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The widespread use of available skin friction relations in engineering and in turbulence research is ample justification to examine their validities and limitations of use. The Prandtl–von Karman logarithmic friction law (Prandtl, 1932), for instance, is vitally important for decades for predicting wall skin friction. There have been, however, rising concerns regarding its accuracy due to the deficit of the flow similarity assumption adopted in the vicinity of the wall and in the core/outer region of the flow field. Hence, the wall skin friction could possibly be doubtful when predicted using the Prandtl–von Karman relation, in particular at high Reynolds numbers compared with values obtained from direct measurements. We therefore briefly review the recent advances in the logarithmic friction relation based on the latest pipe, channel, and boundary layer wall friction data over a wide range of Reynolds numbers. Common similarities and differences among those flows, particularly in terms of the mean flow and the wall skin friction data are highlighted. Recent pipe friction relations (Mckeon et al., 2005) are compared with the Prandtl–von Karman logarithmic friction law and verified with new accurate pipe friction data. A revisited logarithmic skin friction relation for plane-channel flows (Zanoun et al., 2009) is reported, predicting the wall friction with an accuracy of better than ± 1 . 44 % . A modified logarithmic skin friction relation for external flows introduced by Nagib et al. (2007) is presented and discussed. We believe that having a concise review and summary of the often used friction relations in wall-bounded shear flows in a single article will be of interest to fluid mechanics readers.

In light of recent data from hot-wire anemometry and laser Doppler velocimetry, this article explores experimentally the momentum balance and kinetic energy production in fully developed turbulent pipe flow for shear Reynolds numbers in the range from two pipe facilities. It has become common practice to indirectly deduce the Reynolds shear stress via the mean flow data and the mean-momentum balance whenever the simultaneous measurements of the streamwise and wall-normal velocity fluctuations can not be performed precisely. The current assessment underlines, however, the importance of measuring the Reynolds shear stress directly, and the friction velocity independently from the mean-velocity profile to ascertain the quality of the data when utilising the momentum balance. The present analysis also reinforces the universality of the viscous stress gradient to the Reynolds shear stress gradient in the wall vicinity up to the inner limit of the logarithmic layer. The new set of the experimental data shows that Panton's stress function reproduces the measured Reynolds shear stress and kinetic energy production in turbulent pipe flows over a wide Reynolds number range to a high degree.

This study presents findings from a first-of-its-kind measurement campaign that includes simultaneous measurements of the full velocity and vorticity vectors in both pipe and boundary layer flows under matched spatial resolution and Reynolds number conditions. Comparison of canonical turbulent flows offers insight into the role(s) played by features that are unique to one or the other. Pipe and zero pressure gradient boundary layer flows are often compared with the goal of elucidating the roles of geometry and a free boundary condition on turbulent wall flows. Prior experimental efforts towards this end have focused primarily on the streamwise component of velocity, while direct numerical simulations are at relatively low Reynolds numbers. In contrast, this study presents experimental measurements of all three components of both velocity and vorticity for friction Reynolds numbers$Re_{\unicode[STIX]{x1D70F}}$ranging from 5000 to 10 000. Differences in the two transverse Reynolds normal stresses are shown to exist throughout the log layer and wake layer at Reynolds numbers that exceed those of existing numerical data sets. The turbulence enstrophy profiles are also shown to exhibit differences spanning from the outer edge of the log layer to the outer flow boundary. Skewness and kurtosis profiles of the velocity and vorticity components imply the existence of a ‘quiescent core’ in pipe flow, as described by Kwonet al. (J. Fluid Mech., vol. 751, 2014, pp. 228–254) for channel flow at lower$Re_{\unicode[STIX]{x1D70F}}$, and characterize the extent of its influence in the pipe. Observed differences between statistical profiles of velocity and vorticity are then discussed in the context of a structural difference between free-stream intermittency in the boundary layer and ‘quiescent core’ intermittency in the pipe that is detectable to wall distances as small as 5 % of the layer thickness.

Hot-wire measurements of the streamwise velocity are conducted in the large-scale pipe-flow facility CICLoPE in the friction Reynolds number range of 7800 ⩽ R e τ 40000 . Measurements are performed both with a rake of five synchronised probes arranged at different radial locations, and through a radial scan with a single wire traversing the whole pipe radius. Correlation-based analysis is used to extract features of inter-scale modulation in turbulent pipe flows. The R e τ -independence of geometric features is shown with the outer scaling. Very-large-scale motions keep the vertical coherence to the wall through the whole pipe radius, while for the large-scale motions, the local coherence gradually become isotropic as the structure centre moves far away from the wall. From results obtained with the one-point amplitude modulation (AM) correlation function map, the AM effect is characterised by positive correlations observed in the inner region, while an opposite AM effect characterised by negative correlations is observed in the outer region. The strongest AM effect (with the maximum correlations) and the zero net-modulation (with zero correlations) show that the phase difference between large and small scales has a linear relation with the logarithm of the outer-scaled wall distance, but the strongest opposite effect (with the negative maximum correlations) behaves with the phase difference independent of the outer scaled wall-normal distance. Two-point AM coefficient maps, which give richer spatial information in the outer region than the one-point map, present the R e τ -independence for AM and opposite effects within the present R e τ range. In addition, the relative variation of the two effects in the coexisting wall-normal range is characterised by identifying the maximum gradient of the one-point AM coefficient and the peak-to-peak value of the two-point AM coefficient map.

Very-large-scale structures in pipe flows are characterized using an extended Proper Orthogonal Decomposition (POD)-based estimation. Synchronized non-time-resolved Particle Image Velocimetry (PIV) and time-resolved, multi-point hot-wire measurements are integrated for the estimation of turbulent structures in a pipe flow at friction Reynolds numbers of 9500 and 20000. This technique enhances the temporal resolution of PIV, thus providing a time-resolved description of the dynamics of the large-scale motions. The experiments are carried out in the CICLoPE facility. A novel criterion for the statistical characterization of the large-scale motions is introduced, based on the time-resolved dynamically-estimated POD time coefficients. It is shown that high-momentum events are less persistent than low-momentum events, and tend to occur closer to the wall. These differences are further enhanced with increasing Reynolds number.

The present investigation aims at finding out whether there are pipe vibrations in the higher Reynolds number range at the Long Pipe Facility at the CICLoPE facility and to quantify their amplitude and frequency. Since vibrations are natural to any wind-tunnel facility, similar vibration measurements have also been performed in an established high-quality wind-tunnel facility, viz. the Minimum Turbulence Level (MTL) wind tunnel at KTH Royal Institute of Technology, in order to provide reference data. Results affirm that the amplitudes observed in Willert et al. (J Fluid Mech 826, 2017) are most likely the result of an amplification due to the optical set-up that is attached to the window plug, rather than vibrations of the pipe structure.

Streamwise velocity and wall-shear stress are acquired simultaneously with a hot-wire and an array of azimuthal/spanwise-spaced skin friction sensors in large-scale pipe and boundary layer flow facilities at high Reynolds numbers. These allow for a correlation analysis on a per-scale basis between the velocity and reference skin friction signals to reveal which velocity-based turbulent motions are stochastically coherent with turbulent skin friction. In the logarithmic region, the wall-attached structures in both the pipe and boundary layers show evidence of self-similarity, and the range of scales over which the self-similarity is observed decreases with an increasing azimuthal/spanwise offset between the velocity and the reference skin friction signals. The present empirical observations support the existence of a self-similar range of wall-attached turbulence, which in turn are used to extend the model of Baarset al.(J. Fluid Mech., vol. 823, p. R2) to include the azimuthal/spanwise trends. Furthermore, the region where the self-similarity is observed correspond with the wall height where the mean momentum equation formally admits a self-similar invariant form, and simultaneously where the mean and variance profiles of the streamwise velocity exhibit logarithmic dependence. The experimental observations suggest that the self-similar wall-attached structures follow an aspect ratio of$7:1:1$in the streamwise, spanwise and wall-normal directions, respectively.

Experiments have been conducted to assess the sizes and energy fractions of structure in fully developed turbulent pipe flow regime in two pipe facilities, ColaPipe at BTU Cottbus-Senftenberg, and CICLoPE at University of Bologna, for shear Reynolds number in the range \(2.5\cdot {10^3}\le {\mathrm {Re_{\tau }}}\le {{3.7\cdot {10^4}}}\), utilizing a single hot-wire probe. Considerations are given to the spectra of the streamwise velocity fluctuations, and large scale motions and their energy contents from the pipe near-wall to centerline. The analysis of the velocity fluctuations revealed a Reynolds-number dependent inner peak at a fixed wall normal location, however, an outer peak seems not to appear that might be attributed either to low Reynolds number effect or not high enough spatial resolution of the hot-wire probe, motivating further study utilizing nanoscale probes. Sizes of the large scale, and very large scale structures were estimated to have wavelengths of 3R, and 20R at high Reynolds number, respectively. The fractional energy contents in wavelengths associated with the large scale motions at various wall normal locations showed maximum contribution to the turbulent kinetic energy near the outer limit of the logarithmic layer.

Direct numerical simulations of turbulent channel flows at friction Reynolds numbers (Re) of 550, 1000 and 1500 are used to analyse the turbulent production, transfer and dissipation mechanisms in the compound space of scales and wall distances by means of the Kolmogorov equation generalized to inhomogeneous anisotropic flows. Two distinct peaks of scale-energy source are identified. The first, stronger one, belongs to the near-wall cycle. Its location in the space of scales and physical space is found to scale in viscous units, while its intensity grows slowly with $\mathit{Re}$, indicating a near-wall modulation. The second source peak is found further away from the wall in the putative overlap layer, and it is separated from the near-wall source by a layer of significant scale-energy sink. The dynamics of the second outer source appears to be strongly dependent on the Reynolds number. The detailed scale-by-scale analysis of this source highlights well-defined features that are used to make the properties of the outer turbulent source independent of Reynolds number and wall distance by rescaling the problem. Overall, the present results suggest a strong connection of the observed outer scale-energy source with the presence of an outer region of turbulence production whose mechanisms are well separated from the near-wall region and whose statistical features agree with the hypothesis of an overlap layer dominated by attached eddies. Inner–outer interactions between the near-wall and outer source region in terms of scale-energy fluxes are also analysed. It is conjectured that the near-wall modulation of the statistics at increasing Reynolds number can be related to a confinement of the near-wall turbulence production due to the presence of increasingly large production scales in the outer scale-energy source region.

This paper reports on near-wall two-component–two-dimensional (2C–2D) particle image velocimetry (PIV) measurements of a turbulent pipe flow at shear Reynolds numbers up to $Re_{\unicode[STIX]{x1D70F}}=40\,000$ acquired in the CICLoPE facility of the University of Bologna. The 111.5 m long pipe of 900 mm diameter offers a well-established turbulent flow with viscous length scales ranging from $85~\unicode[STIX]{x03BC}\text{m}$ at $Re_{\unicode[STIX]{x1D70F}}=5000$ down to $11~\unicode[STIX]{x03BC}\text{m}$ at $Re_{\unicode[STIX]{x1D70F}}=40\,000$. These length scales can be resolved with a high-speed PIV camera at image magnification near unity. Statistically converged velocity profiles were determined using multiple sequences of up to 70 000 PIV recordings acquired at sampling rates of 100 Hz up to 10 kHz. Analysis of the velocity statistics shows a well-resolved inner peak of the streamwise velocity fluctuations that grows with increasing Reynolds number and an outer peak that develops and moves away from the inner peak with increasing Reynolds number.

The present work aims at investigating the very large-scale structures of turbulent pipe flow in CICLoPE at high Reynolds numbers. According to recent studies, some open questions remain to be answered to identify accurate sizes of these turbulent structures in pipe flow. The CICLoPE facility has been therefore utilized, providing an opportunity to approach high Reynolds number flows with high enough resolution in terms of the viscous length scale, allowing us to investigate the behavior of such turbulent structures. Meandering structures, usually referred as VLSM (very large-scale motions), have been identified with claimed extension up to 20R, where R is the pipe radius.

A new facility to study high Reynolds number wall bounded turbulent flow has been designed. It will be installed in the laboratory of Center for International Collaboration on Long Pipe Experiments ”CICLoPE” in Predappio (Italy). The facility consists of a large pipe, allowing to reach high Reynolds numbers, where all turbulent scales can be resolved with standard measurement techniques. The pipe operates with air at ambient conditions with a maximum speed of 60 m/s in order to avoid any compressibility effect. In order to maintain stable conditions over long period of time the pipe is part of a close loop circuit. The pipe will be located in a tunnel 60 m underground, thus ensuring very low level of external perturbations. The layout resembles an ordinary wind tunnel where the main difference is the long test section, which produces most of the friction losses. This requires the use of a multiple stage axial fan driven by two independent motors. Even though many of the various aerodynamic components are similar to those ordinary used in wind tunnel (corners, diffusers, turbulence manipulators, contraction, etc.) they have been designed aiming at obtaining a very good quality of the flow and minimizing the overall pressure losses.

One of the fundamental characteristics of turbulent flows is that the ratio of large to small scales increases drastically with increasing Reynolds number. In standard laboratory apparatus, the fine structure of turbulence becomes exceedingly small, as compared to the size of standard probes, leading to large experimental uncertainties. Therefore an accurate description of the mutual interactions becomes complicate and many questions remains unanswered. The Center for International Cooperation for Long Pipe Experiment (CICLOPE) was established to design a pipe-flow facility (Long Pipe) that, thanks to its large size, has the potential to eliminate these uncertainties. Here we present the final design of the Long Pipe, which is now complete and operative.

Preliminary results of a Large Eddy Simulation (LES) of rectangular cylinder performed with Open Foam are presented. This is the preliminary part of a longer research project aimed at systematically study the ability of Computational Fluid Dynamics (CFD) techniques in reproducing the flow around slender bodies with sharp edges at high Reynolds numbers. In spite of the simple geometry, the problem is influenced by a number of parameters which makes its correct solution difficult to be achieved. The LES approach presented here appears to be a good candidate for this purpose but further analysis must be performed. Indeed, we highlight the need to adopt a finer resolution in the spanwise direction in order to capture the very anisotropic turbulent dynamics. Furthermore, it emerges the need of Direct Numerical Simulation (DNS) data in order to shed light on the compound role played by the turbulence model, the grid resolution and the inlet conditions.

Hot-wire spatial-resolution effects are investigated in a round jet, using custom-made probes with Platinum wires of 5 and 2.5 \(\upmu \)m of diameter. Characteristic turbulent scales are varied by both moving the probes along the jet centerline (\(10

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.

We report flow visualisations and laser Doppler anemometry (LDA) velocity measurements in the near field of two swirling jets. The Reynolds number based on jet diameter and bulk velocity at the nozzle exit is 1.4 × 105. In the first jet, a small recirculation region is formed around the jet axis, while, in the second, the streamwise velocity remains positive and overshoots near the jet centre. In both cases, flow visualisations show that the vortex core of the jets is depleted of seeding particles. By using time-averaged distributions of the streamwise and tangential velocities measured at the nozzle outlet, the dynamics of the particles is simulated, by integrating their simplified equations of motion. The particles trajectory thus computed agrees well with that observed in the flow visualisations. Although the turbulence intensity is substantially different in the core of the two jets, its effect on the seeding concentration is localised near the edge of the core.

A laminar boundary layer has a relatively low skin-friction drag coefficient (c f ) with respect to a turbulent one, and for increasing Reynolds number the difference in c f rapidly increases, and the difference can easily amount to an order of magnitude in many industrial applications. This explains why there is a tremendous interest in being able to delay transition to turbulence, particularly by means of a passive mechanism, which has the advantage of accomplishing the control without adding any extra energy into the system. Moreover, a passive, control does not have to rely on typically complicated sensitive electronics in sensor-actuator systems. Within the AFRODITE project [3] we now present the first experimental results where we are able to show that miniature vortex generators (MVGs) are really coveted devices in obtaining transition delay.

Mathematical Methods.- Passive Scalar Diffusion as a Damped Wave.- Extremalizing Vector Fields as Guides Toward Understanding Properties of Turbulence.- Low-Wavenumber Forcing and Turbulent Energy Dissipation.- New Potential Symmetries for a Generalised Inhomogeneus Nonlinear Diffusion Equation.- DNS and New Scaling Laws of ZPG Turbulent Boundary Layer Flow.- Linear Instability of a Slowly Divergent Planar Jet.- On How Different Are Genuine and 'Passive' Turbulence.- Upper Bound on the Heat Transport in a Heated From Below Fluid Layer.- Scaling Laws and Intermittency.- Symmetries and Boundary Layer Profiles for Scalar Fields.- Energy and Dissipation Balances in Rotating Flows.- Turbulent Cascade with Intermittency in View of Fragmentation Universalities.- Observational Impact of Surrogacy on the Turbulent Energy Cascade.- Conditional Statistics of Velocity Increments in Fully Developped Turbulence.- A Simple Relation Between Longitudinal and Transverse Increments.- Intermittency Exponent in High-Reynolds Number Turbulence.- Modelling.- An Alternative Model for Turbulent Flow and Forced Convection.- Langevin Models of Turbulence.- Renormalized Perturbation Theory for Lagrangian Turbulence.- Modelling of the Pressure-Strain- and Diffusion-Term in Rotating Flows.- Predicting Probability for Stochastic Processes with Local Markov Property.- Stability of Turbulent Kolmogorov Flow.- Conditional Moment Closure Based on Two Conditioning Variables.- Stochastic Partial Differential Equations as a Tool for Solving PDF Equations.- Non-unique Self-similar Turbulent Boundary Layers in the Limit of Large Reynolds Number.- Experiments.- Two-Point-Correlations in a Zero Pressure Gradient Boundary Layer at Re? = 54600.- Measurements Over a Flat Plate With and Without Suction.- MHD Taylor-Couette Flow for Small Magnetic Prandtl Number and With Hall Effect.- Laser-Cantilever-Anemometer.- Heteroclinic Cycles of Type II in the (2,3) Interaction in the GEOFLOW-Experiment.- Experimental Visualization of Streamwise Streaks in the Boundary Layers of Rayleigh-Benard Convection.- Spatial Correlations in Turbulent Shear Flows.- Hot-Wire and PIV Measurements in a High-Reynolds Number Turbulent Boundary Layer.- Dynamics of Baroclinic Instabilities Using Methods of Nonlinear Time Series Analysis.- Fabrication and Characterization of Miniaturized Thermocouples for Measurements in Flows.- Temperature and Velocity Measurements in a Large-Scale Rayleigh-Benard Experiment.- Statistics and Scaling of the Velocity Field in Turbulent Thermal Convection.- Simulation (DNS and LES).- Nonlinear Stochastic Estimation: A Tool for Deriving Appropriate Wall Models for LES.- Generation of Mean Flows in Turbulent Convection.- Control of a Turbulent Separation Bubble by Periodic Excitation.- Stretching Rate of Passive Lines in Turbulence.- Numerical Study of Particle Motion in a Turbulent Ribbed Channel Flow.- A Fresh Approach to Large Eddy Simulation of Turbulence.- Statistical Analysis of Turbulent Natural Convection in Low Prandtl Number Fluids.- Computational Simulation of Transitional and Turbulent Shear Flows.- Direct Numerical Simulations of Turbulent Rayleigh-Benard Convection in Wide Cylinders.- A Projective Similarity/Eddy-Viscosity Model for Large-Eddy Simulation.- Passive Scalar Transport in Turbulent Supersonic Channel Flow.

There are no measurement techniques for turbulent flows capable of reaching the versatility of hot-wire probes and their frequency response. Nevertheless, the issue of their spatial resolution is still a matter of debate when it comes to high Reynolds number near-wall turbulence. Another, so far unattended, issue is the effect of temperature fluctuations - as they are, e.g. encountered in non-isothermal flows - on the low and higher-order moments in wall-bounded turbulent flows obtained through hot-wire anemometry. The present investigation is dedicated to document, understand, and ultimately correct these effects. For this purpose, the response of a hot-wire is simulated through the use of velocity and temperature data from a turbulent channel flow generated by means of direct numerical simulations. Results show that ignoring the effect of temperature fluctuations, caused by temperature gradients along the wall-normal direction, introduces - despite a local mean temperature compensation of the velocity reading - significant errors. The results serve as a note of caution for hot-wire measurements in wall-bounded turbulence, and also where temperature gradients are more prevalent, such as heat transfer measurements or high Mach number flows. A simple correction scheme involving only mean temperature quantities (besides the streamwise velocity information) is finally proposed that leads to a substantial bias error reduction. © 2014 Springer-Verlag Berlin Heidelberg.

Two coaxial jets which interact and mix belong to the class of flows which are of great interest from both the research and industrial point of view. They have a simple geometry but still the possibility to easily change various parameters makes them suitable for studies of different phenomena related to stability, mixing and turbulence. The definition of the variables able to control the flow dynamics has high applicative potential because coaxial jets are a prototype of many industrial burners but, on the other hand, they are simple enough to be experimentally characterized in laboratory. Due to the high number of parameters able to influence the flow evolution, a complete characterization of such flow is still missing and, therefore, motivates this paper. An experimental analysis of the dominant instabilities in the near field of two coaxial jets has been performed in order to determine the leading processes which start the transition from the initial laminar state, where the two streams are segregated, to the turbulent state. Different inner/outer jet velocity pairs (Ui,Uo) have been tested in order to investigate the effect of both velocity ratio ru = Uo/Ui and Reynolds number. Three main instabilities have emerged depending on the velocity ratio as can be appreciated from figure 1(a) where isocontours of the dominant frequencies have been plotted. For ru < 0.75, the coaxial jets dynamics are driven by the inner shear layer instability, in agreement with [1]. For ru > 1.6, the outer shear layer dominates the near field vortex dynamics [2, 3] while, for ru nearly unitary (i.e. where 0.75 < ru < 1.6), the vortex shedding behind the separating wall imposes its own dynamics [4]. The latter phenomenon is clearly shown in figure 1(b) where a flow visualization of the near field at ru = 1 is reported. Behind the inner separating wall there is a flow pattern composed by staggered vortices which influence the dynamics of the outer shear layer too. An improved scaling relationship for the Strouhal number StŒ tx is proposed in order to calculate the shedding frequency inside the intermediate velocity ratio region. This relationship takes into account Reynolds number as well as velocity ratio effects and leads to a constant Strouhal number value in the whole region where vortex shedding is observable, as showed in figure 2. The present paper confirms experimentally the theoretical results of [5], who proposed that the wake behind the separation wall between the two streams of two coaxial jets creates the conditions for an absolute instability. Evidences of the dominance of the vortex shedding phenomenon in the whole near field inside a certain range of velocity ratios will be presented in the paper. The characterization of this operating region is of utmost importance because the vortex shedding provides a continuous passive forcing mechanism for the control of the flow field, opposite to conventional strategies which use active methods focused to control the outer shear layer dynamics [3]. Therefore, these results allow many improvements in the industrial geometries where coaxial jets are employed by means of just a proper choice of the exit geometry.

High 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.

The flow field around sharp leading edges in subsonic, low Reynolds number flow is investigated experimentally. Separation occurs already at small angles of attack. Under certain conditions reattachment takes place and a separation bubble develops. As this influences the lift and drag of an airfoil decisively, the conditions leading to reattachment and the formation of a laminar boundary layer are of special interest. The influences of the Reynolds number, angle of attack and leading edge apex angle on the boundary layer properties are studied by means of hot-wire anemometry, flow visualisation techniques and measurements of the pressure distribution over the leading edge. For a small parameter range, laminar separation followed by laminar reattachment is observed.

A new facility to study wall bounded turbulent °ow has been designed and will be installed in the laboratory of Center for International Collaboration on Long Pipe Experiments "CICLoPE" in Predappio (Italy). The facility consists of a large pipe, allowing to reach high Reynolds numbers, where all turbulent scales can be resolved with standard measurement techniques. The pipe operates with air at ambient conditions and has a close loop circuit allowing to maintain stable conditions over long period of time. The pipe is located in a tunnel 60 m underground, thus ensuring very low level of external perturbations. Great care has been taken during the design phase to minimize all potential disturbances in order to ensure optimal measuring conditions along the whole pipe.

Measurements in the near field of a coaxial jet under unperturbed and controlled conditions are reported; the Reynolds number (defined with diameter and exit velocity of the inner-jet) is 5600, while the ratios of the velocities and of the diameters of the two jets are 1.8 and 2, respectively. The unperturbed flow, which issues in a nominally laminar state from the contraction nozzles, develops a shear-layer mode instability in the outer-shear layer of the outer-jet, followed by vortex pairing. A control perturbation, composed of sinusoidal acoustic waves at the frequency of the instability and its half, is applied to the flow. The control parameter is the phase difference between the two sinusoidal waves, and is initially varied in the entire range of phases. Then, two values of , providing the maximum attenuation and enhancement of the turbulence intensity, are chosen and, for these, detailed measurements in the axial and radial directions are performed. The data show that the control affects the mean and turbulent fields and is able to redistribute the kinetic energy between them. As a result, the potential core of the inner-jet can be shortened or lengthened. The turbulence intensity can also be modified, thus altering the ratio of the streamwise to the radial velocity components, and the ratio of the fundamental to the subharmonic spectral components.

Although the equations governing turbulent flow of fluids are well known, understanding the overwhelming richness of flow phenomena, especially in high Reynolds number turbulent flows, remains one of the grand challenges in physics and engineering. High Reynolds number turbulence is ubiquitous in aerospace engineering, ground transportation systems, flow machinery, energy production (from gas turbines to wind and water turbines), as well as in nature, e.g. various processes occurring in the planetary boundary layer. High Reynolds number turbulence is not easily obtained in the laboratory, since in order to have good spatial resolution for measurements, the size of the facility itself has to be large. In this paper, we discuss limitations of various existing facilities and propose a new facility that will allow good spatial resolution even at high Reynolds number. The work is carried out in the framework of the Center for International Cooperation in Long Pipe Experiments (CICLoPE), an international collaboration that many in the turbulence community have shown an interest to participate in.