Downstream Shear Research Articles

Experimental control of the flow separation behind a backward facing step using deep reinforcement learning

In this experimental study, a value-based reinforcement learning algorithm (deep Q-network, DQN) is used to control the flow separation behind a backward facing step at a Reynolds number of 2.9 × 104. The flow is forced by a dielectric barrier discharge (DBD) plasma actuator pasted at the upstream of the step edge, and the feedback information of the separation zone is provided by a hotwire sensor submerged in the downstream shear layer. The control law represented by a deep neural network is implemented on a field programable gate array (FPGA), able to execute in real-time at a frequency as high as 1000 Hz. Results show that both open-loop periodical control and DQN control can effectively reduce the reattachment length and the recirculation area. Compared with the former, which requires dozens of trail-and-error measurements lasting for hours, the latter is able to find an optimal control law in only two minutes, achieving a long-term reward 7% higher. Moreover, by introducing a weak penalty term for plasma actuation, the mean actuator power consumption in DQN can be cut down to only 60% of that in the optimal open-loop control, meanwhile sacrificing a negligible amount of control effectiveness. Physically, the open-loop periodical control destabilizes the shear layer earlier, increasing both the area and the peak amplitude of the high turbulent kinetic energy (TKE) zone, whereas under DQN control, only a slight increase in the TKE peak is observed, and the overall spatial distribution remains the same as baseline.

Unsteady vane-rotor secondary flow interaction of the endwall region in a 1.5-stage variable-geometry turbine

Variable-geometry turbines present significant improvements in both off-design cycle efficiency and enhanced engine responsiveness for future adaptive cycle engines. The adjustable vanes regulate the through-flow capacity by varying the installation angle and thus unavoidably require endwall clearance, resulting in profound implications for vane and downstream blade rows. Unsteady numerical simulations are carried out to investigate the vane-rotor flow interaction under the zero and partial clearance type of the adjustable vane in a 1.5-stage variable-geometry turbine. First, specific vortical structures and loss characteristics of upstream adjustable vane leakage flow at turning angles of −5°, 0°, and +5° are described. Then, the basic mechanisms of vane-rotor secondary flow interaction between acquired upstream leakage flow and downstream blade rows secondary flow is explored by the time-resolved method and quantitative detailed analysis method. The results show that adjustable vane vortex core area is the primary cause of internal loss within the vortex, and the mixing and interaction between the leakage vortex and other vortex systems contribute to secondary loss in the vane. Due to the downstream blade shear action caused by different flow velocity on the suction and pressure surfaces, the vane leakage flow fragments upon entering the R1 blade passage and develops downstream of the endwall region. Compared to the vane with zero clearance case at same turning angle, the intensity of the R1 tip leakage vortex at the design turning angle and open turning angle is reduced by 47.4 % and 23.66 % with the upstream partial clearance, respectively. The unsteady vane-rotor secondary flow interaction under different turning angles has large influence on the flow structure and loss characteristics of the downstream blade rows, and results in significant differences in the variation pattern and trend direction of the turbine performance.

Large eddy simulation of combustion characteristics during dual fuel switching process in gas turbine combustor

The present study aims to investigate the combustion characteristics, fuel switching strategy, and flame stability associated with fuel switching processes in a dual-fuel gas turbine combustor. Specifically, the Large Eddy Simulation method is employed in conjunction with a skeletal chemical reaction mechanism to simulate the fuel switching process. It is revealed that the presence of a local rich combustion zone, formed by an increased accumulation of fuel in the head region, leads to the displacement of the heat release position towards the downstream inner shear layer. Additionally, this condition causes a reduction in the recirculation zone area and a deterioration in the uniformity of fuel blending. Simultaneously, the combustor experiences low-frequency, high-amplitude pressure pulsations, as well as pulsations in the heat release rate. Furthermore, a comparative analysis is conducted to assess the discrepancies among three distinct fuel supply strategies employed during fuel switching. The fast-opening strategy is found to demonstrate superior performance indicators, as it accomplishes filling the combustor performance indicators within 80% of the allocated time due to its lower initial fuel changing rate.

Boundary layer transition due to distributed roughness: effect of roughness spacing

The influence of roughness spacing on boundary layer transition over distributed roughness elements is studied using direct numerical simulation and global stability analysis, and compared with isolated roughness elements at the same Reynolds number $Re_h=U_eh/\nu$ ( $U_e$ is the boundary layer edge velocity, h is roughness height and $\nu$ is the kinetic viscosity of the fluid). Small spanwise spacing ( $\lambda _z=2.5h$ ) inhibits the formation of counter-rotating vortex pairs (CVP) and, as a result, hairpin vortices are not generated and the downstream shear layer is steady. For $\lambda _z=5h$ , the CVP and hairpin vortices are induced by the first row of roughness, perturbing the downstream shear layer and causing transition. The temporal periodicity of the primary hairpin vortices appears to be independent of the streamwise spacing. Distributed roughness leads to a lower critical roughness Reynolds number for instability to occur and a more significant breakdown of the boundary layer compared with isolated roughness. When the streamwise spacing is $\lambda _x=5h$ , the high-momentum fluid barely moves downward into the cavities and the wake flow has little impact on the following roughness elements. The leading unstable varicose mode is associated with the central low-speed streaks along the aligned roughness elements, and its frequency is close to the shedding frequency of hairpin vortices in the isolated case. For larger streamwise spacing ( $\lambda _x=10h$ ), two distinct modes are obtained from global stability analysis. The first mode shows varicose symmetry, corresponding to the primary hairpin vortex shedding induced by the first-row roughness. The high-speed streaks formed in the longitudinal grooves are also found to be unstable and interact with the varicose mode. The second mode is a sinuous mode with lower frequency, induced as the wake flow of the first-row roughness runs into the second row. It extracts most energy from the spanwise shear between the high- and low-speed streaks.

Large-Eddy simulations on interaction flow structures of pulsed jets in a crossflow

Numerical investigations of pulsed jets in a crossflow are carried out using large-eddy simulation. The flow mechanism and coherent structure evolution of pulsed jets with different frequencies (f = 20 Hz, 50 Hz, and 100 Hz) are analyzed and compared with the steady-state jet. The proper orthogonal decomposition (POD) method is used to analyze the effect of different vortex structures in the flow field, which further elaborates the influence mechanism of frequency on pulse jets in the crossflow. The results show that under the same velocity ratio, the normal penetration depth of pulsed jets to the crossflow are greater than that of the steady-state jet. Due to the pulsating effect, the pulsed jets will form a large-scale shear layer vortex, which strengthens the turbulence pulsation and promotes the mixing between jets and the crossflow. As the pulse frequency increases, the scale of the vortex rings in the downstream shear layer will become smaller but the number will increase, and fine vortex clusters will be formed around them. At f = 50 Hz, the counter-rotating vortex pair (CVP) and the jet front shear layer vortex have a greater influence in the flow field, followed by the downstream shear layer vortex, and the wake vortex has the least influence in the flow field. At f = 20 Hz and 100 Hz, the wake vortex has a greater influence in the flow field than that at f = 50 Hz.

Numerical Investigation of Bed Shear Stress and Roughness Coefficient Distribution in a Sharp Open Channel Bend

The helical flow leads to the redistribution of the bed shear stress and roughness coefficient in open-channel bends, influencing the transport of sediment and riverbed evolution process. To explore the mechanisms underlying this redistribution, a 3D numerical simulation of a 180° sharp bend was performed by solving the Reynolds-averaged Navier–Stokes (RANS) equations using the Reynolds stress equation model (RSM) as the anisotropic turbulence closure approach. The results indicate that the high bed shear stress zone appeared in the inner bank region from the 50° to 110° sections, and the section maximum bed shear stress gradually shifted outwards, owing to the advective momentum transport by the circulation cells. The quantitative analyses of the terms in depth-averaged Navier-Stokes equations indicate that the contributions of the cross-stream circulation and cross-flow to the downstream bed shear stress were of the same order of magnitude, and the overall contribution rate of the cross-stream circulation was 20%. The contribution rates of the cross-flow, cross-stream circulation, turbulence, and pressure gradient to the transverse bed shear stress were approximately 30%, 7%, 3%, and 60%, respectively, indicating that the pressure gradient term arising from the transverse water surface slope played a dominant role. The Chezy resistance coefficient showed an overall decreasing trend along the bend. Therefore, an effective expression considering the streamwise variation along the centreline and transverse variation was successfully established to predict the uneven distribution of the Chezy resistance coefficient.

Early clinical outcomes and molecular smooth muscle cell phenotyping using a prophylactic aortic arch replacement strategy in Loeys-Dietz syndrome

ObjectivesPatients with Loeys-Dietz syndrome demonstrate a heightened risk of distal thoracic aortic events after valve-sparing aortic root replacement. This study assesses the clinical risks and hemodynamic consequences of a prophylactic aortic arch replacement strategy in Loeys-Dietz syndrome and characterizes smooth muscle cell phenotype in Loeys-Dietz syndrome aneurysmal and normal-sized downstream aorta. MethodsPatients with genetically confirmed Loeys-Dietz syndrome (n = 8) underwent prophylactic aortic arch replacement during valve-sparing aortic root replacement. Four-dimensional flow magnetic resonance imaging studies were performed in 4 patients with Loeys-Dietz syndrome (valve-sparing aortic root replacement + arch) and compared with patients with contemporary Marfan syndrome (valve-sparing aortic root replacement only, n = 5) and control patients (without aortopathy, n = 5). Aortic tissues from 4 patients with Loeys-Dietz syndrome and 2 organ donors were processed for anatomically segmented single-cell RNA sequencing and histologic assessment. ResultsPatients with Loeys-Dietz syndrome valve-sparing aortic root replacement + arch had no deaths, major morbidity, or aortic events in a median of 2 years follow-up. Four-dimensional magnetic resonance imaging demonstrated altered flow parameters in patients with postoperative aortopathy relative to controls, but no clear deleterious changes due to arch replacement. Integrated analysis of aortic single-cell RNA sequencing data (>49,000 cells) identified a continuum of abnormal smooth muscle cell phenotypic modulation in Loeys-Dietz syndrome defined by reduced contractility and enriched extracellular matrix synthesis, adhesion receptors, and transforming growth factor-beta signaling. These modulated smooth muscle cells populated the Loeys-Dietz syndrome tunica media with gradually reduced density from the overtly aneurysmal root to the nondilated arch. ConclusionsPatients with Loeys-Dietz syndrome demonstrated excellent surgical outcomes without overt downstream flow or shear stress disturbances after concomitant valve-sparing aortic root replacement + arch operations. Abnormal smooth muscle cell–mediated aortic remodeling occurs within the normal diameter, clinically at-risk Loeys-Dietz syndrome arch segment. These initial clinical and pathophysiologic findings support concomitant arch replacement in Loeys-Dietz syndrome.

Optimizing Pier Design to Mitigate Scour: A Comprehensive Review and Large Eddy Simulation Study

Scour-induced sediment erosion poses a significant threat to the safety and longevity of infrastructure, including bridges, wind turbines, elevated buildings, and coastal infrastructure. Despite the well-known destructive consequences of scour, accurate models that capture the complexity of its dynamics remain elusive, impeding the development of effective countermeasures. We provide a comprehensive review of existing literature on scour dynamics and examine the fluid dynamics and bed shear stress surrounding bridge piers. We propose CFD (Computational Fluid Dynamics) simulations with LES (Large-Eddy Simulation). The current paper demonstrate that LES is a more effective technique than RANS (Reynolds averaged Navier-Stokes) for investigating bridge scouring. The LES simulations study local scour induced effects and compare the findings with RANS simulation results. Besides, two countermeasures are modeled, delta vane and plate footings, to decrease scour around piers. The results show that both countermeasures effectively reduce the shear stress, and we also suggest a combination of a delta vane and a plate footing as a promising solution to reduce upstream and downstream bed shear stress. The paper highlights the importance of thorough investigations on bridge scouring and the need for effective countermeasures to protect infrastructure from scour-related damage or collapse. The recommended countermeasures hold significant promise to reduce construction and maintenance costs and extend infrastructure longevity.

Triglobal infinite-wing shock-buffet study

This article uses triglobal stability analysis to address the question of shock-buffet unsteadiness, and associated modal dominance, on infinite wings at high Reynolds number, expanding upon recent biglobal work, aspiring to elucidate the flow phenomenon's origin and characteristics. Infinite wings are modelled by extruding an aerofoil to finite aspect ratios and imposing a periodic boundary condition without assumptions on spanwise homogeneity. Two distinct steady base flows, spanwise uniform and non-uniform, are analysed herein on straight and swept wings. Stability analysis of straight-wing uniform flow identifies both the oscillatory aerofoil mode, linked to the chordwise shock motion synchronised with a pulsation of its downstream shear layer, and several monotone (non-oscillatory), spatially periodic shock-distortion modes. Those monotone modes become outboard travelling on the swept wing with their respective frequencies and phase speeds correlated with the sweep angle. In the limiting case of very small wavenumbers approaching zero, the effect of sweep creates branches of outboard and inboard travelling modes. Overall, triglobal results for such quasi-three-dimensional base flows agree with previous biglobal studies. On the contrary, cellular patterns form in proper three-dimensional base flow on straight wings, and we present the first triglobal study of such an equilibrium solution to the governing equations. Spanwise-irregular modes are found to be sensitive to the chosen aspect ratio. Nonlinear time-marching simulations reveal the flow evolution and distinct events to confirm the insights gained through dominant modes from routine triglobal stability analysis.

Study of vibration modes and strain distribution of a flow energy harvester in the wake region of different bluff bodies

This paper aims to analyze mode shapes and corresponding strain distribution in a two-dimensional plane structure exhibiting flow-induced vibration in the wake field of four different upstream bluff bodies such as cylinder, square, triangle, and D-shape. This research is important from the point of view of flow-induced piezoelectric energy harvesting; wherein, induced strain in the structure is directly related to the amount of energy generated. Mainly, all investigations are carried out at low Reynolds number ( Re = 200); however, to widen the scope of the work, other Reynolds numbers are also considered ( Re = 300, 500, and 750 ). The results obtained indicated that the plane structure vibrates in different mode shapes under different wake fields. The square section dominantly gives rise to the fundamental mode vibration, whereas, cylinder, D-shape, and triangular bluff body induce vibration in a mix of fundamental and higher bending modes. Analysis of the corresponding flow regime shows that the position of reattachment point of the downstream shear layer plays an important role in the realization of different vibration modes. The strain distribution under different cases revealed that the wake of a cylindrical bluff body produces highest peak strain, and D-shape bluff body results in highest cumulative strain. From the aspect of energy harvesting, a quantitative comparison of strain-induced and per second charge generation have indicated that for an equivalent flow condition, the D-shape will produce higher energy per unit time than the cases of a square, cylinder, and triangular cross-sections.

Gulf Stream Marine Hydrokinetic Energy Off Cape Hatteras, North Carolina

AbstractMulti-year measurements of current velocity, salinity, and temperature from fixed and vessel-mounted sensors quantify Gulf Stream (GS) marine hydrokinetic energy (MHK) resource variability and inform development off Cape Hatteras, NC. Vessel transects across the GS demonstrate a jet-like velocity structure with speeds exceeding 2.5 m/s at the surface, persistent horizontal shear throughout the jet, and strongest vertical shears within the cyclonic shear zone. Persistent equatorward flow at the base of the GS associated with the Deep Western Boundary Current (DWBC) produces a local maximum in vertical shear where stratification is weak and is postulated to be a site of strong turbulent mixing. Repeated transects at the same location demonstrate that the velocity structure depends upon whether the GS abuts the shelf slope or is offshore.Currents from a fixed acoustic Doppler current profiler (ADCP) deployed on the shoreward side of the GS exceed 1 m/s 64% of the time 40 m below the surface. The 3.75-year time series of currents from the ADCP mooring document large, roughly weekly variations in downstream and cross-stream speed (−0.5 to 2.5 m/s) and shear (± 0.05 s−1) over the entire water column due to passage of GS meanders and frontal eddies. Current reversals from the mean GS direction occur several times a month, and longer period variations in GS offshore position can result in reduced currents for weeks at a time. Unresolved small-scale shear is postulated to contribute significantly to turbulent mixing.

Bi-global stability analysis in curvilinear coordinates

A method is developed to solve biglobal stability functions in curvilinear systems which avoids reshaping of the airfoil or remapping the disturbance flow fields. As well, the biglobal stability functions for calculation in a curvilinear system are derived. The instability features of the flow over a NACA (National Advisory Committee for Aeronautics) 0025 airfoil at two different angles of attack, corresponding to a flow with a separation bubble and a fully separated flow, are investigated at a chord-based Reynolds number of 100 000. The most unstable mode was found to be related to the wake instability, with a dimensionless frequency close to one. For the flow with a separation bubble, there is an instability plateau in the dimensionless frequency ranging from 2 to 5.5. After the plateau and for an increasing dimensionless frequency, the growth rate of the most unstable mode decreases. For a fully separated flow, the plateau is narrower than that for the flow with a separation bubble. After the plateau, with an increased dimensionless frequency, the growth rate of the most unstable mode decreases and then increases once again. The growth rate of the upstream shear layer instability was found to be larger than that of the downstream shear layer instability.

Adjoint sensitivity and optimal perturbations of the low-speed jet in cross-flow

The tri-global stability and sensitivity of the low-speed jet in cross-flow are studied using the adjoint equations and finite-time horizon optimal disturbance analysis at Reynolds number $Re=2000$, based on the average velocity at the jet exit, the jet nozzle exit diameter and the kinematic viscosity of the jet, for two jet-to-cross-flow velocity ratios $R=2$ and $4$. A novel capability is developed on unstructured grids and parallel platforms for this purpose. Asymmetric modes are more important to the overall dynamics at $R=4$, suggesting increased sensitivity to experimental asymmetries at higher $R$. Low-frequency modes show a connection to wake vortices. Adjoint modes show that the upstream shear layer is most sensitive to perturbations along the upstream side of the jet nozzle. Lower frequency downstream modes are sensitive in the cross-flow boundary layer. For $R=2$, optimal analysis reveals that for short time horizons, asymmetric perturbations dominate and grow along the counter-rotating vortex pair observed in the cross-section. However, as the time horizon increases, large transient growth is observed along the upstream shear layer. When $R=4$, the optimal perturbations for short time scales grow along the downstream shear layer. For long time horizons, they become hybrid modes that grow along both the upstream and downstream shear layers.

Direct numerical simulation of high-speed transition due to roughness elements

We study and compare instability mechanisms of a Mach 5.65 laminar boundary layer tripped by an isolated diamond-shaped trip and by an array of diamond-shaped trips using direct numerical simulations. A low-Reynolds-number experiment, consisting of the trip array (Semper & Bowersox, AIAA J., vol. 55 (3), 2017, pp. 808–817), is used to validate our simulations. Three dynamically prominent flow structures are observed in both trip configurations. These flow structures are the upstream vortex system, the shock system, and the downstream shear layers/counter-rotating streamwise vortices that originate from the top and sides of the trips. Analysis of the power spectral density of pressure reveals the source of instability to be an interaction between the shear layers and the counter-rotating streamwise vortices downstream of both trip configurations. The interaction leads to the formation of hairpin-like structures that eventually break down to turbulent flow. This finding contrasts with that of an isolated cylindrical trip (Subbareddy et al., J. Fluid Mech., vol. 748, 2014, pp. 848–878) where the upstream vortex system is found to be the source of instability. Therefore, the shape of a trip plays an important role in the instability mechanism. Furthermore, dynamic mode decomposition (Rowley et al., J. Fluid Mech., vol. 641, 2009, pp. 115–127; Schmid, J. Fluid Mech., vol. 656, 2010, pp. 5–28) of three-dimensional snapshots of pressure fluctuations unveil globally dominant modes consistent with the power spectral density analysis in both diamond-shaped trip configurations.

Mechanisms of flame extinction and lean blowout of bluff body stabilized flames

Lean flame blowout is investigated experimentally within a high-speed combustor to analyze the temporal extinction dynamics of turbulent premixed bluff body stabilized flames. The lean blowout process is induced through fuel flow reduction and captured temporally using simultaneous high-speed particle imaging velocimetry (PIV) and CH* chemiluminescence. The evolution of the flame structure, flow field, and the resulting flame strain rate are analyzed throughout extinction to distinguish the physical mechanisms of blowout. The flame–vortex dynamics are found to be the main driving mechanism of flame extinction; namely, a reduction of the flame-generated vorticity coupled with an increase in the downstream shear layer vorticity. The vorticity dynamics are linked to hydrodynamic instabilities that vary as a function of the decreasing equivalence ratio. A frequency analysis is performed to characterize the dynamic changes of the hydrodynamic instability modes during flame extinction. Various bluff body inflow velocity regimes are investigated to further characterize the extinction instability modes. Both equivalence ratio and flow-driven instabilities are captured through a universal definition of the Strouhal number for the reacting bluff body flow. Finally, a Karlovitz number-based criterion is developed to consistently predict the onset of global extinction for the different inflow velocity regimes.

Time-averaged asymmetries and oscillatory global modes of vortex flows over a slender wing

Previous studies indicate that vortex pairs over slender wings can produce time-averaged asymmetries even before the occurrence of vortex breakdown, but the sources for the asymmetries are still not clear. The present investigation uses numerical simulations and experimental measurements to explore the physical nature for the symmetry breaking of vortices. The results indicate that the time-averaged asymmetries come from a spatial growth of local disturbances (spatial instability) instead of a temporal instability. In numerical simulations, the time-averaged flows around a perfectly symmetric wing exhibit symmetric vortex pairs in which any asymmetric initial perturbation will decay and eventually converge to symmetric flows. However, if a geometric micro-perturbation is asymmetrically placed on the apex, asymmetric vortex pairs can be produced; downstream sectional perturbation energy grows spatially, and the growth rates increase with increasing angle of attack. The spatial perturbation growth mainly comes from antisymmetric modes, whereas symmetric modes have smaller growth rates. The experimental results indicate that artificial apex perturbations can overwhelm natural irregularities on the apex and dominate the orientations of asymmetric vortices. Unsteady aspects of the flows were also studied before vortex breakdown, with the emphasis put on oscillatory global modes by applying dynamic mode decomposition to the entire flow field. It was found that the most energetic modes are related to vortex shedding around trailing edges and instabilities of downstream shear layers, which has significant contributions to fluctuations of aerodynamic forces and moments. The vortex shedding exhibits strong three dimensionality characterized by oblique shedding and a beating phenomenon.

Flow Interactions Between Streamwise-Aligned Tandem Cylinders in Turbulent Channel Flow

The flow interactions between streamwise-aligned wall-mounted circular cylinders is studied experimentally to explore shear-layer interactions that lead to modifications of the flow around the downstream-most cylinder. Two identical finite-length cylinders are arranged in tandem and fully submerged in the log layer of turbulent channel flow, and particle image velocimetry measurements are conducted to document the flow behavior at various streamwise spacings. Comparisons are made to flow around an isolated cylinder as well as the baseline case of smooth-wall turbulent channel flow with no cylinders present. Strong interactions between the shear layers shed by each individual cylinder are noted, whereby the downstream-most wake exhibits a weaker, more spatially compact recirculation region and significantly reduced reattachment length as compared to flow around an isolated cylinder. First- and second-order turbulence statistics show similar wake modifications, particularly for small cylinder spacings where the shear layer shed from the top of the upstream cylinder directly impinges on the downstream-most cylinder and significantly reduces the turbulence levels in the downstream shear layer. Increasing the spacing of the cylinders relaxes the turbulence statistics in the shear layer of the downstream-most cylinder back toward those of an isolated cylinder, although even a spacing of eight cylinder diameters is not sufficient to fully recover isolated cylinder behavior. Finally, the impact of the cylinder aspect ratio is considered is found to have only a modest impact on the observed trends in reattachment and shear-layer interactions between the cylinders.

Influence of the velocity field on scalar transport in gaseous transverse jets

The present experiments explored the dynamical character of the gaseous jet injected flush into cross-flow for variable jet-to-cross-flow momentum flux ratios$J$(5, 12 and 41) and density ratios$S$(0.35 and 1.0). Contoured nozzle and straight pipe injectors were studied here, with the jet Reynolds number fixed at 1900 as other flow parameters were varied. Simultaneous acetone planar laser-induced fluorescence (PLIF) imaging and stereo particle image velocimetry (PIV) were used to study the relationships between scalar and velocity/vorticity fields, with a special focus on comparing PLIF-based extraction of scalar dissipation rates and local strain rates with PIV-based local strain rates in the upstream and downstream shear layers of the jet. There was remarkable similarity between the scalar and vorticity fields for the jet in cross-flow, spanning conditions for absolutely unstable upstream jet shear layers at low$J$or$S$values to conditions for convectively unstable shear layers for larger$J$, equidensity conditions (Megerianet al.,J. Fluid Mech., vol. 593, 2007, pp. 93–129; Getsingeret al.,Exp. Fluids, vol. 53, 2012, pp. 783–801). Proper orthogonal decomposition applied to both scalar and velocity fields revealed strengthening instabilities in both the upstream shear layer and in the jet’s wake as$J$was reduced. The simultaneous measurements allowed PLIF-extracted scalar dissipation rates and strain rates to be determined via a flamelet-like model and compared with PIV-extracted strain rates, each in the diffusion layer-normal direction. There was generally very good qualitative and quantitative agreement for these metrics in both the jet upstream and downstream shear layers for most flow conditions, with excellent correspondence to locations of shear layer vorticity roll up, although downstream shear layer strain rates in some cases showed lesser correspondence between PLIF- and PIV-based data. Such differences are shown to potentially result from diffusion and resolution effects as well as the influence of three-dimensional and transient effects which can be more significant in the lee side of the jet. Nevertheless, the present results reveal interesting dynamics and demonstrate the importance of strain fields in enhanced diffusion and transport phenomena.

Proximal Stenosis Is Associated with Rupture Status in Middle Cerebral Artery Aneurysms.

Hemodynamic factors impact cerebral aneurysm development and progression. Parent vessel architectural features, such as caliber, curvature, and angle, can affect downstream pressure and shear stress. To investigate the association between proximal parent vessel stenosis and aneurysm rupture status at the middle cerebral artery (MCA) bifurcation. Catheter 3-dimensional rotational angiographic datasets from 69 Japanese patients with MCA aneurysms (58 unruptured/11 ruptured) were analyzed. The narrowest cross-sectional area of the M1 segment was evaluated through equidistant cross-sectional plane cuts along the M1 length. The degree of stenosis relative to M1 size (StenosisIndex) and the distance from stenosis to the aneurysm neck (StenosisAnDist) were statistically evaluated. The effects of StenosisIndex and StenosisAnDist were determined in parametric aneurysm models with/without stenosis using computational fluid dynamic and fluid-structure interaction simulations. MCA harboring ruptured aneurysms had significantly greater StenosisIndex (0.31 ± 0.21 vs. 0.17 ± 0.14, P= 0.01), indicative of greater narrowing, and shorter StenosisAnDist (4.26 ± 1.91 vs. 6.94 ± 4.06 mm, P= 0.02) compared with unruptured aneurysms. Multivariate analysis combining StenosisIndex and StenosisAnDist resulted in P= 0.003, area under the curve= 0.81 (80% sensitivity, 74% specificity). Computational fluid dynamic and fluid-structure interaction simulations identified a synergetic effect of high stenosis and short StenosisAnDist in inducing greater aneurysm inflow velocity and deeper jet penetration, greater dome pressure, and greater tensile stress in the aneurysm wall. Ruptured status in bifurcation MCA aneurysms was associated with severity of proximal M1 stenosis and its proximity to the aneurysm neck, a novel risk factor, which acts by increasing aneurysm dome wall tension, and should be considered in investigations of rupture risk stratification.

Multiscale characterization of migrating sand wave in mining induced alluvial channel

Sediment transport mechanism is highly influenced by the hydraulic condition of a river. Sand mining from river bed changes the geometry of the bed and influences the flow parameters, and thus can alter the hydraulic condition of a river. Our study is based on laboratory experiments for sand bed channel with a rectangular mining pit. We present both physical and dynamic characteristics of a mining induced channel from the bed elevation data obtained from Seatek instrument. Physical characteristics shows erosion in the entire channel as the flow in the channel is kept higher the flow required for sediment movement. Two distinct water surface slopes occur in the test section centering the mining pit. Shear stress along the test section is determined and observed higher value of downstream shear stress than upstream of the pit. Erosion at the downstream of the pit is more than the upstream and migration of upstream edge occurs. Dynamic analysis of sand wave depending on statistical approach is carried out to investigate the movement of sediment under the influence of mining. Celerity of sand wave increases with increase in flow velocity during the initial time span (1–4h) but in a later time interval (4–7h) it decreases with increase in flow velocity. It has been found that celerity of sediment initially increases with scale size and then decreases, which indicates a higher resistance is being offered by sediment at high scale.