Horseshoe Structures Research Articles

N-terminal horseshoe conformation of DCC is functionally required for axon guidance and might be shared by other neural receptors

Deleted in colorectal cancer (DCC) is a receptor for the axon guidance cues netrin-1 and draxin. The interactions between these guidance cues and DCC play a key role in the development of the nervous system. In the present study, we reveal the crystal structure of the N-terminal four Ig-like domains of DCC. The molecule folds into a horseshoe-like configuration. We demonstrate that this horseshoe conformation of DCC is required for guidance-cue-mediated axonal attraction. Structure-based mutations that disrupt the DCC horseshoe indeed impair its function. A comparison of the DCC horseshoe with previously described horseshoe structures has revealed striking conserved structural features and important sequence signatures. Using these signatures, a genome-wide search allows us to predict the N-terminal horseshoe arrangement in a number of other cell surface receptors, nearly all of which function in the nervous system. The N-terminal horseshoe appears to be evolutionally selected as a platform for neural receptors.

Numerical Simulation of Coherent Structures over Plant Canopy

This paper reports large eddy simulations of the interaction between an atmospheric boundary layer and a canopy (representing a forest cover). The problem is studied for a homogeneous configuration representing the situation encountered above a continuous forest cover, as well as for a heterogeneous configuration representing the situation similar to an edge or a clearing in a forest. The numerical results reproduces correctly all the main characteristics of this flow as reported in the literature: the formation of a first generation of coherent structures aligned transversally with the wind flow direction, the reorganization and the deformation of these vortex tubes into horse-shoe structures. The results obtained when introducing a discontinuity in the canopy (reproducing a clearing or a fuel break in a forest), are compared with the experimental data collected in a wind tunnel; here, the results confirm the existence of a strong turbulence activity inside the canopy at a distance equal to 8 times the height of the canopy, referenced in the literature as the Enhance Gust Zone (EGZ) characterized by a local peak of the skewness factor.

Large‐eddy simulations of unidirectional water flow over dunes

The unidirectional, subcritical flow over fixed dunes is studied numerically using large‐eddy simulation, while the immersed boundary method is implemented to incorporate the bed geometry. Results are presented for a typical dune shape and two Reynolds numbers, Re = 17,500 and Re = 93,500, on the basis of bulk velocity and water depth. The numerical predictions of velocity statistics at the low Reynolds number are in very good agreement with available experimental data. A primary recirculation region develops downstream of the dune crest at both Reynolds numbers, while a secondary region develops at the toe of the dune crest only for the low Reynolds number. Downstream of the reattachment point, on the dune stoss, the turbulence intensity in the developing boundary layer is weaker than in comparable equilibrium boundary layers. Coherent vortical structures are identified using the fluctuating pressure field and the second invariant of the velocity gradient tensor. Vorticity is primarily generated at the dune crest in the form of spanwise “roller” structures. Roller structures dominate the flow dynamics near the crest, and are responsible for perturbing the boundary layer downstream of the reattachment point, which leads to the formation of “horseshoe” structures. Horseshoe structures dominate the near‐wall dynamics after the reattachment point, do not rise to the free surface, and are distorted by the shear layer of the next crest. The occasional interaction between roller and horseshoe structures generates tube‐like “kolk” structures, which rise to the free surface and persist for a long time before attenuating.

The Asymmetry of the Large-Scale Structures in Turbulent Three-Dimensional Wall Jets Exiting Long Rectangular Channels

The development of the large-scale structures in three-dimensional wall jets formed using long rectangular channels with aspect ratios of 1 and 4 was investigated using measurements of the fluctuating wall pressure and point measurements of the turbulent velocity throughout the near and intermediate field. The instantaneous pressure fluctuations in both jets were laterally asymmetric causing the fluctuating wall pressure to be poorly correlated across the jet centerline. A frequency-dependent proper orthogonal decomposition (POD) of the fluctuating pressure measurements indicated that the first two mode shapes were opposite and each mode made similar contributions to the mean square fluctuations at all frequencies in order to capture the instantaneous asymmetry of the pressure field. The mode shapes in the intermediate field of both jets were strongly frequency dependent, and a subsequent wavelet analysis indicated that there are both large-scale horseshoe structures that span one-half of the jet and separate, smaller, near-wall structures located near the jet centerline. The initial development of the large-scale structures in the two jets differed, with the most energetic fluctuations being more antisymmetric in the square jet.

Heat transfer and rivulet structures formation in a falling thin liquid film locally heated

An experimental investigation of the heat transfer from a local heat source to a liquid film falling down a vertical plate is performed. The thermocapillary counterflow, induced by non-uniform heating, causes a deformation of the film surface having a horizontal bump-shape. This shape becomes unstable above a critical value of the imposed heat flux and deforms into vertical downstream rivulets. This variation of patterns is expected to modify significantly the heat transfer through the film. Experiments are carried out at atmospheric pressure with three varying parameters: the streamwise heater length, the Reynolds number and the imposed heat flux density. Velocimetry, shadowgraphy and infrared thermography are used to study the behavior of the interface and the heat transfer. We put in evidence the presence of a thermocapillary counter flow producing a stagnation line at the upper edges of the horseshoe structures, beyond the instability threshold, and observe a decrease of the heat transfer with the Reynolds number.

The dynamics of a bouncing ball with a sinusoidally vibrating table revisited

The dynamical behavior of a bouncing ball with a sinusoidally vibrating table is revisited in this paper. Based on the equation of motion of the ball, the mapping for period-1 motion is constructured and thereby allowing the stability and bifurcation conditions to be determined. Comparison with Holmes's solution [1] shows that our range of stable motion is wider, and through numerical simulations, our stability result is observed to be more accurate. The Poincare mapping sections of the unstable period-1 motion indicate the existence of identical Smale horseshoe structures and fractals. For a better understanding of the stable and chaotic motions, plots of the physical motion of the bouncing ball superimposed on the vibration of the table are presented.

Multiple wavelength observations of flaring active regions

We present observations of flaring active regions with the Very Large Array (V.L.A. at 6 cm and 20 cm wavelengths) and the Westerbork Synthesis Radio Telescope (W.S.R.T. at 6 cm wavelength). These are compared with photospheric magnetograms (Meudon) and with Hα and offband Hα photographs (Big Bear and Ottawa River Solar Observatories). The 6 cm radiation of these active regions marks the legs of dipolar loops which have their footpoints in lower-lying sunspots. The intense, million degree radiation at 6 cm lies above sunspot umbrae in coronal regions where the longitudinal magnetic field strength H ℓ = 600 Gauss and the height above the sunspot umbrae h = 3.5±0.5 × 10 9 cm. Circularly polarized horseshoe structures at 6 cm ring the sunspot umbrae. The high degree of circular polarization ( ϱ c = 95%) of the horseshoes is attributed to gyroresonant emission above sunspot − penumbrae. The 20 cm radiation of these active regions exhibits looplike coronal structures which extend across regions of opposite magnetic polarity in the underlying photosphere. The 20 cm loops are the radio wavelength counterparts of the X-ray coronal loops. We infer semilengths L = 5 × 10 9 cm, maximum electron temperatures T e(max) = 3 × 10 6 K, emission measures ∫N e 2d l = 10 28 cm −5, and electron densities N e = 10 9 cm −3 (or pressures p = 1 dyn cm −2) for the 20 cm bremsstrahlung. A total of eight solar bursts were observed at 6 cm or 20 cm wavelength with second-of-arc angular resolution. The regions of burst energy were all resolved with angular sizes between 5″ and 30″, brightness temperatures between 2 × 10 7 K and 2 × 10 8 K, and degrees of circular polarization between 10% and 90%. The impulsive phase of the radio bursts are located near the magnetic neutral lines of the active regions, and between the flaring Hα kernels which mark the footpoints of magnetic loops. In one case there was preburst heating in the coronal loop in which a burst occurred. Snapshot maps at 10 s intervals reveal interesting burst evolution including rapid changes of circular polarization and an impulsive burst which was physically separated from both the preburst radio emission and the gradual decay phase of the burst.