Reverse Flow Region Research Articles

Effects of incoming free-stream turbulence on the flow dynamics of a square finite wall-mounted cylinder

This study attempts to describe associated fluid dynamics of a square finite wall-mounted cylinder (FWMC) immersed within free-stream turbulent flow characterized by different turbulence intensities and integral length scales. An improved delayed detached eddy simulation method is adopted to numerically reproduce the fully developed turbulent flow fields. The results reveal that both the turbulence intensities and integral length scales have a significant effect on the separated shear layers, base pressure, and associated aerodynamic forces of the cylinder. Constrained streamlines along with critical point techniques are employed to further illustrate the influence of parameters of interest on a time-averaged flow pattern, including horseshoe vortex, surface flow, and wake topology. Distribution of second-order statistics within the wake region shows a shorter longitudinal length of the reversed flow region and enhanced vortex strength when background turbulence intensity increases. The time-dependent interaction between background turbulence and separated flow around the square FWMC is illustrated based on the phase difference between pressure of opposing side faces and the evolution of the reverse-flow region. In the end, the spectral proper orthogonal decomposition technique is employed to further investigate the effects of incoming flow turbulence on characteristics of the free-end shear flow and Von Kármán vortex shedding in the wake.

Influence of relative thickness on static and dynamic stall characteristics and prediction of airfoils

A study was conducted to investigate the effects of relative thickness on the static and dynamic characteristics of airfoils. A series of NACA symmetric airfoils with relative thickness ratios ranging from 12% to 30% have undertaken a series of sinusoid pitching motion simulations and these airfoils' Glasgow wind tunnel experiment data were analyzed. It was found that the relative thickness of airfoils would affect the stall onset angle of attack (AOA), the severity of the stall, and the dynamic stall model parameters. As the relative thickness of airfoils increases, static and dynamic stall onset AOA decrease and stall occurs earlier. At the static AOA, the thickest airfoil shows a very large reverse flow region. At the dynamic stall onset AOA, the thinner airfoil exhibits a more obvious burst of the laminar separation bubble. When the relative thickness of the airfoil is smaller, a dynamic stall occurs with more severity. On the one hand, the aerodynamic characteristic curve and suction surface pressure curve of airfoils change are more complicated. And the severity of the dynamic stall was quantitatively compared by extracting the difference of lift coefficients corresponding to the same AOA in the upstroke and downstroke stages. On the other hand, vortex occurrence, convection and shedding on the suction surface of the airfoil are more frequent and complicated. This paper also gives a method to solve the problem of the lack of the dynamic stall model parameters of blades section airfoils when predicting the wind turbine's unsteady force.

Deflector effect on flow behavior and drag of an Ahmed body under crosswind conditions

This study investigates the effect of a deflector on Ahmed model drag and the flow on the slant during steady crosswind conditions by experimental methods at a based-height Reynolds number of 2.45 × 105. The length deflector is 0.09 times the length of the slant. The deflector was fixed at the leading edge of the slant with an upward angle of 5° to the horizontal axis. Force, pressure, and global skin-friction measurements were conducted to understand the relation between drag, pressure distribution, and flow fields on the slant surface. The results showed that the deflector reduces aerodynamic drag at low yaw angles. This result is due to the breakdown structure of longitudinal vortexes and the separation bubble, which leads to fully separated flow on the leading edge of the slant. However, at yaw angles above 8°, a large reversed flow region forms on the slant and the drag of the model increases. A similar flow structure on the slant is observed for yaw angles of 12° and 15°. Conversely, the model lift is reduced with the deflector at all yaw angles tested. Detailed pressure distribution and complex flow structure on the slant provide insight in this study.

Quantification of Reynolds-averaged-Navier–Stokes model-form uncertainty in transitional boundary layer and airfoil flows

It is well known that the Boussinesq turbulent-viscosity hypothesis can introduce uncertainty in predictions for complex flow features such as separation, reattachment, and laminar-turbulent transition. This study adopts a recent physics-based uncertainty quantification (UQ) approach to address such model-form uncertainty in Reynolds-averaged Naiver–Stokes (RANS) simulations. Thus far, almost all UQ studies have focused on quantifying the model-form uncertainty in turbulent flow scenarios. The focus of the study is to advance our understanding of the performance of the UQ approach on two different transitional flow scenarios: a flat plate and a SD7003 airfoil, to close this gap. For the T3A (flat-plate) flow, most of the model-form uncertainty is concentrated in the laminar-turbulent transition region. For the SD7003 airfoil flow, the eigenvalue perturbations reveal a decrease as well as an increase in the length of the separation bubble. As a consequence, the uncertainty bounds successfully encompass the reattachment point. Likewise, the region of reverse flow that appears in the separation bubble is either suppressed or bolstered by the eigenvalue perturbations. This is the first successful RANS UQ study for transitional flows.

Simulation of enhanced optical trapping in a perforated dielectric microsphere amplified by resonant energy backflow

Optical energy flow inside a dielectric microsphere exposed to an optical wave is usually codirected with its wave vector. At the same time, if the optical field in a microparticle is in resonance with a high-quality spatial eigenmode, referred to as the whispering-gallery mode (WGM), at least two regions of reverse energy flow emerge in the illuminated and shadow particle hemispheres. These areas are of considerable practical interest due to their enhanced optical trapping potential provided they should be previously cleared from particle material. In this paper, we consider a perforated microsphere with an air-filled pinhole fabricated along the particle diameter and theoretically analyze the conditions for WGMs excitation. A pinhole isolates the energy backflow regions of WGM and multiple enhances the optical pull-in force that transforms a perforated microsphere into an efficient optical tweezer for trapping various nanoobjects.

The interactions of a circular synthetic jet with a turbulent crossflow

The three-dimensional flow of a circular synthetic jet interacting with a turbulent crossflow is investigated with unsteady Reynold-Averaged Navier–Stokes simulations. The effects of jet momentum are examined using three blowing ratios (CB=0.32, 0.67, and 1.10) at constant actuation frequency (f=300 Hz), approach crossflow Reynolds number, Reθ=900, and boundary layer thickness, δ/d=7.25, where d is the jet diameter. The results showed that the expelled jet is accompanied by a reverse flow region on the downstream side which undergoes alternating expansion and contraction during the jet cycle. The size of the reverse flow region and depth of penetration increased with increasing jet momentum. For the low momentum jet, the expelled flow structure evolved into a hairpin vortex which significantly enhanced the wall shear stress in the spanwise direction and near the jet exit. The higher momentum jets, on the other hand, exhibited strong vortex loops around the expelled jet column that transitioned into trailing vortex pairs and a tilted vortex ring further downstream. Along the wall, both horseshoe vortex upstream of the jet exit and tertiary vortices downstream were found attached to the wall. The strong tertiary vortices promoted downwash of fluids which significantly enhanced the wall shear stress along the symmetry plane for the medium and high momentum jets. The impact of the tertiary vortices increased with the jet momentum, offering greater potential for flow separation control.

Pore-scale numerical simulations of flow and convective heat transfer in a porous woven metal mesh

• The fluid dynamics through the woven mesh were studied from the pore scale. • The numerical model was verified in comparison with experimental data. • The flow patterns inside the pores of the woven mesh were studied in detail. • Difference between the numerical and the analytical results of friction factor is 5%. • A new correlation of the Nusselt number was proposed. Woven metal meshes have received considerable attention in several engineering applications owing to their excellent physical properties. Macroscopic parameters, such as flow resistance and heat transfer coefficient through the porous structure, have been extensively investigated. However, the microscale mechanism requires further analysis. In this study, a numerical model was developed to evaluate the fluid dynamics and heat transfer characteristics of a porous woven mesh at the pore scale. The accuracy of the model was verified by comparing the simulated results with experimental data. Flow patterns were comprehensively studied in terms of velocity, helicity, and vorticity. The effects of the fluid properties on the flow behavior were examined. The flow reversal region downstream of the mesh expanded as the Reynolds number increased. Based on the pore-scale numerical results, a Nusselt number correlation was derived for convective heat transfer in the porous woven mesh. This study provides insights into the flow physics within a woven mesh and a framework to optimally design related devices.

Transient nature of secondary vortices in an axial compressor stage with a tandem rotor

An efficient and compact design of a compressor continues to be a challenging area of research. The unavoidable streamwise adverse pressure gradient together with passage transverse pressure gradient restricts the maximum allowable blade turning. Tandem blading is an interesting concept for increasing the pressure rise by permitting a higher blade turning angle. The energized flow through the tandem blade nozzle gap helps to minimize the possibility of flow separation over the suction surface of the aft blade. However, a coherent transient analysis of a tandem rotor stage in an axial compressor is yet to be well explored. In the present paper, the complex flow field over a tandem rotor and the succeeding stator passage is analyzed in detail. Although the tandem rotor increases the flow turning and diffusion effect, the presence of separate trailing edge wakes and hub corner vortex causes early onset of flow separation over the stator suction. The flow structures developed within the rotor and the stator passages at different time instances are highlighted with the help of limiting streamlines and iso-surface Q-criterion superimposed with entropy contours. The results indicate that within the rotor passage, transient flow features are observed near the hub corner region close to the aft blade trailing edge rather than the tip region. When the rotor passes the stator leading edge, the rotor trailing edge leakage flow at the hub is entrained into the stator leading edge reverse flow region. This is then further circumferentially dragged into the mid-passage region. The interference of multiple rotor wakes with the stator leading edge leads to the formation of longitudinal and arch like separation vortices at the stator-hub and the stator-casing regions, respectively. These separation vortices grow in size while being convected downstream. Eventually, as time progresses, the vortices split and shed periodically from the stator surface. The present investigation highlights the requirement of a new stator design in a tandem rotor–conventional stator configuration. Such designs could further magnify the significant aerodynamic performance obtained using a tandem configuration.

Vortical characteristics of the corner separation flow in a high-speed compressor cascade

In aero-engine compressors, corner separations cause blockage, loss generation, and flow instabilities. The aim of the current study is to gain better knowledge of the vortical structures in corner separation flows and their resultant blockage and loss. Numerical simulations at different incidences have been performed for a high-speed linear compressor cascade. First, vortical structures have been visualized with the help of the Q-criterion and eigenvector method, and those associated with the corner separation have been focused on. It was found that the corner separation occurring on either the pressure surface or suction surface corner region was characterized by a large-scale streamwise vortex, named as the corner separation vortex in this paper. The pressure-side corner separation vortex is observed at a great negative incidence initiated from a focus point on the pressure surface, while the suction-side corner separation vortex observed at all concerned incidences originated from a focus on the endwall near the suction surface. Then, the mechanism by which corner separation vortices resulted in blockage and loss has been uncovered. The results showed that low-momentum fluids inside the core region of corner separation vortices led to the flow blockage and the viscous friction between them, and mainstream was responsible for the entropy generation. One major contributor of low-momentum fluids in the suction-side corner region was an off-wall reversed-flow region which was attributed to the breakdown of the suction-side corner separation vortex. Finally, multi-vortex structures in the leading region of the suction-side corner separation at two high positive incidences have been analyzed. It was found that the horseshoe vortex influenced the formation of the suction-side corner separation vortex, which induced a multi-vortex structure, and prevented the onset location of suction-side corner separation vortex from moving upstream with the increased incidence.

Wake flow modification of vehicle external door mirror with slots

A passive flow control method is applied to a vehicle external door mirror by introducing flow from a slot drilling from the front surface to four slots around the rear surface. The wake flow fields behind the door mirror and slotted door mirror are experimentally investigated via particle image velocimetry measurements at a Reynolds number of 12,736. The time-averaged flow fields suggest that the reverse-flow region of the slotted door mirror is reduced and pushed downstream; correspondingly, the turbulent kinetic energy of the slotted door mirror is reduced, implying the suppression of unsteadiness and a reduction in the drag force acting on the door mirror. As a result of the jet flow, the wake flow of the slotted door mirror exhibits a weaker spatial correlation and a smaller length scale, indicating the suppression of large-scale vortex shedding and the enhancement of small-scale flow motion by the slots. The jet flows from the left and right slots of the slotted door mirror prevent the upper and lower shear layers from interacting with each other, which promotes symmetric vortex shedding in the near-wake region. The power spectral density and modal energy distributions suggest that the strength of the primary vortex shedding is reduced by the slotted door mirror. The spatial patterns of the first two proper orthogonal decomposition modes suggest that the jet flow distorts the dominant large-scale structure, reducing the strength of the flow oscillation behind the slotted door mirror. The spatial distributions of modes 3 and 4 suggest that the symmetric vortex shedding is enhanced by the jet flow from the slotted door mirror.

Dynamic characterization of wind pressure fluctuations in separated and reattaching flows

Flow separation and reattachment over bluff bodies has been a primary concern in wind engineering community as it may generate large suction pressure. This study re-visits the characteristics of wind pressure underneath the separated shear layer, with emphasis on diagnosing the underlying dynamic properties. Nonlinear dynamic analysis techniques, namely recurrence plot and recurrence quantification analysis are applied to reveal and compare the wind pressure dynamics associated with different test conditions. The results indicate that the dynamic behaviour of wind pressure is closely tied with leading edge shape, turbulence intensity, as well as turbulence scale. Generally, the underlying dynamics of wind pressure become less deterministic as the measurement position moves from leading edge towards downstream. Such pattern is reversed once the descriptive indicator reaches its minimum. The position of such minimum value corresponds well to the downstream boundary of the reverse flow region. Moreover, the minimum value of dynamic descriptive indicator, as well as the reverse position in its distribution pattern, is correlated with the leading edge shape. On the other hand, the minimum value of dynamic descriptive indicator is also dependent on turbulence scale, while the reverse position in the distribution pattern of dynamic descriptive factor is dependent on turbulence intensity.

Whispering-gallery modes promote enhanced optical backflow in a perforated dielectric microsphere.

Optical energy flow inside a dielectric microsphere is usually co-directed with the optical wave vector. At the same time, if the optical field in a microsphere is in resonance with one of the high-quality spatial eigenmodes (whispering-gallery modes- WGMs), a region of reverse energy flow emerges in the shadow hemisphere. This area is of considerable practical interest due to increased optical trapping potential. In this Letter, we consider a perforated microsphere with an air-filled pinhole fabricated along the particle diameter and numerically analyze the peculiarities of WGM excitation in a nanostructured microsphere. A pinhole isolates the energy backflow region of a resonant mode and changes a perforated microsphere into an efficient optical tweezer. For the first time, to the best of our knowledge, a multiple enhancement of the energy backflow intensity in the pinhole at a WGM resonance is revealed and we discuss the ways for its manipulation.

Numerical investigation of a Mach 6 hypersonic laminar flow on two-dimensional cold-wall compression corners with controlled surface roughness

Numerical simulations are performed in order to investigate the physics of a Mach 6 hypersonic, laminar flow on different configurations of a two-dimensional compression corner. For this geometry, typically characterising control surfaces of re-entry vehicles, the interaction between the incoming boundary layer and the ramp-induced shock wave can lead to the formation of a separated-flow region. The numerical experiments, carried out under adiabatic and a cold-wall conditions, include the presence of a two-dimensional sinusoidal roughness patch, which is placed at various locations upstream of the ramp, in order to evaluate the effects of the surface waviness on the separation bubble. When the roughness elements are placed upstream of the smooth separation point, the recirculation bubble stretches towards the roughness position. This behaviour is induced by the thickening of the boundary layer downstream of the roughness patches, which cause a displacement of the laminar flow. No transition is observed downstream of the patches under the investigated geometrical and flow conditions. For a patch downstream of the smooth separation point, instead, no appreciable effects on the bubble length are noticed. However, the surface roughness inside the bubble can alter the recirculation pattern within the reverse-flow region. Hence, together with the length of the separated flow region, also the heat transfer at the wall is affected by the roughness because of the different recirculation patterns inside the bubble. In particular, the Stanton number highlights a redistribution of the heat-flux intensity between the main peak at reattachment and the secondary peaks inside the separated flow. This effect is more evident for higher ramp angles. These results highlight the importance of roughness-induced effects on the complex phenomenon that shock-wave/boundary-layer interaction is.

Computational Analysis of Active and Passive Flow Control for Backward Facing Step

The internal steady and unsteady flows with a frequency and amplitude are examined through a backward facing step (expansion ratio 2), for low Reynolds numbers (Re=400, Re=800), using the immersed boundary method. A lower part of the backward facing step is oscillating with the same frequency as the unsteady flow. The effect of the frequency, the amplitude, and the length of this oscillation is investigated. By suitable active control regulation, the recirculation lengths are reduced, and, for a percentage of the time period, no upper wall, negative velocity, region occurs. Moreover, substituting the prescriptively moving surface by a pressure responsive homogeneous membrane, the fluid–structure interaction is examined. We show that, by selecting proper values for the membrane parameters, such as membrane tension and applied external pressure, the upper wall flow separation bubble vanishes, while the lower one diminishes significantly in both the steady and the unsteady cases. Furthermore, for the time varying case, the length fluctuation of the lower wall reversed flow region is fairly contracted. The findings of the study have applications at the control of confined and external flows where separation occurs.

Experimental research on space distribution of reverse flow U-tubes in steam generator primary side

For the phenomenon of single-phase reverse flow in the parallel U-tubes of the steam generator (SG), the occurrence and disappearance of the reverse flow are systematically analyzed by experimental methods, as well as the reverse flow rate and the space distribution of the reverse flow U-tube. The experimental results show that there are different critical values corresponding to the occurrence and the disappearance of reverse flow. The reverse flow occurs as the pressure drop of U-tube is reduced to the reverse flow critical pressure drop. Also the flow becomes normal flow as the pressure drop of U-tube is above the normal flow critical pressure drop. When the reverse flow occurred in parallel U-tubes, the flow distribution will have several states. The space distribution of reverse flow U-tube has a certain randomness to a single experiment, but there are also statistical rules with amounts of repeated experiments. In this study, the changes of critical mass flow rate corresponding to the occurrence and disappearance of reverse flow with the inlet temperature are obtained, by which the operation regions of the parallel U-tubes are divided into three parts, namely normal flow region, reverse flow region and blending region, respectively. These research results could be helpful in the development of the reverse flow theories.

A Method of Stall and Surge Prediction in Axial Compressors Based on Three-Dimensional Body-Force Model

Abstract The occurrence of stall and surge in axial compressors has a great impact on the performance and reliability of aero-engines. Accurate and efficient prediction of the key features during these events has long been the focus of engine design processes. In this paper, a new body-force model that can capture the three-dimensional and unsteady features of stall and surge in compressors at a fraction of time required for URANS computations is proposed. To predict the rotating stall characteristics, the deviation of local airflow angle from the blade surface is calculated locally during the simulation. According to this local deviation, the computational domain is divided into stalled and forward flow regions, and the body-force field is updated accordingly; to predict the surge characteristics, the local airflow direction is used to divide the computational domain into reverse flow regions and forward flow regions. A single-stage axial compressor and a three-stage axial compressor are used to verify the proposed model. The results show that the method is capable of capturing stall and surge characteristics correctly. Compared to the traditional fully three-dimensional URANS method (fRANS), the simulation time for multistage axial compressors is reduced by 1–2 orders of magnitude.

Numerical study of hot-gas ventilated supercavitating flow

This paper describes a numerical investigation of the characteristics of supercavitating flow using hot gas. The numerical results are validated through an experimental study under the same conditions at Chungnam National University Cavitation Tunnel. The temperature gradient, velocity profile, and pressure distribution inside the supercavity are investigated. The multiphase numerical study uses the volume of fluid method and the realizable k–ε turbulence model to solve the governing equations. The internal flow structure inside the hot-gas ventilated supercavity can be classified into three distinct regions: the hot-gas ventilation effect region, internal boundary layer, and reverse flow region. The pressure inside the supercavity remains unchanged in the region close to the cavitator, and does not depend on the temperature variation of the ventilation gas; however, a slight increase in pressure occurs in the region near the closure part of the supercavity with maximum increase of 18%. Moreover, the gas leakage mechanism of the hot-gas ventilated supercavity is found to be the twin-vortex tubes, which become a single-vortex tube as the free-stream velocity increases. The temperature distribution is concentrated in the hot-gas effect region. The hot-gas is observed to diffuse very quickly near the injection holes. The present article provides a better understanding of the fluid mechanism inside supercavities, and presents new results on the behavior of hot-gas ventilation. • Simulation of hot-gas ventilated supercavity (the highest temperature of 2773 K) is carried out. • The results reveal findings below. (1) Internal flow structure of the supercavity. (2) Pressure and temperature distributions inside the supercavity. (3) Hot-gas leakage mechanism through the twin-vortex tubes.

Control of flow separation over a circular cylinder using synthetic jet

The development of methods of active separation flow control is of great applied importance for many technical and engineering applications. Understanding the conditions for the flow separation from the surface of a bluff body is essential for the design of aircrafts, cars, hydro and gas turbines, bridges and buildings. Drag, acoustic noise, vibrations and active flow mixing depend drastically on the parameters of the vortex separation process. We investigated the possibility of reducing the longitudinal length of a reverse-flow region using the method of «synthetic jet» active separation flow control. The experiment was carried out on a compact straight-through wind channel with a 1-m long test section of a cross-section of 125x125 mm. The jet was placed at the rear stagnation point of a circular cylinder. The Reynolds number, based on the cylinder diameter and the free-stream velocity, was 5000 and the von Kármán street shedding frequency without the synthetic jet was equal to 64.8 Hz. For the first time, for such a set of parameters, we applied high speed PIV to demonstrate that the injection of the synthetic jet into the cylinder wake region leads to a significant reduction in the longitudinal length of the reverse-flow region.

Tight focusing of circularly polarized light limited by semicircular aperture

In this work, the focusing of a circularly polarized plane wave (wavelength 532 nm) was simulated by a lens with a numerical aperture NA = 0.95. The wave front was considered flat. When integrating according to the Richards-Wolf formulas, the semicircular aperture was set by limiting the azimuthal angle from 0 to π. It was shown that when focusing light with right and left circular polarization, the focal spot turns out to be elliptical - elongated along the y axis, and, depending on the direction of polarization, its center shifts by about 0.05 μm in different directions along the x axis. It was also shown that the reverse flow region is located near the focal spot (at a distance of 0.25 μm from the center). Depending on the direction of polarization, it is located either to the right or to the left of the focal spot. Thus, the polarization state of the incident radiation can be determined from the displacement of the spot in focus.

Physical and numerical study on the transition of gas leakage regime of ventilated cavitating flow

The objective of this paper is mainly to investigate the ventilated cavitating flow around an axisymmetric body with a focus on the gas leakage mechanisms via combining experimental and numerical methods. A high-speed camera technique is used to record the ventilated cavitating flow patterns. The numerical simulation is performed with a free surface model and Filter-based turbulence model (FBM). Good agreement can be obtained between the experimental and numerical results. In the formation of a ventilated cavity, there are three typical gas leakage types with different Froude numbers Fr and gas entrainment coefficients CQ, including RJ closure mode with gas leakage of toroidal vortex shedding (TVS-RJ), TV closure mode with gas leakage of twin-vortex tube entrainment (TVTE-TV), and the coexistence of toroidal vortex shedding and twin-vortex tube entrainment (TVS-TVTE). The internal flow characteristics revealed three distinct regions with the ventilated cavity, including the ventilation influence region, the internal shear layer and the reverse flow region. With the increase of CQ, the incline angle of the cavity bottom surface α gradually decreases. When the incline angle α decrease to a critical value of ∼17.1°, the flow separation disappears completely and the flow regime migrates completely to TV cavity from RJ cavity.