/* DATA DESCRIPTION: Citation: Li, Huixin; Zamani Asl, Mohammad Mehdi; Baeuerlein, Bastian; Avila, Kerstin; Xu, Duo; Avila, Marc (2025): Optical measurements of turbulent mixing in a T-shaped mixer [dataset]. PANGAEA, https://doi.org/10.1594/PANGAEA.982597 Abstract: One of the most widespread canonical devices for fluid mixing is the T-shaped mixer, in which two opposing miscible liquid streams meet at a junction and then mix along a main channel. Laminar steady and time-periodic flows in T-shaped mixers have been thoroughly studied, but turbulent flows have received much less scrutiny despite their prevalence in applications. We here present optical measurements of turbulent mixing at small scales in a novel experimental setup with a hydraulic diameter of four centimetres. Water is used as a working fluid and the Reynolds number,Re, based on the bulk velocity and hydraulic diameter ranges from the laminar (Re=100) to the fully turbulent case (Re=5000). The data comprises two-dimensional particle image velocimetry (PIV) of the velocity field and planar laser-induced fluorescence measurements (PLIF) of the passive scalar (Rhodamine 6G). First, we successfully replicate characteristic flow regimes observed in micro-scale T-shaped mixers at low Reynolds numbers. We then focus on the turbulent regime and characterize the turbulent kinetic energy and dissipation along the mixing channel. Further, we determine the scalar concentration variance and its corresponding probability density function and spectra. The latter exhibits an incipient Batchelor scaling. We estimate the mechanical-to-scalar timescale ratio and examine the link between the turbulent velocity and scalar fields. The measurement data are compared with model predictions and correlations used in engineering practice, and with data from our own direct numerical simulations at Re=1500 performed with a spectral-element code. Keyword(s): Particle Image Velocimetry; Planar laser-induced fluorescence; T-shaped mixer; turbulent mixing Parameter(s): Binary Object (Binary) Figure (Fig) Title (Title) File name (File name) Variable (Variable) File format (File format) Description (Description) License: Creative Commons Attribution 4.0 International (CC-BY-4.0) (URI: https://creativecommons.org/licenses/by/4.0/) Status: Curation Level: Enhanced curation (URI: https://wiki.pangaea.de/wiki/Curation_levels) * Processing Level: PANGAEA data processing level 2 (URI: https://wiki.pangaea.de/wiki/Processing_levels#PANGAEA_data_processing_level_2) Size: 63 data points Files: All files referred to in data matrix can be downloaded in one go as ZIP or TAR (PANGAEA user account required). To download single files, use the filename from data matrix appended to "https://download.pangaea.de/dataset/982597/files/". */ Binary Fig Title File name Variable File format Description Fig2.zip 2 Velocity profiles of inlets obtained by PIV Fig2.zip Streamwise velocity, streamwise velocity rms, critial time. dat (a) Velocity profiles obtained by PIV measurements at Re = 700 and x/H = 2.5 for t/(H/U0) = 0.96 (black up-triangle), 8.68 (blue down-triangle) and 29.26 (cyan circle), where the red line corresponds to the analytical solution from Shah and London. (b) Time series of streamwise velocity at (y/H, z/H) = (2.5, 0) and Re = 700. (c) Critical time for temporally fully developed velocity against the Reynolds number, where the time corresponding to 99% of the final steady velocity in (b) is taken as the critical time tc. (d) Velocity profiles at Re = 1500. (e) Velocity profiles at y/H = 2.5 for Re = 1500, 5000 and t/(H/U0) = 60, where the red line represents the same profile asin (a). (f) Velocity profiles at Re = 5000. (g) Profiles of the root-mean-square streamwise velocity at y/H = 2.5. The red dashed line indicates the mean uncertainty of the PIV measurement. Space-time velocity contours of the streamwise velocity at y/H = 2.5 for Re = 1500 (h) and Re = 5000 (i). Fig3.zip 3 Velocity at center of the junction Fig3.zip Velocity, correlation coefficient and Strouhal number dat Velocity field at the center of the junction for Re = 100 (a), Re = 160 (b), Re = 280 (c) and Re = 360 (d), where the contours show the magnitude of the velocity. (e) The correlation coefficient R(t) against time. (f) Strouhal number corresponding to the periodicity of R(t) against the Reynolds number. Fig4.zip 4 Streamwise velocity at the outlet channel Fig4.zip Time series of velocity at for Re = 1500 and Re = 5000 mat Time series of velocity at for Re = 1500 and Re = 5000 at y/H = 8, 12 and 16. In the mat files, x is the spanwise and y is the streamwise direction. Fig5.zip 5 Average streamwise velocity profiles Fig5.zip Average streamwise velocity and its gradient along spanwise dat Average streamwise velocity profiles uy (a) and their corresponding lateral gradients (b) for several Reynolds numbers at y/H = 16, and as a function of downstream position for Re = 5000 (c). The black square symbols represent fully developed turbulent duct flow (of aspect ratio two) at Re = 5300. Fig6.zip 6 TKE and dissipation rate Fig6.zip TKE, dissipation rate dat (a) Profiles of turbulent kinetic energy (k) at x/H = 16 and several Re, as indicated in the legend. (b) Profiles of the (estimated) turbulent dissipation rate (epsilon) at x/H = 16. (c) Decay of turbulent kinetic energy (circle symbols) and turbulent dissipation rate (square symbols) along the streamwise direction at y/H = 0.0, y/H = 0.5 and y/H = 0.75 in the outlet channel for Re = 5000. (d)-(e) Colormaps of the turbulent kinetic energy and dissipation at x/H = 16 from DNS at Re = 1500. (f) The comparison of the dissipation profiles computed with the usual definition and with the assumption in table II. Fig7.zip 7 Concentration at the outlet channel measured with PLIF Fig7.zip phi or phi1 mat "Time series of concentration at for Re = 1500 and Re = 5000 at x/H = 8, 12 and 16; In the mat files, x is the spanwise and y is the streamwise direction. Note that 2000 snapshot results are shown here because of the very large memory space requirement." Fig8.zip 8 Mean mixture fraction, scalar variance and scalar dissipation rate Fig8.zip mean mixture fraction, scalar variance and scalar dissipation rate dat The profile of mean mixture fraction (a) and scalar variance (b) at x/H = 16. The inset in (b) shows the mean scalar variance averaged along the x direction as a function of Re. (c) The profile of scalar dissipation. (d) Decay of scalar variance (circle symbols) and scalar dissipation (square symbols) at y/H = 0.0, y/H = 0.5 and y/H = 0.75 along the outlet channel for Re = 5000. Fig9.zip 9 PDF of the scalar Fig9.zip PDF dat Probability distribution function (PDF) of the scalar from measurements (symbols) and beta-PDF model (lines) at Re = 1500 and 5000 at selected points in the outlet. From left to right y/H = 0, 0.5 and 0.75 and from top to bottom x/H = 16, 12 and 8. Fig10.zip 10 Mechanical-to-scalar timescale ratio (C_phi) Fig10.zip Turbulent Re, C_phi and spectrum dat "Dependence of _phi on the turbulent Reynolds number ReT extracted from our measurements, with Re belong to [1500, 5000] and y/H belong to [0, 0.4]; One-dimensional longitudinal scalar spectra (x/H belong to [15.8, 16.2]) using a Wiener filter for ReT = 45, (Re, y/H) = (4000, 0.0) and (Re, y/H) = (5000, 0.06)."