Hydrodynamics of trickle-bed reactors involve complex interactions of gas and liquid phases with packed solids. Such complex interactions manifest in different flow regimes occurring in trickle-bed reactors. Knowledge of prevailing flow regime, pressure drop, and liquid holdup is essential for design and performance evaluation of the reactor. Detailed knowledge of fluid dynamics is essential for "a priory" predictions as well as for interpretation of available data. In this study, we have used wall pressure fluctuation measurements to identify prevailing flow regime in trickle beds. Experiments were carried out on two scales of columns (of diameter 10 cm and 20 cm) with two sets of particles (3 min and 6 mm diameter spherical particles). Effects of prewetted and unwetted bed conditions on pressure drop and liquid holdup were reported for a range of operating conditions (V-G = 0.22-0.44 kg/m(2)s, V-L = 2-24 kg/m(2)s). A comprehensive CFD model was developed to predict measured hydrodynamic parameters. The model was evaluated by comparing predictions with the experimental data. The CFD model was then extended to predict the fraction of liquid holdup suspended in the form of drops in the bed. At the end, the CFD model was used to understand hydrodynamics of trickle beds with periodic operation. The experimental data as well as computational models discussed here will have significant implications for understanding and designing of trickle-bed reactors.