Siphon Flow Research Articles

Siphon Removal of Cohesionless Materials

This paper presents the results of an experimental study of the scour created by a siphon flow through a vertical tube of diameter d of 9.7 to 20.4 mm in a cohesionless sand bed of approximately uniform size D of 0.58 mm . The position of the inlet of the siphon tube ranged from −101.6 to 6.4 mm relative to the surface of the sand bed and the velocity of flow through the tube U0 varied from 0.34 to 6.41 m∕s . Data from this and previous studies are used to show the radial extent of the scour hole and the maximum depth of scour depend primarily on the densimetric Froude number F0 , where F0 = U0 ∕ g ( Δρ ∕ρ ) D , g is the gravitational acceleration, Δρ is the difference in density between the sediment and fluid, and ρ is the density of the fluid. A theoretical analysis of the problem is used to further assess the data and it is found that viscous effects are also important to consider. The profile of the scour hole at equilibrium, for all tube positions, is found to be similar when the scour depth is measu...

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Response

Mass Flows in a Disappearing Sunspot Plume

We observed a large sunspot in NOAA Active Region 9535 on 2001 July 12, 15, 17, and 19 with three instruments (CDS, EIT, and MDI) on board SOHO. EUV emission lines of O IV (formed at logarithmic temperature of 5.2), O V (5.4), Ne IV (5.2), Ne V (5.5), Ne VI (5.6), Ne VII (5.7), and Ca X (5.9) revealed a large bright plume within the sunspot penumbra on July 12 and 15, a smaller, dimmer plume on July 17, and no plume on July 19. Downflows of 25 km s-1 or more were measured within the sunspot plume on July 12 and 15. By July 17 the downflow area had shrunk in size, the downflows had diminished in magnitude, and upflows were measured in the umbra and parts of the penumbra outside the plume. By July 19 downflows were no longer observed, but had been replaced entirely with upflows of 15-25 km s-1 in the umbra and portions of the penumbra. This is the first time that the disappearance of a sunspot plume has been observed to occur simultaneously with a dramatic change in flow velocity pattern in the sunspot plume and umbra. On July 12 upflows northeast of the sunspot were observed along with the downflows in the plume. Electron density measurements based on intensity ratios of the O IV lines at 625.8 and 554.5 A indicate a significantly greater value in the upflow zone than in the plume, consistent with siphon flow as the driver of the observed velocities; however, the line at 625.8 A is very weak and blended in the red wing of the Mg X line at 624.9 A, so derived densities are highly uncertain.

Physical Properties of the Solar Magnetic Photosphere under the MISMA Hypothesis. III. Sunspot at Disk Center

Small-scale fluctuations of magnetic field and velocity may be responsible for the Stokes asymmetries observed in all photospheric magnetic structures (the microstructured magnetic atmosphere [MISMA] hypothesis). We support the hypothesis by showing that atmospheres with optically thin microstructure reproduce the polarization of Fe I λλ6301.5 and 6302.5 observed in a sunspot. Ten thousand spectra were fitted by model MISMAs with two magnetic components interleaved along the line of sight. Combining all the fits, we set up a semiempirical model sunspot characterized by two components with very different magnetic field inclinations. The major component, which contains most of the mass, is more vertical than the minor component. The field lines of the minor component are inclined below the horizontal plane throughout the penumbra. Magnetic field lines and mass flows are parallel; consequently, both upflows and downflows are present everywhere on the penumbra. Major and minor components have very different velocities (several hundred meters per second for the major component versus 10 km s-1 for the minor component), but the mass transported per unit time is similar. The similarity between the vertical mass flow and the magnetic flux of the two components suggests that field lines emerging as major components may return to the photosphere as minor components. If so, the observed magnetic field strength difference between components leads to a siphon flow whose magnitude and direction agree with the inferred Evershed flow. Several tests support the internal consistency of the retrieved model sunspot. The magnetic field vector B does not violate the ∇ = 0 condition. The model sunspot reproduces the net circular polarization of the observed lines plus the abnormal behavior of Fe I λ15648. The use of only one magnetic component to interpret the spectra leads to inferring upflows in the inner penumbra and downflows in the outer penumbra, in agreement with previous findings.

What causes solar active region loops to exist at transition region temperatures?

High-lying, dynamic loops have been observed at transition region temperatures since Skylab observations. The nature of these loops has been debated for many years with several explanations having been put forward. These include that the loops are merely cooling from hotter coronal loops, that they are produced from siphon flows, or that they are loops heated only to transition region temperatures. In this paper we will make use of combined SOHO-MDI (Michelson-Doppler Imager), SOHO-CDS (Coronal Diagnostic Spectrometer) and Yohkoh SXT (Soft X-ray Telescope) datasets in order to determine whether the appearance of transition region loops is related to small-scale flaring in the corona, and to estimate the magnetic configuration of the loops. The latter allows us to determine the direction of plasma flows in the transition region loops. We find that the appearance of the transition region loops is often related to small-scale flaring in the corona and in this case the transition region loops appear to be cooling with material draining down from the loop top.

Imaging the velocity profiles in tubeless siphon flow by NMR microscopy

We report on the use of NMR micro-imaging to observe flow within a tubeless siphon. The flow is maintained in a visco-elastic liquid of high extensional viscosity, namely 0.5% w/v 8 million Dalton polyethylene oxide in water. The velocity profiles reveal a significant velocity gradient in the vertical direction as well as a transition from near-Poiseuille flow at the pipe entrance to plug flow far from the pipe entrance towards the base of the tubeless siphon.

X-RAY SPECTRAL ANALYSIS OF SU UMa TYPE DWARF NOVAE OBSERVED WITH ROSAT

X-ray spectral parameters were determined for eight SU UMa type Dwarf Novae observed with the ROSAT PSPC. The raw data were fitted with various spectral models and the best fit spectral models are found to be that of Raymond–Smith and Thermal Bremsstrahlung. The best fit temperatures were estimated to be between kT ~ 1.1-1.8 keV while the Column Densities were found to be between NH ~ 2.4×1020-4.1×1020 cm -2. The estimated 0.1-2.4 keV fluxes were in the range of log FX=-13 to -11 ergs cm-2 s-1. FX/F UV and FX/F opt rates were calculated to be between ~0.09 and ~0.37. This shows that most of the energy is radiated in the Optical and Ultraviolet band from the accretion disk in the quiescent state. Many of the SU UMa type Dwarf Novae show an Ultraviolet lag in their outburst spectrum, the Coronal Siphon Flow Model of Meyer and Meyer-Hofmeister may explain this phenomenon. This model proposes a corona at the boundary layer of a system when it is a quiescent state and suggests that some parts of the X-rays come from the corona. For these reasons, the equations of this model were applied to the results of the spectral analysis. Using this model, the mass accretion rates, the mass evaporation rates, and the radii of the coronas were calculated to be ~10-12.3-10-11.3 M⊙ yr -1, ~10-6.5-10-5.5 g cm -2 s -1 and ~109.1-109.9 cm , respectively. The pressures in the coronas were less than ~1200 g cm -2 s -1 for (z) up to ~10×109 cm . The obtained values suggest that the Corona model can indeed operate in SU UMa type Dwarf Novae.

Temporal Behavior of the Evershed Effect

We study the Evershed flow in the photosphere and the reverse Evershed flow in the chromosphere from simultaneous observations, giving emphasis to the temporal evolution of the phenomena. We compute the components of the velocity vector as a function of distance from the center of the sunspot, assuming an axial symmetry of both the spot and the flow. A five-minute oscillatory pattern is obvious in the penumbra at photospheric level. Our results verify that the velocity of the Evershed flow has a maximum above the penumbra in the photosphere and well outside the penumbra in the chromosphere. We find evidence of temporal variations prominent in the radial component of the average photospheric velocity with a characteristic timescale of 25 minutes. We consider a transient siphon flow or a wave superimposed on a steady flow as possible explanations for the temporal behavior of the photospheric Evershed flow. The radial component of the chromospheric reverse Evershed flow shows a repetitive temporal variation with a typical timescale of 15 minutes. The variation consists of enhanced velocity amplitudes that propagate to an opposite direction from the flow with a velocity of about 5-6 km s-1. This behavior cannot be easily explained in the frame of a transient flow and strongly suggests that it is related to the propagation of a wave. We examine the possibility of its being associated with the propagation of running penumbral waves in the superpenumbra. The temporal evolution of the line-of-sight velocity across superpenumbral fibrils presents alterations that can be associated with a time-dependent flow. However, we also observe propagating velocity packets that can be associated with a wave.

Predator avoidance by nauplii

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 247:137-149 (2003) - doi:10.3354/meps247137 Predator avoidance by nauplii Josefin Titelman1,2,3,*, Thomas Kiørboe1 1Danish Institute for Fisheries Research, Department of Marine Ecology and Aquaculture, Kavalergården 6, 2920 Charlottenlund, Denmark 2Göteborg University, Department of Marine Ecology, Kristineberg Marine Research Station, 450 34 Fiskebäckskil, Sweden 3Present address: Plymouth Marine Laboratory, Prospect Place, The Hoe, Plymouth PL1 3DH, United Kingdom *Email: joti@pml.ac.uk ABSTRACT: We examined anti-predation strategies in relation to motility patterns for early and late nauplii of 6 species of copepods (Calanus helgolandicus, Centropages typicus, Eurytemora affinis, Euterpina acutifrons, Acartia tonsa and Temora longicornis). Remote detection and escape abilities were quantified in siphon flows. Nauplii respond with distinct escape jumps at species- and stage-specific average fluid deformation rates between 0.5 and 4.2 s-1, and escape with average velocities corresponding to ~60 to 130 body lengths s-1. Most of the species and stages orient their escape jumps upwards and also away from the flow, and are thus able to assess direction of flow. Sensitivity to hydrodynamic signals is tightly linked to the general motility pattern. Hydrodynamically conspicuous nauplii that move in a jump-sink pattern are more effective at remotely detecting predators than are continuous swimmers of similar size. We assessed the effects of different motility strategies in terms of detectability and volume encounter with copepod predators by means of simple hydrodynamic models. While jump-sink type nauplii may be at an advantage over swimmers in interactions with sinking and attacking ambush predators, models suggest no immediate relative benefit in terms of escaping flows of filtering copepods. We discuss these predictions in relation to copepod feeding rates. KEY WORDS: Copepod · Nauplii · Escape jump · Predator avoidance · Behavior · Motility · Hydrodynamic signals · Remote detection · Predator-prey interaction · Volume encounter rates · Encounter rates Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 247. Online publication date: February 04, 2003 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2003 Inter-Research.

A Transient Heating Model for Coronal Structure and Dynamics

A wealth of observational evidence for flows and intensity variations in nonflaring coronal loops leads to the conclusion that coronal heating is intrinsically unsteady and concentrated near the chromosphere. We have investigated the hydrodynamic behavior of coronal loops undergoing transient heating with one-dimensional numerical simulations in which the timescale assumed for the heating variations (3000 s) is comparable to the coronal radiative cooling time and the assumed heating location and scale height (10 Mm) are consistent with the values derived from TRACE studies. The model loops represent typical active region loops: 40-80 Mm in length, reaching peak temperatures up to 6 MK. We use ARGOS, our state-of-the-art numerical code with adaptive mesh refinement, in order to resolve adequately the dynamic chromospheric-coronal transition region sections of the loop. The major new results from our work are the following: (1) During much of the cooling phase, the loops exhibit densities significantly larger than those predicted by the well-known loop scaling laws, thus potentially explaining recent TRACE observations of overdense loops. (2) Throughout the transient heating interval, downflows appear in the lower transition region (T ~ 0.1 MK) whose key signature would be persistent, redshifted UV and EUV line emission, as have long been observed. (3) Strongly unequal heating in the two legs of the loop drives siphon flows from the more strongly heated footpoint to the other end, thus explaining the substantial bulk flows in loops recently observed by the Coronal Diagnostic Spectrometer and the Solar Ultraviolet Measurement of Emission Radiation instrument. We discuss the implications of our studies for the physical origins of coronal heating and related dynamic phenomena.

Penumbral fine structure: Theoretical understanding

To understand the dynamic fine structure and the Evershed effect of the sunspot penumbra, we have carried out time-dependent simulations of a thin magnetic flux tube embedded in a static sunspot model (Schlichenmaier et al. 1998a,b). Here, we present new simulation results of the moving tube model which reveal flux tubes that, instead of lying horizontal in the penumbral photosphere, develop waves (photospheric serpent) that reproduce not only the inward migration of penumbral grains, but also their observed outward migration, and which exhibit downflow arches in the outer penumbra. During its evolution the flux tube exhibits quasi-steady states. These states are compared to time-independent solutions of siphon flow models, which have been used to explain the Evershed flow (Meyer & Schmidt 1968, Degenhardt 1991, Montesinos & Thomas 1997).

Variability and dynamic state of active region loops

A set of 218 consecutive CDS rasters taken at the solar limb on October 26–28 1999 has been used to investigate the variability and plasma dynamics of active region loops. Each raster contains simultaneous images in 6 different lines, covering the full temperature range of CDS, 10 000 K (He I) to 2.7 MK (Fe XVI). Activity is seen to go on without breaks at temperatures below 1 MK for the full 39 hours of the series. Transition region loops or extended sections of loops, 50–200 Mm long, appear and disappear in intervals as short as 11 minutes, the observing cadence. In the corona the emission is less variable, but significant changes are seen. Measured Doppler shifts correspond to typical plasma velocities of 20 km s −1 to 100 km s −1, at temperatures 10 000 K to 450 000 K, and siphon flows may occur in some of the loops. High velocities are frequently seen where the emitted intensities are weak, often on the outer edges of loops as defined in that particular spectral line. At coronal temperatures, 1 MK and higher, systematic loop velocities occur only occasionally. Simultaneous observations with EIT and SUMER were made during part of the raster series and are compared with the CDS result.

Velocities in Sunspot Plumes

We have extended our investigation of sunspot regions, with particular attention to the physics of the transition region between the chromosphere and the corona. In sunspot plumes, the most prominent features in the transition region intensity maps, we find downflows exceeding 25 km s−1. The observations show that the flow cannot be maintained by inflow from the corona and prompt a search for the origin of the flow and its possible relation to the origin of the sunspot plume. We find that the downflows in sunspot plumes are maintained by gas at transition region temperatures, streaming in flow channels from locations well outside the sunspot. Hence, the flow pattern in the transition region shows similarities to the inverse Evershed flow in the chromosphere. We present a working hypothesis that is based on the siphon flow mechanism.

Escape response of planktonic protists to fluid mechanical signals

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 214:67-78 (2001) - doi:10.3354/meps214067 Escape response of planktonic protists to fluid mechanical signals Hans Henrik Jakobsen* Danish Institute for Fisheries Research, Department of Marine and Coastal Ecology. Kavalergården 6, 2920 Charlottenlund, Denmark Marine Biological Laboratory, Strandpromenaden 5, 3000 Helsingør, Denmark *E-mail: hhj@dfu.min.dk ABSTRACT: The escape response to fluid mechanical signals was examined in 6 protists, 4 ciliates and 2 dinoflagellates. When exposed to a siphon flow, 3 species of ciliates, Balanion comatum, Strobilidium sp., and Mesodinium pulex, responded with escape jumps. The threshold deformation rates required to elicit an escape ranged between 1.8 and 3 s-1. Escape speeds varied between 100 to 150 body length s-1. Jump directions were non-random in all jumping species and had a negative geotactic component. In a grazing experiment with copepods, the predation mortality of a jumping ciliate was about 15 times lower than that of a non-jumping similar sized protist when the predator was Temora longicornis, which captures prey entrained in a feeding current. However, when the predator was the ambush-feeding copepod Acartia tonsa, the predation mortalities of jumping and non-jumping protists were of similar magnitude. Escape responses may thus be advantageous in some situations. However, jumping behaviour may also enhance susceptibility to some predators, explaining the different predator avoidance strategies (jumping or not) that have evolved in planktonic protists. KEY WORDS: Escape behaviour · Ciliates · Dinoflagellates · Jumping · Strobilidium · Balanion · Mesodinium · Acartia tonsa · Temora longicornis Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 214. Online publication date: April 26, 2001 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2001 Inter-Research.

On the reorientation of non-spherical prey particles in a feeding current

Potentially, non-spherical prey can be re-oriented in a flow field and impact on the predator's feeding structures in a non-random manner. Herein, we quantify a process whereby this passive reorientation occurs, and present a model that predicts the orientation of a spheroidal prey as a function of its shape, size and the characteristics of the fluid flow. For a radial flow field, elongated prey tend to align with their long axis parallel to streamlines. This theory is well supported by our results from a laboratory study of cylindrical particles in a siphon flow. The model is extended to a more realistic representation of copepod feeding currents. In this context, the spatial scale over which this process is active is proportional to « -1/4 where « is the turbulent dissipation rate. For a range of natural turbulence levels, re-orientation efficiency can range from >90% (low turbulence) to <10% (high turbulence).

Effects on UV lines observations of stationary plasma flows confined in coronal loops

We describe a method which uses a rather detailed model of coronal loop hosting a siphon flow as a diagnostic tool to interpret solar UV and X-ray observations in selected bands and lines. We apply the method to investigate the deviations from ionization equilibrium induced by stationary plasma flows confined in coronal loops and their effects on the UV and EUV emission lines observed by the instruments on board SOHO. We present results on the detailed synthesis of loop emission in a set of selected emission lines observed by CDS and SUMER, taking into account the non-equilibrium of ionization effects.

Soho Observations of the Structure and Dynamics of Sunspot Region Atmospheres

We present results from a study of the spatial distributions of line emission and relative line-of-sight velocity in the atmosphere above 17 sunspot regions, from the chromosphere, through the transition region and into the corona, based on simultaneous observations of ten EUV emission lines with the Coronal Diagnostic Spectrometer – CDS on SOHO. We find that the spatial distributions are nonuniform over the sunspot region and introduce the notation 'sunspot loop' to describe an enhanced transition region emission feature that looks like a magnetic loop, extending from inside the sunspot to the surrounding regions. We find little evidence for the siphon flow. Attention is given to the time variations since we observe both a rapid variation with a characteristic time of a few to several minutes and a slow variation with a time constant of several hours to ≈ 1 day. The most prominent features in the transition region intensity maps are the sunspot plumes. We introduce an updated criterion for the presence of plumes and find that 15 out of 17 sunspots contain a plume in the temperature range logT≈5.2–5.6. The relative line-of-sight velocity in sunspot plumes is high and directed into the Sun in the transition region. Almost all the sunspot regions contain one or a few prominent, strongly redshifted velocity channels, several of the channels extend from the sunspot plume to considerable distances from the sunspot. The flow appears to be maintained by plasmas at transition region temperatures, moving from regions located at a greater height outside the sunspots and towards the sunspot. The spatial correlation is high to moderate between emission lines formed in the transition region lines, but low between the transition region lines and the coronal lines. From detailed comparisons of intensity and velocity maps we find transition region emission features without any sign of coronal emission in the vicinity. A possible explanation is that the emission originates in magnetic flux tubes that are too cold to emit coronal emission. The comparisons suggest that gas at transition region temperature occur in loops different from loops with coronal temperature. However, we cannot exclude the presence of transition region temperatures close to the footpoints of flux tubes emitting at coronal temperatures. Regions with enhanced transition region line emission tend to be redshifted, but the correlation between line emission and relative line-of-sight velocity is weak. We extend our conditional probability studies and confirm that there is a tendency for line profiles with large intensities and red shifts (blue shifts) above the average to constitute an increasing (decreasing) fraction of the profiles as the wavelength shift increases.

On the Stability of Siphon Flows Confined in Coronal Loops

We use a time-dependent hydrodynamic model to study the dynamics of siphon flows triggered by differences of pressure or heat deposition asymmetries between the two footpoints of a coronal loop. We show that steady pressure driven flows in a uniformly heated loop are unstable, whereas those driven by asymmetries in the heating function may be stable. We also show that, in these cases, relatively cool loops might be filled far above their static pressure scale height.

On the stability of stationary flows in thin magnetic flux tubes

Based on the almost fibril state of magnetic flux in astrophysical bodies we present a study of the stability of magnetic flux tubes which are described by the thin flux tube approximation. A stationary flow is allowed in the tube which in equilibrium lies in a vertical plane, while the surroundings are assumed to be static. This complements earlier studies with external but no internal flows. The equations governing small adiabatic displacements are given in a canonical form which allows the proof that operators are Hermitian. This form is derived for the first time. The character of operators allows powerful conclusions about the eigensolutions, sufficient conditions for stability as well as the application of variational techniques. The velocity may potentially lead to the phenomenon of overstability. Special cases are discussed analytically. These cover the local approximation, binormal perturbations, static equilibria, and horizontal and vertical flux tubes. Stability of siphon flows along arch-shaped flux tubes describing the Evershed effect in penumbrae of sunspots is analyzed numerically. A standard model is found to be stable. Increasing the flow speed or decreasing the magnetic field strength however leads to monotonic instability. Further astrophysical applications are mentioned.

The Evershed Effect in Sunspots: A Theoretical Explanation

The siphon flow model, consisting in the simulation of a flow of gas moving along a thin magnetic flux tube and driven by the pressure drop between its footpoints, is proposed to explain the observational features of the Evershed effect, one of the longstanding problems in solar physics.