Modified Velocity Field Research Articles

The response of small-scale shear layers to perturbations in turbulence

The perturbation response of small-scale shear layers in turbulence is investigated with direct numerical simulations (DNS). The analysis of shear layers in isotropic turbulence suggests that the typical layer thickness is about four times the Kolmogorov scale $\eta$ . Response for sinusoidal perturbations is investigated for an isolated shear layer, which models a mean flow around the shear layers in turbulence. The vortex formation in the shear layer is optimally promoted by the perturbation whose wavelength divided by the layer thickness is about 7. These results indicate that the small-scale shear instability in turbulence is efficiently promoted by velocity fluctuations with a wavelength of about $30\eta$ . Furthermore, DNS are carried out for decaying turbulence initialised by the artificially modified velocity field of isotropic turbulence. The vortex formation from shear layers is accelerated under the influence of external perturbations with the efficient wavelength to promote the instability. When velocity fluctuations with this wavelength are eliminated by a band-cut filter, the shear layers tend to persist for a long time without producing vortices. These behaviours affect the number of vortices in turbulence, which increases and decreases when velocity perturbations with the unstable wavelength of the instability are artificially amplified and damped, respectively. The increase in the number of vortices results in the enhancement of kinetic energy dissipation, enstrophy production and strain self-amplification. These results indicate that the perturbation response of shear layers is important in the small-scale dynamics of turbulence as well as the modulation of small-scale turbulent motions by external disturbance.

Hydrodynamic analysis of the magnetic field dependent viscous fluid flow and thermosolutal convection between rotating channels

According to research, exposing a person to a magnetic field enhances blood flow and minimizes their risk of suffering a heart attack. Ferrohydrodynamics is the study of fluid motion mechanics that is affected by strong magnetic polarisation forces (FHD). Ferrofluids may transmit heat in a variety of ways by using magnetic fluids. This behaviour is demonstrated by liquid-cooled speakers, which utilise less ferrofluid to prevent heat from reaching the speaker coil. This modification boosts the coil’s ability to expand, which enables the loudspeaker to create high-fidelity sound. It is investigated how the fluid dynamics of spinning, squeezing plates are affected by thermosolutal convection and a magnetic field dependent (MFD) viscosity. Standard differential equations are used to represent the equations of the modified form of Navier Stokes, Maxwell’s, and thermosolutal convection. The magnetic field, modified velocity field equations, and thermosolutal convection equations all yield suitable answers. Additionally computed and thoroughly detailed are the MHD torque and fluid pressure that are imparted to the top plate. To create a technique with quick and certain convergence, the resulting equations for uniform plates are solved using the Homotopy Analysis Method (HAM) with appropriate starting estimates and auxiliary parameters. The validity and reliability of the HAM outcomes are shown by comparing the HAM solutions with the BVP4c numerical solver programme. It has been found that a magnetic Reynolds number lowers the temperature of the fluid as well as the tangential and axial components of the velocity field. Additionally, when the fluid’s MFD viscosity rises, the axial and azimuthal components of the magnetic field behave in opposition to one another. This study has applications in the development of new aircraft take-off gear, magnetorheological airbags for automobiles, heating and cooling systems, bio-prosthetics, and biosensor systems.

On the effect of ionic wind on structure and temperature of laminar premixed flames influenced by electric fields

The general influence of electric fields on flames has been known for many years, the underlying mechanisms, however, are not finally identified yet. The changes observable in the flame structure and the pollutant emission when a flame is exposed to an electric field are mainly attributed to the ionic wind. Yet, especially the resulting flame temperature under the influence of electric fields is still an open research topic. Thus, in the present study the temperature distributions in a laminar premixed Bunsen type flame were measured for different air–fuel-mixtures using point-wise vibrational coherent-anti-Stokes Raman spectroscopy (vibrational-CARS) when a static electric field is activated. Additionally, CH2O- and OH-PLIF (planar laser-induced fluorescence) and particle image velocimetry (PIV) were applied to identify the best-suited locations for the temperature measurements and to visualize changes in the flame structure induced by the electric field for clarifying the underlying mechanisms.The results show that the electric field leads to increased fresh gas and maximum flame temperatures and that this effect is most distinct for rich premixed flames. Both PIV/PLIF and CARS experiments confirm that the flame is constricted and pushed towards the burner due to the ionic wind, which explains the increased temperature of the fresh gas and of the exhaust gas. The increased burner and fresh gas temperatures due to the ionic wind were verified by additional thermocouple measurements. The ionic wind is maximal for the fuel-rich flame, which is explained by the lowest electric resistance due to high charge carrier concentration, and larger flame area, which reduces the gap between the flame and the positively charged electrode. Furthermore, a clear broadening of the hot exhaust gas region was measured, which increases the burn out of UHC and CO reported in the literature. Furthermore, this effect increases the residence time that could also contribute to changed pollutant emission for flames under the influence of electric fields. It can be concluded that the change of the flame temperature is mainly the result of the modified velocity field induced by the ionic wind. Consequently, the ionic wind is the dominant mechanism for many effects observed in flames when applying weak electric fields.

An improved version of the Hughes model for pedestrian flow

Roger Hughes proposed a macroscopic model for pedestrian dynamics, in which individuals seek to minimize their travel time but try to avoid regions of high density. One of the basic assumptions is that the overall density of the crowd is known to every agent. In this paper we present a modification of the Hughes model to include local effects, namely limited vision, and a conviction towards decision making. The modified velocity field enables smooth turning and temporary waiting behavior. We discuss the modeling in the micro- and macroscopic setting as well as the efficient numerical simulation of either description. Finally we illustrate the model with various numerical experiments and evaluate the behavior with respect to the evacuation time and the overall performance.

Non-habitual wearing of high-heeled shoes increases loading coefficient, stress and forefoot-rearfoot load imbalance in females as revealed by the modified velocity field diagram

Summary Study aim: To determine the effects of non-habitual wearing of high-heeled shoes on loading coefficient, loading stress and forefoot-rearfoot load imbalance in females Materials and methods: Fifty young adult female subjects were selected using a convenience sampling technique, and studied utilising crossover control research design. They walked barefoot and thereafter, in high-heeled shoes; for a distance of 10- metre measured out in a gait laboratory. They demonstrated their gait twice for each speed, along a 10 metres walkway, at five speeds varying from very slow to very fast. Mean values of steps and time were recorded and used to calculate the values of velocity, stride length, stride frequency, double-support, swing, single-support, and stance phases of stride. These were adopted to form a modified velocity field diagram (MVFD). The data obtained were statistically analysed using a t-test for correlated means, with alpha set at 0.05. Results: The MVFDs revealed that the F/R load ratios were obtained as 0.67 and 1.5, while loading coefficient was 0.4 and 0.6, for barefoot and high-heeled walking, respectively. Loading coefficient and stress in high-heeled walking was 1.50 and 1.88 of the value in barefoot walking, respectively. Conclusions: Non-habitual wearing of high-heeled shoes increased F/R load imbalance, loading coefficient, and stress in the foot. However, there was no evidence of gait pathology in the subjects when they walked barefoot. Thus, non-habitual use of high-heeled shoes by the subjects did not translate to significant residual biomechanical derangements in the locomotor apparatus otherwise the kinematic data recorded for barefoot walking would have approximated the values obtained during high heeled walking.

Investigation of eddy-viscosity models modified using discrete filters: A simplified “regularized variational multiscale model” and an “enhanced field model”

Subgrid-scale (SGS) models for large-eddy simulation (LES) having the formalism of an effective eddy-viscosity model, but that operates on a modified velocity field, are further evaluated and new ones are proposed. The modified field is obtained using regular filtering of the LES field carried out in physical space. This is actually done by using a discrete and compact operator (only using nearest neighbors values), eventually iterated; this ensures that the proper filtering behavior is preserved, even for near wall points. The first model investigated here is inspired by the variational multiscale approach originally proposed by T. J. Hughes et al. [Phys. Fluids 13, 505 (2001)]. Here, the modelling is simplified, leading to a SGS viscosity effect operating on the “small-scale LES field” that is obtained by subtracting the LES field from its filtered counterpart. Such a model (here called RVMM for short) was already proposed and partially evaluated {e.g., see G. S. Winckelmans and H. Jeanmart [Direct and Large-Scale Eddy Simulation IV (Kluwer, Dordrecht, 2001)] and H. Jeanmart and G. S. Winckelmans (CTR Proceedings of the Summer Program, 2002), the “model M2” of A. W. Vreman [Phys. Fluids 15, L61 (2003)], the “high-pass filtered Smagorinsky model” of S. Stolz et al. [Direct and Large-Eddy Simulation V (Kluwer, Dordrecht, 2004) and S. Stolz et al., Phys. Fluids 17, 065103 (2005)]}. The other model investigated here is an “enhanced field model” (EFM). The SGS viscosity model then operates on a LES field that is artificially enhanced at the small scales; that obtained by adding to the LES field the small-scale field. The two model families are presented in a unified way; they have a behavior that combines viscous and hyperviscous effects, while remaining simple and practical. They however do not naturally have the proper y3 near wall behavior for the SGS dissipation; hence, they need some near wall damping. To ensure the proper near-wall behavior, we use here the dynamic procedure (self-consistent for each model). The performance of both models is compared to that of other models (also dynamic): the Smagorinsky model, hyperviscosity models, and a hybrid model combining explicitly a Smagorinsky term and a hyperviscosity term. The cases here investigated are LES of decaying isotropic turbulence starting at Reλ=90 and LES of turbulent channel flow at Reτ=395. A good behavior of the RVMM and EFM, as compared to the others, is observed in all cases. They constitute an easily implemented and better alternative than the dynamic Smagorinsky model.

Mean flow and spiral defect chaos in Rayleigh-Bénard convection.

We describe a numerical procedure to construct a modified velocity field that does not have any mean flow. Using this procedure, we present two results. First, we show that, in the absence of the mean flow, spiral defect chaos collapses to a stationary pattern comprising textures of stripes with angular bends. The quenched patterns are characterized by mean wave numbers that approach those uniquely selected by focus-type singularities, which, in the absence of the mean flow, lie at the zigzag instability boundary. The quenched patterns also have larger correlation lengths and are comprised of rolls with less curvature. Secondly, we describe how the mean flow can contribute to the commonly observed phenomenon of rolls terminating perpendicularly into lateral walls. We show that, in the absence of the mean flow, rolls begin to terminate into lateral walls at an oblique angle. This obliqueness increases with the Rayleigh number.

Contact modeling in forging simulation

The problem of contact in forging simulation is overviewed and revised in this paper. The integral form of equilibrium equations must be solved subject to the incompressibility and impenetrability or contact conditions. Minimization techniques such as the Lagrange multipliers method, the penalty function method or the augmented Lagrangian method have been used to convert the problem to the unconstrained form. These approaches are based on solving the equilibrium equations while preventing any constraints to be violated. An alternative is to solve the equilibrium equations regardless of the constraints. Then, if any of the constraints is found to be violated, a simple projection is used to force the node back to the contact zone. The major drawback of this simpler approach is the loss of volume/mass in the time steps. A quick remedy is to choose very small loading steps. In this paper, a simple velocity-based projection technique will be proposed to overcome the above mentioned deficiency. In this method the projection procedure is performed iteratively using a modified velocity field in such a way that the final position of the node will be exactly on the contact surface. This is very similar to the so-called “Return Mapping” algorithm in computational plasticity.

Velocity measurements in a turbulent round jet using PTV: the effect of tracer particles

The effects of different tracer particles used in the PTV technique on the velocity field are investigated. The measurements are taken in a water round jet at a Reynolds number equal to about 40000, a flow field widely examined in the literature, in which strong velocity differences are encountered. The interest is focused onto particles with different density. Although the mean and rms values are almost unaffected, both measurements obtained with light and heavy particles (in comparison to the density of the fluid) reveal a modified velocity field. The moments of velocity differences (structure functions) point out that such a modification is felt within each range of flow scales. Particles with density almost equal to that of the fluid reproduce the flow behaviour over all the scales.

The wave structure of turbulent spots in plane Poiseuille flow

The wave packets located at the wingtips of turbulent spots in plane Poiseuille flow have been investigated by hot-film anemometry. The streamwise velocity disturbances associated with the waves were found to be antisymmetric with respect to the channel centreline. The amplitude of the waves had a maximum close to the wall that was about 4% of the centreline velocity. The modified velocity field outside the spot was measured and linear stability analysis of the measured velocity profiles showed that the flow field was less stable than the undisturbed flow. The phase velocity and amplitude distribution of the waves were in reasonable agreement with the theory, which together with the symmetry properties indicate that the wave packet consisted of the locally least stable Tollmien-Schlichting mode.