Turbulent Mixing Layer Research Articles

Variable-density effects in incompressible non-buoyant shear-driven turbulent mixing layers

Abstract

Small transparent prism for the cross-sectional visualization for a turbulent scalar mixing layer using a water-channel PLIF method

This study introduces a simple method that enables cross-sectional visualization in the measurement of the planer-laser induced fluorescence (PLIF) method. The concentration field of the high Schmidt number scalar mixing layer generated by grid-generated turbulence is measured by a water-channel experiment in this tudy. The high Schmidt scalar mixing layer is produced by Rhodamine B aqueous solution and water. A small transparent prism, installed on the outside wall of the test section, aids in applying cross-sectional visualization. The small transparent prism is an isosceles triangular shaped prism filled with the working fluid through the test section of the water channel. The setting and details of the small transparent prism are shown in this study. By using the small transparent prism, the concentration field on the plane set into the test section is shown as a result of the cross-sectional visualization. In this result, a scalar structure like a mushroom is visualized around the mixing interface of the scalar mixing layer, as found in a previous experiment.

Numerical Investigation on Aero-optical Reduction for Supersonic Turbulent Mixing Layer

The tangential injection flow commingled with the mainstream could be abstracted as mixing layer, and profound importance is attached to aero-optical reduction for film-cooling target-seeking vehicles in engineering application. Leveraging the in-house code based on explicit fifth-order Weighted Compact Nonlinear Scheme and ray-tracing method, the supersonic mixing layer is numerically simulated to investigate the effect of inflow disturbance on large-scale structure growth and aero-optical reduction. The vortex growth would be stimulated somewhere when the frequency of inflow perturbation added is close to the local characteristic most unstable value. Based on such cognition, different techniques of super-harmonic and sub-harmonic disturbance are employed for effective aero-optical suppression schemes in the following research. The sub-harmonic one without phase shift shows such ability in the whole turbulent region, and additional perturbation of white noise is also promising for better control consequence.

The Study of the Mixing Layer Particles Distribution in Dependence on the Convective Mach Numbers

The numerical studies of the quasi twodimensional supersonic turbulent gas-particle mixing layer are performed using the 2D-DNS (Direct Numerical Simulations). The system of Navier-Stokes equations of a multi-species flow is solved using the ENO scheme of a third-order accuracy. The dispersion of the particles is simulated by the Lagrangian method following their trajectories in the mixing layer. The study focuses on the roles of the large-scale vortex structures in a particle dispersion dependence on the Stokes numbers and convective Mach numbers.

Multiphase Gas and the Fractal Nature of Radiative Turbulent Mixing Layers

Abstract A common situation in galactic and intergalactic gas involves cold dense gas in motion relative to hot diffuse gas. Kelvin–Helmholtz instability creates a turbulent mixing layer and populates the intermediate-temperature phase, which often cools rapidly. The energy lost to cooling is balanced by the advection of hot high enthalpy gas into the mixing layer, resulting in growth and acceleration of the cold phase. This process may play a major role in determining the interstellar medium and circumgalactic medium phase structure, and accelerating cold gas in galactic winds and cosmic filaments. Cooling in these mixing layers occurs in a thin corrugated sheet, which we argue has an area with fractal dimension D = 5/2 and a thickness that adjusts to match the hot phase mixing time to the cooling time. These cooling sheet properties form the basis of a new model for how the cooling rate and hot gas inflow velocity depend on the size L, cooling time of the mixed phase , relative velocity , and density contrast of the system. Entrainment is expected to be enhanced in environments with short , large , and large . Using a large suite of three-dimensional hydrodynamic simulations, we demonstrate that this fractal cooling layer model accurately captures the energetics and evolution of turbulent interfaces and can therefore be used as a foundation for understanding multiphase mixing with strong radiative cooling.

Characteristics of the turbulent non-turbulent interface in a spatially evolving turbulent mixing layer

The highly convoluted interface separating the turbulent and non-turbulent regions in a turbulent mixing layer is experimentally investigated using the particle image velocimetry (PIV) technique. The mixing layer was generated using a fine screen/mesh in one half of the test section of a low-speed wind tunnel. The PIV data, which were acquired with high spatial resolution in the self-similar regime of the flow, allow us to identify the turbulent/non-turbulent interface (TNTI) using a suitable threshold value of the absolute spanwise vorticity, |ωz|. The threshold values for the top and bottom interfaces of the mixing layer are found to be different, and the probability density function (PDF) of the interface position for both the interfaces is found to follow the Gaussian distribution. Interestingly, the PDF of the interface orientation reveals two clear peaks, and this is attributed to the sustained large-scale motions in a mixing layer, compared to the other free-shear flows, as is also substantiated by further analyses such as the linear stochastic estimation and the conditional analysis of the transverse velocity profile. The linear stochastic analysis also shows the presence of large vorticity structures of the order of the Taylor microscale at the mean TNTI location in a mixing layer. Furthermore, the present work reveals that, using the spanwise component of vorticity alone, we can experimentally identify and estimate the thickness of the viscous superlayer from the conditional profiles of the diffusion term and the correlation coefficient of the dissipation and the diffusion terms in the enstropy transport equation. The present value of the viscous superlayer thickness of 5η–6η (where η is the Kolmogorov length scale) compares well with the values reported in the literature for other shear flows. Although both the interfaces are found to behave like a fractal with a dimension of 1.3 in two dimensions, one can find dominant length scales of the order of the thickness of the viscous superlayer, the thickness of the TNTI and the width of the mixing layer from the pre-multiplied power spectra of the autocorrelation functions of the interface curvature, the normal velocity and the interface position, along the TNTI, respectively. In addition, we find that the TNTI characteristics do not show significant dependence on the velocity ratios and Reλ considered in the present study. Furthermore, the conditional transverse velocity profiles indicate that the entrainment characteristics for the upper and lower TNTIs may be asymmetric in nature.

Analysis of turbulence characteristics in a temporal dense gas compressible mixing layer using direct numerical simulation

This study investigates the effects of a Bethe–Zel’dovich–Thompson (BZT) dense gas (FC-70) on the development of a turbulent compressible mixing layer at a convective Mach number . Three-dimensional direct numerical simulations are performed with both FC-70 and air. The initial thermodynamic state for FC-70 lies inside the inversion region where the fundamental derivative of gas dynamics ( ) becomes negative. The complex Martin–Hou thermodynamic equation of state is used to reproduce thermodynamic peculiarities of the BZT dense gas (DG). The unstable growth phase in the mixing layer development shows an increase of -turbulent stress tensors in DG compared to perfect gas (PG). The following self-similar period has been carefully defined from the time evolution of the integrated streamwise production and transport terms. During the self-similar stage, DG and PG mixing layers at display close values of the momentum thickness growth rate, which seems similarly affected by the well-known compressibility-related reduction for PG. The same mechanisms are at stake, related to the reduction of pressure–strain terms. Turbulent kinetic energy (TKE) spectra show a slower decrease of TKE at small scales for DG compared with PG. The filtered kinetic energy equation balance developed by Aluie (Physica D, vol. 247 (1), 2013, pp. 54–65) is applied for the first time to a compressible mixing layer. The equation is reshaped to better account for TKE transport across the mixing layer. This new formulation brings out the role played by , the pressure strengths power. A detailed comparison of the contributions to the filtered TKE equation is provided for both PG and DG mixing layers.

Instability of supersonic cold streams feeding galaxies – IV. Survival of radiatively cooling streams

ABSTRACT We study the effects of Kelvin–Helmholtz Instability (KHI) on the cold streams that feed massive haloes at high redshift, generalizing our earlier results to include the effects of radiative cooling and heating from a UV background, using analytic models and high resolution idealized simulations. We currently do not consider self-shielding, thermal conduction, or gravity. A key parameter in determining the fate of the streams is the ratio of the cooling time in the turbulent mixing layer which forms between the stream and the background following the onset of the instability, $t_{\rm cool,\, mix}$, to the time in which the mixing layer expands to the width of the stream in the non-radiative case, tshear. This can be converted into a critical stream radius, Rs, crit, such that $R_{\rm s}/R_{\rm s,crit}=t_{\rm shear}/t_{\rm cool,\, mix}$. If Rs < Rs, crit, the non-linear evolution proceeds similarly to the non-radiative case studied by Mandelker et al. If Rs > Rs,crit, which we find to almost always be the case for astrophysical cold streams, the stream is not disrupted by KHI. Rather, background mass cools and condenses on to the stream, and can increase the mass of cold gas by a factor of ∼3 within 10 stream sound crossing times. The mass entrainment induces thermal energy losses from the background and kinetic energy losses from the stream, which we model analytically. Roughly half of the dissipated energy is radiated away from gas with $T \lt 5\times 10^4\, {\rm K}$, suggesting much of it will be emitted in Ly α.

The influence of initial perturbation power spectra on the growth of a turbulent mixing layer induced by Richtmyer–Meshkov instability

This paper investigates the influence of different broadband perturbations on the evolution of a Richtmyer–Meshkov turbulent mixing layer initiated by a Mach 1.84 shock traversing a perturbed interface separating gases with a density ratio of 3:1. Both the bandwidth of modes in the interface perturbation, as well as their relative amplitudes, are varied in a series of carefully designed numerical simulations at grid resolutions up to 3.2×109 cells. Three different perturbations are considered, characterised by a power spectrum of the form P(k)∝km where m=−1, −2 and −3. The growth of the mixing layer is shown to strongly depend on the initial conditions, with the growth rate exponent θ found to be 0.5, 0.63 and 0.75 for each value of m at the highest grid resolution. The asymptotic values of the molecular mixing fraction Θ are also shown to vary significantly with m; at the latest time considered Θ is 0.56, 0.39 and 0.20 respectively. Turbulent kinetic energy (TKE) is also analysed in both the temporal and spectral domains. The temporal decay rate of TKE is found not to match the predicted value of n=2−3θ, which is shown to be due to a time-varying normalised dissipation rate Cϵ. In spectral space, the data follow the theoretical scaling of k(m+2)∕2 at low wavenumbers and tend towards k−3∕2 and k−5∕3 scalings at high wavenumbers for the spectra of transverse and normal velocity components respectively. The results represent a significant extension of previous work on the Richtmyer–Meshkov instability evolving from broadband initial perturbations and provide useful benchmarks for future research.

Evaluating temporal and spatial variability and trend of aerosol optical depth (550 nm) over Iran using data from MODIS on board the Terra and Aqua satellites

Our knowledge of the temporal-spatial variations of atmospheric aerosols is necessary because they can affect public health and air quality. This paper studied Collection 6, the product of Aerosol Optical Depth (AOD 550 nm), using the Deep Blue algorithm of moderate-resolution imaging spectroradiometer (MODIS) on board the Terra and Aqua satellites over Iran from 2003 to 2017. Long-term data of the dust using synoptic stations were also used. The results showed that the southwestern Iran, the Persian Gulf coast from Bushehr to Bandar Abbas, and the Makran coastal area in southeastern Iran have the highest seasonal means of Aerosol optical depth (AOD > 0.3) and that they are among the aerosol hotspot areas in southwestern Asia. Other regions in Iran, especially the northwest and the Zagros Mountains, have the lowest aerosol optical depth (AOD < 0.04). The highest AOD amount of Terra (0.144) and Aqua (0.156) occurs in the spring, and the lowest amount occurs in the autumn season (0.094 and 0.101 Terra and Aqua, respectively). The heights act as a barrier on the particle transport pathway and, as a result, will affect the release of dust on local and regional scales. AOD increase in Iran is more related to aerosol loading in deserts and arid areas around the country. The AOD increase during the cold period of the year in industrial cities is due to the combustion of fossil fuels and the reduction of the boundary layer height (BLH). It is because most atmospheric pollutants are released in the boundary layer and are homogenized through turbulent mixed layer processes. The most prominent active dust sources around Iran, which are the main source of aerosol increase, can be categorized into four sources: (1) Mesopotamian, (2) Northern Arabian Peninsula, (3) Elrob Elkhali desert, and (4) East Arabian Peninsula. The AOD seasonal trend showed that the most significant increasing trend occurs in autumn on the TERRA (42.82%) and Aqua (32.32%) satellites. The largest decreasing trend of AOD in Iran is observed in winter in Kermanshah province (− 0.0011 year−1) in western Iran and in other seasons in southeastern Iran for which meteorological changes play a major role.

A comparative study of Ac-dielectric barrier discharge versus Ns-dielectric barrier discharge plasma actuators in an incompressible turbulent mixing layer

A comparison between ac-dielectric barrier discharge (DBD) and ns-DBD plasma actuators is performed in a low-speed turbulent mixing layer to assess differences in their control mechanisms (momentum versus thermal). The mean flow response to actuation is matched for the two cases at a single downstream location to gauge scaling of relevant parameters (carrier frequency, energy per pulse, etc). The similarity between local behavior downstream is found to extend to the global flow, both in the mean and fluctuating components. However, the ns-DBD requires approximately six times more electrical energy to achieve the same control as the ac-DBD in this specific flow. An analysis of the flow field very near the splitter plate trailing edge sheds light on the difference in momentum versus thermal actuation mechanisms. A velocity deficit is observed for both actuators, but a thermal bump effect is evident in the ns-DBD case while a near surface jet is observed in the ac-DBD case.

A passive model for the evolution of subgrid-scale instabilities in turbulent flow regimes

A zero-dimensional model for the development of turbulent mixing layers driven by Rayleigh–Taylor (RT) or Richtmyer–Meshkov (RM) instabilities is presented, for application in Reynolds-Averaged Navier–Stokes (RANS) simulations. The model approximates the behavior of the turbulent mixing layer at a passively advected tracer particle that sits on the material interface of interest in the simulation, and is used to provide initial conditions for the RANS model once the mixing layer width has developed to a size that may be resolved on the computational mesh. The model relies on approximations to the Besnard–Harlow–Rauenzahn (BHR) RANS closure equations and is thus consistent the turbulent development of the mixing layer, unlike previous models of this type which have focused on the early-time laminar development of RT or RM instabilities. These instabilities will often grow through an initially laminar state before developing into turbulence, and criteria for switching from an existing laminar model to the proposed turbulent model are considered. The proposed model reproduces the mixing layer development of RT and RM instabilities seen in mixed width resolving BHR RANS simulations to a reasonable degree of accuracy, and relaxes the need for the RANS simulation to resolve the mixed width as soon as the mixing layer becomes turbulent, which may be impractical in cases where the mixing layer grows through a wide range of scales.

Parametric study of Ns-DBD plasma actuators in a turbulent mixing layer

Excitation of a low-speed turbulent mixing layer using nanosecond dielectric barrier discharge (ns-DBD) plasma actuators is demonstrated here under various forcing conditions. The mixing layer behavior downstream is examined in the context of observations near the point of actuation (splitter plate trailing edge). Parameters explored include carrier/burst frequency, pulse energy, and duty cycle. Control authority is found to increase with carrier frequency, but a reduction in efficacy is observed for high pulse energy and lower duty cycle. During actuation, a streamwise velocity deficit is found near the trailing edge relative to the baseline. This causes velocity fluctuations that are amplified and lead to the formation of large-scale structures. The results indicate ns-DBD plasma actuators in burst mode acts as an obstruction giving rise to cross-stream perturbations that cause mixing layer growth.

Phytoplankton acclimation to changing light intensity in a turbulent mixed layer: A Lagrangian modelling study

A new individual-based plankton model is used to test the hypothesis that the timescale of photoacclimation of phytoplankton within the surface mixing layer of the ocean is slow relative to mixing, in which case the chlorophyll to carbon (Chl:C) ratio of individual cells shows little adjustment in response to changes in light environment driven by vertical displacement. Rates of photoacclimation are shown to be a strongly non-linear function of light intensity that depends on the balance of intrinsic chlorophyll synthesis at low irradiance versus increasing growth rate at high irradiance. Predicted photoacclimation was negligible for cells experiencing rates of turbulent mixing typical of the open ocean surface boundary layer (10−3 to 10-1 m2 s-1), in which case Chl:C is set by mean light intensity. The model was extended to incorporate a simple ecosystem of nutrient, phytoplankton, zooplankton and detritus and, using two-layer slab physics, used to study photoacclimation in a more realistic setting, the seasonal cycle of plankton dynamics at Ocean Weather Station India in the North Atlantic (59 °N, 20 °W). Results were remarkably similar when compared with an equivalent ecosystem model that used an Eulerian representation of phytoplankton, reinforcing our conclusion that mixing rates within the surface mixed layer of the ocean are typically too fast to permit photoacclimation by phytoplankton to ambient light.

A comprehensive study of flamelet tabulation methods for pulverized coal combustion in a turbulent mixing layer—Part II: Strong heat losses and multi-mode combustion

This paper is a continuation of our work done in Part I, in which the a priori and budget analyses were conducted, Wen et al. (2019). In this work, we focus on addressing specific and recurring issues in flamelet modeling for pulverized coal combustion, including strong heat losses, multi-mode combustion and reaction progress variable definition. First, extended flamelet formulations are developed that can take into account strong heat loss effects in pulverized coal combustion systems. Then, to characterize multi-mode combustion in pulverized coal flames, a coupled premixed and non-premixed flamelet model is developed using the combustion mode index. Finally, the effects of reaction progress variable definition on the flamelet predictions are quantified. A state-of-the-art direct numerical simulation database is employed to challenge the newly developed flamelet models. The tabulated thermo-chemical quantities are compared with the reference direct numerical simulation results through a priori analyses. Comparisons show that the newly developed flamelet models which take into account strong heat loss effects can predict the gas temperature and species mass fractions correctly. The adiabatic flamelet models over-predict the corresponding thermo-chemical quantities in regions where the interphase heat transfer is significant. Coupled with a linear extrapolation method, the prediction of the gas temperature with the adiabatic flamelet models can be improved. The performance of the multi-mode flamelet model depends on whether the local combustion mode can be correctly identified. The conventional combustion mode index based on the gradients of fuel and oxidizer species mass fractions cannot correctly identify the combustion mode in the entire combustion field.

Sudden diffusion of turbulent mixing layers in weakly coupled plasmas under compression.

The rapid growth of viscosity driven by temperature increase in turbulent plasmas under compression induces a sudden dissipation of kinetic energy, eventually leading to the relaminarization of the flow [Davidovits and Fisch, Phys. Rev. Lett. 116, 105004 (2016)PRLTAO0031-900710.1103/PhysRevLett.116.105004]. The interdiffusion between species is also greatly enhanced, so that mixing layers appearing at interfaces between different materials are subjected to strong dynamical modifications. The result is a competition between the vanishing turbulent diffusion and the expanding plasma microscopic diffusion. In direct numerical simulations with conditions relevant to inertial confinement fusion, we evidence regimes where compressed spherical mixing layers are quickly diffused during the relaminarization process. Using one and two-point turbulent statistics, we also detail how mixing heterogeneities are smoothed out.

Experimental analysis of the effect of local base blowing on three-dimensional wake modes

Wake modes of a three-dimensional blunt-based body near a wall are investigated at a Reynolds number . The targeted modes are the static symmetry-breaking mode and two antisymmetric periodic modes. The static mode orientation is aligned with the horizontal major -axis of the base and randomly switches between a positive and a negative state leading to long-time bistable dynamics of the turbulent wake. The modifications of these modes are studied when continuous blowing is applied at different locations through four slits along the base edges (denoted L for left, R for right, T for top and B for bottom) in either four single asymmetric configurations or two double symmetric configurations (denoted LR and TB). Two regimes, referred to as mass and momentum, are clearly identifiable for all configurations. The mass regime, which is fairly insensitive to blowing momentum and location, is characterized by the growth of the recirculating bubble as the total injected flow rate is increased, and is associated with a base drag reduction and interpreted as resulting from the equilibrium between mass fluxes feeding and emptying the recirculating region. A simple budget model is shown to be in agreement with entrainment velocities measured for isolated turbulent mixing layers. The strength of the static mode is reduced up to 20 % when the bubble length is maximum, whereas no change in the periodic mode frequencies is found. On the other hand, the momentum regime is characterized by the deflating of the recirculating bubble, leading to base drag increase, and it is interpreted by the free shear layer forcing, which increases the entrainment velocity, thus emptying the recirculating bubble. In this regime the static mode orientation is imposed by the blowing symmetry. Lateral L and R (respectively top/bottom T and B) blowing configurations select or states in the horizontal (respectively vertical) direction, while bistable dynamics persists for the symmetric LR and TB configurations. The shape of periodic modes follows the changes in wake static orientation. The transition between the two regimes is governed by both the total injected flow rate and the location of the injection.

Split radiographic tracer technique to measure the full width of a high energy density mixing layer

We describe a new radiography–based technique for measuring the width of the mixing layer that develops when high–energy–density (HED) fluids of different density mix at an interface. In a traditional radiography target, the denser material includes an embedded, density–matched, high opacity, tracer layer to focus the radiograph on a 2D slice of the 3D flow field. The high opacity layer enhances the contrast between the dense and light materials that permits a clear identification of the spike front, i.e. penetration of the dense material into the light material, but obscures the bubble front, i.e. penetration of the light material into the heavy when the fluids are highly interpenetrated, as in a turbulent mixing layer. On the other hand, removing the tracer layer makes it difficult to identify the interface at all, due to poor contrast and an increased susceptibility to edge effects. Our solution is a split tracer approach where half the target uses the traditional geometry while the other half utilizes a special tracer layer in the light material to produce a radiograph with inverse x-ray opacity characteristics when backlit at a specified x–ray energy. Hydrodynamic equivalency is maintained between the two halves of the target. By making a single target with the traditional and inverse material pairs set side-by-side, we are able to simultaneously measure the penetration of bubbles and spikes in a deeply nonlinear flow, yielding the full mix width. We discuss this new target concept and present our first experiments at the National Ignition Facility (NIF) demonstrating the viability of this technique.

Effects of different levels of solar radiation and depth-varying vertical diffusion on the dynamics of a reaction–diffusion NPZ model

ABSTRACT Although the effects of solar radiation, diffusion and mixed layer depths on plankton populations have been studied extensively, it is not intuitive what the equilibrium states of a plankton system should be in the absence of seasonal forcing, nor what are the prime determiners of the attracting equilibrium states. In this study, the linear stability of a nutrient–phytoplankton–zooplankton (NPZ) model in reaction–diffusion form is analyzed to understand how solar radiation, diffusion and mixed layer depth influence the predicted equilibrium states. We incorporate parameters that control the response of the phytoplankton growth rate to solar radiation, control the depth of the upper mixed layer and control the strength of the diffusion. The combined effect of the constant in time parameters for these quantities is shown to significantly influence the stability properties of the NPZ reaction–diffusion model. In our parameter range, we find that diffusion and light are stronger determinants of the equilibrium state than mixed layer depth. This linear stability study gives insight into how these quantities affect the stability states of marine planktonic ecosystems and reinforces the importance of understanding light and upper mixed layer turbulence as important factors in determining the state of the plankton system.

ANÁLISIS DEL TRANSPORTE DE OZONO EN LA CUENCA ATMOSFÉRICA DE PUEBLA-TLAXCALA EN EL CENTRO DE MÉXICO

A study on the dynamic of transport and occurrence of ozone in the air basin of the Puebla-Tlaxcala Valley (PTV) was performed between March and April 2012. The ozone (O 3 ), nitrogen oxides (NOx) and several meteorological variables were measured at the surface level in the rural sites of Huaquechula (HUAQ), Chipilo (CHIP), and Amozoc (AMOZ). The vertical O 3 profile and the associated meteorological conditions, were measured intermittently in CHIP. The 1 hour Mexican air quality standard of 0.095 ppm for O 3 , was exceeded in five and one days in CHIP and AMOZ, respectively. A rapid increase in the morning for O 3 levels was observed in the three sites and was associated with the mixing of O 3 above the inversion layer once started the growing of the turbulent mixing layer. Unusual O 3 evening events of around 0.080 ppm were recorded for several hours in the three sites that could be associated with the recirculation of air masses with ozone formed within the VPT and O 3 advectively transported from the Cuernavaca-Cuautla Valley. The vertical O 3 profile showed an increase with height at concentrations even higher than those observed at the surface, and the 2-D analysis of backward trajectories suggested favorable conditions for O 3 transported from the free troposphere to the surface. It is concluded that the implementation of control strategies in the VPT should consider the influence of the inter-basin exchange, the meteorological local and synoptic seasonality, and the possible contributions of O 3 from the free troposphere and not be limited to local conditions.