Authors:Felix J. Förster; Cyril Crua; Martin Davy; Paul Ewart Abstract: Thermometry using laser-induced grating spectroscopy (LIGS) is reported using a high-repetition rate laser system, extending the technique to allow time-resolved measurements of gas dynamics. LIGS signals were generated using the second harmonic output at 532 nm of a commercially available high-repetition rate Nd:YAG laser with nitrogen dioxide as molecular seed. Measurements at rates up to 10 kHz were demonstrated under static cell conditions. Transient temperature changes of the same gas contained in a cell subjected to rapid compression by injection of gas were recorded at 1 kHz to derive the temperature evolution of the compressed gas showing temperature changes of 50 K on a time-scale of 0.1 s with a measurement precision of 1.4%. The data showed good agreement with an analytical thermodynamic model of the compression process. PubDate: 2017-06-07 DOI: 10.1007/s00348-017-2370-6 Issue No:Vol. 58, No. 7 (2017)

Authors:Chuangxin He; Yingzheng Liu Abstract: A proper orthogonal decomposition (POD)-based spatial refinement approach is proposed to increase the resolution of time-resolved particle image velocimetry (TR-PIV) realizations. POD analysis of the high-resolution non-TR-PIV measurements is used to construct the high-resolution POD modes, and the TR-PIV instantaneous realization with relatively low spatial resolution is cross-projected onto the POD modes to obtain the time-varying mode coefficients. Subsequently, the high-resolution time-resolved flow fields, which inherit the information of the superimposed multi-scale structures from the non-TR-PIV measurements, are reconstructed using the linear combination of the cross-projected mode coefficients and the corresponding high-resolution POD modes. Two main sources of error in the novel strategy are discussed: the cross-projected reconstruction of the instantaneous fields, which are excluded in the determination of the high-resolution POD modes, and the interpolation that maps the high-resolution POD modes onto the coarse grid in the cross-projection process when estimating the mode coefficients. To evaluate the method, the flow fields of a free round jet at Reynolds number \(Re\) = 3000 are separately measured in the same field-of-view region using non-TR-PIV and TR-PIV setups with a spatial resolution ratio of 42:1. The high- and low-resolution realizations of the instantaneous flow fields measured by the non-TR-PIV setup are determined by applying fine (32 × 32 pixels) and coarse (128 × 128 pixels) interrogation windows, respectively, to ensure accurate fine-to-coarse mapping of the POD modes and for use as a reference to assess the refinement accuracy. This evaluation shows that the combination of these two error sources can be minimized when the POD-sample size (the number of snapshots employed in the decomposition) and the number of POD modes for refinement are optimized. Finally, application of the present method to TR-PIV data shows that the high-resolution time-resolved fields are successfully achieved with the temporal pattern similar to the original TR-PIV realizations, while much smaller structures are captured by the spatially refined flow. PubDate: 2017-06-07 DOI: 10.1007/s00348-017-2371-5 Issue No:Vol. 58, No. 7 (2017)

Authors:Henning M. Lang; Kilian Oberleithner; C. Oliver Paschereit; Moritz Sieber Abstract: This work investigates the potential of background-oriented schlieren tomography (3D-BOS) for the temperature field reconstruction in a non-isothermal swirling jet undergoing vortex breakdown. The evaluation includes a quantitative comparison of the mean and phase-averaged temperature field with thermocouple and fast-response resistance thermometer as well as a qualitative comparison between the temperature field and the flow field obtained from particle image velocimetry (PIV). Compared to other temperature-measuring techniques, 3D-BOS enables non-invasive capturing of the entire three-dimensional temperature field. In contrast to previous 3D-BOS applications, the present investigation makes use of the special character of the flow, which provides a global instability that leads to a rotational symmetry of the jet. Additionally, the rotational motion of the jet is used to obtain a tomographic reconstruction from a single camera. The quality of 3D-BOS results with respect to the physical setup as well as the numerical procedure is analyzed and discussed. Furthermore, a new approach for the treatment of thin occluding objects in the field of view is presented. PubDate: 2017-06-07 DOI: 10.1007/s00348-017-2367-1 Issue No:Vol. 58, No. 7 (2017)

Authors:Paul Salipante; Steven D. Hudson; James W. Schmidt; John D. Wright Abstract: The accurate measurement of flows in microfluidic channels is important for commercial and research applications. We compare the accuracy of flow measurement techniques over a wide range flows. Flow measurements made using holographic microparticle tracking velocimetry (µPTV) and a gravimetric flow standard over the range of 0.5–100 nL/s agree within 0.25%, well within the uncertainty of the two flow systems. Two commercial thermal flow sensors were used as the intermediaries (transfer standards) between the two flow measurement systems. The gravimetric flow standard was used to calibrate the thermal flow sensors by measuring the rate of change of the mass of liquid in a beaker on a micro-balance as it fills. The holographic µPTV flow measurements were made in a rectangular channel and the flow was seeded with 1 µm diameter polystyrene spheres. The volumetric flow was calculated using the Hagen–Pouiseille solution for a rectangular channel. The uncertainty of both flow measurement systems is given. For the gravimetric standard, relative uncertainty increased for decreasing flows due to surface tension forces between the pipette carrying the flow and the free surface of the liquid in the beaker. The uncertainty of the holographic µPTV measurements did not vary significantly over the measured flow range, and thus comparatively are especially useful at low flow velocities. PubDate: 2017-06-07 DOI: 10.1007/s00348-017-2362-6 Issue No:Vol. 58, No. 7 (2017)

Authors:W. Terra; A. Sciacchitano; F. Scarano Abstract: A method is introduced to measure the aerodynamic drag of moving objects such as ground vehicles or athletes in speed sports. Experiments are conducted as proof-of-concept that yield the aerodynamic drag of a sphere towed through a square duct in stagnant air. The drag force is evaluated using large-scale tomographic PIV and invoking the time-average momentum equation within a control volume in a frame of reference moving with the object. The sphere with 0.1 m diameter moves at a velocity of 1.45 m/s, corresponding to a Reynolds number of 10,000. The measurements in the wake of the sphere are conducted at a rate of 500 Hz within a thin volume of approximately 3 × 40 × 40 cubic centimeters. Neutrally buoyant helium-filled soap bubbles are used as flow tracers. The terms composing the drag are related to the flow momentum, the pressure and the velocity fluctuations and they are separately evaluated. The momentum and pressure terms dominate the momentum budget in the near wake up to 1.3 diameters downstream of the model. The pressure term decays rapidly and vanishes within 5 diameters. The term due to velocity fluctuations contributes up to 10% to the drag. The measurements yield a relatively constant value of the drag coefficient starting from 2 diameters downstream of the sphere. At 7 diameters the measurement interval terminates due to the finite length of the duct. Error sources that need to be accounted for are the sphere support wake and blockage effects. The above findings can provide practical criteria for the drag evaluation of generic bluff objects with this measurement technique. PubDate: 2017-06-07 DOI: 10.1007/s00348-017-2331-0 Issue No:Vol. 58, No. 7 (2017)

Authors:Cheng Yue Wang; Qi Gao; Run Jie Wei; Tian Li; Jin Jun Wang Abstract: Reconstructing pressure fields from 2D or 3D velocimetry data involves a pressure gradient integration procedure. This paper proposes a spectral decomposition-based fast pressure integration (SD-FPI) algorithm to integrate a gridded pressure gradient field. The algorithm seeks the least-square solution for the discrete momentum conservation equation by matrix decompositions, which allows for the use of various difference schemes. The recently proposed fast Fourier transform (FFT) integration method (Huhn et al. Exp Fluids 57:151, 2016) could be viewed as a special example of SD-FPI when adopting a special circulant difference scheme. The inherent relationship between SD-FPI and the Poisson reconstruction is also revealed in theory. An iterative strategy for SD-FPI is developed to integrate the pressure gradient fields with missing data. The accuracy and efficiency of SD-FPI with various difference schemes including the FFT-based approaches are compared based on a synthetic pressure field. We conclude that while the computing efficiency is always high, the accuracy of SD-FPI depends on the difference scheme and error types of the pressure gradients. PubDate: 2017-06-07 DOI: 10.1007/s00348-017-2368-0 Issue No:Vol. 58, No. 7 (2017)

Authors:T. Van Buren; A. J. Smits; M. Amitay Abstract: The flow field generated by suction through a rectangular orifice within a laminar boundary layer is investigated using stereoscopic particle image velocimetry. For orifice aspect ratios of 6, 12, and 18, the impact of suction on the surrounding flow field appears to be self-similar, scaling with aspect ratio and suction velocity. Changing the orifice pitch angle had almost no impact on the surrounding boundary layer, but, as expected, changing the skew angle significantly altered the extent of the suction impact on the flow field. PubDate: 2017-06-05 DOI: 10.1007/s00348-017-2359-1 Issue No:Vol. 58, No. 7 (2017)

Authors:Florian Zentgraf; Michael Stephan; Edouard Berrocal; Barbara Albert; Benjamin Böhm; Andreas Dreizler Abstract: Structured laser illumination planar imaging (SLIPI) is combined with gas phase thermometry measurements using thermographic phosphor (TGP) particles. The technique is applied to a heated jet surrounded by a coflow which is operated at ambient temperature. The respective air flows are seeded with a powder of BaMgAl10O17:Eu2+ (BAM) which is used as temperature-sensitive gas phase tracer. Upon pulsed excitation in the ultraviolet spectral range, the temperature is extracted based on the two-color ratio method combined with SLIPI. The main advantage of applying the SLIPI approach to phosphor thermometry is the reduction of particle-to-particle multiple light scattering and diffuse wall reflections, yielding a more robust calibration procedure as well as improving the measurement accuracy, precision, and sensitivity. For demonstration, this paper focuses on sample-averaged measurements of temperature fields in a jet-in-coflow configuration. Using the conventional approach, which in contrast to SLIPI is based on imaging with an unmodulated laser light sheet, we show that for the present setup typically ~40% of the recorded signal is affected by the contribution of multiply scattered photons. At locations close to walls even up to 75% of the apparent signal is due to diffuse reflection and wall luminescence of BAM sticking at the surface. Those contributions lead to erroneous temperature fields. Using SLIPI, an unbiased two-color ratio field is recovered allowing for two-dimensional mean temperature reconstructions which exhibit a more realistic physical behavior. This is in contrast to results deduced by the conventional approach. Furthermore, using the SLIPI approach it is shown that the temperature sensitivity is enhanced by a factor of up to ~2 at ~270 °C. Finally, an outlook towards instantaneous SLIPI phosphorescence thermometry is provided. PubDate: 2017-06-05 DOI: 10.1007/s00348-017-2364-4 Issue No:Vol. 58, No. 7 (2017)

Authors:Shengxian Shi; Junfei Ding; T. H. New; Julio Soria Abstract: This paper presents a dense ray tracing reconstruction technique for a single light-field camera-based particle image velocimetry. The new approach pre-determines the location of a particle through inverse dense ray tracing and reconstructs the voxel value using multiplicative algebraic reconstruction technique (MART). Simulation studies were undertaken to identify the effects of iteration number, relaxation factor, particle density, voxel–pixel ratio and the effect of the velocity gradient on the performance of the proposed dense ray tracing-based MART method (DRT-MART). The results demonstrate that the DRT-MART method achieves higher reconstruction resolution at significantly better computational efficiency than the MART method (4–50 times faster). Both DRT-MART and MART approaches were applied to measure the velocity field of a low speed jet flow which revealed that for the same computational cost, the DRT-MART method accurately resolves the jet velocity field with improved precision, especially for the velocity component along the depth direction. PubDate: 2017-06-05 DOI: 10.1007/s00348-017-2365-3 Issue No:Vol. 58, No. 7 (2017)

Authors:F. Tuerke; L. R. Pastur; D. Sciamarella; F. Lusseyran; G. Artana Abstract: The flow through two facing, identical cavities (double-cavity) is characterized experimentally, as the inflow velocity and the distance between the cavities is varied. Standard 2D2C particle image velocimetry measurements in the spanwise mid-plane provide information on the instantaneous and mean velocity flow fields. Laser Doppler velocimetry measurements at several points in the double-cavity domain reveal the global character of the streamwise fluctuating velocity spectra. The flow is characterized based on time series, recorded in the shear layer of one of the cavities, for a wide range of inflow velocities and intercavity distances. In a detailed spectral study, we show how the shear layer spectra get affected when the two cavities are brought closer together. Based on the experimental data, a temporal local linear stability analysis was carried out, which was able to explain why the frequency peaks for close intercavity distances broaden and move to higher Strouhal numbers. PubDate: 2017-06-05 DOI: 10.1007/s00348-017-2360-8 Issue No:Vol. 58, No. 7 (2017)

Authors:Nathaniel T. Baker; Alban Pothérat; Laurent Davoust; François Debray; Rico Klein Abstract: This paper introduces an experimental apparatus, which drives turbulence electrically in a liquid metal pervaded by a high magnetic field. Unlike past magnetohydrodynamic setups involving a shallow confinement, the experiment presented here drives turbulence whose dimensionality can be set anywhere between three-dimensional and quasi two-dimensional. In particular, we show that the dimensionality and componentality of the turbulence thus generated are in fact completely fixed by the single parameter \(l_z(l_i) / h\) , which quantifies the competition between the solenoidal component of the Lorentz force and inertia acting on a turbulent structure of the size of the forcing scale \(l_i\) . This parameter is fully tunable thanks to the three operating settings at hand: the injection scale, the intensity of the electric forcing and the magnitude of the magnetic field. Thanks to the very high number of measuring probes and fast acquisition rate implemented in this experiment, it is possible to reliably measure the finest features of the inertial range on a scale-wise basis. PubDate: 2017-06-05 DOI: 10.1007/s00348-017-2363-5 Issue No:Vol. 58, No. 7 (2017)

Authors:Chunxiao Tang; Wenfei Sun; Hayi He; Hongqiang Li; Enbang Li Abstract: Spurious vectors (also called “outliers”) in particle image velocimetry (PIV) experiments can be classified into two categories according to their space distribution characteristics: scattered and clustered outliers. Most of the currently used validation and correction methods treat these two kinds of outliers together without discrimination. In this paper, we propose a new technique based on a penalized least-squares (PLS) method, which allows automatic classification of flows with different types of outliers. PIV vector fields containing scattered outliers are detected and corrected using higher-order differentials, while lower-order differentials are used for the flows with clustered outliers. The order of differentials is determined adaptively by generalized cross-validation and outlier classification. A simple calculation method of eigenvalues of different orders is also developed to expedite computation speed. The performance of the proposed method is demonstrated with four different velocity fields, and the results show that it works better than conventional methods, especially when the number of outliers is large. PubDate: 2017-06-05 DOI: 10.1007/s00348-017-2350-x Issue No:Vol. 58, No. 7 (2017)

Authors:Sebastian Bürkle; Lukas G. Becker; Maria Angela Agizza; Andreas Dreizler; Volker Ebert; Steven Wagner Abstract: For improving the design of combustors, the knowledge of residence-time distributions (RTD) is important as they influence exhaust gas compositions. Measuring RTDs in combustors is challenging, due to high temperatures, chemical reactions, the presence of particles or corrosive species as well as high turbulence levels. This paper presents a technique for the in situ measurement of RTDs in combustors. Based on tunable diode laser absorption spectroscopy (TDLAS), the temporal evolution of the concentration of tracers is tracked simultaneously at the combustion chamber inlet and outlet. Using either air or mixtures of oxygen and carbon dioxide (oxy-fuel atmosphere) as oxidants, the method is applied to reacting and non-reacting operating conditions in a 20-kWth methane combustor. For reacting conditions, hydrogen chloride is used as a tracer, whereas for non-reacting conditions methane was chosen. Depending on the tracer, for a repetition rate of approximately 2 kHz detection limits of 16–40 ppmV are achieved. For deriving RTDs, low-pass filtering is compared to reactor network modeling. Different RTDs observed for varying operating conditions are discussed. PubDate: 2017-06-05 DOI: 10.1007/s00348-017-2366-2 Issue No:Vol. 58, No. 7 (2017)

Authors:Alessandro Masullo; Raf Theunissen Abstract: The measurement of displacements near the vicinity of surfaces involves advanced PIV algorithms requiring accurate knowledge of object boundaries. These data typically come in the form of a logical mask, generated manually or through automatic algorithms. The automatic detection of masks usually necessitates special features or reference points such as bright lines, high contrast objects, and sufficiently observable coherence between pixels. These are, however, not always present in experimental images necessitating a more robust and general approach. In this work, the authors propose a novel method for the automatic detection of static image regions which do not contain relevant information for the estimation of particle image displacements and can consequently be excluded or masked out. The method does not require any a priori knowledge of the static objects (i.e., contrast, brightness, or strong features) as it exploits statistical information from multiple PIV images. Based on the observation that the temporal variation in light intensity follows a completely different distribution for flow regions and object regions, the method utilizes a normality test and an automatic thresholding method on the retrieved probability to identify regions to be masked. The method is assessed through a Monte Carlo simulation with synthetic images and its performance under realistic imaging conditions is proven based on three experimental test cases. PubDate: 2017-05-23 DOI: 10.1007/s00348-017-2357-3 Issue No:Vol. 58, No. 6 (2017)

Authors:Laura Haya; Stavros Tavoularis Abstract: Flow characteristics past a bileaflet mechanical heart valve were measured under physiological flow conditions in a straight tube with an axisymmetric expansion, similar to vessels used in previous studies, and in an anatomical model of the aorta. We found that anatomical features, including the three-lobed sinus and the aorta’s curvature affected significantly the flow characteristics. The turbulent and viscous stresses were presented and discussed as indicators for potential blood damage and thrombosis. Both types of stresses, averaged over the two axial measurement planes, were significantly lower in the anatomical model than in the axisymmetric one. This difference was attributed to the lower height-to-width ratio and more gradual contraction of the anatomical aortic sinus. The curvature of the aorta caused asymmetries in the velocity and stress distributions during forward flow. Secondary flows resulting from the aorta’s curvature are thought to have redistributed the fluid stresses transversely, resulting in a more homogeneous stress distribution in the anatomical aortic root than in the axisymmetric root. The results of this study demonstrate the importance of modelling accurately the aortic geometry in experimental and computational studies of prosthetic devices. Moreover, our findings suggest that grafts used for aortic root replacement should approximate as closely as possible the shape of the natural sinuses. PubDate: 2017-05-23 DOI: 10.1007/s00348-017-2361-7 Issue No:Vol. 58, No. 6 (2017)

Authors:Meraj Mohebi; David H. Wood; Robert J. Martinuzzi Abstract: The influence of post-stall angles of attack, \(\alpha\) , on the turbulent flow characteristics behind a thin high aspect ratio flat plate was investigated experimentally. Time-resolved stereo particle image velocimetry was used in an open-section wind tunnel at a Reynolds number of 6600. The mean field was determined along with the wake topology, force coefficients, vortex shedding frequency, and the terms in the transport equation for the turbulent kinetic energy k. Coherent and incoherent contributions to the Reynolds stress and k-transport terms were estimated. Over the measured range of \(20^\circ \le \alpha \le 90^\circ\) , quasi-periodic vortex shedding is observed and it is shown that most of the fluctuation energy contribution in the wake arises from coherent fluctuations associated with vortex shedding. As the angle of attack is reduced from \(90^\circ\) , the length of the recirculation region and the drag decrease, while the shedding frequency increases monotonically. In contrast, mean lift and k are maximized at \(\alpha \approx 40^\circ\) , suggesting a relationship between the bound vortex circulation and the levels of k. Structural differences in the mean strain field, wake topology, relative contributions to the k-production terms, and significant differences in the incoherent field suggest changes in the wake dynamics for \(\alpha > 40^{\circ }\) and \(20^{\circ } \le \alpha \le 40^{\circ }\) . For \(\alpha > 40^\circ\) , coherent contributions to the fluctuation field result in a large region close to the plate exhibiting small levels of negative mean production and generally low levels of advection, despite very high levels of production just downstream of the recirculation region. PubDate: 2017-05-23 DOI: 10.1007/s00348-017-2352-8 Issue No:Vol. 58, No. 6 (2017)

Authors:Olivier Léon; Estelle Piot; Delphine Sebbane; Frank Simon Abstract: The present study provides theoretical details and experimental validation results to the approach proposed by Minotti et al. (Aerosp Sci Technol 12(5):398–407, 2008) for measuring amplitudes and phases of acoustic velocity components (AVC) that are waveform parameters of each component of velocity induced by an acoustic wave, in fully turbulent duct flows carrying multi-tone acoustic waves. Theoretical results support that the turbulence rejection method proposed, based on the estimation of cross power spectra between velocity measurements and a reference signal such as a wall pressure measurement, provides asymptotically efficient estimators with respect to the number of samples. Furthermore, it is shown that the estimator uncertainties can be simply estimated, accounting for the characteristics of the measured flow turbulence spectra. Two laser-based measurement campaigns were conducted in order to validate the acoustic velocity estimation approach and the uncertainty estimates derived. While in previous studies estimates were obtained using laser Doppler velocimetry (LDV), it is demonstrated that high-repetition rate particle image velocimetry (PIV) can also be successfully employed. The two measurement techniques provide very similar acoustic velocity amplitude and phase estimates for the cases investigated, that are of practical interest for acoustic liner studies. In a broader sense, this approach may be beneficial for non-intrusive sound emission studies in wind tunnel testings. PubDate: 2017-05-23 DOI: 10.1007/s00348-017-2348-4 Issue No:Vol. 58, No. 6 (2017)

Authors:Dell Olmstead; Patrick Wayne; Jae-Hwun Yoo; Sanjay Kumar; C. Randall Truman; Peter Vorobieff Abstract: An experimental study examines shock acceleration with an initially diffuse cylindrical column of sulfur hexafluoride surrounded by air and inclined with respect to the shock front. Three-dimensional vorticity deposition produces flow patterns whose evolution is captured with planar laser-induced fluorescence in two planes. Both planes are parallel to the direction of the shock propagation. The first plane is vertical and passes through the axis of the column. The second visualization plane is normal to the first plane and passes through the centerline of the shock tube. Vortex formation in the vertical and centerline planes is initially characterized by different rates and morphologies due to differences in initial vorticity deposition. In the vertical plane, the vortex structure manifests a periodicity that varies with Mach number. The dominant wavelength in the vertical plane can be related to the geometry and compressibility of the initial conditions. At later times, the vortex interaction produces a complex and irregular three-dimensional pattern suggesting transition to turbulence. Highly repeatable experimental data are presented for Mach numbers 1.13, 1.4, 1.7, and 2.0 at column incline angles of 0 \(^{\circ }\) , 20 \(^{\circ }\) , and 30 \(^{\circ }\) for about 50 nominal cylinder diameters (30 cm) of downstream travel. PubDate: 2017-05-23 DOI: 10.1007/s00348-017-2358-2 Issue No:Vol. 58, No. 6 (2017)

Authors:Sandra Coumar; Viviana Lago Abstract: This paper presents an experimental investigation, carried out at the Icare Laboratory by the FAST team, focusing on plasma flow control in supersonic and rarefied regime. The study analyzes how the Mach number as well as the ambient pressure modify the repercussions of the plasma actuator on the shock wave. It follows previous experiments performed in the MARHy (ex–SR3) wind tunnel with a Mach 2 flow interacting with a sharp flat plate, where modifications induced by a plasma actuator were observed. The flat plate was equipped with a plasma actuator composed of two aluminum electrodes. The upstream one was biased with a negative DC potential and thus, created a glow discharge type plasma. Experimental measurements showed that the boundary layer thickness and the shock wave angle increased when the discharge was ignited. The current work was performed with two nozzles generating Mach 4 flows but at two different static pressures: 8 and 71 Pa. These nozzles were chosen to study independently the impact of the Mach number and the impact of the pressure on the flow behavior. In the range of the discharge current considered in this experimental work, it was observed that the shock wave angle increased with the discharge current of \(+15\%\) for the Mach 2 flow but the increase rate doubled to \(+28\%\) for the Mach 4 flow at the same static pressure, showing that the discharge effect is even more significant when boosting the flow speed. When studying the effect of the discharge on the Mach 4 flow at higher static pressure, it was observed that the topology of the plasma changed drastically and the increase in the shock wave angle with the discharge current of \(+21 \%\) . PubDate: 2017-05-23 DOI: 10.1007/s00348-017-2346-6 Issue No:Vol. 58, No. 6 (2017)

Authors:Daniele Simoni; Davide Lengani; Marina Ubaldi; Pietro Zunino; Matteo Dellacasagrande Abstract: The effects of free-stream turbulence intensity (FSTI) on the transition process of a pressure-induced laminar separation bubble have been studied for different Reynolds numbers (Re) by means of time-resolved (TR) PIV. Measurements have been performed along a flat plate installed within a double-contoured test section, designed to produce an adverse pressure gradient typical of ultra-high-lift turbine blade profiles. A test matrix spanning 3 FSTI levels and 3 Reynolds numbers has been considered allowing estimation of cross effects of these parameters on the instability mechanisms driving the separated flow transition process. Boundary layer integral parameters, spatial growth rate and saturation level of velocity fluctuations are discussed for the different cases in order to characterize the base flow response as well as the time-mean properties of the Kelvin–Helmholtz instability. The inspection of the instantaneous velocity vector maps highlights the dynamics of the large-scale structures shed near the bubble maximum displacement, as well as the low-frequency motion of the fore part of the separated shear layer. Proper Orthogonal Decomposition (POD) has been implemented to reduce the large amount of data for each condition allowing a rapid evaluation of the group velocity, spatial wavelength and dominant frequency of the vortex shedding process. The dimensionless shedding wave number parameter makes evident that the modification of the shear layer thickness at separation due to Reynolds number variation mainly drives the length scale of the rollup vortices, while higher FSTI levels force the onset of the shedding phenomenon to occur upstream due to the higher velocity fluctuations penetrating into the separating boundary layer. PubDate: 2017-05-23 DOI: 10.1007/s00348-017-2353-7 Issue No:Vol. 58, No. 6 (2017)