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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.

Flush-mounted cavity hot-wire probes have emerged as an alternative to classical hot-wire probes mounted several diameters above the surface for wall-shear stress measurements. They aim at increasing the frequency response and accuracy by circumventing the well-known issue of heat transfer to the substrate that hot-wire and hot-film probes possess. Their use, however, depends on the assumption that the cavity does not influence the flow field. In this study, we show that this assumption does not hold, and that turbulence statistics are modified by the presence of the cavity with sizes that are practically in use. The mean velocity and fluctuations increase near the cavity while the shear stress decreases in its surroundings, all seemingly stemming from the fact that the no-slip condition is not present anymore and that flow reversal occurs. Overall, the energy spectra and the probability density function of the wall shear stress fluctuations indicate a change of nature of turbulence by the presence of the cavity.

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.

This paper reports on the effects of a series of fluid-dynamic dielectric barrier discharge plasma actuators on a NACA0015 airfoil at high angle of attack. A set of jet actuators able to produce plasma jets with different directions (vectoring effect) and operated at different on/off duty cycle frequencies are used. The experiments are performed in a wind tunnel facility. The vectorized jet and the transient of the flow induced by unsteady duty cycle operation of each actuator are examined and the effectiveness of the actuator to recover stall condition in the range of Reynolds numbers between 1.0 × 105 and 5.0 × 105 (based on airfoil chord), is investigated. The actuator placed on the leading edge of the airfoil presents the most effective stall recovery. No significant effects can be observed for different orientations of the jet. An increase of the stall recovery is detected when the actuator is operated in unsteady operation mode. Moreover, the frequency of the on/off duty cycle that maximizes the stall recovery is found to be a function of the free stream velocity. This frequency seems to scale with the boundary layer thickness at the position of the actuator. A lift coefficient increase at low free stream velocities appears to linearly depend on the supply voltage.

A theoretical and experimental study has been undertaken, under ESA contract, by a team including GMV, the University of Bologna and EADS-Astrium, to assess the feasibility of an entry descent and landing (EDL) vehicle based on unpowered rotary decelerator. A deployable rotor with telescopic blades is considered, aimed at replacing all the standard deceleration devices for a descent module (parachutes, airbags, and retrorockets) except for the heat shield, with Mars as the main planetary target.This paper deals with the aerodynamic assessment of the proposed vehicle configuration. The objectives are twofold. First, a proof of concept of the main autorotation related events, viz. blades deployment from folded configuration in supersonic or transonic regime and telescopic blades extension in subsonic regime, is experimentally analyzed. Second, a characterization of the vehicle performances in terms of aerodynamic drag, lift to drag ratio and rotor angular speed is accomplished. Experiments are performed in the supersonic wind tunnel of Von Karman Institute of Fluid Dynamics and in the low speed wind tunnel of the University of Bologna. The vehicle is found to feature longitudinal static stability within the tested range of angles of attack. Multivalued rotor angular rates are found at subsonic speeds, corresponding to stalled and unstalled operation, with major differences in the achieved decelerating performances. Overall, the autorotation with telescopic blades is regarded as a viable concept for Mars EDL.

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.

Theory - "In memory of Prof. Rudolf Friedrich".- Stability.- Wall bounded flows.- Complex flows.- Acoustic.- Numerical methods.

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.

Miniature vortex generators (MVGs) are able to delay the transition to turbulence in a flat plate boundary layer if properly designed. Unfortunately, the natural recovery of the modulated laminar base flow in the streamwise direction is of exponential space scale and hence the passive laminar control fades away fairly rapidly. Here we show that by placing a second array of MVGs downstream of the first one it is possible to nourish the counter-rotating streamwise vortices responsible for the modulation, which results in a prolonged streamwise extent of the control. With this control strategy it is possible to delay the transition to turbulence, consecutively, by reinforcing the control effect and with the ultimate implication of obtaining a net skin-friction drag reduction of at least 65%. © 2014 American Physical Society.

This paper reports the first turbulence measurements performed in the Long Pipe Facility at the Center for International Cooperation in Long Pipe Experiments (CICLoPE). In particular, the Reynolds stress components obtained from a number of straight and boundary-layer-type single-wire and X-wire probes up to a friction Reynolds number of 3.8×10 4 are reported. In agreement with turbulent boundary-layer experiments as well as with results from the Superpipe, the present measurements show a clear logarithmic region in the streamwise variance profile, with a Townsend–Perry constant of A 2 ≈1.26. The wall-normal variance profile exhibits a Reynolds-number-independent plateau, while the spanwise component was found to obey a logarithmic scaling over a much wider wall-normal distance than the other two components, with a slope that is nearly half of that of the Townsend–Perry constant, i.e. A 2, w ≈ A 2 /2. The present results therefore provide strong support for the scaling of the Reynolds stress tensor based on the attached-eddy hypothesis. Intriguingly, the wall-normal and spanwise components exhibit higher amplitudes than in previous studies, and therefore call for follow-up studies in CICLoPE, as well as other large-scale facilities. This article is part of the themed issue ‘Toward the development of high-fidelity models of wall turbulence at large Reynolds number’.

The present work describes the multidimensional behaviour of wall-bounded turbulence in the space of cross-scales (spanwise and wall-normal) and distances from the wall. This approach allows us to understand the cascade mechanisms by which scale-energy is transmitted scale-by-scale away from the wall, through the overlap layer, and into the bulk flow. Two distinct cascades are identified involving the attached and detached scales of motion, respectively. From the near-wall region, scale-energy is transferred towards the bulk, flowing through the attached scales of motion, while among the detached scales it converges towards small scales, ascending again to the channel centre. It is then argued that the attached scales of wall-bounded turbulence are involved in a reverse cascade process that starts from the wall and ends in the bulk flow. On the other hand, the detached scales belong to a direct forward cascade process towards dissipation. Hence, at a given distance from the wall the attached motion is fed by smaller attached scales located closer to the wall. In turn this attached motion is responsible for creating the scale-energy that sustains larger attached scales farther from the wall and smaller detached scales that are responsible for connecting the scale-energy at large scales to the dissipation at small scales through a forward cascade. © Published under licence by IOP Publishing Ltd.

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.

The effect of temperature fluctuations—as they are e.g. encountered in non-isothermal flows—on the mean, variance and spectra in wall-bounded turbulent flows when utilising hot-wire anemometry has recently been documented. The present work extends these efforts on the effect of temperature fluctuations on the skewness and flatness factors as well as assesses the performance of commonly employed temperature compensations.

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 study on the generation and development of high-amplitude steady streamwise streaks in a flat-plate boundary layer is presented. High-amplitude streamwise streaks are naturally present in many bypass transition scenarios, where they play a fundamental role in the breakdown to turbulence process. On the other hand, recent experiments and numerical simulations have shown that stable laminar streamwise streaks of alternating low and high speed are also capable of stabilizing the growth of Tollmien–Schlichting waves as well as localized disturbances and to delay transition. The larger the streak amplitude is, for a prescribed spanwise periodicity of the streaks, the stronger is the stabilizing mechanism. Previous experiments have shown that streaks of amplitudes up to 12 % of the free stream velocity can be generated by means of cylindrical roughness elements. Here we explore the possibility of generating streaks of much larger amplitude by using a row of miniature vortex generators (MVGs) similar to those used in the past to delay or even prevent boundary layer separation. In particular, we present a boundary layer experiment where streak amplitudes exceeding 30 % have been produced without having any secondary instability acting on them. Furthermore, the associated drag with the streaky base flow is quantified, and it is demonstrated that the streaks can be reinforced by placing a second array of MVGs downstream of the first one. In this way it is possible to make the control more persistent in the downstream direction. It must be pointed out that the use of MVGs opens also the possibility to set up a control method that acts twofold in the sense that both transition and separation are delayed or even prevented.

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.

The laminar breakdown of a three-dimensional flat-plate boundary-layer flow with favourable and ensuing adverse pressure gradient (APG), generic for an infinite swept wing, is investigated in detail by means of spatial direct numerical simulations. Emphasis is on transition mechanisms in the region of adverse pressure gradient where the undistorted laminar base flow also becomes unstable with respect to Tollmien– Schlichting (TS) waves that are most amplified if two-dimensional with respect to the local streamwise direction. The influence of finite-amplitude crossflow vortices, coming from the region of favourable pressure gradient, on the TS instability properties is investigated. It turns out that crossflow-vortex-induced secondary instabilities are the most amplified disturbances even for low-amplitude vortex modes. The TS waves act as generators of fundamental low-frequency secondary modes, but are neither important for their growth nor for breakdown. As for active disturbance control, any method aiming at attenuating two-dimensional TS waves must fail. On the other hand, the passive upstream-flow-deformation technique also delays transition in the APG region.

An experimental and theoretical investigation aimed at describing the nonmodal growth of steady and spanwise periodic streamwise streaks in a flat plate boundary layer is presented. Stable laminar streaks are experimentally generated by means of a spanwise periodic array of small cylindrical roughness elements fixed on the plate. The streamwise evolution of the streaks is measured and it is proved that, except in a small region near the roughness elements, they obey the boundary layer scalings. The maximum achievable amplitude is mainly determined by the relative height of the roughness elements. Results are compared with numerical simulations of optimal and suboptimal boundary layer streaks. The theory is able to elucidate some of the discrepancies recently noticed between experimentally realizable nonmodal growth and optimal perturbation theory. The key factor is found to be the wall normal location and the extension of the laminar standing streamwise vortices inducing the streaks. The differences among previous experimental works can be explained by different dominating streak generation mechanisms which can be linked to the geometry and to the ratio between the roughness height and the boundary layer scale.

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.

This work reports an experimental investigation performed in wind tunnel facility on a NACA0015 airfoil equipped with a set of vectorizable fluid-dynamic DBD plasma actuators. The ability of the actuators to recover the stall condition with free stream velocities in the range 5-23 m/s has been investigated. The discharge has been generated by feeding the actuators with a sinusoidal voltage waveform with a frequency of 15 kHz and voltages up to 7.5 kV peak. The actuator positioned on the leading edge exhibits highest ability in the stall recovery. Unsteady actuation obtained by alternatively switching on and off the discharge is more effective in the stall recovery when compared with a steady actuation. The frequency that optimizes the stall recovery has been found to be a function of the velocity with the power of 1.5. This result leads to the determination of a constant Strouhal number if the boundary layer thickness is used as characteristic length. Finally the lift coefficient, obtained when the plasma actuator has been turned on, has been found to be a linear function of the applied voltage.

A new approach for the study of the overlap layer of wallturbulence is proposed. The multi-dimensional description of turbulence given by the Kolmogorov equation generalized to wall-flows is used and shown relevant for the identification of very robust features of the overlap layer. Numerical data of a turbulent channel are used. Despite the low Reynolds, a well-defined outer energy source region appears. The possibility to analyze its effects on the energy tranfer among scales and wall-distances is shown important to clarify the influences of the external region on the inner layer.

The response of hot-wire X-probes in regions of strong velocity gradients (such as in the near wall region of boundary layer flows) is investigated experimentally. Although the wall-normal velocity component should be close to zero near the wall, one usually encounters an increased absolute value of this component when the wall is approached. Moreover some physically inconsistent behaviour in other measured quantities, for instance the Reynolds stresses, may be found. These effects can be due to a physical displacement of the wires (e.g. a wall-normal displacement of the two wires so that they do not cross at their mid-points), but also due to the influence of the probe on the local flow field. The latter might be an effect of blockage or wall interference and can be treated as a virtual displacement. The response equations of an X-probe with different wall-normal displacement of the wires are derived and applied in order to correct the measured data. A systematic experimental investigation of the effect of varying the physical displacement of the wires is also made, and it is shown that both the first and second order correction terms of the probe response equations can be estimated from this experiment. A correction procedure for measurements close to the wall is proposed and used to correct Reynolds stress profiles in a flat plate boundary-layer. It is also shown that the present experimental set-up can be used to estimate some turbulence correlations which otherwise are unaccessible with standard measurement techniques.

The flow field of a coaxial jet configuration having inner to outer diameter ratio Di/Do≅0.5 is studied for two values of the velocity ratio, Ui/Uo=0.30 and Ui/Uo=0.67, both with a 5 mm thick and with a sharp inner duct wall. LDA and hot-wire measurements are used to obtain the statistical moments up to the fourth order and the time histories of the axial and radial velocity components. As expected, the inner jet core length is seen to decrease with decreasing velocity ratio, and for Ui/Uo=0.30 it is comparable to that of the outer jet, a condition that gives rise to large fluctuations and to a high level of mixing between the two streams. The sharpening of the inner duct wall produces a reduction in the radial fluctuations and Reynolds stresses in the near field, particularly for the case Ui/Uo=0.67, at which a regular vortex shedding occurs with the 5 mm thick duct; however, the differences in the velocity moments between the two configurations rapidly decrease, and become negligible after the end of both potential cores. The prevailing frequencies of the fluctuating flow field, estimated from wavelet spectra of the time histories of both velocity components, decrease regularly moving downstream from the jet outlet, and seem to confirm the dominance of the stronger vortices of the outer shear layer. Finally, the trends of the higher order moments may be interpreted in terms of plausible physical schemes of the mixing processes of the developing shear layers.

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.

Spanwise arrays of miniature vortex generators (MVGs) are used to generate energetic transient disturbance growth, which is able to modulate the boundary layer flow with steady and stable streak amplitudes up to 32 % of the free-stream velocity. This type of modulation has previously been shown to act in a stabilizing manner on modal disturbance growth described by classical instability theory. In an attempt to reproduce a more realistic flow configuration, in the present experimental set-up, Tollmien–Schlichting (TS) waves are generated upstream of the MVG array, allowing for a complete interaction of the incoming wave with the array. Fifteen new MVG configurations are investigated and the stabilizing effect on the TS waves is quantified. We show that the streak amplitude definition is very important when trying to relate it to the stabilization, since it may completely bypass information on the mean streamwise velocity gradient in the spanwise direction, which is an essential ingredient of the observed stabilization. Here, we use an integral-based streak amplitude definition along with a streak amplitude scaling relation based on empiricism, which takes the spanwise periodicity of the streaks into account. The results show that, applying the integral definition, the optimal streak amplitude for attenuating TS wave disturbance growth is around 30 % of the free-stream velocity, which corresponds to ${\sim }20\hspace{0.167em} \% $ in the conventional definition when keeping the spanwise wavelength constant. The experiments also show that the disturbance energy level, based on the full velocity signal, is significantly reduced in the controlled case, and that the onset of transition may be inhibited altogether throughout the measured region in the presence of an MVG array.

Classical vortex generators, known for their efficiency in delaying or even inhibiting boundary layer separation, are here shown to be coveted devices for transition to turbulence delay. The present devices are miniature with respect to classical vortex generators but are tremendously powerful in modulating the laminar boundary layer in the direction orthogonal to the base flow and parallel to the surface. The modulation generates an additional term in the perturbation energy equation, which counteracts the wall-normal production term and, hence, stabilizes the flow. Our experimental results show that these devices are really effective in delaying transition, but we also reveal their Achilles' heel.

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.

Basic turbulence aspects.- Particle laden flows - Lagrangian turbulence.- Modeling and simulation.- Experimental methods.- Special flows. Atmospheric boundary layer.- Boundary layer.- Wind energy.- Convection.