Structures In Shear Layer Research Articles

Experimental characterization of non-premixed turbulent jet propane flames

Abstract This paper reports an experimental study conducted on turbulent jet propane flames aiming at further understanding of turbulent structure in non-premixed slow-chemistry combustion systems. Measurements of mean and fluctuating velocity and temperature fields, mean concentration of major chemical species, correlation between velocity and temperature fluctuations, and dissipation of temperature fluctuations are reported in a turbulent round jet non-premixed propane flame, Re =20 400 and 37 600, issuing vertically in still air. The experimental conditions were designed to provide a complete definition of the upstream boundary conditions in the measurement domain for the purpose of validating computational models. The measured data depicts useful flow field information for describing turbulent non-premixed slow-chemistry flames. Velocity–temperature correlation measurements show turbulent heat fluxes tended to be restricted to the mixing layer where large temperature gradients occurred. Observations of non-gradient diffusion of heat at x / D =10 were verified. Temperature fluctuation dissipation, χ , showed the highest values in the shear layer, where the variance of temperature fluctuations was maximum and combustion occurred. The isotropy between the temperature dissipation in the radial and tangential directions was confirmed. By contrast, the observed anisotropy between axial and radial directions of dissipation suggests the influence of large structures in the entrainment shear layer on the production of temperature fluctuations in the flame region. The value of the normalized scalar dissipation at the stoichiometric mixture fraction surface, χ st , was calculated, and ranges between 2 and 4 s −1 . The measured data were used to estimate the budgets in the balance equations for turbulent kinetic energy, Reynolds shear stresses, turbulent heat flux and temperature variance, quantifying the mechanisms involved in the generation of turbulence as well as in the transport of the temperature.

Large-eddy simulation of separated flow over a bluff rectangular plate

Abstract The turbulent separated-reattaching flow over a bluff rectangular plate is investigated using the large-eddy simulation (LES) technique. Simulations are presented for a Reynolds number (Red) of 50,000 and a blockage ratio (Br) of 5.6%. Three subgrid-scale models are used: structure function, selective structure function and Smagorinsky models. The performance of these models is examined by comparing the mean flow and turbulence statistics, and the dynamics of the flow with experimental observations. With both structure-function and Smagorinsky models, the break-up and three-dimensionalization of the separated shear layer are delayed. The dynamics of the reattaching flow is altered by the persistence of small-scale structures in the Smagorinsky model simulation, while excessive subgrid-scale dissipation is evident in the structure function simulation. Both models yield deficient mean flow structures and turbulence statistics. The selective version of the structure function model, which allows a localization of the subgrid-scale contribution, produces separated shear layer instabilities, dynamical patterns, and structures which are physically consistent with flow visualization. The mean flow and turbulent statistics obtained with this model are also found to be in excellent agreement with measurements. Using structure identification techniques based on the vorticity modulus |ω| and the eigenvalue λ2 of the tensor S ik S kj +Ω ik Ω kj , horseshoe vortices hypothesized in earlier experimental work are clearly identified in the reattachment region. Wavelet signal analysis reveals the persistence of scales associated with shear layer flapping and the intermittent nature of the pseudo-periodic shedding of vortices in the reattachment region.

Turbulence control in wall jets

A new method for the control of mixing of a plane turbulent wall jet has been investigated. A thin wire, mounted in the vicinity of the wall-jet nozzle, changes the formation of the shear-layer structures in the early stages of the development of the wall jet. The wire is operated in two ways: (1) a still wire inhibits the natural shear layer roll-up and reduces the size of the turbulent structures and thereby the mixing; (2) a self-excited oscillating wire introduces large coherent structures and thereby enhances the mixing. The size of these structures does not depend on the shear-layer instability but rather on the wire frequency. Measurements of the mean and fluctuating velocities have been performed with hot-wire anemometry as well as measurements of the skin friction by means of Preston tubes, surface fences and wall hot-wires. The Reynolds number based on the slot width was Re j =10000.

Low Reynolds number two-dimensional separated and reattaching turbulent shear flow

Pulsed-wire measurements of the streamwise mean velocity and Reynolds stress, mean and fluctuating surface shear stress, and other statistics have been made in a sharp- edged turbulent separation bubble formed behind a normal flat plate mounted on the front of a long splitter plate, covering nearly a decade range of low Reynolds number. The streamwise Reynolds stress increases appreciably with Reynolds number, from a level comparable with that in an isolated plane mixing layer to more than twice that level, while the change in the mean flow is at most slight. It is inferred using previous measurements that the other stresses do not change as much, thereby leaving the mean flow relatively unaffected. The mean wall shear stress and the r.m.s. of the (streamwise) fluctuations decrease in fixed proportion with increasing Reynolds number. Normalized p.d.f. distributions of velocity and wall shear stress do not change, except in the vicinity of the secondary separation bubble. At low Reynolds number the development length of the overlying mixing layer is an appreciable proportion of the bubble length, the development length being a function of the momentum thickness at separation. It is argued that the observed changes with Reynolds number in the bulk of the flow arise primarily as a result of a change in response of this layer to the fluctuating rates of strain imposed by the recirculating flow; at a low Reynolds number the response of the shear layer structures to the fluctuating rates of strain is less than it is at a high Reynolds number. As a consequence of the sensitivity to fluctuating strain rates the structure of the layer differs fundamentally from that of an isolated mixing layer.The measurements help to reconcile previous measurements in this flow geometry which otherwise appear to be inconsistent. Moreover, in most of the previous measurements the flow width was not sufficient for end effects to have been negligible, most noticeably in the near-wall flow, where such effects are largest.

Velocity field of a bluff-body wake

The characteristics of shear-layer vortex shedding and toroidal wake behind a circular disc are experimentally studied. Five characteristic flow structures, laminar, subcritical, transitional, supercritical, and turbulent modes, are identified in the shear layer evolving from the edge of the disk. The Strouhal numbers of the vortex shedding are about 0.12 and 0.20 in the subcritical and supercritical shear layers, respectively. The Taylor integral length scales of the shed vortices in the subcritical and supercritical shear layers are about one and 0.5 disc diameters, respectively. The flow patterns, recirculation length, as well as turbulence properties are closely related to the characteristic modes of shear-layer structures in different ranges of Reynolds numbers. The measured distributions of Reynolds stresses show that the simple Boussinesq approximation of gradient-type stress model is inappropriate for this flow field. The intensity of turbulence fluctuation superimposed on the vortices dominates the distributions of the Reynolds stresses.

Aeroacoustic properties of supersonic elliptic jets

The aerodynamic and acoustic properties of supersonic elliptic and circular jets are experimentally investigated. The jets are perfectly expanded with an exit Mach number of approximately 1.5 and are operated in the Reynolds number range of 25 000 to 50 000. The reduced Reynolds number facilitates the use of conventional hot-wire anemometry and a glow discharge excitation technique which preferentially excites the varicose or flapping modes in the jets. In order to simulate the high-velocity and low-density effects of heated jets, helium is mixed with the air jets. This allows the large-scale structures in the jet shear layer to achieve a high enough convective velocity to radiate noise through the Mach wave emission process.Experiments in the present work focus on comparisons between the cold and simulated heated jet conditions and on the beneficial aeroacoustic properties of the elliptic jet. When helium is added to the jet, the instability wave phase velocity is found to approach or exceed the ambient sound speed. The radiated noise is also louder and directed at a higher angle from the jet axis. In addition, near-field hot-wire spectra are found to match the far-field acoustic spectra only for the helium/air mixture case. These results demonstrate that there are significant differences between unheated and heated asymmetric jets in the Mach 1.5 speed range, many of which have been found previously for circular jets. The elliptic jet was also found to radiate less noise than the round jet at comparable operating conditions.

Flow field and noise characteristics of a supersonic impinging jet

This paper describes the results of a study examining the flow and acoustic characteristics of an axisymmetric supersonic jet issuing from a sonic and a Mach 1.5 converging–diverging (C–D) nozzle and impinging on a ground plane. Emphasis is placed on the Mach 1.5 nozzle with the sonic nozzle used mainly for comparison. A large-diameter circular plate was attached at the nozzle exit to measure the forces generated on the plate owing to jet impingement. The experimental results described in this paper include lift loss, particle image velocimetry (PIV) and acoustic measurements. Suckdown forces as high as 60% of the primary jet thrust were measured when the ground plane was very close to the jet exit. The PIV measurements were used to explain the increase in suckdown forces due to high entrainment velocities. The self-sustained oscillatory frequencies of the impinging jet were predicted using a feedback loop that uses the measured convection velocities of the large-scale coherent vortical structures in the jet shear layer. Nearfield acoustic measurements indicate that the presence of the ground plane increases the overall sound pressure levels (OASPL) by approximately 8 dB relative to a corresponding free jet. For moderately underexpanded jets, the influence of the shock cells on the important flow features was found to be negligible except for close proximity of the ground plane.

Large Eddy Simulation of Flow Transition in a Compressible Flat-Plate Boundary Layer at Mach 4.5

Large eddy simulation (LES) has been carried out to investigate the oblique transition process of a flat-plate boundary layer at a free-stream Mach number of M∞ = 4.5 and a Reynolds number of 10,000 based on the free-stream velocity and inflow displacement thickness. The numerical simulation is performed using a spatial approach to solve a full compressible Navier-Stokes system in the curvilinear coordinates. A compact sixth-order central difference scheme (Lele, 1992) is applied to the wall-normal direction and streamwise direction, the pseudo-spectral method is used in the spanwise direction. A compact storage third order Runge-Kutta scheme (Wray, 1986) is adopted for lime-integration. The sub-grid scales are formulated according to the filtered structure function model (Ducros et al., 1996). A pair of oblique first-mode perturbation is imposed on the inflow boundary. Several stages of transition process can be identified, i.e., the linear and weak nonlinear growth of disturbance, the appearance of staggered A-vortex pattern, the evolution of A-vortex into hairpin vortex, and the break-down of hairpin vortical structures. The organized motions during the transition are discussed in detail. The vortical and the shear layer structures are compared with the results obtained by Adams and Kleiser (1996) in their direct numerical simulation based on a temporal approach. The evolution of the averaged quantities, such as the boundary layer thickness are studied. The skin friction coefficient and the turbulent mean velocity profile obtained from the simulation agree well with the flat-plate theory of Van Driest (1956).

Three-Dimensional Structure of Lifted, Turbulent-Jet Flames

Previous planar imaging measurements in lifted, turbulent jet flames provide evidence for the three-dimensionality of these flows and the importance of flame propagation in the azimuthal direction normal to the image plane. The primary objective of the present paper is to obtain data in a plane normal to the flow direction that will provide a better understanding of the three-dimensional features of the flow. In addition, the results of the previous measurements were limited to a Reynolds number of 7,000. imaging data at a Reynolds number of 12,100 will be presented to extend the previous findings to a higher Reynolds number regime. Images of the radial cross section in the low Reynolds number flame show that considerable asymmetry and structure exist. At upstream locations, the flame is typically not continuous around the circumference of the jet but consists of multiple flame segments. Farther downstream, the flame extends over a larger percentage of the circumference with breaks confined to smaller lengths along the circumference. The CH chemiluminescence images show numerous breaks in the flame sheet. These breaks coincide with outward bulges in the flame sheet that result from the formation of vortical structures in the jet shear layer. The images clearly show the three-dimensionality of the flow, since the breaks in the flame are localized to the area of the bulges and do not extend around the circumference of the central jet. Many of the same features found in the lower Reynolds number flame are also seen at the higher Reynolds number. Consistent with the larger turbulence scales existing farther downstream where this flame is stabilized, the flame surface is more convoluted and breaks in the flame sheet are more extensive. Rapid discontinuous jumps upstream by the flame are seen considerably more often than in the lower Reynolds number flame, which is indicative of the higher turbulence levels and greater three dimensionality of this flame.

Time-Resolved Temperature Measurements in the Developing Region of an Elliptic, Jet-Diffusion Flame at a Reynolds Number of 6000

Abstract Time resolved temperature measurements were performed in the developing region of an elliptic, jet-diffusion flame using fine-wire thermocouples compensated for thermal inertia effects. Measurements of mean and rms temperature profiles, as well as power spectral densities and probability density functions of temperature were obtained along the centerline and radially along the major and minor axes for nondimensional axial stations in the range of 5 = z/Dh = 30. Significant differences in the thermal structure of this elliptical jet flame along the minor and major axes were observed. Higher temperature fluctuations along the minor axis in the fuel-side mixing layer and higher temperature fluctuations along the major axis in the air-side mixing layer were observed. Stronger, buoyancy-driven flow structures in the air-side shear layer caused more radial movement of the reaction zone along the major axis as evidenced by rms temperature profiles with substantially higher values (200 K.) farther out ra...

The effect of counterflow on the development of compressible shear layers

A compressible countercurrent shear layer was investigated experimentally by establishing reverse flow around the perimeter of a supersonic jet. Measurements demonstrate that spatial growth rates of the countercurrent shear layer significantly exceed those of the classical coflowing layer at comparable density ratios and levels of compressibility. Experiments also reveal the presence of coherent three-dimensional structures in the countercurrent shear layer at convective Mach numbers where similar structures are not present in coflowing layers. It is argued that these kinematic differences are responsible for the enhanced diffusion of the shear layer with counterflow. The spatio-temporal theory is used to examine the connection between the experimental observations and the existence of a transition from convective to absolute instability in high-speed shear layers.

Turbulent Structure and Generation Mechanism of Coherent Vortex in Cavity Shear Layer of Open-channel Flows

Turbulence measurements in and over the cavity of open-channel flows were conducted by making use of a two component fiber-optic laser Doppler anemometer (FLDA). Mean Structures in the cavity shear layer were examined in detail and it was revealed that the turbulent structures depended on the length of cavity significantly.The visualization of coherent vortices was carried out by the use of dye injection techniques, and the characteristics of coherent and unstable vortices around separation point were clarified by means of unstable analysis. The fundamental frequency was determined and unstability of coherent vortices was examined.

Near-Wall Bursting Phenomena and Quasi-Streamwise Vortex Models.

Near-wall bursting phenomena have been detected both by the VITA technique and by a quadrant analysis. In addition to these detection schemes, visual burst detection has been performed in order to characterize the difference between VITA events and Q2 (second quadrant) events. It is shown that the spanwise yaw angle of visualized low-speed streaks for VITA events is greater than for Q2 events. Based on the experimental findings and the Euler equation, quasi-streamwise vortex models are proposed, and the results are compared with the experiments. Fluctuating velocity signals computed from the models are in good agreement with the experimental data for VITA events. Contour plots of the calculated axial velocity, spanwise vorticity and instantaneous Reynolds shear stress are presented. The generation processes of shear layer structures and Q2 and Q4 motions are demonstrated.

Near-Wall Bursting Phenomena and Quasi-Streamwise Vortex Models.

Near-wall bursting phenomena have been detected both by the VITA technique and by quadrant analysis. In addition to these detection schemes, visual burst detection has been performed in order to characterize the difference between VITA events and Q2 (second quadrant)events. It is shown that the yaw angle of visualized low-speed streaks for VITA events is greater than for Q2 events. Based on the experimental results and Euler equation, quasi-streamwise vortex models are proposed, and the results are compared with the experiments. Fluctuating velocity signals computed from the models are in good agreement with the experimental data for VITA events. Contour plots of the calculated streamwise velocity, spanwise vorticity and instantaneous Reynolds shear stress are presented. The generation processes of shear layer structures and Q2 and Q4 motions are shown.

Turbulent structure and entrainment in heated jets: The effect of initial conditions

The turbulent structure in the near field of heated jets was investigated at a Reynolds number of 10 000 for density ratios between 1.0 and 0.5; the corresponding mixing and entrainment of these low density jets was examined. The exit conditions of the jet were carefully controlled using extension tubes to alter the nozzle-exit boundary layer thickness, as well as using screens to generate turbulent conditions while keeping the boundary layer thickness approximately constant. Heated jets with initially laminar exit conditions were dominated by the formation and pairing of vortex structures. Increasing the boundary layer thickness produced longer wavelength structures which saturated and paired at farther downstream distances; under these conditions jet momentum mixing was reduced, giving rise to an increase in the jet potential core length. The shear layer structures also became more organized as the jet density was reduced relative to the density of the ambient fluid, resulting in increased momentum mixing and a dramatic visual spreading of the jet. Turbulent exit conditions disrupted the formation of these large-scale vortex structures for all of the density ratios investigated, producing smaller spreading rates and significantly lower mixing and entrainment.

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.

The interaction between a plane shear layer and a slender body

The interaction of a plane shear layer with a thin flat plate located in the nonlinear region of the shear layer has been investigated experimentally. The shear layer's velocity ratio is 0.375, and its Reynolds number is ϱΔUθ 0/ μ = 625. It is found that for different angles of attack of the plate, the mixing layer is deflected toward the slower stream. In addition, the plate attenuates turbulent fluctuations of the shear layer structures.

Phase decorrelation of coherent structures in a free shear layer

The vortices near the origin of an initially laminar mixing layer have a single frequency with a well-defined phase; i.e. there is little phase jitter. Further downstream, however, the phase jitter increases suddenly. Even when the flow is forced, this same transition is observed. The forcing partially loses its influence because of the decorrelation of the phase between the forcing signal and the passing coherent structures. In the present investigation, this phenomenon is documented and the physical mechanism responsible for the phase decorrelation is identified.

Evolution of coherent structures in a plane shear layer

A two-dimensional plane shear layer perturbed by small-amplitude plane acoustic waves was studied experimentally. The perturbation consists of two different modes with equal amplitude, a fundamental mode and its first subharmonic. The evolution of the shear layer is influenced substantially by the phase difference between the two perturbation modes, resulting in two distinct vortex merging processes. For the pairing process, two neighboring vortices rotate around one another and gradually amalgamate. In a certain range of the phase difference, every other vortex weakens and vanishes within a few wavelengths while all the vortices remain aligned. In this tearing process of amalgamation, the subharmonic mode does not develop as large amplitude as during the pairing process. The energy distributions as well as the growth of the shear layer are different for these two kinds of vortex amalgamation.

On turbulent spots in plane Poiseuille flow

Turbulence characteristics inside a turbulent spot in plane Poiseuille flow are investigated by analysing a database obtained from a direct numerical simulation. The spot is found to consist of two distinct regions — a turbulent area and a wave area. The flow inside the turbulent area has a strong resemblance to that found in the fully developed turbulent channel. Suitably defined mean and r.m.s. fluctuations as well as the internal shear-layer structures are found to be similar to the turbulent counterpart. In the wave area the inflexional mean spanwise profiles cause a rapid growth of oblique waves, which break down to turbulence. The breakdown process of the oblique waves is reminiscent of the secondary instability observed during transition to turbulence in channel and boundary-layer flows. Other detailed characteristics associated with the Poiseuille spot are presented and are compared with experimental results.