Marc Michard

Combining PIV, POD and vortex identification algorithms for the study of unsteady turbulent swirling flows

Particle image velocimetry (PIV) measurements are made in a highly turbulent swirling flow. In this flow, we observe a coexistence of turbulent fluctuations and an unsteady swirling motion. The proper orthogonal decomposition (POD) is used to separate these two contributions to the total energy. POD is combined with two new vortex identification functions, Γ1 and Γ2. These functions identify the locations of the centre and boundary of the vortex on the basis of the velocity field. The POD computed for the measured velocity fields shows that two spatial modes are responsible for most of the fluctuations observed in the vicinity of the location of the mean vortex centre. These two modes are also responsible for the large-scale coherence of the fluctuations. The POD computed from the Γ2 scalar field shows that the displacement and deformation of the large-scale vortex are correlated to these modes. We suggest the use of such a method to separate pseudo-fluctuations due to the unsteady nature of the large-scale vortices from fluctuations due to small-scale turbulence.

Space-Time Correlations in Two Subsonic Jets Using Dual Particle Image Velocimetry Measurements

Dual particle image velocimetry (dual PIV) measurements have been performed to investigate the space-time correlations in two subsonic isothermal round jets at Mach numbers of 0.6 and 0.9. The correlation scales are analyzed along the centerline and in the shear-layer center over the first 11 jet diameters from the nozzle exit To provide robust results over a wide range of flow conditions, these correlation scales are given in terms of their appropriate quantities, namely, the mean or rms velocity in reference to velocity and the momentum thickness or the half-velocity diameter in reference to length in the shear layer and on the jet axis, respectively. From these results, a discussion on the modeling of turbulence in jets is addressed. The self-similarity of some space correlation functions in the shear layer and on the jet axis is shown. Furthermore, far enough downstream in the shear layer, some of the ratios between the space and time scales are relatively close to the values expected in homogeneous and isotropic turbulence. It is also found that the ratio between the integral length and the time scales in the fixed frame is of the order of the local mean flow velocity. In the convected frame, the appropriate scaling factor is the rms velocity.

Changing lift and drag by jet oscillation : experiments on a circular cylinder with turbulent separation

Abstract Oscillating jet actuators have been implemented and tested on a circular cylinder. Their action on the separation of turbulent boundary layers is investigated using complementary approaches. Wall pressure distribution shows that a large lift is generated, at the expense, however, of a slightly increased drag. Particle image velocimetry measurements provide the mean and fluctuating velocity fields in the near-wake. The control jet deflects the mean flux lines towards the wall, illustrating that the separation is delayed. This effect appears more and more powerful as the pulsed jet velocity increases. Phase averaging of the PIV fields shows that periodic structures are generated by the control, and how these structures modify the aerodynamic forces by entraining the external flow towards the wall. Finally, a few comparisons are made with laminar boundary layers and some general mechanisms are presented for the lift increase.

Space-time correlations in two subsonic jets using dual-PIV measurements

Dual Particule Image Velocimetry (dual-PIV) measurements have been performed to investigate the space-time correlations in two subsonic isothermal round jets at Mach numbers 0.6 and 0.9. Measurements are obtained along the centerline and the shear-layer region which are closely connected with the noise generation. Integral scales have been calculated with as much as 2000 samples which provides high-quality data. The correlation scales are given in function of appropriate references, namely the local momentum thickness (respectively the jet diameter) for the shear-layer (respectively the jet axis) and the mean or rms local velocity. Far enough downstream in the shear-layer, some of the ratios between the space scales and between the time scales are relatively close to the values expected in isotropic an homogeneous turbulence. Furthermore, the relation between the time and space scales follows well the Taylor’s assumption in the shear-layer.

Combining LDA and PIV for turbulence measurements in unsteady swirling flows

Combining LDA and PIV techniques to investigate unsteady swirling flows permits a more detailed interpretation of velocity fluctuations than that from LDA alone. This can be necessary when comparing measurements with numerical simulations which attempt to capture the unsteadiness. One possible method of combining the measurement data is introduced and discussed with application to swirling flowfields which exhibit a precessing vortex core. The analysis indicates that in such flows the unsteadiness accounts for up to 70% of the total energy in the velocity fluctuations.

An Experimental Characterization of the Flow Past an Airfoil in the Wake of a Circular Rod

An experimental study of the flow past an airfoil in the wake of a rod shows that, at high Reynolds numbers, the vortices shed by the rod are strongly stretched and split near the leading edge and affected by small scale turbulence structures. These are shown to enhance three-dimensional effects, and to broaden the spectrum around the shedding frequency. The airfoil leading edge is the dominant acoustic source region. Post-processing tools combining Proper Orthogonal Decomposition and new vortex identification algorithms are applied to PIV measurements in order to extract the main vortical structures from snapshots, and study their variability.Copyright

Analysis of the swirl effect on turbulent length scales in an ICE cylinder by two-point LDV

Abstract Two-point laser Doppler velocimetry is used to investigate the turbulent flow behind an axisymmetric cylinder head simulating the intake stroke of an internal combustion engine. Profiles of axial, radial and tangential mean and RMS velocities as well as profiles of integral length scales are measured at several distances from the cylinder head. The effect of non-homogeneity of the flow on integral length scale is investigated. The effects of swirl and valve lift on integral scales are also analysed. The temporal spectra of the tangential fluctuating velocity are measured to identify the swirl centre precession.