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We investigate oscillatory instability and routes to chaos in Rayleigh-B\'enard convection of electrically conducting fluids in presence of external horizontal magnetic field. Three dimensional direct numerical simulations (DNS) of the governing equations are performed for the investigation. DNS shows that oscillatory instability is inhibited by the magnetic field. The supercritical Rayleigh number for the onset of oscillation is found to scale with the Chandrasekhar number $\mathrm{Q}$ as $\mathrm{Q}^{\alpha}$ in DNS with $\alpha = 1.8$ for low Prandtl numbers ($\mathrm{Pr}$). Most interestingly, DNS shows $\mathrm{Q}$ dependent routes to chaos for low Prandtl number fluids like mercury ($\mathrm{Pr} = 0.025$). For low $\mathrm{Q}$, period doubling routes are observed, while, quasiperiodic routes are observed for high $\mathrm{Q}$. The bifurcation structure associated with $\mathrm{Q}$ dependent routes to chaos is then understood by constructing a low dimensional model from the DNS data. The model also shows similar scaling laws as DNS. Bifurcation analysis of the low dimensional model shows that origin of different routes are associated with the bifurcation structure near the primary instability. These observations show similarity with the previous results of convection experiments performed with mercury.

Oscillatory instability and routes to chaos in Rayleigh-Bénard convection: Effect of external magnetic field