Large-scale Coherent Vortices Research Articles

Large eddy simulation of shock wave/boundary layer interactions in a transonic compressor cascade

In this paper, large eddy simulation (LES) was performed to investigate the shock wave/boundary layer interaction (SBLI) phenomenon in transonic compressor cascades with a chord Reynolds number of 2.12 × 106. A comprehensive analysis was conducted on both the SBLI structures inherent to the transonic compressor cascade and the coherent vortex structures within the boundary layer. The underlying mechanisms of the shock-induced boundary layer transition and the shock low-frequency unsteadiness in the transonic compressor cascade were elucidated through spectral and dynamic mode decomposition (DMD) analysis. The results revealed that boundary layer separation induced by the SBLI cannot reattach, leading to the formation of large-scale coherent vortex structures. Spectral analysis revealed that the shock-induced boundary layer transition in the transonic compressor cascade was dominated by inviscid Kelvin–Helmholtz (K–H) and secondary instability mechanisms, characterized by a dimensionless Strouhal number of 0.06. Additionally, pressure signals showed the variations in sub-frequency from the separated shear layer to the main flow. The oscillation amplitude of the shock foot was significantly greater than that of the shock main body, and the oscillation frequency of the shock foot was consistent with the sub-frequency. The oscillation frequency of the shock main body coincided with that of the compression ramp and flat plate configurations. Finally, DMD modal analysis indicated that high-frequency modes were correlated with turbulent fluctuations in the boundary layer, while medium- and low-frequency modes corresponded to shedding motion in the separated shear layer and low-frequency motion of the shock. This work promotes the understanding of the complex flow mechanisms of SBLI in the transonic compressor cascades.

Influence of Coherent Vortex Rolls on Particle Dynamics in Unstably Stratified Turbulent Channel Flows

This work investigates the dynamics of heavy particles dispersed in turbulent channel flows under unstable thermal stratification conditions using point-particle direct numerical simulations (PP-DNS), to quantify the influence of large-scale coherent vortex rolls, arising from the combined effects of shear and buoyancy, on the spatial distribution and preferential sampling behavior of inertial particles. We examined three particle Stokes numbers (St+=0.6,60,120) and two friction Richardson numbers, Riτ=0.272 and Riτ=27.2, which exemplify the regimes below and above the critical condition for vortex roll formation, respectively. The results indicate that the flow reorganization into large-scale longitudinal vortices significantly alters the topological features of small scales in the near-wall region impinged by the thermal plumes, resulting in a prevalence of vorticity-dominated topologies. The interplay between this phenomenon and the tendency of particles to preferentially sample strain-dominated topologies leads to a distinctive asymmetric particle distribution in the near-wall planes. Inertial particles markedly accumulate in the strain-dominated regions where the coherent thermal plumes emerge from the walls, while avoiding the vorticity-dominated impingement zones. This peculiar particle response to the vortex rolls is most pronounced when the particle response time matches the characteristic timescale of the large-scale coherent motions in the cross-stream planes.

How tidal waves interact with convective vortices in rapidly rotating planets and stars

Context. The dissipation of tidal inertial waves in planetary and stellar convective regions is one of the key mechanisms that drive the evolution of star–planet and planet–moon systems. This dissipation is particularly efficient for young low-mass stars and gaseous giant planets, which are rapid rotators. In this context, the interaction between tidal inertial waves and turbulent convective flows must be modelled in a realistic and robust way. In the state-of-the-art simulations, the friction applied by convection on tidal waves is commonly modeled as an effective eddy viscosity. This approach may be valid when the characteristic length scales of convective eddies are smaller than those of the tidal waves. However, it becomes highly questionable in the case where tidal waves interact with potentially stable large-scale vortices such as those observed at the poles of Jupiter and Saturn. The large-scale vortices are potentially triggered by convection in rapidly-rotating bodies in which the Coriolis acceleration forms the flow in columnar vortical structures along the direction of the rotation axis. Aims. We investigate the complex interactions between a tidal inertial wave and a columnar convective vortex. Methods. We used a quasi-geostrophic semi-analytical model of a convective columnar vortex, which is validated by numerical simulations. First, we carried out linear stability analysis using both numerical and asymptotic Wentzel–Kramers–Brillouin–Jeffreys (WKBJ) methods. We then conducted linear numerical simulations of the interactions between a convective columnar vortex and an incoming tidal inertial wave. Results. The vortex we consider is found to be centrifugally stable in the range –Ωp ≤ Ω0 ≤ 3.62Ωp and unstable outside this range, where Ω0 is the local rotation rate of the vortex at its center and Ωp is the global planetary (stellar) rotation rate. From the linear stability analysis, we find that this vortex is prone to centrifugal instability with perturbations with azimuthal wavenumbers m = {0,1, 2}, which potentially correspond to eccentricity, obliquity, and asynchronous tides, respectively. The modes with m > 2 are found to be neutral or stable. The WKBJ analysis provides analytic expressions of the dispersion relations for neutral and unstable modes when the axial (vertical) wavenumber is sufficiently large. We verify that in the unstable regime, an incoming tidal inertial wave triggers the growth of the most unstable mode of the vortex. This would lead to turbulent dissipation. For stable convective columns, the wave-vortex interaction leads to the mixing of momentum for tidal inertial waves while it creates a low-velocity region around the vortex core and a new wave-like perturbation in the form of a progressive wave radiating in the far field. The emission of this secondary wave is the strongest when the wavelength of the incoming wave is close to the characteristic size (radius) of the vortex. Incoming tidal waves can also experience complex angular momentum exchanges locally at critical layers of stable vortices. Conclusions. The interaction between tidal inertial waves and large-scale coherent convective vortices in rapidly-rotating planets (stars) leads to turbulent dissipation in the unstable regime and complex behaviors such as mixing of momentum and radiation of new waves in the far field or wave-vortex angular momentum exchanges in the stable regime. These phenomena cannot be modeled using a simple effective eddy viscosity.

INVESTIGATION OF THE VORTEX PICTURE OF A FREE JET FLOW BOUNDED BY FLAT WALLS

It is shown that the aerodynamics of turbulent jets flowing from rectangular nozzles is significantly influenced not only by the presence or absence of limiting end walls, but also by the development of large-scale coherent vortices formed at the initial section of the jet. It is established that in the case of large-scale vortices formed at the initial section with axes parallel to the free edge of the nozzle, they can either rest against the end walls with their ends, or close on themselves at the end walls, covering the initial section of the jet. It is shown that the initially formed annular vortices covering the initial section of the jet deform as they move away from the nozzle and turn into vortex cords with axes parallel to the direction of the jet. As a rule, the vortex cords are arranged symmetrically relative to the axis and have opposite directions. These vortices are the main cause of the appearance of an uneven velocity and temperature profile along the axis. At the same time, large-scale primary vortices with their ends rest against the end walls and cannot cause the appearance of maxima and minima of the velocity profile along the axis. In this case, the development of the boundary layers at the end walls is similar to the development of the boundary layer of a homogeneous flow in a flat channel.

Turbulent Transition of a Flow from Small to O(1) Rossby Numbers

Abstract Oceanic flows are energetically dominated by low vertical modes. However, disturbances in the form of atmospheric storms, eddy interactions with various forms of boundaries, or spontaneous emission by coherent structures can generate weak high-baroclinic modes. The feedback of the low-energy high-baroclinic modes on large-scale energetically dominant low modes may be weak or strong depending on the flow Rossby number. In this paper we study this interaction using an idealized setup by constraining the flow dynamics to a high-energy barotropic mode and a single low-energy high-baroclinic mode. Our investigation points out that at low Rossby numbers the barotropic flow organizes into large-scale coherent vortices via an inverse energy flux while the baroclinic flow accumulates predominantly in anticyclonic barotropic vortices. In contrast, with increasing Rossby number, the baroclinic flow catalyzes a forward flux of barotropic energy. The barotropic coherent vortices decrease in size and number, with a strong preference for cyclonic coherent vortices at higher Rossby numbers. On partitioning the flow domain into strain-dominant and vorticity-dominant regions based on the barotropic flow, we find that at higher Rossby numbers baroclinic flow accumulates in strain-dominant regions, away from vortex cores. Additionally, a major fraction of the forward energy flux of the flow takes place in strain-dominant regions. Overall, one of the key outcomes of this study is the finding that even a low-energy high-baroclinic flow can deplete and dissipate large-scale coherent structures at O(1) Rossby numbers.

Influence of flow structures and combustion on cross-scale turbulent kinetic energy transfer in premixed swirl flames.

This paper experimentally analyzes the simultaneous influence of combustion and large-scale vortex structures on the transfer of kinetic energy across scales around the laminar flame thickness δL0 in a turbulent premixed swirl flame and a non-reacting swirl flow. High-resolution tomographic particle image velocimetry and formaldehyde planar laser induced fluorescence measurements are used to obtain 3D velocity fields and estimates of the progress variable and density fields. The kinetic energy transfer across a filter scale of Δ=1.5δL0 was then quantified using physical space analysis. Coherent flow structures were identified using the swirling strength and proper orthogonal decomposition was used to identify the dominant periodic flow structure. While non-reacting regions of the flow show mean down-scale energy transfer (forward-scatter), mean back-scatter is observed internal to the flame. Importantly, the back-scatter magnitude in the flame increases in regions undergoing flame/vortex interaction. That is, the mean back-scatter magnitude at locations simultaneously inside the flame and a large-scale coherent vortex is higher than regions in the flame and not in a vortex, with the mean back-scatter magnitude increasing with the swirling strength. This increased back-scatter may be due to locally higher heat release rates in locations of flame/vortex interaction. Overall, the results demonstrate the importance of kinetic energy back-scatter at scales around the flame thickness and the complicated relationship between back-scatter and local flame/flow structure; these results should be considered when modeling turbulence in flames.

Geometric scale effect of the subsonic-supersonic shear layer based on a sinusoidal lobed splitter plate

Passive flow control tools have been widely used to enhance shear layer mixing in ramjet engines. Lobed mixer is an excellent method in mixing enhancement performance, while there is no comprehensive consensus on its mixing enhancement mechanism in a subsonic-supersonic shear layer. In this work, a sinusoidal lobed mixer is used to increase mixing between the subsonic and supersonic flows, and large eddy simulation (LES) is used to research the potential flow control mechanisms. Parameter experiments were carried out and the mixing performance was obtained. The numerical conclusions show good similarity with experimental conclusions. The impact of the geometric scales of the modified lobed mixer in the evolution of shear layer is investigated. It is found that sinusoidal lobed splitter plate with wavelength 10 mm and amplitude 2 mm shows the best mixing thickness experimentally and numerically, where compared with other cases, it is 3.3 times thicker than that of the flat plate. It is demonstrated that the sinusoidal lobed mixer performance is affected by its amplitude and wavelength. Large amplitude increases the initial disturbance, induces large scale streamwise vortex and enhances its mixing effectively. The number and size of large scale coherent vortices are inversely proportional to the wavelength. Smaller wavelength can accelerate the transformation process of the vortex structure, but it causes the slow development of shear layer downstream. Sinusoidal lobed mixer can promote the generation of large-scale streamwise vortices, significantly shorten the transition distance, and achieve better shear layer mixing.

Predicting rocker shoe parameters for reducing plantar pressure using machine learning methods

The formation and evolution of quadruple synthetic jets issuing into quiescent air are experimentally investigated. Four different configurations (monopole-like, dipole-like, quadrupole-like and 90-degrees-circularly-shifted-jets) have been studied by varying the phase lags between the ejection strokes of the four jets, at constant values of the source Reynolds and Strouhal numbers, equal to 4000 and 0.2, respectively. For each configuration instantaneous three-component in-plane velocity fields in the most relevant streamwise planes are measured with stereoscopic particle image velocimetry. The evolution of the turbulent jet flow is analyzed by the velocity field triple decomposition, thus separating the time-averaged component from the forced periodic oscillation and the turbulent fluctuation. It is observed that the near region of the flow domain is dominated by the formation and advection of large-scale coherent vortex structures, which are originated by the interaction of the vortex rings inherent to the single synthetic jets. The vortex dynamics is captured by phase-averaged measurements and its role in determining the flow field structure of each of the investigated configurations is widely discussed. The vortex structures are observed to lose their coherence and break down into turbulence while propagating downstream. The decay of the large-scale coherent motions is accompanied by the establishment of a self-preserving behavior of both the mean flow and turbulent quantities. Nevertheless, it is shown that the far-field topology depends critically on the source conditions and near-field evolution.

Rotating co-flow jets with horizontal lip and shape transition

Purpose Control over large-scale coherent structures and stream-wise vortices lead to enhanced entrainment/conservation of the jet which is desirable for most free jet applications such as design of combustion chamber in jet engines and flame length elongation of welding torch used for metal cutting. Design/methodology/approach A co-flow nozzle with lip thickness of 2 mm, between the primary (inner) and secondary (outer) flow, is selected. Three nozzle combinations are used, i.e. C–C (circle–circle), C–E (circle–ellipse) and C–S (circle–square) for acquiring comparative data. For these nozzle combinations, inner nozzle exit plane is kept as a circle, whereas the outer nozzle exit planes are varied to circle, ellipse and square. The exit plane area of outer nozzle for the nozzle combinations has equivalent diameter, De. The nozzles are fabricated in a way that the outer nozzle can be rotated along the longitudinal axis, keeping the inner nozzle intact. Findings The C–C nozzle combination is effective in low Mach number regime in decaying the jet, when the rotational component is introduced. Around 30% reduction in the jet core length is observed for the C–C nozzle combinations without any lip. The C–E nozzle shows sedative result in decaying or preserving the jet. The C–S nozzle combination shows interesting phenomenon, whereby the low subsonic case tends to conserve the jet by 15% and the higher subsonic case tends to decay the jet by 10%. Originality/value The developed nozzle systems show both conservative and destructive effect on the jet, which is desirable for the mentioned applications.

Flow mechanism of bimodal phenomenon in intermittent corner separation in a linear compressor cascade using a hybrid RANS/LES method

To gain a better knowledge of the bimodal phenomenon in intermittent corner separation, a hybrid RANS/LES turbulence model is applied to simulate the unsteady flow in a linear cascade. After the validation of the Stress Blended Eddy Simulation (SBES) model with the detailed experimental data, the flow mechanisms of intermittent coherent vortical structures and bimodal phenomenon are clarified according to unsteady numerical results. An indicator is proposed to evaluate the degree of the bimodal phenomenon. Firstly, the intermittency in corner separation is found, which arises from the formation of coherent vortical structures. The collision of the boundary layer and backflow near the trailing edge gives rise to large-scale coherent vortices around the separation surface. Then, according to the histogram of velocity probability density functions (PDF), the bimodal phenomenon found near the separation surface indicates that there are two non-periodic switching flow modes. Combination of vortical structures and velocity distributions manifests that the intermittency of coherent vortical structure is responsible for the switching flow modes around the separation surface. Thirdly, three characteristic parameters are defined to describe the features of two flow modes, corresponding to two Gaussian distributions that PDF decomposed into. The indicator Ra, which measures the difference between two flow modes, is used to evaluate the degree of the bimodal phenomenon. A larger Ra represents the higher probability of bimodal phenomenon occurring in flow fields, which can be used to quantitatively determine the position and fluctuation of the corner separation surface.

Large-scale vortices and zonal flows in spherical rotating convection – CORRIGENDUM

Motivated by understanding the dynamics of stellar and planetary interiors, we have performed a set of direct numerical simulations of Boussinesq convection in a rotating full sphere. The domain is internally heated with fixed temperature and stress-free boundary conditions, but fixed heat flux and no-slip boundary conditions are also briefly considered. We particularly focus on the large-scale coherent structures and the mean zonal flows that can develop in the system. At Prandtl number of unity, as the thermal forcing (measured by the Rayleigh number) is increased above the value for the onset of convection, we find a relaxation oscillation regime, followed by a geostrophic turbulence regime. Beyond this we see for the first time the existence of large-scale coherent vortices that form on the rotation axis. All regime boundaries are well described by critical values of the convective Rossby number $Ro_c$, with transitions from oscillatory to geostrophic turbulence, and then to the large-scale vortex regime at values $Ro_c\approx 0.2$ and $Ro_c\approx 1.5$, respectively. The zonal flow is controlled by the convective Rossby number and changes its direction when the flow transitions from the geostrophic turbulence regime to the large-scale vortex regime. While the non-zonal flow speed and heat transfer can be described by the so-called inertial scaling in the geostrophic turbulence regime, the formation of large-scale vortices appears to reduce both the non-zonal flow speed and the efficiency of convective heat transfer.

Large-scale vortices and zonal flows in spherical rotating convection

Abstract

Mixing enhancement in a subsonic-supersonic shear layer with a cavity splitter plate

The development of more efficient ramjet engines has motivated the study of enhanced mixing in compressible shear layers. Cavity-mixer is a widely used passive flow control tool, but its mixing enhancement mechanisms in compressible subsonic-supersonic shear layers has yet to be comprehensively explored. In this work, a cavity-mixer is used to increase mixing in a subsonic-supersonic shear layer, and the underlying flow control mechanisms are examined. Parametric experiments were performed on a laboratory-scale blowdown wind tunnel and an extraction wind tunnel. Numerical simulations were performed in a large eddy simulation framework. Schlieren and nanoparticle-based planar laser scattering techniques were adopted for mixing flow visualization. It is found that the growth rate of the cavity (length-to-depth ratio of 5) disturbed shear layer is 31% higher than the benchmark configuration at convective Mach number of 0.49∼0.56. It is demonstrated that the compressibility and length-to-depth ratio affect the cavity-mixer performance. High convective Mach number suppresses the cavity disturbance and weakens its mixing enhancement efficiency. Cavity-mixer with a length-to-depth ratio of 3 shows the highest experimental and numerical growth rate, compared to 5 and 7 under a convective Mach number around 0.49. Large-scale coherent vortices are confirmed in the subsonic-supersonic mixing process. Cavity disturbance promotes the three-dimensional characteristics of the flow field, increases the vorticity of large coherent structures, and advances the K-H instability position of shear layer, which plays a key contribution to mixing enhancement.

Electrical and Flow Characteristics of a Double-Side Sliding Pulsed Discharge Plasma Actuator

The development of next-generation high-maneuverability aircraft requires new technologies for controlling asymmetric separated vortices at high angles of attack. Plasma flow control is an emerging type of active flow control technology that can be used to control asymmetric vortices over slender bodies by means of pulsed dielectric barrier discharge (DBD) plasma actuators. However, some problems worthy of further study remain unsolved. For example, the stimulation strength is inadequate, the region of discharge is small, and the control mechanism is unclear. This paper proposes a double-side sliding pulsed discharge (DSPD) plasma actuator that combines a sinusoidal alternating current (AC) pulsed plasma actuator with two high-voltage direct current (DC) components. This actuator can generate two groups of unsymmetrical large-scale coherent vortex structures to balance the vorticity of the asymmetric vortices over the slender forebody of an aircraft and increase the distance between the cores of the asymmetric vortices, weakening the interaction between the vortices and hindering the emergence of the asymmetric phenomenon. Focusing on the key scientific issue of DSPD plasma actuation, the discharge morphologies and electrical properties under different DC conditions are studied, and the unsteady vortex structures induced by the DSPD plasma actuator under a static atmosphere are revealed through high-speed schlieren system and particle image velocimetry (PIV) measurements. The flow control ability of the DSPD plasma actuator is verified through wind tunnel experiments, and the main rules governing its influence on the external flowfield are determined. It is speculated that this actuator will further improve the aerodynamic flow control of slender aircraft.

Three-dimensional visualization of destruction events of turbulent momentum transfer in a plane jet

Destruction events of momentum transfer as represented by the Reynolds (shear) stress via the pressure–rate-of-strain interaction in a plane jet are investigated using direct numerical simulation data. To obtain three-dimensional pictures of the turbulent structures, pseudo-three-dimensional data are constructed on the basis of Taylor’s frozen hypothesis from time series of the relevant quantities. The coherent motion in the plane jet is extracted by means of conditional sampling analogous to variable-interval time averaging. We obtain a large-scale coherent structure consisting of horseshoelike vortices and a Kármán-vortex-like very large spanwise vortex street. Furthermore, we find that the active Reynolds stress destruction is localized around the large-scale coherent vortex structure. However, the coherent structure itself does not contribute to the Reynolds stress destruction, but the remaining incoherent motions do. For the significant pressure–rate-of-strain events in the incoherent motion, we display invariant maps of the velocity gradient tensor and the principal axes of strain to facilitate a discussion of the consistency of the numerical results with an illustration by Hinze [Turbulence, 2nd ed. (McGraw-Hill, 1975), pp. 325–328] and with our own experimental results [Takahashi et al., “Experimental investigation on destruction of Reynolds stress in a plane jet,” Exp. Fluids 60, 46 (2019)]. Consequently, the present results suggest that the illustration of the Reynolds stress destruction by Hinze is not completely applicable to a plane jet.

Vortex evolution in the near wake behind polygonal cylinders

The near wake of the polygonal cylinder with the side number N = 3~∞ is systematically studied using particle image velocimetry (PIV) at Re = 1.6 × 104. The proper orthogonal decomposition (POD) analysis is carried out to extract the large-scale coherent vortex structures and their evolution. It has been found that the vortex circulation grows to the maximum at the vortex formation length by entraining the vorticity from the separated shear layer and then undergoes a two-stage decay. The maximum circulation scales with the wake width, defined as the vertical distance between the two peaks of streamwise velocity fluctuation at vortex formation length. The vortex center trajectory indicates that the vortices move towards the centerline first and then away, with the vortex size monotonically increasing over the examined streamwise range. The vortex size at the maximum circulation also scales with the wake width. The vortex convection velocity increases gradually in the streamwise direction, and the ratio of the lateral and streamwise components of the vortex convection velocity, when scaled by wake width and vortex formation length respectively, approaches asymptotically 0.18 in the downstream, irrespective of the cylinder orientation or N.

Formation of Turbulence at Astronomical Observatories in Southern Siberia and North Caucasus

The main characteristics of the astroclimate (the structure of turbulent fields and conditions for their formation) are studied for several Russian astronomical observatories in southern Siberia and North Caucasus, including Sayan Solar, Baikal Astrophysical, Special Astrophysical, and Kolyvansky Ridge Observatories and the Center for Laser Atmosphere Sensing of Institute of Atmospheric Optics, Siberian Branch, Russian Academy of Sciences. The experimental results have been received from long-term astroclimate observations (expeditions in 2006–2017) with the use of small-size ultrasonic weather stations; and theoretical results, from the numerical solution of different boundary problems for the Navier–Stokes equations. The presence of regions with coherent (non-Kolmogorov) turbulence over the observatories has been ascertained, the formation of which is caused by the topography and the nonuniform heating of the underlying surface. Large-scale coherent vortex structures and coherent turbulence have been detected inside dome rooms. Numerical simulation allows us to analyze the influence of design features and temperature regimes of the telescope components and to test the telescope optical characteristics, including the minimization of the effects of external and under-dome turbulence. The paper includes the review of the history and evolution of the “coherent structure” and “coherent turbulence” concepts based on the world scientific literature and our own researches.

The study of the unstably-stratified marine atmospheric boundary layer by direct numerical simulation

Prediction of the properties of the atmospheric flows over sea is important for local and regional weather forecast. This prediction typically relies on the performance of large-scale meteorological models based on bulk formulae for air velocity, temperature and humidity. The bulk formulas relate turbulent fluxes of momentum, heat and vapor to bulk air velocity, humidity and air-sea temperature difference. The most widely used parameterizations used to compute the coefficients of proportionality in the bulk formulae are formulated on the basis of the Monin-Obukhov similarity theory (MOST) for different types of air stratification (stable, neutral and unstable). In our previous studies we showed that MOST quite accurately predicts the properties of the air-flow over waved water surface under neutral and stable stratification conditions provided that stratification effects are relatively weak and flow is in a statistically stationary state. In the present work, we perform direct numerical simulation and study the air flow over a waved water surface under unstable stratification conditions where the sea surface temperature is larger than the bulk air temperature. Such situation occurs in the tropical cyclone conditions as well as in polar lows at high latitudes. Our results show that in this case, the air flow dynamics is dominated by the development of large-scale cylindrical coherent vortex structures elongated in the direction of the mean wind. These structures cause a notable deviations from the MOST predictions for the air velocity and temperature profiles.

Large-Scale Coherent Vortex Formation in Two-Dimensional Turbulence

The evolution of a vortex flow excited by an electromagnetic technique in a thin layer of a conducting liquid was studied experimentally. Small-scale vortices, excited at the pumping scale, merge with time due to the nonlinear interaction and produce large-scale structures—the inverse energy cascade is formed. The dependence of the energy spectrum in the developed inverse cascade is well described by the Kraichnan law k–5/3. At large scales, the inverse cascade is limited by cell sizes, and a large-scale coherent vortex flow is formed, which occupies almost the entire area of the experimental cell. The radial profile of the azimuthal velocity of the coherent vortex immediately after the pumping was switched off has been established for the first time. Inside the vortex core, the azimuthal velocity grows linearly along a radius and reaches a constant value outside the core, which agrees well with the theoretical prediction.

Experimental investigation of flow structure due to truncated prismatic rib turbulators using particle image velocimetry

An experimental investigation of detailed flow field measurement in a rectangular duct with truncated prismatic ribs mounted on the bottom wall has been carried out using 2-dimensional particle image velocimetry (2-D PIV) technique. The truncated prismatic ribs have been manufactured by tapering the square rib from both the sides up to the center to provide rib height at the ends (h) of 0–8 mm with an increment of 2 mm. The PIV measurements have been performed in vertical as well as in horizontal streamwise planes for different Reynolds numbers and a fixed rib pitch to height ratio of 12. In addition, a number of PIV measurements in vertical symmetry plane have also been performed while covering the streamwise region of 46≤x/e≤120 to observe the nature of the flow in the test section and the flow periodicity. Overall, the emphasis is to investigate the flow structures and their dynamics in the inter-rib region in terms of time-averaged streamlines, mean velocities, vorticity fields, instantaneous velocity vector and corresponding vorticity fields, and fluctuation statistics in both the measurement planes. The close-up inspection based on critical point theory shows the significant evidence of salient critical points and limit cycles such as saddle point, stable and unstable node, bifurcation line and stable and unstable focus in both just upstream and downstream rib regions. The critical flow patterns around the rib confirm three-dimensionality and unsteady nature of the flow. The instantaneous flow characteristics reveal the presence of large-scale coherent vortices in the separated shear layer region, the existence of periodical processes such as intermittent ejection of the recirculation zone into the main flow and motion of the reattachment point. Significant reduction in the size of primary recirculation bubble and thereby reduction in reattachment length is observed for truncated prismatic ribs as compared to square rib in spanwise direction.