Remote acoustic diagnostics of a wake behind a heated cylinder

The possibilities of remote acoustic diagnostics of flows are experimentally and theoretically studied. A Karman vortex street behind a cylinder is used as a model. The vortex temperature exceeded that of the surrounding medium. A laboratory experiment carried out in a low-turbulence wind tunnel at Re100 has shown that vorticity, stall rate, spatial period, and vortex temperature can be found from the characteristics of sound scattered by the flow.Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 38, No. 8, pp. 832–840, August, 1995.     

Modification of the wake behind a circular cylinder by using synthetic jets

Experimental tests were conducted to control the flow around a cylinder by means of unsteady blowing (synthetic jet) through a single slot disposed on the wall of the model. The flow Reynolds number (based on the diameter of the model) wasR _D=10₅. The efficiency of the synthetic jet is quantified in terms of delaying separation and modifying the drag coefficient. The investigations were of three types: measurements of the mean pressure distribution, wall visualizations of the separation line position and measurements of the mean flow-field in the wake.     

3D simulation and visualization of unsteady wake flow behind a cylinder

In this paper we focus on 3D simulation of unsteady wake flow behind a circular cylinder. We show that in addition to accurate formulations and sufficiently-refined meshes, efficient computing methods are essential components of an effective simulation strategy. We use the Multi-Domain Method (MDM) we developed recently in computation of two cases. At Reynolds number 300, we demonstrate how the MDM enables us to use highly-refined meshes to capture wake patterns which we otherwise cannot fully represent. At Reynolds number 140, we show that with the MDM we can extend our computations sufficiently downstream, and with sufficient accuracy, to successfully capture the second phase of the Karman vortex street, which has been observed in laboratory experiments, and which has double the spacing between the vortices compared to the first phase.     

Self-similarity of far wake behind tandem of two disks

In this work we used digital particle image visualization (PIV) to experimentally establish the self-similarity of far wake behind a tandem of two disks of a diameter D (300 mm) with a common axis along the incident flow. The research was performed in a water flume (Re ≈ 2 · 10⁵) with variation of L, the longitudinal dimension of the tandem. The self-similarity of the velocity profile in the wake behind the tandem has been established; the level of turbulent fluctuations of the profile has been measured. Due to the influence of the second disk, the velocity deficit in the wake behind the tandem exceeded the corresponding value for a single disk, being independent of the distance between the disks (L = 4–8D). The velocity fluctuations behind the tandem did not differ much from the level of fluctuations in the case of a single disk up to a distance of forty calibers downstream, where the wake ceased to differ from the background of natural turbulent fluctuations of the incident flow. It has been found that the position of the second disk in the tandem affects the energy loss in the wake due to its expansion but does not influence the decay. The revealed patterns in the wake development behind tandems of bodies will enable optimization of construction of systems of repetitive elements and their movement in different flows.     

Coherence and Reynolds stresses in the turbulent wake behind a curved circular cylinder

The turbulent wake behind a curved circular cylinder is investigated based on data obtained from a direct numerical simulation. Here, emphasis is placed in the assessment of two approaches for simplified modelling: reduced-order modelling (ROM) and Reynolds-averaged Navier–Stokes equations. To this end, the instantaneous vortical structures, the proper orthogonal decomposition (POD) of the flow, and relevant Reynolds stress components have been analysed. The results show that despite the complexity of the instantaneous vortical structures, the wake dynamics are governed by the quasi-periodic shedding of primary vortices. Between 24% and 50% of the kinetic energy in the POD is captured by the two most energetic modes, and about 200 modes are needed to capture 90% of the kinetic energy. These findings suggest that, as long as the large-scale structures of the von Kármán vortex shedding are concerned, the present case can be approached by ROM; but a detailed representation of the flow dynamics without an eddy viscosity model that accounts for the unresolved turbulent fluctuations would require a large amount of degrees of freedom. Concerning the Reynolds stresses, their magnitude varies considerably depending on the depth at which they have been sampled. This dependence is related to the strength of the vortex shedding, and the intensity of the secondary flows induced by the curvature of the cylinder. As a consequence of the combination of these two effects, the correlation between streamwise and vertical velocity fluctuations is highest in the wake behind the midspan of the curved cylinder, and the correlation between cross-flow and vertical velocity fluctuations reaches large values in the lower wake.     

Experimental investigation on wake profile detection based on laser scattering by bubbles

The optical system for detecting wake profiles based on laser backscattering by bubbles at 180° is reported, in which the monostatic optical geometry is adopted and the power density estimation is used to process bubble scattering signal.     

Investigation of a wake decay behind a circular disk in a hydro channel at high Reynolds numbers

LDA and PIV techniques were used to study the decay of an axisymmetrical turbulent wake originated downstream an immobile disk in a water flume for Reynolds numbers Re = 1.5?2.4?10⁵. Data were compared with experiments performed with a set of thermo-anemometers behind a disk in a wind tunnel at lower Reynolds numbers (Re = 1.3?2.6?10⁴). Observations for a new range of Reynolds number confirmed that the velocity distribution in the disk wake keeps self-similarity. The decay of a wake by the power law ?2/3 in the presented experiments remains until the maximum deficit of velocity becomes comparable with the turbulent pulsation level in the free-stream (less than 2 %).     

Experimental study of the evolution of periodic controlled disturbances in a hypersonic viscous shock layer on a flat plate

Characteristics of the fields of mean density and density fluctuations measured with introduction of periodic disturbances into a hypersonic viscous boundary layer on a flat plate are presented. The experiments are performed for a flow Mach number M_∞ = 21, Reynolds number per meter Re_1∞ = 6·10⁵ m⁻¹, and temperature factor of the surface T _w /T ₀ = 0.26. The disturbances are introduced into the shock layer by an oblique gasdynamic whistle. The work was financially supported by the Russian Foundation for Basic Research (Grants Nos. 04-01-00474 and 05-08-33436).     

Effect of small total pulse on development of a wake behind the self-propelled bodies

Experimental studies on the wakes behind the self-propelled bodies of revolution have been analysed, and results of stereometric studies on the co-current flows behind autonomous streamlined and bluff bodies of revolution are presented for the times of their development corresponding to the coordinate system of a distant wake of 2400 and 1200 body diameters, correspondingly. Possible modelling of axisymmetric co-currents by the flows generated by a string at its pulse motion along the axis is examined. It is shown that supposed solutions based on the model of interferential development of wakes behind the self-propelled bodies of revolution can approximate both the known results of experimental studies of wakes behind the self-propelled bodies of revolution and the flows generated by corresponding pulse transportation of a string.     

Two-dimensional structure of the wake behind a pitching airfoil with higher non-dimensional pitching rate

Two-dimensional flow patterns have been visualized by two kinds of dyes and a VTR camera in a water tunnel atRe=4.0×10³ in order to study how flow patterns of vortex changed behind a pitching airfoil with various pitching rates and pitching amplitudes. The tested airfoil was NACA0010. The experiment was performed under the conditions such as the pitching motions with sinusoidal or triangular wave around its mid- or quarter-chord axis. The non-dimensional pitching rates werek=1.97, 2.81 and 4.22. The pitching amplitudes were changed asA=±2, ±6 and ±12, however the mean angle of attack was fixed atμ _m =0°. As a result, the followings were clarified. The flow pattern of thrust producing vortex street was observed in the wake behind the pitching airfoil with a higher nondimensional pitching rate. Similar flow patterns of the vortex were observed in spite of the differences in the location of pitching motion axis, in the pitching motion wave profile and in the pitching amplitude. Moreover, the pitching motion around the quarter-chord axis with triangular wave could realize the thrust producing vortex street more easily.     

Experimental investigation of the limits of ethanol combustion in the boundary layer behind an obstacle

Experimental data on the flow structure and mass transfer near the boundaries of the region existence of the laminar and turbulent boundary layers with combustion are considered. These data include the results of in-vestigation on reacting flow stability at mixed convection, mass transfer during ethanol evaporation “on the floor” and “on the ceiling”, when the flame surface curves to form the large-scale cellular structures. It is shown with the help of the PIV equipment that when Rayleigh–Taylor instability manifests, the mushroom-like structures are formed, where the motion from the flame front to the wall and back alternates. The cellular flame exists in a narrow range of velocities from 0.55 to 0.65 m/s, and mass transfer is three times higher than its level in the standard laminar boundary layer.     

Plasma ion evolution in the wake of a high-intensity ultrashort laser pulse

Experimental investigations of the late-time ion structures formed in the wake of an ultrashort, intense laser pulse propagating in a tenuous plasma have been performed using the proton imaging technique. The pattern found in the wake of the laser pulse shows unexpectedly regular modulations inside a long, finite width channel. On the basis of extensive particle in cell simulations of the plasma evolution in the wake of the pulse, we interpret this pattern as due to ion modulations developed during a two-stream instability excited by the return electric current generated by the wakefield.