Effect of flat wall length on decay and shock structure of a supersonic square wall jet

Experiments were conducted to find the effect of wall length on the decay behaviour and shock structure of a supersonic wall jet issuing from c-d nozzle of the square-shaped exit. A straight flat wall of width same as the side length of the square was attached to the lip of the nozzle such that the leading edge of the wall and the side of the square aligned properly which allowed the supersonic jet to graze past the flat wall. Experiments were conducted with five different wall lengths, that is, [Formula: see text] = 0.5, 1, 2, 4 and 8. Wall pressure measurements were made from leading edge to the trailing edge of the wall along its centreline. Schlieren flow visualization of the jet flow over the wall for the different wall lengths revealed the shock pattern and the effect of the wall length on the shock structure. The shock structure and jet deflection were significantly affected due to the presence of the wall. There was an upward jet deflection for [Formula: see text] up to [Formula: see text] whereas a downward jet deflection was observed for [Formula: see text]. Noticeable changes in the shock structure were observed for the wall lengths up to 2 D h. The wall length also significantly affected the jet decay characteristics and supersonic core length. Maximum enhancement in jet decay and maximum reduction in supersonic core length resulted when the wall length was [Formula: see text]. However, when the wall length was increased to [Formula: see text], there was a significant reduction in jet decay and a recovery of [Formula: see text]. Presence of wall always resulted a reduction in Lsc irrespective of wall length. The wall effect was to induce a more precipitous pressure drop closer to the nozzle exit, and a more gradual drop farther from it for [Formula: see text] > [Formula: see text].

Thin-film Rayleigh–Taylor instability in the presence of a deep periodic corrugated wall

Rayleigh–Taylor instability of a thin liquid film overlying a passive fluid is examined when the film is attached to a periodic wavy deep corrugated wall. A reduced-order long-wave model shows that the wavy wall enhances the instability toward rupture when the interface pattern is sub-harmonic to the wall pattern. An expression that approximates the growth constant of instability is obtained for any value of wall amplitude for the special case when the wall consists of two full waves and the interface consists of a full wave. Nonlinear computations of the interface evolution show that sliding is arrested by the wavy wall if a single liquid film residing over a passive fluid is considered but not necessarily when a bilayer sandwiched by a top wavy wall and bottom flat wall is considered. In the latter case interface tracking shows that primary and secondary troughs will evolve and subsequently slide along the flat wall due to symmetry-breaking. It is further shown that this sliding motion of the interface can ultimately be arrested by the top wavy wall, depending on the holdup of the fluids. In other words, there exists a critical value of the interface position beyond which the onset of the sliding motion is observed and below which the sliding is always arrested.

Magnetohydrodynamics with Applications to the Study of Electrolysis and Turbulence

The equations of magnetohydrodynamics (MHD) are presented as continual modeling for slow motions. The original equations of the MHD environment are linearized, reduced, and applied to the analysis of environments characterized by the phenomena of electrolysis and turbulence. A continual approach for electrolysis and turbulence is presented, and the real-life ongoing studies are considering local models. The formulation of the problem and its analysis are presented as the density of the MHD-field decreases from a flat wall. Experimental studies with respect to propulsion devices in sea water are characterized.

Discrimination of 2D wall textures by passive echolocation for different reflected-to-direct level difference configurations

In this work, we study people’s ability to discriminate between different 2D textures of walls by passive listening to a pre-recorded tongue click in an auralized echolocation scenario. In addition, the impact of artificially enhancing the early reflection magnitude by 6dB and of removing the direct component while equalizing the loudness was investigated. Listening test results for different textures, ranging from a flat wall to a staircase, were assessed using a 2 Alternative-Forced-Choice (2AFC) method, in which 14 sighted, untrained participants were indicating 2 equally perceived stimuli out of 3 presented stimuli. The average performance of the listening subjects to discriminate between different textures was found to be significantly higher for walls at 5m distance, without overlap between the reflected and direct sound, compared to the same walls at 0.8m distance. Enhancing the reflections as well as removing the direct sound were found to be beneficial to differentiate textures. This finding highlights the importance of forward masking in the discrimination process. The overall texture discriminability was found to be larger for the walls reflecting with a higher spectral coloration.

Active colloids under geometrical constraints in viscoelastic media

We study the behavior of active particles (APs) moving in a viscoelastic fluid in the presence of geometrical confinements. Upon approaching a flat wall, we find that APs slow down due to compression of the enclosed viscoelastic fluid. In addition, they receive a viscoelastic torque leading to sudden orientational changes and departure from walls. Based on these observations, we develop a numerical model which can also be applied to other geometries and yields good agreement with experimental data. Our results demonstrate, that APs are able to move through complex geometrical structures more effectively when suspended in a viscoelastic compared to a Newtonian fluid. Graphic Abstract

Free Convection Heat Transfer in a Micropolar Fluid Confined Between a Long Vertical Wavy Wall and a Parallel Flat Wall

The free convection heat transfer in a micropolar fluid confined between a long vertical wavy wall and a parallel flat wall has been studied. Analysis of fluid flow over a wavy wall is of interest because of its physical applications such as transpiration cooling of re-entry vehicles, rocket booster and film vaporization, in combustion chambers etc. The equations governing the flow and the heat transfer have been solved subject to the relevant boundary conditions by assuming that the solution consists of two parts viz. a mean part and a perturbed one. To obtain the perturbed part of the solution, use has been made of the long Wavy approximation. The sets of differential equations have been solved by Finite Element Method. The zeroth order, the first order and total solution of the problem have been evaluated numerically for several sets of values of the various parameters entering the problem and are depicted graphically.

Viscous fluid microflows in cells of a porous medium in the presence of a gradient pressure

A simulation of the flow of a viscous fluid with a given pressure gradient through a porous structure, which was represented as a system of fixed particles, was carried out. Inside the porous structure there are moving particles, which are markers of microflows in the cells. The viscous fluid flows along a flat wall bounding the porous structure on one side. The calculations take into account the hydrodynamic interaction of all particles, both moving and stationary between themselves and with the plane. Computer simulations of this kind of flows through model structures formed, respectively, of 441, 567 periodically and 478 randomly located motionless particles of effective size and different positions of the flat wall, were carried out. The size of the moving particles placed in a viscous liquid was 0.2 of the size of the effective particles. The results of numerical simulation showed that microflows with an opposite direction of velocity are realized inside the structure, which follows from Darcy’s law. Such a complex dynamics of the flow inside the porous structure means that the use of averaged equations of fluid filtration gives an incorrect picture of the flow at the pore size and can serve as an explanation of the nonlinear dependence of the average filtration rate on the applied pressure gradient.