When background wind is moderate, urban environment may face great challenges as removal ability of airborne pollutants, anthropogenic heat and moisture reduces. Under this condition, buoyancy-driven flow can break the dominant role of the background wind and affect the urban environment greatly. Knowledge of buoyant flow dynamics is essential for understanding urban wind/thermal environment and related pollutant transport. As one important buoyant flow, thermal plumes generated by a cluster of high-rise compact buildings are modelled using a laboratory water channel in this study. Flow similarity, when the buoyancy is prominent, is well satisfied. Mean flows, turbulent statistics, and rising features under different Froude (Fr) numbers are systematically explored by two-dimensional particle image velocimetry (PIV) technique. Overall, the transition of buoyancy-dominated and wind-dominated flow dynamics is observed when Fr number varies. For horizontal and vertical velocity components, peak magnitude and position are quantitatively determined on four building poles. Fr number obviously changes the relative magnitude of v-peak among four poles. Turbulence statistics, including velocity variance, turbulence kinetic energy, power spectral density, turbulent momentum flux, and turbulence production, are systematically analyzed, which show consistent results. As Fr number decreases, the plume mixing is enhanced. When particularly aiming at the plume region, smaller Fr numbers usually make the maximum velocities occur more in the plume central part rather than the boundaries. A quantitative rational function is given to describe the relation between Fr number and the rising angle, as a = 8.76/(Frq + 0.097).