We report experimental investigations of an unconfined liquid-solid fluidized bed at low to moderate Reynolds number. The fluidization velocity $U_f$ is measured when an upward flow is imposed, and the sedimentation velocity $U_{\mathrm{sed}}$ is measured when the flow is stopped. Only when the level of inlet flow fluctuation is sufficiently low will these two velocities coincide, as expected for an unbound system. A systematic comparison between $U_f$ and $U_{\mathrm{sed}}$ should therefore be made when attempting to determine fluidization laws. Comparisons with previous studies show that $U_f$ measurements gather on a single function of the ratio $\mathrm{\Phi }/{\mathrm{\Phi }}_{\mathrm{pack}}$ between the particle volume fraction $\mathrm{\Phi }$ and its packing value ${\mathrm{\Phi }}_{\mathrm{pack}}$, except for a factor $K$ that encompasses all effects other than concentration: Stokes number, Reynolds number, confinement, and inlet condition. Time-resolved measurements of the fluidized bed surface show two contributions: uncorrelated high-frequency fluctuations and low-frequency oscillations that remain correlated over minutes. The coherent oscillations correspond to the arrival of upward concentration waves at the bed surface. Their wavelength, which characterizes the length scale of large-scale heterogeneities, was known to diverge as the packing state is approached. Here, we found that it also increases with decreasing concentration, suggesting that the loss of global bed stability observed in previous work at high expansions may be related to the development of heterogeneous structures of a size comparable to that of the column.

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• Fluid-particle interactions
• Fluidized beds
• Interparticle interactions
• Multiphase flows
• Particle-laden flows
• Waves and free surface flows