A theoretical study of the effect of gravity modulation on a nanofluid filled porous medium subjected to a constant temperature gradient across is made. In particular the onset of convection in water based nanofluids containing conventional metallic and metal oxide particles (Cu‐water, Al2O3‐water, TiO2‐water, and Ag‐water) is investigated. The nanofluid behaves more like a single‐phase fluid rather than a conventional solid–liquid mixture. Darcy's law of flow through porous media and Boussinesq approximation are employed. Stability analysis based on the method of small perturbations is performed using the normal mode assumption. The resulting synchronous and asynchronous, restricted to subharmonic, instability modes are predicted directly through the Mathieu functions for different nanoparticle concentrations. It is found that Al2O3‐water nanofluid can be effectively used in applications involving gravity modulation.

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Abstract The effect of gravity modulation in controlling the onset of convection in a porous medium saturated with a second-grade fluid of Rivlin–Ericksen type is studied. The Brinkman equation of flow through porous media is considered with effective viscosity different from fluid viscosity. Necessary conditions for the occurrence of instability due to infinitesimal perturbations are found using the method of normal modes. Following Floquet analysis, the thresholds corresponding to synchronous and subharmonic solutions and the transition between them are predicted using the Mathieu functions.