Shear Layer Width Research Articles

Mixing augmentation of supersonic streams

The use of passive mixing devices to enhance the mixing of supersonic streams is of importance in many aerospace and allied applications. A comparative experimental study of two such non-conventional axisymmetric supersonic nozzles for mixing augmentation is undertaken in the present study. The nozzles studied are conventional convergent divergent (c-d), ramp, and nozzle with secondary tabs at the Mach number of 1.7. The freejet characteristics of the jets from the nozzles and the confined mixing of the jets under isothermal and non-isothermal conditions were experimentally investigated. In addition, schlieren and laser light sheet methods are employed for flow visualization. Results of experiments on freejet showed rapid total pressure decay, higher jet width, and increased shear layer width in the case of jets from non-conventional nozzles. Also visualized were complex shock patterns in these jets. The confined mixing studies in both cold and hot flows confirm the increased mixing behavior of non-conventional nozzles within shorter mixing duct lengths. Laser sheet visualization clearly demonstrated the formation of streamwise vortices, which accounts for the enhanced mixing while employing the nozzles under study.

Plasma rotation and finite Larmor radius losses in a reversed field pinch

A model is proposed to calculate the effect of particle and momentum losses due to unconfined ion orbits at the edge of the plasma in a reversed field pinch device. The value of the shearing rate can be obtained, requiring that the electromagnetic force due to the return current in the confined plasma is equilibrated by the reaction force due to direct ion momentum losses. A natural scaling of the shearing rate with the ion cyclotron frequency is obtained by assuming that this equilibrium is achieved through the effect of orbit squeezing caused by the finite value of the radial electric field gradient. The extension of the shear layer has been obtained requiring that the contribution of charge exchange viscosity to the perpendicular momentum balance is negligible. A shear layer width scaling as the Larmor radius is obtained. The scaling relations derived account for recent observations obtained in the reversed field experiment, RFX [G. Rostagni, Fusion Eng. Des. 25, 301 (1995)]. A quantitative comparison with experimental data indicates that impurities in low ionization stages can play an important role in the shear layer dynamics.

Anomalous ion mixing within an MHD scale Kelvin‐Helmholtz vortex: 2. Effects of inhomogeneity

Hybrid code simulations of the MHD scale Kelvin‐Helmholtz (K‐H) instability have been conducted. In contrast to the homogeneous background case where the large‐scale ion mixing across the shear layer occurs inside the vortex obeying the hydrodynamic similarity law, the ion mixing is found to be reduced when the background magnetic field is inhomogeneous. This stabilization effect is twofold: it is an ion kinetic effect which reduces the saturation vortex size and so the mixing length when the shear layer width and the wavelength of the growing mode are small. The reduction effect still works even when the shear layer width/wavelength are large and a well defined vortical motion as in the fluid model is evident. The mixing is suppressed for this case because the ions on the lower magnetic field (magnetosheath) side cannot enter within the vortex island: The rolled‐up magnetopause current sheet block them out electrostatically. The geophysical implication of the present result is that the K‐H mixing alone may not be as efficient as to produce the large‐scale mixing blob of the low‐latitude boundary layer (LLBL) when the magnetic field intensity is highly different across the magnetopause.

Freja UV imager observations of spatially periodic auroral distortions

Freja UV imager measurements of distortions of discrete arcs within the auroral distribution reveal a number of examples of periodic intensifications. Analysis of these features reveals a clear peak at 152 km separation between the intensifications with a less well defined peak at approximately twice that value. No observations at separations less than 100 km were found. Theories relating such features to shear flow regions suggest a relationship between wavelength and shear layer width. A statistical analysis indicates that the product of the wave number and the arc system width yielded a value of .7 which is consistent with theoretical predictions. The periodic features were found to occur throughout the afternoon/midnight sector with no apparent dependence on MLT. As well they appeared at a variety of locations within the auroral distribution implying the common existence of shear layers.

Three-Dimensional Simulations of Compressible Mixing Layers: Visualizations and Statistical Analysis

The effects of compressibility on the planar mixing layer are investigated by means of visualization and statistical analysis of the flowfield quantities computed using three-dimensional temporally evolving inviscid simulations. The levels of compressibility studied range from relative Mach numbers of Mr — 0.2-2.4. The objectives of this research are to identify large-scale structures present in the mixing layer at different levels of compressibility as well as to examine the statistical description of the flowfield, in order to gain understanding of the physical entrainment and mixing processes. Three-dimensional simulation visualizations of both the passive scalar and pressure fields show the nature of the large-scale structure present in the planar mixing layer to change from nearly two dimensional and spanwise at low compressibility to highly three dimensional and oblique at increased compressibility. Statistical analysis of the flowfield quantities shows that the shear layer width, Reynolds shear stress, and transverse turbulence intensity decrease with increasing compressibility levels, whereas the streamwise turbulence intensity remains nearly constant and the spanwise turbulence intensity increases. These statistics support the increasingly three-dimensional nature of the large-scale motion of the mixing layer with increasing Mr.

Asymptotic intensity of the quasi-periodic oscillations in fully developed turbulent shear layers

Turbulent shear layers generated from a common splitter plate as well as from a half-frame screen are investigated experimentally in the developing regime and in the asymptotic regime. The phase-averaged means with time delay of velocity fluctuations are analysed in terms of Fourier modes in the frequency domain to give both the amplitude and frequency of the local fundamental mode due to the presence of the large-scale organized motion. The amplitudes of both the streamwise and the transverse components tend to relax to asymptotic values that are independent of the velocity ratio as well as the shear-layer apparatus. The Strouhal number St, defined as fL/Ua, where f is the local fundamental mode frequency, L is the shearlayer width and Ua, is the average convection velocity of the structures is found to be initially dependent on the local Reynolds number. In the asymptotic regime, the orientation of the large-scale structures is tilted backward toward the higher-speed side in all cases.

Aerodynamic mechanisms in jet‐edge‐resonator oscillation

Undulatory motions of a separated shear layer in a rectangular slot are found to give rise to three forces that are believed responsible for jet‐edge and jet‐edge‐resonator oscillation at low Mach number. The three are (1) a transverse force on the downstream edge which gives rise to the transverse dipole feedback radiation postulated by Powell, (2) a longitudinal force on the downstream wall, which results in longitudinal dipole radiation believed responsible for oscillation in shallow cavities, and (3) a longitudinal force in the body of the fluid which is associated with pipetone drive. By direct integration of the Lighthill tensor over the entire region of fluid motion between the edges, the magnitude of each force is estimated. In every case the results are directly proportional to shear layer width, and cannot therefore be derived from theories using the vortex sheet approximation.

Effects of the initial condition on the axisymmetric free shear layer: Effect of the initial fluctuation level

An axisymmetric air free shear layer was experimentally investigated in order to determine the effect of the initial peak fluctuation level u′pe/Ue on the characteristic measures of the free shear layer. The shear layer width and the similarity parameter depend noticeably on u′pe/Ue. The asymptotic peak turbulence intensity u′p∞ increases with u′pe/Ue but appears to have a limiting value of about 0.18. The location for achievement of self-preservation progressively moves upstream with increasing u′pe/Ue, suggesting that higher initial fluctuations hasten the spectral evolution of the velocity fluctuations and development of the shear layer. The results show that the initial fluctuation level has a much more dramatic effect on the evolution and the average characteristic measures of the free shear layer than the initial momentum thickness has, and may have been the primary reason for discrepancies among results from different previous inverstigations.