Vortex Dynamics in the Wake of Planetary Ionospheres

Measurements conducted with spacecraft around Venus and Mars have shown the presence of vortex structures in their plasma wake. Such features extend across distances of the order of a planetary radius and travel along their wake with a few minutes rotation period. At Venus, they are oriented in the counterclockwise sense when viewed from the wake. Vortex structures have also been reported from measurements conducted by the solar wind-Mars ionospheric boundary. Their position in the Venus wake varies during the solar cycle and becomes located closer to Venus with narrower width values during minimum solar cycle conditions. As a whole there is a tendency for the thickness of the vortex structures to become smaller with the downstream distance from Venus in a configuration similar to that of a corkscrew flow in fluid dynamics and that gradually becomes smaller with increasing distance downstream from an obstacle. It is argued that such process derives from the transport of momentum from vortex structures to motion directed along the Venus wake and that it is driven by the thermal expansion of the solar wind. The implications of that momentum transport are examined to stress an enhancement in the kinetic energy of particles that move along the wake after reducing the rotational kinetic energy of particles streaming in a vortex flow. As a result, the kinetic energy of plasma articles along the Venus wake becomes enhanced by the momentum of the vortex flow, which decreases its size in that direction. Particle fluxes with such properties should be measured with increasing distance downstream from Venus. Similar conditions should also be expected in vortex flows subject to pressure forces that drive them behind an obstacle.

Examination of Reynolds number effect on the development of round jet flow

In this study, the Reynolds number effect on the development of round jet flow is presented. The jet is produced from a smoothly contracting round nozzle and the flow structure is controlled by varying the air blower speed in order to obtain various Reynolds numbers (Re). The flow Reynolds number considered varies between 1140 and 9117. Mean velocity measurements were taken using hot-wire probe at different axial and lateral distances (0≤x/d≤50, where x is the downstream distance and d is the nozzle diameter) for the jet flow and at for 0≤x/d≤30 in long pipe attached to the nozzle. Measurements reveal that Reynolds number dictate the potential core length such that the higher the Reynolds number, the lower the potential core which is a measure of mixing of jet and ambient fluid. It shows that further away from the jet exit section, potential core decreases as Reynolds number increases, the velocity profile has a top hat shape very close to the nozzle exit and the shape is independent of Reynolds number. It is found that potential core extends up to x/d=8 for Reynolds number of 1140 as against conventional near field 0≤x/d≤6. This may suggest effect of very low Reynolds number. However, further investigation is required to ascertain this at extremely low Reynolds numbers. It is also observed that further away from the jet exit section, the higher the downstream distance, the higher the jet half-width (R1/2). Furthermore, the flow in the pipe shows almost constant value of normalised axial centerline velocity for a longer distance and this clearly indicates that there is mass redistribution rather than entrainment of ambient fluid. Overall, the Reynolds number controls the magnitude rather than the wavelength of the oscillation

Field measurements of wake meandering at a utility-scale wind turbine with nacelle-mounted Doppler LiDARs

Wake meandering is a low-frequency oscillation of the entire wind turbine wake that can contribute to power and load fluctuations of downstream turbines in wind farms. Field measurements of two Doppler LiDARs mounted on the nacelle of a utility-scale wind turbine were used to investigate relationships between the inflow and the wake meandering as well as the effect of wake meandering on the temporally averaged wake. A correlation analysis showed a linear relationship between the instantaneous wake position and the lateral velocity that degraded with the evolution of the turbulent wind field during the time of downstream advection. A low-pass filter proportional to the advection time delay is recommended to remove small scales that become decorrelated even for distances within the typical spacing of wind turbine rows in a wind farm. The results also showed that the velocity at which wake meandering is transported downstream was slower than the inflow wind speed, but faster than the velocity at the wake center. This indicates that the modelling assumption of the wake as an passive scalar should be revised in the context of the downstream advection. Further, the strength of wake meandering increased linearly with the turbulence intensity of the lateral velocity and with the downstream distance. Wake meandering reduced the maximum velocity deficit of the temporally averaged wake and increased its width. Both effects scaled with the wake meandering strength. Lastly, we found that the fraction of the wake turbulence intensity that was caused by wake meandering decreased with downstream distance contrary to the wake meandering strength.

Solar Cycle Variations in the Position of Vortex Structures in the Venus Wake

Measurements conducted with the Venus Express (VEX) spacecraft at its entry and exit through vortex structures in the Venus wake reveal that their position varies with the solar cycle. Both crossings are consistently measured closer to Venus during minimum solar cycle conditions and are gradually encountered at larger distances downstream from the planet along the solar cycle. At the same time their width along the VEX trajectory on the plane transverse to the solar wind direction is larger during minimum solar cycle conditions and show a gradual decrease along the solar cycle. As a result the vortex structures are envisioned as features that gradually become thinner as they extend along the Venus wake and agree with the geometry of a vortex flow in fluid dynamics whose thickness decreases with the downstream distance from an obstacle. Similar conditions should also be applicable to Mars and other bodies within the solar system and also possibly to exo-planets in external stellar systems.

Shallow free-surface Stokes flow around a corner

The steady lateral spreading of a free-surface viscous flow down an inclined plane around a vertex from which the channel width increases linearly with downstream distance is investigated analytically, numerically and experimentally. From the vertex the channel wall opens by an angle α to the downslope direction and the viscous fluid spreads laterally along it before detaching. The motion is modelled using lubrication theory and the distance at which the flow detaches is computed as a function of α using analytical and numerical methods. Far downslope after detachment, it is shown that the motion is accurately modelled in terms of a similarity solution. Moreover, the detachment point is well approximated by a simple expression for a broad range of opening angles. The results are corroborated through a series of laboratory experiments and the implication for the design of barriers to divert lava flows are discussed. This article is part of the theme issue ‘Stokes at 200 (Part 1)’.

Case Study of Transient Dynamics in a Bypass Reach

The operating conditions of Nordic hydropower plants are expected to change in the coming years to work more in conjunction with intermittent power production, causing more frequent hydropeaking events. Hydropeaking has been shown to be detrimental to wildlife in the river reaches downstream of hydropower plants. In this work, we investigate how different possible future hydropeaking scenarios affect the water surface elevation dynamics in a bypass reach in the Ume River in northern Sweden. The river dynamics has been modeled using the open-source solver Delft3D. The numerical model was validated and calibrated with water-surface-elevation measurements. A hysteresis effect on the water surface elevation, varying with the downstream distance from the spillways, was seen in both the simulated and the measured data. Increasing the hydropeaking rate is shown to dampen the variation in water surface elevation and wetted area in the most downstream parts of the reach, which could have positive effects on habitat and bed stability compared to slower rates in that region.

Experiments on the grain size gap across gravel-sand transitions

<p>An abrupt transition in river bed grain size occurs from gravel to sand over a short downstream distance, often only a few channel widths, and is termed the gravel-sand transition. At this point, the bed structure also changes from framework- to matrix-supported. Whether the gravel-sand transition is externally imposed, a result of internal dynamics (sediment sorting, abrasion, suspension deposition) or due to some other emergent property is unclear. Interestingly, there is a general absence of rivers beds with median surface grain sizes between ~1 and 5 mm. Here we present a new global compilation of gravel-sand transition characteristics across a diverse range of settings. We identify commonalities in the location of gravel-sand transitions, finding they occur at upstream extents of externally imposed backwater effects, where the gravel supply is exhausted (i.e. downstream of mountain ranges), or where both effects are coincident. A series of laboratory channel experiments, examining changes in fluid and sediment dynamics across a gravel-sand transition, show systematic changes in near bed turbulence that control sand deposition patterns. Gravel coarser than ~10 mm prevents sand deposition at the bed surface. We also find that gravel-sand transitions cannot form where river beds contain substantial amounts of ~1 to 5 mm particles, because these grain sizes enhance the mobility of coarser gravel, preventing a shift to a sand bed.</p>

A Swirl Cooling Flow Experimental Investigation on a Circular Chamber Using Three-Dimensional Stereo-Particle Imaging Velocimetry

An experimental investigation is presented using three-dimensional (3-D) stereo-particle image velocimetry (stereo-PIV) of a swirl flow that models a gas turbine blade internal cooling configuration. The study is intended to provide an evaluation of the developments of the swirl cooling flow methodology utilizing the 3-D stereo-PIV. The objective is to determine the critical swirl number that has the potential to deliver the maximum axial velocity results. The swirl cooling flow methodology comprises cooling air channeling through the blade’s internal passages lowering the temperature; therefore, the experimental circular chamber is made of acrylic allowing detailed measurements and includes seven discrete tangential jets designed to create the swirl flow. An oil particle seeder (LAVision) is used to provide the particles for velocity measurements while the clear acrylic chamber allows visualization of the flow phenomena. The post-processed data are completed using davis, velocity calculations are conducted in matlab, and techplot is used for data visualization. The focus of this investigation is on the continuous swirl flow that must be sustained via continuous injection of tangential flow at three different Reynolds number, 7000, 14,000, and 21,000, where the swirl flow is generated with seven inlets. Important variations in the swirl number are present near the air inlets and decreases with downstream distance as predicted, since the second half of the chamber has no more inlets. The axial velocity reaches the maximum downstream in the second half of the chamber. The circumferential velocity decreases the downstream distance and reaches the highest toward the center of the chamber.

Numerical study of plume emission from an elevated line source in a wall-bounded flow

Purpose This paper aims to study turbulent dispersion of a passive plume emitting from a single elevated line source of different elevations in a plane channel flow by using direct numerical simulation (DNS). Design/methodology/approach The investigation was conducted in both physical and spectral spaces, which includes an analysis of statistical moments and pre-multiplied spectra of the velocity and concentration fields. The pre-multiplied power spectra of the velocity and concentration fields are compared to identify the transition of the plume development from the turbulent convective stage to the turbulent diffusive stage. Findings It is observed that due to the presence of wall shear, the mean plume drifts toward the wall for the near-wall source release case. It is also observed that streamwise development of the plume is sensitive to both the source elevation and the downstream distance from the source. For the line source placed near the center of the channel, the plume development is dominated by the bulk meandering effects. However, for the plume emitting from the near-wall line source, it hits the ground soon after its release and becomes dominated by the wall shear. As the downstream distance from the line source increases, the streamwise development of the plume released from the near-wall line source transitions from a turbulent convective stage to a turbulent diffusive stage. Originality/value This paper represents an original DNS study of turbulent mixing and dispersion of a passive plume emitting from a line source of different elevations in a wall-bounded flow. This paper proposes a practical method to identify the transition of the plume development from the turbulent convective to the turbulent diffusive stages.

On the mean structure of sharp-fin-induced shock wave/turbulent boundary layer interactions over a cylindrical surface

Interactions between an oblique shock wave generated by a sharp fin placed on a cylindrical surface and the incoming boundary layer are investigated to unravel the mean features of the resulting shock/boundary layer interaction (SBLI) unit. This fin-on-cylinder SBLI unit has several unique features caused by the three-dimensional (3-D) relief offered by the cylindrical surface that noticeably alter the shock structure. Complementary experimental and computational studies are made to delineate both the surface and off-body flow features of the fin-on-cylinder SBLI unit and to obtain a detailed understanding of the mechanisms that dictate the mean flow and wall pressure features of the SBLI unit. Results show that the fin-on-cylinder SBLI exhibits substantial deviation from quasi-conical symmetry that is observed in planar fin SBLI. Furthermore, the separated flow growth rate appears to decrease with downstream distance and the separation size is consistently smaller than the planar fin SBLI with the same inflow and fin configurations. The causes for the observed diminution of the separated flow and its downstream growth rate were investigated in the light of changes caused by the cylinder curvature on the inviscid as well as separation shock. It was found that the inviscid shock gets progressively weakened in the region close to the triple point with downstream distance due to the 3-D relief effect from cylinder curvature. This weakening of the inviscid shock feeds into the separation shock, which is also independently impacted by the 3-D relief, to result in the observed modifications in the fin-on-cylinder SBLI unit.