We analyze the effect of large-scale coherent structures on the deposition of low-inertia particles in a turbulent pipe flow using extended proper orthogonal decomposition (EPOD) and spectral analysis. We perform direct numerical simulations (DNSs) at two the Reynolds numbers 5300 and 10 300 (based on bulk parameters) with the particles released at the pipe inlet. The equilibrium Eulerian model is employed for calculating particle velocity, and the analysis is limited to particles with Stokes number (based on wall units) less than 1. Increasing the Stokes number increases the energy at small streamwise wavelengths (due to inertial clustering), and the spectral energy peak moves from ${\lambda }_z^{+}\approx 1000$ to ${\lambda }_z^{+}\approx 150$. The spectral peak in the $({\lambda }_z^{+},y^{+})$ plane, where $y^{+}$ is the wall-normal distance, moves from the buffer layer to the logarithmic region. Gravity has a substantial effect on the POD mode shapes. For the downward flow, a second peak appears closer to the center. A new Fukagata-Iwamoto-Kasagi (FIK) identity is derived for the wall deposition rate coefficient (Sherwood number, Sh) and employed to quantify the contributions of the mean and fluctuating velocity and particle concentration fields for different Stokes, Froude, and Reynolds numbers. Modes with azimuthal wave numbers $k_{\theta }$ equal to three or four are found to contribute most to deposition. Application of the developed methodology to higher Reynolds number can elucidate the role of large- and very-large-scale flow structures on particle deposition to the wall. It is well known these structures leave their footprint at the wall but their contribution to deposition is not well understood.

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• Fluid-particle interactions
• Multiphase flows
• Particle-laden flows
• Turbulent boundary layers
• Turbulent multiphase flows