Streams Of Different Velocities Research Articles

Parametric analysis of the flow past a square cylinder in the interface of two different-velocity streams

We address the linear stability of the two-dimensional incompressible flow past a square cylinder immersed in the wake of an upstream splitter plate, which separates two streams of different velocities, UT (top) and UB (bottom), to three-dimensional perturbations. The analysis is done in the so-called wake transition regime, across which two-dimensional vortex shedding incorporates spanwise modulation. In addition to the top and bottom stream Reynolds numbers (ReT,B=DUT,B/ν, with D the square cylinder side and ν the kinematic viscosity of the fluid), a parametric analysis is conducted to gauge the effects of splitter plate length (S) and the gap between the trailing edge of the splitter plate and the front face of the cylinder (G) on the leading three-dimensionalizing instabilities. Modes akin to the A- and B-type instabilities that characterize the wake transition regime past circular and square cylinders in homogeneous incoming flow are observed only at very small values of the top-to-bottom Reynolds number ratio R=ReT/ReB, while a third mode, mode C, is ubiquitous beyond more than very mild values of R. Increasing S at constant small G has a stabilizing effect on mode C, whose onset is pushed to higher values of R. Only for long S is mode A observed. Fixing a short S and increasing G results instead in a destabilization of mode C, and mode B is favored over mode A.

Improvements to the Empirical Solar Wind Forecast (ESWF) model

The empirical solar wind forecast (ESWF) model in its current version 2.0 runs as a space-safety service in the frame of ESA’s Heliospheric Weather Expert Service Centre. The ESWF model forecasts the solar-wind speed at Earth with a lead time of 4 days. The algorithm uses an empirical relation found between the area of solar coronal-holes (CHs), as observed in EUV within a 15^{circ} meridional slice, and the in-situ measured solar-wind speed at 1 AU. This relation however, forecasts Gaussian type speed profiles, as the CH rotates in and out of the meridional slice, causing some discrepancy in the timing between forecasted and observed solar-wind speed. With adaptations to the ESWF 2.0 algorithm we improve the precision and accuracy of the ESWF speed profiles. For that we implement compression and rarefaction effects occurring between solar-wind streams of different velocities in interplanetary space. By considering the propagation times for plasma parcels between the Sun and Earth and their interactions, we achieve the asymmetrical shape of the speed profile that is characteristic of high-speed streams (HSS). By further implementing CH segmentation, co-latitude information and dynamic thresholding, we find that the newly developed ESWF 3.2 performs significantly better than ESWF 2.0. For a sample of 294 different HSSs, we derive a relative increase in hits of the timing and peak velocity by 13.9%. The Pearson correlation coefficient increases by 14.3% from 0.35 to 0.40.

Square cylinder in the interface of two different velocity streams

We investigate the incompressible flow past a square cylinder immersed in the wake of an upstream nearby splitter plate separating two streams of different velocity. The bottom stream Reynolds number, based on the square side, $Re_B=56$ is kept constant while the top-to-bottom Reynolds numbers ratio $R\equiv Re_T/Re_B$ is increased in the range $R\in [1,6.5]$ , corresponding to a coupled variation of the bulk Reynolds number $Re\equiv (Re_T+Re_B)/2\in [56,210]$ and an equivalent non-dimensional shear parameter $K\equiv 2(R-1)/(R+1)\in [0,1.4667]$ . The onset of vortex shedding is pushed to higher $Re$ as compared with the square cylinder in the classic configuration. The advent of three dimensionality is triggered by a mode-C-type instability, much as reported for open circular rings and square cylinders placed at an incidence. The domain of minimal spanwise-periodic extension that is capable of sustaining spatiotemporally chaotic dynamics, namely one accommodating about twice the wavelength of the dominant eigenmode, has been chosen for the analysis of the wake transition regime. The path towards spatiotemporal chaos is, in this minimal domain, initiated with a modulational period-doubling tertiary bifurcation that also doubles the spanwise periodicity. At slightly higher values of $R$ , the flow has become spatiotemporally chaotic, but the main features of mode C are still clearly distinguishable. Although some of the nonlinear solutions found along the wake transition regime employing the minimal domain might indeed be unstable to long wavelength disturbances, they still are solutions of the infinite cylinder problem and are apt to play a relevant role in the inception of spatiotemporally chaotic dynamics.

Aerodynamic performances and wake topology past a square cylinder in the interface of two different-velocity streams

We analyze the incompressible flow past a square cylinder immersed in the wake of an upstream splitter plate, which separates two streams of different velocities, UT (top) and UB (bottom). The Reynolds number associated with the flow below the plate is kept constant at ReB=DUB/ν=56, based on the square cylinder side D as characteristic length. The top-to-bottom flow dissymmetry is measured by the ratio R≡ReT/ReB∈[1,5.3] between the Reynolds numbers above and below the plate. The equivalent bulk Reynolds taken as the mean between top and bottom changes with R in the range Re≡(ReT+ReB)/2∈[56,178]. A Hopf bifurcation occurs at R=2.1±0.1 (Re=86.8±2.8), which results in an asymmetric Kármán vortex street with vortices only showing on the high-velocity side of the wake. A spanwise modulational instability is responsible for the three-dimensionalization of the flow at R≃3.1 (Re≃115) with the associated wavelength of λz≃2.4. For velocity ratios R≥4, the flow becomes spatiotemporally chaotic. The migration of the mean stagnation and base pressure points on the front and rear surfaces of the cylinder as R is increased determine the boundary layer properties on the top and bottom surfaces and, with them, the shear layers that roll up into the formation of Kármán vortices, which in turn help to clarify the evolution of the lift and drag coefficients. The symmetries of the different solutions across the flow transition regime are imprinted on the top and bottom boundary layers and can, therefore, be analyzed from the time evolution and spanwise distribution of trailing edge boundary layer displacement thickness at the top and bottom rear corners.

Transition from shear-dominated to Rayleigh–Taylor turbulence

Turbulent mixing layers in nature are often characterised by the presence of a mean shear and an unstable buoyancy gradient between two streams of different velocities. Depending on the relative strength of shear versus buoyancy, either the former or the latter may dominate the turbulence and mixing between the two streams. In this paper, we present a phenomenological theory that leads to the identification of two distinct turbulent regimes: an early regime, dominated by mean shear, and a later regime dominated by buoyancy. The main theoretical result consists of the identification of a cross-over timescale that distinguishes between the shear- and the buoyancy-dominated turbulence. This cross-over time depends on three large-scale constants of the flow, namely, the buoyancy difference, the velocity difference between the two streams and the gravitational acceleration. We validate our theory against direct numerical simulations of a temporal turbulent mixing layer compounded with an unstable stratification. We observe that the cross-over time correctly predicts the transition from shear- to buoyancy-driven turbulence, in terms of turbulent kinetic energy production, energy spectra scaling and mixing layer thickness.

Design and analysis of Coanda effect nozzle with two independent streams

ABSTRACT This project aims at presenting a method of making use of the ACHEON Coanda effect based on velocity. The main component of the model is HOMER nozzle. It consists of two inlets, in which streams of different velocity can be made to flow. It is made possible by using the PEACE concept. This project is an analysis work of six different 2D models varying in their geometry. Deflection at various angles is produced. From the base design, five designs are made by varying their domain and they are analysed at different conditions. Higher deflection angle is produced in either direction. The graphs are plotted for pressure and also in terms of velocity. The concept of thrust vectoring is used here. This project helps in creating more flexibility of thrust vectoring nozzle. It overcomes the limitations of traditional systems related to jet deflection exhausts.

Suppression mechanism of Kelvin-Helmholtz instability in compressible fluid flows.

The transformative influence of compressibility on the Kelvin-Helmholtz instability (KHI) at the interface between two fluid streams of different velocities is explicated. When the velocity difference is small (subsonic), shear effects dominate the interface flow dynamics causing monotonic roll-up of vorticity and mixing between the two streams leading to the KHI. We find that at supersonic speed differentials, compressibility forces the dominance of dilatational (acoustic) rather than shear dynamics at the interface. Within this dilatational interface layer, traveling pressure waves cause the velocity perturbations to become oscillatory. We demonstrate that the oscillatory fluid motion reverses vortex roll-up and segregates the two streams leading to KHI suppression. Analysis and illustrations of the compressibility-induced suppression mechanism are presented.

LE of shallow mixing interfaces: A review

Eddy-resolving techniques have become a powerful tool to investigate shallow flows at both laboratory and field scale. In this paper several examples are given where high-resolution 3D numerical simulation are used to investigate the spatial development of mixing interfaces (MIs) forming in shallow environments like open channels with idealized and natural bathymetry where the bed friction plays a major role in the spatial development of the MI and associated large-scale turbulence. The focus is on the coherent structures forming within the MI and in its vicinity that control the momentum and mass exchange and heat transfer between the two sides of the MI. Examples include: (1) a MI developing in a flat-bed open channel downstream of a splitter wall separating two parallel fully-turbulent streams of different velocities, (2) a MI developing in a flat-bed open channel downstream of a 60\(^{\circ }\) wedge separating two non-parallel fully turbulent streams of different velocities, (3) a MI developing downstream of a river confluence for cases with a large and, respectively, a small difference between the mean velocities of the two streams. Stratification effects due to unequal densities of the two incoming streams are also discussed, (4) a MI developing between a main rectangular straight channel and a series of shallow embayments present at one of the channel banks. Besides using available experimental data to demonstrate that eddy resolving techniques can accurately predict the structure of the MI and its development, the paper discusses new insights into the physics of these flows obtained based on the simulations. The paper also provides an overview of the main numerical approaches that can be used to simulate the unsteady dynamics of the large scale turbulence in flows containing shallow MIs.

Direct numerical simulation of a mixing layer downstream a thick splitter plate

In this numerical study, the flow obtained behind a trailing edge separating two streams of different velocities is studied by means of direct numerical simulation. The main originality of this work is that the splitter plate itself is included in the computational domain using an immersed boundary method. The influence of the trailing-edge shape is considered through the analysis of the destabilizing mechanisms and their resulting effect on the spatial development of the flow. The streamwise evolution of the different flows is found to be very different for each of the configurations considered, both in terms of mean quantities and flow dynamics. Present results suggest that the wake component, which dominates the flow close to the trailing edge, is still influential further downstream, as already observed in pure wake flows but only conjectured in mixing layer. A detailed analysis of the vortex dynamics is proposed using instantaneous visualizations, statistical/stability analysis considerations, and proper orthogonal decomposition in order to better understand how the transition regime from the wake to the mixing layer occurs and why it can influence the self-similarity, in a region where no wake influence can be locally detected.

Interplanetary Scintillation Observations of Stream Interaction Regions in the Solar Wind

We present a summary of results from ten years of interplanetary scintillation (IPS) observations of stream interaction regions (SIRs) in the solar wind. Previous studies had shown that SIRs were characterized by intermediate-velocity solar wind and – in the case of compressive interactions – higher levels of scintillation. In this study we considered all cases of intermediate velocities in IPS observations from the European Incoherent SCATter (EISCAT) radar facility made at low- and mid-heliographic latitudes between 1994 and 2003. After dismissing intermediate-velocity observations which were associated with solar-wind transients (such as coronal mass ejections) we found that the remaining cases of intermediate velocities lay above coronal structures where stream interaction would be expected. An improved ballistic mapping method (compared to that used in earlier EISCAT studies of interaction regions) was used to identify the regions of raypath in IPS observations which might be expected to include interaction regions and to project these regions out to the distances of in-situ observations. The early stages of developing compression regions, consistent with their development on the leading edges of compressive stream interaction regions, were clearly detected as close to the Sun as 30 R ⊙, and further ballistic projection out to the distances of in-situ observations clearly associated these developing structures with density and velocity features characteristic of developed interaction regions in in-situ data in the cases when such data were available. The same approach was applied to study non-compressive interaction regions (shear layers) between solar-wind streams of different velocities where the stream interface lay at near-constant latitude and the results compared with those from compressive interaction regions. The results confirm that intermediate velocities seen in IPS observations above stream boundaries may arise from either detection of intermediate-velocity flow in compression regions, or from non-compressive shear layers. The variation in velocity about the mean determined from IPS measurements (representing the spread in velocity across that part of the raypath associated with the interaction region in the analysis) was comparable in compressive and non-compressive regions – a potentially interesting result which may contain important information on the geometry of developing SIRs. It is clear from these results that compressive and non-compressive interaction regions belong to the same class of stream – stream interaction, with the dominant mode determined by the latitudinal gradient of the stream interface. Finally, we discuss the results from this survey in the light of new data from the Heliospheric Imagers (HI) on the Solar TErrestrial RElations Observatory (STEREO) spacecraft and other instruments, and suggest possible directions for further work.

On the combustion of sprays of liquid fuels and liquid oxidizers in a diffusion flame in a shear layer flow

Two unidirectional streams of different velocities are considered here. One of the streams carries a multisize (polydisperse) spray of fuel droplets, and the other stream carries a multisize spray of droplets of a liquid oxidizer. In both streams, droplets of different sizes are assumed to travel at different longitudinal velocities, that is, drop-size-correlated velocities are assumed here. The lateral spread of the fuel vapor and the vapors produced by the droplets of the liquid, oxidizer are analyzed and their effect on the spray diffusion flame is examined. The present results indicate that not only does the fuel drop-size distribution have a profound effect on flame location and flame temperature, but the liquid oxidizer's drop-size distribution also plays an important role in determining flame properties. Specifically when the droplet distribution of the liquid oxidizer is polydisperse, a higher flame temperature can be obtained if larger amounts of smaller droplets are present in the drop-size histogram.

Particle dispersion in variable density and viscosity shear flows

The dispersion of small dense particles by an unsteady planar shear layer formed between two streams of different velocity, density, and viscosity is investigated numerically. The two-phase flow is assumed to be in the dilute regime. The Lagrangian transport element method is employed to provide two-dimensional unaveraged simulations of the carrier flow. The evolution of the particle field is captured by computing the trajectories of individual particles using a reduced form of the equation of particle motion. The analysis focuses on the impact of the densities and viscosities of the two streams on the dispersion of particles of different Stokes numbers (St). The results verify the well established behavior between St and dispersion: maximized dispersion for intermediate St due to centrifugation of particles by the vortices into the irrotational streams, reduced dispersion at low St with particles following the flow and minimal dispersion for large St with particles remaining unaffected by the flow. Variation of the viscosities of the irrotational streams does not substantially alter the above trends except for a shift in the value of St where different behaviors are experienced. This shift is linked to the existence of an effective viscosity that controls the dispersion process. This viscosity is the one characterizing the well mixed vortical structures. Variations in the densities of the two streams decreases the dispersion of intermediate St particles as compared to the uniform density case. This behavior is due to the impact of baroclinic vorticity generation that creates asymmetries in the overall vorticity field that diminish the centrifugation of the particles into one of the two streams.

Characteristics of flow instabilities from unstable stratification of density in channel shear layers at low Reynolds numbers

The instabilities which form due to globally unstable stratification of density in the shear layer which develops between two coincident streams of different velocity in a channel are described. A portion of the flow is contaminated with dye to visualize flow features. The channel employed is 1.59 cm thick, Reynolds numbers range from 10 0–10 1, and Richardson number ranges from 10 −3–2×10 1. A variety of flow features are evident in unstable, marginally stable and unstable flow domains, including streamwise vortices, hairpin vortices, spanwise structures, and localized turbulent motions. Spanwise and streamwise wave number variations in these structures are also described, along with the mixing onset boundary, and a scaling factor which accounts for the dependence of the mixing onset boundary on channel thickness.

Onset and effects of instabilities from unstable stratification of density on mass transfer in channel shear layers at low Reynolds numbers

The onset of mixing in the shear layer located between two coincident laminar streams of different velocity in a thin, ribbon-like channel is studied. Globally unstable density stratification is employed, which, with certain experimental conditions, results in augmented mixing and entrainment, and disruption of mass transfer of solid particles compared to channels with stable density gradients, and laminar, two-dimensional flow. The mixing onset boundary is quantified in terms of the Reynolds number, Richardson number, and channel flow rate ratios using a newly developed mixing onset parameter R. The results indicate that unstable density gradients, rather than shear gradients, are most responsible for inducing the mixing.

A theoretical study of polydisperse liquid sprays in a shear-layer flow

A shear layer formed by two unidirectional gas streams of different velocities with a multisize (polydisperse) spray of evaporating droplets suspended in one of the gas streams is considered here. Similarity solutions are presented for the evolution in droplet size distributions across the shear layer and the effects of various initial droplet size distributions on the profiles of vapor concentrations are examined. A qualitative comparison between the present results for typical computed total mass distributions of the liquid phase and experimental data reported by Lazaro and Lasheras [Phys. Fluids A 1, 1035 (1989); Proceedings of the 22nd Symposium (International) on Combustion (The Combustion Institute, Pittsburgh, 1988), pp. 1991–1998; J. Fluid Mech. 235, 143 (1992)] shows strong similarity between the two sets of profiles. This supports the assumptions and boundary conditions employed in the present theoretical study. The general behavior of the theoretical SMD (Sauter mean diameter) distribution of the spray across the shear layer also compares well with the reported experimental results of Lazaro and Lasheras.

Viscous interaction between parallel radial streams

Similarity solutions of the appropriate boundary-layer equations are obtained for the steady laminar shear layer between adjacent parallel radial streams of different velocities. The dependence of the resulting velocity distributions upon the radial pressure gradient is investigated. A minimal favourable pressure gradient is necessary for inward radial flows to produce shear layers of finite displacement thickness. The latter requirement determines a unique solution in the case of outward flows. The maximal adverse pressure gradient for which the present problem admits of physically acceptable similarity solutions is significantly larger than the corresponding value for boundary layers on a solid wall.

An Experimental Investigation of the Blowout Limits of a Jet Diffusion Flame in Co-Flowing Streams of Different Velocity and Composition

The blowout limits of a methane diffusion flame in a co-flowing air-fuel or air-diluent stream were determined for a range of surrounding co-flow stream velocities, both laminar and turbulent, up to ~ 1.50 m/s. Methane, ethylene, propane and hydrogen were used as the fuels in the surrounding co-flow stream while nitrogen and carbon dioxide were used as diluents. The experimental results show that the velocity of the surrounding stream affects the blowout phenomena significantly. An increase in the stream velocity has a detrimental effect on the blowout limits at very low velocities up to 0.30 m/s (essentially laminar flow) and at velocities higher than 1.50 m/s (turbulent flow). The addition of a fuel to the air stream in most cases enhances the blowout limit of a methane diffusion flame. However, different trends in the variation of the blowout limits with the surrounding fuel concentration were observed, depending on the type of fuel used and on whether the surrounding coflow stream was laminar or turbulent. The addition of nitrogen or carbon dioxide to the air stream results in decreasing the blowout limits. The effect is more severe at the higher velocities.

Confinement and Bed‐Friction Effects in Shallow Turbulent Mixing Layers

The transverse development of the turbulent mixing layers in an open channel flow of shallow water depth was investigated experimentally to study the confinement and bed-friction effects. Mean and r.m.s. velocity profiles were obtained, using a hot-film anemometer, at a number of cross sections downstream of a splitter plate between two streams of different velocities. In the confinement between the free surface and the channel bed, the transverse spreading rate of the shallow mixing layer was initially twice as large as the nominal rate for the free mixing layer. The spreading rate reduces with distance from the splitter plate under the stabilizing influence of bed-friction, and diminishes to zero in the far field region when the bed-friction number exceeds a critical value of about 0.09.

Large-scale structures in a forced turbulent mixing layer

The large-scale structures that occur in a forced turbulent mixing layer at moderately high Reynolds numbers have been modelled by linear inviscid stability theory incorporating first-order corrections for slow spatial variations of the mean flow. The perturbation stream function for a spatially growing time-periodic travelling wave has been numerically evaluated for the measured linearly diverging mean flow. In an accompanying experiment periodic oscillations were imposed on the turbulent mixing layer by the motion of a small flap at the trailing edge of the splitter plate that separated the two uniform streams of different velocity. The results of the numerical computations are compared with experimental measurements.When the comparison between experimental data and the computational model was made on a purely local basis, agreement in both the amplitude and phase distribution across the mixing layer was excellent. Comparisons on a global scale revealed, not unexpectedly, less good accuracy in predicting the overall amplification.

Upstream Growing Shear Layer in Stratified Flow

A numerical solution for an upstream growing free shear layer formed at the boundary between two uniform streams of different velocity in a strongly stratified fluid is given.