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A DNS study on the Mach number effect for a supersonic microramp
No full textMicrovortex Generators (MVGs) are passive control devices with heights below the boundary layer thickness that have been proposed to mitigate the detrimental effects of Shock Wave/Boundary Layer Interaction (SBLI).
Although their control effectiveness has been largely demonstrated, several aspects of the flow generated by MVGs in supersonic boundary layers still need to be characterized. In this work, we present a campaign of Direct Numerical Simulations (DNSs) of a turbulent boundary layer on a microramp, to investigate the effect of the Mach number, from subsonic to the supersonic regime. An in-depth analysis characterizes the properties of the flow generated by the microramp, showing that the flow topology changes significantly because of compressibility effects, and that typical wake features do not scale linearly with the geometry dimensions but rather depend on the incoming flow conditions as well
A Neural Networks Approach to Detecting Lost Heritage in Historical Video
Full text linkDocumenting Cultural Heritage through the extraction of 3D measures with photogrammetry is fundamental for the conservation of the memory of the past. However, when the heritage has been lost the only way to recover this information is the use of historical images from archives. The aim of this study is to experiment with new ways to search for architectural heritage in video material and to save the effort of the operator in the archive in terms of efficiency and time. A workflow is proposed to automatically detect lost heritage in film footage using Deep Learning to find suitable images to process with photogrammetry for its 3D virtual reconstruction. The performance of the network was tested on two case studies considering different architectural scenarios, the Tour Saint Jacques which still exists for the tuning of the networks, and Les Halles to test the algorithms on a real case of an architecture which has been destroyed. Despite the poor quantity and low quality of the historical images available for the training of the network, it has been demonstrated that, with few frames, it was possible to reach the same results in terms of performance of a network trained on a large dataset. Moreover, with the introduction of new metrics based on time intervals the measure of the real time saving in terms of human effort was achieved. These findings represent an important innovation in the documentation of destroyed monuments and open new ways to recover information about the past
Planing Hull Hydrodynamic Performance Prediction Using LincoSim Virtual Towing Tank
Full text linkThis work shows the performance of LincoSim, a web-based virtual towing tank enabling automated and standardized calm water computational fluid dynamics (CFD) data sampling, extending previous published applications to the case of a high-speed hull. The calculations are performed for a 1:10 scale model of a 43 ft powerboat hull form in the Froude number range from 0.3 to 2.0. The counterpart physical model is the experimental fluid dynamics (EFD) campaign performed at the University of Naples Federico II, where the resistance, sinkage and trim data have been measured. The EFD/CFD data comparison is performed and shown with a discussion of the spotted differences. The average percentage differences between the EFD and CFD data for the whole speed range are 1.84, 6.87 and 6.94 for the resistance, dynamic trim, and sinkage, respectively. These results confirm the maturity of the standardized and automated CFD modeling for calm water hydrodynamic analysis included in LincoSim, even at very high Froude numbers. The wetted length of the keel and chine and the wetted surface are calculated from numerical data using the advanced post-processing. Finally, as a work in progress, we test a first comparison for the same hull of the EFD and CFD data, considering two seakeeping conditions for head waves at a given wavelength for two velocity conditions. Also, this kind of analysis confirms the tight correlation between the measured and computed outcomes. This synergic interplay of EFD and CFD can link the advantages of both methods to support hull design but also requires experiment planning and final data analysis to obtain physical parameters not easily measurable in laboratory, such as the wetted surface, wetted lengths, proper viscous contribution, and pressure distribution both in calm water and in waves
Drag increase and turbulence augmentation in two-way coupled particle-laden wall-bounded flows
Full text linkThe exact regularized point particle method is used to characterize the turbulence modulation in two-way momentum-coupled direct numerical simulations of a turbulent pipe flow. The turbulence modification is parametrized by the particle Stokes number, the mass loading, and the particle-to-fluid density ratio. The data show that in the wide region of the parameter space addressed in the present paper, the overall friction drag is either increased or unaltered by the particles with respect to the uncoupled case. In the cases where the wall friction is enhanced, the fluid velocity fluctuations show a substantial modification in the viscous sub-layer and in the buffer layer. These effects are associated with a modified turbulent momentum flux toward the wall. The particles suppress the turbulent fluctuations in the buffer region and concurrently provide extra stress in the viscous sub-layer. The sum of the turbulent stress and the extra stress is larger than the Newtonian turbulent stress, thus explaining the drag increase. The non-trivial turbulence/particles interaction turns out in a clear alteration of the near-wall flow structures. The streamwise velocity streaks lose their spatial coherence when two-way coupling effects are predominant. This is associated with a shift of the streamwise vortices toward the center of the pipe and with the concurrent presence of small-scale and relatively more intense vortical structures near the wall
Unsteadiness characterisation of shock wave/turbulent boundary-layer interaction at moderate Reynolds number
Full text linkA direct numerical simulation of an oblique shock wave impinging on a
turbulent boundary layer at Mach number 2.28 is carried out at moderate
Reynolds number, simulating flow conditions similar to those of the experiment
by Dupont et al. (2006). The low-frequency shock unsteadiness, whose
characteristics have been the focus of considerable research efforts, is here
investigated via the Morlet wavelet transform. Owing to its compact support in
both physical and Fourier spaces, the wavelet transformation makes it possible
to track the time evolution of the various scales of the wall-pressure
fluctuations. This property also makes it possible to define a local
intermittency measure, representing a frequency-dependent flatness factor, to
pinpoint the bursts of energy that characterise the shock intermittency scale
by scale. As a major result, wavelet decomposition shows that the broadband
shock movement is actually the result of a collection of sparse events in time,
each characterised by its own temporal scale. This feature is hidden by the
classical Fourier analysis, which can only show the time-averaged behaviour.
Then, we propose a procedure to process any relevant time series, such as the
time history of the wall-pressure or that of the separation bubble extent, in
which we use a condition based on the local intermittency measure to filter out
the turbulent content in the proximity of the shock foot and to isolate only
the intermittent component of the signal. In addition, wavelet analysis reveals
the intermittent behaviour also of the breathing motion of the recirculation
bubble behind the reflected shock, and allows us to detect a direct, partial
correspondence between the most significant intermittent events of the
separation region and those of the wall-pressure at the foot of the shock
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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