G.R. Grek

Influence of initial conditions at the nozzle exit on the structure of round jet

Experimental data concerning the influence of initial conditions at the nozzle exit on the structure and development characteristics of round jets are reported. Features in the development of laminar and turbulent round jets emanating from variously elongated nozzles at identical Reynolds numbers are revealed. Smoke visualization pictures obtained for jets formed under different initial conditions (with different distributions of mean and pulsating flow velocities at the nozzle exit) are discussed. It is shown possible to make the zone of laminar flow in the jet stream more extended, and to delay the jet turbulization process in space, by making the flow-velocity profile more parabolic at the exit of elongated nozzle. Features in the development of vortical structures in a jet under an acoustic action are identified. It is shown that, for a turbulent round jet to be produced right at the nozzle exit, the nozzle length must be increased in excess of a certain value so that to provide for spatial growth of turbulent boundary layer thickness, finally ending in the formation of a fully turbulent flow velocity profile across the channel.

ROLE OF LOCALIZED STREAMWISE STRUCTURES IN THE PROCESS OF TRANSITION TO TURBULENCE IN BOUNDARY LAYERS AND JETS (REVIEW)

Results of the analysis of specific features of the laminar–turbulent transition in various subsonic shear flows, which are caused by localized stationary and nonstationary streamwise structures, are presented. One mechanism of flow turbulization is considered, which involves the origination and development of secondary high‐frequency disturbances in regions of flow instability generated by its modulation by streamwise structures. It is shown that this process is identical in different types of shear flows (boundary layers and jets) and in flows of the type of localized streamwise structures (stationary or nonstationary).

Round and plane jets in a transverse acoustic field

Results of experimental studies on round and plane, macro- and microjets subjected to transverse acoustic field at low Reynolds numbers are presented. A new phenomenon associated with transformation of the round microjet to the plane one under acoustic forcing is revealed. Also, bifurcation of the round microjet is observed. It is shown that both plane macro- and microjets are prone to a sinusoidal instability. As is found, the plane microjet becomes twisted at its periphery in the direction of the oscillatory flow velocity induced by acoustic waves. Acoustic influence both upon the pseudo-plane and the planemicrojets results in their sinusoidal oscillations and bifurcation. New phenomena observed in the present experiments on the round and plane microjets are caused by a proportionality of the acoustic energy with that of the microjets.

Subsonic round and plane macrojets and microjets in a transverse acoustic field

26 In comparison with microjets, the investigation of free macrojets has attracted greater attention because of their wide use in various fields of science and engi� neering. However, interest in studying free microjets has recently increased considerably (1) because of, in particular, the development of MEMS technologies. The possibility of the potential use of microjets in var� ious processes appeared, for example, in microcool� ing, jet burning, nanopowder manufacture, etc. Spe� cial attention is given to the investigation of the acous� ticfield action on the microjet (2-5), which is of importance both for understanding the physics of the process and for the possibility of practical use of the phenomenon, for example, in aviation, astronautics, and the chemical industry. The purpose of these investigations consists in experimental study of the mechanism of developing round and plane macroand microjets under the acousticfield action and comparison of the obtained results with the results of recent works in this direction. In this study, we discussed the results of the experi� mental investigations of the mechanism of developing jet flows with small Reynolds numbers in a transverse acoustic field. We investigated the round macrojet (Re d = 5300, d = 20 mm) and microjet (Red = 20-60, d = 200-600 μm) and the plane macrojet (Reh = 3600, h = 14.5 mm) and microjet (Reh = 75, h = 200- 700 μm) depending on the velocity profile at the noz� zle end (from the shock one with the narrow velocity� gradient region and gradientfree jet core to the para� bolic one). Thermoanemometric measurements and the smoke visualization of flows were carried out. The patterns as a result of the smoke visualization of flows were obtained with the help of laser illumination of the jet synchronized with the acousticaction frequency, which enabled us to obtain new data on the mecha� nism of the development of jets.

The influence of riblets on the development of a Λ structure and its transformation into a turbulent spot

144 Riblets are passive elements mounted on a smooth wall surface in a turbulent boundary layer, which can reduce friction drag up to 10%. These elements have the form of streamwise grooves with triangular or hemispherical cross section whose dimensions are comparable with those of a viscous sublayer. The systematic investigations of riblets as a means of reducing turbulent friction began in the late 1970s at the NASA Research Center in Langley [1‐4]. These investigations showed that the friction drag decreases when the dimensionless parameter of riblets s + = is approximately equal to 15. However, the drag increases at s + = 30. For the maximum drag reduction, the riblets must be oriented in the direction of the local velocity of the viscous sublayer. The net drag reduction is almost linearly proportional to the coverage of the entire streamlined surface by the riblets. The near-wall turbulent structure of flows on riblets was extensively studied using physical [3, 5, 6] and numerical experiments (DNS) [7, 8]. According to one of the hypotheses put forward in order to explain the drag reduction in the turbulent boundary layer on riblets, they modify coherent structures in the viscous sublayer. It was found that riblets operate as an obstacle for the transversal (spanwise) oscillations of streamwise vortices, which results in drag reduction [9]. In other words, riblets reduce the friction drag in the turbulent boundary layer by changing the sequence of the nearwall vortex dynamics by means of a passive spanwise su* ν

Visualization of conventional and combusting subsonic jet instabilities

Results of experimental studies of round and plane propane micro jet combustion in a transverse acoustic field at small Reynolds numbers are presented in this paper. Features of flame evolution under the given conditions are shown. Based on the new information obtained on free micro jet evolution, new phenomena in flame evolution in a transverse acoustic field with round and plane propane micro jet combustion are discovered and explained.

Nonlinear Sinusoidal and Varicose Instability in a Boundary Layer

It is well known [1] that the laminar‐turbulent transition at a low turbulence level of the free flow is associated with the development of instability waves, the so-called Tollmien‐Schlichting waves. When a twodimensional Tollmien‐Schlichting wave reaches a certain amplitude at the nonlinear stage of its development, it undergoes three-dimensional distortion and, as a result, characteristic three-dimensional Λ structures arise [1]. Owing to certain features of the appearance and development of these structures, they are not only typical for the classical laminar‐turbulent transition, but are also inevitable attributes of a transition to more complex flows, e.g., flows modulated with longitudinal streaky structures, such as Hertler vortices, transverseflow vortices on sliding wings, etc., as well as flows in the viscous sublayer of a turbulent boundary layer. In these cases, they arise in particular due to the secondary high-frequency instability of such flows and may be manifested not only as Λ structures, but also in the form of horseshoe vortices ( Ω structures), hairpin vortices, etc. A characteristic feature of the development of such structures, e.g., on a sliding wing, is the disappearance of one of the counter-rotating vortices due to the transverse flow, whereas the development of a classical Λ structure can be observed on a straight wing [1]. The high-frequency secondary instability of transition and turbulent near-wall flows in the presence of streaky structures is often attributed to so-called sinusoidal and varicose instability. Both instability modes were investigated under controlled conditions at the linear and initial stages of nonlinear development. When the transverse size of the streaky structure was larger than the thickness of the shear layer, growth of varicose instability was observed. At the same time, when the transverse size of the streaky structure was comparable to or smaller than the thickness of the shear layer, it became more instable with respect to antisymmetric (sinusoidal) modes than to symmetric (varicose) modes. The experiment reported in [2] clearly shows that the growth of the symmetric mode leads to the formation of hairpin vortices, which are a pair of counterrotating longitudinal vortices that are connected by a head, i.e., a Λ vortex, while an antisymmetric mode is developed to a train of quasi-longitudinal vortices with alternating-sign vorticity. Unfortunately, the experiments reported in [2] concerned only the initial stage of the nonlinear development of disturbances, and spatial resolution was insufficiently high to reveal the structure of the flow in more detail. In this paper, we report on our experimental investigations of the nonlinear stage of the varicose and sinusoidal instability of the streaky structure in the Blasius boundary layer. In contrast to the experiment reported in [2], the study is more detailed (thermal anemometer measurements of the longitudinal velocity component and velocity pulsations in space ( xyz ) at 5 × 10 4 points) in order to reveal the features of the dynamics of the appearance, development, and internal structure of coherent formations up to the later stages of their nonlinear development. The experiments were carried out under controlled conditions in a low-turbulent wind tunnel. A plane plate 14 mm in thickness, 1000 mm in width, and 2000 mm in length was placed in parallel in the operation part of the tunnel. The streaky structure was generated by means of a cylindrical roughness element, which had a height of 1.1 mm and a diameter of 5.8 mm and was placed in the center of the plate at a distance of x 0 = 438 mm from the fore. The velocity of the flow was equal to U ∞ = 7.8 m/s, and the turbulence level was no higher than 0.04%. In the absence of the roughness element, the laminar boundary layer was developed without any waves and the velocity profile was close to the Blasius profile. A roughness-element height of h = 1.1 mm is close to the thickness of the displacement of

Experimental study of a Λ-structure development and its transformation into the turbulent spot

Results of an experimental research on study of the characteristics of the Λ-structures development and mechanism of their transformation into the turbulent spots are presented in this work. It is shown, that the isolated Λ-structure can be both decreasing, and increasing with transformation it into the turbulent spot downstream. Structurally both disturbances represent from self two counter-rotating vortices (legs of disturbance), closed in the field of leading edge by a head. Configuration distinction is, that if the decreasing Λ-structure reminds more hairpin vortex and develops within of a boundary layer, head of the increasing Λ-structure goes out far away over the border of a boundary layer and the disturbance gets a kind of the Greek letter — Λ. It is shown, that the increase of Λ-structure is connected with the development of secondary, high-frequency disturbances on the legs of a structure. The reason it, apparently, is presence of a local velocity gradient ∂u/∂z in area of the Λ-structure legs, which creates conditions for development on it secondary disturbances. It is shown, that the frequency of secondary disturbance decreases owing to a continuous stretching of a localized disturbance at its downstream development. The mechanism of secondary, high-frequency breakdown of the Λ-structures is observed and at their periodic generation.

Effect of riblets on nonlinear disturbances in the boundary layer

Results of experimental investigations of the nonlinear stage of sinusoidal and varicose instability of a streaky structure, which leads to multiplication of streaky structures and origination of coherent structures (such as Λ-structures), are presented. Riblets suppress the intensity of streaky structures, stabilize the flow against the development of the secondary high-frequency instability of streaky structures, and, for this reason, delay spatial turbulization of the flow. The results of these investigations can be useful for understanding the flow structure in such situations and for possible controlling of the coherent structures aimed at flow stabilization.

The flame structure in round and plane propane microjet combustion in a transverse acoustic field at low Reynolds numbers

The results of experimental studies of the structure and features of flame evolution under propane combustion in round and plane microjet flows at low Reynolds numbers in a transverse acoustic field are discussed in this paper. The specific features of flame evolution under these conditions are shown. Based on the new information obtained on free microjet evolution, new phenomena in flame evolution in a transverse acoustic field with round and plane propane microjet combustion are discovered and explained.