The influence of near-wall density and viscosity gradients on turbulence in channel flows

The influence of near-wall density and viscosity gradients on near-wall turbulence in a channel are studied by means of Direct Numerical Simulation (DNS) of the low-Mach number approximation of the Navier--Stokes equations. Different constitutive relations for density and viscosity as a function of temperature are used in order to mimic a wide range of fluid behaviours and to develop a generalised framework for studying turbulence modulations in variable property flows. Instead of scaling the velocity solely based on local density, as done for the van Driest transformation, we derive an extension of the scaling that is based on gradients of the semi-local Reynolds number $Re_\tau^*$. This extension of the van Driest transformation is able to collapse velocity profiles for flows with near-wall property gradients as a function of the semi-local wall coordinate. However, flow quantities like mixing length, turbulence anisotropy and turbulent vorticity fluctuations do not show a universal scaling very close to the wall. This is attributed to turbulence modulations, which play a crucial role on the evolution of turbulent structures and turbulence energy transfer. We therefore investigate the characteristics of streamwise velocity streaks and quasi-streamwise vortices and found that, similar to turbulent statistics, the turbulent structures are also strongly governed by $Re_\tau^*$ profiles and that their dependence on individual density and viscosity profiles is minor. Flows with near-wall gradients in $Re_\tau^*$ ($d {Re_\tau^*}/dy \neq 0$) showed significant changes in the inclination and tilting angles of quasi-streamwise vortices. These structural changes are responsible for the observed modulation of the Reynolds stress generation mechanism and the inter-component energy transfer in flows with strong near-wall $Re_\tau^*$ gradients.Comment: Submitted manuscript under review in JF

Hydrodynamic stability of a sheared liquid film

We study the hydrodynamic stability of a thin layer of liquid that is sheared by a gas. First, the interface conditions for the free surface approximation of the problem are discussed. We then study the stability of the flow to disturbances with phase speeds smaller than the maximum velocity in the liquid film, i.e. the internal mode, extending previous results and resolving some apparent contradictions. The dynamic effect of the gas is studied by dropping the free surface approximation and solving the Orr-Sommerfeld equation for the gas together with that for the liquid. The effect on the stability of the liquid film is very large, which is explained by the fact that the imaginary part of the wave speed (which determines the stability of the film) is very small. Consequently the free surface approximation is, in general, not correct. We then study the dependence of the critical Reynolds number on the Weber number, on the curvature of the liquid velocity profile and on the properties of the gas. With the gas included, a second mode of instability is found which has a phase velocity that is, in general, larger than the maximum liquid velocity and corresponds to capillary-gravity waves. We compare results with experiments from the literature; good agreement is found. Finally, a suggestion on the relevance of this study to the generation of ‘roll waves’, which are important from a practical point of view, is given

Direct numerical simulation of turbulent pipe flow up to a Reynolds number of 61,000

In this paper we will present results of several direct numerical simulations of turbulent pipe flow. The highest Reynolds number simulated in this study was 61, 000. Our numerical model uses Fourier expansions in axial and circumferential directions and 6th order staggered compact finite difference in the wall normal direction. Apart form standard turbulent statistics we also will present 1D energy spectra and autocorrelation functions.Process EnergyMechanical, Maritime and Materials Engineerin

Reprocessing Zamak laryngoscope blades into new instrument parts; an ‘all-in-one’ experimental study

Introduction: Disposable instruments in healthcare have led to a significant increase of medical waste. The aim of this study is to validate the recycling of disposable Zamak laryngoscope blades into new medical components by using a new ‘all-in-one’ affordable reprocessing setup as alternative for die-casting. Methods: A n “all-in-one” casting set-up was designed and built. Laryngoscope blades, recovered from two hospitals, were disinfected, melted and cast into dog-bones and into new instrument parts. The quality of the cast material was evaluated using X-ray fluorescence spectrometry. The mechanical properties were obtained by assessing the Ultimate Tensile Strength (UTS) and tensile tests. Results: A recovery of 93 % Zamak was obtained using a melting temperature of 420 °C for 3 h. The XRF Spectro data showed higher Zinc and silicon concentrations when compared with Virgin Zamak. The dog-bones tests resulted in an average UTS, Yield Strength (YS) and Young's Modulus (YM) of 236 ± 61 (MPa), 70 ± 43 and 9 ± 3, respectively, representing 82 %, 103 % and 64 % of the UTS, YS and YM of standard Zamak. Functional instrument parts with extensions and inner chambers were cast with a maximal shrinkage percentage of 1 ± 1 %. Discussion: This study demonstrates that the created “all-in-one” reprocessing method can process contaminated disposable Zamak laryngoscope blades into new raw base material and new instrument parts. Although material and surface properties can deteriorate, reprocessed Zamak still has sufficient mechanical properties and can be used to cast complex parts with sufficient dimensional tolerances and minimal shrinkage. Conclusion: A micro reprocessing method was designed and used to turn disposed laryngoscope blades into new basis material and semi-finished components. Follow up studies are needed to scale and optimize this process towards a functional alternative for die casting. It should be further investigated how this process can contribute to further medical waste reduction and a circular healthcare economy

Reprocessing Zamak laryngoscope blades into new instrument parts; an ‘all-in-one’ experimental study

Introduction: Disposable instruments in healthcare have led to a significant increase of medical waste. The aim of this study is to validate the recycling of disposable Zamak laryngoscope blades into new medical products by using a new ‘all-in-one’ affordable reprocessing setup as alternative for die-casting. Methods: An “all-in-one” casting set-up was designed and built. Laryngoscope blades, recovered from two hospitals, were disinfected, melted and casted into dog-bones and into new instrument parts. The quality of the casted material was evaluated using X-ray fluorescence spectrometry. The mechanical properties were obtained by assessing the Ultimate Tensile Strength (UTS) and tensile tests. Results: A recovery of 93% Zamak was obtained using a melting temperature of 420 0C for three hours. The XRF Spectro data showed higher Zinc and silicon concentrations when compared with Virgin Zamak. The dog-bones tests resulted in an average UTS, Yield Strength (YS) and Young’s Modulus (YM) of 236 ±61 (MPa), 70 ±43 and 9 ±3, respectively, representing 82%, 103% and 64% of the UTS, YS and YM of standard Zamak. Functional instrument parts with extensions and inner chambers were casted with a maximal shrinkage percentage of 1±1%. Discussion: This study demonstrates that the created “all-in-one” reprocessing method can process contaminated disposable Zamak laryngoscope blades into new raw base material and new instrument parts. Although material and surface properties can deteriorate, reprocessed Zamak still has sufficient mechanical properties and can be used to cast complex parts with sufficient dimensional tolerances and minimal shrinkage. Conclusion: A circular micro reprocessing method was designed and used to turn disposed laryngoscope blades into new basis material and semi-finished products. Follow up studies are needed to scale and optimize this process towards a functional alternative for die casting. It should be further investigated how this process can contribute to further medical waste reduction and a circular healthcare economy