1,721,010 research outputs found
Turbulence Enhancement in Coaxial Jet Flows by Means of Vortex Shedding
No full textFor many years investigations have been conducted to understand the flow instabilities that lead to transition in jets. The inviscid linear stability analysis of Batchelor & Gill [1] has shown that immediately
at the jet exit, where the velocity profile is close to a ’top-hat’ one, all the instability modes are able to be exponentially amplified while in the far field region only the helical mode seems to be unstable. The transition between these two different instability regions is still unclear and the analysis is complicated by the presence of several unstable modes embedded in the turbulence background. Therefore, several experiments have been done with active or passive excitation methods in order to highlight the role of a
single or few modes in the evolution of the flow.
Previous investigations in naturally and artificially excited jets have determined the importance of two instability lengthscales: one associated with the initial shear-layer thickness at the exit of the nozzle, and the other associated with the jet diameter which governs the shape of the mean velocity profile at the end
of the potential core. The instability modes in the first region develop through continuous and gradual frequency and phase adjustments to produce a smooth merging with the second region. This process makes this problem from a fundamental point of view interesting, and for that reason it has received a great deal of attention.
Axisymmetric excitation by means of acoustic forcing has been able to highlight several important aspects of the complex dynamics involved, like the role played by the so called shear layer and jet column mode acting in the near field of the jet at the nozzle exit and at the end of the potential core, respectively.
However, fewer works have been devoted to the investigation of higher azimuthal modes principally due to the higher complexity of the excitation facility (see [2]).
This work is aimed to show the effect of two oblique modes (m = ±1) at the nozzle lip generated by means of acoustic forcing. Different excitation amplitudes and frequencies have been tested regarding their effects on the flow dynamics. The relative decrease, with respect to the unexcited case, in the rms value of the streamwise velocity fluctuations in the shear layer centerline at x/D =3 has been reported in figure 1. It is evident that the amplitude of the exciting wave does not play an important role (except
at very high levels) and that the rms shows a maximum reduction (around 15%) close to 1500 Hz that is equal to a Strouhal number St = f/U0 = 0.029, i.e. a superharmonic of the shear layer mode.
Velocity measurements have been performed in the natural (A) and excited cases with f =1500 Hz (B) and with f =180 Hz (C). (A) corresponds to the unexcited case, (B) to the one with maximum
turbulence suppression and (C) to the one associated to the jet column mode. It is evident from figure 2(a) that only in case (B) several nonlinear interactions take place in the near field region leaving some imprint further downstream (see figure 2(b)).
The measurements in the mixing layer region confirm this reduction showing that in case (B) (reported in figure 3(b)) the shear layer attains earlier a self similar state in the velocity fluctuation after a faster evolution inside the first diameter and a peak value of the fluctuation intensity 20% less than in case (A)
(reported in figure 3(a)) and (C), underlining the important effect of this kind of excitation.
This reduction of turbulence could be connected to the same phenomenon described by Elofsson & Alfredsson [3] about the effect of oblique waves in laminar boundary layers, where the authors showed that the interaction of two waves is able to generate streamwise streaks by means of nonlinear interaction.
Whether or not oblique waves can be observed in jets will be further investigated by means of several measurement techniques and will be reported in th..
A study of swirling turbulent pipe and jet flows
No full textAxially rotating turbulent pipe flow is an example in which the rotation strongly affects the turbulence, which then also influences the mean flow properties. For instance, in the fully developed flow as well, the fluid is not in solid body rotation due to the influence of the cross-stream Reynolds stress. The present paper reports new measurements from a rotating pipe flow and the streamwise mean velocity distribution is compared with recent scaling ideas of Oberlack [J. Fluid Mech. 379, 1 (1999)] and good agreement is found. A second part of the paper deals with the initial stages when the flow leaves the pipe and forms a swirling jet. The measurements in the jet show that at some distance downstream (approximately five jet diameters) the central part of the jet actually rotates in the opposite direction as compared to the rotation of the pipe. This effect is explained by the influence of the cross-stream Reynolds shear stress (45 refs.
CICLoPE -A large pipe facility for detailed turbulence measurements at high Reynolds number
No full textCICLoPE – A Large Pipe Facility for Detailed Turbulence Measurements at High Reynolds Number
No full textHigh Reynolds number turbulence is ubiquitous in a number of flow of practical interest and crucial to draw conclusions regarding the physics of turbulence. Although recent laboratory experiments, measurements in planetary boundary layer and direct numerical simulations provide a huge amount of information, none of these data sets provide high Reynolds number, high spatial resolution and well converged statistics at the same time. As a response to this problem, an international collaboration between a group of universities and research centers started some years ago to build large scale infrastructures for high Reynolds number experiments. The Center for International Cooperation in Long Pipe Experiments, CICLOPE (www.ciclope.unibo.it) at the University of Bologna, was created for this purpose and will be open to international scientists through different collaboration programs. The laboratory is currently under construction and the first facility, which will be installed there is a large pipe flow experiment that will allow fully resolved turbulence measurements even at high Reynolds number
Scaling of mixed structure functions in turbulent boundary layers
Full text linkWe address the issue of the scaling of the anisotropic components of the hierarchy of correlation tensors in the logarithmic region of a turbulent boundary layer over a flat plate, at Re?15000. We isolate the anisotropic observables by means of decomposition tools based on the SO(3) symmetry group of rotations. By employing a dataset made of velocity signals detected by two X probes, we demonstrate that the behavior of the anisotropic fluctuations throughout the boundary layer may be understood in terms of the superposition of two distinct regimes. The transition is controlled by the magnitude of the mean shear and occurs in correspondence with the shear scale. Below the shear scale, an isotropy-recovering behavior occurs, which is characterized by a set of universal exponents which roughly match dimensional predictions based on Lumley's argument [J. L. Lumley, Phys. Fluids 8, 1056 (1965)]. Above the shear scale, the competition between energy production and transfer mechanisms gives rise to a completely different scenario with strong alterations of the observed scaling laws. This aspect has significant implications for the correct parametrization of the anisotropy behavior in the near wall region since, approaching the wall, an increasingly larger fraction of the scaling interval tends to conform to the shear-dominated power laws
The diagnostic plot - a new way to appraise turbulent boundary-layer data
No full textIt has been shown that the diagnostic plot may distinguish between accurately measured data in the near-wall region and data which may suffer from various problems. The diagnostic plot also has the interesting property that both the inner (y+ < 10) and outer regions can be made to collapse in the same plot. There is unfortunately not yet enough well-resolved data of u! for high Reynolds numbers to clarify how u! scales in the outer region although in Ref. [1] scaling with U# gives the smallest scatter when plotted against the wall distance normalized with the boundary-layer thickness. When better data becomes available it will be interesting to see how the diagnostic plot varies with Re in the outer region. If the Reynolds number variation is small the diagnostic plot may be used to estimate the boundary-layer free-stream velocity from a few measurement points in the outer region of u! and U, which may be helpful when studying atmospheric boundary layers
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
The effect of oblique waves on jet turbulence
No full textCoaxial jets are used in many technological applications such as burners, turbofans, etc. They are important also for fundamental research, since they represent a basic flow configuration. However, its full characterisation is governed by numerous parameters [1]. The prevailing model for more than a decade was that coaxial jets could be considered as a simple combination of single jets [2], where the two shear layers develop independently from each other. This simple view was however modified when Dahm et
al. [3] in their flow visualisation study evinced the existence of different topological flow regimes for different velocity ratios, ru = Uo/Ui (hereby Ui and Uo denote the maximum absolute velocity of the inner and outer streams at the nozzle exits, respectively), and absolute velocities.
One of the most important results of Dahm et al. [3] was the finding of a mutual interaction between the two shear layers. The authors found that the development of the inner shear layer was ‘locked’ into the development of the outer shear layer and is henceforth known as the ‘locking phenomenon’. Their flow visualisation study revealed that the vortices from the inner shear layer were trapped into the free spaces left in between the consecutive outer layer vortices.
Buresti et al. [1] showed that the each other, has a crucial role in the evolution of transitional coaxial jets. They found that for a certain range of ru, two trains of alternating vortices are shed from both sides of the inner wall with a frequency, which is related to the vortex shedding frequency.
In a recent study Talamelli and Gavarini [4] formulated a theoretical background for this experimental finding. They showed, by means of linear stability analysis, that the alternate vortex shedding behind
the inner wall can be related to the presence of an absolute instability, which exists for a specific range of velocity ratios and for a finite thickness of the wall separating the two streams. The authors proposed that this absolute instability may provide a continuous passive forcing mechanism for the destabilisation
of the whole flow field even if the instability is of limited spatial extend.
In the present abstract we show results from an experimental investigation, which aims to prove the possibility to use this mechanism to control the development of coaxial jets not only in the inner mixing region, but also in the outer one, where the annular jet interacts with the ambient fluid. The absence and
presence of the vortex shedding phenomenon behind the inner separating wall is hereby introduced by a sharp and thick wall geometry, respectively. It is evident from figures 1–2 that the ‘locking phenomenon’ is reversed, i.e. the outer shear layers vortices are trapped into the spaces left free by the inner ones and
their passage frequency collapses with that of the vortex shedding frequency (cf. left plot in fig. 3). To our knowledge this is the first time that the reverse mechanism of the ‘locking phenomenon’ was shown experimentally, confirming the results of the linear stability analysis of Talamelli and Gavarini [4] and underlining the importance of the geometry of the inner separating wall. The experiments also showed that the vortex shedding mechanism enhances the turbulence intensity both within the inner and outer
shear layer (cf. right plot in fig. 3). Further insight into this passive control mechanism and its effect on the turbulence statistics will be presented in the paper
New opportunities for detailed flow measurements at high Reynolds numbers
No full textHigh Reynolds number turbulence is ubiquitous in many flows of practical interest
and its understanding is crucial to draw conclusions on the physics of turbulence. Although recent
laboratory experiments, measurements in the planetary boundary layer and direct numerical
simulations provide a huge amount of information, none of these data sets provide high Reynolds
number, high spatial resolution and well converged statistics at the same time. As a response to this
problem, an international collaboration between a group of universities and research centers started
some years ago to build large scale infrastructures for high Reynolds number experiments. The
Center for International Cooperation in Long Pipe Experiments, CICLoPE (www.ciclope.unibo.it)
at the University of Bologna, was created for this purpose and will be open to international scientists
through different collaboration programs. The first facility, which will be ready in 2010, is
a large pipe flow experiment that will allow fully resolved turbulence measurements even at high
Reynolds number
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