Convective Vortices Research Articles

Effect of morphed trailing edge on the acoustic and flow characteristics of National Advisory Committee for Aeronautics (NACA) 4418 airfoil

Trailing-edge profile has an important impact on the aeroacoustic behavior of an airfoil, particularly the tonal noise in the low-medium Reynolds number range. Three profiles, NACA 4418 defined by the National Advisory Committee for Aeronautics (NACA) and its morphed variants with the trailing-edge deflections at ±8°, are investigated by wind tunnel experiments to reveal the acoustic and flow characteristics at the chord-based Reynolds number ranging from 1.2×105 to 3.1×105. The consistent observation across all profiles is the dominant feedback loop located on the pressure surface. The aeroacoustic findings show that the morphed trailing edge alters the tonal frequency spacing, mainly due to the change of the vortex convection velocity in the feedback loop. Furthermore, the trailing-edge morphing is observed to modulate the mode switching of the dominant tones. The local flow results reveal that the laminar separation bubble on the pressure side amplifies the flow oscillation. Notably, the profile with the trailing-edge upward deflection, characterized by a laminar separation bubble on the pressure surface, is particularly susceptible to the low-frequency broadband oscillation. The joint analysis of the acoustic and flow fields suggests that the laminar separation bubble heightens the sensitivity of the dominant tone mode switching to the incoming flow velocity.

Convective vortices in collapsing stars

Abstract Recent studies show that non-radial structures arising from massive star shell convection play an important role in shaping core-collapse supernova explosions. During the collapse phase, convective vortices generate acoustic waves that interact with the supernova shock. This amplifies turbulence in the post-shock region, contributing to explosion. We study how various physical parameters influence the evolution of these convective vortices during stellar collapse using simplified simulations. We model the collapsing star with a transonic Bondi flow and represent convection as solenoidal velocity perturbations. Our results are consistent with previous studies, demonstrating that the peak perturbation amplitude scales linearly with the pre-collapse convective Mach number and inversely with the angular wavenumber of convection. While the shell radius and width primarily determine the timescale of accretion, they have little impact on the peak perturbation amplitudes. Finally, we show that when the convective Mach number is below approximately 0.2, the dynamics remain within the linear regime.

On the Use of Different Sets of Variables for Solving Unsteady Inviscid Flows with an Implicit Discontinuous Galerkin Method

This article presents a comparison between the performance obtained by using a spatial discretization of the Euler equations based on a high-order discontinuous Galerkin (dG) method and different sets of variables. The sets of variables investigated are as follows: (1) conservative variables; (2) primitive variables based on pressure and temperature; (3) primitive variables based on the logarithms of pressure and temperature. The solution is advanced in time by using a linearly implicit high-order Rosenbrock-type scheme. The results obtained using the different sets are assessed across several canonical unsteady test cases, focusing on the accuracy, conservation properties and robustness of each discretization. In order to cover a wide range of physical flow conditions, the test-cases considered here are (1) the isentropic vortex convection, (2) the Kelvin–Helmholtz instability and (3) the Richtmyer–Meshkov instability.

Vorticity skewness of finite-amplitude rapidly rotating Rayleigh–Bénard convection

Rotating Rayleigh–Bénard convection denotes the convection between a warm plate and a cold plate in a rotating environment. It is a classic model for understanding convective vortices in the atmosphere and ocean. The influence of background rotation on fluid inertia breaks the symmetry between cyclones and anticyclones. Such a symmetry breaking could be represented by vorticity skewness, which still lacks a systematic theory. Rapidly rotating convection with stress-free boundaries and unit Prandtl number is a convenient starting point. The investigation starts from the convective onset stage, where the vortices grow stationarily. Asymptotic analysis shows that the volumetric vorticity skewness $S$ is produced by the interaction between the $n=0,1$ and $n=1,2$ vertical eigenmodes. The $n=0$ (barotropic) mode contributes positively to $S$ mainly by stretching the vertical relative vorticity, an ageostrophic effect. The $n=2$ mode makes a minor negative contribution to $S$ by preferentially intensifying the outflow over the inflow, a non-hydrostatic effect. The theory predicts $S$ to be proportional to the global Rossby number defined with the volumetric standard deviation of vorticity, ${Ro_g}$ . The proportional factor does not depend on the Rayleigh and Ekman numbers, agreeing with direct numerical simulations. Then the system enters the equilibrium stage. The stretching of vertical vorticity still contributes to $S$ dominantly. At ${Ro_g}\gtrsim 0.5$ , the emergent unsteady flow significantly suppresses the asymmetry between the inflow and outflow strength, and weakens its influence on $S$ .

Effects of Mach number on space-time characteristics of wall pressure fluctuations beneath turbulent boundary layers

Wall pressure fluctuations beneath turbulent boundary layers are a fundamental source of aerodynamic noise by exciting the wall structure, with their space-time characteristics serving as the basic ingredient for predicting the wall structural response. To this end, direct numerical simulations of fully developed compressible turbulent boundary layers at Mach numbers of 0.5, 1.2, and 2.0 are conducted to investigate wall pressure fluctuations comprehensively. The effects of Mach number on the single-point statistics of wall pressure fluctuations, such as the root mean square, skewness and flatness factors, probability density function, and frequency spectrum, are assessed to be very weak. Regarding the space-time characteristics, the convection velocity Uc determined by the space-time correlation of wall pressure fluctuations increases slightly with the Mach number, which only reflects the convective behavior of turbulent vortices. On the wavenumber–frequency spectrum, characteristic peaks of both the acoustic wave and convective vortices are identified. At Mach 0.5, the peaks of the fast (Uc+c) and slow (Uc−c) acoustic waves are unattached to others with c denoting acoustic speed, while only the peak of the fast acoustic wave is distinguishable from the convective peak at Mach 1.2 and 2.0. Due to the aerodynamic heating at supersonic conditions, the thermal effect on acoustic speed should be taken into account in determining the acoustic wavenumber. By introducing a convective Prandtl–Glauert parameter, a refined relation is proposed to provide a more accurate depiction of the acoustic domain in the wavenumber–frequency spectrum.

Self-excited force characteristics and flow mechanisms of a 5:1 rectangular cylinder in torsional vortex-induced vibration and flutter

In this paper, the self-excited force characteristics, flow rules and vibration mechanisms of a 5:1 rectangular cylinder in torsional vortex-induced vibration (VIV) and flutter were investigated through wind tunnel tests and numerical simulations. The nonlinearity of the self-excited lifting moment is not monotonous but exhibits a peak within a specific reduced wind speed and amplitude range. The amplitude-dependent characteristics of dimensionless work and aerodynamic damping are the primary reasons for non-divergent vibration, which is mainly influenced by the amplitude-dependent phase difference. The reduced wind speed significantly affects the formation zone length and convection speed of the leading-edge vortex, thereby determining the distribution of surface pressure phase difference and dimensionless work. As the reduced wind speed increases, there is a positive and negative alternation in dimensionless work, leading to the occurrence of torsional VIV and flutter. While the torsional amplitude does not affect the distribution of phase difference, it merely influences the formation zone length, resulting in amplitude-dependent dimensionless work and accounting for the non-divergent vibration. The characteristics of vortex convection, pressure phase variation and dimensionless work of the 5:1 rectangular cylinder in torsional VIV and flutter follow similar rules, indicating that the two vibration forms share the same inherent essence.

Rainfall analysis of the May 2021 southeastern Texas and southern Louisiana flood

ABSTRACT This paper analyzes the heavy rainfall on 16–20 May 2021 across southeastern Texas and southern Louisiana. In Louisiana, five fatalities were attributed to the event and > $1 billion in losses. The rainfall was caused by two slow-moving, mesoscale convective vortices that concentrated precipitation south of Port Arthur, TX, Lake Charles, LA, and Baton Rouge, LA, in 6 to 12-hour periods. Using calibrated radar data, the Storm Precipitation Analysis System estimated 6-h rainfall totals of 254–362 mm south of Lake Charles, surpassing 200–500-year average recurrence intervals (ARI) for those locations, with an isolated area receiving 367.5 mm, equating to a > 800-year ARI. In East Baton Rouge Parish, LA, 203–268 mm was estimated in 6-h (200–500-year ARI), with an isolated area receiving 274 mm, equivalent to a 750-year ARI. A comparison of the May 2021 and August 2016 events, which both affected southern Louisiana, is made using a depth-area-duration analysis. While the forcing mechanisms differed, 1–6 h rainfall depths were similar at 1 –25,900 km2. The May 2021 event exceeded rainfall depths produced by the August 2016 event for 2–6 h durations for area sizes of 1–388 km2, highlighting the precipitation intensity produced by the May 2021 event.

Numerical simulation of heat transfer performance and convective vortex evolution in a phase change thermal storage device with dispersed heat sources

Numerical simulation of heat transfer performance and convective vortex evolution in a phase change thermal storage device with dispersed heat sources

Quasi-Linear Convective Systems and Derechos across Europe: Climatology, Accompanying Hazards, and Societal Impacts

Abstract In this work, we use 8 years (2014–21) of Operational Programme for the Exchange of Weather Radar Information (OPERA) radar data, ESWD severe weather reports, and arrival time difference (ATD) lightning detection network (ATDnet) data to create a climatology of quasi-linear convective systems (QLCSs) across Europe. In the first step, 15-min radar scans were used to identify 1475 QLCS polygons. Severe weather reports, lightning data, and morphological properties were used to classify QLCSs according to their intensity into 1151 marginal (78.0%), 272 moderate (18.5%), and 52 derecho (3.5%) events. The manual evaluation led to the recognition of QLCS morphological and precipitation archetypes, areal extent, duration, speed, forward motion, width, length, accompanying hazards, injuries, and fatalities. Results indicate that QLCSs are the most frequent during summer in central Europe, while in southern Europe, their occurrence is extended to late autumn. A bow echo feature occurred in around 29% of QLCS cases, while a mesoscale convective vortex occurred in almost 9%. Among precipitation modes, trailing and embedded stratiform types accounted for around 50% of QLCSs. The most frequent hazard accompanying QLCSs was lightning (taking up on average 94.4% of the area impacted by QLCS), followed by severe wind gusts (7.9%), excessive precipitation (6.1%), large hail (2.9%), and tornadoes (0.5%). Derechos had the largest coverage of severe wind reports (49.8%), while back-building QLCSs were the most prone to excessive precipitation events (13.5%). QLCSs caused 104 fatalities and 886 injuries. Severe wind gusts were responsible for 87.6% of fatalities and 73.6% of injuries. Nearly half of all fatalities and injuries were associated with only the 10 most impactful QLCS events, mostly warm-season derechos.

A stochastic Galerkin lattice Boltzmann method for incompressible fluid flows with uncertainties

Efficient modeling and simulation of uncertainties in computational fluid dynamics (CFD) remains a crucial challenge. In this paper, we present the first stochastic Galerkin (SG) lattice Boltzmann method (LBM) built upon the generalized polynomial chaos (gPC). The proposed method offers an efficient and accurate approach that depicts the propagation of uncertainties in stochastic incompressible flows. Formal analysis shows that the SG LBM preserves the correct Chapman–Enskog asymptotics and recovers the corresponding macroscopic fluid equations. Numerical experiments, including the Taylor–Green vortex flow, lid-driven cavity flow, and isentropic vortex convection, are presented to validate the solution algorithm. The results demonstrate that the SG LBM achieves the expected spectral convergence and the computational cost is significantly reduced compared to the sampling-based non-intrusive approaches, e.g., the routinely used Monte Carlo method. We obtain a speedup factor of 5.72 compared to Monte Carlo sampling in a randomized two-dimensional Taylor–Green vortex flow test case. By leveraging the accuracy and flexibility of LBM and the efficiency of gPC-based SG, the proposed SG LBM provides a powerful framework for uncertainty quantification in CFD practice.

Dust Emissions on Mars From Phoenix Lidar Measurements in Relation to Local Meteorological Conditions

AbstractThe diurnal cycle of dust aerosols on Mars is studied by analyzing lidar observations at the Phoenix landing site under cloud‐ and fog‐free conditions and in the absence of elevated, long‐range transported dust layers. There is a pronounced diurnal cycle in the dust‐layer height with minimum heights of 4–6 km occurring between 11:00 and 17:00 local time. The ratio of the aerosol optical depth (AOD) within the lowermost 2 km to the total AOD reaches peak values at the same time. This can be explained by local dust emissions driven by the diurnal cycle of heating and cooling in the boundary layer. Analysis of wind and pressure measurements show that the gustiness of surface winds and the frequency of convective vortices undergo diurnal variations resembling those of AOD, indicating that these processes are the main drivers for local dust emissions.

Hybrid compressible lattice Boltzmann method for supersonic flows with strong discontinuities

Within the framework of the hybrid recursive regularized lattice Boltzmann (HRR-LB) model, we propose a novel hybrid compressible LB method to ensure the conservation of total energy in simulating compressible flows with strong discontinuities. This method integrates a LB solver to handle the mass and momentum conservation equations via collision-streaming steps on standard lattices, while a finite volume method (FVM) is employed for the conservation of the total energy equation. The flux reconstruction in the FVM is achieved through a momentum coupled method (MCM). The interface momentum, crucial for reconstructing the convective fluxes and determining the upwind extrapolation of passive scalar quantities in MCM, is derived from the LB method. The validity and accuracy of the proposed method are evaluated through six test cases: (I) isentropic vortex convection in subsonic and supersonic regimes; (II) non-isothermal acoustic pulse; (III) one-dimensional Riemann problems; (IV) two-dimensional Riemann problem; (V) double Mach reflection of a Mach 10 shock wave; and (VI) shock–vortex interaction. Numerical results demonstrate that this method surpasses the previous HRR-LB model by Guo et al. [“Improved standard thermal lattice Boltzmann model with hybrid recursive regularization for compressible laminar and turbulent flows,” Phys. Fluids 32, 126108 (2020)] in terms of accuracy and robustness when dealing with strong shock waves.

Line-Based High-Order Hybrid Scheme on Unstructured Grids for Compressible Flows

This study explores the use of line-based finite difference techniques on unstructured grids employing high-order stencils for practical aerodynamic applications. The primary focus is on finite-difference explicit and compact spatial discretization up to sixth order. Problems studied include compressible flows containing discontinuities, which necessitated the use of a hybrid finite difference–finite volume approach, where the former is shown to be effective and accurate for smooth regions, while the latter is robust in capturing discontinuities and shocks. To differentiate between discontinuous and smooth zones, two distinct shock indicators are used, one based on pressure gradients and the other on the divergence of velocity. To achieve faster convergence, spatial discretization is applied with line-based alternating direction implicit (ADI) time integration. A key focus of this work is to demonstrate the proposed methodology in flow scenarios relevant to aircraft applications that also test various aspects of the methodology. Consequently, the cases studied are isentropic vortex convection, standing normal shock, transonic NACA0012 airfoil, supersonic cylinder, unsteady flow past tandem circular cylinders, and supersonic flow past a forward-facing step. Comparisons were also done against an existing line-based finite-volume technique for unstructured grids that employed Monotonic Upstream-Centered Scheme for Conservation Laws (MUSCL) and fifth-order Weighted Essentially NonOscillatory (WENO5) spatial reconstruction schemes. It is observed that the hybrid finite difference–finite volume approach performed well with excellent agreement against available results, sometimes with far fewer degrees of freedom. Furthermore, the implicit ADI scheme is effective over unstructured grids and provides a three-time speed-up over the traditional Runge–Kutta fourth-order scheme.

Development of an accurate central finite-difference scheme with a compact stencil for the simulation of unsteady incompressible flows on staggered orthogonal grids

In scale-resolving simulations such as Large Eddy Simulations (LES), the spatial discretization scheme of the convective term plays a crucial role in avoiding interference between the numerical errors and the subgrid-scale model. Accurate schemes lead to lower truncation errors and better predictions of turbulent flows without the need for an excessively refined grid. To this end, a new second-order finite-difference scheme (HCDS6) has been developed for incompressible flows and orthogonal staggered grids. Compared to the standard second-order scheme, the new scheme has significantly lower dispersion errors. Compared to existing high-order schemes, the numerical stencil of HCDS6 is more compact, which makes it easier to implement, especially considering boundary conditions around complex geometries using Immersed Boundary Methods (IBM). The HCDS6 scheme conserves the discrete momentum with limited production or dissipation of discrete kinetic energy, which guarantees its numerical stability. Its performance is evaluated using an open-source CFD package called REEF3D. Three benchmarks demonstrate the key properties and performance of the scheme: the convection of an isentropic vortex, the Taylor-Green vortex flow, and turbulent channel flow. Its relatively low dispersion errors, combined with ease of implementation, make the HCDS6 scheme a promising candidate for efficient scale-resolving simulations of turbulent flows.

Influence of variable gravity on the phase change material for enclosed vertical channels with helical fins

This study numerically investigates the influence of variable gravity on the melting behaviour of a phase change material (PCM- paraffin wax RT27) contained within an enclosed vertical channel under different gravity conditions, specifically those found on the Earth, the Moon, and under the microgravity environments. Furthermore, the impacts of incorporating various number of helical fins within the PCM unit under different gravitational conditions are assessed for the first time. The findings indicate that the melting efficiency of PCM is considerably affected by the reduced gravity conditions, due to changes in the natural convection, velocity, and temperature; the melting time is twice as long under the moon-gravity conditions and sixfold longer under the microgravity conditions compared to the earth-gravity. It is also observed that reduced efficiency can be remedied by the insertion of fins into the PCM unit. The breakdown of the natural convection vortices with different number and orientation of fins expedites the melting process for all gravity conditions thus leading to improved performance in melting and energy storage.

An experimental investigation of constrained melting of a phase change material (PCM) in circular geometries, part I: Velocity characterization

This study is focused on the detailed characterization of the transient velocity fields of a phase change material (PCM) enclosed in a geometry with a circular cross-section subjected to uniform external heating. Particle image velocimetry (PIV) was used to study the velocity behavior related to the buoyancy-driven convection. The PCM was initially subcooled, and the walls suddenly heated to exceed the melting temperature. Experiments were conducted using Rubitherm RT26 in a quasi-2D circular enclosure at three applied heating conditions. Detailed velocity fields show strong symmetric convective vortices at the sides of the PCM chamber, and a three-dimensional recirculation was detected near the bottom. These structures strongly influence the melting behavior. The results suggest self-similarity in flow behavior between heating conditions. Detailed transient temperature fields and an extended discussion of the associated heat transfer behavior are presented in the companion paper (Part II).

Comparative Assessment on Unsteady Aerodynamics of Thin and Thick Airfoils Subjected to Pitching Motion

This article presents the comparison between the unsteady flow characteristics of thick (t/c ≥ 6%) and thin (t/c ≤ 6%) airfoils performing pitching motion at Reynolds number Re = 3.6 × 10 5 . For the analysis, the considered airfoils are NACA 0012 as thick and symmetric airfoil and GOE 417A as thin and highly cambered airfoil. The analysis in this study is restricted to only two airfoils because from the preliminary investigations, similar aerodynamic characteristics with slight variation in their flow field development under same conditions were observed. The influence of reduced frequency, pitching amplitude and mean angle of attack on the airfoil’s aerodynamic performance in terms of instantaneous force coefficients is thoroughly investigated. Further, the flow surrounding the airfoils is also investigated. It is noticed that the aforementioned parameters substantially affect the instantaneous force coefficients (both qualitatively and quantitatively). Under certain conditions, the formation of reverse Karman vortex street was observed indicating airfoil’s thrust force generation. It must be noted that the conditions where the thrust force generation occurs for thick and symmetric airfoils like NACA 0012 are quite different from that of thin and cambered airfoils like GOE 417A. Additionally, a drastic change in the flow field around the airfoils is observed under similar conditions. For NACA 0012 airfoil, the formation and convection of the Leading Edge Vortex (LEV) under certain conditions has detrimental effects on its aerodynamic characteristics as it encourages early flow separation. However, the same cannot be true for thin and highly cambered airfoils like GOE 417A. Under the similar conditions with negligible variation, the LEV contributes toward providing extra suction over the GOE 417A airfoil’s suction side. This significantly improves airfoil’s aerodynamic characteristics.

Model for the cyclonic bias of convective vortices in a rotating system

In convective fluids, small vortices develop between neighboring convective cells. Familiar in the atmosphere in the form of dust devils and water spouts, these convective vortices have been seen in simulations of oceanic convection where the vortices exhibit an unexplained bias towards cyclonic rotation despite having a large Rossby number. Here we use large eddy simulations (LES) of an idealized oceanic convective mixed layer and vary independently the Coriolis acceleration and the surface buoyancy flux to investigate the development of the cyclonic bias of convective vortices. While large convective vortices are biased for sufficiently large values of the Coriolis parameter, small convective vortices do not exhibit a clear bias. Using Lagrangian particles, we find that the large convective vortices develop through the merger of many small convective vortices. We propose a statistical theory to predict the cyclonic bias of large convective vortices composed of many small unbiased convective vortices and test the theory using LES results. We apply the theory to typical convective conditions and find that convective vortices in upper ocean mixed layers are expected to exhibit a bias, while convective vortices in the terrestrial and Martian atmospheres are expected to be largely unbiased. Published by the American Physical Society 2024

An Accurate and Automated Convective Vortex Detection Method for Long-Duration Infrasound Microbarometer Data

Abstract Heating of the surficial layer of the atmosphere often generates convective vortices, known as “dust devils” when they entrain visible debris. Convective vortices are common on both Earth and Mars, where they affect the climate via dust loading, contribute to wind erosion, impact the efficiency of photovoltaic systems, and potentially result in injury and property damage. However, long-duration terrestrial convective vortex activity records are rare. We have developed a high-precision and high-recall method to extract convective vortex signatures from infrasound microbarometer data streams. The techniques utilizes a wavelet-based detector to capture potential events and then a template matching system to extract the duration of the vortex. Since permanent and temporary infrasound sensors networks are present throughout the globe (many with open data), our method unlocks a vast new convective vortex dataset without requiring the deployment of specialized instrumentation. Significance Statement Convective vortices, or “dust devils,” contribute to regional dust loading in Earth’s atmosphere. However, long-duration convective vortex activity records are rare. We came up with a way to autonomously detect the pressure signatures left by convective vortices striking low-frequency sound, or “infrasound,” sensors. Since permanent infrasound stations have been active for decades, our method has the potential to add orders-of-magnitude more events than previously catalogued.

Numerical study of the scattering of acoustic waves by an elliptic vortex.

The scattering of the acoustic waves generated by a monopolar source propagating through a two-dimensional elliptic vortex, fixed or convected by a uniform flow, is studied by solving the Linearized Euler Equations in Cartesian coordinates using the Discontinuous Galerkin Method. For a fixed vortex position, the number, amplitudes, and angular spreads of the acoustic interference beams resulting from the sound scattering are found to significantly depend on the orientation of the vortex major axis with respect to the direction of the incident waves and on the vortex maximum tangential velocity. In particular, additional interference beams are obtained at large observation angles for a more elliptical vortex. For a convected elliptic vortex, the interference beams are curved as the angle between the incident acoustic wave and the vortex major axis varies when the vortex travels in the downstream direction. As expected, the scattering of the acoustic waves leads to spectral broadening in this case. Moreover, the widths and the frequencies of the lateral lobes obtained in the spectra on both sides of the peak at the source frequency are different for elliptic and round vortices.