Strong Ejections Research Articles

Open channel flow past a train of rib roughness

In this paper, the effect of depth on turbulent open channel flow past a train of rib elements is examined. The two-dimensional square ribs spanning the width of the channel are located throughout the length of the flume. The experiments are conducted in the fully rough regime to document the turbulence characteristics of the flow at two different rib spacing (pitch-to-height ratios [ p/k ] of 9 and 18) conforming to the classical definition of k-type roughness. The streamwise mean velocity profiles demonstrate a shift from a smooth wall flow but varied marginally between the two cases of p/k and were also not affected by the change of flow depth. At shallow depths, roughness with p/k = 9 show a substantial increase of turbulence intensities and Reynolds shear stress in the outer layer compared to the reference flow case on a smooth bed. In the case of p/k = 18, the roughness effect is confined to the region less than 3 k from the wall and an outer layer similarity is confirmed with no effect of flow depth. Quadrant analysis shows an increase of ejection and sweep events in the outer layer at shallow depths for p/k = 9. Near the reattachment point, strong sweeps penetrate into the outer layer for both rough wall flows only at shallow depth. Strong ejections, on the other hand, seem to be more sensitive to the roughness and p/k ratio. The present results indicate that at shallow depths with p/k = 9, most of the flow in the outer layer is affected, while with a larger spacing (p/k = 18), the flow characteristics are less sensitive to the change of depth.

The overall configuration of the interplanetary magnetic field upstream of Saturn as revealed by Cassini observations

The Cassini spacecraft approached Saturn during the declining phase of the solar cycle, at a time when the heliosphere was highly structured by compressions and rarefactions associated with corotating interaction regions (CIRs). We examine in detail the hourly averaged interplanetary magnetic field (IMF) data prior to Saturn Orbit Insertion and during one subsequent orbit of Cassini closer to solar minimum, where the spacecraft spent several months in the solar wind. We observe the effects of CIRs on IMF structure and show examples of where this structure is disturbed by the passage of strong coronal mass ejections and by solar cycle effects. We examine the field directions and find that in general, they correspond well to the predictions of the Parker model, but with several notable deviations, which we discuss.

The role of plasma ejections in the development of large solar flares of various durations

Recent observations indicate that relatively strong plasma ejections are accompanied by the formation of systems of coronal loops with two glowing ribbons near their footpoints. However, while two-ribbon flares can sometimes last for many hours, for example, soft X rays, they sometimes decay within tens of minutes. We study here factors affecting the durations of flares using four major flares occurring in July 15–18, 2002, as examples. Various ground-based and satellite observations are used to show that short-duration events involved collimated (narrow) plasma ejections directed to the north and the subsequent formation of compact loops in the leading part of the active region. During one event, a powerful eastward ejection in a wide solid angle was followed by the formation of an extended arch system in the trailing part, which determined the long duration of the flare. It is proposed that in events involving collimated jets and corresponding narrow features in coronal mass ejections (CMEs), systems of coronal loops do form, but post-eruptive energy release either does not occur or is expressed very faintly. So the energy does not go downward from this region, and the plasma is emitted free in the coronal loops. In contrast to such rapid flares, wide ejections and bright, large-scale CMEs are accompanied by the formation and prolonged existence of an extended arch system. Thus, powerful nonstationary solar processes involve a large-scale CME and the flare itself, with the pattern of a particular event determined by the reconnection scenario and the evolution of the ejected plasma.

The role of coherent structures in subgrid-scale energy transfer within the log layer of wall turbulence

The present effort documents the relationship between dominant subgrid-scale energy transfer events and coherent motions within the log layer of wall turbulence. Instantaneous velocity fields in the streamwise–wall-normal plane of a zero-pressure-gradient turbulent boundary layer acquired by particle-image velocimetry at Reτ≡u*δ∕ν=2350 are spatially filtered to generate an ensemble of resolved-scale velocity fields in the spirit of large-eddy simulation. The relationship between subgrid-scale dissipation and embedded coherent structures is then studied using instantaneous realizations and conditional averaging techniques. This analysis reveals that strong forward- and backward-scatter events occur spatially coincident to individual hairpin vortices and their larger-scale organization into vortex packets. In particular, large-scale regions of forward scatter are observed along the inclined interface of the packets, coincident with strong ejections induced by the individual vortices. The most intense forward-scatter events are found to occur when these ejections are opposed by sweep motions. Strong backward scatter of energy is observed at the trailing edge of the vortex packets and weaker backscatter is also noted locally around the individual heads of the hairpin structures. The collective observations presented herein demonstrate that hairpin vortices and their organization into larger-scale packets are important contributors to interscale energy transfer in the log layer of wall turbulence.

Simulation of small-scale coronal explosives due to magnetic reconnections

The dynamics of small-scale explosive phenomena in the lower corona have been simulated by solving the compressible magnetohydrodynamic equations. Numerical results show that the magnetic reconnections in a long coronal current sheet consist of a series of discrete small reconnection events, coalescence of magnetic islands, and plasmoid ejections, corresponding to the explosive events occurring intermittently and as bursts in a mentioned observational case. The generation of magnetic islands via multiple-X-point reconnection and their coalescence processes, to some extent, are qualitatively similar to the sequence of brightenings in the active region NOAA 8668. The strong ejections are possibly related to the recorded extreme ultraviolet (EUV) emitting structures. Morphological comparison and quantitative check of the plasma parameters support this candidate mechanism, and the idea that explosive events that appear to last long may not be single events, but a succession of explosive events either resolved or unresolved. The temporal energy conversion process is also examined.

Prediction and investigation of the turbulent flow over a rotating disk

Large-eddy simulation (LES) has been used to predict the statistically three-dimensional turbulent boundary layer (3DTBL) over a rotating disk. LES predictions for six parameter cases were compared to the experimental measurements of Littell & Eaton (1994), obtained at a momentum thickness Reynolds number of 2660. A signal-decomposition scheme was developed by modifying the method of Spalart (1988) to prescribe time-dependent boundary conditions along the radial direction, entrainment towards the disk surface was prescribed by satisfying global mass conservation. Predictions of the mean velocities and r.m.s. fluctuations are in good agreement with data, with the largest discrepancy occurring in the prediction of the wall-normal intensities. The primary and two secondary shear stresses are also in good agreement with the measurements and one-dimensional energy spectra of the velocity fluctuations agree well with established laws, i.e. a −1 slope in the buffer region and −5/3 slope near the edge of the boundary layer.Conditionally averaged velocities provide new evidence in support of the structural model of Littell & Eaton (1994) concerning the interaction of mean-flow three-dimensionality and shear-stress producing structures. Inside the buffer region under strong ejections, the conditionally averaged crossflow (radial) velocity is larger than the unconditioned mean, and the profile conditioned on strong sweeps is smaller than the mean. This is consistent with the notion that streamwise vortices having the same sign as the mean streamwise vorticity, and beneath the peak crossflow location, are mostly responsible for strong sweep events; streamwise vortices with opposite sign as the mean streamwise vorticity promote strong ejections. Comparison of two-point spatial correlations with previous measurements in two-dimensional turbulent boundary layers (2DTBLs) indicates interesting structural similarities, e.g. the correlation of wall pressure and surface-normal velocity fluctuations is an odd function of streamwise separation, being positive downstream and negative upstream. These similarities offer quantitative indirect support to the hypothesis advanced by Littell & Eaton (1994) and Johnston & Flack (1996) that structural models describing 2DTBLs may be employed as a baseline in (equilibrium) 3DTBL structural studies.

Turbulent structure in the three-dimensional boundary layer on a swept wing

Abstract The effects of mean flow three-dimensionality on the turbulent structure of the boundary layer on a swept wing are investigated experimentally. The configuration of the test model used to simulate the infinite swept wing condition is similar to that used by Van den Berg, B., Elsenaar, A., Lindhout, J.P.F., Wesseling, P., 1975. Measurements in an incompressible three-dimensional turbulent boundary layer, under infinite swept-wing conditions, and comparison with theory. J. Fluid Mech. 70 (1), 127–148. Mean flow measurements show that the location of the maximum cross-flow velocity is within the log-law region for the streamwise velocity profiles. In the near-wall region, the streamwise turbulence intensity, u 2 /U τ , decreases with increasing three-dimensionality. A four quadrant analysis is applied to the fluctuating velocity components in the local mean flow and wall-normal directions. At downstream stations, the contributions to the turbulent shear stress from quadrant 4 (sweeps) are predominant in the region near the wall. It is thus revealed that strong ejections are suppressed by the effects of increased three-dimensionality.

LDA measurements in a turbulent boundary layer over a d-type rough wall

Two-component LDA measurements in smooth wall and d-type rough wall boundary layers indicate that there are important differences in the turbulence intensities over a significant distance above the two surfaces. All the Reynolds stresses—especially the shear stress—are larger over the d-type rough wall, reflecting the relatively strong momentum exchange which occurs over the roughness cavities. Flow visualisations indicate that this exchange is associated with strong ejections of cavity fluid into the outer flow.

Possible connection between surface winds, solar activity and the Earth's magnetic field

RECORDS of 14C in tree rings provide a proxy for changes in solar activity on timescales of decades to centuries1,2. Here I report an association between Maunder-type solar cycles recorded in tree rings and fluctuations in surface wind intensity on a roughly 200-year timescale over a period of about 2,000 years in the mid-Holocene. The wind record is preserved as changes in the thickness of varved sediments in Elk Lake, Minnesota, which contain a large aeolian component. Changes in cyclonic activity and tropospheric winds have been reported1–5 to occur in this region a few days after strong coronal mass ejections (CMEs); increases of up to 7% in zonal flow have been reported5 at an altitude of ~ 300 m after strong CMEs in winter. The implication of this association is that century-scale changes in surface winds over Minnesota were the long-term (Maunder-scale) counterpart of short-term (daily) changes in winds after CMEs. The weak magnetic field strength of the Earth at this time6,7 might be relevant to a possible mechanism for the association.

Large-scale turbulent structures and the stabilization of lifted turbulent jet diffusion flames

The physical mechanisms responsible for the stabilization of lifted axisymmetric turbulent jet diffusion flames remain uncharacterized. Flame stabilization has been shown to take place at radial locations in the corresponding isothermal jets where the flow has an intermittent character. Recently developed flow diagnostics capable of simultaneous multi-point concentration and velocity measurements are used to investigated large-scale turbulent structures in the intermittent region of an isothermal jet of propane. The findings, in conjunction with earlier literature results, show that large-scale structures form as the results of organized turbulent motion during which strong outward ejections of fluid form central regions of the jet flow into the ambient surroundings. Once formed, the large-scale structures are convected downstream for long distances. The downstream edges of the structures are regions of significantly higher concentration gradients and shear than regions further upstream. Occasionally, the jet fluid loses its turbulent energy and is no longer transported by the flow. This fluid can be reentrained by the passage of later large-scale structures. It is concluded that the processes responsible for flame stabilization must occur in these large-scale turbulent structures. Flame extinction is most probable on the downstream edges of the structures while the ignition and stabilization of the flame is likely on the upstream sides. Reentrained fluid provides a possible mechanism for ignition of flammable mixtures on the upstream sides of the structures.

Use of hollow electrodes in melting periclase

The use of hollow electrodes in melting of periclase makes it possible to create the evacuation necessary for stable arcing, to reduce the number of strong ejections of molten material from the furnace, to introduce the quantity necessary according to the method of loose additions, particularly of alumina, directly into the molten material, and to accelerate the melting process with the use of an oxygen enriched draft.