Spreading Width Research Articles

Flow and mixing characteristics of double-concentric jets pulsed at annular flow

The flow and mixing characteristics of double concentric jets pulsed at the annular flow were experimentally studied. The evolution process of the flow was observed and analyzed by means of the laser-light sheet assisted flow visualization method using the instantaneous streak pictures. The jet spread width was obtained using the binary boundary detection method based on the long-exposure pictures of the flow. The average velocities and turbulence intensities in both the centerline and radial directions were measured using the hot-wire anemometry. The mixing properties were determined using the tracer gas concentration detection technique. Three characteristic flow modes were identified in the domain of the central jet and the annular flow Reynolds numbers, namely the annular flow dominated mode, central jet dominated mode-low shear, and central jet dominated mode-high shear. In the annular flow dominated mode, the annular flow broke up the central jet in the centerline, where the axial velocity decayed quickly and the turbulence intensity increased to the maximum value. The larger jet spread in the radial direction enhanced the mixing characteristics in the recirculation region. In the central jet dominated mode-low shear, the central jet penetrated the recirculation region, resulting in significant entrainment of the annular flow. The axial velocity was modest and the turbulence intensity was insignificant; therefore, low mixing was observed in the recirculation region. In the central jet dominated mode-high shear, the recirculation bubble shrunk rapidly toward the jet exit, while the jet spread was controlled by the strong central jet entrainment. Although the annular flow interrupted the central jet and the turbulence intensity reached a peak after the combined jet, the enhancement of the mixing was quite seen to be lower than in the case of the annular flow dominated mode.

Flow and mixing characteristics of a forward-inclined stack-issued jet in crossflow

The flow and mixing characteristics of a forward-inclined stack-issued jet at various inclination angles (θ) and jet-to-crossflow momentum flux ratios (R) were experimentally studied in an open-loop wind tunnel. Flow behaviors were examined using the laser-assisted smoke flow visualization technique. The instantaneous velocities of the upwind-side shear-layer were digitized by a hot-wire anemometer using a high-speed data acquisition system. The instability frequencies in the upwind-side shear-layer vortices were obtained by the fast Fourier transform method. Long-exposure flow images were processed using the binary edge-detection technique to obtain the jet spread width. Transverse dispersion of jet fluids was determined using tracer gas concentration detection. The upwind-side shear-layer vortices revealed four characteristic flow modes: the High impingement-crossflow dominated mode (about θ < 15° and low R), the High impingement-jet flow dominated mode (about θ < 25° and high R), the Low impingement-crossflow dominated mode (about θ > 15° and low R), and the Low impingement-jet flow dominated mode (about θ > 25° and high R). Increasing θ in the crossflow dominated regimes eliminated the upwind-side shear-layer vortices, while increasing θ in the jet flow dominated regimes emphasized the upwind-side shear-layer vortices. Increasing θ at a fixed value of R increased jet spread width in the far field in all modes. In the near field, at x/d < 5 in the High impingement-crossflow dominated regime, the jet spread width was greater than in the Low impingement-crossflow dominated regime. In the jet flow dominated regimes, higher θ values led to greater jet spread width. Transverse dispersion of the jet fluids approached the jet spread width results. In the Low impingement-jet flow dominated regime, transverse dispersion of the jet fluids was significantly increased compared to the other regimes. In addition, the maximum tracer gas concentration was severely reduced at all axial stages, which implied better dispersion of the jet fluids in this regime.

Effect of Vane Shape on Fertilizer Distribution for a Dual-Disc Spinner Spreader

Abstract.Spinner-disc spreaders are commonly used for application of granular fertilizers with components (divider, spinner-discs, and vanes) influencing material flow behavior and distribution. Fertilizer ricocheting off these components is an uncontrolled aspect of material flow that negatively impacts spread uniformity. Therefore, an investigation using potash (KCl) was conducted to understand the impact of vane shape on fertilizer flow, ricocheting and distribution for a dual-disc spinner spreader. Four different vane shapes were used in this study. The first two vanes (Vanes 1 and 2) were common to this type of spinner spreader with a tapered and open-faced design. However, Vane 1 had a forward tapered top edge at an angle of 32° while Vane 2 had a top edge that was tapered backwards at 15°. Vanes 3 and 4 both had C-channel cross sections with Vane 3 tapered from inside out but Vane 4 had a constant height. Treatments included application rates of 220 and 440 kg/ha using three spinner disc speeds (600, 700, and 800 rpm). Stationary tests were conducted using a collection device that mounted around the spinner discs and vanes in order to estimate fertilizer particle exit points off the vanes. Standard pan tests were conducted to characterize resulting spread patterns, effective spread width, and spread uniformity. Results indicated that the level of ricocheting was significantly impacted by top edge design of a vane and increased with disc speed. The forward, upward facing top edge of Vane 1 caused on average, 26% of the material flow to be ricocheted by the vanes thereby inducing an uncontrolled nature of spread. However, the rearward facing top edge of Vane 2 reduced ricocheting by 13% plus generated a backward particle rotation for those contacting it. The majority of ricocheting occurred when particles contacted the vanes compared to the spinner discs. Ricocheting generated an uncontrollable aspect of the spread pattern with these particles applied along the centerline of the spreader. The effective spread width increased with disc speed. All four vane shapes generated equal effective spread widths of 18.3 and 21.3 m at 600 and 700 rpm, respectively. However, at 800 rpm, Vane 4 generated the greatest effective spread width of 24.4 m compared to 22.9 m for the other three vanes. The wider spread width for Vane 4 was contributed to the rectangular U cross-section maximizing the horizontal velocity of potash particles when exiting the vanes compared to the other three, more open faced vanes. Finally, the spread uniformity varied by vane shape with Vane 2 consistently generating the lowest CVs. Keywords: Distribution, Ricocheting, Vane Design, Spread uniformity, pattern.

Experimental testing of a helical rotor for compost distribution

It is well known that a manure spreader must be able to apply manure consistently, effectively and uniformly over the time. An overapplying doze will cause serios environmental problems (pollution) and a sub-applying doze will not contribute to plants growth and the applying of the fertilizer will be a loss. Corroborate with others problems such as maintaining a constant tractor speed, a constant rotational speed of the rotor, a constant and continuous flow of the material make the manure spreading machines an important subject for study and continuously improving . The propose of this study is to test a vertical helicoidal rotor for manure spreading in order to improve the machine distribution uniformity. The relation between the rotor position and rotation sense is a definitory issue for the machine performance. In order to achieve a high distribution uniformity a single helical rotor is tested and a series of possible situations of placing four rotors on the machine are analyzed. The optimal solution is given by two indicators: uniformity of spreading and distribution width. The test were made with an helical rotor having the following characteristics: high=1,1 m, maximum diameter of helical spiral=0.345 m, minimum diameter of the helical spiral= 0.114m, helical pitch=0.3m, inclination angle = 15°. After analyzing four possible situations it is observe that the combination of the maximum uniformity with minimum distribution with is the optimal solution. Experimental were carried out with compost and can be repeatable with others type of solid organic fertilizers, but a new calibration should be done. It is desirable that a trapezoidal, oval or triangle pattern with uniform sloping sides to be achieved. Because most of the spreaders actually produce an oval or triangle distribution pattern it is necessary subsequent passes that increase the time for fertilization operation.

Gross, intermediate and fine structure of nuclear giant resonances: Evidence for doorway states

We review the phenomenon of fine structure of nuclear giant resonances and its relation to different resonance decay mechanisms. Wavelet analysis of the experimental spectra provides quantitative information on the fine structure in terms of characteristic scales. A comparable analysis of resonance strength distributions from microscopic approaches incorporating one or several of the resonance decay mechanisms allows conclusions on the source of the fine structure. For the isoscalar giant quadrupole resonance (ISGQR), spreading through the first step of the doorway mechanism, i.e.\ coupling between one particle-one hole ($1p1h$) and two particle-two hole ($2p2h$) states is identified as the relevant mechanism. In heavy nuclei it is dominated by coupling to low-lying surface vibrations, while in lighter nuclei stochastic coupling becomes increasingly important. The fine structure observed for the isovector giant dipole resonance (IVGDR) arises mainly from the fragmentation of the $1p1h$ strength (Landau damping), although some indications for the relevance of the spreading width are also found.

Falling liquid film periodical fluctuation over a superhydrophilic horizontal tube at low spray density

Falling film on horizontal tube banks is widely used in heat transfer exchangers and absorbers due to its high heat and mass transfer performance. The superhydrophilic surface can effectively ameliorate the wettability of the tube wall and maintain thin film even at a very low spray density. Both of them play the dominating role in heat and mass transfer process. In this paper, a thermal tracing method was employed to investigate the liquid spreading feature and film fluctuation characteristics over a horizontal superhydrophilic tube. The results showed that the liquid film spreading width for the superhydrophilic tube was two times higher than that of the plain surface. Interestingly, as the liquid film extended continuously along the tube surface, the spreading width exhibited little relevance to the spray density under the current experimental condition for the superhydrophilic tube. Furthermore, in the case of droplet mode, the increment of flow rate leaded to an increase in the frequency of the impacting droplet instead of the droplet volume. Finally, the periodic behavior of liquid film fluctuation in droplet mode was observed as well, which demonstrated higher fluctuation intensity than that of other flow modes. The superhydrophilic-aided drop mode is of value in exploring the efficient, precise, and feasible technique for falling liquid film on a horizontal tube especially for ultralow spray density.

Determining the Performance Quality of Combine Harvesters Operating

Modern methods of monitoring the performance quality of combine harvesters during their testing are applied in a «swath laying» mode. The authors have shown the relevance of a similar assessment method used in the most common “crushing-and-spreading” mode and developed a simple and reliable method and a technical tool for assessing grain losses in the field during the combine harvester operation. (Research purpose) To develop a method for controlling grain loss of a combine harvester operating in a “chopping-and-spreading” mode using a two-chamber sampler prototype. (Materials and methods) The authors have proposed a new design of the prototype of a two-chamber sampler. They have proved that it ensures separate collection of the lost grain resulting from combine harvester units. A limited number of samplers were installed in the field and a method for sampling grain losses from working units of a combine harvester based on the developed sampler was applied. The applied method of control consists in evaluating the test results of a two-chamber sampler prototype during a combine harvester test in a «chopping-and-spreading» mode. (Results and discussion) The authors have developed the prototype of a two-chamber sampler of a fundamentally new design. They offer a method for determining the performance quality of a combine harvester. They have obtained graphs of grain losses distribution across the width of a threshing-and-separating unit and the spreading width equal to the operating width of a header. (Conclusion) It has been established that the use of a two-chamber sampler will ensure separate sequential sampling behind a header, a threshing unit, and a chopping spreader, while excluding sequential overlapping of errors. It has been revealed that the proposed scheme for sampling a grain losses resulting from the working units of a combine harvester will reduce the total number of samplers used - no more than 7 in each replication, regardless of the width of a threshing-and-separating unit or a header.

Wake modelling via actuator-line method for exergy analysis in openFOAM

ABSTRACT In this paper, first and second laws of thermodynamics are performed on MEXICO wind turbine based on a generalized Actuator Line method (ALM). In order to simulate the flow field around wind turbine’s rotor and to predict the wake’s field behind it, large eddy simulation (LES) coupled with a novel Gaussian spreading width is employed in ALM. Simulation has been done for two different conditions including design conditions and stalled conditions. Due to the wake deficit results, exergy analysis conducted. Some metrological variables such as velocity, pressure, and temperature which are measured in the wind tunnel are used to evaluate wind turbine’s performance based on energy and exergy efficiencies. Obtained results are compared to experimental data. It was observed that the ALM results agreed well with measurements. Stall condition’s results were in better agreement with experimental data. It is shown that the axial and radial wake velocities showed a difference of 5.2 and 11% respectively. Also, it is shown that output results from ALM have a good ability to analysis wind turbine based on the second law of thermodynamics. The maximum difference from exergy efficiency analysis compared to measurements were only 6.2%.

Low-Cost Gait Analysis for Behavioral Phenotyping of Mouse Models of Neuromuscular Disease.

Measurement of animal locomotion is a common behavioral tool used to describe the phenotype of a given disease, injury, or drug model. The low-cost method of gait analysis demonstrated here is a simple but effective measure of gait abnormalities in murine models. Footprints are analyzed by painting a mouse's feet with non-toxic washable paint and allowing the subject to walk through a tunnel on a sheet of paper. The design of the testing tunnel takes advantage of natural mouse behavior and their affinity for small dark places. The stride length, stride width, and toe spread of each mouse is easily measured using a ruler and a pencil. This is a well-established and reliable method, and it generates several metrics that are analogous to digital systems. This approach is sensitive enough to detect changes in stride early in phenotype presentation, and due to its non-invasive approach, it allows for testing of groups across life-span or phenotypic presentation.

Performance Analysis of MIMO STBC System in Flat Fading and Frequency Selective Fading Channels

Multiple Input Multiple Output (MIMO) technology is a technique that can be used to overcome multipath fading. The multipath fading is caused by signals coming from several paths that experience different attenuations, delays and phases. In a multipath condition, an impulse that sent by the transmitter, will be received by the recipient not as an impulse but as a pulse with a spread width that called delay spread. Delay spread can cause intersymbol interference (ISI) and bit translation errors from the information received. To determine the effect of delay spread on the MIMO system, then MIMO system performance research was performed on flat fading and frequency selective fading channels using the Space Time Block Code (STBC) coding technique. This research was conducted using MatLab 2018a software. The simulation results show that the MIMO STBC system performance on flat fading channels is better than the MIMO STBC system performance on the frequency selective fading channel. This result is analyzed based on the value of BER vs. Eb/No and eye diagram.

Demonstration of gold foam plasma as bright x-ray source and slow ion emitters

In this paper, enhancement in x-ray emission and reduction of kinetic energy of ions from low density foam plasma is demonstrated by performing experiment and hydrodynamic simulation. The plasma is produced by irradiation of solid gold and gold foam targets (densities 0.2 g/cc, 0.13 g/cc and 0.1 g/cc) at intensities in the range of 4 × 1013–1 × 1014 W cm−2. Time resolved x-ray emission is measured by an x-ray streak camera with 10 ps resolution. The x-ray flux measured by the streak camera from low density gold foam shows a 13% enhancement in comparison to solid gold in the spectral range >0.8 keV and above. Decrease in velocity of ions is observed in low density gold foam. In solid gold, thermal ions peak velocity is 31 × 104 m s−1 and spread in narrow energy width, however, in case of 0.1 g/cc, peak velocity reduces to 6 × 104 m s−1 towards target normal and emitted in broad energy range. Shadowgraphy results also provide evidence of narrower expansion of plasma from solid gold. However, total ion flux from low density gold foam is comparable to ion flux of solid gold indicating the process of volumetric absorption.

Shire/Takeda: How deal risks create M&amp;A opportunities

AbstractM&amp;A activity in 2018 has been concentrated––in terms of news flow, deal length, spread width, size and volume––around two main mega‐deals, the competing bids of Fox and Comcast for Sky and Takeda's acquisition of Shire, which both ended successfully. The US‐China trade war and European‐wide political concerns, including the uncertainty and lack of visibility around Brexit and the Italian budget, may have discouraged large transactions. Overall deal risk has also increased with enhanced regulatory scrutiny over national interest in sensitive sectors. In this article, we will focus on one of them: Takeda's acquisition of Shire, whose interest lies in the number and nature of issues involved, as well as the attractiveness of the trade until the very end.

Numerical Study on the Flow Characteristics of 2-D Free Jet at a Low Reynolds Number

Two-dimensional free jets have been numerically studied in the laminar flow regime. The numerical solution was obtained by solving the incompressible Navier-Stokes equations with SMAC finite-difference method. The variation of the axial velocity, potential core and jet spreading width were investigated to make clear the characteristics of free jet at a low Reynolds number. It was found that the fully developed region where the self-similarity is satisfied exists in the downstream of the collapsing point of the potential core. The variation of the core is depended on the Reynolds number. It showed that the half-width of geometry is useful in identifying the similarity. The predicted velocity distributions were in good agreement with those of experimental and theoretical results.

Statistical study of particle flux footprint widths with tungsten divertor in EAST

The study of the divertor heat flux is of great importance for the operation of ITER and future fusion devices. In this paper, the behaviors of the widths of the particle flux footprints on the tungsten divertor in EAST have been statistically studied for the first time. A large number of divertor Langmuir probe (Div-LP) data from EAST general discharges in the 2016 campaign have been stepwise selected and analyzed for both L- and H-mode plasmas. It is found that the scaling dependences on poloidal magnetic field for the particle flux fall-off width at the inner tungsten divertor agree well with those at the outer graphite divertor in both L- and H-mode plasmas. The difficulty of extracting reliable footprint widths has been overcome by fitting data from two Div-LP arrays distributed toroidally. The nonlinear regressions of the particle flux fall-off width and particle flux spreading width at the inner tungsten divertor in H-mode plasmas have then been performed. A negative dependence on the plasma stored energy is found in the regression of the particle flux fall-off width and the exponent of the stored energy in the regression is consistent with previous experimental studies. The comparison between the particle flux fall-off width at the tungsten divertor with that at the graphite divertor indicates that statistically the divertor material and the direction of the toroidal magnetic field seem to have no significant influence on the particle flux fall-off width at the inner divertor. In addition, the similar particle flux fall-off width in L- and H-mode plasmas and the in–out asymmetry of the particle flux fall-off width at the graphite divertor have also been shown.

Experimental Investigation of Nozzle Spacing Effects on Characteristics of Round Twin Free Jets

The objective of this paper is to investigate the effects of nozzle spacing on the mean velocity and higher-order turbulent statistics of free twin round jets produced from sharp contraction nozzles. The experiments were performed in an air chamber where four nozzle spacing ratios, S/d = 2.8, 4.1, 5.5, and 7.1, were investigated at a fixed Reynolds number of 10,000. A planar particle image velocimetry (PIV) system was used to conduct the velocity measurements. The results show that downstream of the potential core, a reduction in spacing ratio leads to an earlier and more intense interaction between the jets, indicated by enhanced half-velocity width spread rate in the inner shear layers and a significant rise of turbulent intensities and vorticity thickness along the symmetry plane. A reduction in spacing ratio, however, confines the ambient fluid entrainment along the inner shear layers leading to a reduced core jet velocity decay rate. The closer proximity of the jets also leads to the decrease of Reynolds stresses in the inner shear layers but not in the outer shear layers. The Reynolds stress ratios along the jet centerline reveal the highest anisotropy in the potential core region.

The effect of overlapping adjacent passages of the unit on the uniformity of the application of mineral fertilizers

The article presents some results of studies on the effect of overlapping widths on the uniform distribution of fertilizers on adjacent passages of an aggregate conducted at the N.P.Ogarev Mordovia State University. Recommendations are offered on reducing the ratio between the working width and the spreading width during centrifugal apparatus operation. Observations showed that spreaders with centrifugal type working elements distribute fertilizers unevenly across the working width. Most of them fall in the middle, and towards the edges their number decreases. To achieve a more uniform application, they are usually scattered with overlap. The studies are conducted to determine the effect of overlap on the uniform distribution of mineral fertilizers. To assess the uniformity of the surface distribution of mineral fertilizers and lime by the experimental setup along the working width and in the direction of travel, the fertilizers were collected in baking sheets of 0,5×0,5×0,1 m in size. The baking sheets were placed on an even area tightly to each other in three rows with row spacing of 5 m. During the experiments, the non-uniformity of fertilizer application at a machine grip width of 10 m with an overlap of 2 m was higher than the agrotechnical permissible values - 25 %. Studies have shown that it is possible to increase the uniformity of application while reducing the width of the aggregate to 8 m, while the overlap should be at least 4 m. Studies have shown that it is possible to increase the uniformity of application while reducing the width of the aggregate to 8 m, while the overlap should be at least 4 m. At the same time, the problem of ensuring accurate driving of fertilizer applicators is even more acute with an increase in the working width of modern high-performance equipment. Obviously, in this case, in the absence of special stands for the operational tuning of machinery, it is very difficult to correctly adjust them. Therefore, each time you change the type of fertilizer and doses you need to check the position of the regulator with the recommended operating instructions. The working width of the grip depends on this, and, consequently, the distance between adjacent passages of the unit is dependent. The quality of centrifugal machines, in addition, depends on the quality of the applied mineral fertilizers (range of sizes and shapes of granules, flow-ability), the state of the field topography, the evenness of the soil, wind speed, the qualification of the machine operator and its good faith (strict adherence to a given speed), operating speed and etc. Obviously, in order to introduce mineral fertilizers with permissible unevenness (permissible coefficient of variation for nitrogen fertilizers ±10 %, for potash and phosphorus ±20 %), centrifugal working bodies must strictly comply with the requirements of the work execution regulations.

On the spreading width of isobaric analog resonances

A description of the spreading width of the Isobaric Analog Resonances in medium-heavy mass spherical nuclei is proposed within a combined approach. This latter consists in incorporating the "Coulomb description" of isospin-forbidden processes into the newly developed particle-hole dispersive optical model. Within this approach, the observed spreading width of the resonances based on the 208,209Pb parent-nuclei ground state is quantitatively estimated without the use of specific adjustable parameters.

A dosimetric analysis of proton beam therapy using different snouts

AbstractPurposeThis exploration is intended to analyse the dosimetric characteristics of proton beams of multiple energies using different snout sizes.Materials and methodsA synchrotron was used for the extraction of eight proton beam energies (100–250 MeV). Dosimetric measurements were taken in a water phantom that was irradiated with a proton beam emanating from the gantry system at angles 0, 90, 180 and 270 degree using a large and a medium snout. The range of beam energies in the phantom, their corresponding centre modulation depth (CMD) and the width of spread out Bragg peak (SOBP) were measured by Markus chamber. Double scattering technique was employed for the creation of SOBPs.ResultsThe range of proton beams varied from 4·3 cm for 100 MeV beam to 28·5 cm for 250 MeV beam with the medium snout and from 4·3 cm for 100 MeV to 25 cm for 250 MeV beam with large snout in the water phantom. SOBP width showed a variation from 4 to 10 cm with medium and large snout. While determining the output with medium snout, the discrepancy of 1·1% was observed between the maximum and minimum mean values of output for all the given set of energies and angles. There occurred a difference of 0·9% between the maximum and minimum mean values of output with the large snout. Beam output at SOBP centre was 12% higher with large snout as compared to that with medium snout for all the given beam energies. Flatness and symmetry were found within ±2·5% tolerance limits with medium and large snouts.ConclusionFlatness and symmetry were found within explicit limits with both medium and large snouts. Large snout produced higher beam output than that of medium snout at the centre of SOBP. This exploration can be extended to the determination of beam output, flatness and symmetry with a small snout.

Damped transient response of the giant dipole resonance in the lead region

We analyze thoroughly the statistical properties of the 1- spectrum in energy interval 9.5-18.5 MeV for Hg206, Pb204,206,208, and Po210 nuclei. To this aim we use the Skyrme interaction SLy4 and the volume pairing interaction, treated in the BCS approximation, as our model Hamiltonian to create a single-particle spectrum and to analyze excited states. The excited states are obtained in the quasiparticle random phase approximation taking into account the coupling between one- and two-phonon states. The obtained spectra (studied by means of the nearest-neighbour spacing distribution, Δ3-statistics and the Porter-Thomas distribution) indicate on the onset of statistical properties close to those of the Gaussian Orthogonal Ensembles. The comparison of the results, obtained with the aid of the coupling calculated microscopically and by means of the Gaussian random distribution, demonstrates a close similarity in the description of the spreading widths of the Isovector Dipole Giant Resonanse of the considered nuclei. Furthermore, we show that employing the random distribution for the coupling between microscopic one-phonon states and two-phonon states, generated by the Gaussian Orthogonal Ensembles distribution, a good agreement is also obtained with the microscopic description of the decay widths. (Less)

Systematic study of giant quadrupole resonances with the subtracted second random-phase approximation: Beyond-mean-field centroids and fragmentation

A systematic analysis of giant quadrupole resonances is performed for several nuclei, from $^{30}\mathrm{Si}$ to $^{208}\mathrm{Pb}$, within the subtracted second random-phase-approximation (SSRPA) model in the framework of the energy-density-functional theory. Centroid energies and widths of the isoscalar giant quadrupole resonances are compared with the corresponding random-phase-approximation (RPA) values. We find lower SSRPA centroid energies compared to the RPA values leading, in general, to a better agreement with the experimental data. As far as the widths are concerned, we observe for both SSRPA and RPA cases a global attenuation of the single-particle Landau damping going from lighter to heavier nuclei, and we obtain, systematically, larger widths in the SSRPA model compared to the RPA case. For some selected nuclei for which high-resolution ($p,{p}^{\ensuremath{'}}$) experimental data are available, namely $^{40}\mathrm{Ca}$, $^{90}\mathrm{Zr}$, $^{120}\mathrm{Sn}$, and $^{208}\mathrm{Pb}$, the theoretical strength distributions are directly compared with the experimental spectra. We observe a significant improvement, with respect to RPA results, in the description of the spreading widths and of the fragmentation of the obtained spectra, due to the coupling between one-particle--one-hole and two-particle--two-hole configurations.