Particle streak velocity field measurements in a two-dimensional mixing layer

Using digital image processing of particle streak photography, the streamwise and perpendicular components of the velocity field were investigated, in the mid‐span plane of a two‐dimensional mixing layer, with a 6:1 velocity ratio. The Reynolds number of the flow, based on the local vorticity thickness and the velocity difference across the layer, ranged from 1360 to 2520, in the plane of observation. The significant result of this experiment was that the region of vorticity bearing fluid is confined to a small fraction of the flow. A second finding, consistent with the small regions of concentrated vorticity, was the observation of instantaneous streamwise velocity reversal, in the laboratory frame, in small regions of the flow

Effects of a downstream disturbance on the structure of a turbulent plane mixing layer

Using a two-dimensional airfoil, a disturbance was introduced into a plane mixing layer some distance downstream of the splitter plate trailing edge. Results indicate that it is possible to induce very large changes in the layer growth rate downstream of the disturbance location, while leaving the portion of the shear layer between the splitter plate and the disturbance source essentially unaffected. Furthermore, the use of forcing for modification of the mixing layer in the region upstream of the disturbance is demonstrated. It Is shown that two different mechanisms are responsible for coupling such disturbances to the flow in the present forcing of upstream and downstream regions

Mixing and chemical reactions in a turbulent liquid mixing layer

An experimental investigation of entrainment and mixing in reacting and non-reacting turbulent mixing layers at large Schmidt number is presented. In non-reacting cases, a passive scalar is used to measure the probability density function (p.d.f.) of the composition field. Chemically reacting experiments employ a diffusion-limited acid–base reaction to directly measure the extent of molecular mixing. The measurements make use of laser-induced fluorescence diagnostics and high-speed, real-time digital image-acquisition techniques. Our results show that the vortical structures in the mixing layer initially roll-up with a large excess of fluid from the high-speed stream entrapped in the cores. During the mixing transition, not only does the amount of mixed fluid increase, but its composition also changes. It is found that the range of compositions of the mixed fluid, above the mixing transition and also throughout the transition region, is essentially uniform across the entire transverse extent of the layer. Our measurements indicate that the probability of finding unmixed fluid in the centre of the layer, above the mixing transition, can be as high as 0.45. In addition, the mean concentration of mixed fluid across the layer is found to be approximately constant at a value corresponding to the entrainment ratio. Comparisons with gas-phase data show that the normalized amount of chemical product formed in the liquid layer, at high Reynolds number, is 50% less than the corresponding quantity measured in the gas-phase case. We therefore conclude that Schmidt number plays a role in turbulent mixing of high-Reynolds-number flows

Inviscid instability characteristics of free shear layers with non-uniform density

The linear spatial instability of two-dimensional two-stream plane mixing layers has been studied extensively in the past. In the case of uniform density, Michalke (1965) investigated the single-stream shear layer and Monkewitz & Huerre (1982) considered the effect of the velocity ratio. Maslowe & Kelly (1971) studied the stratified (non-uniform density) shear layers and showed that density variations can be destabilizing. In all these studies, the mean velocity profile has been assumed to be monotonically increasing from the value on the low-speed stream to that on the high-speed stream and usually the hyperbolic tangent form is used. It should be noted, however, that under experimental conditions the initial mean velocity profile almost always has a wake component due to the boundary layers on the two sides of the splitter plate. The effect of the wake component has only recently come into consideration with the investigations of Miau 1984 and Zhang et al. 1984 for the uniform density case. The purpose of the present work is to study the instability characteristics of both uniform and non-uniform density plane shear layers taking into account the wake component of the initial velocity profile. The inviscid, linear, parallel-flow stability analysis of spatially growing disturbances is utilized to numerically calculate the range of unstable frequencies and wave-numbers

A Cancellation Experiment in a Forced Turbulent Shear Layer

Results are presented which demonstrate that it is possible to cancel, using feedback control techniques, the effects of an externally generated disturbance in a fully-developed turbulent two-dimensional shear layer

Control of Turbulent Mixing Layers

This the final report of research conducted at the California Institute of Technology, by Paul E. Dimotakis, in collaboration with Dr. M. M. Moochesfahani as co-investigator, and with the assistance of Mr. P. Tokumaru during the last year. The primary goal was to explore ways in which open loop and closed feedback loop control methods can be utilized to affect the qualitative and quantitative behavior of turbulent shear layers. In particular, we attempted to i. investigate the dynamic behavior and response of these flows through a study of the feedback control schemes required to produce a given desired outcome, ii. explore the extent to which specific properties of turbulent shear layer flows, such as growth rate profile and mixing, can be manipulated and altered by such means, and, iii. devise schemes for producing turbulent shear layer flows with specific desirable properties, as might be dictated, for example, by the flow specifications for the efficient operation of a combustion device. In the course of this work, other derivative and closely related efforts were also undertaken, some of which will be described below. The work conducted under the sponsorship of this Grant was primarily experimental and in close collaboration with a broader experimental, numerical and theoretical effort at Caltech to study unsteady separated flows, and the evaluation and use of control techniques in these flows in particular

Effect of initial acceleration on the development of the flow field of an airfoil pitching at constant rate

We present results from a series of experiments where an airfoil is pitched at constant rate from 0 to 60 degrees angle of attack. It is well documented that the dynamic stall behavior of such an airfoil strongly depends on the nondimensional pitch rate K = dot-alpha C/(2U(sub infinity)), where C is the chord, dot-alpha the constant pitch rate, and U(sub infinity) the free stream speed. In reality, the actual motion of the airfoil deviates from the ideal ramp due to the finite acceleration and deceleration periods imposed by the damping of drive system and response characteristics of the airfoil. It is possible that the pitch rate alone may not suffice in describing the flow and that the details of the motion trajectory before achieving a desired constant pitch rate may also affect the processes involved in the dynamic stall phenomenon. The effects of acceleration and deceleration periods are investigated by systematically varing the acceleration magnitude and its duration through the initial acceleration phase to constant pitch rate. The magnitude and duration of deceleration needed to bring the airfoil motion to rest is similarly controlled

Two-Point LDV Measurements in a Plane Mixing Layer

Investigations into the nature of the large structures in a two-dimensional shear layer were carried out using laser Doppler velocimetry in the GALCIT free-surface water tunnel. By simultaneous measurements of velocity at two points outside the turbulent region, above and below the shear layer, it was possible to measure the strength (total circulation) and location of the vorticity center of the large structures. It was found that structures not in the process of pairing convect downstream with the center of their cores close to the ray y/x along which the mean velocity is given by U_m = l/2(U_1 + U_1 ). The determined value of the mean circulation is consistent with the independent measurements of the mean spacing between the structures. Results indicate that if the large structure vorticity distribution is elliptical, the inclination angle of its axis of symmetry with respect to the now direction is small

Influence of Effective Angle of Attack Oscillation Amplitude on Force Generation by Pitching-Plunging Flat Plates

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/97081/1/AIAA2012-2665.pd