Viscoelastic thermovibrational flow driven by sinusoidal and pulse (square) waves

The present study aims to probe the role of an influential factor heretofore scarcely considered in earlier studies in the field of thermovibrational convection, that is, the specific time-varying shape of the forcing used to produce fluid motion under the effect of an imposed temperature gradient. Towards this end, two different temporal profiles of acceleration are considered, namely, a classical (sinusoidal) and a pulse (square) wave. Their effects are analyzed in conjunction with the ability of a specific category of fluids to accumulate and release elastic energy, i.e. that of Chilcott–Rallison finitely extensible nonlinear elastic (FENE-CR) liquids. Through solution of the related governing equations in time-dependent, three-dimensional and non-linear form for a representative set of parameters (generalized Prandtl number Pr_g = 8, normalized frequency Ω = 25, solvent-to-total viscosity ratio ξ = 0.5, elasticity number θ = 0.1, and vibrational Rayleigh number Ra_ω = 4000), it is shown that while the system responds to a sinusoidal acceleration in a way that is reminiscent of modulated Rayleigh-Bénard (RB) convection in a Newtonian fluid (i.e. producing a superlattice), with a pulse wave acceleration, the flow displays a peculiar breaking-roll mode of convection that is in-between classical (un-modulated) RB in viscoelastic fluids and purely thermovibrational flows. Besides these differences, these cases share important properties, namely, a temporal subharmonic response and the tendency to produce spatially standing waves.