The objective of this experimental investigation is to characterize the gas–surface interaction under different flow conditions. Therefore, the mass flow rates driven by a pressure gradient under isothermal conditions and by only a temperature gradient under constant pressure conditions are measured in the same microchannel for five different gases: helium, neon, nitrogen, argon and krypton. The pressure driven experiments are carried out in the hydrodynamic and slip flow regimes, $0.0016< \text{Knudsen number } (Kn) <0.12$, while the temperature driven experiments in the slip and transitional flow regimes have $0.05< Kn<0.45$. Using a previously developed methodology, the velocity and thermal slip coefficients are derived from the measured mass flow rates. By adopting the classical Maxwell boundary condition, the accommodation coefficients are found to be very different for both types of flows, with a significantly lower value for polyatomic nitrogen in the case of temperature gradient driven flows. An attempt to calculate the tangential momentum and normal energy accommodation coefficients in the frame of the Cercignani–Lampis model was successful only for the tangential momentum accommodation coefficient, which was found to be very close to that derived with the Maxwell model. However, it was not possible to obtain the values of the normal energy accommodation coefficient due to a lack of numerical results which connect the thermal slip and normal energy accommodation coefficients for very low values of the latter.

Shock-flame interactions are fundamental problems in many combustion applications ranging from flame acceleration to flame control in supersonic propulsion applications. The present paper seeks to quantify the rate of deformation of the flame surface and burning velocity caused by the interaction and to clarify the underlying mechanisms. The interaction of a single shock wave with a cellular flame in a Hele-Shaw shock tube configuration was studied experimentally, numerically and theoretically. A mixture of stoichiometric hydrogen-air at sub-atmospheric pressure was chosen such that large cells can be isolated and their deformation studied with precision subsequent to the interaction. Following passage of the incident shock and vorticity deposition along the flame surface, the flame cusps are flattened and reversed backwards into the burned gas. The reversed flame then goes through four stages. At times significantly less than the characteristic flame burning time, the flame front deforms as an inert interface due to the Richtmyer–Meshkov instability with nonlinear effects becoming noticeable. At times comparable to the laminar flame time, dilatation due to chemical energy release amplifies the growth rate of the Richtmyer–Meshkov instability. This stage is abruptly terminated by the transverse burnout of the resulting flame funnels, followed by a longer front re-adjustment to a new cellular flame evolving on the cellular time scale of the flame. The proposed flame evolution model permits us to predict the evolution of the flame geometry and burning rate for arbitrary shock strength below the shock-induced auto-ignition point and flames with unit Lewis number in two dimensions.

This work reports on modelling unsteady gas flow in porous media at the macroscopic scale in the slip regime, a topic of interest in a wide range of applications. The slip effect is modelled by means of a Navier-type boundary condition. A macroscopic model is derived from the initial-boundary-value problem governing unsteady, single-phase flow of a Newtonian fluid through homogeneous porous media in the creeping, isothermal and slightly compressible slip regime. For momentum transport, the macroscopic model involves two terms. The first consists of a convolution product between the macroscopic pressure gradient and the temporal derivative of an apparent dynamic permeability tensor; the second accounts for the memory of the initial condition. Both contributions are predicted from the solution of a unique closure problem that is independent of the initial flow configuration and of the macroscopic pressure gradient. The accuracy of the model is assessed by comparisons with direct numerical simulations performed at the pore-scale, which find excellent agreement. The simulations also show that a classical heuristic model, which is the consequence of assuming a separation of time scales between the pore-scale and the macroscale, is inadequate, in general, to correctly predict the macroscopic velocity. Results from this work provide a formal clear insight about unsteady flow in porous media in the slip regime, motivating further theoretical and experimental work.

We experimentally and theoretically study impacts into dense cornstarch and water suspensions. We vary impact speed as well as intruder size, shape and mass, and we characterize the resulting dynamics using high-speed video and an onboard accelerometer. We numerically solve previously proposed models, most notably the added-mass model as well as a class of viscous-like models. In the viscous-like models, the intruder dynamics is dominated by large, viscous-like forces at the boundary of the jammed front where large shear rates and accompanying large viscosities are present. We find that our experimental data are consistent with this class of models and inconsistent with the added-mass model. Our results strongly suggest that the added-mass model, which is the dominant model for understanding the dynamics of impacts into shear-thickening dense suspensions, should be updated to include these viscous-like forces.