Adjacent Flow Research Articles

Fragility curves for road embankments exposed to adjacent debris flow

Studies of recent decades have shown thousands of kilometers of transportation networks that have presented damage or failure from different types of flows, causing important traffic disruptions. Debris flows running adjacent (or in parallel) to river channels often explain the structural damage to road embankments caused by slope erosion. The probability of expected structural damage caused by a natural hazard may be modeled using fragility curves, which have been developed for transportation infrastructures like bridges and roads exposed to debris flows and are used in risk assessment. There are even fragility curves available to estimate the fragility of road embankments exposed to perpendicular debris flows. However, currently no model is available to estimate the road damage probability of embankments exposed to adjacent debris flows despite their important effects on traffic. This paper aimed to develop fragility curves for road embankments exposed to adjacent debris flows, considering mechanisms of exposed slopes subject to erosion and subsequent embankment instability. Models were calibrated considering flow characteristics in straight and bend channels including three damage states in terms of road traffic capacity loss. Monte Carlo simulations were performed to model potential damage, obtaining fragility curves for two types of roads. Curves were fit to log-normal distributions with a 99.5% confidence level. The analysis demonstrated that the geometric characteristics of road embankments explain their fragility, wherein lower heights and platform widths result in more probable expected damage. The analytical model developed confirmed that the erosive process intensifies in bend zones of channels, resulting in higher damage probability.

Research on the Relationship between Stall Propagation and Flange Leakage of Mixed-Flow Pumps

In order to explore the internal relationship between stall core propagation and flange leakage flow in the rotating stall of a mixed-flow pump, based on the k-ε turbulence model, a SIMPLEC algorithm and hexahedral structured grid are used to simulate the internal flow field of the mixed-flow pump. By setting the flange clearance as 0.2 mm, 0.5 mm and 0.8 mm, the propagation characteristics of the rotating stall and the unsteady characteristics of flange leakage flow of the mixed-flow pump under the condition of near stall are studied, and the influence of the flange clearance on the pressure fluctuation characteristics of the mixed-flow pump under the condition of near stall is analyzed. The results show that the stall core is located at the outlet of the impeller and propagates from the leading edge of the adjacent blade along the opposite direction of blade rotation to the next flow channel. The pressure gradient in the stall channel and the energy loss are large. When the flange clearance is 0.5 mm and 0.8 mm, the stall core changes from one to two, and the propagation mechanism of the stall core tends to be complex in the two adjacent flow channels. When the flange clearance is 0.8 mm, the propagation period decreases. The variation law of leakage flow is consistent with the propagation law of stall core. When the flow passage changes from stall state to non-stall state, the leakage flow also changes from one state to another, so the leakage flow can be used as a form of apparent stall. Under the condition of near stall, the pressure fluctuation curve of the adjacent monitoring points has a large phase difference consistent with the propagation period of the stall core, and has a strong pressure drop. When the flange clearance is 0.5 mm and 0.8 mm, the time domain curve of pressure fluctuation has two wave troughs in one cycle. In the near stall condition, the main frequency of the pressure fluctuation at the monitoring point is the stall frequency, and the amplitude of the main frequency at the middle of the outlet is the largest. The characteristics of flange leakage flow and pressure fluctuation can better reflect the flow situation in the pump when rotating stall occurs. The research results can provide a basis for judging whether stall occurs in the flow passage of the pump.

Effects of asymmetric stroke deviation on the aerodynamic performance of flapping wing

The kinematics of insect flapping flight are complex and asymmetric, which are contributed to their superior flying capabilities, and the design of novel flapping micro air vehicles can draw inspiration from relevant researches. Previous studies usually focus on the wing with asymmetric stroke or pitch motions. A trajectory with asymmetric deviation motion, named as “pear-shaped” pattern, is proposed in current work. The hovering aerodynamics and vortex dynamics of a rigid flapping wing have been numerically investigated by comparing with that of “line-shaped” pattern with no deviation. In order to have a better insight into the influences of the asymmetric deviation, we change the kinematic parameters, that is, stroke amplitude, pitching amplitude, deviation amplitude, and phase lag between stroke and pitch angles. The results show that the wing with asymmetric deviation exhibits superior capability in lift enhancement for most of the cases analyzed, which is accompanied by the extra power cost and slight reduction in efficiency. The asymmetric deviation in cases with high stroke amplitude or low pitching amplitude may be considered as a cost-saving strategy, subject to slight damage on lift generation (if acceptable). Additionally, the asymmetric deviation brings a strong asymmetry into the instantaneous forces during one flapping cycle. The underlying lift-enhancing mechanism is explored by examining the dominant vortex structures in the adjacent flow field of the wing, which is mainly attributed to the changes in the effective angle of attack, increasing with downward deviation and decreasing with upward deviation.

Characteristics of the wall temperature field in a mixed convection turbulent boundary layer

The behavior of the heated wall temperature field adjacent to a mixed convection turbulent boundary layer flow was investigated. The wall temperature field was captured using an infrared camera. The Richardson number was varied between experiments to produce a range of mixed convection conditions ranging from buoyancy dominant (RiL = 2.0) to inertia dominant (RiL = 0.01). From the 2D temperature fields recorded, the fluctuating temperature fields and associated low-order statistics were computed. A Fourier-based analysis of the fluctuating temperature field indicated the presence of multiple modes of energy exchange between heated wall and the adjacent turbulent boundary layer flow including energy input, dissipation, and a viscous-convective subrange for RiL ≤ 0.3. The Proper Orthogonal Decomposition (POD) was performed on the fluctuating temperature field. Results show cell-like patterns in low-order POD modes at all tested RiL suggesting a convective cell-like structure in the fluid region adjacent to the interface. The manifestation of these cell-like patterns was found to occur at increasing POD mode number as buoyant contributions decreased relative to inertial contributions.

Customizable Microfluidic Origami Liver-on-a-Chip (oLOC).

The design and manufacture of an origami-based liver-on-a-chip device are presented, together with demonstrations of the chip's effectiveness at recapitulating some of the liver's key in vivo architecture, physical microenvironment, and functions. Laser-cut layers of polyimide tape are folded together with polycarbonate nanoporous membranes to create a stack of three adjacent flow chambers separated by the membranes. Endothelial cells are seeded in the upper and lower flow chambers to simulate sinusoids, and hepatocytes are seeded in the middle flow chamber. Nutrients and metabolites flow through the simulated sinusoids and diffuse between the vascular pathways and the hepatocyte layers, mimicking physiological microcirculation. Studies of cell viability, metabolic functions, and hepatotoxicity of pharmaceutical compounds show that the endothelialized liver-on-a-chip model is conducive to maintaining hepatocyte functions and evaluation of the hepatotoxicity of drugs. Our unique origami approach speeds chip development and optimization, effectively simplifying the laboratory-scale fabrication of on-chip models of human tissues without necessarily reducing their structural and functional sophistication.

SHARAD detection of sedimentary infilling within an unnamed crater near Mangala Fossa region, Mars

We present the first Shallow Radar (SHARAD) based observations of subsurface reflections within an unnamed crater centered at 21.0° S, 150.6° W. It is situated south of the Mangala Fossa and Mangala Valles, a major fluvial outflow channel system. We informally name the crater Mangala crater for this paper. In contrast with adjacent Tharsis lava flow, our analysis of radar propagation in the Mangala crater reveals a low loss tangent (0.008–0.009) and lower dielectric subsurface material (average 5.6) for a subsurface unit that is ~40 m thick. These values are unexpected in this region. Lava flows dominate the surface and surrounding region, and SHARAD investigations of lava flows in the Tharsis region have detected higher values, creating a discrepancy that we discuss. Based on observed stratigraphy, and subsurface geophysical properties, we propose that the crater infilling is composed of moderate density sedimentary material buried by a layer of lava in the eastern portion of Mangala crater but left exposed in the west. Our measurements are also supported by previously done geologic mapping. We identify two subsurface reflections that provide details to determine one of the layers is sediment rather than lava. Detection of multiple reflections also indicates that infilling occurred in at least four successive events. These results are significant because it adds more context to this highly studied region and provides compelling evidence that water moved large volumes of sediment into this basin.

Is Westward Travelling Surge Driven by the Polar Cap Flow Channels?

AbstractFollowing substorm auroral onset, the active aurora region usually expands poleward toward the poleward auroral boundary. Such poleward expansion is often associated with a bulge region that expands westward and forms the westward travelling surge. In this study, we show all‐sky imager and Poker Flat Advanced Modular Incoherent Scatter Radar observations of two surge events to investigate the relationship between the surge and ionospheric flows that likely have polar cap origin. For both events, we observe auroral streamers, with an adjacent flow channel consisting of decreased density and low electron temperature plasma flowing equatorward. This flow channel appears to impinge and lead/feed surge formation, and to stay connected to the surge as it moves westward. Also, for both events, streamer observations indicate that, following initial surge development, similar flows led to explosive surge enhancements. The observation that the streamers are connected to the auroral polar boundary and that the flow channels consisted of low density, low electron temperature plasma suggests the possibility that the impinging plasma came from the polar cap. For both events, the altitude variations of F region plasma within the surges are related with aurora emission and the poleward/equatorward flow, and the surges develop strong auroral streamers that initiate along the poleward auroral boundary when contacted with the flow. These results suggest that the flow of polar cap origin, which maps to underlying processes in the magnetotail, may play a crucial role in auroral surges by feeding low entropy plasma into surge initiation and development, and also playing an important role in the dynamics within a surge.

A convection-enhanced flow field for aqueous redox flow batteries

In this work, we propose and fabricate a convection-enhanced flow field for aqueous redox flow batteries. Similar to the conventional single serpentine flow field, the new flow field here has an inlet and outlet, but the repatterned flow path induces higher pressure differences between adjacent flow channels over the entire electrode surface, thereby enhancing the under-rib convection between adjacent flow channels. The application of the new flow field to a vanadium redox flow battery enables substantial performance improvements. At 1.67 mL min−1 cm−2, which is a typical flow rate for a kilowatt-class VRFB stack, the energy efficiency, and electrolyte utilization reach 75.3% and 49.6% at a current density of 250 mA cm−2, increasing by 10.0% and 33.3% as opposed to the use of the conventional serpentine flow field, respectively. Three-dimensional numerical modelling shows that the improved performance is ascribed to the more uniform in-plane distribution of reactants and current with the new flow field. Another important finding of this work is that there is a critical flow rate above which energy efficiency tends to stabilize for a given current density.

Incorporating Practical Degree of Saturation in Capacity Estimation of On-Street, Mid-Block, Off-Line Bus Stops

The effectiveness of an on-street bus facility depends on the general traffic that shares the lane used by buses. The Transit Capacity and Quality of Service Manual (TCQSM) methodology was used to estimate facility bus capacity based on critical stop operation. Hisham et al. provided an improved understanding of performance of an on-street, mid-block, off-line bus stop by relating bus stop capacity to the adjacent lane traffic flow rate. Using the TCQSM methodology, bus stop capacity was determined by adopting an estimated operation margin that relates to a design bus stop failure rate. However, failure rate is theoretically ambiguous and difficult to quantify in practice, particularly under high volume traffic conditions. In contrast, degree of saturation is a direct measure of operating conditions experienced by buses using the stop, and by the adjacent lane general traffic, so it directly affects approach delay and queuing. The aim of this study was to better understand performance at on-street, mid-block, off-line bus stops by considering degrees of saturation of loading areas and the adjacent lane rather than design failure rate according to the TCQSM methodology, and to ensure that waiting times upstream of bus stops are kept to acceptable levels by determining bus stop maximum working capacity.

Statistical Properties of Current, Energy Conversion, and Electron Acceleration in Flux Ropes in the Terrestrial Magnetotail

AbstractWe perform a statistical analysis on current density, energy conversion, and electron acceleration in the primary flux ropes (PFRs) and the secondary flux ropes (SFRs) distinguished by out‐of‐plane electron current, respectively. It is found that current filaments are plentiful in both PFRs and SFRs. The closer to the center of the SFRs, the stronger the |J| is. The J•E′ is intermittent in the PFRs. However, J•E′ almost stays positive in the SFRs. The local adiabatic acceleration of the electrons in the PFRs and the SFRs are explored. Fermi and betatron acceleration are closely related to the adjacent magnetic field and bulk flow which can lead to the contraction/relaxation of the PFRs and SFRs and enhance/reduce the gradient of the magnetic field. The acceleration by the parallel electric field is significant in the SFRs. Our results can help to improve the understanding of the PFRs and SFRs and their roles in the magnetotail dynamics.

Reference-Free DIBR-Synthesized Video Quality Metric in Spatial and Temporal Domains

Depth image-based rendering (DIBR) techniques play an important role in free viewpoint videos (FVVs), which have a wide range of applications including immersive entertainment, remote monitoring, education, etc. FVVs are usually synthesized by DIBR techniques in a &#x201C;blind&#x201D; environment (without a reference video). Thus, an effective reference-free synthesized video quality assessment (VQA) metric is vital. At present, many image quality assessment (IQA) algorithms for DIBR-synthesized images have been proposed, but limited researches have been concerned about the quality assessment of DIBR-synthesized videos. To this end, this paper proposes a novel reference-free VQA method for synthesized videos, which operates in Spatial and Temporal Domains, dubbed as STD. The design fundamental of the proposed STD metric considers the effects of two major distortions introduced by DIBR techniques on the visual quality of synthesized videos. First, considering the geometric distortion introduced by DIBR technologies can increase high-frequency contents of the synthesized frame, the influence of the geometric distortion on the visual quality of a synthesized video can be effectively evaluated by estimating high-frequency energies of each synthesized frame in spatial domain. Second, temporal inconsistency caused by DIBR techniques brings the temporal flicker distortion, which is one of the most annoying artifacts in DIBR-synthesized videos. In temporal domain, we quantify temporal inconsistency by measuring motion differences between consecutive frames. Specifically, optical flow method is first used to estimate the motion field between adjacent frames. Then, we calculate the structural similarity of adjacent optical flow fields and further adopt the structural similarity value to weight the pixel differences of adjacent optical flow fields. Experiments show that the above two features are able to well perceive the visual quality of DIBR-synthesized videos. Furthermore, since the two features are extracted from spatial and temporal domains, respectively, we integrate them using a linear weighting strategy to obtain our STD metric, which proves advantageous over two components and the competing state-of-the-art I/VQA methods. The source code is available at <uri>https://github.com/wgc-vsfm/DIBR-video-quality-assessment</uri>.

Investigation of Low-Frequency Phenomena within Flow Pattern in Standard Mixing Vessel Induced by Pitched Blade Impeller

An experimental study on the flow pattern dynamics in a standard mixing vessel with radial baffles filled with water and induced by a pitched blade impeller pumping downward is presented. Investigation is mainly focused on detection and analysis of quasi-periodical or periodical low-frequency phenomenon connected with time- and length-scales considerably exceeding the Blade Passage Frequency (hereinafter BPF) and common turbulent eddies. This phenomenon, which is expressed as large-scale mean-flow variations, is generally known as flow Macro-Instability (hereinafter MI). It could break-down just below the liquid surface, or it crashes to the liquid surface and causes its Macro-Swelling (hereinafter MS). Our investigation was based on classical two-dimensional (2D) Particle Image Velocimetry (hereinafter PIV) measurement within 3 selected vertical planes in the vessel and subsequent analysis of the velocity field. The dominant frequencies evaluated in the selected points and overall analysis of the quasi-periodical macro-flow pattern behavior is to be shown. Identification of the quasi-periodical substructures appeared within the flow pattern was performed using the Oscillation Pattern Decomposition (hereinafter OPD) method. Observation of the macro-flow patterns confirmed presence of the macro-flow structures detected within flow pattern at the identical mixing pilot plant setup by previous investigations of the MIs phenomenon, i.e., the primary circulation loop and strong impeller discharge jet located in the lower vessel segment and the strong ascending wall current at the baffle, which break-down below the surface very often. A further important contribution of the presented work is the investigation of both flow pattern within the baffles vicinity and in the middle of the sector far from the baffle, showing a significant difference. Low-frequency periodical (or quasi-periodical) behavior of the investigated macro-structures was qualitatively confirmed by the presented results and it was quantified using the velocity dominant frequencies evaluation, as noted below. This frequency analysis brings insight into the supposed and detected interconnections between dynamics of the adjacent flow structures. Detected different flow patterns within the main plane near the baffles and in the inclined plane reveal a strong influence of the baffle presence to the local vertical flow, especially within the upper part of the vessel. Quite a different flow pattern appears tangentially in front of and behind the baffle, where a wake is revealed, indicating significant influence of the baffle on the local tangential flow. The new findings represent a contribution to better understanding the physical phenomena behind the standard mixing process.

Late Pleistocene Boulder Slumps Eroded from a Basalt Shoreline at El Confital Beach on Gran Canaria (Canary Islands, Spain)

This study examines the role of North Atlantic storms degrading a Late Pleistocene rocky shoreline formed by basaltic rocks overlying hyaloclastite rocks on a small volcanic peninsula connected to Gran Canaria in the central region of the Canary Archipelago. A conglomerate dominated by large, ellipsoidal to angular boulders eroded from an adjacent basalt flow was canvassed at six stations distributed along 800 m of the modern shore at El Confital, on the outskirts of Las Palmas de Gran Canaria. A total of 166 individual basalt cobbles and boulders were systematically measured in three dimensions, providing the database for analyses of variations in clast shape and size. The goal of this study was to apply mathematical equations elaborated after Nott (2003) and subsequent refinements in order to estimate individual wave heights necessary to lift basalt blocks from the layered and joint-bound sea cliffs at El Confital. On average, wave heights in the order of 4.2 to 4.5 m are calculated as having impacted the Late Pleistocene rocky coastline at El Confital, although the largest boulders in excess of 2 m in diameter would have required larger waves for extraction. A review of the fossil marine biota associated with the boulder beds confirms a littoral to very shallow water setting correlated in time with Marine Isotope Stage 5e (Eemian Stage) approximately 125,000 years ago. The historical record of major storms in the regions of the Canary and Azorean islands indicates that events of hurricane strength were likely to have struck El Confital in earlier times. Due to its high scientific value, the outcrop area featured in this study is included in the Spanish Inventory of Geosites and must be properly protected and managed to ensure conservation against the impact of climate change foreseen in coming years.

Frame Deletion Detection Based on Optical Flow Orientation Variation

Frame deletion detection is an arrestive field for video forensics in recent years. This paper presents a frame deletion detection algorithm based on optical flow orientation variation. Optical flow field has been proved to be effective for the detection of the frame deletion operation. However, current optical-flow based methods have relatively poor detection performance for real-world videos. Our research aims to develop a new forensic method to detect frame deletion for real-world videos. The proposed algorithm firstly develops an analytical model for flow orientation variation between adjacent optical flow fields, and exhibits a novel forensic clue for frame deletion detection, under an ideal but reasonable assumption. Accordingly, an effective descriptor, namely pseudo flow orientation variation (PFOV), is created to approximate the flow orientation variation, as a discriminative description feature to capture the frame deletion trace. The Frobenius norm and a smooth function are also introduced to quantify each descriptor to form an one-dimensional time series, and further statistics technique is adopted to detect the anomalous points in the time series caused by frame deletion. In addition, to improve the descriptor performance, the Robust Principal Component Analysis (RPCA) is applied to extract moving objects from video sequences and compute the proposed descriptor on them. The tests are made on 324 real-world videos, and experiment results demonstrate that the true positive rate can reach 90.12%, meanwhile the false alarm rate is 7.71%, which indicates the superiority of the proposed method in the forensic algorithms for real-world video.

A numerical analysis for a damage propagation in a plate-type fuel assembly of a research reactor

If several flow channels are completely blocked in a plate-type fuel assembly of research reactors with high power density, a flow instability (FI) can occur in an unblocked channel and can be propagated to the other adjacent flow channels. Such FI propagation can cause a propagation of fuel plate damages during the flow blockage accident. Hence, the thermal hydraulic behaviors of flow channels have been analyzed in order to investigate the FI propagation process by the simulation of CFD code for a multiple flow channel blockage in KiJang Research Reactor (KJRR) which is the model reactor assumed for this paper. Simulation results show that a FI occurs at the first unblocked channel if 8 flow channels are completely blocked and most of fuel plates are significantly damaged in 29 s after a complete blockage of 8 flow channels occurs in a plate type fuel assembly of the model reactor.

Reciprocating Subsurface Wetlands for Drainage Water Treatment A Case Study in Egypt (Dept.C)

A new patented constructed wetlands treatment technology for enhanced nutrients reduction has been developed by Tennessee Valley Authority. This new treatment technology is known as Reciprocating Flow Constructor Wetland (RFCW) through increasing oxygen content in the treatment processes. The RFCW has a process of operating two adjacent subsurface flow gravel beds by alternately draining and filling the wastewater on a recurrent basis to achieve tertiary levels of wastewater treatment. In Egypt, A pilot-scale RFCW was built near Lake Manzala as a demonstration of low cost treatment technology to treat a portion of Bahr El-Baqar drain wastewater that carries drainage water contains domestic and industrial pollution loads. The pilot wetland consists of one sedimentation basin followed by two subsurface flow cells working in a fill-empty sequence mechanism. The treatment capacity is 100 m per day. Treatment detention time is 4 days including 2 days in the gravel bed RFCW cells. Objectives of this paper are to describe operational flow inside the alternate cells, and to evaluate the treatment performance of reciprocating wetland for such polluted drainage water and to estimate removal rate constants (K) of the treated pollutants applying mixed flow reactor model. Water samples were collected from both influent and effluent wetland cells. Samples were analyzed to measure concentrations and loads of DO, BOD, TSS, NH4-N, NO3-N, TP, Fe, and FC. Results showed that RFCW managed to polish pollutants of drainage influent especially ammonia and nitrates reaching a limit which it could be used in a safe manner for the agricultural purpose or dumping it into Lake Manzala without negative impacts on its ecosystem.

Simulating waves on a horizontal liquid film entrained by a gas flow

Nonlinear waves on a horizontal liquid film surface are considered. The effect of adjacent gas flow is taken into account through the data on shear stress at the film-gas interface obtained by the Boussinesq model of turbulence. A model nonlinear equation for the film thickness deviation from the undisturbed level is used to simulate nonlinear wave modes. Weakly nonlinear steady-state travelling solutions of this equation with wave numbers located in the vicinity of neutral wave numbers are constructed analytically. The evolution of periodic perturbations with wave numbers lying in the depth of the linear instability region is also considered numerically. Several typical scenarios of their evolution have been identified.

Visualization and characterization of thermals in a heated turbulent boundary layer

This research study investigated the three-dimensional nature and characteristics of buoyant thermals in the mixed convection turbulent boundary layer. Thermals were visualized using a multi-plane flow visualization technique. Experiments were performed in a variety of conditions where the bulk Richardson number was varied to cover a range describing both inertia dominant flow (RiL=0.05) and buoyancy dominant flow (RiL=2.0). Qualitative results define key interactions between thermals and the adjacent turbulent boundary layer flow. A robust algorithm for automatic thermal detection was developed and validated. This algorithm was then applied to experimental images to detect thermals and facilitate a quantitative analysis on the size and spatial distribution of buoyant thermals. Based on these findings an overall flow structure is proposed featuring convective cell like patterns at high Richardson numbers. A first approximation on the shape of thermals and velocity of key thermofluid interactions between thermals and turbulent boundary layer is made. Results indicate these thermal interactions can exhibit motion substantially different from the surrounding turbulent boundary layer flow.

An Explicit Scheme to Represent the Bidirectional Hydrologic Exchanges Between the Vadose Zone, Phreatic Aquifer, and River

AbstractUnderstanding groundwater storage variations as a result of effects from both the vadose zone and the river is of critical importance in the hydraulically connected surface‐subsurface system. In this study, we present a novel Bidirectional Exchange Scheme in Surface and Subsurface (BE3S) that represents bidirectional exchanges between the vadose zone, phreatic aquifer, and river. The approach enables explicit representations of each flow regime while conserving mass, and successfully yields solutions of the coupled system for multiple temporal resolutions. We test the scheme by comparing the BE3S‐derived outputs against other models and corresponding observational data. We apply the scheme to simulate hydrologic states in a reach of the Brazos River in Southeast Texas, such as soil moisture content, groundwater level, and river stage as well as net subsurface discharge fluxes. Good agreements between the simulated and observed data for all the components show the suitability of the proposed scheme in modeling the bidirectional flows and exchanges. We also assess how the bidirectional hydrologic exchanges are affected by adjacent flow domains. We find that vertical percolation is significantly affected by unsaturated soil thickness, resulting in the spatial variability of vertical percolation across sloping topography. Hilltop‐to‐valley convergent groundwater flow is also found to impede vertical percolation in river valleys due to shallow water table while facilitating percolation in the hilltops. The capability of the presented scheme that accounts for the topographically driven lateral groundwater flow and drainage dynamics provides the potential to enhance the representation of the surface‐subsurface system in Earth System Models (ESMs).

Local statistics of laminar expanding flames subjected to Darrieus–Landau instability

We, herein, report an experimental study to investigate the role of local flamefront dynamics on the propagation and acceleration of cellularly unstable premixed expanding laminar flames. Using simultaneous Mie-scattering imaging and Particle Image Velocimetry, we measured the flame edge location and its adjacent flow field, which were subsequently processed to quantify the evolution of the probability density functions (pdfs) of flow velocities, curvature, normal strain rate and tangential strain rate. We showed that appropriate normalizaton of the measured quantities can unify the data from different pressures when identified with the corresponding Peclet number, defined as the ratio of the flame radius to the flame thickness. Since the flamefront is stable and smooth at lower Peclet numbers, the flame-induced flow field and stretch rates are almost uniform over the flamefront, resulting in narrow pdfs. At higher Peclet numbers, however, the flame becomes progressively more wrinkled and hence the variations in the local quantities increase leading to wider pdfs. Furthermore, while the mean curvature was found to be inversely proportional to the mean flame radius and insensitive to the cellular structure, the mean normal strain rate was strongly influenced by the cellular structure.