Velocity Variables Research Articles

A Polarimetric Radar Operator and Application for Convective Storm Simulation

In this study, a polarimetric radar forward model operator was developed for the Weather Research and Forecasting (WRF) model that was based on a scattering algorithm using the T-matrix methodology. Three microphysics schemes—Thompson, Morrison 2-moment, and Milbrandt-Yau 2-moment—were supported in the operator. This radar forward operator used the microphysics, thermodynamic, and wind fields from WRF model forecasts to compute horizontal reflectivity, radial velocity, and polarimetric variables including differential reflectivity (ZDR) and specific differential phase (KDP) for S-band radar. A case study with severe convective storms was used to examine the accuracy of the radar operator. Output from the radar operator was compared to real radar observations from the Weather Surveillance Radar–1988 Doppler (WSR-88D) radar. The results showed that the radar forward operator generated realistic polarimetric signatures. The distribution of polarimetric variables agreed well with the hydrometer properties produced by different microphysics schemes. Similar to the observed polarimetric signatures, radar operator output showed ZDR and KDP columns from low-to-mid troposphere, reflecting the large amount of rain within strong updrafts. The Thompson scheme produced a better simulation for the hail storm with a ZDR hole to indicate the existence of graupel in the low troposphere.

Discovery and analysis of three magnetic hot subdwarf stars: evidence for merger-induced magnetic fields

ABSTRACT Magnetic fields can play an important role in stellar evolution. Among white dwarfs, the most common stellar remnant, the fraction of magnetic systems is more than 20 per cent. The origin of magnetic fields in white dwarfs, which show strengths ranging from 40 kG to hundreds of MG, is still a topic of debate. In contrast, only one magnetic hot subdwarf star has been identified out of thousands of known systems. Hot subdwarfs are formed from binary interaction, a process often associated with the generation of magnetic fields, and will evolve to become white dwarfs, which makes the lack of detected magnetic hot subdwarfs a puzzling phenomenon. Here we report the discovery of three new magnetic hot subdwarfs with field strengths in the range 300–500 kG. Like the only previously known system, they are all helium-rich O-type stars (He-sdOs). We analysed multiple archival spectra of the three systems and derived their stellar properties. We find that they all lack radial velocity variability, suggesting formation via a merger channel. However, we derive higher than typical hydrogen abundances for their spectral type, which are in disagreement with current model predictions. Our findings suggest a lower limit to the magnetic fraction of hot subdwarfs of $0.147^{+0.143}_{-0.047}$ per cent, and provide evidence for merger-induced magnetic fields which could explain white dwarfs with field strengths of 50–150 MG, assuming magnetic flux conservation.

Shear induced fluid flow path evolution in rough-wall fractures: A particle image velocimetry examination

Rough-walled fractures in rock masses, as preferential pathways, largely influence fluid flow, solute and energy transport. Previous studies indicate that fracture aperture fields could be significantly modified due to shear displacement along fractures. We report experimental observations and quantitative analyses of flow path evolution within a single fracture, induced by shear displacement. Particle image velocimetry and refractive index matching techniques were utilized to determine fluid velocity fields inside a transparent 3D-printed shear-able rough fracture. Our analysis indicate that aperture variability and correlation length increase with the increasing shear displacement, and they are the two key parameters, which govern the increases in velocity variability, velocity longitudinal correlation length, streamline tortuosity, and variability of streamline spacing. The increase in aperture heterogeneity significantly impacts fluid flow behaviors, whilst changes in aperture correlation length further refine these impacts. To our best knowledge, our study is the first direct measurements of fluid velocity fields and provides insights into the impact of fracture shear on flow behavior.

Influence of Wall Compliance on the Flow Patterns in a Patient-Specific Brachio-Cephalic Arterio-Venous Fistula

Patients with end stage renal disease require some form of vascular access for treatment, with Arterio-Venous Fistulas (avf) being the preferred form available due to better patency rates. However, they continue to present complications after creation, leading to early or late failure. While many studies are examining the flow in patient-specific fistulas, they often neglect the influence of vessel compliance on its hemodynamics. The objective of this study is to investigate the effect of wall compliance on the complex hemodynamics of a patient-specific brachio-cephalic avf and how it differs from a rigid fistula. Particle Image Velocimetry (piv) was used to capture the flow pattern within the fistula for both steady (Re = 1817) and pulsatile (Reav=1817, Remax=2232) flow conditions. The results were compared to rigid model measurements performed under the same Reynolds number. The streamline plots and coefficient of variation results did not differ significantly between the models; however, the non-dimensional velocity and directional variability results did vary between the two fistulas. A difference of approximately 8% was seen between the two models for both steady and pulsatile flow. The findings of this study suggest that to determine the bulk flow, a rigid model is adequate, but to capture the finer details of the flow, a compliant model is necessary.

Intraseasonal, Annual, and Interannual Variabilities of Subsurface Currents at 4.7°N in the Western Pacific Ocean

AbstractStrong multi‐timescale variabilities of subsurface zonal velocity over 300–600 m depth are observed beneath the North Equatorial Countercurrent (NECC) by the mooring observation at 4.7°N, 140°E during 2014–2018. The observed subsurface velocity shows peaks at intraseasonal, semiannual, and annual periods, and is interconnected with the El Niño‐Southern Oscillation (ENSO) cycle at a nearly zero lag. Aided by a reanalysis product, we reveal mechanisms for the above variabilities. Forced by wind stress curl over 150°–155°E and 160°–170°E along 4.7°N, the vertical propagation of the third meridional mode long Rossby waves is primarily responsible for the observed semiannual and annual cycles, respectively. The subsurface intraseasonal variability with peak periods of 50‐ and 80‐day are induced by short Rossby waves with the southeastward and downward group velocity. Such waves interconnect observed 50‐ and 80‐day ISV of the subsurface current to that of surface currents at 8°N, 128°–130°E and 6.5°N, 132°–134°E, respectively. The ENSO variability results mainly from the interannual change of the winds that force the long Rossby waves. The wind forcing leads the Niño 3.4 index and the subsurface velocity by a similar time, resulting in the subsurface instantaneous response to ENSO.

Overcoming temporal dispersion for measurement of activity-related impedance changes in unmyelinated nerves

Objective. Fast neural electrical impedance tomography is an imaging technique that has been successful in visualising electrically evoked activity of myelinated fibres in peripheral nerves by measurement of the impedance changes (dZ) accompanying excitation. However, imaging of unmyelinated fibres is challenging due to temporal dispersion (TP) which occurs due to variability in conduction velocities of the fibres and leads to a decrease of the signal below the noise with distance from the stimulus. To overcome TP and allow electrical impedance tomography imaging in unmyelinated nerves, a new experimental and signal processing paradigm is required allowing dZ measurement further from the site of stimulation than compound neural activity is visible. The development of such a paradigm was the main objective of this study. Approach. A finite element-based statistical model of TP in porcine subdiaphragmatic nerve was developed and experimentally validated ex-vivo. Two paradigms for nerve stimulation and processing of the resulting data—continuous stimulation and trains of stimuli, were implemented; the optimal paradigm for recording dispersed dZ in unmyelinated nerves was determined. Main results. While continuous stimulation and coherent spikes averaging led to higher signal-to-noise ratios (SNRs) at close distances from the stimulus, stimulation by trains was more consistent across distances and allowed dZ measurement at up to 15 cm from the stimulus (SNR = 1.8 ± 0.8) if averaged for 30 min. Significance. The study develops a method that for the first time allows measurement of dZ in unmyelinated nerves in simulation and experiment, at the distances where compound action potentials are fully dispersed.

Transcranial Doppler ultrasonography detects the elevation of cerebral blood flow during ictal-phase of pentetrazol-induced seizures in dogs.

To investigate the association between changes in cerebral blood flow and electrographic epileptic seizure in dogs using transcranial Doppler ultrasonography (TCD). 6 healthy Beagle dogs. Each dog was administered pentetrazol (1.5 mg/kg/min) or saline (0.9% NaCl) solution under general anesthesia with continuous infusion of propofol. Both pentetrazol and saline solution were administered to all 6 dogs, with at least 28 days interval between the experiments. Blood flow waveforms in the middle cerebral artery and the basilar artery were obtained using TCD at baseline, after pentetrazol administration, and after diazepam administration. TCD velocities, including peak systolic velocity, end-diastolic velocity, and mean velocity and resistance variables, were determined from the Doppler waveforms. During ictal-phase of pentetrazol-induced seizures, the TCD velocities significantly increased in the basilar and middle cerebral arteries while TCD vascular resistance variables did not change in either artery. The TCD velocities significantly decreased after diazepam administration. Systemic parameters, such as the heart rate, mean arterial pressure, systemic vascular resistance, cardiac index, end-tidal carbon dioxide, oxygen saturation, and body temperature, did not change significantly during seizures. This study showed that cerebral blood flow, as obtained from TCD velocities, increased by 130% during ictal-phase of pentetrazol-induced seizures in dogs. The elevated velocities returned to baseline after seizure suppression. Thus, TCD may be used to detect electrographic seizures during the treatment of status epilepticus in dogs, and further clinical studies clarifying the association between changes in cerebral blood flow and non-convulsive seizure cases are needed.

Some Regularity Properties on Bolza problems in the Calculus of Variations

The paper summarizes the main core of the last results that we obtained in [8, 4, 17] on the regularity of the value function for a Bolza problem of a one-dimensional, vectorial problem of the calculus of variations. We are concerned with a nonautonomous Lagrangian that is possibly highly discontinuous in the state and velocity variables, nonconvex in the velocity variable and non coercive. The main results are achieved under the assumption that the Lagrangian is convex on the one-dimensional lines of the velocity variable and satisfies a local Lipschitz continuity condition w.r.t. the time variable, known in the literature as Property (S), and strictly related to the validity of the Erdmann–Du-Bois Reymond equation.

Pengaruh Kecepatan Udara Primer Dan Sekunder Terhadap Kenerja Kompor Biomassa Berbahan Bakar Cangkang Kemiri

Household energy need in Indonesia, especially for cooking, are still dominated by LPG fossil fuels with 70% of LPG needs being imported. On the other hand, the availability of biomass waste, especially candlenut shells, is abundant. One way to optimize the use of candlenut shells is to use candlenut shells as fuel for biomass stoves by optimizing the efficiency of biomass stoves. In this study, a biomass stove with primary and secondary airflow was used with a combustion chamber diameter of 200 mm and a height of 340 mm. The biomass waste used is candlenut shells as fuel. The purpose of this study was to explain the increase in the performance of a biomass stove fueled with candlenut shell which is influenced by primary and secondary air velocity. Biomass stove testing includes thermal efficiency, specific fuel consumption, CO and CO2 emissions. The performance test results show that the biomass stove with a primary air velocity of 3.7 m/s, a secondary air velocity of 3.7 m/s has the highest thermal efficiency of 45.77%. The lowest specific fuel consumption is obtained from a biomass stove with a primary air velocity of 1.5 m/s, a secondary air velocity of 1.5 m/s, which is 0.583 kg/hour, and the emission test results show that complete combustion occurs with 0% CO emotion. on all air velocity variables studied.

Heat Pump Drying of Kelp (Laminaria japonica): Drying Kinetics and Thermodynamic Properties

The main objective of this research is the study of the drying kinetics and thermodynamic properties of kelp using heat pump drying technology. The effects of the independent variables of temperature (20–50 °C), air velocity (0.3–1.3 m/s), humidity (20–50%), and thickness (0.8–4.2 mm) on the drying time, moisture uniformity, effective moisture diffusivity (Deff), activation energy (Ea), enthalpy (ΔH), entropy (ΔS), and Gibbs free energy (ΔG) were investigated. The results show that the Page model was effective in describing the moisture content change of kelp during heat pump drying. The Deff varied from 1.00 × 10−11 to 13.00 × 10−11 m2/s and the temperature, air velocity, humidity, and thickness had significant effects on drying time and moisture uniformity. Higher temperature and air velocity with proper humidity shortened the drying time and lessened the influence of thickness on moisture uniformity. The Ea (16.38–26.66 kJ/mol) and ΔH (13.69–24.22 kJ/mol) were significantly increased by thickness. When the temperature was 40 °C, air velocity 1.3 m/s, and air humidity 40%, the moisture content was reduced to 18% in 5 h, with a homogeneous moisture content. This study clarifies the regularity of moisture change inside kelp and provides a theoretical reference for the development of macroalgae drying technology.

The results of physical modeling of fires for road tunnels

The paper gives the results of a fire development study using physical models of inclined vehicular traffic tunnels scaled 1:60. The tunnel inclination varied within 0-10% with a 2% increment. The fire strength initiated and developed in a natural tunnel varied within 5-15 MW with a 5 MW increment. The length of the natural tunnel was 360 m, width: 8 m, height: 6 m, cross-sectional area: 48 m2, and the tunnel width and height ratio: 1.33. The model of the natural tunnel of the given geometry was made of a 2-mm-thick stainless steel sheet with an appropriate scale. The sizes of the tunnel model are: length: 6 m, width: 0.16 m, height: 0.12 m, and cross-sectional area: 0.0192 m2. By doing experiments on this model, we studied the nature of propagation of gases (smoke) emitted during the fire initiation and development, specified the impact of the gradient factor on the rate of variability of the critical velocity and backlayering distance. The said characteristics were studied according to the air temperature variability in the tunnel model. The air temperature was measured with K-type thermocouples equipped with open and closed detectors. Maximum measuring temperature was 800°C. The thermocouples were installed in the ceiling of the tunnel model along the entire length, with 5 and 10 cm increments. The fire was modeled with natural gas. Ventilation air was supplied into the model by an axial fan from one portal of the model. The velocity of the ventilation air was measured with an anemometer, and the air discharge was calculated at every moment according to the velocity. The required strength of the modeled fire was provided by natural gas, and the measurement was done with a volumetric and mass regulatory meter. Simultaneous data collection, processing, analysis and digital transmission of data from the K-type thermocouples, anemometers and natural gas flow meters were provided with DT-85 Datataker. The obtained results can be used to develop road tunnel ventilation projects that take into account the impact of fires on ventilation.

Sinking velocity of small particles in the Black Sea: Vertical distribution and seasonal variability from continuous Bio-Argo measurements of backscattering

High-resolution Bio-Argo measurements of backscattering ( bbp ) were used to study the vertical distribution and time variability of the sinking of small, highly reflective particles in the Black Sea in the 0–200 m layer. The evolution of particles is divided into three stages: formation, sinking, and removal. The formation of small, highly reflective particles is mainly related to early summer and early winter coccolithophore blooms and the formation of suspended oxidized manganese in the suboxic zone. The removal rate of the particles was estimated from the vertical distribution of bbp in relation to its values at the layers of particle formation. Further, contour analysis of the time-depth diagram of bbp anomalies was used to automatically estimate the vertical distribution and seasonal and interannual variability of sinking velocity ( w ). The average value of w is ~0.8 m/day, which agrees with laboratory estimates of coccolith sinking ( Fritz and Balch, 1996 ). Physical processes significantly affect w : mixed layer deepening in late autumn-early winter increase w up to 1–1.5 m/day; warming and a decrease in density of upper layers promote coccolithophores deepening in summer ( w = 0.9–1.1 m/day). Minimum sinking velocity is observed in the warm period of a year in the strongly stratified layers of the main pycnocline at 40–80 m. On interannual time scales, we observed an increase in w during the strongest coccolithophore blooms, which is probably related to the enhanced particle aggregation. Sinking particles after winter coccolithophore blooms are traced up to the suboxic zone, where it probably affects the settling of manganese-containing particles and related biochemical processes. • Temporal and vertical variability of sinking velocity (w) is estimated using Bio-Argo bbp data. • Mixed layer deepening, stratification, and warming affect variations in w. • The estimated average sinking velocity of coccoliths is ~0.8 m/day. • Winter particles formed in the upper layers affect particles dynamics in the suboxic zone. • On interannual time scales w increases during strongest coccolithophore blooms.

Machine-learning identification of the variability of mean velocity and turbulence intensity for wakes generated by onshore wind turbines: Cluster analysis of wind LiDAR measurements

Light detection and ranging (LiDAR) measurements of isolated wakes generated by wind turbines installed at an onshore wind farm are leveraged to characterize the variability of the wake mean velocity and turbulence intensity during typical operations, which encompass a breadth of atmospheric stability regimes and rotor thrust coefficients. The LiDAR measurements are clustered through the k-means algorithm, which enables identifying the most representative realizations of wind turbine wakes while avoiding the imposition of thresholds for the various wind and turbine parameters. Considering the large number of LiDAR samples collected to probe the wake velocity field, the dimensionality of the experimental dataset is reduced by projecting the LiDAR data on an intelligently truncated basis obtained with the proper orthogonal decomposition (POD). The coefficients of only five physics-informed POD modes are then injected in the k-means algorithm for clustering the LiDAR dataset. The analysis of the clustered LiDAR data and the associated supervisory control and data acquisition and meteorological data enables the study of the variability of the wake velocity deficit, wake extent, and wake-added turbulence intensity for different thrust coefficients of the turbine rotor and regimes of atmospheric stability. Furthermore, the cluster analysis of the LiDAR data allows for the identification of systematic off-design operations with a certain yaw misalignment of the turbine rotor with the mean wind direction.

Relationships between highly skilled golfers’ clubhead velocity and kinetic variables during a countermovement jump

ABSTRACT Previous research has sought to establish the relationship countermovement jump (CMJ) performance has with clubhead velocity (CHV). However, these investigations either assessed lower skilled golfers, or utilised field-based protocols, which are unable to assess a number of biomechanical variables. Fifty highly skilled golfers performed CMJs on Kistler force platforms in laboratory conditions. The CMJ variables included positive impulse, net impulse, average power, peak power, peak force, force at zero velocity and jump height. Clubhead velocity was measured using a TrackMan 3e launch monitor at a driving range. A Pearsons correlation was employed to measure the strength and direction of the relationships between CHV and CMJ derived performance variables. Results indicated strong positive relationships (all p’s <0.001) between CHV and positive impulse (r = 0.695), net impulse (r = 0.689), average power (r = 0.645), peak power (r = 0.656), peak force (r = 0.517) and force at zero velocity (r = 0.528) with no significant relationship with jump height. However, if investigators only have access to field-based protocols, it is recommended that they measure jump height and utilise inverse dynamics to calculate take-off velocity. By multiplying take-off velocity by mass, this allows the attainment of net impulse.

Tidal dynamics in palaeo‐seas in response to changes in physiography, tidal forcing and bed shear stress

AbstractSimulating hydrodynamic conditions in palaeo‐ocean basins is needed to better understand the effects of tidal forcing on the sedimentary record. When combined with sedimentary analyses, hydrodynamic modelling can help inform complex temporal and spatial variability in the sediment distribution of tide‐dominated palaeo‐ocean basins. Herein, palaeotidal modelling of the epicontinental Upper Jurassic (160 Ma, lower Oxfordian) Sundance and Curtis seas of North America reveals possible regional‐scale variations in tidal dynamics in response to changes in ocean tidal forcing, physiographic configuration and bottom drag coefficient. A numerical model forced with an M2 tidal constituent at the open boundary shows that the magnitude and location of tidal amplification, and the variability in current velocity and bed shear stress in the basin, were controlled by palaeophysiography. Numerical results obtained using a depth of 600 m at the ocean boundary of the system enable the prediction of major facies trends observed in the lower Curtis Formation. The simulation results also highlight that certain palaeophysiographic configurations can either permit or prevent tidal resonance, leading to an overall amplification or dampening of tides across the basin. Furthermore, some palaeophysiographic configurations generated additional tidal harmonics in specific parts of the basins. Consequently, similar sedimentary successions can emerge from a variety of relative sea‐level scenarios, and a variety of sedimentary successions may be deposited in different parts of the basin in any given relative sea‐level scenario. These results suggest that the interpretation of sedimentary successions deposited in strongly tide‐influenced basins should consider changes in tidal dynamics in response to changing sea level and basin physiography.

Postural Stability in Single-Leg Quiet Stance in Highly Trained Athletes: Sex and Sport Differences.

This study aimed to determine if there is a difference in postural stability in highly trained adolescents and young adult athletes regarding sex and sport. The participants were young athletes (n = 464) from seven different sports. We considered the center of pressure (CoP) velocity (total, anterior–posterior (AP) and medial–lateral (ML)), CoP amplitude (AP and ML), and CoP frequency (AP and ML), as assessed by single-leg quiet stance test. Significant interactions were found between sex and sport for all CoP variables (p < 0.02). Additionally, a significant main effect of sport was also found in all CoP variables (p = 0.01). Regarding sex, significant effects were found for all CoP amplitude variables (p = 0.01), as well as for CoP velocity variables, except for CoP ML (p = 0.06). Moreover, there was no sex effect for CoP frequency AP (p = 0.18). The results of the current study confirm the claim that the criteria for optimal postural strategies for elite athletes likely depend on a given sport.

Investigation of radiation and MHD on non-Newtonian fluid flow over a stretching/shrinking sheet with CNTs and mass transpiration

The inclined magnetohydrodynamics and heat transfer on non-Newtonian fluid flows through a stretching/shrinking sheet is investigated the present analysis. The conservation of mass, momentum and energy equations are non-dimensionalised into the set of highly nonlinear ODEs by using similarity transformation. An analytical expression is obtained for velocity. The consequence of several flow physical constraints on the fluid velocity variable, the skin friction coefficient, the temperature distribution, and the Nusselt number, including nanoparticle volume fraction constraint are discussed. Laplace transform is utilized to explain the heat transfer equation with radiation and get the analytical result in expressions of incomplete gamma function. The differences in velocity and temperature profiles with respect to several relevant physical parameters are investigated.

Robust Composite Dynamic Event-Triggered Control for Multiple USVs with DLLOS Guidance

In this paper, a robust composite dynamic event-triggered formation control scheme is proposed for multiple underactuated surface vehicles (USVs) from two aspects, i.e., guidance and control. In the guidance module, a novel dual-layer line-of-sight (DLLOS) guidance principle is incorporated into the leader–follower framework to generate the reference path. To overcome the problem of unavailable leader velocity information, an adaptive speed controller is designed to adjust the navigational speed of followers. As for the control part, by utilizing the dynamic event-triggered method, the operational frequency of actuators can be reduced in a flexible manner. That can effectively avoid the excessive wear and chattering phenomenon of actuators. Furthermore, by the fusing of the radial basis function neural networks (RBF NNs) and the robust neural damping technique, the model uncertainty, environmental disturbances and some unknown parameters can be remodeled, and only two gain-related adaptive laws need to be updated online. The serial–parallel estimation model (SPEM) is established to predict the velocity variables, and the approximation performance of NNs can be enhanced by virtue of the derived prediction error. Through the Lyapunov stable theorem, all control signals in the closed-loop system are guaranteed semi-globally uniformly ultimately bounded (SGUUB) stability. Finally, digital simulations are illustrated to verify the effectiveness and superiority of the proposed algorithm.

Dynamics of Water, Salt, and Nutrients Exchange at the Inlets of Three Coastal Lagoons

The intertidal patterns at the inlet of three coastal lagoons (Agiasma, Porto Lagos, and Xirolimni) in Northern Greece were investigated by combining in situ samplings and computational efforts. These lagoons are Mediterranean, microtidal coastal systems, connected with the adjacent open sea (Thracian Sea) through their inlet canals and are highly affected by the lagoon–sea exchange processes. Limited freshwater enters their basins, mostly due to precipitation and agricultural drainage. An intense monitoring program of water flow and quality at the mouth of the three lagoons was carried out, aiming to quantify the transport mechanisms of water, salt, and nutrients across the inlet canal under different tidal/meteorological conditions. Ebb currents were recorded higher than flood currents, and the temporal variability of the longitudinal velocity was characterized by asymmetries. Residual currents were important to the water exchange, with the Eulerian water, salt, and nutrient fluxes being an order of magnitude larger than the Stokes drift. Eulerian transport and tidal pumping are considered as important mechanisms for salt and nutrients exchange through the inlets. The return flow factor varied from 1 to 17.5% of the water exiting the lagoons in ebb, while the residence time ranged from 0.7 days to 4.2 days.

Dramatic Changes in the Observed Velocity of the Accretion Disk Wind in MCG-03-58-007 Are Revealed by XMM-Newton and NuSTAR

Abstract Past X-ray observations of the nearby Seyfert 2 MCG-03-58-007 revealed the presence of a powerful and highly variable disk wind, where two possible phases outflowing with v out1/c ∼ −0.07 and v out2/c ∼ −0.2 were observed. Multi-epoch X-ray observations, covering the period from 2010 to 2018, showed that the lower-velocity component is persistent, as it was detected in all the observations, while the faster phase outflowing with v out2/c ∼ −0.2 appeared to be more sporadic. Here we present the analysis of a new monitoring campaign of MCG-03-58-007 performed in 2019 May–June and consisting of four simultaneous XMM-Newton and NuSTAR observations. We confirm that the disk wind in MCG-03-58-007 is persistent, as it is detected in all the observations, and powerful, having a kinetic power that ranges between 0.5% and 10% of the Eddington luminosity. The highly ionized wind (log(ξ/erg cm s−1) ∼ 5) is variable in both the opacity and, remarkably in its velocity. This is the first time where we have observed a substantial variability of the outflowing velocity in a disk wind, which dropped from v out/c ∼ −0.2 (as measured in the first three observations) to v out/c ∼ −0.074 in just 16 days. We conclude that such a dramatic and fast variability of the outflowing velocity could be due to the acceleration of the wind, as recently proposed by Mizumoto et al. Here, the faster wind, seen in the first three observations, is already accelerated to v out/c ∼ −0.2, while in the last observation our line of sight intercepts only the slower, pre-accelerated streamline.