1,721,135 research outputs found
Scaling properties of the Ffowcs-Williams and Hawkings equation for complex acoustic source close to a free surface
No full textWe perform a scaling analysis of the terms composing the Ffowcs-Williams and Hawkings (FWH) equation, which rules the propagation of noise generated by a rigid body in motion. Our analysis extends the seminal work of Lighthill (Proc. R. Soc. Lond. A, vol. 211, 1952, pp. 564-587) and the dimensional analysis of classical sources (monopole, dipole and quadrupole) considering all the FWH integral terms. Scaling properties are analysed in light of perfect/imperfect similarity when laboratory-scale data are used for full-scale predictions. As a test case we consider a hydrodynamic example, namely a laboratory-scale ship propeller. The data, obtained numerically in a previous study, were post-processed according to the scaling analysis presented herein. We properly scale the speed of sound to obtain perfect similarity and quantify the error with respect to the imperfect scaling. Imperfect similarity introduces errors in the acoustic response related both to the linear terms and to the nonlinear terms, the latter of great importance when the wake is characterized by robust and organized vorticity. Successively, we analyse the effect of a free surface, often present in hydrodynamic applications. We apply the method of images to the FWH equation. The free surface may generate a frequency-dependent constructive/destructive interference. The analysis of an archetypal acoustic field (monopole) provides robust explanation of these interference effects. Finally, we find that imperfect similarity and the absence of a free surface may introduce errors when model-scale data are used to obtain the full-scale acoustic pressure. The error is small for microphones placed in the near field and becomes relevant in the far field because of the nonlinear terms
A New Small Drifter for Shallow Water Basins: Application to the Study of Surface Currents in the Muggia Bay (Italy)
Full text linkA new small drifter prototype for measuring current immediately below the free surface in a water basin is proposed in this paper.
Thedrifter dimensionsmake it useful for shallow water applications.The drifter transmits itsGPS location viaGSMphone network.
The drifter was used to study the trajectory of the surface current in the Muggia bay, the latter containing the industrial harbor of
the city of Trieste (Italy).The analysis has been carried out under a wide variety of wind conditions. As regards the behavior of the
drifter, the analysis has shown that it is well suited to detect the water current since its motion is marginally affected by the wind.
The study has allowed detecting the main features of the surface circulation within the Muggia bay under different meteorological
conditions. Also, the study has shown that the trajectory of the surface current within the bay is weakly affected by the Coriolis
force
Laboratory-scale investigation of a periodically forced stratified basin with inclined endwalls
Full text linkWe present results of numerical simulations of a stratified reservoir with a three-layer stratification, subject to an oscillating surface shear stress. We investigate the effect of sloped endwalls on mixing and internal wave adjustment to forcing within the basin, for three different periods of forcing. The simulations are carried out at a laboratory scale, using large-eddy simulation. We solve the three-dimensional Navier-Stokes equations under the Boussinesq approximation using a second-order-accurate finite-volume solver. The model was validated by reproducing experimental results for the response of a reservoir to surface shear stress and resonant frequencies of internal waves. We find interesting combinations of wave modes and mixing under variation of the forcing frequencies and of the inclination of the endwalls. When the frequency of the forcing is close to the fundamental mode-one wave frequency, a resonant internal seiche occurs and the response is characterized by the first vertical mode. For forcing periods twice and three times the fundamental period, the dominant response is in terms of the second vertical mode. Adjustment to forcing via the second vertical mode is accompanied by the cancellation of the fundamental wave and energy transfer to higher-frequency waves. The study shows that the slope of the endwalls dramatically affects the location of mixing, which has a feedback on the wave field by promoting the generation of higher vertical modes
On the analysis of sloshing of water in rectangular containers: numerical study and experimental validation
No full textTurbulent oscillating channel flow subjected to wind stress
No full textThe channel flow subjected to a wind stress at the free surface and an oscillating pressure gradient is investigated using large-eddy simulations (LES). A slowly pulsating mean flow occurs, with the turbulent mechanics essentially being quasi steady. Logarithmic boundary layers are present at both the bottom wall and the free-surface. Turbulent streaks are observed in the bottom and free-surface layer. The mean velocity and the structures emerging in the flow have been discussed
Assessment of methodologies for the solution of the Ffowcs Williams and Hawkings equation using LES of incompressible single-phase flow around a finite-size square cylinder
Full text linkThe acoustic analogy represents a powerful tool for the prediction of noise generated by the
interaction between the flow and a moving body. It is based on decoupling the acoustic problem
from the fluid dynamic one: the velocity and pressure fields, obtained through a separate numerical
simulation, are used as source terms in an inhomogeneous wave equation whose solution reconstructs
the noise in the far field. When the method is based on the fundamental Ffowcs Williams and
Hawkings (FW-H) equation, different solving methodologies may be adopted.
The present work considers the original FW-H equation and gives the advective formulation of
the volume integral terms. The results are compared with those obtained with the Curle and porous
formulations.
To account for volume integrals, the assumption of compact noise source is needed. This assumption
is common in literature, however, in the present work, a dimensional analysis is proposed, in
order to indicate in a rigorous way the cases in which the compressibility delays can be avoided.
The dimensional analysis is tested in the case of an acoustic monopole field. Successively, the FW-H
porous formulation is compared with the original FW-H equation in the case of an irrotational advected
vortex. This example puts in evidence the different response of the two methods in the case
of a vortex crossing the acoustic domain.
Then, different solution strategies of the FW-H are evaluated using a fluid dynamic dataset
obtained through large eddy simulation of a turbulent flow around a finite-length cylinder with square
section. The analysis allows to point out the strengths and drawback of the different techniques and
to achieve, through the comparison of the different solutions, an accurate understanding of the noise
source mechanisms taking place in the flow. Finally, a mixed procedure, merging the advantages of
the porous formulation with the direct evaluation of the volume integral terms is proposed. It may
be used in presence of significant time delays. Overall, the present study is oriented to the analysis
of very low Mach number flows, although the complete porous method might be applicable in a more
general framework. This aspect will require additional research in the future
Turbulence structures over irregular rough surfaces
No full textTurbulent flow in a channel with irregular two-dimensional rough surfaces is analysed
through wall-resolving large eddy simulation (LES). Both walls of the channel are
roughened through the superimposition of sinusoidal functions having random amplitude
and four different wavelengths. The downward shift of the velocity profile in the
log region due to the roughness, known as roughness function, is well captured in the
simulations. The spanwise and wall-normal turbulence intensities are found to increase
with the roughness height, while the streamwise component decreases. The analysis
of the Reynolds stress anisotropy tensor highlights a tendency towards isotropisation,
confirmed by the vorticity rms. The analysis of the statistics shows that the effects of
the roughness on the turbulent flow are greatly related to the increase of the height
of the maximum peaks of the corrugations. Although the inner layer is dramatically
affected by the wall irregularities, the outer layer appears not affected by the specific
wall shape and a collapse of the turbulence statistics in smooth- and rough-wall conditions
is observed. Much of the present results are consistent with the observations
made over regular roughness, nevertheless the turbulence structures and the turbulent
kinetic energy production analysis shows that the turbulence dynamics are directly
affected by the local shape of the irregularities. The spatial inhomogeneities of the
height of the roughness peaks and of the cavity region locally modifies the structures
introducing variable length-scales. Overall the irregular roughness destroys the coherent
pattern of the flow selectively, depending on the spatial distribution of the higher
roughness peaks
The effect of the slope of irregularly distributed roughness elements on turbulent wall-bounded flows
No full textWall roughness produces a downward shift of the mean streamwise velocity profile in the log region, known as the roughness function. The dependence of the roughness function on the height and arrangement of roughness elements has been confirmed in several studies where regular rough walls were analysed; less attention has been paid to non-regular rough walls. Here, a numerical analysis of turbulent flows over irregularly shaped rough walls is performed, clearly identifying the importance of a parameter, called the effective slope (ES) of the wall corrugations, in characterizing the geometry of non-smooth irregular walls. The effective slope proves to be one of the fundamental geometric parameters for scaling the roughness function. Specifically, for a moderate range of roughness heights, both in the transitionally and in the fully rough regime, ES appears to scale the roughness function for a wide range of irregular rough geometric configurations. The effective slope determines the relative importance of friction drag and pressure drag. For ES similar to 0.15 we find that the friction contribution to the total wall stress is nearly in balance with the pressure-drag contribution. This value separates the region where the roughness function Delta U+ = f(ES) is linear from that where a smooth nonlinear behaviour is observed. In the cases investigated, value ES similar to 0.15 also separates the transitionally rough regime from the fully rough regime
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