On the Use of Hydrodynamic Instability Test as an Efficient Tool for Evaluating Viscoelastic Fluid Models

Abstract The so-called ''second-order'' rheological model has been of widespread use in studies related to viscoelastic fluids. The model, however, has been shown by Mathematics Subject Classification: 76E2

A novel heat transfer switch using the yield stress

International audienceWe explore the feasibility of a novel method for the regulation of heat transfer across a cavity, by using a controllable yield stress in order to suppress the convective heattransfer. Practically, this type of control can be actuated with electro-rheological ormagneto-rheological fluids. We demonstrate that above a given critical yield stressvalue only static steady regimes are possible, i.e. a purely conductive unyielded fluidfills the cavity. We show that this limit is governed by a balance of yield stressand buoyancy stresses, here described by B. With proper formulation the criticalstate can be described as a function of the domain geometry, and is independent ofother dimensionless flow parameters (Rayleigh number, Ra, and Prandtl number, Pr).On the theoretical side, we examine the conditional stability of the static regime.We derive conservative conditions on disturbance energy to ensure that perturbationsfrom a static regime decay to zero. Assuming stability, we show that the kineticenergy of the perturbed field decays to zero in a finite time, and give estimatesfor the stopping time, t0. This allows us to predict the response of the system insuppressing advective heat transfer. The unconditional stability is also considered forthe first time, illustrating the role of yield stress. We focus on the hydrodynamiccharacteristics of Bingham fluids in transition between conductive and convectivelimits. We use computational simulations to resolve the Navier–Stokes and energyequations for different yield stresses, and for different imposed controls. We showthat depending on the initial conditions, a yield stress less than the critical value canresult in temporary arrest of the flow. The temperature then develops conductivelyuntil the fluid yields and the flow restarts. We provide estimates of the hydrodynamictimescales of the problem and examples of flow transitions. In total, the theoreticaland computational results establish that this methodology is feasible as a control, atleast from a hydrodynamic perspective