Flow Separation Zone Research Articles

Modal Analysis of Separation Bubble Unsteadiness in Conical Shock Wave/Turbulent Boundary Layer Interaction

The frequency properties and the typical flow features within the interaction zone in conical shock-wave/turbulent boundary-layer interactions (CSBLIs) are analyzed by performing proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) on a time-resolved direct numerical simulation database. The streamwise velocity and pressure fluctuations are processed according to their energy and frequency, respectively. As is the case of planar shock-wave/boundary-layer interactions, fluctuations in some bands of frequencies are more energetic downstream of the flow separation zone. The energetic low-frequency modes of the streamwise velocity fluctuations are reminiscent of Görtler-like vortices, whereas those of pressure fluctuations are connected with the “breathing” motion of the separation bubble. Higher-frequency modes, on the other hand, are related to wrinkling of the impinging conical shock. Although they bear resemblance with those in planar shock/turbulent boundary-layer interactions, it was found that the flow structures herein are less coherent owing to spanwise inhomogeneity of the impinging shock strength. The POD and DMD results of CSBLI support the notion that Görtler-like vortices might be an intrinsic mechanism of the interaction zone, which is probably responsible for the low-frequency unsteadiness of CSBLI. The present results may serve as a basic guidance for the design of fluidic control strategies of shock-induced flow separation.

Direct Numerical Simulation of Heat Transfer of Lead–Bismuth Eutectic Flow Over a Circular Cylinder at Re = 500

The flow and heat transfer characteristics of the lead–bismuth eutectic (LBE) (Prandtl number Pr = 0.025) passing over a circular cylinder at Re = 500 are studied by direct numerical simulation and compared with the case of the air (Pr = 0.71). This article makes two major contributions: (1) heat transfer characteristics for the LBE flowing past a circular cylinder. For the case of air, the results show that a similarity exists among the wake structure of instantaneous temperature, fluctuating temperature, and velocity. However, these fields differ severely for the case of LBE. The local time-averaged Nusselt number (Nu¯) and circumferentially averagedNu¯are smaller in the LBE than that in the air. The circumferential and spanwise distributions ofNu¯in the LBE show a greater uniformity. The regions with larger circumferential or spanwise inhomogeneity appear in the flow separation zone. Besides, a resemblance between the distribution of the root mean square of fluctuation temperature and the turbulence kinetic energy can be recognized in the air; however, the similarity disappears for the case of LBE and in which the temperature fluctuation is smaller than that in the air. (2) Study on the temperature and velocity defect laws in the wake. By introducing the defect scales, it is concluded that the velocity field has not entered the self-preserving state yet, while the self-preserving state starts at the location of five times the diameter of the cylinder downstream of the cylinder for the temperature in the LBE and that of 21 times for the temperature in the air. In summary, even if without taking the buoyancy force into consideration, this article provides a fruitful description of the flow and heat transfer characteristics when the LBE flows past a cylinder, which is a typical flow in a helical coil steam generator of lead–bismuth alloy-cooled fast reactors. These highly resolved data on velocity and temperature are valuable for turbulence and heat fluxes modeling in the future and may facilitate the in-depth understanding of such flow and heat transfer characteristics within a limited variation range of operating temperature.

Structure of shear-induced platelet aggregated clot formed in an in vitro arterial thrombosis model

AbstractThe structure of occlusive arterial thrombi is described herein. Macroscopic thrombi were made from whole blood in a collagen-coated, large-scale stenosis model with high shear flow similar to an atherosclerotic artery. The millimeter-sized thrombi were harvested for histology and scanning electron microscopy. Histological images showed 3 distinctive structures of the thrombus. (1) The upstream region showed string-like platelet aggregates growing out from the wall that protrude into the central lumen, with red blood cells trapped between the strings. The strings were >10 times as long as they were wide and reached out to join the strings from the opposite wall. (2) Near the apex, the platelet strings coalesced into a dense mass with microchannels that effectively occluded the lumen. (3) In the expansion region, the thrombus ended abruptly with an annulus of free blood in the flow-separation zone. Scanning electron microscopy showed dense clusters of spherical platelets upstream and downstream, with amorphous platelets in the occluded throat consistent with prior activation. The total clot is estimated to contain 1.23 billion platelets with pores 10 to 100 μm in diameter. The results revealed a complex structure of arterial thrombi that grow from their tips under high shear stress to bridge the 2.5-mm lumen quickly with von Willebrand factor platelet strings. The occlusion leaves many microchannels that allow for some flow through the bulk of the thrombus. This architecture can create occlusion or hemostasis rapidly with minimal material, yet can remain porous for potential delivery of lytic agents to the core of the thrombus.

Evaluation of the SU2 Open-Source Code for a Hypersonic Flow at Mach Number 5

This paper presents the evaluation of the Stanford University Unstructured (SU2) open-source computational software package for a high Mach number 5 flow. The test case selected is an impinging shock wave turbulent boundary layer interaction (SWTBLI) on a flat plate where the experimental data of Sch¨ulein et al. [27] is used for validation purposes. Two turbulence models, the Spalart–Allmaras (SA) and the k-ω Shear Stress Transport (SST) within the SU2 code are evaluated in this study. Flow parameters, such as skin friction, wall pressure distribution and boundary layer profiles are compared with experimental values. The results demonstrate the performance of the SU2 code at a high Mach number flow and highlight its limitations in predicting fluid flow physics. At higher shock generator angles, the discrepancy between experimental and CFD data is more significant. Within the interaction and flow separation zones, a smaller separation bubble and delayed separation are predicted by the SA model while the k-ω SST model predicts early separation. Both models are able to predict wall pressure distribution correctly within the experimental values. However, discrepancies were observed in the prediction of skin friction due to the inability of the models to capture the boundary layer recovery after shock impingement.

Проблемы обнаружения отрыва потока датчиками давления на беспилотных летательных аппаратах с пропеллером

An experimental study of pulsations characteristics of the zone of flow separation arising at a small airplane-type UAV with a pushing two-blade propeller were carried out. The measurements were done in wind tunnel by unsteady pressure sensors and microphones built into the skin of the UAV for the test cases with and without a rotating propeller. A significant effect of the propeller on the level of pulsations was found. An increase of the incoming flow velocity led to a weakening of this effect. Analysis of the spectral data of the disturbances did not reveal a direct relationship between the propeller noise and the unsteady characteristics of the separation zone.

Problems of flow separation detection by pressure sensors on a unmanned aerial vehicles with a propeller

An experimental study of pulsations characteristics of the zone of flow separation arising at a small airplane-type UAV with a pushing two-blade propeller were carried out. The measurements were done in wind tunnel by unsteady pressure sensors and microphones built into the skin of the UAV for the test cases with and without a rotating propeller. A significant effect of the propeller on the level of pulsations was found. An increase of the incoming flow velocity led to a weakening of this effect. Analysis of the spectral data of the disturbances did not reveal a direct relationship between the propeller noise and the unsteady characteristics of the separation zone. Keywords: UAV, flow separation, unsteady pressure sensor, propeller

Bedload infilling and depositional patterns in chute cutoffs channels of a gravel‐bed river: The Ain River, France

AbstractAlthough abandoned channels are common and identifiable features in alluvial plains, their detailed internal architecture remains overlooked, particularly their coarse permeable compartment, with implications for underground water flow. The actively shifting gravel‐bed Ain River (France) provides an opportunity to study the geometry and architecture of bedload deposits (plug) during channel disconnection, in relation to river discharge and planform evolution. In this study, combined geomorphic and grain‐size surveys were conducted on the bedload deposits associated with the closure phase of four cutoff channels. We find that bedload accumulates mainly through (i) the initiation of a bar at the mouth of the abandoned channel in the flow separation zone, which reduces flow connectivity; (ii) lateral accretion of coarse‐grained bars, resulting in channel narrowing downstream of the initial upstream bar; and (iii) merging of coarse‐grained longitudinal bars—anchored to the initial plug bar. The channel plug progrades downstream until occupying ca. 0.5–0.7 times the channel surface and then starts thickening. We also find that channel plug volume is controlled by sediment supply and channel inherited topography, which are partially controlled by the dynamics of neighbouring channel bends.

Numerical investigation of the nozzle expansion state and its effect on the performance of the steam ejector based on ideal gas model

Many research works have generally divided the back pressure range of steam ejectors into critical, subcritical and malfunction modes based on the change of the entrainment ratio. In this study, we innovatively define and compare the abnormal and normal modes of the over-expanded, fully expanded and under-expanded states for a steam ejector using the CFD method and find that the steam ejector can maintain almost the same the secondary fluid mass flow rate under the normal mode of different expanded states. A comparison of different nozzle expansion states is also carried out. The results demonstrate that huge flow separation zones occur in the mixing chamber and diffuser in the under-expanded state, causing a drop in the secondary fluid mass flow rate compared with other expanded states. In addition, the irreversibility analysis illustrates that the steam ejector could minimize entropy production under the over-expansion state to obtain the optimal entrainment rate. More importantly, this paper provides a theoretical reference for maximizing the entrainment ratio or the secondary fluid mass flow rate from the perspective of the nozzle expansion state.

Numerical Study on Natural Ventilation Characteristics of a Partial-Cylinder Opening for One-Sided-Windcatcher of Variable Air-Feeding Orientations in Taif, Saudi Arabia

To provide a clean and cheap source of natural ventilation in windy and arid zones, a windcatcher facility is the best option. This paper aims to study the effect of the inlet opening angle of a new windcatcher model with different values ranging from 60° to 90° for three different feeding orientations at leading-down, central-up, and trailing-down locations. The ventilation performance of the new one-sided windcatcher is numerically examined using CFD simulations, where the 3D RANS and k-epsilon equations are applied at different wind speeds. The flow features of the new models are analyzed and compared with a basic traditional model based on the induced air distribution, aerodynamic losses, and ventilation rates. Results revealed that the sharp edge of the inlet opening leads to an increase in the flow separation and recirculation zone, especially when the opening angle is increased. The highest pressure coefficient is achieved by the trailing-down model compared with the other windcatcher models at an opening angle of 90°. The total pressure drop and ventilation rates increase in all the new windcatcher models due to the increase in the opening angle from 60° to 90°. At identical conditions, with an opening angle of 90° and wind speed of 5 m/s, the trailing-down model achieved a higher pressure coefficient than the leading-down and central-up models by 20.55% and 37.37%, respectively. Furthermore, the trailing-down model could provide higher ventilation rates than the central-up and leading-down models by 31% and 42%, respectively. Finally, the trailing-down windcatcher model can be recommended as the best choice to provide natural ventilation at Taif City in Saudi Arabia.

Large eddy simulation of separated flow to investigate heat transfer characteristics in an asymmetric diffuser subjected to constant wall heat flux

In the present study, heat transfer characteristics of an asymmetric diffuser with separated flow has been studied. The flow separation is triggered due to wall expansion in two directions. Large Eddy Simulation (LES) approach is adopted to solve the turbulent separated flow and heat transfer in such diffuser at Reynolds number of 10000. For this purpose, a finite volume solver is extended in the OpenFOAM framework to solve the energy equation for incompressible flow. The extended solver has been adjusted to deal with backscatter phenomena and to prevent non-physical heat transfer results and numerical instability. An appropriate grid resolution is employed to perform LES calculations and predict the characteristics of the heat transfer within the separated flow. The numerical results are validated against measurements and Direct Numerical Simulation (DNS) results. The present study showed that the low mean velocity and turbulent kinetic energy (TKE) in a separated flow region are responsible for generating high temperature hot spots resulted from significant reduction of heat transfer from the walls. It has been observed that the heat transfer from the wall is increased slightly before the flow re-attachment region. The applicability of the Reynolds analogy in the separated flow zone for this problem has been examined. Moreover, the analysis of the computational performance showed that increasing the number of computational cells can improve, to certain extent, the convergence rate of the solver and therefor, reduced the computation cost.

Reduction of the Flow Separation Zone at Combining Open-Channel Junction by Applying Alternate Suction and Blowing

The flow separation zone (FSZ) is one of the main flow characteristics of the combining open channel junction and is associated with energy dissipation. Bed shear stress in the contracted flow region increases as the channel’s effective width is reduced. Reduction of the FSZ at the junction to improve efficiency is desirable. The present numerical study focuses on the effect of continuous suction and blowing perturbations in reducing the FSZ at a right-angled open channel junction. The flow field is simulated using computational fluid dynamics (CFD) software. The numerical model is validated by comparing the simulated velocity field, water surface elevation, and energy loss for the unperturbed junction flow with corresponding experimental results available in the literature. The continuous suction and blowing perturbation is generated by a sinusoidal function with zero net discharge and is applied through a rectangular slit. Three different slit locations are considered downstream of the junction. The simulated results (time-averaged) show that the sinusoidal perturbation is effective in reducing the dimensions of the FSZ and, consequently, the energy loss and bed shear stress. The results demonstrate the enhanced effectiveness in reducing the energy loss when the slit is more proximate to the junction for the chosen flow configuration and the sinusoidal perturbation characteristics. The results of the time-averaged depth ratios show only a marginal reduction due to the flow perturbation.

Compressor Airfoil Separation Control Using Nanosecond Plasma Actuation at Low Reynolds Number

Flow control effects of three types of nanosecond (NS) dielectric barrier discharge (DBD) plasma actuations in suppressing the flow separation of a compressor airfoil at low Reynolds number are investigated using large-Eddy simulation. It has been found that the NS DBD plasma actuation can effectively suppress the flow separation and two mechanisms behind the flow control effect have been uncovered. First, the NS DBD plasma actuation can induce distorted flow structure (DFS) within the flowfield, and the induced DFS of plasma actuations located upstream of the flow separation zone can bring the Kelvin–Helmholtz instability of the shear layer between mainstream flow and separated flow froward to the separation point. Then, a large-scale spanwise vortex, which promotes the mixing of the main flow and the separated flow, is induced within the flow separation zone, thus resulting in suppression of the flow separation. Second, the plasma actuation located at the blade leading edge can improve the momentum of the laminar boundary layer, owing to the propagations of compressive waves as well as DFS along the blade surface. The plasma actuation located within the separation zone fails to suppress the boundary-layer flow separation effectively because the induced DFS cannot influence the shear layer between mainstream flow and separated flow.

Swimming behavior of juvenile silver carp near the separation zone of a channel confluence

Knowledge of locomotion of fish near river confluences is important for prediction of fish distribution in a river network. The flow separation zone near the confluence of a river network is a favorite habitat and feeding place for silver carp, which is one of the four major species of Chinese carp and usually provides positive rheotaxis to water flow. In the current study, a series of laboratory experiments were done to determine the behavioral responses of juvenile silver carp to the hydrodynamic forces near the separation zone of a channel confluence. The locomotion and trajectory of juvenile silver carp were recorded by infrared thermal imaging, while the flow velocity field near the separation zone was measured by a Particle Image Velocimetry (PIV) system. A total of 60 juvenile silver carp were released near the separation zone among which 40 carp swam in the upstream direction. Amongst them, 24 carp swam to the tributary and the remaining 16 swam into the main channel. Almost all these 24 carp travelled initially along the boundary of the separation zone near the corner, where flow shear was strongest, and then swam to the tributary. Instead of avoiding zones of strong vorticity, they chose and followed a trajectory along which the flow vorticity was large. On encountering these vortical flows, they increased the tail-beat frequency and decreased the tail-beat amplitude to maintain body stability. These observations provide important knowledge on locomotion of fish near river confluences and are beneficial for the fish habitat protection.

Effects of flow separation and shape factor of nanoparticles in heat transfer fluid for convection thorough phase change material (PCM) installed cylinder for energy technology applications

In this study, flow separation effects on the performance of PCM embedded thermo-fluid system is numerically analyzed by using the finite element method. In the heat transfer fluid, shape effects of nanoparticles are considered. Spherical, blade, brick and cylindrical shaped alumina nanoparticles are used in water. The numerical work is performed for different values of Reynolds number (between 100 and 300), expansion ratio of the channel (0.35 and 1), nanoparticle volume fraction (between 0 and 2%) and different shape of particles. The PCM material is paraffin wax with spherical shaped capsules of 20 mm in diameter. There is significant impact of channel expansion and flow separation zone on the performance of the system. When the area expansion is introduced in the straight channel, the configuration with the highest Reynolds number resulted in performance degradation due to flow recirculation extended in the PCM region. The charging time for a straight channel is reduced by about 84% when comparison is made with the channel having an expansion ratio of 0.35. The shape factor of the alumina nanoparticles significantly affects the flow recirculations and thermal exchange between the PCM and heat transfer fluid. Among various particles, cylindrical shaped one performs best while 23.8% reduction in charging time is obtained at the highest solid volume fraction when comparison is made with pure water as heat transfer fluid.

Experimental influence of a splitter vane on the flow fields in a wide-angled diffuser

Based on PIV and high-speed photography technology, the lengths and area ratios of flow separation zone under different Reynolds numbers Re in a diffuser with a fixed angle (28º), with and without a splitter vane were obtained. The flow field, vorticity and the flow distribution (hydrogen bubble tracing) at the downstream vicinity of the outlet of the diffuser under three representative conditions (Re = 1000, 6000 and 12000) were given. The experimental results show that with increasing Re, the area proportion of the separation zone decreases, but the length proportion of the separation zone increases, because the thickness of the separation zone along the vertical flow direction is reduced. Due to the existence of vane, the area of the corresponding separation zone is always smaller than that without the vane.

A 3D flow separation model for open propellers with blunt trailing edge

The panel method is widely used in the design and analysis of marine propellers. However, this method cannot handle propellers with a blunt trailing edge due to a lack of proper models for the flow separation downstream. In this study, a numerical scheme, i.e., the three-dimensional flow separation model, is proposed to enable the panel method to be applied to propellers with a blunt trailing edge. In this model, the flow separation zone is represented as an extension of the trailing edge. The criteria of zero local lifts and zero local moments over the extension of all blade sections are used to establish the equations that determine the shape of the flow separation zone. Those equations are solved either by the Newton–Raphson method or as an optimization problem. A low-order panel method coupled with a boundary layer solver (the viscous/inviscid interaction method) is used to predict the pressure distributions and forces for the extended geometry. To validate this model, the pressure distributions of the propeller blade are compared with those from solving the Reynolds-Averaged Navier–Stokes equations. The predicted open water characteristics of a controllable pitch propeller are compared with the experimental measurements at a wide range of advance ratios with different pitches. This model requires much less computational effort while preserving high accuracy, and thus can be used reliably in designing and analyzing propellers with a blunt trailing edge.

Aerodynamic analysis of the small-scaled centrifugal compressor for micro-turbojet engine applications

This paper presents the results of an aerodynamic analysis of a small-scaled transonic centrifugal compressor for micro turbojet engine (TJE) applications. The analysis was conducted using a CFD model validated by experimental data collected for the gaged JetCat P200-RX micro-TJE. The loss coefficients of the impeller, vaned diffuser and deswirler were estimated for 4 design points corresponding to 70%, 80%, 90% and 100% rotation speeds to perform the loss balance diagram. The flow angle spanwise variation demonstrated an intensive flow separation zone at the top 25% of the vaned diffuser span due to the pressure shock appearance. It was shown that the main source of the losses in the investigated compressor is the deswirler due to non-optimal flow turning conditions. The diffuser loss coefficient was estimated as 0.18 at the compressor full load.

Implementation and characterization of a physiologically relevant flow waveform in a 3D microfluidic model of the blood–brain barrier

Previous in vitro studies interrogating the endothelial response to physiologically relevant flow regimes require specialized pumps to deliver time-dependent waveforms that imitate in vivo blood flow. The aim of this study is to create a low-cost and broadly adaptable approach to mimic physiological flow, and then use this system to characterize the effect of flow separation on velocity and shear stress profiles in a three-dimensional (3D) topology. The flow apparatus incorporates a programmable linear actuator that superposes oscillations on a constant mean flow driven by a peristaltic pump to emulate flow in the carotid artery. The flow is perfused through a 3D in vitro model of the blood-brain barrier designed to induce separated flow. Experimental flow patterns measured by microparticle image velocimetry and modeled by computational fluid dynamics reveal periodic changes in the instantaneous shear stress along the channel wall. Moreover, the time-dependent flow causes periodic flow separation zones, resulting in variable reattachment points during the cycle. The effects of these complex flow regimes are assessed by evaluating the integrity of the in vitro blood-brain barrier model. Permeability assays and immunostaining for proteins associated with tight junctions reveal barrier breakdown in the region of disturbed flow. In conclusion, the flow system described here creates complex, physiologically relevant flow profiles that provide deeper insight into the fluid dynamics of separated flow and pave the way for future studies interrogating the cellular response to complex flow regimes.

Study on the mesoscale causes of the influence of surface tension on material erosion in a cavitation field

ABSTRACT Cavitation erosion is a fundamental concern in many fields of engineering, e.g. the flow separation zone in high speed flow and the blade area of hydraulic machinery. However, the influence of the physical characteristics of the environmental medium in these engineering facilities on cavitation erosion is still unclear. In this paper, we adopted underwater low-voltage discharge technology to generate cavitation bubbles, and studied the mesoscale causes of material erosion in solutions with different surface tensions. It is found that with the decrease of liquid surface tension, the radial velocity of cavitation bubbles decreases significantly during the shrinkage of the cavitation bubble and the minimum radius to which the cavitation bubble shrinks increases, the micro-jet generated by the cavitation bubble develops more slowly, and the impact strength of cavitation bubble collapses on the wall decreases. The findings reveal new physical mechanisms of the cavitation-induced material erosion in liquids with different surface tensions at the macroscopic level. These will provide significant guidance for cavitation prevention in various applications such as aeration cavitation-alleviation, hydraulic machinery, ultrasonic cleaning and the medical industry.

The Influence of Slipface Angle on Fluvial Dune Growth

AbstractDunes dominate the bed of sandy rivers and they respond to flow by changing shape and size, modifying flow, and sediment transport dynamics of rivers. Our understanding of and ability to predict dune adaptation, particularly dune growth and decay, remain incomplete. Here, we investigate dune growth from an initial flatbed in a laboratory setting by continuously mapping the 3D bed topography using a line laser scanner combined with a 3D camera. High‐resolution profiles of flow velocity and sediment concentration providing both bedload and suspended sediment fluxes were obtained by deploying Acoustic Concentration and Velocity Profiler technology. Our analysis reveals that the magnitude of the dune slipface angle, which determines flow separation and controls turbulence production, adjusts to the imposed flow at time scales similar to the evolution of dune height and length. The initiation of a flow separation zone intensifies through scour, and results in acceleration of the dune growth. Gradients in sediment transport and the rate of dune growth are inherently linked to spatial variations in slipface angles. During dune growth, the slipface angle evolves differently than the ratio of dune height to length, which immediately reaches its equilibrium value after dune initiation.