Leading Edge Effect Research Articles

Influence of large-scale free-stream turbulence on bypass transition in air and organic vapour flows

The free-stream turbulence (FST) induced transition in perfect and non-ideal gas zero-pressure-gradient flat-plate boundary layers is investigated by means of large-eddy simulations. The study focuses on the influence of large incoming disturbances over the laminar-to-turbulent transition, by comparing two different integral length scales $L_f$ , which differ by a factor of seven, at different FST intensities $T_u$ . High-subsonic dense-gas boundary layers of the organic vapour Novec649, representative of organic Rankine cycle applications, are compared with air flows at Mach numbers 0.1 and 0.9. Compressibility and non-ideal gas effects are shown to be of minor importance in comparison to the influence of the FST integral length scale $L_f$ . An increase of the inlet turbulent intensity always promotes transition, whereas an increase of $L_f$ has a double effect on the transition onset. At $T_u=2.5\,\%$ , increasing $L_f$ promotes the transition, while it tends to delay transition for an FST intensity of 4 %. Larger FST integral scales tend to increase the spanwise distance between laminar streaks generated in the boundary layer. Two competing transition scenarios are observed. When the incoming turbulence intensity and length scale are moderate, the classical bypass route consists in the linear non-modal growth of streaks, which then experience secondary instabilities (sinuous or varicose) and lead to the generation of turbulent spots. The second scenario is characterized by the appearance of $\Lambda$ -shaped structures near the inlet, which are further stretched to hairpin vortices before breaking down to turbulence. Spot inceptions can therefore occur at earlier locations than the streak growth. We are then faced with a competition between the classical bypass transition and nonlinear response mechanisms that ‘bypass’ this route. The present case at high $L_f$ and low $T_u$ is an example of a competing scenario, but even for the higher $T_u$ and $L_f$ conditions, only approximately one-third of turbulent spots are due to the $\Lambda$ -shaped events. The nonlinear alternative route has strong similarities with scenarios described previously in the literature in the presence of leading edge effects or due to passing wakes. Such a path is governed by the turbulence intensity, but also by the integral length scale, with both parameters playing a critical role in the generation of the $\Lambda$ -shaped structures near the inlet. This alternative mechanism is found to be robust under varying flow and thermodynamic conditions.

LES investigation of the wavy leading edge effect on cavitation noise

This paper investigates the noise reduction performance of biomimetic hydrofoils with wavy leading edge and the corresponding mechanisms. We employ Large Eddy Simulation (LES) approach and permeable Ffowcs Williams-Hawkings (PFW–H) method to predict cavitation noise around the baseline and biomimetic hydrofoils. The results show that the wavy leading edge can effectively reduce the high-frequency noise, but has little effect on the low-frequency noise. Further analyses and discussions deal with the noise reduction mechanisms. The main source for the low-frequency noise is the cavity volume acceleration, while the wavy leading edge has little effect on it. The high-frequency noise sources, related to the surface pressure fluctuations and the turbulence characteristics, are significantly suppressed by the wavy leading edge, thus decreasing the high-frequency noise intensity. Our investigation indicates that the wavy leading edge has great prospects for cavitation noise reduction technique.

Leading-Edge Effects on Freestream Turbulence Induced Transition of an Organic Vapor

Leading-Edge Effects on Freestream Turbulence Induced Transition of an Organic Vapor

Wavy Leading Edge (WLE) Influence on a Rectangular Wing Using an Unsteady Analysis Approach

A rectangular wing with Wavy Leading Edge (WLE) effect was investigated experimentally and numerically. This research was carried out with the NACA 0018 profile. The morphology of humpback whale flippers, which are blunt and rounded in a specific pattern, inspired the design of the WLE. The rectangular wing was explored in pitching motion with a reduced frequency of k = 0.25 and varied aspect ratios. Multiple aspect ratios (AR) of the rectangular wing have been evaluated to determine the best wing aspect ratio, notably 3.9, 5.1, and 7.9. Only at AR 3.9 and 5.1 does the WLE perform efficiently in both upstroke and downstroke motion. WLE has a sinusoidal function shape. The improvement of lift force was stronger during upstroke motion than during downstroke motion. The stall is minimized during the pitching motion of the WLE wing, according to the numerical simulation. This result could be applied to fin stabilizers or wind turbines.

Effect of Leading-Edge Cranks on Stability and Control of Active-Flow-Control-Enabled Tailless Aircraft

The Swept Wing Flow Test (SWIFT) is a tailless unmanned combat aerial vehicle (UCAV) model to be tested at high Reynolds numbers in NASA’s National Transonic Facility. The model is designed around a -shaped wing with a single, large crank at its leading edge (LE). It suffers from an unstable nose-up pitch departure resulting from flow separation augmented by the LE crank. A small-scale, modular wind tunnel model () was built that allowed for changes in the crank angle by increasing the outboard wing sweep. Eliminating the crank entirely increased the and changed the sign of pitch departure, thus exposing the significance of the LE crank. The model was equipped with sweeping jet actuators that could be individually enabled by valves located at the actuator inlets, allowing one to explore the role of active flow control (AFC) in expanding the model’s longitudinal stability margins and controlling its yaw while being cognizant of the coupling between changes in the model’s planform and their effect on AFC. Test results indicated that selective actuation depending on the model’s attitude modified the flow and dramatically increased the trimmed , while further suggesting that the actuation should dynamically change with incidence to improve AFC efficacy.

Blunt leading-edge effect on spanwise-varying leading-edge contours of an UCAV configuration

The vortex system on the SACCON model is studied under low Reynolds number by using the flow visualization technique and the surface pressure testing measurement in a low-speed water channel and a wind tunnel, respectively. A main topic is discussed below. The blunt leading-edge section on this SACCON model induces various flow phenomena under different angles of attack. The attached flow appearing around the upstream leading-edge region at a low angle of attack induces different flow phenomena downstream, so does the branched vortices appearing near the trailing-edge. Although this attached flow is vanished at higher angle of attack, the stability of forming an outboard vortex can be shown in the results of pressure coefficients as a function of an angle of attack. Moreover, how the blunt leading-edge contour on SACCON model and other models affecting flow field can be compared with the findings given in the previous studies. The models with only blunt leading-edge contour can prompt an outboard vortex and an inner vortex, compared to the flow fields on the models with spanwise-varying leading-edge contours. This inner vortex is formed due to the attached flow inboard passing downstream and being affected by the outboard vortex. However, this attached flow is different from the attached flow observed on SACCON model. Therefore, flow phenomena occurring on different blunt leading-edge models can be differentiated.

Effect of the Wavy Leading Edge to Aerodynamic Performance Improvement in A Nuclear Steam Turbine Last Stage Blade

Effect of the Wavy Leading Edge to Aerodynamic Performance Improvement in A Nuclear Steam Turbine Last Stage Blade

Optimizing Effect of Wavy Leading Edge (WLE) in Rectangular Wing and Taper Wing

Experimental and numerical research have been performed to investigate the Wavy Leading Edge (WLE) effect on the rectangular wing. The WLE is inspired by humpback whale flipper morphology which is blunt and rounded in certain form pattern. This flipper shape plays an important role for its behaviour specially capturing their prey. This advantage could be applied to other systems such as fin stabilizers or wind turbines. Steady cases in various aspect ratios were conducted to find out the optimum effect of WLE with baseline NACA 0018 profile at Reynolds number 1.4 x 105. The chord length of the wing (c) was 125 mm. The WLE shape defined as wavelength (W) 8% of c and amplitude (d) is 5% of c. The aspect ratio (AR) variations were 1.6; 3.9; 5.1; 7.9 and 9.6. A simple rectangular form of the wing was selected to analysis the WLE effect on the various ARs. The taper wing shape is applied to find out the WLE effect at the AR 7.9. three types of taper ratio (TR) are 0.1; 0.3 and 0.5. The results show that the WLE on the taper wing has better advantage to control the stall in steady case. Another impressive result was the WLE wing with AR 7.9 and TR 0.3 has the best lift coefficient and pressure distribution.Keywords: stall, wavy leading edge, steady case, rectangle wing, taper wing, aspect ratio.

No leading-edge effect in North Atlantic harbor porpoises: Evolutionary and conservation implications.

Understanding species responses to past environmental changes can help forecast how they will cope with ongoing climate changes. Harbor porpoises are widely distributed in the North Atlantic and were deeply impacted by the Pleistocene changes with the split of three subspecies. Despite major impacts of fisheries on natural populations, little is known about population connectivity and dispersal, how they reacted to the Pleistocene changes, and how they will evolve in the future. Here, we used phylogenetics, population genetics, and predictive habitat modeling to investigate population structure and phylogeographic history of the North Atlantic porpoises. A total of 925 porpoises were characterized at 10 microsatellite loci and one quarter of the mitogenome (mtDNA). A highly divergent mtDNA lineage was uncovered in one porpoise off Western Greenland, suggesting that a cryptic group may occur and could belong to a recently discovered mesopelagic ecotype off Greenland. Aside from it and the southern subspecies, spatial genetic variation showed that porpoises from both sides of the North Atlantic form a continuous system belonging to the same subspecies (Phocoena phocoena phocoena). Yet, we identified important departures from random mating and restricted dispersal forming a highly significant isolation by distance (IBD) at both mtDNA and nuclear markers. A ten times stronger IBD at mtDNA compared with nuclear loci supported previous evidence of female philopatry. Together with the lack of spatial trends in genetic diversity, this IBD suggests that migration–drift equilibrium has been reached, erasing any genetic signal of a leading‐edge effect that accompanied the predicted recolonization of the northern habitats freed from Pleistocene ice. These results illuminate the processes shaping porpoise population structure and provide a framework for designing conservation strategies and forecasting future population evolution.

날갯짓 초소형 비행체의 앞전 및 시맥 탄성이 공력 성능에 미치는 영향

본 연구의 날갯짓 초소형 비행체는 실제 생명체의 날개를 모방하여, 매우 유연한 재질의 캠버날개를 활용한다. 캠버 날개는 생명체와 유사하게 앞전, 시맥, 박막과 같이 특성이 서로 다른 세 가지 재질로 구성되어 있고 다양한 방식으로 구속되어 있다. 날개의 유연성을 활용한 수동 회전(passive rotation) 방식은 앞전과 시맥의 재질이 날갯짓 궤적에 매우 큰 영향을 미치는 요소이기 때문에 적절한 유연성을 갖는 재질의 선정이 필수적이다. 이러한 날개의 재질들과 복잡한 형상을 사실적으로 모델링하여 정밀하게 해석할 수 있는 유체-구조 연성해석 프로그램을 개발하고, 날개의 앞전과 시맥의 탄성 계수의 변화에 따른 공력탄성학 효과를 정밀하게 분석하였다. 결과적으로 재료의 탄성 계수 변화만으로도 날개의 비틀림각 궤적을 적절히 발생시킴으로써 날갯짓 비행체의 추력 및 효율을 크게 증가시킬 수 있음을 보였다.

Effects of Leading Edge of a Pump-Turbine on Turbine Performance

Effects of Leading Edge of a Pump-Turbine on Turbine Performance

A lateral flow immunoassay with self-sufficient microfluidic system for enhanced detection of thyroid-stimulating hormone

We report a self-sufficient microfluidic paper-based lateral flow immunoassay device (μLFD) for highly sensitive detection of the thyroid-stimulating hormone (TSH). Fabrication of the paper microchannels involves engraving the nitrocellulose membrane with a CO2 laser to create narrow flow paths, which constrain the fluid flow over the test zone. The proposed microchannel modified devices were studied for detection of the TSH using gold nanoparticles as labels. The effect of such microchannel modified LFDs has led to an improvement in sensitivity by nine times and the limit of detection by 6.6 times due to the slow flow rate of the sample compared with the traditional LFD. In addition, the binding of gold nanoparticles over the test line is more uniform in the case of the μLFD, thus minimizing leading-edge effects, resulting in more accurate quantitative analysis. The proposed strategy offers great potential for multiplex detection of biomarkers with increased sensitivity without introducing any hydrophobic materials to the LFD.

Effect of Wavy Leading Edge with Various Aspect Ratios on a Rectangular Wing

Learning from nature, the humpback whale can swim faster than other baleen whales. The humpback whale has the flipper with wavy leading edges (WLE), which has an improvement of hydrodynamic performance. The WLEs have the function to generate vortices to maintain lift and to prevent stall at high angles of attack. This research aim is to find out the WLE effect on hydrodynamic performance in different wing aspect ratios (AR), whose profile is the rectangular wing of NACA 0018 as a baseline wing. The WLE profile was designed by a sinusoidal function. We focused on steady flow conditions in both the experiments and numerical simulations. The experimental work was conducted in the circular water channel at Hiroshima University, Japan. Meanwhile, the numerical work was employed using RANS simulation with SST k-ω turbulence model. Based on the results, we clarified that the increase of AR could possess a higher lift coefficient (Cl), and then physically interpreted the WLE effect on various aspect ratios. This knowledge could be expanded into the application of eco-friendly energy-saving devices such as fin stabilizers of ships and wind turbines to improve hydrodynamic performance.

Comparison between temporal and spatial direct numerical simulations for bypass transition flows

ABSTRACT Temporally developing direct numerical simulations (T-DNS) are performed for free-stream turbulence induced bypass transition flow over a flat-plate under zero-pressure gradient, and results are validated using experimental data and spatially developing DNS (S-DNS) results. The temporal simulations predict the growth of near-wall Klebanoff modes in the pre-transition regime and their subsequent breakdown due to the sinuous-like instability, where the increase in free-stream turbulence intensity and length scale enhanced the instability. A formulation for the domain translation velocity is developed using the momentum integral boundary layer equation, to estimate the translation of the domain along the plate. The formulation was found to be robust for a range of free-stream turbulence intensities, and both the boundary layer growth and free-stream decay compared well with spatial simulations, confirming that the same translation velocity can be used for the entire domain. The optimal domain size for a temporal simulation is dictated by the length scale of the turbulent spots and errors due to streamwise periodicity, and is estimated to be to , where is the boundary layer thickness at the transition onset. The T-DNS prediction of the integral boundary layer parameters, mean flow, second and higher-order statistics, stress budgets, and free-stream decay were comparable to those of S-DNS, even though they were obtained on ten times smaller streamwise domain and numerical grid. However, T-DNS are limited for bypass transtion flows for which leading-edge effects are unimportant, and may have limitations for the accurate predictions of the near-wall Klebanoff modes that extend up to breakdown. Nonetheless, temporal approach is identified to be a viable inexpensive alternative to the spatial approach for canonical test cases for transition flow physics analysis.

Modeling the interaction of numerosity and perceptual variables with the diffusion model

Ratcliff and McKoon (2018) proposed integrated diffusion models for numerosity judgments in which a numerosity representation provides evidence used to drive the decision process. We extend this modeling framework to examine the interaction of non-numeric perceptual variables with numerosity by assuming that drift rate and non-decision time are functions of those variables. Four experiments were conducted with two different types of stimuli: a single array of intermingled blue and yellow dots in which both numerosity and dot area vary over trials and two side-by-side arrays of dots in which numerosity, dot area, and convex hull vary over trials. The tasks were to decide whether there were more blue or yellow dots (two experiments), more dots on which side, or which dots have a larger total area. Development of models started from the principled models in Ratcliff and McKoon (2018) and became somewhat ad hoc as we attempted to capture unexpected patterns induced by the conflict between numerosity and perceptual variables. In the three tasks involving numerosity judgments, the effects of the non-numeric variables were moderated by the number of dots. Under a high conflict, judgments were dominated by perceptual variables and produced an unexpected shift in the leading edge of the reaction time (RT) distributions. Although the resulting models were able to predict most of the accuracy and RT patterns, the models were not able to completely capture this shift in the RT distributions. However, when subjects judged area, numerosity affected perceptual judgments but there was no leading edge effect. Based on the results, it appears that the integrated diffusion models provide an effective framework to study the role of numerical and perceptual variables in numerosity tasks and their context-dependency.

Lateral aerodynamic interference between an interior train and a flat box bridge-deck

This paper studies the crosswind aerodynamics of an interior train and flat box bridge-deck system via a series of wind tunnel experiments in wind angle of attack α = [−12°, 12°]. Aerodynamics of the train- and bridge-only models is measured as benchmarks under similar conditions. Upon comparing with the train- and bridge-only cases, the lateral aerodynamic interference between the train and bridge-deck is qualitatively identified by smoke-wire visualization and quantitatively detected by surface pressure and flow profiles located 8 mm upstream from the train model and 490 mm downstream from the bridge-deck model. Generally, the lateral aerodynamic interference on the train model is primarily manifested by suppression of underbody vortex shedding, shielding effect of bridge-deck leading-edge, and quasi-Reynolds number effect. Compared to the train-only case, the suppression of underbody vortex shedding reduces fluctuating drag, lift, and moment coefficients, whereas the shielding effect decreases the mean drag coefficient. The quasi-Reynolds number effect, owing to the accelerated and turbulent approaching flow over the rounded shoulder of the train model as α varies, abruptly alters drag, lift, and moment coefficients mimicking the classical Reynolds number effect, while the inflow Reynolds number remains unchanged. On the other hand, the lateral aerodynamic interference on the bridge-deck model is chiefly presented as flow transition promoting effect and intensifying effect on bridge-deck trailing-edge flow separating. Compared to the bridge-only case, the flow transition promoting effect leads to the global centerline of the bridge-deck aerodynamics to be parallel shifted about 4° to the positive direction of α, while the intensifying effect increases fluctuating drag, lift, and moment coefficients. Finally, the aerodynamics of the whole train-bridge system is briefly discussed.

Effect of Wavy Leading Edge on Pitching Rectangular Wing

Effect of Wavy Leading Edge on Pitching Rectangular Wing

Simulation of disruptions triggered by Vertical Displacement Events (VDE) in tokamak and leading edge effect in plasma energy deposition to material surfaces

The paper describes two major non-linear properties of the vertical instability of a tokamak plasma, which has a vertical elongation: (a) inductive excitation of surface (edge) currents stabilizing the instability and converting it into fast equilibrium evolution, and (b) creation of a wetting zone without the normal component of the magnetic field when the plasma has contact with material surfaces. Two major disruption effects for both mitigated and non-mitigated disruptions, important for JET and ITER, were considered: (a) excitation of vertical disruption during the current quench (i.e., abnormal plasma current ramp down) and (b) related to the wetting zone, potential leading edge effect in plasma energy deposition to the in-vessel tiles during disruptions. Our considerations together with a 2-D version of the VDE (Vertical Disruption Event) code are based on a new mathematical model, called Tokamak MHD (TMHD), as a replacement for the conventional model, a model that cannot solve numerical problems related to extreme plasma anisotropy and negligible mass. The code includes a 3-D model of surface currents on a thin conductive wall and has a well-specified algorithm for extension to vertical disruptions that excite asymmetric kink modes.

Scaling of convective boundary layer flow induced by linear thermal forcing at Pr1.

The transient convective boundary layer flow induced by heating an initially isothermal and stationary fluid with a linearly heated semi-infinite vertical plate is comprehensively investigated by a scaling analysis in the present study. The local temperature at the leading edge of the specified linear thermal profile can be any value that is no lower than the ambient fluid, which significantly differentiates the present study from previous similar investigations. Additionally, the convective flow associated with both Pr<1 and Pr>1 (where Pr is the Prandtl number) fluids is analyzed and quantified. The derived scaling relations demonstrate that the dynamics of the Pr<1 and Pr>1 fluids are fundamentally different and the two flows are described by different sets of scaling laws. The study also shows that the convective boundary layer flow first experiences an initial growth state, and it eventually transits to a fully steady state after the leading edge effect has been completely convected away. It is further revealed that unlike the extensively studied homogenously heating problem which is featured by a one-dimensional initial growth state and a two-dimensional fully developed state, the present flow is consistently two-dimensional if a nonzero background temperature stratification presents. The derived scaling relations are validated against the direct numerical simulation results, and a good agreement is obtained.

Representation of Horizontal Transport Processes in Snowmelt Modeling by Applying a Footprint Approach

The energy balance of an alpine snow cover significantly changes once the snow cover gets patchy. The local advection of warm air causes above-average snow ablation rates at the upwind edge of the snow patch. As lateral transport processes are typically not considered in models describing surface exchange, e.g. for hydrological or meteorological applications, small-scale variations in snow ablation rates are not resolved. The overall model error in the hydrological model Alpine3D is split into a contribution from the pure “leading edge effect” and a contribution from an increase in the mean air temperature due to a positive snow-albedo feedback mechanism. We found an overall model error for the entire ablation period of 4 % for the almost flat alpine test site Gletschboden and 14 % for the Wannengrat area, which is located in highly complex terrain including slopes of different aspects. Terrestrial laser scanning measurements at the Gletschboden test site were used to estimate the pure “leading edge effect” and reveal an increase in snow ablation rates of 25-30 % at the upwind edge of a snow patch and a total of 4-6 % on a catchment scale for two different ablation days with a snow cover fraction lower than 50 %. The estimated increase of local snow ablation rates is then around 1-3 % for an entire ablation period for the Gletschboden test site and approximately 4 % for the Wannengrat test site. Our results show that the contribution of lateral heat advection is smaller than typical uncertainties in snow melt modelling due to uncertainties in boundary layer parameters but increases in regions with smaller snow patch sizes and long-lasting patchy snow covers in the ablation period. We introduce a new temperature footprint approach, which reproduces a 15 % increase of snow ablation rates at the upwind edge of the snow patch, whereas observations indicate that this value is as large as 25 %. This conceptual model approach could be used in hydrological models. In addition to improved snow ablation rates, the footprint model better represents snow mask maps and turbulent sensible heat fluxes from eddy-covariance measurements.