This paper presents an investigation on natural convection in a cavity with imposed modulated thermal gradients or modulated gravity forces. Numerical computations are presented, which are based on the finite element solution of the transient Navier-Stokes and energy balance equations, along with appropriate thermal boundary conditions or time-varying gravity forces. In parallel with numerical development, an experimental system is setup where oscillating wall temperatures are prescribed to produce modulated temperature gradients and the velocity fields are measured by a laser-based particle image velocimetry (PIV) system. Computed results compare well with experimental measurements for various conditions. With the mathematical model, so verified by experimental measurements, numerical simulations are carried out to study the effects of modulation frequency and Prandtl number on the fluid flow. Results show strong non-linear interaction in a fluid with a relative high Prandtl number within the intermediate range of modulated frequency. It is also found that for a fluid with a small Prandtl number typical of molten metals and semiconductor melts, modulated gravity and thermal gradients produce almost the same flow field both in structure and in magnitude. (C) 2004 Elsevier Ltd. All rights reserved.