Perturbed Free Shear Layers

Laser-Doppler measurements of turbulent-flow parameters in a stirred mixer

A new method is introduced for estimating the convection velocity of individual modes in turbulent shear flows that, in contrast to most previous ones, only requires spectral information in the temporal or spatial direction over which a modal decomposition is desired, while only using local derivatives in other directions. If no spectral information is desired, the method provides a natural definition for the average convection velocity, as well as a way to estimate the accuracy of the frozen-turbulence approximation. Existing data from numerical turbulent channels at friction Reynolds numbers Reτ 1900 are used to validate the new method against classical ones, and to characterize the dependence of the convection velocity on the eddy wavelength and wall distance. The results indicate that the small scales in turbulent channels travel at the local mean velocity, while large ‘global’ modes travel at a more uniform speed proportional to the bulk velocity. To estimate the systematic deviations introduced in experimental spectra by the use of Taylor’s approximation with a wavelengthindependent convection velocity, a semi-empirical fit to the computed convection velocities is provided. It represents well the data throughout the Reynolds number range of the simulations. It is shown that Taylor’s approximation not only displaces the large scales near the wall to shorter apparent wavelengths but also modifies the shape of the spectrum, giving rise to spurious peaks similar to those observed in some experiments. To a lesser extent the opposite is true above the logarithmic layer. The effect increases with the Reynolds number, suggesting that some of the recent challenges to the k −1 x energy spectrum may have to be reconsidered.

Estimation of turbulent convection velocities and corrections to Taylor's approximation

This paper reports on an experimental investigation to determine the structure and mean flow quantities of round zero-net-mass-flux (ZNMF) jets. These jets are generated by a piston oscillating in a cavity behind a circular orifice. Several different flow patterns were observed with dye flow visualization and a parameter map of these was generated. Cross-correlation digital particle image velocimetry was used to measure instantaneous two-dimensional in-plane velocity fields in a plane containing the orifice axis. These velocity fields are used to investigate the existence of a self-preserving velocity profile in the far field of the ZNMF jet. The mean flow quantities and turbulent statistics of the ZNMF jets were compared with measurements for ‘equivalent’ continuous jets in the same apparatus. Phase-averaged velocity measurements were obtained in the near field of the ZNMF jets and were used to determine the radial entrainment. The out-of-plane vorticity fields were also investigated to gain an understanding of the mechanisms responsible for the difference in spreading rate of ZNMF jets compared to conventional continuous jets. A conceptual model of the ZNMF jet structure in the near field for Strouhal numbers much less than one is proposed that explains the observed behaviour of these ZNMF jets.

The evolution of round zero-net-mass-flux jets

Conditionally sampled velocities have been measured in a reattaching turbulent shear layer behind a rearward facing step in an effort to understand unsteady behaviour of reattaching flows. Laser-Doppler velocimeter measurements were conditionally sampled on the basis of instantaneous flow direction near reattachment. Conditions of abnormally short reattachment and abnormally long reattachment were considered. Ensemble-averages of measurements made during these conditions were used to obtain mean velocities and Reynolds stresses. In the mean flow, conditional streamlines show a global change in flow pattern which correlates with wall-flow direction. This motion can loosely be described as a "flapping" of the shear layer. Stresses shown also vary with the change in flow pattern. Yet, the global "flapping" motion does not appear to contribute much to the fluctuating energy in the flow. A second type of fluctuating motion (vortical) was observed. Spectral analysis of both wall static pressure and streamwise velocity show that the majority of energy in the flow resides in frequencies characteristic of roll-up and pairing of vortical structure seen in free shear layers (St = 0.2). Two-point velocity correlations also indicate a vortical behaviour of the flow. It is conjectured that the "flapping" is a disorder of the roll-up and pairing process occurring in the shear layer.

Time-dependent behavior of a reattaching shear layer

An experimental study of the developing structural characteristics of a two-dimensional jet in an extremely quiet environment was performed. The jet, at an exit Reynolds number of 6000 and with fluctuation intensity under 0.2% at the mouth, was operated within a large anechoic room. Measurements of energy spectra, fluctuation phase angles and two-dimensionality led to the inference of structural patterns in the flow. These patterns are initially characterized by relatively strong symmetric modes exhibiting limited two-dimensionality and oriented parallel to the mouth of the jet. Subsequent downstream evolution led to the formation of an antisymmetric pattern beyond the jet potential core and the associated development of extended structures possessing a definite large lateral inclination. The results of this work suggest a developing large-scale structural pattern more complicated than previously supposed.

Structural characteristics of a developing turbulent planar jet

Measurements of four types of correlations in the self preserving region of a turbulent plane jet are presented. Negative correlations are noted between the two halves of the jet, exhibiting an apparent flapping like motion. That motion is noted to be self-preserving and most likely attributed to the presence of large scale organized structures.

The Apparent Flapping Motion of a Turbulent Plane Jet—Further Experimental Results

An investigation into the magnitude and direction of the convective velocity in a plane air jet was performed. Convective velocities were obtained from cross-correlation measurements. They are defined as the ratio of the spacing between two hot-wire probes and the time delay between their signals to reach maximum correlation. These velocities were larger in magnitude than the local mean velocities for lateral distances greater than the half-width of the jet. Frequency analysis of the convective velocity indicates that the large-scale eddies move slower than the mean flow while the small scales move faster. Based on the convective velocity vector, broadband ‘convection lines’ were defined and found to point outward with respect to the streamlines for all values of y/b [Gt ] 0·5. Likewise, frequency investigation indicates that ‘convection lines’ point outward for all y/b [Lt ] 1·3 and then inward for larger values of y/b.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022112081003236

Turbulent convective velocities /broadband and wavenumber dependent/ in a plane jet

Mean Temperature and Velocity in a Plane Turbulent Jet

Etude experimentale visant a verifier l'hypothese selon laquelle une allee de tourbillons de Von Karman serait responsable du comportement de battement apparent dans l'ecoulement d'un jet plan

V. W. Goldschmidt

Papers

Structural characteristics of a developing turbulent planar jet

The Apparent Flapping Motion of a Turbulent Plane Jet—Further Experimental Results

Turbulent convective velocities /broadband and wavenumber dependent/ in a plane jet

Mean Temperature and Velocity in a Plane Turbulent Jet

Coherent Structures in the Similarity Region of Two-Dimensional Turbulent Jets