Turbulent Motion Research Articles

Self-similarity of scalar isosurface area density in a temporal mixing layer

Understanding the behaviour of scalar isosurfaces in a turbulent flow is of particular interest for many problems in turbulent mixing that contain sharp interfaces between regions of the flow. Common examples include combustion, where the chemical reactions occur in thin regions within the flow, and the turbulent/non-turbulent interface in shear flows, where a thin region separates the rotational, turbulent motions from the irrotational, non-turbulent background. Recent advances in computing technology allow for in-depth analysis of these interface problems that are difficult to quantify in a laboratory setting. In this paper, the results of a direct numerical simulation of a passive scalar $\varPhi$ evolving on a turbulent, temporally developing mixing layer are described. A novel approach has been taken to calculate the surface area of individual scalar isosurfaces, $A_{iso}$ , throughout the simulation, as well as the mean isosurface area density, $\varSigma$ , as a function of the cross-stream direction and time. A notable finding is that the profiles of $\varSigma$ develop in a self-similar manner when scaled by the Taylor scale of the scalar field, $\lambda _\phi$ . Remarkably, the scaling appears to hold for a wide range of isovalues. A rough scaling argument based on the formal definition of $\varSigma$ and properties of a temporal mixing layer is presented which also exposes a dependence on $\lambda _\phi$ . Based on these results, a possible scaling for the isosurface area is presented as $A_{iso}/A_0 \sim (Re \, Sc)^{1/2}$ , where $Re$ and $Sc$ are local Reynolds and Schmidt numbers, respectively.

Effects of form-induced velocity in rough-wall turbulent channel flows

Wall roughness induces form-induced (or dispersive) velocity and pressure perturbations inside the roughness sublayer of a wall-bounded turbulent flow. This work discusses the role played by the form-induced velocity in influencing turbulence statistics and structure, using existing direct numerical simulation data of transient half channels in response to an impulse acceleration (Mangavelli et al. Effects of surface roughness topography in transient channel flows. J Turbul 2021;22:434–460). Focuses are given to (i) reshaping of turbulent coherent motions by the rate-of-strain of the mean velocity, and (ii) contributions of different velocity sources to turbulent pressure fluctuations. Half-channel flows in both fully-developed and non-equilibrium, transient states are discussed. Results show that form-induced velocity gradients not only form an important source of turbulent pressure in an equilibrium flow, but also lead to turbulence production and potentially direct structural change of turbulent eddies in a non-equilibrium flow under acceleration.

Temporally resolving premixed turbulent flame structures using self-supervised adversarial reconstruction of CH-PLIF

Understanding the turbulence-flame interaction is crucial to model the low-emission combustors developed for energy and propulsion applications. To this end, a novel frame interpolation (FI) method is proposed to better resolve the spatiotemporal evolution of premixed turbulent flame structures. The framework is completely self-supervised, agnostic to optical flow, and driven by leveraging transferrable feature knowledge at lower speeds and adversarial learning to statistically map the flame dynamics across frames. The method is successfully applied on a 10 kHz CH planar laser-induced fluorescence (PLIF) dataset of highly wrinkled premixed flames with turbulent Reynolds numbers (ReT) of 1100, 1400, and 7900, by down-sampling the image sequence to 5 kHz and restoring the sequence back to 10 kHz via FI. All reconstructions recovered important flame events and displayed excellent resemblance of the corrugated CH-layer geometries to that of the ground truths, with average intersection over union (IoU) and structural similarity index (SSIM) scores of 0.49 and 0.82, which are above the high-similarity baselines of 0.36 and 0.75, respectively. The wrinkling parameters (WP) of the flames also matched the ground truths, wherein R2 was roughly 0.95 for ReT = 1100 and 1400 and 0.85 for ReT = 7900 (lower due to the turbulence-induced uncertainties). The FI is further iteratively repeated to 40 kHz on the ReT = 7900 flames to facilitate pocket analysis by confidently linking their origin of formation, thus, enabling distinction from 3D tunnels, and improving statistical characterization of their consumption speeds. Given that the object features do not exhibit highly turbulent motions with regard to the initial time step, the proposed FI method is shown to be highly accurate and useful to analyzing finite-resolution experimental image sets including, but not restricted to, CH-PLIF.

Jet–ISM Interaction in NGC 1167/B2 0258+35, an LINER with an AGN Past

We report the results of joint Chandra/ACIS—NuSTAR deep observations of NGC 1167, the host galaxy of the young radio jet B2 0258+35. In the ACIS data, we detect X-ray emission, extended both along and orthogonal to the jet. At the end of the southeast radio jet, we find lower-energy X-ray emission that coincides with a region of CO turbulence and fast outflow motions. This suggests that the hot interstellar medium (ISM) may be compressed by the jet and molecular outflow, resulting in more efficient cooling. Hydrodynamic simulations of jet–ISM interaction tailored to NGC 1167 are in agreement with this conclusion and with the overall morphology and spectra of the X-ray emission. The faint hard nuclear source detected with Chandra and the stringent NuSTAR upper limits on the harder X-ray emission show that the active galactic nucleus (AGN) in NGC 1167 is in a very low-accretion state. However, the characteristics of the extended X-ray emission are more consonant to those of luminous Compton-thick (CT) AGNs, suggesting that we may be observing the remnants of a past high accretion rate episode, with sustained strong activity lasting ∼2 × 103 yr. We conclude that NGC1167 is presently a Low-Ionization Nuclear Emission-line Region (LINER) , but was an AGN in the past, given the properties of the extended X-ray emission and their similarity with those of CT AGN extended emission.

Horizontal meandering in direct numerical simulations of the stable boundary layer

AbstractLarge‐scale nonturbulent motions are investigated using high‐resolution direct numerical simulations of the turbulent Ekman layer at a moderate Reynolds number. In particular, the role of stable stratification effects in the generation and amplification of large‐scale oscillations in the wind‐speed components and buoyancy is analysed. Eulerian autocorrelation functions (EAFs) and spectral analysis help to describe and characterize such modes as meandering‐like structures. Focus is given to the strong stability cases, where vertical turbulent motions are damped and large‐scale modes drive near‐wall patterns of flow laminarization. Horizontal meandering arises in the near‐wall region when the ratio of vertical wind‐speed variance to horizontal wind‐speed variance decreases to small orders of magnitude. In the case of strong stratification, the characteristic features of meandering motion were identified as negative lobes in the EAF, with corresponding low‐frequency peaks in the horizontal wind speed and buoyancy spectra. The Ekman configuration used reproduced the development of meandering‐like structures satisfactorily in strong stable conditions, as indicated by field observations of the stable boundary layer and its dependence on the level of stratification. At strong stability, the Rotta model constant, which represents the relationship between the dissipation and return to isotropy terms in the velocity variance budget, is shown to vary with stability.

Direct numerical simulation of planar turbulent jets: Effect of a pintle orifice

The effects of a pintle-shaped orifice on a planar turbulent jet flow at Reynolds number 4000, based on the inlet bulk mean velocity and the jet width, are studied using direct numerical simulations. Flapping of the jet along with a low-frequency modulation of the Kelvin–Helmholtz (KH) instability, in the presence of a pintle-shaped orifice, is observed. To compare the pintle-jet behavior, a free-jet is simulated as a reference case. The effects of the near-field region on the far-field flow characteristics have been investigated. In both the cases, the KH instability in the near-field influences the far-field jet, whereas the pintle-jet also exhibits a low-frequency flapping. In addition, oblique vortex pattern has been observed in the case of pintle-jet. The far-field flow statistics of the pintle-jet with a top-hat inlet interestingly agree with those of the free-jet with a hyperbolic tangent inlet. Temporal variation of the jet characteristics has been analyzed using spatiotemporal plots. In addition, the large- and small-scale turbulent motion have been studied using three anisotropic invariant maps (turbulence triangles, eigenvalue, and barycentric maps). Moreover, that the barycentric map gives a better visualization of the anisotropic behavior has been observed in the current study.

Millimeter gap contrast as a probe for turbulence level in protoplanetary disks

Turbulent motions are believed to regulate angular momentum transport and influence dust evolution in protoplanetary disks. Measuring the strength of turbulence is challenging through gas line observations because of the requirement for high spatial and spectral resolution data, and an exquisite determination of the temperature. In this work, taking the well-known HD 163296 disk as an example, we investigated the contrast of gaps identified in high angular resolution continuum images as a probe for the level of turbulence. With self-consistent radiative transfer models, we simultaneously analyzed the radial brightness profiles along the disk major and minor axes, and the azimuthal brightness profiles of the B67 and B100 rings. By fitting all the gap contrasts measured from these profiles, we constrained the gas-to-dust scale height ratio $\Lambda$ to be $3.0_{-0.8}^{+0.3}$, $1.2_{-0.1}^{+0.1}$ and ${\ge}\,6.5$ for the D48, B67 and B100 regions, respectively. The varying gas-to-dust scale height ratios indicate that the degree of dust settling changes with radius. The inferred values for $\Lambda$ translate into a turbulence level of $\alpha_{\rm turb}\,{<}\,3\times10^{-3}$ in the D48 and B100 regions, which is consistent with previous upper limits set by gas line observations. However, turbulent motions in the B67 ring are strong with $\alpha_{\rm turb}\,{\sim}1.2\,{\times}\,10^{-2}$. Due to the degeneracy between $\Lambda$ and the depth of dust surface density drops, the turbulence strength in the D86 gap region is not constrained.

Effect of injection strategy optimization on PCCI combustion and emissions under engine speed extension in a heavy-duty diesel engine

To mitigate the consequent emissions of greenhouse gases and to conform to increasingly strict vehicle emission requirements, experiments and numerical simulation were used to study effects of multiple injection strategy on premixed charge compression ignition (PCCI) combustion and emissions under speed extension in a heavy-duty diesel engine fueled with diesel. As the speed increases, the injection duration is prolonged, the injection rate is decreased, the fuel-oxygen mixing degree is decreased. In order to enhance fuel-oxygen mixing, the injection timing of single injection should firstly be optimized to fully utilize the shape of the chamber. Compared with single injection, the double injections can increase the mixing time, the optimized multiple injections can further increase the mixing time, making the premixing more sufficient and the combustion more complete. Compared with double injections, the optimized multiple injection strategy used in this paper adds a short pulse pilot injection before the main injection, which prolongs the duration of turbulent motion in the cylinder, and enhances the mixing rate of the main injection fuel and air. Results reveals that for single injection with injection timing of −15°CA ATDC, at low speed, through optimizing the injection strategy, the NOx and soot emissions are reduced by 38.5% and one order of magnitude, respectively, and the gross indicated thermal efficiency (ITEg) is increased by 8.66%; at medium speed, by optimizing the injection strategy, the NOx and soot emissions are reduced by 59.3% and 70.4%, respectively, and the ITEg is increased slightly by 0.33%; at high speed, by optimizing the injection strategy, the soot emission is decreased significantly, the NOx emissions and the ITEg are increased slightly. It was found that with the increase of speed, the effect of multiple injection strategy on the ITEg and emissions is gradually increased, and the optimization effect of multiple injection strategy is more obvious than that of low speed.

Experimental investigation on reactivity-controlled compression ignition (RCCI) combustion characteristic of n-heptane/ammonia based on an optical engine

Ammonia as a carbon-free fuel used in IC engines always suffers from large cyclic variations and high NOx emissions. Ammonia blending with high-reactivity fuels is recognized as an effective attempt to overcome these shortcomings. In this work, for the first time, the combustion and emission characteristics of n-heptane/ammonia were investigated under reactivity-controlled compression ignition (RCCI) conditions in an optical engine. An intake adapter was introduced to modify turbulent flow motion and n-heptane was directly injected to regulate mixture reactivity based on energy ratios. Particle image velocimetry (PIV) was employed for in-cylinder flow measurement, high-speed photography and instantaneous pressure acquisition were used for combustion evolutions, and Fourier transform infrared (FTIR) spectroscopy was adopted for NOx and NH3 emissions. The results show both mixture reactivity and turbulence have significant impacts on ammonia combustion. Specifically, n-heptane addition manifesting mixture reactivity dominates combustion processes at early-injection conditions, while the turbulence with increasing intensity becomes significant at late-injection conditions. Visualization images show that the initiation of initial flame kernels is controlled by the local distribution of n-heptane fraction, and that high n-heptane fraction at early injection or low n-heptane fraction at late injection conditions tends to induce homogeneous combustion. Flame stretch analysis confirms the promotion of mixture reactivity and turbulence motion on turbulent flame propagation and identifies that a high n-heptane fraction with swirl-tumble motion exhibits lower flame stretch sensitivity, especially for late-injection scenarios. Besides, low mixture reactivity with early injection and high mixture reactivity with late injection strategies can achieve high-efficiency combustion. However, when considering NOX/NH3 emissions, a high n-heptane fraction at late injection with swirl-tumble motion has the most potential to achieve the compromise between thermal efficiency and emissions. The present work demonstrates that with optimized fuel injection and flow arrangement, ammonia blending with high-reactivity fuels can achieve high-efficient combustion while maintaining NOx/NH3 emissions within reasonable levels.

Shock-Induced Mesoparticles and Turbulence Occurrence

The development of a new approach to describe turbulent motions in condensed matter on the basis of nonlocal modeling of highly non-equilibrium processes in open systems is performed in parallel with an experiment studying the mesostructure of dynamically deformed solids. The shock-induced mesostructure formation inside the propagating waveform registered in real time allows the transient stages of non-equilibrium processes to be qualitatively and quantitatively revealed. A new nonlocal approach, developed on the basis of the nonlocal and retarded transport equations obtained within the non-equilibrium statistical physics, is used to describe the occurrence of turbulence. Within the approach, the reason for the transition to turbulence is that the non-equilibrium spatiotemporal correlation function generates the dynamic structures in the form of finite-size clusters on the mesoscale, with almost identical values of macroscopic densities moving as almost solid particles that can interact and rotate. The fragmentation of spatiotemporal correlations upon impact forms the mesoparticles that move at different speeds and transfer mass, momentum and energy-like wave packets. The movements recorded simultaneously at two scale levels indicate the energy exchange between them. Its description required a redefinition of the concept of energy far from local thermodynamic equilibrium. The experimental results show that the irreversible part of the dynamic mesostructure remains frozen into material as a new defect.

Applications of wide-ranging PIV measurements for various turbulent statistics in artificial atmospheric turbulent flow in a wind tunnel

Particle image velocimetry (PIV) is currently the only method which can experimentally quantify the spatial distributions of velocity with high sampling frequencies without causing intrusions into the airflow of the measuring devices. To extend the use of PIV systems for determining the distributions of velocity for wind environmental and engineering purposes, this study showed various applications of wide-ranging PIV systems for an artificially created turbulent flow at the wind-tunnel scale. Various turbulent statistics, which are commonly employed for evaluating the wind structures and environments, were compared among three conditions of the measurement areas covered by PIV systems and a hot-wire anemometry (HWA). The largest measurement covering 500 × 500 mm2 can capture the turbulent motions extending to 40% of the entire boundary layer. In addition, the discrepancies on two-length scales could be discussed using spatial and temporal correlation functions owing to the wide-ranging PIV. Moreover, the limitations of the wide-ranging PIV systems were clarified. While the turbulent statistics compared among three PIV conditions and HWA demonstrated adequate agreement, the PIV with the largest measurement area failed to capture the small-scale turbulent structures due to the coarse grid resolutions. Moreover, the measurement resolutions of PIV systems demonstrated sensitivity in determining the spectrum and probability density functions, whereas a wide-ranging PIV system can cover wide-ranging measurement areas.

Lip height effects on pool fire: An experimental investigation

This work studies lip height effects on the heat transfer and combustion characteristics of pool fire. Fire experiments of n-heptane and ethanol with pool diameters (D) of 0.1 m, 0.15 m and 0.2 m were carried out, in which the lip height (h) was increased from 0 until the burning could not be sustained. Results show that for both fuels, as the lip height increases, the flame enters the pool and the flame sheet becomes wrinkled and deformed. The n-heptane fire shows two distinct burning behaviors inside and outside the pool, and the burning rate decreases with the lip height due to heat feedback attenuation and air entrainment restriction. The conductive feedback rate remains almost unchanged with different lip heights for both fuels. For n-heptane, the radiative and convective heat feedback rates generally decrease with the lip height, and the convective heat feedback fraction increases as the flame enters into the pool, suggesting intensified turbulence motion. For ethanol, the radiative heat feedback rate decreases monotonically with the lip height, and the convective heat feedback rate increases slightly, followed by a decrease with h/D. A mass flux correlation based on the modified stagnant film theory is developed for the convection-dominated pool fires, which is in good agreement with the experimental data.

Dimensional transmutation and nonconventional scaling behavior in a model of self-organized criticality

This paper addresses two unusual scaling regimes (types of critical behavior) predicted by the field-theoretic renormalization group analysis for a self-organized critical system with turbulent motion of the environment. The system is modeled by the anisotropic stochastic equation for a “running sandpile” introduced by Hwa and Kardar in [Phys. Rev. Lett. 62, 1813 (1989)]. The turbulent motion is described by the isotropic Kazantsev–Kraichnan “rapid-change” velocity ensemble for an incompressible fluid. The original Hwa–Kardar equation allows for independent scaling of the spatial coordinates [Formula: see text] (the coordinate along the preferred dimension) and [Formula: see text] (the coordinates in the orthogonal subspace to the preferred direction) that becomes impossible once the isotropic velocity ensemble is coupled to the equation. However, it is found that one of the regimes of the system’s critical behavior (the one where the isotropic turbulent motion is irrelevant) recovers the anisotropic scaling through “dimensional transmutation.” The latter manifests as a dimensionless ratio acquiring nontrivial canonical dimension. The critical regime where both the velocity ensemble and the nonlinearity of the Hwa–Kardar equation are relevant simultaneously is also characterized by “atypical” scaling. While the ordinary scaling with fixed IR irrelevant parameters is impossible in this regime, the “restricted” scaling where the times, the coordinates, and the dimensionless ratio are scaled becomes possible. This result brings to mind scaling hypotheses modifications (Stell’s weak scaling or Fisher’s generalized scaling) for systems with significantly different characteristic scales.

Influence of ratio of nozzle length to diameter on local heat transfer study of an unconfined circular air jet impingement

The influence of ratio of nozzle length to diameter on local heat transfer characteristics of an unconfined circular air jet impingement on flat plate is studied experimentally. The ratio of nozzle length to diameter (L/D = 0.5 to 83), nozzle to plate surface (z/D = 0.2 to 2) and Reynolds number (Re = 12,000 to 28,000) is investigated. The local heat transfer characteristics are reported at stagnation and wall jet region by using IR thermal imager and thin foil technique. Peak Nu occurs at wall jet area for z/D = 0.2 rather than at stagnation point for all L/D ratios and is substantially dependent on z/D and Re. It is caused by flow acceleration due to venacontraction effect between the nozzle and target surfaces which results in higher Nu in wall jet area than in stagnation area. For all Reynolds number, long pipe nozzle gives higher stagnation Nu value at z/D < 0.75 due to fully developed flow at nozzle outlet and L/D = 0.5 gives higher stagnation Nu value at z/D > 0.75 due to higher turbulence motion at nozzle outlet. At z/D < 0.4, L/D = 0.5 has a lower Nu at all L/D ratios in wall jet area (R/D > 2) and it decreases as Re increases. The influence of L/D ratio in wall jet zone (R/D > 1.8) is nearly comparable at all Re for z/D > 0.75. The L/D ratios are least influenced at lower Re and higher z/D value. The correlation of stagnation Nu was obtained with maximum deviation of ±16% to the maximum heat transfer rate for optimum conditions.

Experimental study on the evolution of droplet size distribution during the fog life cycle

Abstract. The evolution of the droplet size distribution (DSD) during the fog life cycle remains poorly understood and progress is required to reduce the uncertainty of fog forecasts. To gain insights into the physical processes driving the microphysical properties, intensive field campaigns were conducted during the winters of 2010–2013 at the Instrumented Site for Atmospheric Remote Sensing Research (SIRTA) in a semi-urban environment southwest of Paris city center to monitor the simultaneous variations in droplet microphysical properties and their potential interactions at the different evolutionary stages of the fog events. Liquid water content (LWC), fog droplet number concentration (Nd) and effective diameter (Deff) show large variations among the 42 fog events observed during the campaign and for individual events. Our findings indicate that the variability of these parameters results from the interaction between microphysical, dynamical and radiative processes. During the formation and development phases, activation of aerosols into fog droplets and condensational growth were the dominant processes. When vertical development of radiation fog occurred under the influence of increasing wind speed and subsequent turbulent motion, additional condensational growth of fog droplets was observed. The DSDs with single mode (around 11 µm) and double mode (around 11 and 22 µm) were observed during the field campaign. During the development phase of fog with two droplet size modes, a mass transfer occurred from the smaller droplets into the larger ones through collision–coalescence or Ostwald ripening processes. During the mature phase, evaporation due to surface warming induced by infrared radiation emitted by fog was the dominant process. Additional droplet removal through sedimentation is observed during this phase for fog with two droplet size modes. Because of differences in the physical processes involved, the relationship between LWC and Nd is largely driven by the DSD. Although a positive relationship is found in most of the events due to continuous activation of aerosol into fog droplets, LWC varies at a constant Nd in fog with large Deff (&gt;17 µm) due to additional collision–coalescence and Ostwald ripening processes. This work illustrates the need to accurately estimate the supersaturation for simulating the continuous activation of aerosols into droplets during the fog life cycle and to include advanced parameterizations of relevant microphysical processes such as collision–coalescence and Ostwald ripening processes, among others, in numerical models.

A Gradient Tensor–Based Subgrid-Scale Parameterization for Large-Eddy Simulations of Stratified Shear Layers Using the Weather Research and Forecasting Model

Abstract The transition process from laminar stratified shear layer to fully developed turbulence is usually captured using direct numerical simulations, in which the computational cost is extremely high and the numerical domain size is limited. In this work, we introduce a scale-aware subgrid-scale (SGS) parameterization, based on the gradient tensor of resolved variables, which is implemented in the Weather Research and Forecasting (WRF) Model. With this new SGS model, we can skillfully resolve the characteristics of transition process, including formation of vortex cores, merging vorticity billows, breaking waves into smaller scales, and developing secondary instability in the stratified shear layer even at coarse-resolution simulations. Our new model is developed such that the time scales of the eddy viscosity and diffusivity terms are represented using the tensor of the gradient and not that of the rate-of-strain, which is commonly used in the parameterization of turbulent-viscosity models. We show that time scales of unresolved transition processes in our new model are correlated with those of vorticity fields. At early times, the power-law slopes in the kinetic and available potential energy spectra are consistent with the process of formation and merging waves with an upscale energy transfer. At later times, the power-law slopes are in line with the process of breaking waves into small-scale motions with a downscale transfer. More importantly, the efficiency of turbulent mixing is mainly high at the edge of vortex filaments and not at the vortices’ eyes. These findings can improve our understanding of turbulent mixing process in large-scale wind-induced events, such as tropical cyclones, using the WRF Model. Significance Statement The evolution of instabilities in stratified shear layers has significant impacts on the structure of large-scale geophysical flows and also on the energy pathway to smaller-scale motions in internal waves and turbulence. Resolving transition processes in stratified shear layers requires very high-resolution simulations in climate models. We propose a new subgrid-scale parameterization that is implemented in the Weather Research and Forecasting Model to capture the dynamics of transition process from laminar to three-dimensional turbulence in stratified shear layers at coarse-resolution simulations. Our new scale-aware parameterization can reduce biases in climate models by skillfully representing unresolved fluxes, leading to higher accuracy in weather predictions of temperature, precipitation, and surface fluxes with an affordable computational cost.

Energetics and vortex structures near small-scale shear layers in turbulence

Vortices and kinetic energy distributions around small-scale shear layers are investigated with direct numerical simulations of isotropic turbulence. The shear layers are examined with the triple decomposition of a velocity gradient tensor. The shear layers subject to a biaxial strain appear near vortices with rotation, which induce energetic flow that contributes to the shear. A similar configuration of rotating motions near the shear layers is observed in a multi-scale random velocity field, which is free from the dynamics of turbulence. Therefore, the mechanism that sustains shearing motion is embedded as a kinematic nature in random velocity fields. However, the biaxial strain is absent near the shear layers in random velocity because rotating motions appear right next to the shear layers. When a random velocity field begins to evolve following the Navier–Stokes equations, the shear layers are immediately tilted to the nearby rotating motions. This misalignment is a key for the vortex to generate the compressive strain of the biaxial strain around the shear layer. As the configuration of shearing and rotating motions arises from the kinematic nature, the shear layers with the biaxial strain are formed within a few times the Kolmogorov timescale once the random velocity field begins to evolve. The analysis with high-pass filtered random velocity suggests that this shear layer evolution is caused by small-scale turbulent motions. These results indicate that the kinematic nature of shear and rotation in velocity fluctuations has a significant role in the formation of shear layers in turbulence.

Simulating Slosh Induced Damping, with Application to Aircraft Wing-like Structures

The added damping generated by liquid sloshing in a tank has been utilized in a number of civil applications, including aviation, to reduce the vibration of the system. As part of a wider EU H2020 project called SLOWD (Sloshing Wing Dynamics), the presented study performed numerical simulations on the slosh-induced damping of liquid in tanks that were under free decay oscillations and embedded in an aircraft wing-like structure. A new open-source partitioned fluid–structure interaction software framework is presented and employed for the numerical simulations. Periodic sloshing waves and violent vertical fluid motions are observed in the study. These demonstrate the effects of slosh-induced damping under different excitation amplitudes of the structure and a varying number of baffled regions within the tank. Various sloshing patterns caused by different combinations of the excitation amplitude and compartment numbers lead to different induced dampings of the free decay motion. We observed a distinctly non-monotonic function on the slosh damping when the initial excitation amplitude is small (i.e., 0.25), with a 59% reduction when we increase the number of baffled compartments from one to four, and a 153% increase when moving from one to eight compartments. This is due to the change in the sloshing wave frequency, resulting in a significant change in the impact of the fluid between the tank ceiling and the wave crests. When the initial excitation amplitude is large (i.e., 1.0), there is no significant change in the slosh-induced damping when changing the number of compartments in the tank, for the range of parameters considered, due to the highly turbulent fluid motion. This work is expected to form the basis of further, more detailed studies within the context of the SLOWD project and its ever-expanding experimental data output.

Identification and analysis of very-large-scale turbulent motions using multiscale proper orthogonal decomposition

A multiscale proper orthogonal decomposition (mPOD) has been used to decompose the multiscale features of very long turbulent channel flow. The very-large-scale motions (VLSM) can be clearly visualized, even for flow at relatively low Reynolds number. A new energetic mode, called eVLSM, has been identified, which contains substantial energy. The large-scale structures (apart from eVLSM) are inclined to the streamwise direction and appear to be responsible for the typical meandering behavior or even for the breakup of VLSM.

Multi-scale simulation of vertical dust transport considering turbulent motions

Analysis of observational data shows that the different scale dust transportation events have different contributions to dust transportation. The large-scale dust concentration fluctuations (dimensionless based on dust concentration) have a good linear relationship with large-scale shear wind speed (dimensionless based on frictional wind speed), and small-scale fluctuations have the similar results. Therefore, this multi-scale characteristic needs to be considered in the dust transport prediction. Guided by this idea, a multi-scale dust transport model based on the turbulent motions at different scales is proposed. The prediction results are in good agreement with the observation results. The analysis of the prediction results reveals the spatial variation of the dust concentration, the large-scale and small-scale vertical dust flux along the height. At the same time, our results show that under the action of large-scale turbulent motions, a channel for dust upward transportation will be formed, but not all large-scale turbulence events can transport dust to the upper part of the atmospheric surface layer. The small-scale turbulent motions does not affect the location of the dust transport channel, however mainly affects the size of the dust concentration, that is, ignoring the effect of small-scale turbulent motions will cause an overestimation of the dust concentration.