Turbulent Shear Flows Research Articles

Scrutinizing Dynamic Cumulant Lattice Boltzmann Large Eddy Simulations for Turbulent Channel Flows

This technical paper outlines the predictive performance of a recently published dynamic cumulant lattice Boltzmann method (C-LBM) to model turbulent shear flows at all resolutions. Emphasis is given to a simple strategy that avoids a frequently observed velocity overshoot phenomenon near rigid walls when combining the C-LBM with an all-resolution (universal) wall function. The examples included are confined to turbulent channel flow results for a variety of friction Reynolds numbers within 180 and 50,000, obtained on a sequence of isotropic, homogeneous grids that feature non-dimensional lattice spacings using inner coordinates from 4 to 2200. The results indicate that adjusting the near-wall distance of the first fluid node, i.e., the intersection of the wall with the first lattice edge, to the resolution provides a reasonably simple, robust, and accurate supplement to the all-resolution C-LBM approach. The investigated wall function/C-LBM combination displays a remarkable predictive performance for all investigated resolutions.

Observation of water droplet motion in a shear flow

Although numerous studies have investigated droplets in shear flows, the features of these droplets are incompletely understood. This paper reports our experimental visualization results of sessile droplets in fully developed turbulent shear flows and the onset of their movement. The visualization was performed from the side and bottom views. Furthermore, the droplet height is much smaller than the boundary-layer thickness, and the droplet motions are strongly affected by turbulent velocity fluctuation. The droplets were deformed toward the front and spread in a spanwise direction. The front and rear-side contact angles recover the balance position, that changes symmetrically over time and are not clearly related to sessile droplet oscillation. The droplet oscillation frequencies were well predicted by the model based on the resonant vibrations of the standing waves except for the lowest mode. Based on the critical air velocity and droplet height, the motion onset was adequately approximated by a simple empirical relation between the Weber and Reynolds numbers.

Near-cancellation of up- and down-gradient momentum transport in forced magnetized shear-flow turbulence

Visco-resistive magnetohydrodynamic turbulence, driven by a two-dimensional unstable shear layer that is maintained by an imposed body force, is examined by decomposing it into dissipationless linear eigenmodes of the initial profiles. The down-gradient momentum flux, as expected, originates from the large-scale instability. However, continual up-gradient momentum transport by large-scale linearly stable but nonlinearly excited eigenmodes is identified and found to nearly cancel the down-gradient transport by unstable modes. The stable modes effectuate this by depleting the large-scale turbulent fluctuations via energy transfer to the mean flow. This establishes a physical mechanism underlying the long-known observation that coherent vortices formed from nonlinear saturation of the instability reduce turbulent transport and fluctuations, as such vortices are composed of both the stable and unstable modes, which are nearly equal in their amplitudes. The impact of magnetic fields on the nonlinearly excited stable modes is then quantified. Even when imposing a strong magnetic field that almost completely suppresses the instability, the up-gradient transport by the stable modes is at least two-thirds of the down-gradient transport by the unstable modes, whereas for weaker fields, this fraction reaches up to 98%. These effects are persistent with variations in magnetic Prandtl number and forcing strength. Finally, continuum modes are shown to be energetically less important, but essential for capturing the magnetic fluctuations and Maxwell stress. A simple analytical scaling law is derived for their saturated turbulent amplitudes. It predicts the falloff rate as the inverse of the Fourier wavenumber, a property which is confirmed in numerical simulations.

Effects of pipeline geometry, sample volume, and flow rate on pb monitoring outcomes in copper pipe drinking water supply systems

Pb in drinking water is one of the main sources of Pb in human bodies. Besides excess lead incidents caused by lead service lines, Pb contamination in non-lead pipe systems (e.g. copper and PVC) is also on the rise. Brass fixtures and lead-solder connections are the primary sources of lead in non-lead systems, which cause intermittent peaks of Pb at the consumer tap. The concentrations of lead in tap water depend on pipeline geometry, sampling methods, and the characteristic of the pipe flow. Using a 3-dimensional computational fluid dynamics model, this study analyzes the Pb concentration variations at taps of copper water supply systems. The turbulent diffusion and shear flow dispersion are fully simulated in the model to provide the most accurate prediction. Water parcels containing lead (clouds) are formed adjacent to lead sources during stagnation and are then dispersed with the flow of water through the pipe when the tap is opened. The geometry of the pipeline has a significant impact on the monitored Pb levels. The complex flow condition in elbow areas leads to a more intense mixing of lead ions. Therefore, the Pb levels at consumer taps in complex plumbing systems in high-rise buildings are higher than in straight pipelines. When the sampling flow rate is large, the peak height of Pb is slightly higher due to higher turbulent intensity. Pb concentrations in sequential samples are predicted with sample volumes of 50, 250, 500, and 1000 mL. Lead levels may be diluted significantly when samples are taken in large volumes (e.g. 500 mL or 1 L), resulting in an underestimation of Pb levels at taps. A sample volume of 250–500 mL is recommended in sequential sampling programs on non-lead service lines in order to identify Pb sources.

Onset of large-scale convection in wall-bounded turbulent shear flows

Large-scale coherent rolls are observed frequently in unstably stratified turbulent wall-bounded flows where they influence strongly the turbulent transport and the mean flow properties. Here we address the question of their genesis by means of a linear stability analysis of the turbulent mean flow where a model of turbulent Reynolds stresses is embedded in the linear stability operator. We use the unstably stratified turbulent channel flow as a testbed for the analysis. We show that the onset of large-scale convection is associated to the linear instability of the mean flow to large-scale streamwise-uniform coherent rolls of aspect ratio$A \approx 3\unicode{x2013}3.3$when the friction Richardson number exceeds$|Ri_{\tau,c}|=0.86$. This corresponds to critical Rayleigh numbers that increase with the Reynolds number approximately as$Ra_c \approx 0.04\,Re_b^{1.8}$. These results are consistent with those obtained in recent direct numerical simulations performed in the same setting. It is also found that if turbulent Reynolds stresses are not modelled in the linear operator used in the stability analysis, then predictions are not consistent with direct numerical simulations results.

Stochastic Model for Quasi-One-Dimensional Transitional Turbulence with Streamwise Shear Interactions.

The transition to turbulence in wall-bounded shear flows is typically subcritical, with a poorly understood interplay between spatial fluctuations, pattern formation, and stochasticity near the critical Reynolds number. Here, we present a spatially extended stochastic minimal model for the energy budget in transitional pipe flow, which successfully recapitulates the way localized patches of turbulence (puffs) decay, split, and grow, respectively, as the Reynolds number increases through the laminar-turbulent transition. Our approach takes into account the flow geometry, as we demonstrate by extending the model to quasi-one-dimensional Taylor-Couette flow, reproducing the observed directed percolation pattern of turbulent patches in space and time.

Mechanism for sequestering magnetic energy at large scales in shear-flow turbulence

Straining of magnetic fields by large-scale shear flow, which is generally assumed to lead to intensification and generation of small scales, is reexamined in light of the persistent observation of large-scale magnetic fields in astrophysics. It is shown that, in magnetohydrodynamic turbulence, unstable shear flows have the unexpected effect of sequestering magnetic energy at large scales due to counteracting straining motion of nonlinearly excited large-scale stable eigenmodes. This effect is quantified via dissipation rates, energy transfer rates, and visualizations of magnetic field evolution by artificially removing the stable modes. These analyses show that predictions based upon physics of the linear instability alone miss substantial dynamics, including those of magnetic fluctuations.

Scale-resolving turbulent channel flow simulations using a dynamic cumulant lattice Boltzmann method

This paper reports the assessment of a modified cumulant lattice Boltzmann method in turbulent channel flows. The suggested approach is of interest for its resolution-spanning capabilities, which are scrutinized for grid resolutions between 4 and 430 wall units. With the emphasis on a recently published parameterized cumulant collision operator, we suggest a resolution-sensitive regularization supplemented by a dynamic subgrid-scale model based upon third-order cumulant expressions, which seamlessly vanishes for direct numerical simulations. The strategy is optionally supplemented by a wall function approach to adjust the wall treatment. The analysis involves the mean flow field, resolved and modeled second moments, two-point correlations, and spectral data and correlation lengths obtained for four friction Reynolds numbers (180, 550, 2000, and 5200). Results display a resolution-independent capability to simulate the attached turbulent shear flow.

Linearised Reynolds-averaged predictions of secondary currents in turbulent channels with topographic heterogeneity

A rapid predictive tool based on the linearised Reynolds-averaged Navier–Stokes equations is proposed in this work to investigate secondary currents generated by streamwise-independent surface topography modulations in turbulent channel flow. The tool is derived by coupling the Reynolds-averaged momentum equation to the Spalart–Allmaras transport equation for the turbulent eddy viscosity, using a nonlinear constitutive relation for the Reynolds stresses to capture correctly secondary motions. Linearised equations, describing the steady flow response to arbitrary surface modulations, are derived by assuming that surface modulations are shallow. Since the equations are linear, the superposition principle holds and the flow response induced by an arbitrary modulation can be obtained by combining appropriately the elementary responses obtained over sinusoidal modulations at multiple spanwise length scales. The tool permits a rapid exploration of large parameter spaces characterising structured surface topographies previously examined in the literature. Here, channels with sinusoidal walls and with longitudinal rectangular ridges are considered. For sinusoidal walls, a large response is observed at two spanwise wavelengths scaling in inner and outer units respectively, mirroring the amplification mechanisms in turbulent shear flows observed from transient growth analysis. For longitudinal rectangular ridges, the model suggests that the analysis of the response and the interpretation of the topology of secondary structures is facilitated when the ridge width and the gap between ridges are used instead of other combinations proposed in the literature.

The effect of modulated driving on non-rotating and rotating turbulent plane Couette flow

Direct numerical simulations of turbulent non-rotating and rotating plane Couette flow with a periodically modulated plate velocity are conducted to study the effect of modulated forcing on turbulent shear flows. The time-averaged shear Reynolds number is fixed at $Re_S = 3 \times 10^4$ , which results in a frictional Reynolds number of approximately $Re_\tau \approx 400$ . The modulating frequency is varied in the range $Wo\in (20,200)$ , while the modulating amplitude is kept fixed at $10\,\%$ of the shear velocity except to demonstrate that varying this parameter has minimal effect. The resulting shear at the plates is found to be independent of the forcing frequency, and equal to the non-modulated baseline. For the non-rotating simulations, two clear flow regions can be seen: a near-wall region that follows Stokes’ theoretical solution, and a bulk region that behaves similar to Stokes’ solutions but with an increased effective viscosity. For high driving frequencies, the amplitude response follows the scaling laws for modulated turbulence of von der Heydt et al. (Phys. Rev. E, vol. 67, 2003, 046308). Cyclonic rotation is not found to modify the system's behaviour in a substantial way, but anti-cyclonic rotation changes significantly the system's response to periodic forcing. We find that the persistent axial inhomogeneities introduced by mild anti-cyclonic rotation make it impossible to measure the propagation of the modulation adequately, while stronger anti-cyclonic rotation creates regions where the modulation travels instantaneously.

Perturbative model for the second-order velocity structure function tensor in turbulent shear flows

A model for the second order velocity structure function tensor, based on Kolmogorov's theory and valid for the inertial range, is proposed, incorporating the effect of anisotropy as a linear perturbation to the standard isotropic form. Structure functions measured from direct numerical simulations of channel flow and from experimental measurements in turbulent boundary layers are compared with predicted behavior and reasonable agreement is found.

Impact of Breathing Pattern and Nebulization on Expelled Viral Content During Mechanical Ventilation Using an Ex Vivo Porcine Lung System.

Little is known about the fate of expelled viral particulates during the aerosolization of inhaled medications during mechanical ventilation. We hypothesized that breathing patterns that generate a greater degree of shear stress and turbulent air flow will produce a greater concentration of exhaled viral RNA with the presence of a nebulizer during mechanical ventilation. Eight ex vivo pig lungs were utilized as the physiological model. Each lung was dedicated to a specific breathing pattern that consisted of tidal breathing, respiratory distress, cough, and sneeze. Breath simulations were carried out through a commercial mechanical ventilator. Ninety mL of a bacteriophage stock at a concentration of 108 PFU/mL were introduced into the lungs during a 10-min sample collection session. The number of viral particles collected in exhalate was measured using quantitative polymerase chain reaction. The impact of breathing pattern on measured viruses was analyzed through two-way analysis of variance. The interaction effect between nebulization and breath pattern on exhaled viral quantity was not statistically significant P = .80, partial η2 = 0.167. The analysis of the main effects indicated that the effects of the breathing pattern and nebulization phase were not statistically significant P = .26, partial η2 = 0.519; P = .98, partial η2 = 0, respectively. There were no statistically significant differences among the breathing patterns related to measurable viral RNA. Coughing produced the most measurable increase in measured viral quantity during the nebulization phase and non-nebulization phase with a mean exhaled viral quantity (3.5 × 105 ng/μL [95% CI 1.6 × 105-5.5 × 105] and 2.7 × 105 ng/μL [95% CI 7.1 × 103-5.5 × 105], respectively). Tidal breathing with the presence of a nebulizer and respiratory distress without a nebulizer produced the lowest measured viral quantities (M = 1.1 × 105 ng/μL [95% CI -1.7 × 105 to 3.9 × 105]; M = 1.2 × 105 ng/μL [95% CI -1.6 × 105 to 4.0 × 105]). In this ex vivo porcine model, the introduction of a nebulizer did not increase the mean viral RNA captured throughout all of the breathing patterns.

Migration behaviors of microplastics in sediment-bearing turbulence: Aggregation, settlement, and resuspension

The interaction between microplastics (MPs) and suspended sediment (SS) is important for the environmental fate of MPs. This study explored the interaction of MPs with SS and the vertical migration behavior of MPs in sediment-bearing turbulence. The turbulent shear flow caused MPs to aggregate. This aggregation resulted in a rapid increase in particle size, which peaked when the shear rate was 19.94 s−1, and then declined with a further increase in the shear rate. Compared to large MPs, small MPs were more prone to aggregation, which formed heterogeneous aggregate MPs-SS in sediment-bearing turbulence. Owing to the formation of heterogeneous aggregates, small MPs had a much higher settlement rate in sediment-bearing turbulence than in sediment-free turbulence. MPs in bottom sediments may resuspend owing to turbulent shear flow acting on sediments, causing secondary pollution. These results provide new insights into the aggregation, settlement, and resuspension behaviors of MPs in natural waters.

Extreme events in transitional turbulence.

Transitional localized turbulence in shear flows is known to either decay to an absorbing laminar state or to proliferate via splitting. The average passage times from one state to the other depend super-exponentially on the Reynolds number and lead to a crossing Reynolds number above which proliferation is more likely than decay. In this paper, we apply a rare-event algorithm, Adaptative Multilevel Splitting, to the deterministic Navier-Stokes equations to study transition paths and estimate large passage times in channel flow more efficiently than direct simulations. We establish a connection with extreme value distributions and show that transition between states is mediated by a regime that is self-similar with the Reynolds number. The super-exponential variation of the passage times is linked to the Reynolds number dependence of the parameters of the extreme value distribution. Finally, motivated by instantons from Large Deviation theory, we show that decay or splitting events approach a most-probable pathway. This article is part of the theme issue 'Mathematical problems in physical fluid dynamics (part 2)'.

Periodic hill flow simulations with a parameterized cumulant lattice Boltzmann method

AbstractThe article is concerned with the assessment of a cumulant lattice Boltzmann method in wall‐bounded, separated turbulent shear flows. The approach is of interest for its resolution‐spanning success in turbulent channel flows without using a specific turbulence treatment. The assessment focuses upon the flow over a periodic hill, which offers a rich basis of numerical and experimental data, for Reynolds numbers within . The analysis involves the mean flow field, second moments and their invariants, as well as spectral data obtained for a wide range of resolutions with . With the emphasis on a recently published parameterized cumulant collision operator, the universality of the value assigned to a stability preserving regularization parameter is assessed. Reported results guide resolution‐dependent optimized values and indicate a required minimum resolution of . Analog to the findings for attached turbulent shear flows, the approach appears to adequately resolve complex turbulent flows without the need for ad hoc modeling for a range of scale resolving resolutions.

High-fidelity modeling of turbulent shear flow downstream of a 3-D airfoil with spanwise ice contamination leading stall

The characteristics of turbulent shear flow downstream of NACA23012 airfoil with ridge-ice contamination leading stall are investigated in this paper. A low-dissipated compressible N–S solver is employed to simulate its unsteadiness via Improved Delayed Detached Eddy Simulation (IDDES) with sufficient spatial and temporal resolution. Nonlinear phenomena of eddy dynamics, such as vortex pairing, shedding, merging and breaking down, are fully resolved with extremely fine grid scale over 0.2 billion. The calculated Reynolds number based on the airfoil chord length is near 2.1×106. High-fidelity results show that the turbulent shear stream quickly develops into tubular geometry after leaving the ridge. The shedding-off tubular geometries quickly show anisotropy, twist in all directions, and finally decomposed into turbulence. Large-scale structural interaction is found above the trailing edge zone, determined by the shedding-off vortex and the rolling-up flow from the lower surface of the airfoil. Using POD and DMD analysis, we make sense it as the cause of low-frequency oscillation of the shear layer and even the lift fluctuation.

Large eddy simulation of aerosol particle dispersion mechanism in aircraft exhaust plume

Aerosol Infrared Stealth Technology is a novel method to suppress the infrared by releasing aerosol particles around the hot exhaust plume. However, the Infrared Radiation (IR) suppression rate is much lower in flight tests, which is caused by the unclear particle dispersion mechanism in high-speed non-isothermal shear flow. Hence, the large eddy simulation (LES) and particle discrete phase model (DPM) are coupled to solve the turbulent shear flow and the aerosol particle motion. The effects of aircraft flight speed on the turbulent flow characteristics and particle motion behaviors are systematically studied. It is found that the Mach rings appear in flight conditions, and they are getting blurry when increasing the flight speed. Both γ-like vortex and horseshoe-like vortex are observed when Ma < 1, and the horse-like could disappear when Ma > 1. The aerosol particle distribution is closely related to the turbulent gas vortices. The particle lateral dispersion is obviously restricted with the rising flight speed. Besides, the drag force plays a dominant role, the Saffman force and the thermophoretic force are of the same order of magnitude, all of these three forces are much greater than the gravity.

Computational Study of Abdominal Aortic Aneurysms with Severely Angulated Neck Based on Transient Hemodynamics Using an Idealized Model

An abdominal aortic aneurysm (AAA) is an enlargement of the abdominal aorta that can become a life-threatening disease. The pulsatile blood flow exhibits intricate laminar patterns in the abdominal portion of the human aorta under normal resting conditions, whereas secondary flows are caused by adjacent branches and abnormal vessel geometries. If a pathological disorder (e.g., aneurysm) alters the structural composition of the artery wall, the flow dynamics become more complex. In this study, we analyzed the hemodynamics of pulsatile blood flow in three-dimensional AAA models. Computational predictions of hemodynamic changes were performed considering idealized models for four severe proximal neck angulations of symmetric aneurysms assuming conditions of laminar flow and a rigid artery wall. The predictions were based on computational fluid dynamics throughout the cardiac cycle. Postprocessing was used to visualize the numerical findings. The hemodynamic changes in factors such as velocity, flow streamline, pressure, and wall shear stress were obtained and visualized. The resulting blood flow through the severely angulated proximal neck of the abdominal aorta caused strong turbulence and asymmetric flow inside the aneurysm sac, leading to blood recirculation, especially during diastole. The simulation results showed the formation of regions with high and low wall shear stress, turbulent flow, and recirculation in the aneurysm sac depending on the angulation, which could have led to aortic wall weakness.

RETRACTED: Effects of wavy porous fins on the flow, thermal fields, and entropy of the magnetic radiative non-Newtonian nanofluid confined inclined enclosures

In this paper, the numerical investigation on the micropolar non-Newtonian nanofluid transient flow within inclined enclosures containing wavy porous fins is carried out. Electromagnetic forces together with thermal radiation are taken into account. Also, the impacts of the Brownian motion and thermophores on the suspension flow are surveyed. The finite volume method is employed to solve nonlinear coupled partial differential equations. The enclosure is partially heated from below and the second law of the thermodynamic is analyzed. The cases where the microelements close to the boundaries are unable to rotate are assumed and a heat source/sink is taken place within the flow domain. The main outcomes revealed that the presence of the radiation enhances the convection situation while the heat generation and vortex parameters reduce the heat transfer rate. Also, there is a diminishing in the rate of the heat transfer up to 5.62% when the Hartmann number is altered from 0 to 50. This numerical investigation is germane to the engine lubricants, paint rheology, turbulent shear flows, direct absorption of the sunlight in solar collectors, cement slurries, and combustion chambers.

Meandering Motions and Their Relation to the Energy Spectrum in Turbulent Shear Flows

Meandering Motions and Their Relation to the Energy Spectrum in Turbulent Shear Flows