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The velocity and mixing field of two coaxial jet configurations has been experimentally characterized by means of hot- and cold-wire anemometry to investigate the effect of the initial conditions on the flow development, and to determine the leading processes in the ``mixing transition'' of the two streams. Amongst the possible operating conditions, four configurations with different inner and outer jet bulk velocity pairs $(U_i,U_o)$ have been spatially characterized in terms of velocity and mixing field covering a region where $0.5le r_u=U_o/U_ile 3$ with two different separation wall geometries, namely a sharp and a thick separating wall. For high velocity ratios the difference between the two geometries appears small and inside the experimental accuracy. On the other hand, for nearly unitary velocity ratio and with the thick wall configuration, the presence of a strong wake instability increases the interpenetration between the two streams enhancing the mixing process.

Despite the rapid development of optical velocimetry methods (like LDV, PIV, etc.) the hot-wire anemometer remains the main instrument used in wind tunnel studies of turbulence. To obtain precise results close to walls in turbulent boundary layers, requires the user to have accurate procedures for a good calibration at low velocities, knowledge of effects of blockage and heat conduction to the wall, and how spatial resolution influences the results. We have carried out measurements in three different wind tunnels (at KTH, Univ. Melbourne and IIT) with various hot-wire probes (stubbed and stubless, as well as straight and boundary layer type) operated with commercially available and home-made anemometer systems. The use of different facilities enabled measurements at similar Reynolds numbers, but with different free stream velocities, resulting in a wide range of viscous scales for the hot-wire sensor lengths. The results indicate that poor spatial resolution influences the measured fluctuating velocity distribution well into the overlap region and clarifies controversial aspects regarding the scaling of the near-wall peak and the apparent existence of an outer peak in the rms distribution. The mean velocity within the buffer region has been found to be affected by probe geometry and size, an influence that is especially important when correcting for the absolute wall position by means of common correction methods.

A new approach to evaluate turbulence intensity and transverse Taylor microscale in turbulent flows is presented. The method is based on a correction scheme that compensates for probe resolution effects and is applied by combining the response of two single hot-wire sensors with different wire lengths. Even though the technique, when compared to other correction schemes, requires two independent measurements, it provides, for the same data, an estimate of the spanwise Taylor microscale. The method is here applied to streamwise turbulence intensity distributions of turbulent boundary layer flows but it is applicable generally in any turbulent flow. The technique has been firstly validated against spatially averaged DNS data of a zero pressure-gradient turbulent boundary layer showing a good capacity to reconstruct the actual profiles and to predict a qualitatively correct and quantitatively agreeing transverse Taylor microscale over the entire height of the boundary layer. Finally, the proposed method has been applied to available higher Reynolds number data from recent boundary layer experiments where an estimation of the turbulence intensity and of the Taylor microscale has been performed.

Spatial averaging, resulting from the finite size of a hot-wire probe, significantly affects the accuracy of velocity measurements in turbulent flows close to walls. Here, we extend the theoretical model, introduced in Segalini et al. (Meas Sci Technol 22:104508, 2011) quantifying the effect of a linear spatial filter of hot-wire probes on the mean and the variance of the streamwise velocity in turbulent wall-bounded flows, to describe the effect of the spatial filtering on the third- and fourth-order moments of the same velocity component. The model, based on the three-(four) point velocity-correlation function for the third-(fourth-) order moment, shows that the filtering can be related to a characteristic length scale which is an equivalent of the Taylor transverse microscale for the second-order moment. The capacity of the model to accurately describe the attenuation is validated against direct numerical simulation (DNS) data of a zero pressure-gradient turbulent boundary layer. The DNS data allow the filtering effect to be appraised for different wire lengths and for the different moments. The model shows good accuracy except for the third-order moment in the region where a zero-crossing of the third-order function is observed and where the equations become ill-conditioned. An “a posteriori” correction procedure, based on the developed model, to correct the measured third- and fourth-order velocity moments is also presented. This procedure, based on combining the measured data by two single hot-wire sensors with different wire lengths, is a natural extension of the one introduced by Segalini et al. (Exp Fluids 51:693–700, 2011) to evaluate both the turbulence intensity and the transverse Taylor microscale in turbulent flows. The technique is validated against spatially averaged simulation data showing a good capacity to correct the actual profiles over the entire height of the boundary layer except, as expected, for the third-order moment in the region where the latter exhibits a zero-crossing. Moreover, the proposed method has been tested on experimental data from turbulent pipe flow experiments.

Experiments are carried out to characterize the mixing field of two coaxial jet configurations, having, respectively, a thick and a sharp inner duct wall. The influence of the separation wall thickness on the initial development of the “mixing transition” of the two streams is analysed as a function of the jet velocity ratio through measurements with hot- and cold-wire anemometry and by using temperature as a passive scalar. To study the mixing at the near-exit region, a new thickness measure based on the mean scalar concentration is introduced. It is shown that the presence of a sufficiently thick wall significantly increases the interpenetration between the two streams, effectively enhancing the mixing process. However, this is observed only for nearly unitary velocity ratios, which correspond to the existence of a regular alternate vortex shedding from the two sides of the inner duct wall. Conversely, for small and large velocity ratios, the difference in mixing between the two geometries greatly decreases and becomes of the order of the experimental accuracy.

Coaxial 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.

For 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...