Dust devil dynamics

Buoyancy-induced secondary vortex and reverse flows, flow transition, and the associated heat transfer processes in mixed convection of air through a bottom heated inclined rectangular duct were investigated experimentally. The local spanwise-averaged heat transfer coefficient and air temperature variations with time at selected locations were measured and the cross plane secondary flow was visualized for the Reynolds number ranging from 35 to 186, Grashof number up to 5 × 106, and −20° ≤ Φ ≤ 26°. The results indicated that the heat transfer enhancement is due to the presence of the buoyancy-driven secondary vortex flow and/or reverse flow. The onset of thermal instability was found to move upstream for a larger negative inclined angle (opposing convection) and/or a higher Grashof number and to be delayed for a larger positive inclined angle (aiding convection) and/or a higher Reynolds number. At increasing Grashof number, the instantaneous flow visualizations clearly showed the changes of the vortex flow and/or reverse flow structures in the downstream section of the duct. At slightly supercritical Grashof numbers the secondary flow is in the form of two pairs of longitudinal rolls with the vortex flow ascending along the side walls. For higher Grashof numbers the vortex rolls rotate in the opposite direction with the secondary flow descending near the side walls and the flow was found to be time periodic. At even higher Grashof numbers in opposing convection the buoyancy induced reverse flow exists and the flow is quasi-periodic. Further raising the Grashof number or lowering the Reynolds number causes the flow to change from a transitional quasi-periodic state to a chaotic turbulent state.

Presently, the Strait of Gibraltar is undergoing an unprecedented invasion of the alien alga Rugulopteryx okamurae of North Pacific origin. According to the scarce literature, the algae first settled in the south shore, probably following commercial exchanges with French ports where it was accidentally introduced together with Japanese oysters imported for mariculture. There is no certainty, however, that the algae first colonized the south shore of the Strait and, from there, spread to the north. It could well have been the opposite. Whatever the case, it spread all over the Strait and surrounding areas with amazing rapidity. Human-mediated vectors (algae attached to ship hulls or fishing nets, for example) can be behind the spread from the shore initially settled to the algae-free shore on the opposite side. But it could also have happened by means of hydrodynamic processes without direct human intervention. This possibility is assessed in this paper by revisiting historical current meter profiles collected in the Strait of Gibraltar searching for secondary cross-strait flows. All the stations present an intermediate layer of northward cross-strait velocity near the interface of the mean baroclinic exchange along with a surface layer above of southward velocity, whose lower part also overlaps the interface zone. The first one would back the south-to-north transport of algal fragments, the second one, the north-to south. In both cases, algae must reach the depth of the interface. The vertical velocity field in the area, which far exceeds the small sedimentation velocity of the algae, allows their vertical displacements throughout the water column. Its endurance to survive under the weak or no light conditions that will prevail during the cross-strait transport and its capability of reactivating the metabolism after this unfavorable period, offers chances for colonizing the opposite shore. Therefore, the propagation of the algae by hydrodynamic processes, without human intervention, cannot be ruled out.

In this study, the cause of rotation in simulated dust-devil-like vortices is investigated. The analysis uses a numerical simulation of an initially resting, dry, atmosphere, in which uniform surface heating leads to the development of a growing convective boundary layer (CBL). As soon as convective mixing sets in, regions of weak vertical vorticity develop at the lowest model level. Using forward trajectories, this vorticity is shown to originate from horizontal baroclinic production and simultaneous reorientation into the vertical within the descending branches of the convective cells. The requirement for vertical vorticity production in the downdraft cells is shown to be a nonaxisymmetric horizontal footprint of the downdraft regions. The resulting vertical vorticity is not initially associated with rotation. However, as the CBL matures, like-signed vortex patches merge, the vertical vorticity magnitude increases due to stretching, and deformation in the vortex patch decreases, leading to the development of vortices. The ultimate origin of the vortices is thus initially horizontal vorticity that has been produced baroclinically and that has subsequently been reoriented into the vertical in sinking air. Significance Statement Dust devils are concentrated vortices consisting of rapidly rising buoyant air, which may pose a risk to small aircraft and light structures on the ground. Although these vortices are a common occurrence in convective boundary layers, the origin of the vorticity within these vortices has not yet been fully established. The present study uses a numerical simulation of an evolving convective boundary layer and analyzes air parcel trajectories to identify the origin of vertical vorticity at the surface during dust-devil formation. The work contributes an answer to the long-standing question of what causes dust devils to spin.

Large eddy simulations of vertical convective vortices and dust devils in the Martian convective boundary layer are presented, employing a version of the Mars MM5 mesoscale model, adapted to use periodic boundary conditions and run at resolutions of 10 to 100 m. The effects of background horizontal wind speed and shear on dust devil development are studied in four simulations, each extending over the daytime portion of one Martian day. The general vorticity development in all cases is similar, with roughly equal positive and negative vorticity extrema. Two dust devils were found to develop in the highest wind speed case and in a case run without background wind. The dust devil structures were found to agree well qualitatively with terrestrial dust devil observations, including the prediction of greatly diminished vertical velocities in the vortex core. Thermodynamic scaling theory of dust devils was found to provide good prediction of the relationship between central pressure and temperature in the modeled vortices. Examination of the turbulent kinetic energy budgets suggests balance between buoyancy generation and loss through dissipation and transport. The vorticity for the dust devils is provided by twisting of horizontal vorticity into the vertical. The horizontal vorticity originates from horizontal variations in temperature at the lower boundary (thermal buoyancy). While the horizontal winds generated by the modeled dust devils were likely insufficient to lift dust, this study provides a solid starting point for dynamic modeling of what may be an important component of the Martian dust cycle.

Radial and tangential velocity distributions are measured for laminar flow of two Newtonian and one viscoelastic fluids in the disk and cylinder system. Velocities are determined from time lapse photographs of small particles suspended in the fluid which is illuminated in thin sections by a collimated beam of light. Measurements are made at Reynolds numbers of 0.0616, 24.4, and 97.6 for the Newtonian fluids and at 0.0371 for the viscoelastic fluid. The distributions are compared with the analyses of Kramer and Johnson in Part I. A toroidal secondary flow is observed which circulates in opposite directions for Newtonian and viscoelastic fluids, and a study of the dependence of the secondary flow direction on fluid properties and disk speed is reported. Complex flow patterns which appear in the region in which the direction of secondary flow changes are described. For Newtonian fluids tangential velocity computations agree to ±5% with the experimental measurements up to a Reynolds number of 24.4, and predicted radial velocities agree with experimental data within 20% at the Reynolds number of 24.4. At a Reynolds number of 97.6 predicted tangential velocities differ from experimental measurements by as much as ±50% while radial velocities differ from experimental values by a factor of two to three. The second order fluid predicts tangential velocities which are twice as large as experimental values and radial velocities which are in error by as much as a factor of six. The WJFLMB model predicts tangential and radial velocities which agree with experiment to within ±20% and ±40%, respectively. A Weissenberg Rheogoniometer is used to measure non-Newtonian viscosity, complex viscosity, and primary normal stress differences of the viscoelastic fluids, and to measure viscosities of the Newtonian fluids.

Secondary flows in transonic centrifugal compressor impellers affect their aerodynamic performance. In open-type impellers, low energy fluids can accumulate on the suction surfaces near the trailing edge tip side since the secondary flows and tip leakage flows interfere each other and complex flow phenomena can be generated around the impellers. Therefore, designers must consider the effect of secondary flows to avoid the aerodynamic performance degradation while designing compressor impellers. In this paper, a novel design concept about suppression of secondary flows in centrifugal compressor impellers to improve their aerodynamic performance. A transonic centrifugal compressor impeller was redesigned with the present design concept by a two-dimensional inverse method based on a meridional viscous flow calculation in this study. A design concept was introduced in above calculation process. As the design concept, by bending vortex filaments with controlling peak positions of the blade loading distributions, induced velocity due to bound vortices at the blades was generated in radial opposite direction of the secondary flows on the suction surface. Due to investigate the effect of the design concept in this paper, three-dimensional Reynolds Averaged Navier-Stokes simulations were carried out, and the vortex cores were visualized by a critical point theory and colored by non-dimensional helicity. In the conventional transonic centrifugal compressor impeller, the secondary flow vortices were confirmed and one of the vortices was broken down. In the redesigned impeller, the breakdown of the secondary flow vortices was not observed and the accumulation of the low energy fluids was suppressed compared with the conventional impeller. The total pressure ratio and adiabatic efficiency of the redesign impeller were higher than that of the conventional impeller, and the secondary flows were successfully suppressed in this research.

The relative positions of leading edges and trailing edges of rectangular vortex generators (VG) in the winglet form play a crucial role in modifying thermal and hydraulic boundary layers thereby affecting the heat transfer augmentation and pressure drop in a sudden expansion channel having expansion ratio (ER) 2:1. A numerical simulation was carried out for solving the momentum and energy equations of a three dimensional vortex- laminar flow (Reynolds number < 190) with a finite volume method based commercial code FLUENT 16.2. Reynolds number, angle of attack and VG configuration have been considered as influential parameters affecting the thermo-hydraulic performance of the sudden expansion channel due to the combined effect of primary recirculation and secondary vortex flow. The heat transfer enhancement along with corresponding pressure drop penalty was compared for the toe-out and toe-in configurations at various Reynolds (Re) numbers and angle of attacks (β). Eventually irrespective of configurations, rectangular winglet VGs are found to have better overall performance at β = 30° and β = 45° in both toe-out and toe-in configurations due to strong secondary vortex flow.

A previous Doppler-lidar observation near a sea-breeze front revealed that small-scale vertical vortices, similar to dust devils, have a preferred direction of rotation, which suggests that their rotation was affected by meso-scale vertical vorticity associated with the front. In contrast, planetary vorticity is believed to have a negligible effect on dust devils. This paper investigates the effects of ambient rotation on dust devils by means of a large eddy simulation, which yielded the following findings: when the ambient rotation is as small as the earth's rotation, only a tiny asymmetry is found in the occurrences of vertical vortices of different signs; as the ambient rotation is increased, it significantly affects the rotational direction of the vortices, and the magnitude of their vertical vorticity attains a maximum for an ambient vorticity of about 10-3 s-1; a further increase in the ambient rotation changes the structure of convection in the convective mixed layer and suppresses horizontal convergence, so that vertical vorticity of the vortices is reduced.

In the present work, the critical aspects of how particles influence the structure of secondary flows are explored. Despite its significance in the manipulation and detection of particles and cells in microfluidic applications, this topic has received limited attention until now. Employing a fluid-structure interaction method, this research examines the two-way coupling between particles and secondary flows at a Reynolds number (Re) of 80, revealing significant insights into the behaviour of particles under such conditions. The investigation explores the intricate relationships between particle transverse migration, rotation, and secondary flow fields, emphasizing how these interactions amplify with both particle size and secondary flow intensity. The findings underscore the critical role of secondary flow vortices in generating drag forces, which in turn influence particle motion. Notably, the study demonstrates that for larger particles (a/H=0.375), the vortex-induced additional force acts repulsively, discouraging their movement along with the secondary flow. Through detailed analysis, this paper elucidates the lateral migration behaviour of particles, their rotational dynamics in response to secondary flows, and the overarching effects of their interactions with secondary flow structures. This research provides insights into the fundamental mechanisms governing particle behaviour in microfluidic environments, offering potential advancements in particle manipulation and detection.

Using a pressure controlled vortex design method to control secondary flow losses in a turbine stage

The present paper investigates the effects of a fan-shaped hole endwall cooling geometry on the aero-thermal performance of a nozzle vane cascade. Two endwall cooling geometries with four rows of holes were tested, for different mass flow rate ratios: the first configuration is made of cylindrical holes, whereas the second one features conical expanded exits and a reduced number of holes. The experimental analysis is mainly focused on the variations of secondary flow phenomena related to different injection rates, as they have a strong relationship with the film cooling effectiveness. Secondary flow assessment was performed through downstream 3D aerodynamic measurements, by means of a miniaturized 5-hole probe. The results show that at high injection rates, the passage vortex and the 3D effects tend to become weaker, leading to a strong reduction of the endwall cross flow and to a more uniform flow in spanwise direction. This is of course obtained at the expense of a significant increase of losses. The thermal behavior was then investigated through the analysis of adiabatic effectiveness distributions on the two endwall configurations. The wide-banded thermochromic liquid crystals (TLC) technique was used to determine the adiabatic wall temperature. Using the measured distributions of film-cooling adiabatic effectiveness, the interaction between the secondary flow vortices and the cooling jets can be followed in good detail all over the endwall surface. Fan-shaped holes have been shown to perform better than cylindrical ones: at low injection rates, the cooling performance is increased only in the front part of the vane passage. A larger improvement of cooling coverage all over the endwall is attained with a larger mass flow rate, about 1.5% of core flow, without a substantial increase of the aerodynamic losses.

Correlations on heat transfer rate and friction factor of a rectangular toothed v-cut twisted tape exhibiting the combined effects of primary and secondary vortex flows

Numerical investigation of the thermal-hydraulic performance of horizontal supercritical CO2 flows with half-wall heat-flux conditions

In this paper the global eigenmode structures of linear ion temperature gradient (ITG) modes in tokamak plasmas are obtained using a novel technique which combines results from the local gyrokinetic code GS2 with analytical theory to reconstruct global properties. Local gyrokinetic calculations are performed for a range of radial flux surfaces, x, and ballooning phase angles, p, to map out the local complex mode frequency, Ω0(x, p) = ω0(x, p) + iγ0(x, p) for a single toroidal mode number, n. Taylor expanding Ω0 about a reference surface at x = 0, and employing the Fourier-ballooning representation leads to a second order ODE for the amplitude envelope, A(p), which describes how the local results are combined to form the global mode. The equilibrium profiles impact on the variation of Ω0(x, p) and hence influence the global mode structure. The simulations presented here are based upon a global extension to the CYCLONE base case and employ the circular Miller equilibrium model. In an equilibrium with radially varying profiles of a/LT and a/Ln, peaked at x = 0, and with all other equilibrium profiles held constant, including ηi = Ln/LT, Ω0(x, p) is found to have a stationary point. The reconstructed global mode sits at the outboard mid-plane of the tokamak, with global growth rate, γ ∼ Max[γ0]. Including the radial variation of other equilibrium profiles like safety factor and magnetic shear, leads to a mode that peaks away from the outboard mid-plane, with a reduced global growth rate. Finally, the influence of toroidal flow shear has also been investigated through the introduction of a Doppler shift, , where Ωϕ is the equilibrium toroidal flow, and a prime denotes the radial derivative. The equilibrium profile variations introduce an asymmetry into the global growth rate spectrum with respect to the sign of , such that the maximum growth rate is achieved with non-zero shearing, consistent with recent global gyrokinetic calculations.

For flame stabilization, burner systems are usually equipped with swirlers which incite a defined air rotation, quantified by the swirl number. In consequence, the swirl number, which significantly influences the flame dynamics, is fixed. Applications exist in which the swirl number can be varied through mechanically adjustable swirler geometries. Such systems, however, comprise moving parts and actuators, the operation of which might be disadvantageous in a combustor environment. Hence, a swirler is developed that allows for continuous swirl number changes by means of fluidic flow control. The direction of a primary air flow is changed through the injection of an adjacent wall jet which follows a curved surface. This causes a directional change of the primary flow due to entrainment effects. The swirler consists of a cylindrical plenum into which a primary air flow is led through azimuthally distributed channels. Each channel comprises a slotted wall. A secondary air mass flow can be induced passing through the slots, hence creating an adjacent wall jet, which subsequently follows a curved surface. The resulting azimuthal direction change of the primary flow depends on the induced secondary air mass flow. The angular flow momentum and thus the swirl number can be therefore continuously changed. The development process of the proposed swirler is presented and the efficiency of swirl variation of different geometries is qualitatively assessed through computational flow simulations. The effects of stationary operation and unsteady swirl number variations are herein investigated and, moreover, the variation performance is assessed for a range of different air mass flows. The simulation results show the potential of a linear swirl number variation with respect to the induced secondary air mass flow. Moreover, the simulation results show that the achieved swirl numbers are comparable to those of state-of-the-art swirlers.