The main objective of this work is the study and analysis of non-linearities forced through oscillating magnetic fields in a conducting fluid where the instabilities are triggered due to magnetohydrodynamic forces. Different geometries have been studied and different surface patterns that break the symmetries have been observed. First, an InGaSn drop of fluid where the system breaks the azimuthal and radial symmetries depending on the volume is observed. Second, we extend the study to an InGaSn annular configuration where the presence of patterns opens the door to discuss the possibility to extend these results to other configurations as biological systems, where the conducting fluid is an electrolyte. This configuration has an added interest, as it has been proposed that the vertigoes triggered on patients in an MRI test could be generated by the interaction of the magnetic field with the electrolyte present in the inner ear.

We evaluate the effect of the aspect ratio, i.e., the distance between the propellers H divided by the diameter D, on the slow dynamics of a von Karman swirling flow driven by two propellers in a closed cylinder. We use a cell with a fixed diameter D but where the distance between the propellers can be turned continuously and where the inertia from the propellers can also be changed using different gears. No change on the dynamics is observed when the momentum of inertia is modified. Some dramatic changes of the shear layer position are observed modifying the aspect ratio Gamma = H/D. A bifurcation of the shear layer position appears. Whereas for low Gamma the shear layer position has a smooth evolution when turning the asymmetry between the rotation frequency of the propellers, for high Gamma the transition becomes abrupt and a symmetry breaking appears. Secondly we observe that the spontaneous reversals with large residence times already observed in this experiment for Gamma = 1 [de la Torre and Burguete, Phys. Rev. Lett. 99, 054101 (2007)] exist only in a narrow window of aspect ratio. We show using an experimental study of the mean flow structure and a numerical approach based on a Langevin equation with colored noise that the shear layer position seems to be decided by the mean flow structure, whereas the reversals are linked to the spatial distribution of the turbulent fluctuations in the cell.

This study aimed to describe the commissioning of small field size radiosurgery cones in a 6-MV flattening filter free (FFF) beam and report our measured values. Four radiosurgery cones of diameters 5, 10, 12.5, and 15¿mm supplied by Elekta Medical were commissioned in a 6-MV FFF beam from an Elekta Versa linear accelerator. The extraction of a reference signal for measuring small fields in scanning mode is challenging. A transmission chamber was attached to the lower part of the collimators and used for percentage depth dose (PDD) and profile measurements in scanning mode with a stereotactic diode. Tissue-maximum ratios (TMR) and output factors (OF) for all collimators were measured with a stereotactic diode (IBA). TMR and the OF for the largest collimator were also acquired on a polystyrene phantom with a microionization chamber of 0.016¿cm3 volume (PTW Freiburg PinPoint 3D). Measured TMR with diode and PinPoint microionization chamber agreed very well with differences smaller than 1% for depths below 20¿cm, except for the smaller collimator, for which differences were always smaller than 2%. Calculated TMR were significantly different (up to 7%) from measured TMR. OF measured with diode and chamber showed a difference of 3.5%. The use of a transmission chamber allowed the measurement of the small-field dosimetric properties with a simple setup. The commissioning of radiosurgery cones in FFF beams has been performed with essentially the same procedures and recommended ...

We present the experimental analysis of the instabilities generated on a large drop of liquid metal by a time-dependent magnetic field. The study is done exploring the range of tiny values of the control parameter (the ratio between the Lorentz forces and inertia) avoiding nonlinear effects. Two different instabilities break the symmetries generating spatial patterns that appear without a threshold for some specific frequencies (up to the experimental precision) and have been observed for parameter values two orders of magnitude lower than in previously published experiments [J. Fluid Mech. 239, 383 (1992)]. One of the instabilities corresponds to a boundary condition oscillation that generates surface waves and breaks the azimuthal symmetry. The other corresponds to a parametric forcing through a modulation of the Lorentz force. The competition between these two mechanisms produces time-dependent patterns near codimension-2 points.

BACKGROUND: There are not reliable methods for measuring laryngo-pharyngeal mechano-sensitivity (LPMS). We aimed to determine the reliability of a new method for measuring LPMS using a new laryngo-pharyngeal esthesiometer (LPEER) in a prospective cohort of dysphagic stroke and non-dysphagic patients. The patients underwent clinical and endoscopic evaluations of swallowing (FESSST). The LPMS assessments consisted of measurements by an expert and a novel rater of the laryngeal-adductor reflex threshold (LART), cough reflex threshold (CRT) and gag reflex threshold (GRT) using the LPEER. We assessed the Bland-Altman limits of agreement, the intraclass correlation coefficients (ICCs) and Spearman correlation coefficients (SCCs). For the inter-rater comparisons, we contrasted the expert and novel raters. A total of 1608 measurements were obtained from 34 dysphagic stroke patients and 33 non-dysphagic patients. The intra-rater ICCs for all reflex thresholds were >0.90. The inter-rater ICCs were 0.87 for the LART, 0.79 for the CRT and 0.70 for the GRT. The intra-rater SCCs for all reflex thresholds were above 0.88 (P¿<¿0.0001). The inter-rater SCC were 0.80 for the LART, 0.79 for the CRT and 0.70 for the GRT (all P¿<¿0.0001). The Bland-Altman plots revealed good agreement for the LART and CRT and moderate agreement for the GRT. The median normal value was 0.14 mN for the LART, 4.4 mN for the CRT and 11.9 mN for the GRT. The median thresholds values in patients with aspiration were LART: 1.31 mN; CRT: 32.9 mN and GRT: 32.9 mN (all P¿<¿0.006 vs normal thresholds). The LPEER exhibited substantial to excellent intra- and inter-rater reliability.

BACKGROUND: Laryngo-pharyngeal mechano-sensitivity (LPMS) is involved in dysphagia, sleep apnea, stroke, irritable larynx syndrome and cough hypersensitivity syndrome among other disorders. These conditions are associated with a wide range of airway reflex abnormalities. However, the current device for exploring LPMS is limited because it assesses only the laryngeal adductor reflex during fiber-optic endoscopic evaluations of swallowing and requires a high degree of expertise to obtain reliable results, introducing intrinsic expert variability and subjectivity. METHODS: We designed, developed and validated a new air-pulse laryngo-pharyngeal endoscopic esthesiometer with a built-in laser range-finder (LPEER) based on the evaluation and control of air-pulse variability determinants and on intrinsic observer variability and subjectivity determinants of the distance, angle and site of stimulus impact. The LPEER was designed to be capable of delivering precise and accurate stimuli with a wide range of intensities that can explore most laryngo-pharyngeal reflexes. RESULTS: We initially explored the potential factors affecting the reliability of LPMS tests and included these factors in a multiple linear regression model. The following factors significantly affected the precision and accuracy of the test (P < 0.001): the tube conducting the air-pulses, the supply pressure of the system, the duration of the air-pulses, and the distance and angle between the end of the tube conducti

The long time dynamics of large particles trapped in two inhomogeneous turbulent shear flows is studied experimentally. Both flows present a common feature, a shear region that separates two colliding circulations, but with different spatial symmetries and temporal behaviors. Because large particles are less and less sensitive to flow fluctuations as their size increases, we observe the emergence of a slow dynamics corresponding to back-and-forth motions between two attractors, and a super-slow regime synchronized with flow reversals when they exist. Such dynamics is substantially reproduced by a one dimensional stochastic model of an over-damped particle trapped in a two-well potential, forced by a colored noise. An extended model is also proposed that reproduces observed dynamics and trapping without potential barrier: the key ingredient is the ratio between the time scales of the noise correlation and the particle dynamics. A total agreement with experiments requires the introduction of spatially inhomogeneous fluctuations and a suited confinement strength.

We compare the dynamics obtained in two intermediate aspect ratio (diameter over height) experiments. These systems have rotational symmetry and consist of fluid layers that are destabilized using two different methods. The first one is a classical Bénard Marangoni experiment, where the destabilizing forces, buoyancy and surface tension, are created by temperature gradients. The second system consists of a large drop of liquid metal destabilized using oscillating magnetic fields. In this configuration, the instability is generated by a radial Lorentz force acting on the conducting fluid. Although there are many important differences between the two configurations, the dynamics are quite similar: the patterns break the rotational symmetry, and different azimuthal and radial wavenumbers appear depending on the experimental control parameters. These patterns in most cases are stationary, but for some parameters they exhibit different dynamical behaviours: rotations, transitions between different solutions or cyclic connections between different patterns

This paper presents a method to obtain the pencil-beam kernels that characterize a megavoltage photon beam generated in a flattening filter free (FFF) linear accelerator (linac) by deconvolution from experimental measurements at different depths. The formalism is applied to perform independent dose calculations in modulated fields. In our previous work a formalism was developed for ideal flat fluences exiting the linac's head. That framework could not deal with spatially varying energy fluences, so any deviation from the ideal flat fluence was treated as a perturbation. The present work addresses the necessity of implementing an exact analysis where any spatially varying fluence can be used such as those encountered in FFF beams. A major improvement introduced here is to handle the actual fluence in the deconvolution procedure. We studied the uncertainties associated to the kernel derivation with this method. Several Kodak EDR2 radiographic films were irradiated with a 10 MV FFF photon beam from two linacs from different vendors, at the depths of 5, 10, 15, and 20cm in polystyrene (RW3 water-equivalent phantom, PTW Freiburg, Germany). The irradiation field was a 50mm diameter circular field, collimated with a lead block. The 3D kernel for a FFF beam was obtained by deconvolution using the Hankel transform. A correction on the low dose part of the kernel was performed to reproduce accurately the experimental output factors. Error uncertainty in the kernel derivation procedure was estimated to be within 0.2%. Eighteen modulated fields used clinically in different treatment localizations were irradiated at four measurement depths (total of fifty-four film measurements). Comparison through the gamma-index to their corresponding calculated absolute dose distributions showed a number of passing points (3%, 3mm) mostly above 99%. This new procedure is more reliable and robust than the previous one. Its ability to perform accurate independent dose calculations was demonstrated.

In this work we review localized structures appearing in thermo-convective experiments performed in extended (large "aspect ratio") fluid layers. After a brief general review (not exhaustive), we focus on some results obtained in pure fluids in a Benard-Marangoni system with non-homogeneous heating where some structures of this kind appear. The experimental results are compared in reference to the most classical observed in binary mixtures experiments or simulations. In the Benard-Marangoni experiment we present the stability diagram where localized structures appear and the typical situations where these local mechanisms have been studied experimentally. Some new experimental results are also included. The authors want to honor Prof. H. Brand in his 60th. birthday and to thank him for helpful discussions.

We show evidence of the frozen dynamics (Kibble-Zurek mechanism) at the transition one-dimensional (1D) front of an extended 1D array of convective oscillators that undergo a secondary subcritical bifurcation. Results correspond to a global synchronization process from nonlocal coupling between the oscillating units. The quenched dynamics exhibits defect trapping at the synchronization front according to the Kibble-Zurek mechanism, predicted for condensed matter systems. A stronger subcriticality prevents the fronts from freezing defects during the quenched transitions. A synchronization model of supercritical oscillating units is proposed to explain differentiation mechanisms in morphogenesis above a critical crossing rate when the frequency of the individual oscillators becomes coherent. The phases of such oscillators are spatially coupled through a Kuramoto-Battogtokh term that leads to the experimentally observed subcriticality. As a consequence, we show that the Kibble-Zurek mechanism overcomes non-locality of a geometrical network above a critical crossing rate.

The existence of energy cascades as signatures of conserved magnitudes is one of the universal characteristics of turbulent flows. In homogeneous 3D turbulence, the energy conservation produces a direct cascade from large to small scales, although in 2D, it produces an inverse cascade pointing towards small wave numbers. In this Letter, we present the first evidence of an inverse cascade in a fully developed 3D experimental turbulent flow where the conserved magnitude is the angular momentum. Two counter-rotating flows collide in a central region where very large fluctuations are produced, generating a turbulent drag that transfers the external torque between different fluid layers.

We present new experimental results on the quenching dynamics of an extended thermo-convective system (a network array of approximately 100 convective oscillators) going through a secondary subcritical bifurcation. We characterize a dynamical phase transition through the nature of the domain walls (1D-fronts) that connect the basic multicellular pattern with the new oscillating one. Two different mechanisms of the relaxing dynamics at the threshold are characterized depending on the crossing rate mu = d epsilon/dt vertical bar(epsilon=0) of the quenched transition. From the analysis of fronts, we show that these mechanisms follow different correlation length scales xi similar to mu(-sigma). Below a critical value mu(c), a slow response dynamics yields a spatiotemporal coherent front with weak coupling between oscillators. Above mu(c), for rapid quenches, defects are trapped at the front with a strong coupling between oscillators, similarly to the Kibble-Zurek mechanism in quenched phase transitions. These defects, pinned to the fronts, yield a strong decay of the correlation length.

We present numerical simulations of the kinematic induction equation in order to examine the dynamo efficiency of an axisymmetric von Karman-like flow subject to time-dependent nonaxisymmetric velocity perturbations. The numerical model is based on the setup of the French von Karman-sodium dynamo (VKS) and on the flow measurements from a water experiment conducted at the University of Navarra in Pamplona, Spain. The principal experimental observations that are modeled in our simulations are nonaxisymmetric vortexlike structures which perform an azimuthal drift motion in the equatorial plane. Our simulations show that the interactions of these periodic flow perturbations with the fundamental drift of the magnetic eigenmode (including the special case of nondrifting fields) essentially determine the temporal behavior of the dynamo state. We find two distinct regimes of dynamo action that depend on the (prescribed) drift frequency of an (m=2) vortexlike flow perturbation. For comparatively slowly drifting vortices we observe a narrow window with enhanced growth rates and a drift of the magnetic eigenmode that is synchronized with the perturbation drift. The resonance-like enhancement of the growth rates takes place when the vortex drift frequency roughly equals the drift frequency of the magnetic eigenmode in the unperturbed system. Outside of this small window, the field generation is hampered compared to the unperturbed case, and the field amplitude of the magnetic eigenmode is modulated with approximately twice the vortex drift frequency. The abrupt transition between the resonant regime and the modulated regime is identified as a spectral exceptional point where eigenvalues (growth rates and frequencies) and eigenfunctions of two previously independent modes collapse. In the actual configuration the drift frequencies of the velocity perturbations that are observed in the water experiment are much larger than the fundamental drift frequency of the magnetic eigenmode that is obtained from our numerical simulations. Hence, we conclude that the fulfillment of the resonance condition might be unlikely in present day dynamo experiments. However, a possibility to increase the dynamo efficiency in the VKS experiment might be realized by an application of holes or fingers on the outer boundary in the equatorial plane. These mechanical distortions provoke an anchorage of the vortices at fixed positions thus allowing an adjustment of the temporal behavior of the nonaxisymmetric flow perturbations.

In this work, we analyze recent results concerning the instabilities created in a layer of liquid metal by the action of time-dependent magnetic fields. The experimental setup allows the characterization of different patterns very close to the threshold. For very low frequencies of the forcing field, the axisymmetric fluid layer destabilizes with different azimuthal wavenumbers. An improved analysis allows the characterization of different patterns for interaction parameter values as low a

Purpose: This article presents an improved pencil-beam dose calculation formalism based on an experimental kernel obtained by deconvolution. The new algorithm makes it possible to calculate the absorbed dose for all field sizes. Methods: The authors have enhanced their previous work [J. D. Azcona and J. Burguete, Med. Phys. 35, 248-259 (2008)] by correcting the kernel tail representing the contribution to the absorbed dose far from the photon interaction point. The correction was performed by comparing the calculated and measured output factors. Dose distributions and absolute dose values calculated using the new formalism have been compared to measurements. The agreement between calculated and measured dose distributions was evaluated according to the gamma-index criteria. In addition, 35 individual intensity-modulated radiation therapy (IMRT) fields were calculated and measured in polystyrene using an ionization chamber. Furthermore, a series of 541 IMRT fields was calculated using the algorithm proposed here and using a commercial IMRT optimization and calculation software package. Comparisons were made between the calculations at single points located at the isocenter for all the beams, as well as between beams grouped by anatomic location. Results: The percentage of points passing the gamma-index criteria (3%, 3 mm) when comparing calculated and measured dose distributions is generally greater than 99% for the cases studied. The agreement between the calculations and the experimental measurements generally lies in the +/- 2% interval for single points, with a mean value of 0.2%. The agreement between calculations using the proposed algorithm and using a commercial treatment planning system is also between +/- 5%. Conclusions: An improved algorithm based on an experimental pencil-beam kernel obtained by deconvolution has been developed. It has been validated clinically and promises to be a valuable tool for IMRT quality assurance as an independent calculation system for monitor units and dose distributions. An important point is that the algorithm presented here uses an experimental kernel, which is therefore independent of Monte-Carlo-calculated kernels.

In a quasi-1D thermal convective system consisting of a large array of nonlinearly coupled oscillators, clustering is the way to achieve a regime of mostly antiphase synchronized oscillators. This regime is characterized by a spatiotemporal doubling of traveling modes. As the dynamics is explored beyond a spatiotemporal chaos regime (STC) with weak coupling, new interacting modes emerge through a supercritical bifurcation. In this new regime, the system exhibits coherent subsystems of antiphase synchronized oscillators, which are stationary clusters following a spatiotemporal beating phenomena (ZZ regime). This regime is the result of a stronger coupling. We show from a phase mismatch model applied to each oscillator, that these phase coherent domains undergo a global phase instability, meanwhile the interactions between oscillators become nonlocal. For each value of the control parameter we find out the time-varying topology (link matrix) from the contact interactions between oscillators. The new characteristic spatiotemporal scales are extracted from the antiphase correlations at the time intervals defined by the link matrix. The interpretation of these experimental results contributes to widen the understanding of other complex systems exhibiting similar phase chaotic dynamics in 2D and 3D