词汇
##普通场景
###be prone to: 容易的，易于
Droplets featuring a large diameter are prone to deformation under the influence of aerodynamic shear forces, resulting in a decidedly nonspherical shape.
Because of current limitations in computational capacity, an industrially viable model of droplet–wall interactions must be based on a semi-empirical formulation of the impingement process.
Although principally feasible, such an extension is complicated by the fact that droplets in the SLD regime violate fundamental assumptions regarding the impingement behavior of droplets within the envelope of Appendix C regulations.
For the case of droplet distributions featuring an MVD within the Appendix C envelope, an approaching droplet’s kinetic energy is sufficiently low to justify the assumption of negligible bouncing and splashing effects. However, empirical as well as computational studies have convincingly demonstrated the occurrence of droplet splash and rebound phenomena for droplet size distributions beyond the Appendix C limit. Under such circumstances, the approaching droplet mass is only partially deposited at the predicted impingement location while the splashed or rebounded mass fraction is reintroduced into the flow field, potentially resulting in reimpingement on aircraft lift and control surfaces located downstream of actively protected regions .
The current certification envelope defined by 14 CFR Part 25 Appendix C: Atmospheric Icing Conditions for Aircraft Certification, accounts for an icing envelope characterized by water droplet mean volume diameters up to 50 micrometers. International airworthiness authorities have jointly developed an extension of Appendix “C”, named “Appendix O”, to account for SLD conditions in the aircraft certification process. Freezing drizzle and freezing rain conditions were included, leading aircraft manufacturers to demonstrate that their products can safely operate in an SLD environment.
Icing occurs when an aircraft flies through clouds in which super cooled droplets are suspended in atmosphere with an ambient air temperature below the freezing point. The droplets impinge on the aircraft surfaces and freeze leading to ice accretion. The resulting change in the aircraft geometry can modify wing aerodynamic characteristics (loss of lift, increase in drag), and can affect the probes’ measurements or even damage the engine by ice ingestion. In order to comply with certification regulation rules (Part 25), airframers have to demonstrate safe aircraft operation in icing conditions. However, the whole icing envelope cannot be flight tested or even tunnel tested because of the associated costs and time. Airframers, with the support of research institutes, have developed numerical methods and tools to cover their needs.
In order to comply with this new rule, the simulation tools, as Acceptable Means of Compliance, have to be improved and validated for these conditions.
Before entering in the details of the models, a few definitions will be reminded: …
The ONICE2D tool is a comprehensive 2D ice accretion suite developed by ONERA and comprising the following components: …
The Eulerian formulation used within DROP3D is recalled (ContinuityandMomentumequations): [eq]
In contrast to the Lagrangian formulation employed by the ONERA [11] and NASA LEWICE [12] droplet impingement codes, DROP3D relies on a purely Eulerian model.
In-flight icing accretion poses a serious threat to the safty of aircraft, particularly during climbing, holding, and landing. Strict regulations in terms of ice protection and operating envelopes are in effect to minimize the risk of incidents and accidents. Standards for in-flight icing certification based on temperature, pressure altitude, droplet sizes, and their water content have been laid out by the Federal Aviation Administration (FAA) in Appendix C of Federal Aviation Regulation (FAR) 25 and 29 [1,2], with which all commercial passenger-carrying aircraft must comply to demonstrate safe operation. …. Their modeling is different from that of small droplets because aerodynamic and surface collision phenomena can distort their shape, break them up before impingement, splatter, bounce, and shatter them. In the SLD regime, a postimpact flow of droplets may thus reenter the airstream and deposit on unprotected surfaces downstream of the primary impact points.
airworthiness: 适航性
The traditional means of demonstrating airworthiness include numerical, dry and icing-tunnel simulations, followed by a natural icing flight campaign. Replicating SLD conditions in icing tunnels or locating them in nature is a difficult, if not elusive, task. Numerical tools have fewer constraints in simulating SLD encounters; however, the fine-grain modeling of individual droplet impacts is impractical, hence the droplet–wall interaction is introduced in the form of empirical coarse-grain models. Splashing and bouncing water is generally assumed to be carried away by the airflow and pose no further threat, at least on simple geometries.
###perceived: 感知，认为，理解，感知到的 dwarfed: 侏儒，使矮小
In the Lagrangian method, droplets are seeded into the airflow solution and tracked on an individual basis, using Newton’s equations of motion to determine if and where impingement occurs. Any perceived advantages of this approach are dwarfed by the difficulty of applying it in three dimensions, for complex geometries, and for a distribution of droplets mimicking atmospheric conditions.
While this approach may be relatively safe for some two-dimensional (2-D) geometries, it may not be accurate enough for a complete aircraft, or, for that matter, even for more complex 2-D shapes such as high-lift configurations with slat and flap(s) deployed, where water could re-impinge further back on unprotected regions.
###assess: 估算，评价
The computation of reinjected droplets is a costly, but necessary, step needed to assess the true impact of SLD on the sizing and analysis of anti-icing or de-icing equipment. It was noted by Rutkowski et al. [11] in their work on a Lagrangian re-impingement methodology that the tracking of even a few droplets reinjected from the surface took a great deal of time.
Even with the use of local subgrids, the solution of the reinjected droplets increases the computational cost significantly. At each reinjection site, only individual surface-element faces were defined as inflow boundaries. An average increase of five to nine times the cost of the primary impingement solution was observed. Although this may appear to be a steep price to pay, it will be shown that the effect of reinjection considerably alters the surface impingement and vividly demonstrates the need for a more careful approach to IPS design.
Figure 17 (top left) shows the LWC contours of the DROP3D solution after post-processing to remove the bouncing and splashing water. The slat acts as a screen for the main element by removing the larger incoming droplets; however, the smaller droplets can still impact the leading edge of the main element. Figure 17 (top right) shows that most of the reinjected water droplets bouncing off the suction surface of the slat travel over the upper surface of the main element without re-impinging and are lost downstream, whereas those originating from the pressure surface re-impinge on the main element. Details of the slat and flap regions are shown in the image at the bottom-left. The figure also shows that a significant amount of water rebounds from the main element and re-impinges on the flap, not just on its pressure surface, but also on its leading edge. These phenomena are also clearly visible in the collection efficiency curves of the individual elements shown in Fig. 18. Figure 18a shows that some of the droplets rebounding from the leading edge of the slat reimpinge on both the pressure and suction sides of its trailing edges. Figure 18b shows the contribution of re-impingement on the collection efficiency on the leading edge and the pressure surface of the main element and the flap cove. This again is mostly due to water coming from the front of the main element. Figure 18c shows that the peak collection efficiency on the flap increases by nearly 10% due to the reinjected water coming from the main element. With the inclusion of the reinjected water, ice growth and the roughness associated with it, especially in the flap cove and on the leading edge of the flap, may have a very pronounced effect on the aerodynamic performance of the airfoil.
###progressively: 逐渐地 ballistic: 弹道的；射击的
The impact angle decreases and the postcollision droplet size increases with distance from the leading edge, resulting in droplets whose trajectories are progressively more ballistic.
albeit: 虽然，即使
The larger bouncing droplets from the lower surface of the slat follow nearly ballistic trajectories, albeit with reduced kinetic energy, and impact the lower surface of the leading edge of the main element.
In the case where the initial velocity of a splashed droplet is below the escape velocity, it is not guaranteed that the droplet will re-impinge close to the initial impingement location, so redistribution of water mass due to splashing may occur.
###lend itself to: 适合于 conceive: 想出，构思，怀孕
Although limited in computational elegance, the Lagrangian formulation lends itself well to the description of droplet–wall interaction processes. Droplet trajectories may be developed throughout the computational domain until a solid boundary is encountered. Subsequently, secondary droplet sizes and velocity components obtained from empirical correlations may be imposed as initial conditions for further time integration of droplet trajectories. However, as the notion of individual droplets does not exist in an Eulerian formulation, an adequate mathematical description of the physical phenomena observed during droplet–wall collisions is not easily conceived.
incur: 招致，引起
Strictly speaking, the time derivative in Eq. (20) represents the instantaneous change in momentum incurred by the secondary droplet mass at the time of impact due to the impulse delivered by the target surface.
pertain: 从属，有关
The most comprehensive collection of experimental data pertaining to SLD specific droplet impingement originates from a recent study performed by Papadakis et al. [22] at NASA Glenn’s Icing Research Tunnel.
The most important of these violations pertain to the following simplifying assumptions originally made by Bourgault et al. [3] in the derivation of the mathematical equations governing droplet behavior: …
###performance penalty
The slat located ahead of the main element delays flow separation which enables the wing to operate at higher angles of attack without significant performance penalties. Most commercial aircraft, due to maintenance and manufacturing complexity, employ three elements, rather than four or more elements.
A five-stage Runge–Kutta temporal discretization is implemented.
The approach introduces the effect of the postimpact droplets by successive solutions of the conservation equations.
This paper presents the details of the proposed Eulerian reinjection approach with the two droplet–wall interaction models recast in an Eulerian framework, discusses the computational cost and related mitigation techniques, and gives concrete examples of its capabilities.
###duality: 二元性
By decoupling the pre- and postimpact simulations, the boundary condition duality of trying to model the surface as both inlet and exit simultaneously is avoided.
The simulation of droplet impingement with splashing, bouncing, and reinjection in a mass-conservative Eulerian framework can be described as follows:
1) The incident droplet impingement locations and mass caught are computed in the customary Eulerian manner.
###manifested: 证明的，已显示的，显然的 pronounced: 显著的；断然的；讲出来的
Another effect of large droplet MVDs is a tendency for droplets to deform under the influence of aerodynamic shear forces, resulting in increased aerodynamic drag. The effect of aerodynamic shear may become sufficiently manifested in the vicinity of aircraft surfaces to cause eventual droplet breakup, resulting in a reduction of the droplet distribution’s MVD before impingement. Both of these effects result in a more pronounced aerodynamic influence on droplet trajectories.
##比较的语法
###vicinity: 邻近地区 in the vicinity of: 在附近
Excellent agreement between numerical and experimental collection efficiency distributions is observed throughout the droplet impingement region at MVDs of 20, 52, and 111 m, whereas a deviation from experimental data within the vicinity of the airfoil leading edge becomes increasingly apparent at elevated MVDs of 154 m and 236 m.
###attributable: 可归因于某人/某事物 deem: 认为
At this point it is unclear whether this discrepancy is attributable to the original mathematical model or a deficiency in the experimental measurement methods, which produce fluctuations in the value of the maximum collection efficiency by up to 10% as reported by Papadakis et al.. In any case, it would appear sensible that the maximum collection efficiency obtained under consideration of droplet–wall interactions should not exceed the value obtained without the consideration of mass loss. As such, the proposed mathematical model may be considered mathematically consistent and physically representative. Considering the overall quality of the numerical results, the mathematical model of droplet–wall interactions may be deemed sufficiently calibrated.
In particular, based on the author’s knowledge, IDDES has not been applied to iced modern wings.
In-depth: 彻底地，深入地
Poll et al. provided an in-depth analysis on the separation characteristics of a swept wing.
###lateral: 横向的
Shallow flow or thin liquid film models are used for a wide range of physical and engineering problems. They are applicable to free surface flows, when the flow thickness is distinctly smaller than the lateral dimension.
in the context of: 在……情况下，在……背景下
In the context of shallow granular flows, it is a convenient tool to derive a solution for basal pressure.
The model derived herein exploits the thinness of the fluid layer and leads to a lubrication model of the dynamics whereby the unknown fluid fields are parameterized only by the thickness of the fluid layer. Thus, a considerable simplification of the governing equations is achieved when compared to the full Navier–Stokes equations. Such thin-geometry or long-wave models have previously been derived in many physical contexts, such as in the mechanics of beams, plates and shells (Timoshenko & Woinowsky-Krieger 1959; Roberts 1993), coating flows (Levich 1962; Benney 1966; Roskes 1969; Atherton & Homsy 1976; Tuck & Schwartz 1990), shallow-water waves (Mei 1989), viscous fluid sheets (van de Fliert, Howell & Ockenden 1995), etc.
We do not discuss boundary conditions at the lateral extremes of the substrate, as the substrate is assumed to be so large in extent, when compared to the fluid thickness, that the dynamics of the fluid film are largely unaffected by the edge boundary conditions.
Clean smooth surfaces on aircraft wings increase the efficiency of aerodynamics and thus reduce fuel consumption. However, challenges to these surfaces such as ice accretion, insect adhesion and rain erosion negatively affect aerodynamic performance and need to be overcome to implement technologies like hybrid laminar flow control for the next generation of aircraft. The purpose of this overview is to describe the background of fouling media on aerofoil, the current state of art used to mitigate these challenges and potential of hybrid sol-gel technology to be used in this field.
For all thin film approximation, it is first assumed that the film thickness is much smaller than the characteristic geometrical size of the vehicle wall (radius of curvature). This assumaption is valid for most surface areas of a car, but might brak down at very sharp corners, for example at the trailing edge of the rear-mirror.
First, we discuss the basic assumptions invoked in this study for the film modeling.
##glossaries: 术语表
The discrete solution is determined by Eq. (3.4) given later, but first, to make it easier for the readers to follow the discussion, we briefly summarize some glossaries used later.
##perpendicular：垂直于
The unit vector perpendicular to the coating surface, denoted by n, is defined by $Equation$.