Search

The role of hydrodynamics resulting in the mixed-mode oscillations (MMO) of unsteady pressure in a backward facing step combustor with partially premixed flame is investigated using time-resolved particle image velocimetry and CH* chemiluminescence imaging. This mode of pressure oscillations comprises switching between high-amplitude oscillations dictated by acoustic mode and/vortex mode and low-amplitude oscillations. The high-amplitude oscillations are marked by intense flow and flame modulation, while the low-amplitude oscillations are accompanied by small-scale structures on the shear layer, that are sedate in driving the heat release oscillations. The small-scales override on the elongated flame that resides in the recirculation zone of the step. The key aspect of the present work is in identifying the transitional events that result in the decay/ onset of high-amplitudes. This is achieved by tracking vortex cores at each instant during the transition. The small-scale shear layer structures, that are present during low-amplitude events merge as they convect with the flow. The identified vortex cores on the shear layer and in the recirculation zone reduce in their multiplicity as the merger progresses. This subsequently results in the genesis of a large-scale structure that modulates the flame coherently, thereby triggering high-amplitude oscillations. This is corroborated with identifying the presence of a single vortex core resulting in high amplitude events.

Vegetation occurring along river margins deeply alters the hydrodynamic flow structure, inducing a characteristic shear layer profile and triggering the onset of large scale vortices, which, in turn, govern the lateral exchanges across the vegetated interface. In such systems, vegetative drag plays a key role, regulating the velocity difference between the vegetated area and the adjacent main channel. In this study, the link between vegetation density, differential velocity ratio, and presence of large scale coherent structures is experimentally explored. Tests were performed considering different vegetation density and focusing on the lateral distributions of velocity statistics and presence of coherent turbulent structures. Results showed that, with decreasing vegetation density, the favorable conditions for the onset of large scale structures can disappear with direct effects on velocity distributions and efficiency of lateral momentum transport. Based on the test results, a critical value of differential velocity ratio is suggested.

We study the flow of a viscous incompressible liquid in a cylindrical duct with two serial diaphragms by finding the numerical solutions of nonstationary Navier–Stokes equations. It is shown that, within a certain range of Reynolds numbers, the flow of liquid in the region between the diaphragms is characterized by the presence of an unstable shear layer. A series of sequential vortices formed in the shear layer generates self-sustained oscillations of the velocity profile in the opening of the second diaphragm. It is shown that the analyzed system also has two stable modes of periodic oscillations with different frequencies. These oscillations may play the role of an acoustic source in the duct.

The delayed detached-eddy simulation with adaptive coefficient (DDES-AC) method is used to simulate the baseline and leading-edge undulation control of dynamic stall for the reverse flow past a finite-span wing with NACA0012 airfoil. The numerical results of the baseline configuration are compared with available measurements. DDES and DDES-AC perform differently when predicting the primary and secondary dynamic stalls. Overall, DDES-AC performs better owing to the decrease of grey area between the strong shear layer and the fully three-dimensional separated flow. Moreover, the effects of the undulating leading-edge on the forces, lift gradients, and instantaneous flow structures are explored. Compared with the uncontrolled case, the lift gradient in the primary dynamic stall is reduced from 18.4 to 8.5, and the secondary dynamic stall disappears. Therefore, periodic unsteady air-loads are also reduced. Additionally, the control mechanism of the wavy leading edge (WLE) is also investigated by comparison with the straight leading edge (SLE). No sudden breakdown of strong vortices is the main cause for WLE control.

Flows around a dual step cylinder are numerically investigated at a Reynolds number of 200 for three diameter ratios (D/d) of 2, 1.43 and 1.19. Simulation results reveal three distinctive vortex shedding modes: (i) the N-S mode for D/ d = 2 , (ii) the transition mode for D/ d = 1 . 43 , and (iii) the L-S mode for D/ d = 1 . 19 . The vortex shedding frequency transition from the smaller-diameter cylinder to the larger-diameter one, the dominant shedding frequency downstream of the larger cylinder, and the vortex interactions exhibit significant differences in the three vortex shedding modes. As D/ d = 2 , the direct cross-boundary and half-loop connections take place downstream of the step where vortex dislocations are essentially fixed. When decreasing D/d, two new vortex interactions, the double-half-loop and three-half-loop connections, appear with a larger spanwise vortex dislocation occurrence range. In addition, flows in different D/d cases are governed by a distinctive vortex shedding law. A pair of streamwise vortices occur downstream the centroid of the larger cylinder. This reduces the cylinder step effect on the local vortex shedding, absorbing the shear layer of the spanwise N-cell vortices, and making the hydrodynamic force coefficients smaller. Interestingly, these fluid force coefficients further decrease as D/d increases.

The focus of this work is on assessing the ability of an array of discrete finite-core vortices to model vorticity-dominated flows where vortex sheets that have not fully rolled up into isolated vo...

International audience; The presence of elongated streaks of high and low streamwise velocity in the shear layer of circular jets breaks the axisymmetry of their steady-state solution. If the streaks are considered to be part of the base flow, for the purpose of linear instability analysis, the instability eigenmodes are thus affected by their presence. The resulting changes of growth rate and spatial shapes of eigenmodes, related to the shear instability in jets, are investigated here for parallel base flows. Optimal streamwise vortices (rolls) with prescribed azimuthal periodicity are computed, such that the transient temporal growth of the streaks that they produce is maximal. The presence of nite-amplitude streaks requires the formulation of eigenvalue problems in a two-dimensional cross-plane. Sinuous rolls and streaks are found to have a stabilising effect on Kelvin-Helmholtz instability, whereas the varicose rolls and streaks have a destabilising effect. Absolute instability is not found to occur. This work shows that the effects of rolls and streaks need to be taken into account for more precise modelling of jet instability.

This paper addresses the effect of expansion ramps on the decay characteristics of a controlled supersonic rectangular jet. A Mach 1.8 rectangular jet issuing from a converging-diverging nozzle of aspect ratio 2 and Reynolds number 1.861?105 is considered as the base model for the study. Two separate nozzle models with expansion ramps placed at the diagonal ends on major and on the minor internal surface are considered for the present study. The diagonal placements of ramps induce additional vortices near the nozzle exit apart from the corner vortices emanating from the rectangular corners. Experimental and computational investigations at different expansion levels are carried to prove the enhanced mixing rate due to the induced vortex. The ramps on the minor side caused maximum supersonic core length reduction of 44% at an inlet total pressure of 4 bar and a minimum of 27% at an inlet total pressure of 8 bar. The maximum and minimum core length reductions caused by the ramps placed on the major side are 22% and 11% at inlet total pressures 6 and 8 bar respectively. Both experimental and numerical studies show that shock waves are rendered weak by the minor side placement of ramps at all expansion levels.

In this study, wake of an elliptic cylinder is analyzed in the presence of a fluid–fluid interface. The interactions between the interface and flow affect each other and hence different wake dynamics and interface topologies are observed. The numerical solver developed for this study has immersed boundary method (IBM) coupled with level-set method (LSM). The proximity of the elliptical cylinder to the interface (H/D), Froude number (Fr), and angle of incidence (AOI) are the parameters considered. Three different Froude number regimes are considered for this study, namely, subcritical (Fr 1) regime. In this paper, the interface topology, wake dynamics, and force values are compared for the variation of these parameters.

International audience; A possible interaction between vortical structures in a round jet shear layer, viz. vortex rings and streamwise vortices, is explored following Davoust et. al (J. Fluid Mech., vol. 709, 2012, pp. 408-444). These authors reported a radial organization of streamwise vorticity in a high diameter-based Reynolds number (Re) jet, contrary to the classically observed azimuthal organization. They hypothesized that the observed weaker vortex rings in such jets could be deformed by streamwise vortices and further reoriented and stretched in the streamwise direction. As this hypothesis was based on the observations of one configuration of a jet flow, our study aims at assessing it by varying the key parameter i.e. the relative strength of the vortex rings and the streamwise vortices, through forcing, along with various jet configurations. We first analyze a low Mach number, Re = 1.5 × 10 5 transitional jet, using high-speed stereo particle image velocimetry in a cross-sectional plane at two jet-exit diameters from the nozzle exit. The axisymmetric mode is acoustically excited at various amplitudes to increase the strength of the rings relative to streamwise vortices, at a Strouhal number (St) of 0.49, the most energetic frequency in the unforced jet. Starting from a radial array in the unexcited jet, a gradual shift towards an azimuthal configuration is obtained with increasing excitation level. Quantification of the relative strengths of the streamwise vortices and vortex rings confirms that a radial array is observed whenever the streamwise vortices are more intense than the rings, and conversely for the azimuthal configuration. We then extend the analysis to other jet cases in terms of Re, St and state of the exiting boundary layer. We observe that the correlation between the radial or azimuthal organization of streamwise vortices and the relative strengths of the vortical structures holds systematically, confirming the possibility of the proposed interaction. A detailed analysis of the forced jets also sheds light on some interesting effects of acoustic excitation on the vortical organization in round jets.

The turbulent flow around a circular cylinder has been investigated at R e = 5000 using direct numerical simulations. Low frequency behavior, vortex undulation, vortex splitting, vortex dislocations and three dimensional flow within the wake were found to happen at this flow regime. In order to successfully capture the wake three dimensionality, different span-wise lengths were considered. It was found that a length L Z = 2 π D was enough to capture this behavior, correctly predicting different aspects of the flow such as drag coefficient, Strouhal number and pressure and velocity distributions when compared to experimental values. Two instability mechanisms were found to coexist in the present case study: a global type instability originating in the shear layer, which shows a characteristic frequency, and a convective type instability that seems to be constantly present in the near wake. Characteristics of both types of instabilities are identified and discussed in detail. As suggested by Norberg, a resonance-type effect takes place in the vortex formation region, as the coexistence of both instability mechanisms result in distorted vortex tubes. However, vortex coherence is never lost within the wake.

In this study, a dynamic stall control strategy, called the co-flow jet (CFJ), is applied to the rotor airfoil. The effect of the CFJ on the unsteady dynamic stall characteristics of the rotor airfoil is numerically investigated via numerical simulations of the unsteady Reynolds-averaged Navier-Stokes (URANS) equations coupled with the Spalart-Allmaras (S-A) turbulence model. The numerical methods are validated by a NACA0012 pitching airfoil case and a NACA6415 airfoil case based on the CFJ, and good agreement with experiments is found. Via the study of the typical conditions of CFJ control to suppress the dynamic stall of the OA212 rotor airfoil, it is verified that this method has a good effect on dynamic stall suppression. The diffusion and blending of the turbulent shear layer between the CFJ injection jet and the main flow excite the main flow and enhance its ability to resist the reverse pressure gradient; this suppresses the generation and development of the separation vortex, thereby enhancing the aerodynamic characteristics, improving the hysteresis effect, and increasing the system stability. On this basis, the control parameters of the CFJ are further studied, including the influences of the jet momentum coefficient and the positions and sizes of the injection and suction slots on suppressing the dynamic stall of the rotor airfoil. It is found that there is a jet momentum coefficient that optimizes the suppression effect of the dynamic stall of the rotor airfoil. Moreover, the position of the injection slot is found to have a greater effect on the dynamic stall suppression, while the size of the injection slot and the position and size of the suction slot have little effect.

We investigate ambient fluid entrainment and near-field flow characteristics of a free surface plunging jet for five Reynolds numbers ranging from 3000 to 10000 using time-resolved stereo particle image velocimetry (SPIV). We present time-averaged velocities, RMS velocity fluctuations, mean entrainment and unsteady flow features and compare them with previous studies on free jets. We find that plunging jets have a smaller potential core length, and earlier decay of the mean centerline velocity. The peak RMS velocity fluctuations occur at a location significantly upstream compared to the free jets reported in the literature. Near-field ambient fluid entrainment of plunging jets is measured for the first time and is found to be considerably higher than free jets in the low Reynolds number range. For the plunging jet case at Re = 3000, faster jet decay, higher levels of turbulent intensity in the near-field, and augmented mass entrainment result from strong primary vortices that give the turbulent/non-turbulent interface (TNTI) its convoluted shape which facilitates both bulk entrapment of ambient fluid and small scale nibbling because of larger surface area. These primary vortices occur right below the free surface and disintegrate into secondary structures at axial locations that are upstream compared to those of free jets. At higher Reynolds numbers, primary vortices are smaller in size, weak in swirling strength, and disintegrate prematurely, resulting in suppressed mixing and reduced entrainment efficiency.

An experimental study on the compact lean premixed flames stabilized on a bluff-body and swirl burner is performed. The flames are featured with a strengthened Inner Recirculation Zone (IRZ). The objective is to identify the critical causes of blow-off near the lean limit. Meanwhile, the effects of hydrogen addition on the flame stabilization are also examined. Premixed CH4/H2/air mixtures are adopted with varied hydrogen fractions up to 80%. Flames far from and close to the blow-off conditions are both obtained for comparison. Simultaneous PIV/OH-PLIF measurements are performed, and flow-flame dynamics are analyzed. Results show that the flow straining near the flame root is dominant to induce the root extinction and initiate the blow-off. The strain is observed to mainly result from the shear deformation. The weakened root burning intensity is demonstrated with the CH* chemiluminescence. With hydrogen addition, the flame root is strengthened due to the enhanced resistance to the strain. The increased burning velocity and temperature with hydrogen are also beneficial for the flame stabilization. The inner shear layer vortices are anticipated to promote the blow-off by convecting cold unburned mixture into the IRZ.

A constantly accelerating circular plate was investigated towards understanding the effect of non-stationarity on shear-layer entrainment and topology. Dye visualizations and time-resolved particle image velocimetry measurements were collected for normalized accelerations spanning three orders of magnitude. Increasing acceleration acts to organize shear-layer topology. Specifically, the Kelvin–Helmholtz instabilities within the shear layer better adhered to a circular path and exhibited consistent and repeatable spacing. Normalized starting-vortex circulation was observed to collapse with increasing acceleration, which one might not expect due to increased levels of mixing at higher instantaneous Reynolds numbers. The entrainment rate was shown to increase nonlinearly with increasing acceleration. This was attributed to closer spacing between instabilities, which better facilitates the roll-up of fluid between the shear layer and vortex core. The shear-layer organization observed at higher accelerations was associated with smaller spacings between instabilities. Specifically, analogous point-vortex simulations demonstrated that decreasing the spacing between instabilities acts to localize and dampen perturbations within an accelerating shear layer.

Riparian shrubs and trees present a complex, seasonally variable morphology, with flexible stems and leaves efficiently adapting to the flow forcing (reconfiguration). The aim of this paper is to investigate how foliage and reconfiguration affect the flow and mixing in a partly vegetated channel. Specific attention was placed on the velocity statistics, onset and coherence of turbulent structures, and lateral momentum transport at the horizontal interface between vegetation and open water. The experimental flume arrangement was novel in that it allowed investigating the lateral shear layer induced by flexible riparian plants. The natural-like vegetation consisted of emergent woody plants and a grassy understory, with density, morphology and reconfiguration behavior comparable to those found in real riparian areas. Investigations were conducted under foliated and leafless conditions to determine the seasonality effects. The mean and turbulent flow structure was determined with acoustic Doppler velocimetry, and dynamic plant motions were investigated from video footage. The presence of foliage enhanced the drag discontinuity at the interface, resulting in more pronounced velocity gradients between the vegetated and open areas compared to the leafless conditions. Foliation induced stronger shear layer-scale mixing, whereas, under leafless conditions, the local mixing induced by stems was more important. The reconfiguration decreased the coherence of the two-dimensional large-scale vortices at the interface while their characteristic frequency was consistent with the canonical mixing layer theory. Our results indicated that shear layer dynamics in partly vegetated channels was influenced strongly by morphology and reconfiguration of complex plants, with more efficient lateral momentum transport at the interface in the foliated conditions than previously reported for shear layers induced by simpler vegetation.

An experimental results on the spatio-temporal variation of velocity field and vortex structure, generated from the separated boundary layers on the offshore side of the still-water shoreline, during the run-down process of non-breaking solitary waves over a 1:3 sloping beach are presented. Three waves having the incident wave-height to water-depth ratios (H0/h0) of 0.363, 0.263, and 0.171 were generated in a wave flume. Two flow visualization techniques and high-speed particle image velocimetry were employed. The primary topics and new findings are: (1) Mechanism of the incipient flow separation, accompanied by formation of the separated shear layer from the beach surface, is elucidated under the adverse pressure gradient, using the fine data of velocity measurements very close to the sloping boundary. (2) Occurrence of hydraulic jump subsequently followed by development of the tongue-shaped free surface and projecting jet is demonstrated through spatio-temporal variation in the Froude number. It is confirmed by a change in the Froude number from supercritical to subcritical range as the free surface rapidly rises from the onshore to offshore side. (3) A complete evolution of the primary vortex structure (including the core position, vortex size, and velocity distribution passing through the vortex core) is first introduced systematically, together with the illustration of temporal variation in the topological structure. The non-dimensional shoreward distance of the vortex core section decreases with the increase in the non-dimensional time. However, the non-dimensional size height of the primary vortex increases with increasing non-dimensional time. (4) Two universal similarity profiles for both the wall jet flow and the shear layer flow demonstrate independency of the two similarity profiles of the wave-height to water-depth ratio and the beach slope. The similarity profiles indicate the promising collapse of the data from three previous studies for 1:20, 1:10, and 1:5 sloping beaches.

In this second part of a two article study, the coherent flow structure of a high Reynolds number, strongly swirling jet that includes in its core an energetic shear layer and that discharges in a square shaped cross-section duct is experimentally investigated. Selecting the jet diameter and average jet discharge velocity as characteristic scales, the swirl number, the duct cross section size, and the embedded shear layer circulation are all order one, whereas the flow Reynolds number is O ( 10 5 ) . The selected flow configuration and non-dimensional parameters are similar to those characterizing the flow in compact, lean premixed combustors presently being considered for aeroengine applications. Two space fixed, hot-wire probes are used as coherent mode detection and phase determination sensors to conduct detailed mode conditioned, LDV measurements. Spectral analysis of the hot-wire velocity signals shows the presence of three spatially and temporally coherent flow modes. The structure of the different modes is investigated using conditional post-processing specifically designed to analyze this high Reynolds number, complex flow field. The lowest frequency mode is seen to be linked to a precession instability of the swirling jet vortex breakdown region. The two other, more higher frequency modes are connected to the Kelvin–Helmholtz instability of the shear layer that develops in the swirling jet core. Symmetry arguments are used to partially reconstruct the vorticity structure linked to the shear layer instability modes. A close relation between the precession and the shear layer instability modes is found to occur.

International audience; Particle image velocimetry (piv) measurements are made at the trailing edge of a piezoelectric actuated aileron in order to investigate the physical effect on the flow via high-frequency low-amplitude actuation at high Reynolds numbers. The measurements at different actuation frequencies show the modification of the primary frequency components of the flow with the actuation frequency. A statistical analysis reveals the reduction of the Reynolds stress components which increases with the actuation frequency. Proper orthogonal decomposition (pod) analysis shows the modification of the spatial modes illustrating the vortex breakdown in the shear-layer and the reduction of the temporal mode spectral energy depending on the actuation. It has been shown that a specific low amplitude actuation frequency produces a significant reduction of the predominant shear-layer frequency.

Combustion instability, the coupling between flame heat release rate oscillations and combustor acoustics, is a significant issue in the operation of gas turbine combustors. This coupling is often driven by oscillations in the flow field. Shear layer roll-up, in particular, has been shown to drive longitudinal combustion instability in a number of systems, including both laboratory and industrial combustors. One method for suppressing combustion instability would be to suppress the receptivity of the shear layer to acoustic oscillations, severing the coupling mechanism between the acoustics and the flame. Previous work suggested that the existence of a precessing vortex core (PVC) may suppress the receptivity of the shear layer, and the goal of this study is to first, confirm that this suppression is occurring, and second, understand the mechanism by which the PVC suppresses the shear layer receptivity. In this paper, we couple experiment with linear stability analysis to determine whether a PVC can suppress shear layer receptivity to longitudinal acoustic modes in a nonreacting swirling flow at a range of swirl numbers. The shear layer response to the longitudinal acoustic forcing manifests as an m = 0 mode since the acoustic field is axisymmetric. The PVC has been shown both in experiment and linear stability analysis to have m = 1 and m = −1 modal content. By comparing the relative magnitude of the m = 0 and m = −1,1 modes, we quantify the impact that the PVC has on the shear layer response. The mechanism for shear layer response is determined using companion forced response analysis, where the shear layer disturbance growth rates mirror the experimental results. Differences in shear layer thickness and azimuthal velocity profiles drive the suppression of the shear layer receptivity to acoustic forcing.

Three-dimensional flow fields downstream of micro VGs in backward-facing step (BFS) flow are experimentally investigated by time-resolved tomographic particle image velocimetry (PIV). Streaks within the separated shear layer are visualized by time-averaged velocity field. Complex streamwise vorticity fields are analyzed by proper orthogonal decomposition (POD) and coherent vortex structures are reconstructed by predominant POD mode pairs.

The experimental method of PIV (Particle image velocimetry) is used to study the air flow over a forward facing step creating shear layer between the outer flow and the recirculation bubble. This layer decays into vortices, which are highlighted by using Reynolds (temporal) decomposition or Agrawal (spatial) decomposition. The later can be tuned to different-sized structures, and, additionally, their energies roughly follow the Kolmogorov -5/3 law.

The flows around a cylinder in infinite domain are simulated using CFD method. The simulation results are found to be in agreement with the data in literatures. The flow stability is investigated with the energy gradient theory. It is found that the two zero velocity gradient points at lateral sides of the cylinder lead to flow instability which causes vortex shedding. The flow transition to turbulence are dominated by two aspects, i.e. the zero velocity gradient at the lateral sides of the cylinder, and the zero velocity gradient inner the region of the vortex in the cylinder wake.

A variety of different flame configurations and heat release distributions exist in high swirl, annular flows, due to the existence of inner and outer shear layers as well a vortex breakdown bubble. Each of these different configurations, in turn, has different thermoacoustic sensitivities and influences on combustor emissions, nozzle durability, and liner heating. This paper presents findings on the sensitivities of the outer shear layer- stabilized flames to a range of parameters, including equivalence ratio, bulkhead temperature, flow velocity, and preheat temperature. There is significant hysteresis for flame attachment/detachment from the outer shear layer and this hysteresis is also described. Results are also correlated with extinction stretch rate calculations based on detailed kinetic simulations. In addition, we show that the bulkhead temperature near the flame attachment point has significant impact on outer shear layer detachment. This indicates that understanding the heat transfer between the edge flame stabilized in the shear layer and the nozzle hardware is needed in order to predict shear layer flame stabilization limits. Moreover, it shows that simulations cannot simply assume adiabatic boundary conditions if they are to capture these transitions. We also show that the reference temperature for correlating these transitions is quite different for attachment and local blow off. Finally, these results highlight the deficiencies in current understanding of the influence of fluid mechanic parameters (e.g. velocity, swirl number) on shear layer flame attachment. For example, they show that the seemingly simple matter of scaling flame transition points with changes in flow velocities is not understood.

By numerically solving the Navier-Stokes equations, the response characteristics of inflow-stimulated Kelvin-Helmholtz vortex in compressible shear layer arestudied. The mixing characteristics and the unique growth mechanism of the vortex structure are clearly revealed. By employing the index of vorticity thickness, the mixing properties are quantitatively analyzed. Based on the flow visualization results, the spatial size and the structure angle of the flow coherent structure are investigated by utilizing spatial correlation analysis. The evolution mechanism of the vortex structure in supersonic mixing layer induced by inlet forcing is revealed by analyzing the dynamical performances of the flow structure under different frequency disturbances. The numerical results show that with low forcing frequency at f = 5 kHz, the mixing efficiency is remarkably increased in the near-field of the flow. Whereas, in the far-field downstream the flow, the size of the structure reaches saturation state and the vortex passage frequency is locked, which causes the vorticity thickness to stabilize from 12mm to 14mm. Meanwhile, in a free mixing layer, the pairing and merging process occur in the flow field to promote the growth of the vortex structure, while in mixing layer with inlet forcing, the growth mechanism is that the vortex core engulfs a string of vortices induced by Kelvin-Helmholtz instability. The process of engulfment contributes much to the growth of the vortex structure. The analysis of spatial correlation distribution shows that in the area where engulfment occurs, the contour line shows the property of long and narrow ellipse instead of full ellipse and the structure in the area possesses the characteristics of intense rotation and inclination. Besides, with high inlet forcing frequency at f = 20 kHz, the size of the vortices becomes full in the near-field, and the vorticity thickness stabilizes between 3mm and 4 mm downstream the flow field. Meanwhile, the size of the vortex in controlled supersonic mixing layer is dominated by the imposed high-frequency forcing. An equation describing the quantitative relationship between the vortex characteristics and the imposed forcing frequency is derived, that is, the size of the uniform distribution vortex is approximately equal to the ratio of the value of convective velocity to inlet forcing frequency.

The flows in a jet entering a crossflow are unsteady and the droplet movement is of vital importance for the two-phase mixing effect. In this paper, the hollow cone spray into the crossflow is investigated experimentally by using the PIV visualization system and the image-processing techniques. The experiments are carried out inside a rectangular duct (95 mm × 95 mm in cross-section) at the ambient temperature and pressure. Different nozzle injection angles and crossflow velocities are experimented on. The instantaneous droplet distributions and the velocity vector fields are obtained. Our results show that the flow field falls into three main domains and their effects on the movement and distribution of the droplet are varied. The coherent structure which breaks the stability of the upper counter-rotating vortex pair (CVP) structure is induced on the interface between the upper CVP and the mainstream zone. When the spray is against the crossflow the larger coherent structures are induced and impose greater influences on the mixing process. The turbulence intensity on the shear layer increases and the dispersion of the droplet is promoted. The experimental findings will benefit the understanding of the mixing mechanism of the hollow cone spray in the crossflow and the achievement of an optimum mixing.

This paper investigates the use of a newly improved fifth-order weighted ENO scheme on the simulation of 2D Richtmyer-Meshkov instability. The new scheme, which we called WENO-Z-P, arises from the WENO-Z scheme of Borges et al. WENO-Z-P scheme achieves fifth-order accuracy near the first-order and second-order critical points and possesses higher resolution. We obtain an amazing high-resolution result of two-dimensional Richtmyer-Meshkov instability by WENO-Z-P scheme. More numerous small vortices are found in the shear layer with higher contrast and clarity. We can conclude that the accuracy lose near the second-order critical points play a significant role.The accuracy lose near the second-order critical points deeply drop the resolution of numerical scheme.

The effect of sidewall in self-sustained oscillation of an open cavity has been analyzed to understand the flow instability and influence of acoustic waves for transonic flow using Large-Eddy Simulation (LES). The present simulation resolves the formation of shear layer, its rollup resulting in large-scale structures apart from shock-shear layer interactions and evolution of acoustic waves. Presence of sidewall generates streamwise vortices that grow and convect downstream, while interacting with the spanwise rollup of shear layer. This results in coherent structures that finally impinge on the aft wall generating acoustics.

Micro vortex generators are a new kind of passive flow control instruments for shock-boundary layer interaction problems. In contrary to the conventional vortex generator, they have heights approximately 20-40% (more or less) of the boundary layer. Among them, Mircoramp vortex generators (MVG) are given special interest by engineers because of their structural robustness. The mechanism of the flow control was thought that a pair of streamwise vortex is generated by MVG and remains in the boundary layer for relatively long distance; the down-wash effect by the streamwise vortices will bring about momentum exchange, which makes the boundary layer less liable to separation. During such process, a specific phenomenon called as momentum deficit will happen [1], i.e., a cylindrical region consisted of low speed flows will be formed after the MVG. It was pointed out by Li and Liu [2] that the origin of deficit comes from the shedding of boundary layer over MVG.

Accepted: 2012-09-24, 資料番号: SA1004409000

In this paper, we present a visualization and tracking system for coherent structures. For this purpose, we propose to consider shear stress-the stretching and shear of particles inside a flow-in vortex dynamics. Based on a discussion and comparison of recent methods for computing shear stress, we introduce visualization techniques in order to provide a representation of shear layers according to their physical interpretation. This paper contributes a combination of theory in fluid mechanics and the corresponding visualization: 1) shear layer criteria are assessed according to how well they can be combined with common vortex identification criteria; 2) sheets of maximal shear are introduced as an appropriate visual representation of shear layers; 3) a visualization method is described for simultaneous tracking of vortices and shear layers as well as their interaction; and 4) the relevance of shear layers in vortex dynamics is demonstrated by means of several examples. We have implemented these new techniques in an interactive visualization system for time-dependent 3D flow. The system is used by fluid mechanics experts in their research of shear-vortex interaction.

In this study, we investigate flow control around a triangular prism in a uniform flow by utilizing receptivity to an external acoustic disturbance; we also investigate the mechanism of this flow control. The mean pressure fields on a prism surface are measured and the velocity fields around it are carefully investigated using Single type and X-type hot-wire probes. When a disturbance corresponding to the instability of a separated shear layer is added to a flow, the growth and the three-dimensionalization of the shear layer vortices related to flow randomization changes. Consequently, as the position of the Karman vortex shedding and the spanwise correlation of the vortex structure are changed, the force exerted by a fluid increases or decreases depending on the frequency of the disturbance.

The bypass transition of flat-plate boundary layer induced by a circular cylinder wake under the influence of roughness elements is experimentally investigated. The hydrogen-bubble visualization results show that the boundary layer separation occurs upstream of the roughness, and the separated shear layer is incised by roughness to different extent, resulting in different kinds of secondary vortices formed immediately downstream of the roughness. During the evolution of the secondary vortex, two types of instabilities are observed, which are denoted as large- and small-scale instabilities, respectively, according to different spatial scale of the hairpin vortices formed afterward. A merging process of hairpin vortices is also observed when the secondary vortices undergo the small-scale instability, and a potential new transition control strategy based on the present observation is proposed.

The flow over a deep cavity at low subsonic velocity is considered in the present paper. The cavity length-to-depth aspect ratio is L/H = 0.2. Single hot-wire measurements characterized the incident turbulent boundary layer and show the influence of the cavity on the streamwise statistic components just downstream from the cavity. The streamwise mean and fluctuating velocity profiles are affected by the cavity. PIV measurements reveal the presence for ejection-like events responsible of local perturbations of the skewness and the flatness coefficients. Time-resolved PIV technic is also used to characterize phase properties of shear layer oscillating cycle. It is shown that for deep cavity with first Rossiter mode, only one vortical structure is formed at the cavity leading edge. Then, it grows while convecting downstream along the shear layer. A well-defined ejection process begins after the vortex impact near the cavity downstream corner. A cylinder device placed spanwisely near the cavity leading edge eliminates the resonance and highly modifies the behavior of the shear layer flow. In fact, the shear layer could be divided into upper and lower parts with different structure aspects.