To investigate the forced-oscillation-frequency responses on the three-dimensional thermal convection inside a cube heated from one wall and chilled from its opposite wall in the non-gravitational field, the authors conduct computations at Plandtl number Pr = 7.1 (water). The direction of the forced sinusoidal oscillation is parallel to the temperature gradient inside the cube. As a result, five kinds of flow structures S2, S4, S5, S6 and S appear in the tested ranges of vibrational Rayleigh

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... mber Ra η and non-dimensional forced-oscillation frequency ω. The S consists of a pair of trident currents. Whenever it is not conductive but convective for ω < 5.0×10 2 , convective motion always starts with the S4 from the rest at each forcing cycle. The authors find out the optimum frequency where the amplitude of a spatially-averaged kinetic energy attains the maximum at each Ra η . At the optimum frequency, flow structure is always characterised by the S4. So, the optimum frequency can be related with the S4.