Michel Stanislas

Experimental study of eddy structures in a turbulent boundary layer using particle image velocimetry

Particle image velocimetry experiments have been performed in a turbulent boundary-layer wind tunnel in order to study the coherent structures taking part in the generation and preservation of wall turbulence. The particular wind tunnel used is suitable for high-resolution experiments (

PIV optimization for the study of turbulent flow using spectral analysis

\delta \gt 0.3

Some considerations on the accuracy and frequency response of some derivative filters applied to particle image velocimetry vector fields

m) at high Reynolds numbers (up to

Vortical structures in the turbulent boundary layer: a possible route to a universal representation

R_{\theta} = 19\,000

Main results of the First International PIV Challenge

in the present results). Eddy structures were identified in instantaneous velocity maps in order to determine their mean characteristics and possible relationships between these structures. In the logarithmic region, the results show that the observed eddy structures appear to organize like elongated vortices, tilted downstream, mainly at an angle of about 45° and having a cane shape. The characteristics of these vortices appear here to be universal in wall units for

The organization of near-wall turbulence: a comparison between boundary layer SPIV data and channel flow DNS data

R_{\theta}\,{\leq}\,19\,000

Two-point correlations in high Reynolds number flat plate turbulent boundary layers

. They seem to find their origin at a wall distance of about 25 wall units as quasi-streamwise vortices and to migrate away from the wall while tilting to form a head and a leg. Away from the wall, their radius increases and their vorticity decreases very slowly so that their circulation is nearly constant. Near the wall, the picture obtained is in fair agreement with existing models. The analysis of the results indicates a universality of the buffer-layer mechanism, even at low Reynolds number, and a sensitivity of the logarithmic region to low-Reynolds-number effects.

PIV Measurements in the Impeller and the Vaneless Diffuser of a Radial Flow Pump in Design and Off-Design Operating Conditions

Particle image velocimetry (PIV) is a measurement technique which is well adapted to the study of the structure of turbulent flows, as it allows us to obtain quantitative information on the spatial structure of the velocity field. This contribution presents an experimental approach to characterize the measurement noise of a PIV system and the spatial response of such a method. This approach is based on a specific spectral analysis of the velocity vector field deduced from several PIV experiments. This study was done in two steps. The first step was to measure the noise level of PIV and to determine a model for the PIV transfer function from a series of displacement fields measured in a quiet liquid. This model shows the effect of the interrogation window size and introduces a spectral noise density which is constant for a given recording set-up. The second step was to compute spectra from velocity fields obtained in a turbulent boundary layer in a plane parallel to the wall. These spectra show that PIV behaves as a band pass filter. This series of experiments allows us to build a model for the prediction of the PIV spectrum knowing the real one. This model confirms that the PIV noise is white. It allows us to optimize the interrogation window size in order to obtain the best compromise between the spectral response and the spatial resolution. The rms value of the noise can be estimated from the noise density, allowing us to quantify the measurement accuracy. The improvement of sub-pixel window shift is also discussed, leading to a small decrease in the noise level. An analysis is proposed to identify the main sources of noise: particles cut by the border of the interrogation window, isolated particles, etc.

Practical aspects of image recording in particle image velocimetry

This paper concerns the computation of derivatives from particle image velocimetry (PIV) velocity fields with the goal of obtaining the vorticity component normal to the plane. A variety of derivative schemes are characterized by their transfer function, taking into account the truncation and noise amplification. The PIV measurement noise is supposed to be a white one in the Fourier space. A spectral approach is used in order to choose the best filter for turbulent flows. The derivative spectra are discussed. An application is presented on a real turbulent flow with two interrogation window sizes and different derivative schemes. The most significant schemes are also applied to a velocity field containing a single vortex. A comparison of the maximum of vorticity obtained with each scheme and through a least-square fit with an Oseen vortex, allows us to quantify the effect of the band pass filter and to select the best scheme.

Incompressible Navier-Stokes computations with SUPG and GLS formulations—A comparison study

A study of streamwise oriented vortical structures embedded in turbulent boundary layers is performed by investigating an experimental database acquired by stereoscopic particle image velocimetry (SPIV) in a plane normal both to the mean flow and the wall. The characteristics of the experimental data allow us to focus on the spatial organization within the logarithmic region for Reynolds numbers Re θ up to 15000. On the basis of the now accepted hairpin model, relationships and interaction between streamwise vortices are first investigated via computation of two-point spatial correlations and the use of linear stochastic estimation (LSE). These analyses confirm that the shape of the most probable coherent structures corresponds to an asymmetric one-legged hairpin vortex. Moreover, two regions of different dynamics can be distinguished: the near-wall region below y + =150, densely populated with strongly interacting vortices; and the region above y + =150 where interactions between eddies happen less frequently. Characteristics of the detected eddies, such as probability density functions of their radius and intensity, are then studied. It appears that Reynolds number as well as wall-normal independences of these quantities are achieved when scaling with the local Kolmogorov scales. The most probable size of the detected vortices is found to be about 10 times the Kolmogorov length scale. These results lead us to revisit the equation for the mean square vorticity fluctuations, and to propose a new balance of this equation in the near-wall region. This analysis and the above results allow us to propose a new description of the near-wall region, leading to a new scaling which seems to have a good universality in the Reynolds-number range investigated. The possibility of reaching a universal scaling at high enough Reynolds number, based on the external velocity and the Kolmogorov length scale is suggested.