Single Shear Layer Research Articles

The flow and heat transfer behavior of synthetic jets with star shaped orifice of different lobes

In the present experimental investigation, the flow and heat transfer characteristics of synthetic jet (SJ) issuing from different lobed (3–10) star-shaped orifices are studied. Tests are performed by actuating the SJ actuator at constant input power of 4Vrms, with varied range of orifice to surface spacings (1–16) and numerous values of Reynolds number (8692–10746). The synthetic jet's flow characteristics are studied using hot-wire anemometry, while the thermal characteristics are analyzed using an IR camera. Distinct peaks in turbulence intensity are observed in major and minor planes for different lobed (3–6) star orifices; this may be due to the development of two shear layers in such orifices, which consequently merges into a single shear layer for higher lobed (8–10) star orifice. The 6-lobed star orifice is found to exhibit maximum heat transfer performance at z/d varying between 2 and 14. At z/d = 2, the 6-lobed star orifice exhibit 7.6% and 10.1% improvement in average Nusselt number compared to circular and 10-lobed star orifice respectively, which may be due to formation of vortical structures along the lobed edges and subsequent switching of axes between the major and minor planes. To comprehend the energy associated with the issuing vortices, the power spectral density of the velocity signal at the orifice exit is examined. A unified correlation for the average Nusselt number is also derived for various star-shaped orifices.

Buzz Flows in an External-Compression Inlet with Partially Isentropic Compression

A rectangular external-compression supersonic inlet with partially isentropic compression is experimentally studied at a freestream Mach number of 2.0. During the test, the inlet operates at the little buzz state after going through the supercritical state and the steady subcritical state. A striking feature of the little buzz state is that both the most upstream and downstream positions of the unsteady separation point of the forebody boundary layer take on a linear variation trend with the increase of throttling ratio, yielding an intensifying flow oscillation. Then, the mixed buzz phenomenon that the little and big buzz coexist appears. As the throttling ratio further rises, the occurrence probability of little buzz is reduced, and finally the big buzz state takes place. In this state, two unique transient flow patterns with the inflow and supersonic backflow coexisting at the duct entrance can be observed. Different from the prevailing viewpoint, the present little buzz belongs to the Dailey instability and shares a common origin with the big buzz in essence. Furthermore, a theoretical analysis based on Fisher’s criterion indicates that a single shear layer from a -shaped terminal shock with an upstream Mach number below 1.60 is unable to trigger the Ferri instability of a supersonic inlet.

초음속 2차원 2단 혼합층에서 중간층의 역할

The basic flow configuration is composed of a plane, double shear layer where relatively thin mid gas layer is sandwiched between air and fuel stream. The present study describes numerical investigations concerning the combustion enhancement according to a variation of mid layer thickness. In this case, the effect of heat release in turbulent mixing la yers is important. For the numerical solution, a fully conservative unsteady 2 nd order time accurate sub-iteration method and 2 nd order TVD scheme are used with the finite volume method including k-ω SST model. The results consists of three categories; single shear layer consists of fuel and air, inert gas sandwiched between fuel and air, cold fuel gas sandwiched between fuel and air. The numerical calculations has been carried out in case of 1, 2, 4 mm of mid layer thickness. The height of total gas stream is 4 cm. The combustion region is broadened in case of inert gas layer of 2, 4 mm thickness and cold fuel layer of 4 mm thickness compared with single shear layer.

The Kelvin-Helmholtz instability in weakly ionized plasmas - II. Multifluid effects in molecular clouds

We present a study of the Kelvin-Helmholtz instability in a weakly ionised, multifluid MHD plasma with parameters matching those of a typical molecular cloud. The instability is capable of transforming well-ordered flows into disordered flows. As a result, it may be able to convert the energy found in, for example, bowshocks from stellar jets into the turbulent energy found in molecular clouds. As these clouds are weakly ionised, the ideal magnetohydrodynamic approximation does not apply at scales of around a tenth of a parsec or less. This paper extends the work of Jones & Downes (2011) on the evolution of the Kelvin-Helmholtz instability in the presence of multifluid magnetohydrodynamic effects. These effects of ambipolar diffusion and the Hall effect are here studied together under physical parameters applicable to molecular clouds. We restrict our attention to the case of a single shear layer with a transonic, but super-Alfvenic, velocity jump and the computational domain is chosen to match the wavelength of the linearly fastest growing mode of the instability. We find that while the introduction of multifluid effects does not affect the linear growth rates of the instability, the non-linear behaviour undergoes considerable change. The magnetic field is decoupled from the bulk flow as a result of the ambipolar diffusion, which leads to a significant difference in the evolution of the field. The Hall effect would be expected to lead to a noticeable re-orientation of the magnetic field lines perpendicular to the plane. However, the results reveal that the combination with ambipolar diffusion leads to a surprisingly effective suppression of this effect.

Structures of separation on a circular cylinder in periodic flow

The structures of separation on a smooth circular cylinder immersed in a sinusoidally oscillating flow are examined in detail for a constant value of the frequency parameter )-plane. The positions of the separation points are measured using sublayer fences, flush-mounted hot-film sensors, and extensive high-speed video recordings. The variations of the length scales of the resulting structures, the irregularity of the dye concentration fields, and measurements with two sensors have shown that the separation is three-dimensional, time-dependent, often turbulent, and far from being an eruption of a double-sided single shear layer, or a self-contained bubble. The increase of the three-dimensionality of the flow, evolution of various sizes of structures, secondary separations within the primary separation zone, and the occasional eruption of multiple shear layers are quite similar to the first direct numerical simulation of a laminar separation bubble in the presence of an oscillating inlet flow.

The counterpropagating Rossby wave perspective on Kelvin Helmholtz instability as a limiting case of a Rayleigh shear layer with zero width

The Kelvin Helmholtz (KH) problem, with zero stratification, is examined as a limiting case of the Rayleigh model of a single shear layer whose width tends to zero. The transition of the Rayleigh modal dispersion relation to the KH one, as well as the disappearance of the supermodal transient growth in the KH limit, are both rationalized from the counterpropagating Rossby wave perspective.

Properties of single shear layer instabilities and vortex-induced excitation mechanisms of thick plates

The basic findings of an experimental investigation of flow-induced vibrations of gate plates with multiple degrees of freedom are presented. The study focused on the fluid dynamic behaviour of the single shear layer separating from a thick rectangular plate. The principal aim of the study was to further the physical understanding of instability-induced excitation mechanisms (IIE) involving shear layer instabilities and vortex generation. It is shown that this type of gate vibration is caused by two dominant excitation mechanisms: cross-flow and streamwise impinging-leading-edge-vortex (ILEV) excitation and streamwise body-resonant leading-edge-vortex-shedding (BR-LEVS) excitation. The first mechanism is caused by the local interaction of the shear layer underneath the gate plate with the trailing edge of the gate lip. The second mechanism is produced by the instability of the shear layer in the tailwater of the gate, with the instability being necessarily triggered by the elastic motion of the flow separation at the leading edge of the gate.

Wake of a Self-Propelled Body, Part 1: Momentumless Wake

Experiments wereperformedin theturbulent boundary layerand nearwakeofan axisymmetricbody propelled by a jet to study the evolution of the momentumless wake. Comparisons with measurements in the drag wake of the body(without the jet )and in the isolated jet provide an understanding of initial mixing between the two e ows. Triple-sensor hot wires and multitube pressure probes were used to measure the mean velocity, turbulence, and pressure e elds from the jet exit to a distance of over 15 jet diameters. It is found that the evolution of the wake takes place in three distinct stages: a zone close to the jet exit, about 4 jet diameters long, where the jet shear layer mixes with e uid from the wall region of the boundary layer; an intermediate region, about 12 jet diameters long, where there is mixing between the boundary layer and the jet up to the axis; and the third region where the two e ows lose their identities to become a single shear layer and the mean e ow acquires some of the characteristics of self-similar e ows. However, the momentumless wake does not conform to the assumptions and results of classical similarity analysis.

Wake of a Self-Propelled Body, Part 2: Momentumless Wake with Swirl

Experiments were performed in the near wake of an axisymmetric body propelled by a swirling jet. Comparisons with parallel experiments in the drag wake of the bare body without the jet and in the isolated swirling jet provide insights on the mixing between the component flows and their evolution into the momentumless wake with swirl. Comparisons with the momentumless wake without swirl analyze the effect of swirl in this type of wake. Comparison with experimental data in the wake of the same body propelled by a marine propeller are also presented. Hot-wire and pressure probes were used in a wind tunnel to measure the mean velocity, turbulence, and pressure fields from the jet exit to a distance of over 15 jet diameters. The wake evolves in at least three distinct stages: a zone close to the jet exit where the jet periphery mixes with the wall region of the body boundary layer, an intermediate region where the mixing between the boundary layer and the jet spreads up to the axis, and a region where the two flows lose their identities to become a single shear layer in which there is negligible turbulence production

NOISE FROM SUPERSONIC COAXIAL JETS, PART 2: NORMAL VELOCITY PROFILE

Instability waves have been established as noise generators in supersonic jets. Recent analysis of these slowly diverging jets has shown that these instability waves radiate noise to the far field when the waves have components with phase velocities that are supersonic relative to the ambient speed of sound. This instability wave noise generation model has been applied to supersonic jets with a single shear layer and is now applied to supersonic coaxial jets with two initial shear layers. In this paper the case of coaxial jets with normal velocity profiles is considered, where the inner jet stream velocity is higher than the outer jet stream velocity. To provide mean flow profiles at all axial locations, a numerical scheme is used to calculate the mean flow properties. Calculations are made for the stability characteristics in the coaxial jet shear layers and the noise radiated from the instability waves for different operating conditions with the same total thrust, mass flow and exit area as a single reference jet. The effects of changes in the velocity ratio, the density ratios and the area ratio are each considered independently.

Experimental and Numerical Studies of Shear Layers in Granular Shear Cell

The stability of a shear layer inside a granular material in a gravity field is studied experimentally and numerically. A shear cell is built of transparent acrylic to visualize the motion of the granular material. This shear cell consists of two concentric cylinders containing layers of uniform spheres in the annular space between the cylinders. The shearing motion of the spheres is produced by rotating the bottom boundary of the cell. Friction of the cylinder walls resists the shear motion, thus creating a single shear layer adjacent to the bottom boundary, while the rest of the layers above move with constant speed as a solid body. As the rotation speed of the bottom boundary increases, two layers adjacent to the bottom boundary begin to shear. This shearing zone quickly thickens and dilates as the rotational speed increases. The transition of this shear motion from a single layer to many layers of shearing is studied by video recording. The initiation of this transition is observed to depend on the material properties and the number of layers overlaying the shear layer. A one-dimensional numerical model is constructed to bring insight into this transitional phenomenon.

Characteristics of single and clusters of bursting events in the inner layer

A new method of identification of bursting events in a turbulent boundary layer is presented and applied to VITA. The philosophy adapted here is similar to that of Tiederman (1988). It consists of the classification of individual events into groups corresponding to multiple breakups of the same streak (Bogard and Tiederman, 1986). The strategy is however different, in that it uses differences in the signatures of the events belonging to the same cluster. The iterative procedure finds out these differences and classifies events which have similar signatures. It is found that the distribution of the frequency of groups of events identified this way differs from the distribution of the solitary ones. The “multiple shear layer events” (MSLs) have strikingly different signatures compared to “single shear layer events” (SSLs). These differences are mostly present near the wall and disappear progressively at the beginning part of the constant shear layer. The contribution of the MSLs to the Reynolds shear stress is larger. The differences in the conditional averages of the streamwise and wall normal velocities are highly suggestive that the regeneration mechanism of the clusters of events near the wall is dynamically different.

Vortex shedding mechanism from prisms having H and I sections

In case of the flow around the H-section prism, a single and double shear layer instabilities coexist. It is found that the fundamental frequency due to the single shear layer instability along the shear layer between the upper (or lower) leading edge and upper (or lower) trailing edge is adjusted so as to coincide with that due to the double shear layer instability behind the prism, and vice versa. On the other hand, in case of the flow around the I-section prism, there exists only a double shear layer instability, and asymmetry of the vortex pair in the vortex formation region and the subsequent transformation into the unstable system of the three vortices, viz., upper main vortex, lower main vortex and third vortex, leads to the alternate vortex shedding. It is realized that the vortex shedding mechanism from the I-section prism is essentially same as that from a circular, triangular or square prism.

Two-Dimensional Large-Scale Structures in a Compressible Shear Layer.

The effect of compressibility on the two-dimensional large-scale structures in a single shear layer was investigated for the temporally developing case. Each eddy is elongated by compressibility through (i) the formation of volumetric source and sink distribution and (ii) the emergence of unstable travelling waves under the condition that the Mach number of free streams, M, exceeds unity. The dilatation accelerates the streamwise velocity and decelerates the transverse velocity, so that the convection of vorticity makes the resulting eddy elongated in the streamwise direction and reduces its own angular velocity of rigid-body rotation. The eddy formed at M>1 takes an airfoil-like shape and has almost the same thickness as the original shear layer, due to an existing phase velocity different from the mean value of the two free stream velocities. Each eddy can propagate by itself in such a way that its leading edge rolls up the tail of the preceding eddy.

Sound production at the edge of a steady flow

The theory initiated by Lighthill (1952) to describe the sound radiated by turbulence embedded in an uniform fluid at rest is here extended to the case where the turbulence exists on the edge of a uniformly moving stream. An exact analogy is developed between the distant real sound field and that which would be radiated by a particular quadrupole distribution adjacent to a vortex sheet positioned in the linearly disturbed flow. The equivalent sources in this analogy are quadrupoles identical in strength with those in Lighthill's model, but the quadrupoles are now shown to convect with the fluid-particle velocity. There is no amplifying effect of shear. The particular case of a plane shear layer is worked out in detail for sound waves of scale large in comparison with the shear-layer thickness.A downstream zone of silence is predicted as is the formation of highly directional beams associated with the interference of sound radiated directly and sound reflected from the fluid interface. A distinct structure results in which the variation of sound with flow velocity, density and angle is not easily accounted for by simple power-law scaling. Finally a comparison is made with some features of jet noise; the modelling of the high frequency jet noise problem by a single shear layer yields some features consistent with experiment.