Outer Peaks Research Articles

Streamwise energy-transfer mechanisms in zero- and adverse-pressure-gradient turbulent boundary layers

The present study investigates streamwise ( $\overline {u^2}$ ) energy-transfer mechanisms in the inner and outer regions of turbulent boundary layers (TBLs). Particular focus is placed on the $\overline {u^2}$ production, its inter-component and wall-normal transport as well as dissipation, all of which become statistically significant in the outer region with increasing friction Reynolds number ( $Re_{\tau }$ ). These properties are analysed using published data sets of zero, weak and moderately strong adverse-pressure-gradient (APG) TBLs across a decade of $Re_{\tau }$ , revealing similarity in energy-transfer pathways for all these TBLs. It is found that both the inner and outer peaks of $\overline {u^2}$ are always associated with local maxima in the $\overline {u^2}$ production and its inter-component transport, and the regions below/above each of these peaks are always dominated by wall-ward/away-from-wall transport of $\overline {u^2}$ , thereby classifying the $\overline {u^2}$ profiles into four distinct regimes. This classification reveals existence of phenomenologically similar energy-transfer mechanisms in the ‘inner’ and ‘outer’ regions of moderately strong APG TBLs, which meet at an intermediate location coinciding with the minimum in $\overline {u^2}$ profiles. Conditional averaging suggests existence of similar phenomena even in low $Re_{\tau }$ canonical and/or weak APG TBLs, albeit with the outer-region mechanisms weaker than those in the inner region. This explains the absence of their $\overline {u^2}$ outer peak and the dominance of $\overline {u^2}$ wall-normal transport away from the wall, which potentially originates from the inner region. Given that the wall-ward/away-from-wall transport of $\overline {u^2}$ is governed by the $Q_4$ (sweeps)/ $Q_2$ (ejections) quadrants of the Reynolds shear stress, it is argued that the emergence of the $\overline {u^2}$ outer peak corresponds with the statistical dominance of $Q_4$ events in the outer region. Besides unravelling the dynamical significance of $Q_2$ and $Q_4$ events in the outer region of TBLs, the present analysis also proposes new phenomenological arguments for testing on canonical wall-turbulence data at very high $Re_{\tau }$ .

Study on the Impact of Pole Spacing on Magnetic Flux Leakage Detection under Oversaturated Magnetization.

Magnetic flux leakage (MFL) inspection employs leakage magnetic fields to effectively detect and locate pipeline defects. The spacing between magnetic poles significantly affects the leakage magnetic field strength. While most detectors typically opt for moderate pole spacing for routine detection, this study investigates the propagation characteristics of MFL signals at small pole spacings (under specimen oversaturated magnetization) and their impact on MFL detection. Through finite element simulation and experiments, it reveals a new signal phenomenon in the radial MFL signal By at small pole spacings, the double peak-valley (DPV) phenomenon, characterized by outer and inner peaks and valleys. Theoretical analysis based on the simulation results elucidates the mechanisms for this DPV phenomenon. Based on this, the impact of defect size, pipe wall thickness, and magnetic pole and rigid brush height on MFL signals under small magnetic pole spacings is examined. It is demonstrated that, under a smaller magnetic pole spacing, a potent background magnetic field manifests in the air above the defect. This DPV phenomenon is generated by the magnetic diffusion and compression interactions between the background and defect leakage magnetic fields. Notably, the intensity of the background magnetic field can be mitigated by reducing the height of the rigid brush. In contrast, the pipe wall thickness and magnetic pole height exhibit a negligible influence on the DPV phenomenon. The emergence of the DPV precipitates a reduction in the peak-to-valley difference within the MFL signal, constricting the depth range of detectable defects. However, the presence of DPV increases the identification of defects with smaller opening sizes. These findings reveal the characterization of the MFL signal under small pole spacing, offering a preliminary study on identifying specific defects using unconventional signals. This study provides valuable guidance for MFL detection.

Direct numerical simulation of adverse pressure gradient turbulent boundary layer up to [formula omitted

A direct numerical simulation of a moderate adverse pressure gradient turbulent boundary layer flow on a flat plate was performed on a fairly long domain with a Reynolds number from Reθ≃2000 to Reθ≃8000. The mean streamwise velocity profile does not exhibit a clear log region even at the highest Reynolds number. The Reynolds stresses exhibit a well defined peak in the outer region which moves away from the wall before to stabilise at a wall distance y of 0.45 boundary layer thickness after evolving over 20 local boundary layer thicknesses. These outer peaks are the consequences of an excess of production over dissipation and they all scale with the shear stress velocity for wall distance 0.2δ<y<0.85δ. The different terms of the Reynolds stresses equations are analysed in the outer region and the analysis confirms that the leading terms are the same as for a zero pressure gradient turbulent boundary layer except that the production and dissipation are not negligible in the outer region. The most significant contribution of scales smaller than two Taylor scales is on the pressure strain term of the shear stress equation. The probability of the occurrence of small scale vortices conditioned by the presence of an ejection or a sweep shows that the shear stress contained in ejections is associated with the production and transport of near wall vortices away from the wall. The local mean shear generated by the sweeps increases the production of near wall vortices. Lastly, the large scale structures analysed by means of two point correlations of the streamwise fluctuating velocity are shown to be strongly modified by the adverse pressure gradient. The correlation reveals a decoupling between the near wall streaks and the large scale streamwise structures enhanced by the adverse pressure gradient.

Velocity spectra and scale decomposition of adverse pressure gradient turbulent boundary layers considering history effects

Experiments at relatively large Reynolds number were conducted to better understand the influence of an adverse pressure gradient on the turbulence in two-dimensional boundary layer flows. One focus is on documenting the scale contributions to the Reynolds normal and shear stress profiles under the influence of a positive streamwise pressure gradient. Toward these aims, velocity spectra at friction Reynolds numbers (7100≤Reτ≤8600) are presented, analyzed, and compared to a zero pressure gradient case from the same facility at nominally matching Reynolds number. Distance-from-the-wall scaling is central to numerous theoretical and modeling considerations of the canonical flat plate flow. Consistently, the present spectral measurements reveal clear evidence for its preservation on the inertial sublayer under modest adverse pressure gradients. Increases are seen in the viscous-scaled premultiplied spectra in the outer region as the pressure gradient increases — commensurate with the increases observed in the outer peaks of the streamwise and wall-normal velocity variances, as well as the Reynolds shear stress. Effects of varying Reτ on the streamwise and wall-normal velocity variances and the Reynolds shear stress are studied by comparing the present data at varying Clauser pressure gradient parameter, β, with previous experiments having similarly varying β at significantly lower Reτ (≈1900). This allows to study the effects of changing Reynolds number at matched β. Cases at similar β and Reynolds number, but with varying flow history are also examined, wherein the same β value is arrived either by starting from a smaller or larger initial β. The effects of β>0 on the large and small scale motions are investigated by segregating the signal contributions according to a cut-off wavelength established in previous zero pressure gradient flows. Under a normalization that uses a hybrid velocity scale this analysis reveals similarity between the outer regions of the spectrally-decomposed variances and Reynolds shear stress. These analyses also reveal a more dramatic reduction in the near-wall large scale contribution to the Reynolds shear stress gradient in the β>0 flow when compared to the β=0 flow.

Cross-correlation effects in the solution NMR spectra of near-equivalent spin-1/2 pairs.

The nuclear magnetic resonance (NMR) spectra of spin-1/2 pairs contain four peaks, with two inner peaks much stronger than the outer peaks in the near-equivalence regime. We have observed that the strong inner peaks have significantly different linewidths when measurements were performed on a 13C2-labelled triyne derivative. This linewidth difference may be attributed to strong cross-correlation effects. We develop the theory of cross-correlated relaxation in the case of near-equivalent homonuclear spin-1/2 pairs, in the case of a molecule exhibiting strongly anisotropic rotational diffusion. Good agreement is found with the experimental NMR lineshapes.

Assessment of Turbulence Models over a Curved Hill Flow with Passive Scalar Transport

An incoming canonical spatially developing turbulent boundary layer (SDTBL) over a 2-D curved hill is numerically investigated via the Reynolds-averaged Navier–Stokes (RANS) equations plus two eddy-viscosity models: the K−ω SST (henceforth SST) and the Spalart–Allmaras (henceforth SA) turbulence models. A spatially evolving thermal boundary layer has also been included, assuming temperature as a passive scalar (Pr = 0.71) and a turbulent Prandtl number, Prt, of 0.90 for wall-normal turbulent heat flux modeling. The complex flow with a combined strong adverse/favorable streamline curvature-driven pressure gradient caused by concave/convex surface curvatures has been replicated from wind-tunnel experiments from the literature, and the measured velocity and pressure fields have been used for validation purposes (the thermal field was not experimentally measured). Furthermore, direct numerical simulation (DNS) databases from the literature were also employed for the incoming turbulent flow assessment. Concerning first-order statistics, the SA model demonstrated a better agreement with experiments where the turbulent boundary layer remained attached, for instance, in Cp, Cf, and Us predictions. Conversely, the SST model has shown a slightly better match with experiments over the flow separation zone (in terms of Cp and Cf) and in Us profiles just upstream of the bubble. The Reynolds analogy, based on the St/(Cf/2) ratio, holds in zero-pressure gradient (ZPG) zones; however, it is significantly deteriorated by the presence of streamline curvature-driven pressure gradient, particularly due to concave wall curvature or adverse-pressure gradient (APG). In terms of second-order statistics, the SST model has better captured the positively correlated characteristics of u′ and v′ or positive Reynolds shear stresses (&lt;u′v′&gt; &gt; 0) inside the recirculating zone. Very strong APG induced outer secondary peaks in &lt;u′v′&gt; and turbulence production as well as an evident negative slope on the constant shear layer.

Pyromellitic diimide based bipolar molecule for total organic symmetric redox flow battery

Non-aqueous redox flow batteries based on total organic electrolytes, consisting of earth-abundant elements, i.e., C, H, N, O, are regarded as a promising technology for sustainable and large-scale energy storage. In particular, bipolar redox-active organic materials (BROMs) are distinctly fascinating as electroactive species because they have multiple oxidation states and can undergo multiple redox processes. By virtue of this unique characteristic, BROMs can be utilized as both anolyte and catholyte in symmetric flow batteries, consequently, helping alleviate cross-contamination issues. Herein we report an all-organic symmetric redox flow battery based on diimide molecule as a bipolar electroactive material. The potential interval between the cathodic peak and the inner and outer anodic peaks led to a promising cell voltage of 2.22 V. The symmetric battery can be operated at a current density of 20 mA cm−2, with a coulombic efficiency of 90% for over 100 cycles. This work proposes an alternative approach to designing multi-electron organic redox active materials to facilitate the advancement of high-density symmetric flow batteries.

Enhanced outer peaks in turbulent boundary layer using uniform blowing at moderate Reynolds number

Uniform blowing through a permeable surface acts as an active flow control method in wall bounded flows. Such control technique was investigated in a zero pressure gradient turbulent boundary layer over a flat plate. Measurement data were obtained with the help of Laser Doppler Anemometry technique. Besides the drag reduction characteristics of such flow control method, time averaged measurement of stream-wise and wall-normal velocity components was taken at Reynolds number based on momentum thickness () of 1100–3670. Due to the difference in surface condition with and without blowing, mean properties of the boundary condition at wall influence the flow properties when scaled with outer scaling properties. Enhanced turbulence is observed in Reynolds stresses using statistical analysis including the thickening of the boundary layer.

Decomposition of the mean friction drag on an NACA4412 airfoil under uniform blowing/suction

The application of drag-control strategies on canonical wall-bounded turbulence, such as periodic channel and zero- or adverse-pressure-gradient boundary layers, raises the question on how to distinguish consistently the origin of control effects under different reference conditions. We employ the RD identity (Renard &amp; Deck, J. Fluid Mech., vol. 790, 2016, pp. 339–367) to decompose the mean friction drag and investigate the control effects of uniform blowing and suction applied to an NACA4412 airfoil at chord Reynolds numbers $Re_c=200\,000$ and $400\,000$ . The connection of the drag reduction/increase by using blowing/suction with the turbulence statistics (including viscous dissipation, turbulence kinetic energy production and spatial growth of the flow) across the boundary layer, subjected to adverse or favourable pressure gradients, is examined. We found that the inner and outer peaks of the contributions associated with the friction-drag generation show good scaling with either inner or outer units, respectively. They are also independent of the Reynolds number, control scheme and intensity of the blowing/suction. The small- and large-scale structures are separated with an adaptive scale-decomposition method, namely the empirical mode decomposition (EMD), which aims to analyse the scale-specific contribution of turbulent motions to friction-drag generation. Results unveil that blowing on the suction side of the airfoil is able to enhance the contribution of large-scale motions and to suppress that of small scales; however, suction behaves contrarily. The contributions related to cross-scale interactions remain almost unchanged with different control strategies.

Scaling of global properties of fluctuating streamwise velocities in pipe flow: Impact of the viscous term

We extend the procedure outlined in Basse [“Scaling of global properties of fluctuating and mean streamwise velocities in pipe flow: Characterization of a high Reynolds number transition region,” Phys. Fluids 33, 065127 (2021)] to study global, i.e., radially averaged, scaling of streamwise velocity fluctuations. A viscous term is added to the log-law scaling, which leads to the existence of a mathematical abstraction, which we call the “global peak.” The position and amplitude of this global peak are characterized and compared to the inner and outer peaks. A transition at a friction Reynolds number of order 10 000 is identified. Consequences for the global peak scaling, length scales, non-zero asymptotic viscosity, turbulent energy production/dissipation, and turbulence intensity scaling are appraised along with the impact of including an additional wake term.

Bipolar Diimide Based Molecule for Nonaqueous Symmetric Redox Flow Battery

Redox flow batteries (RFBs) allow decoupling of power and energy capacity, a unique feature that makes them a promising energy storage technology for large scale applications[1]. In recent years, there has been a massive development in aqueous organic RFBs. However, their commercialization has been hindered by low energy density owing to low solubility of redox active materials and narrow electrochemical stability window of water which limits the cell voltage[2] .Nonaqueous redox flow batteries based on organic redox active material are a regarded as an alternative to aqueous organic RFBs[3] . Apart from enjoying the advantages of organic redox active materials such as low cost, diverse molecular structure and tailorability, nonaqueous RFBs also benefits from using organic supporting electrolytes which provide a wider voltage window (2-5V) facilitating a high energy capacity and low-cost storage system[4, 5] .Herein, a diimide based molecule is designed, chemically modified, analysed and employed as bipolar redox active materials for a total organic symmetric redox flow battery. Cyclic voltammetry studies show three redox couples, two anodic one electron transfer and one cathodic two electrons transfer redox transitions which are electrochemically reversible. The potential difference between the cathodic peak and the inner and outer anodic peaks leads to a promising cell voltage of 1.51V and 2.22V respectively in redox flow battery application. This work suggests an alternative way to design a multielectron redox active materials for symmetric batteries.Figure 1: Cyclic voltammogram of 0.002M diimide based molecule in 0.1M tetrabutylammonium hexafluorophosphate in dimethylformamide recorded on a glassy carbon working electrode with a scan rate of 50 mV/s. Acknowledgement The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. T23-601/17-R) References [1] P. Alotto, M. Guarnieri, F. Moro, Renewable and Sustainable Energy Reviews. 29 (2014) 325-335.[2] J. Luo, B. Hu, M. Hu, Y. Zhao, T.L. Liu, ACS Energy Letters. 4 (2019) 2220-2240.[3] S. Shin, S. Yun, S. Moon, Rsc Advances. 3 (2013) 9095-9116.[4] W. Wang, V. Sprenkle, Nature Chemistry. 8 (2016) 204-206.[5] M. Hayyan, F.S. Mjalli, M.A. Hashim, I.M. AlNashef, T.X. Mei, Journal of Industrial and Engineering Chemistry. 19 (2013) 106-112. Figure 1

Decomposition of the mean friction drag in adverse-pressure-gradient turbulent boundary layers

From an energy budget perspective, the decomposition of mean friction drag in adverse-pressure-gradient turbulent boundary layers is conducted, obtaining contributions associated with dissipation, production, and convection across the boundary layer. In the wall-normal distributions of the decomposed constituents, positions of inner peaks are well scaled in viscous units, whereas outer peaks scale in outer units, regardless of Reynolds number and pressure-gradient magnitude.

Linking the small-scale relativistic winds and the large-scale molecular outflows in the z = 1.51 lensed quasar HS 0810+2554

ABSTRACT We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of the quadruply lensed z = 1.51 quasar HS 0810+2554 which provide useful insight on the kinematics and morphology of the CO molecular gas and the ∼ 2 mm-continuum emission in the quasar host galaxy. Lens modelling of the mm-continuum and the spectrally integrated CO(J = 3→2) images indicates that the source of the mm-continuum has an eccentricity of e ∼ 0.9 with a size of ∼1.6 kpc and the source of line emission has an eccentricity of e ∼ 0.7 with a size of ∼1 kpc. The spatially integrated emission of the CO(J = 2→1) and CO(J = 3→2) lines shows a triple peak structure with the outer peaks separated by Δv21 = 220 ± 19 km s−1 and Δv32 = 245 ± 28 km s−1, respectively, suggesting the presence of rotating molecular CO line emitting gas. Lensing inversion of the high spatial resolution images confirms the presence of rotation of the line emitting gas. Assuming a conversion factor of αCO = 0.8 M⊙ (K km s−1 pc2)−1 we find the molecular gas mass of HS 0810+2554 to be MMol = (5.2 ± 1.5)/μ32 × 1010 M⊙, where μ32 is the magnification of the CO(J = 3→2) emission. We report the possible detection, at the 3.0–4.7σ confidence level, of shifted CO(J = 3→2) emission lines of high-velocity clumps of CO emission with velocities up to 1702 km s−1. We find that the momentum boost of the large-scale molecular wind is below the value predicted for an energy-conserving outflow given the momentum flux observed in the small-scale ultrafast outflow.

Azimuthal organization of large-scale motions in a turbulent minimal pipe flow

Direct numerical simulation data for turbulent minimal pipe flows with Reτ = 927, 1990, and 2916 are examined to explore the azimuthal (or spanwise) organization of their large-scale structures. We chose a streamwise-minimal unit with a streamwise domain length of Lx+≈1000, which is the characteristic streamwise length of near-wall streaks. The spanwise scales of most of the energetic motions and their contributions to the total energy are comparable with those of the streamwise long-domain simulation. In the azimuthal energy spectra of the streamwise velocity fluctuations (u), the large-scale energy increases with Reτ and three outer peaks (λθ = 0.7–0.8, π/2 and π) become evident when Reτ = 2916. The presence of the outer peaks at λθ = 0.7–0.8 and π/2 is consistent with the results of the long-domain simulation. The peak at λθ = 0.7–0.8 is associated with large-scale motions and the other two peaks are associated with very-large-scale motions (VLSMs). The maximum spanwise wavelength increases linearly with the wall-normal distance from the wall. A kz−1 region is evident in the range 0.3R &amp;lt; λz (=rλθ) &amp;lt; R, which indicates the presence of self-similar motions. The conditional two-point correlation with a cut-off wavelength of λz = 0.9R shows that there is a strong correlation between the enhanced energy in the outer region and the wall-attached structures, which were extracted from the time evolution of the streamwise-averaged u field (u2D). The spanwise sizes (lz) of the attached u2D structures scale with their height (ly) in the log region and their time scales (lt) follow ltuτ/lz = 2, which is consistent with the bursting time scale. Their spanwise sizes lie in the range R &amp;lt; lz &amp;lt; 3R, for which lt increases significantly, which indicates that these structures are associated with VLSMs and make the dominant contributions to the enhanced energy in the outer region. These structures penetrate to the wall region as a manifestation of the footprint and modulate the small-scale energy. The negative-u2D structures induce congregative motions in the near-wall region.

Rotations of elliptic vortex beams in media with and without anisotropy

We introduce the anisotropy into the evolutions of the vortex, and discuss three different kinds of dynamics: circular shaped vortex in linear anisotropic media, elliptic shaped vortex in linear isotropic media, and elliptic shaped vortex in linear anisotropic media. The evolutionary processes dramatically depend on both the anisotropic parameter δM of the media and the initial aspect ratio ρ0 of the elliptic vortex. Under the critical condition , the elliptic vortex will not rotate, and keeps its aspect ratio invariant during the evolutions. Generally, when , the dynamics of the elliptic vortex can be divided into three stages: nonuniform process, rotational process and uniform process. In the nonuniform and the uniform processes of the vortex, the positions of the outer elliptic peaks and the inner elliptic holes do not change in spite of the nonzero energy flow. Also, the energy distributions become nonuniform in the former process, and uniform in the latter process. In the rotational process, the outer peaks and the inner holes can both rotate, but with the opposite rotational directions.

Effect of divergence angle of ejector nozzle on aerosolisation of powdered nanoparticles

ABSTRACTA combined experimental-numerical study was performed to reveal the mechanisms of powdered SiO2 nanoparticle (NP) breakage and establish the relationship between the divergent angle (α) of a convergent-divergent (C-D) nozzle and nanoparticle size distribution. All distributions of aerosolised SiO2 NPs obtained through ejectors with varied α were identically log-normal and unimodal. As α increased (from 8° to 20°), there were identified decreases in Geometric Mean Distribution (GMD)(from 216 to 189 nm), mode diameter, and median diameter, while Geometric Standard Deviation (GSD) increased slightly (from 1.59 to 1.65). The parallel negative growth trends of D25 and D75, with slightly decreasing interquartile (D25–D75) ranges, indicated that an increase in α could result in a clear shift of Particle Sizes Distribution(PSD) towards smaller particle sizes. This implied that NP deagglomeration was strengthened and the ejector became more efficient as α increased. Three cross-sections including the C-D nozzle outlet (Section 1), mixer tube inlet (Section 2), and central section (Section 3) were selected to estimate the influence of shear stress (|τ|) distribution in the ejector on NP aerosolisation. Increasing α hardly changed the location of the two inner peaks, but it significantly decreased the values of inner peaks and smoothened |τ| distributions. |τ| in Section 2 with different α exhibited a flat bimodal distribution and the two outer steep peaks (from Section 1) disappeared. For |τ0| = 0.5 and |τ0| = 1 Pa, the ratio of the |τ| distribution range of |τ| > |τ0| (0.5 Pa < |τ0|< 5 Pa) to the entire section span, η, increased significantly as α increased. The growth rate of η decreased apparently for |τ0| ranging from 2 to 4 Pa, but for |τ0| ≥ 5 Pa, η stopped growing in Section 1. The maximum value of |τ| exerted no significant influence on NP dispersion. The smoothness and flatness of the |τ| distribution determined whether the powdered NPs could experience enough shear aerodynamics and be dispersed thoroughly. This played a crucial role in NP aerosolisation. The mechanism of the effect of α on NP dispersion resulted in smooth and flat |τ| distributions. This helped expand |τ| distribution to increase the possibility of powdered NPs experiencing stronger shear flow as α increased.

Photoelectron momentum distribution of ground- and excited-state lithium atoms induced by extreme-ultraviolet photon absorption

We perform a theoretical investigation of the photoelectron momentum distribution (PMD) after ionization of a lithium (Li) atom, initially prepared in the ground (1s22s) or excited (1s22p) state, by extreme ultraviolet (XUV) radiation of frequency 85 eV that was previously measured in an experiment with the free-electron laser in Hamburg (2009 Phys. Rev. Lett. 103 103008). The experiment revealed three distinct peaks in PMD with absolute momentum values of 2.4, 1.2 and 0.8 atomic units (a.u.), respectively. We carry out an all-electron calculation based on time-dependent density functional theory (TDDFT) and find that the two outer PMD peaks (2.4 and 1.2 a.u.) are well described by the single ionization from the valence (2s) and core (1s) electron shells, while the innermost peak (0.8 a.u.) is not reproduced. However, the peak at this position is found in the TDDFT calculation of a Li+ ion (1s2) subject to the same XUV radiation, thus the TDDFT calculations are in good agreement with the experimental measurements if one assumes that the innermost peak originates from a Li+ ion rather than a neutral atom. We estimate possible production of Li+ ions by the pumping infrared laser used in the experiment to prepare the target atoms in the 1s22p excited state prior to interaction with the XUV field.

Robust features of a turbulent boundary layer subjected to high-intensity free-stream turbulence

The influence of the large scale organisation of free-stream turbulence on a turbulent boundary layer is investigated experimentally in a wind tunnel through hot-wire measurements. An active grid is used to generate high-intensity free-stream turbulence with turbulence intensities and local turbulent Reynolds numbers in the ranges $7.2\,\%\leqslant u_{\infty }^{\prime }/U_{\infty }\leqslant 13.0\,\%$ and $302\leqslant Re_{\unicode[STIX]{x1D706},\infty }\leqslant 760$, respectively. In particular, several cases are produced with fixed $u_{\infty }^{\prime }/U_{\infty }$ and $Re_{\unicode[STIX]{x1D706},\infty }$, but up to a 65 % change in the free-stream integral scale $L_{u,\infty }/\unicode[STIX]{x1D6FF}$. It is shown that, while qualitatively the spectra at various wall-normal positions in the boundary layer look similar, there are quantifiable differences at the large wavelengths all the way to the wall. Nonetheless, profiles of the longitudinal statistics up to fourth order are well collapsed between cases at the same $u_{\infty }^{\prime }/U_{\infty }$. It is argued that a larger separation of the integral scale would not yield a different result, nor would it be physically realisable. Comparing cases across the wide range of turbulence intensities and free-stream Reynolds numbers tested, it is demonstrated that the near-wall spectral peak is independent of the free-stream turbulence, and seemingly universal. The outer peak was also found to be described by a set of global scaling laws, and hence both the near-wall and outer spectral peaks can be predicted a priori with only knowledge of the free-stream spectrum, the boundary layer thickness ($\unicode[STIX]{x1D6FF}$) and the friction velocity ($U_{\unicode[STIX]{x1D70F}}$). Finally, a conceptual model is suggested that attributes the increase in $U_{\unicode[STIX]{x1D70F}}$ as $u_{\infty }^{\prime }/U_{\infty }$ increases to the build-up of energy at large wavelengths near the wall because that energy cannot be transferred to the universal near-wall spectral peak.

Direct numerical simulation of a turbulent Couette–Poiseuille flow: Turbulent statistics

The direct numerical simulation of a fully developed turbulent Couette–Poiseuille flow is performed to investigate the modification of turbulent statistics on the bottom and top walls compared to those in a pure Poiseuille flow. The streamwise mean velocity profile shows that an extended logarithmic layer for a Couette–Poiseuille flow is developed from each wall to the centerline. In addition, the turbulent intensities and Reynolds shear stress on the bottom wall are found to be larger than those in the Poiseuille flow, whereas it is reversed on the top wall due to reduction of the velocity shear. The quadrant analysis of the Reynolds shear stress reveals that large Q2 and Q4 event motions are continuously created throughout the entire flow near the centerline, leading to active momentum transport between the bottom and top walls for the C-type. Inspection of the pre-multiplied streamwise and spanwise energy spectra shows that distinct secondary outer peaks are created for all velocity components and a plateau, called the kx−1 region, is presented in the logarithmic region. Based on an analysis of the net force spectra, three spectral ranges in the wavelength space, corresponding to small- (λx/δ ≤ 1), large- (1 ≤ λx/δ ≤ 10) and very-large-scale (λx/δ ≥ 10) motions for a Couette–Poiseuille flow, are proposed, and very-large-scale structures are highly energetic and contribute more than half of the streamwise turbulent kinetic energy and Reynolds shear stress, where λx is the streamwise wavelength and δ is the channel half height.

(Invited) Plasmon-Induced Photocurrent Generation for Exploring the Near-Field Ofstrongly Coupled Plasmonic Systems

[Introduction] Metallic nanostructures which show localized surface plasmon resonances (LSPRs) have received considerable attention as a light harvesting optical antenna for light-energy conversion systems such as solar cells as well as artificial photosynthesis [1,2]. To construct efficient light harvesting optical antennae, an optimization of structural design is one of the most important research topics in plasmon-induced light energy conversions. Coupled plasmonic systems such as nanogap heptamer, dolmen, metal-insulator-metal nanostructures, and so on is promising as a photoelectrode design because of their strong near-field enhancement and wide wavelength responsibility. In this study, a coupled plasmonic system based on a waveguide-LSPR coupling system is employed to investigate whether the photocurrent response extends over a wide wavelength range and is promoted by near-field enhancement in the plasmon-induced photocurrent generation using gold nanostructured titanium dioxide (TiO2) photoelectrodes. Near-field spectrum and photocurrent action spectrum are compared to elucidate the effect of near-field enhancement on the photocurrent generation. [Experimental] TiO2 photoelectrodes supporting periodic gold nanogratings (AuNGs) with different pitch sizes were fabricated by a deposition of TiO2 on a glass substrate with a thickness of 250 nm using an atomic layer deposition (ALD) reactor, and subsequent electron beam lithography and lift-off processes. A conventional photoelectrochemical measurement was performed. The AuNGs/TiO2 photoelectrode, a platinum wire and a saturated calomel electrode (SCE) were employed as working, counter and reference electrodes, respectively. An aqueous Ar-gas-bubbled KClO4 (0.1 mol/dm3) solution was used as a supporting electrolyte solution. A plasmon-induced water oxidation as a half reaction of water splitting was explored [3]. [Results and Discussion] Figure 1(a) shows a scanning microscope (SEM) image of AuNGs/TiO2 photoelectrode with a pitch size of 300 nm. Extinction spectra of the AuNGs/TiO2 photoelectrode with a different pitch size are shown in Fig. 1(b). There is only one peak corresponding to the LSPR mode of periodic Au-NGs with 200 nm and 225 nm pitch sizes. Starting with a pitch size of 250 nm, three peaks can be observed, and the peaks show a spectral shift with increasing the pitch size. The two outer peaks can be assigned as coupled modes (P+ and P-) deriving from the symmetric waveguide mode Ew and LSPR mode Ep as shown in the illustration of the waveguide-LSPR coupling system (Fig. 1(c)). The center peak can be considered as a coupling mode between the asymmetric waveguide mode and the waveguide-LSPR coupling system. Internal quantum efficiency (IQE) spectra and simulated near-field spectra of the AuNGs/TiO2 photoelectrodes with 300 nm and 350 nm pitch sizes are shown in Fig. 2. The near-field intensity in the spectra was calculated by monitoring at the interface between the AuNGs and the TiO2 film by the electromagnetic simulations using a finite-difference time-domain (FDTD) method. It was clearly elucidated that IQE spectrum has successfully reproduced the near-field spectrum under the coupling conditions. This indicates that the photocurrent response extended over a wide wavelength range utilizing the coupled plasmonic systems and near-field enhancement effects promoted the plasmon-induced water oxidation because IQE values increased responding to the near-field spectra [4]. References Ueno, K., Oshikiri, T., Sun, Q., Shi, X., Misawa, H. Chem. Rev., DOI: 10.1021/acs.chemrev.7b00235.Ueno, K., Oshikiri, T., H. Misawa, ChemPhysChem, 2016, 17, 199−215.Nishijima, Y., Ueno, K., Yokota, Y., Murakoshi, K., Misawa, H. J. Phys. Chem. Lett. 2010, 1, 2031−2036.Guo, J., Ueno, K., Shi, X., Oshikiri, T., Misawa, H. et al., J. Phys. Chem. C 2017, 121, 21627−21633. Figure 1