Spatio-temporal structure and cycle to cycle variations of an in-cylinder tumbling flow

The aim of this paper is to make use of PIV and high-speed PIV in a research engine of moderate tumbling ratio in order to analyze both the spatial structure of the flow and its temporal evolution during series of consecutive cycles. Appropriate analyzing tools are introduced, and four different points are addressed: (1) the chain of events driving the generation of the three-dimensional mean tumbling motion is investigated; (2) a Lagrangian analysis of the roll-up of the tumbling jet in individual cycles demonstrates a strong cycle to cycle variation during the compression phase (the rms of the position of the jet front being approximately 10% of the piston stroke); (3) focussing on the “breakdown” phase, phase invariant proper orthogonal decomposition enables us to distinguish cycles according to their structure near top dead center (TDC). We show that when the coherent energy of the flow is conserved, there is no increase in the fluctuating kinetic energy; (4) finally, the phase-averaged Reynolds stresses is decomposed into a contribution of the in-cycle coherence and the turbulence carried by the flow states. Approximately 30% of the fluctuating kinetic energy is due to cycle to cycle fluctuations in this chamber near TDC.     

Extended Proper Orthogonal Decomposition for Analysis of Unsteady Flames

The aim of this work is to present the use of proper orthogonal decomposition (POD) and extended proper orthogonal decomposition (EPOD) for revealing flame dynamics as a set of statistical quantities referred as modes. The flame fluctuations are used to derive an empirical functions base representing the most important features of the flame. The capabilities of the technique are exemplified in the case of an unsteady laminar flame. The flame is naturally unsteady and can be excited to amplify the fluctuations. The data base consists of synchronous Particle Image Velocimetry (PIV) and OH-Planar Laser Induced Fluorescence (OH-PLIF) measurements. It was found that the POD based upon the PIV vectors only reveals flow features when the excitation is added. On contrary, the OH-PLIF based POD performs well in any case and constitutes a suitable base for the EPOD analysis.     

Coherent and turbulent process analysis in the flow past a circular cylinder at high Reynolds number

This article presents an experimental study of the turbulent flow past a circular cylinder at high Reynolds number by means of advanced optical measurements techniques. Following previous studies using standard PIV and stereoscopic PIV (3C PIV), TRPIV and 3C-TRPIV have been employed in low subsonic wind tunnel environment. The database consisting of statistical and time-dependent fields aims at providing a physical analysis of the coherent and turbulent part, as well as a proper basis for validation and improvement of recent turbulence modelling approaches for strongly detached flow at high Reynolds number. As the nonlinear interaction between the coherent and turbulent dynamics have to be taken into account in a model, particular attention is paid to a decomposition of the flow into a coherent and a turbulent part, and to the analysis of their dynamics. This is achieved both using phase averaging and Proper Orthogonal Decomposition. For phase averaging, the two first POD coefficients are used for the evaluation of the vortex shedding phase angle. Furthermore, selected results of a Detached Eddy Simulation which had been validated by means of the experiment, are also presented to contribute to the physical analysis. The present study's experimental data resolved in space and time allow the confirmation of the conditional averaging for the turbulent stresses evaluation, by alleviating their overestimation due to phase jitter that occurs between the trigger signal and the velocity, when the phase angle is determined from a wall pressure signal. A more accurate physical analysis of the flow is achieved, particularly regarding the occurrence of irregular vortex shedding.     

Coherent and turbulent processes in the bistable regime around a tandem of cylinders including reattached flow dynamics by means of high-speed PIV

The turbulent flow around two cylinders in tandem at the sub-critical Reynolds number range of order of 10⁵ and pitch to diameter ratio of 3.7 is investigated by using time-resolved Particle Image Velocimetry (TRPIV) of 1 kHz and 8 kHz. The bi-stable flow regimes including a flow pattern I with a strong vortex shedding past the upstream and the downstream cylinder, as well as a flow pattern II corresponding to a weak alternating vortex shedding with reattachment past the upstream cylinder are investigated. The structure of this “reattachment regime” has been analyzed in association with the vortex dynamics past the downstream cylinder, by means of POD and phase-average decomposition. These elements allowed interconnection among all the measured PIV planes and hence analysis of the reattachment structure and the flow dynamics past both cylinders. The results highlight fundamental differences of the flow structure and dynamics around each cylinder and provide the ‘gap’ flow nature between the cylinders. Thanks to a high-speed camera of 8 kHz, the shear-layer vortices tracking has been possible downstream of the separation point and the quantification of their shedding frequency at the present high Reynolds number range has been achieved. This issue is important regarding fluid instabilities involved in the fluid–structure interaction of cylinder arrays in nuclear reactor systems, as well as acoustic noise generated from the tandem cylinders of a landing gear in aeronautics.     

POD investigation of the unsteady turbulent boundary layer developing over porous moving flexible fishing net structure

Particle image velocimetry (PIV) measurements are made to investigate the boundary layer developing over a modeled bottom trawl. The random motion of the fishing net structure as well as the flexibility and the porosity of this structure means that it is not enable to access the main characteristics of such a flow, using classical post-processing mathematical tools. An innovative post-treatment tool based on proper orthogonal decomposition (POD) is then developed to extract the mean velocity flow field from each available PIV instantaneous unsteady velocity field. In order to do so, the whole available velocity database is used to compute POD eigenfunctions and the first POD modes are identified as representing the mean flow field. It is then possible to deduce the mean boundary layer flow field for each position of the fishing net structure during PIV measurements. It is then observed that the mean flow field strongly depends on multiple parameters such as surface curvature, structure porosity, random motion of the structure. Streamwise evolution of classical thicknesses of boundary layer flow are also analyzed. The present work also provides benchmark PIV data of the unsteady flow developing on fishing net porous structures, which helps the progress in unsteady numerical codes for this investigation.     

Integral Length Scales and Time Scales of Turbulence in an Optical Spark-Ignition Engine

In-cylinder air flow structures are known to play a major role in mixture preparation and flame development in spark-ignition engines. In this paper both LDV and PIV measurements were undertaken in an optical spark-ignition at 1500 RPM 0.5 bar inlet plenum pressure. One of the primary PIV planes was vertical cutting through the centrally located spark plug (tumble plane) inside the pentroof at ignition timing. The other plane was horizontal inside the pentroof 1 mm below the spark plug LDV was conducted 1 mm below the spark plug on a line from inlet to exhaust but also on a lower line 14 mm below the spark plug. In-cylinder PIV data at specific crank angles in the intake and compression strokes were also analysed on the central tumble plane and on a horizontal plane 14 mm below the spark plug. The combination of both techniques allowed high spatial and temporal resolution as the two data sets complemented each other to provide details of mean flow and turbulence characteristics on different levels, aiming ultimately for quantification of integral time scales and length scales. LDV cycle-resolved analysis distinguished between the classic approach of using the time integral of the autocorrelation function to obtain the integral time scale and a high-frequency cut-off analysis to obtain high- and low-frequency fluctuations about an in-cycle mean.     

Three-dimensional vortex wake structure of a flapping-wing micro aerial vehicle in forward flight configuration

The unsteady flow field past a backward-facing step in a rectangular duct is investigated by adopting time-resolved particle image velocimetry (PIV) in the Reynolds number range of 2,640–9,880 based on step height and the inlet average velocity. The PIV realizations are subjected to post-processing techniques, namely, proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD). At low Reynolds numbers, the second spatial POD modes indicate the presence of the shear layer mode, whereas this feature shifts to higher modes at higher Reynolds numbers. The corresponding temporal modes are Fourier-transformed to obtain the dominant frequency, whose Strouhal number corroborates the above observation. Short-time windows in the transverse velocity component along the shear layer are selected to investigate the temporal stability of the flow field by DMD to quantify the growth rate of the shear layer mode. The higher harmonics of this mode are also observed to grow, albeit at lesser rate. By relating to POD analysis, the most energetic structures were found to correspond to the unstable modes. The correlation between these unstable DMD modes and the Fourier-filtered flow fields for the same frequencies indicate better match for the lower operating Reynolds number case as compared to higher ones. The spatial stability analysis demonstrates the growth of the shear layer vortices, which is combined with the temporal stability analysis to evaluate the phase velocity of the identified shear layer structures. The calculated phase velocity magnitude of the shear layer is found to be reasonably below the local velocity as expected.     

Multi-time-delay LSE-POD complementary approach applied to unsteady high-Reynolds-number near wake flow

This investigation compared the application and accuracy of single- and multi-time-delay linear stochastic estimation-proper orthogonal decomposition (LSE-POD) methods in the temporal domain. These methods were considered for low-dimensional estimations of the dynamics of the energy-containing structures in a high Reynolds number flow. The near wake dynamics of a bluff body were used to demonstrate the robustness and accuracy of the investigated LSE-POD methods. Statistically independent two-dimensional particle image velocimetry (PIV) measurements were used to determine spatial POD modes, and time-resolved surface pressure measurements were used to determine LSE coefficients required for estimating the time-varying POD coefficients. A low-order, time-resolved reconstruction of the wake dynamics was accomplished using these estimated time-varying POD coefficients. The paper also provides details concerning the accuracy of the estimation using multi-time-delay LSE-POD. The results demonstrate that the multi-time LSE-POD technique is successful in capturing and reconstructing the important near wake dynamics. It is also shown that optimizing the time delays used for the estimations increases the accuracy of the reconstruction. As a result of its capabilities, the multi-time-delay implementation of the LSE-POD approach offers an alternate method for low-dimensional modeling that is attractive for real-time flow estimation.     

Flow field decomposition applied to experimental data obtained for a transitional boundary layer

Using experimental data from Particle Image Velocimetry (PIV) measurements, coherent structures of a transitional spatially developing boundary layer are determined. The coherent structures are determined utilizing the Proper Orthogonal Decomposition (POD), which is based on an expansion of the flow field variables into a set of eigenfunctions or modes. For having constant and reproducible flow field conditions, the flow is artificially excited by means of periodic velocity fluctuations. The used excitation device allows the generation of different transition scenarios, where this paper focuses on the case of thefundamental transition. Phase locked excitation signals allow the recording of instantaneous velocity fields of the flow field at certain instants of time. It can be shown that PIV is a suitable technique to provide experimental data for POD. The results of the POD show that already a small number of modes cover most of the kinetic energy of the flow.     

Uncovering large-scale coherent structures in natural and forced turbulent wakes by combining PIV,POD, and FTLE

Planar velocity data of the unsteady separated flow in the turbulent wake of a circular cylinder obtained by particle image velocimetry (PIV) are analyzed in order to visualize the large-scale coherent structures associated with alternating vortex shedding at a Reynolds number of 2,150. Two different cases are examined: unforced vortex shedding in the natural wake and vortex lock-on incited by forced perturbations superimposed in the inflow velocity. Proper orthogonal decomposition (POD) is employed to reconstruct the low-order wake dynamics from randomly sampled snapshots of the velocity field. The reconstructed flow is subsequently used to determine the evolution of the finite-time Lyapunov exponent (FTLE) fields which identify the Lagrangian coherent structures. The results demonstrate that the combination of methods employed offers a powerful visualization tool to uncover large-scale coherent structures and to exemplify vortex dynamics in natural and forced bluff-body wakes.     

Obtaining phase averaged turbulence properties in the near wake of a circular cylinder at high Reynolds number using POD

The flow past a circular cylinder at high Reynolds number is studied by means of PIV, 3C-PIV and Time-Resolved PIV techniques. One of the goals of this study was to allow comparisons with numerical simulations on a domain identical to that of the experiment. For this reason, the cylinder was placed in a confined environment, with a high blockage and a low aspect ratio, thereby allowing computations on a mesh of reasonable size, and avoiding “infinite conditions”. This paper deals with the decomposition of the flow in a coherent and random parts. To this aim, phase averaged quantities were first obtained using the wall pressure signal on the cylinder as a trigger signal. This was achieved using both conditional sampling and LSE with similar results. This decomposition is then analysed using the Time Resolved PIV measurements, as well as by comparison of the contributions of the organised and turbulent fluctuations to the time-independent Reynolds stress tensor with those estimated from velocity spectra by interpolation and integration of the continuous part. In agreement with other studies, it is found that the contribution of the turbulent motion is overestimated as a result of the occurence of phase jitter between the trigger and velocity signal. A POD analysis was then performed to extract the coherent motion and to compare this decomposition with that obtained by phase averaging. Similarly to the phase averaging, the POD allows the decomposition of the time-independent stress tensor as the sum of two contributions corresponding to the first N modes, and the rest of the modes. This decomposition is then analysed by comparing these contributions to those obtained from the velocity spectra, according to the value N chosen. It is found that these contributions are strongly dependent on N, and the contribution of the first modes greatly overestimate the coherent motion if N is too large. In order to obtain a good decomposition of the flow in coherent and random parts, the difficulty in this case lies in the choice of the modes. Finally, the POD coefficients of the first two modes are used instead of the pressure signal to determine the phase of the vortex shedding, and the phase averaging is reconsidered. It is found that the phase averaged vortices are less smeared by the averaging process, the turbulent stresses better follow the evolution of the vortices, and the contributions of both coherent and turbulent fluctuations are found to agree well with those evaluated from the velocity spectra. This enhancement is obtained because the phase angle is determined directly from the velocity fields to be averaged, thereby reducing the phase-jitter effect. A comparison with a detached eddy simulation is also briefly shown and demonstrates the high level of agreement obtainable between simulation and experiment, as well as confirming the enhancement of the phase averaging using this procedure.     

Proper orthogonal decomposition analysis for cycle-to-cycle variations of engine flow. Effect of a control device in an inlet pipe

This paper aims to investigate cycle-to-cycle variations of non-reacting flow inside a motored single-cylinder transparent engine in order to judge the insertion amplitude of a control device able to displace linearly inside the inlet pipe. Three positions corresponding to three insertion amplitudes are implemented to modify the main aerodynamic properties from one cycle to the next. Numerous particle image velocimetry (PIV) two-dimensional velocity fields following cycle database are post-treated to discriminate specific contributions of the fluctuating flow. We performed a multiple snapshot proper orthogonal decomposition (POD) in the tumble plane of a pent roof SI engine. The analytical process consists of a triple decomposition for each instantaneous velocity field into three distinctive parts named mean part, coherent part and turbulent part. The 3rd- and 4th-centered statistical moments of the proper orthogonal decomposition (POD)-filtered velocity field as well as the probability density function of the PIV realizations proved that the POD extracts different behaviors of the flow. Especially, the cyclic variability is assumed to be contained essentially in the coherent part. Thus, the cycle-to-cycle variations of the engine flows might be provided from the corresponding POD temporal coefficients. It has been shown that the in-cylinder aerodynamic dispersions can be adapted and monitored by controlling the insertion depth of the control instrument inside the inlet pipe.     

Dependence of combustion dynamics in a gasoline engine upon the in-cylinder flow field, determined by high-speed PIV

We apply time-resolved high-speed particle image velocimetry (PIV) in an optically accessible gasoline engine to determine the effect of the in-cylinder flow field upon combustion dynamics. Our PIV setup involves solid particles as tracer, which enables also measurements at firing top dead center and during the combustion process itself. We analyze the flow field for the entire intake and compression phase, as well as the decay of a prominent large-scale tumble structure in the flow field. The data indicate significant cycle-to-cycle flow field variations, characterized by detection of kinetic energy and tumble center. Measurements in fired engine operation demonstrate the influence of the flow field on combustion dynamics. At stoichiometric operation, we find that variations in the kinetic energy of the flow field are a major cause of cycle-to-cycle variations. From simultaneous imaging of the combustion flame and PIV at lean operation, we find that the velocity distribution in the flow field induces a macroscopic motion of the flame kernel??which significantly effects the combustion process.     

Phase-resolved characterization of vortex shedding in the near wake of a square-section cylinder at incidence

The vortex formation and shedding process in the near wake region of a 2D square-section cylinder at incidence has been investigated by means of particle image velocimetry (PIV). Proper orthogonal decomposition (POD) is used to characterize the coherent large-scale flow unsteadiness that is associated with the wake vortex shedding process. A particular application of the POD analysis is to extract the vortex-shedding phase of individual velocity fields, which were acquired at asynchronous low rate with respect to the vortex shedding cycle. The phase of an individual flow field is determined from its projection on the first pair of POD modes, allowing phase averaging of the measurement data to be performed. In addition, a low-order representation of the flow, constructed from the mean and the first pair of POD modes, is found to be practically equivalent to the phase-averaged results. It is shown that this low-order representation corresponds to the basic Fourier component of the flow field ensemble with respect to the reconstructed phase. The phase-averaged flow representations reveal the dominant flow features of the vortex-shedding process and the effect of the angle of incidence upon it.