1,720,961 research outputs found
Scale interaction in a mixing layer: The role of the large-scale gradients
No full textThe interaction between scales is investigated in a turbulent mixing layer. The large-scale amplitude modulation of the small scales already observed in other works depends on the crosswise location. Large-scale positive fluctuations correlate with a stronger activity of the small scales on the low speed-side of the mixing layer, and a reduced activity on the high speed-side. However, from physical considerations we would expect the scales to interact in a qualitatively similar way within the flow and across different turbulent flows. Therefore, instead of the large-scale fluctuations, the large-scale gradients modulation of the small scales has been additionally investigated
Streamwise fences for the reduction of trailing-edge noise in a NACA633018 airfoil
No full textStreamwise fences for the reduction of the trailing-edge noise are experimentally investigated on a NACA633018 airfoil. Interchangeable trailing-edge inserts with streamwise fences of different spacing and height are tested in an anechoic wind tunnel. Far-field trailing-edge noise was measured by an array of microphone and the airfoil drag was calculated from the wake profiles acquired by a wake rake. The transversal spacing between the fences has a much stronger impact on noise reduction than the fences height. A maximum noise reduction of 5-6dB is obtained from fences having a spacing of 2 mm, and it is achieved in the range of Strouhal numbers based on the chord of 15-40, equivalent to frequencies 1-3 kHz. When increasing the spacing between the fences from 2 mm to 4 mm, a different aeroacoustic behavior is observed, with a lower noise reduction at high frequencies, and a higher noise reduction and low frequencies. Increasing the angle of attack from α=0° to α=6° does not lead to any significant deterioration of the noise reduction performance. From a wake survey, the coefficient of drag was found to increase of only 6-7%when installing trailing-edge inserts with fences
Buoyancy-induced transitional flows around an enclosed horizontal cylinder: an experiment
No full textAn experiment is performed to detect the onset of time-dependent
flow regimes within a water-filled square-sectioned cavity containing a horizontal cylindrical heat source. Data are recorded on the cavity cross-section, where velocity fields and local temperature values are detected using a 2D PIV rig,
and thermocouples, respectively. Numerical predictions of the experimental runs are carried out, where the system is modelled as a 2D feature.
Results are provided for nine values of the leading parameter, the modified Rayleigh number Raq, ranging from 1.48x10^4 to 1.21 x10^5. The system is witnessed to undergo a transition from steady-state, laminar flow to unsteady oscillatory flow at Raq = 5.88x10^4.
Experimental data and numerical predictions are compared and cross-validated. The evolution of the flow throughout the bifurcation is described by velocity and temperature profile plots, 2D field visualizations and Fourier analysis of experimental time series. The influence of the flow regime on the overall heat transfer rate of the system is finally discussed
Amplitude and frequency modulation of the small scales in a jet
No full textThe present study is an experimental investigation of the relationship between the large- and small-scale motions in the far field of an air jet at high Reynolds number. In the first part of our investigation, the analysis is based on time series of hot-wire anemometry (HWA), which are converted into space series after applying the Taylor hypothesis. By using a spectral filter, two signals are constructed, one representative of the large-scale motions (2?T?L, where ?T is the Taylor length scale, and L is the integral length scale) and the other representative of the small-scale motions (1.5?5?, where ? is the Kolmogorov length scale). The small-scale signal is found to be modulated both in amplitude and in frequency by the energy-containing scales in an analogous way, both at the centreline and around the centreline. In particular, for positive fluctuations of the large-scale signal, the small-scale signal is locally stronger in amplitude (amplitude modulation), and it locally exhibits a higher number of local maxima and minima (frequency modulation). The extent of this modulation is quantified based on the strength of the large-scale fluctuations. The response of the small-scale motions to amplitude modulation can be considered instantaneous, being on the order of one Kolmogorov time scale. In the second part of our investigation we use long-range ?PIV to study the behaviour of the small-scale motions in relation to their position in either high-speed or low-speed regions of the flow. The spatially resolved velocity vector fields allow us to quantify amplitude modulation directly in physical space. From this direct estimation in physical space, amplitude modulation is only 25 % of the value measured from HWA. The remaining 75 % comes from the fixed spectral band filter used to obtain the large- and small-scale signals, which does not consider the local convection velocity. A very similar overestimation of amplitude modulation when quantified in the time-frame is also obtained analytically. Furthermore, the size of the structures of intense vorticity does not change significantly in relation to the large-scale velocity fluctuation, meaning that there is no significant spatial frequency modulation. The remaining amplitude modulation in space can be explained as a statistical coupling between the strength of the structures of vorticity and their preferential location inside large-scale high-velocity regions. Finally, the implications that the present findings have on amplitude and frequency modulation in turbulent boundary layers (TBLs) are discusse
Small-scale motions in turbulent boundary-free shear flows
No full textThe present work is an experimental and numerical investigation of the small-scale motions in turbulent free-shear flows. In the far-field turbulence of a jet at high Reynolds number (Re? = 350) hot-wire anemometry (HWA) is applied to measure time series of flow velocity. By filtering these time series, large- and small-scale velocity fluctuations are obtained. Both the amplitude and the frequency of the small-scale signals are locally stronger (weaker) for positive (negative) fluctuations of the large-scale signal, which is refered to as amplitude and frequency modulation. The local amplitude and frequency of the small-scale signals increase monotonically with the strength of the large-scale velocity fluctuations. The same flow is also investigated with long-range ?PIV (microscopic Particle Image Velocimetry). The measurement is validated against the HWA signals by comparison of the turbulence statistics. A validation based on the topological content is also performed. The coherent structures of vorticity and of intense dissipation are adequately resolved, and their characteristic size is assessed. It is found that the size of the vortical structures does not change significantly when conditioned on strongly-positive or strongly-negative large-scale velocity fluctuations. Using the PIV results the amplitude and frequency modulation observed from HWA signals is explained as an inhomogeneous distribution of the small-scale structures within the flow. In particular, the analysis of ?PIV data reveals that the intense vortical and dissipation structures tend to be preferentially located in high-velocity regions, hence they are characterized by convection velocities higher than the mean velocity of the flow. Furthermore, the spatially resolved velocity vector fields allow to quantify amplitude modulation directly in physical space. From this direct estimation in physical space, amplitude modulation is only 25% of the value measured from hot-wire anemometry. The remaining 75% comes from the fixed spectral band filter used to obtain the large- and the small-scale signals, which does not consider the local convection velocity (Taylor hypothesis of frozen turbulence). A very similar overestimation of amplitude modulation when quantified in the time-frame is also confirmed analytically. Based on the experimental analysis on the jet an explanation for amplitude and frequency modulation is developed, which can be extended to other free-shear flows. The validity of this interpretation is assessed based on the analysis of Direct Numerical Simulations of a mixing layer, at the Reynolds number based on the Taylor microscale (Re? =) of 250. The local vorticity rms, taken as a measure of the small-scale activity, is found to be modulated by the large-scale velocity fluctuations depending on the position within the flow. In particular, on the low-speed side of the mixing layer, positive large-scale velocity fluctuations correspond to a stronger vorticity rms, whereas on the high-speed side, they correspond to a weaker vorticity rms. This is consistent with previous studies on a mixing layer. Important differences are found in the strength of the scale interaction from time series and in physical space, consistent with the predictions developed from the analysis of the jet. On the high-speed side of the mixing layer, amplitude modulation from time series largely underestimates the value obtained from spatial series, and overestimates it on the low-speed side. Therefore, the interaction between large-scale velocity fluctuations and small scales is dependent on the flow position within the mixing layer, similar to a turbulent boundary layer. Nonetheless, when the vorticity rms is correlated with the large-scale shear velocity gradients, the correlation coefficient is found to be nearly constant throughout the mixing layer, and close to unity. This reveals that the large and the small scales present a strong interaction independent of the position when the large-scale shear velocity gradients are considered, instead of the large-scale velocity fluctuations, as in the existing literature on amplitude modulation. The strong correlation between the large-scale gradients and the small scales suggests to investigate possible evidence of the so called “scale invariance” (Meneveau and Katz 2000). The alignment between the local vorticity and the large-scale vorticity is examined within the vortical tubes. It is found that the vorticity from unfiltered (representing the small scales) and from low-pass-filtered velocity vector fields (representing the larger scales) tend to be aligned within the vortical tubes. This suggests that the direction of vorticity does not vary significantly across the scales. Therefore, the anisotropy of the large scales is partially preserved at the small-scale level, which is in contrast with the Kolmogorov’s hypothesis of local isotropy.Process and EnergyMechanical, Maritime and Materials Engineerin
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
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Streamwise fences for the reduction of trailing-edge noise in a NACA633018 airfoil
No full textStreamwise fences for the reduction of the trailing-edge noise are experimentally investigated on a NACA633018 airfoil. Interchangeable trailing-edge inserts with streamwise fences of different spacing and height are tested in an anechoic wind tunnel. Far-field trailing-edge noise was measured by an array of microphone and the airfoil drag was calculated from the wake profiles acquired by a wake rake. The transversal spacing between the fences has a much stronger impact on noise reduction than the fences height. A maximum noise reduction of 5-6dB is obtained from fences having a spacing of 2 mm, and it is achieved in the range of Strouhal numbers based on the chord of 15-40, equivalent to frequencies 1-3 kHz. When increasing the spacing between the fences from 2 mm to 4 mm, a different aeroacoustic behavior is observed, with a lower noise reduction at high frequencies, and a higher noise reduction and low frequencies. Increasing the angle of attack from α=0° to α=6° does not lead to any significant deterioration of the noise reduction performance. From a wake survey, the coefficient of drag was found to increase of only 6-7%when installing trailing-edge inserts with fences.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Wind EnergyReflection & LifestyleAircraft Noise and Climate Effect
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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