaged, time-periodic base flow. The first approach leads to the well-known resolvent analsysis, through which we compute the optimal forcing and response modes at a given frequency and spanwise wavenumber. The latter approach leads to the formulation of the harmonic transfer function, a linear operator that governs the dynamics of fluctuations about time-periodic base flows. Within this framework, perturbations at different temporal frequencies are coupled to one another through the base flow, and we can therefore study the cross-frequency and energy transfer mechanisms. For the harmonic transfer function, we compute the optimal global forcing and response modes, which are full spatio-temporal flow fields. The cross-frequency modes provide insight into the spatial patterns that arise from the scattering of perturbations from the base flow. [Preview Abstract] |
Monday, November 25, 2019 6:08PM - 6:21PM |
P40.00005: Turbulent boundary layer injected with low blowing ratio effusion film Jeremy Basley, Kevin Gouder, Jonathan F Morrison Effusion cooling of turbine blades is used in jet engines to alleviate the thermal and shear strain they sustain while beneath the hot flow exiting the combustion chamber. This study focuses on the mechanisms underlying the interactions between the film-injected momentum and the incoming high Reynolds number turbulent flow. A large-scale low-velocity experiment is carried out in the closed-loop 10x5 wind-tunnel facility. The scaled-up effusion device consists of a plenum located directly underneath a thick plate pierced along a staggered grid of inclined $D =$ 16mm diameter holes with a pitch of 5$D$. This setup is placed in a turbulent boundary layer, tripped and developing over 15 m. A range of injected velocities with respect to free-stream velocity (blowing ratio) is investigated with time-resolved planar PIV, complemented with hot-wire anemometry profiles, and wall-pressure measurements. The resulting time-resolved and space-extended data sets explain the favourable outcome of low blowing ratios, for which the shear-driven mixing of the effusion film is limited to near-wall region of the boundary layer. Results also suggest the effusion film effectively restricts the penetration of fluid from the outer region into the near-wall region of the boundary layer. [Preview Abstract] |
Monday, November 25, 2019 6:21PM - 6:34PM |
P40.00006: Classification of forcing conditions in pulsatile turbulent pipe flow using Reynolds shear stress co-spectra Zijin Cheng, Thomas O. Jelly, Simon J. Illingworth, Ivan Marusic, Andrew S.H. Ooi The turbulence dynamics of pulsatile pipe flow are investigated using data obtained from direct numerical simulations at a mean friction Reynolds number of 180, 270 and 360. The forcing conditions are achieved by applying a time-harmonic axial pressure gradient. This study directs attention towards the frequency response of single- and two-point turbulence statistics to systematic variations in the forcing frequency. We propose a classification of the applied forcing conditions based on the Reynolds shear stress frequency co-spectra and the applied forcing frequency. We perform simulations based on this classification to extend the physical understanding of the phase dependence of single- and two-point turbulence statistics under high, very-high and ultra-high forcing frequencies, focussed around the frequency response of turbulence dynamics in time (frequency) and space (wavenumber) domains. Results also reveal a decoupling behaviour when the frequency of the forcing is higher than the highest frequency (smallest time-scale) of the energy containing motions in the Reynolds shear stress co-spectra. [Preview Abstract] |
Monday, November 25, 2019 6:34PM - 6:47PM |
P40.00007: Manipulating Near-wall Turbulent Boundary Layer by Unsteady Air-films Cong Wang, Morteza Gharib Previously we have demonstrated that wall-attached air-films can be sustained in turbulent boundary layer (TBL) and dynamically modulated by pressure wave. This technique is effective in manipulating the near-wall turbulent shear flow. Here we show that in the presence of modulated air-films, the phase-averaged streamwise velocity demonstrates a Stokes type oscillatory motion. The near-wall viscous shear stress ($\nu\frac{\partial \overline{U}}{\partial y}$) is suppressed and negative Reynolds shear stress ($-\overline{u^{\prime}v^{\prime}}$) can be generated in the vicinity of air-films. Through a quadrant analysis, we identify a potential mechanism for the generation of negative Reynolds shear stress. [Preview Abstract] |
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