Size-resolved particle-decay measurements in Rayleigh–Bénard turbulence within the Pi Convection-Cloud Chamber

<p dir="ltr">This dataset underpins the paper <i>“Observed deviation from Stokes’ Law in the dry deposition of heavy particles in Rayleigh–Bénard turbulence.”</i> It contains every raw time-series measurement and all derived decay constants used to test whether small, heavy particles settle in buoyancy-driven turbulence according to Stokes' Law.</p><h4><b>Scientific context</b></h4><p dir="ltr">Dry deposition governs how long aerosols remain airborne, influencing air-quality, weather and climate. Theory predicts that, for very low-inertia particles (St ≪ 1), the settling and deposition velocity scales with the square of the particle diameter (Stokes settling), even in turbulent flows. We performed laboratory tests of this assumption in anisotropic, buoyancy-driven turbulence generated by Rayleigh–Bénard (RB) convection. Contrary to Stokesian expectations, we find an approximately linear diameter–dependence.</p><h4><b>Experimental apparatus</b></h4><ul><li><b>Facility:</b> 3.14 m³ Pi Convection-Cloud Chamber at Michigan Technological University.</li><li><b>Flow:</b> Dry RB turbulence driven by ΔT = 10 K and 20 K (Ra ≈ 1–2 × 10⁹). The large-scale circulation period (~60 s) is our characteristic mixing time τₘ.</li><li><b>Particles:</b></li><li><ul><li>DEHS oil droplets, 1–10 µm, ρ = 912 kg m⁻³, generated continuously with a Palas MAG-3000.</li><li>Solid, hollow, and ultra-hollow glass microspheres, 1–38 µm, ρ = 600–2500 kg m⁻³, injected in pulses with a compressed-air “air-cannon.”</li><li>Resulting Kolmogorov-scale Stokes numbers 3 × 10⁻⁵ – 0.1.</li></ul></li><li><b>Optical particle counters (OPCs):</b></li><li><ul><li>Palas WELAS 2000 (up to 128 bins, 0.2–100 µm, 5 L min⁻¹)</li><li>Alphasense OPC-N3 (24 bins, 0.35–40 µm, 0.28 L min⁻¹)<br>Counters were mounted at top and bottom ports; most data sampled at 1 Hz, a subset at ~1 min cadence.</li></ul></li><li><b>Trials:</b> 68 experiments (65 usable), July 2023 and May/June 2025, yielding 1,322 fitted decay constants across 83 size–density classes.</li></ul><h4><b>Methodology & processing workflow</b></h4><ol><li>Inject particles.</li><li>Record number concentration decay for each size bin for 30–120 min.</li><li>Store raw data in a pandas MultiIndex DataFrame with levels<br><code>particle-counter type [welas/opc] → location (top/bottom) → date → trial</code><br>(see README for indexing examples).</li><li>Apply centered rolling averages and censor start/end segments that deviate from log-linearity.</li><li>Perform ordinary-least-squares fits to ln C(t) to obtain decay time-constant τ and regression diagnostics (r-value, points-used, etc.).</li><li>Aggregate τ values by size-density class and compare against Stokes theory; all figure generation is scripted in <b>plots.ipynb</b>.Reproducibility notes</li></ol><h4><b>Reproducibility notes</b></h4><p dir="ltr">Running <code>plots.ipynb</code> end-to-end in Google Colab reproduces all figures in ~5 minutes on a free GPU/CPU runtime. All graphical operations use publicly available libraries; no proprietary software is required.</p><table><tr><th><p dir="ltr"><b>File</b></p></th><th><p dir="ltr"><b>Format</b></p></th><th><p dir="ltr"><b>Contents</b></p></th><th><p dir="ltr"><b>Typical loading command</b></p></th></tr><tr><td><b>particle_decay_data.pkl</b></td><td><p dir="ltr">Pickle</p></td><td><p dir="ltr">Raw concentration time-series for every trial (MultiIndex DataFrame, units # cm⁻³)</p></td><td><code>pd.read_pickle()</code></td></tr><tr><td><b>metadata.pkl</b></td><td><p dir="ltr">Pickle</p></td><td><p dir="ltr">Trial-level, bin-by-bin metadata and all fitted parameters (1,322 rows)</p></td><td><code>pd.read_pickle()</code></td></tr><tr><td><b>plots.ipynb</b></td><td><p dir="ltr">Jupyter Notebook</p></td><td><p dir="ltr">Recreates every figure from the paper (Colab-ready)</p></td><td><p dir="ltr">Open in Colab or Jupyter; requires </p><i>pandas ≥ 2.2, numpy ≥ 1.26, scipy ≥ 1.13, matplotlib ≥ 3.9</i></td></tr><tr><td><b>README.pdf</b></td><td><p dir="ltr">PDF</p></td><td><p dir="ltr">Detailed file schema, example queries, software dependencies, and Colab setup instructions</p></td><td><p>—</p></td></tr></table><p><br></p>