V. V. Kozlov

Transition experiments in a boundary layer with embedded streamwise vortices

The stability of a flat plate boundary layer modulated by stationary streamwise vortices was studied experimentally in the T‐324 low speed wind tunnel in Novosibirsk. Vortices were generated inside the boundary layer by means of roughness elements arranged in a regular array along the spanwise (z‐) direction. Transition is not caused directly by these structures, but by the growth of small amplitude traveling waves riding on top of the steady vortices. This situation is analogous to the transition process in Gortler and cross‐flows. The waves were found to amplify up to a stage where higher harmonics are generated, leading to turbulent breakdown and disintegration of the spanwise boundary layer structure. For strong modulations, the observed instability is quite powerful, and can be excited ‘‘naturally’’ by small uncontrollable background disturbances. Controlled oscillations were then introduced by means of a vibrating ribbon, allowing a detailed investigation of the wave characteristics. The instability...

An experimental study of the influence of riblets on transition

An experimental study of the effect of riblets on three-dimensional nonlinear structures, the so-called Λ-vortices on laminar-turbulent transition showed that riblets delay the transformation of the Λ-vortices into turbulent spots and shift the point of transition downstream. This result is opposite to the negative influence of such ribbed surfaces on two-dimensional linear Tollmien-Schlichting waves (the linear stage of transition). Thus, the ribbed surface influences laminar-turbulent transition structures differently : a negative influence on the linear-stage transition structures and a positive influence on the nonlinear-stage transition structures. It is demonstrated that transition control by means of riblets requires special attention to be paid to the choice of their location, taking into account the stage of transition.

The influence of riblets on a boundary layer with embedded streamwise vortices

The present study shows that surface manipulation makes it possible to suppress longitudinal vortex structures in a boundary layer, and thereby stabilize it with respect to high‐frequency traveling waves and delay the transition to turbulence. The results should be of interest for the control of transition in cross‐flow and Gortler flow.

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

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.

Adverse pressure gradient effect on nonlinear varicose instability of a streaky structure in an unswept wing boundary layer

Nonlinear varicose instability of a streaky structure is experimentally investigated in a wing boundary layer. Spatial evolution of the streaky structure perturbed by high-frequency traveling waves is compared at zero and adverse streamwise pressure gradients. The process of streak multiplication and generation of ?-shaped vortices is under examination. Results obtained testify to the strong influence of pressure gradient upon the breakdown of the streaky structure due to its varicose instability.

Combustion of a high-velocity hydrogen microjet effluxing in air

This study is devoted to experimental investigation of hydrogen-combustion modes and the structure of a diffusion flame formed at a high-velocity efflux of hydrogen in air through round apertures of various diameters. The efflux-velocity range of the hydrogen jet and the diameters of nozzle apertures at which the flame is divided in two zones with laminar and turbulent flow are found. The zone with the laminar flow is a stabilizer of combustion of the flame as a whole, and in the zone with the turbulent flow the intense mixing of fuel with an oxidizer takes place. Combustion in these two zones can occur independently from each other, but the steadiest mode is observed only at the existence of the flame in the laminar-flow zone. The knowledge obtained makes it possible to understand more deeply the features of modes of microjet combustion of hydrogen promising for various combustion devices.

Subsonic Round and Plane Macro/Micro‐jets in a Transverse Acoustic Field

Results of experimental studies on the mechanism of plane evolution and round macro‐and micro‐jet flows at low Reynolds numbers in a transverse acoustic field are discussed. New data on the jet development mechanism are obtained through hot‐wire measurements. Smoke visualization of the jet flow with the use of stroboscopic laser illumination of the jet at frequencies of the acoustic influence on the latter. A lot of the new phenomena in development of the macro and micro‐jets in a cross acoustic field are found out.