Abrupt Change In Surface Roughness Research Articles

Research on the generation mechanism of surface and subsurface damage in loose abrasive lapping of RB-SiC ceramics

Reaction-bonded silicon carbide ceramics (RB-SiC) are extensively utilized in aerospace, space optics, and other fields due to their superior physical and chemical properties. Loose abrasive lapping plays a crucial role in the optical manufacturing of large-diameter RB-SiC mirrors. Previous research predominantly focused on grinding processes and overlooked the removal mechanism during lapping and their impact on surface and subsurface damage. In this study, a three-body brittle fracture removal model was established to explore the removal mechanisms of RB-SiC. Additionally, experiments were carried out to investigate the influence of abrasive particle size on the surface and subsurface damage. Experimental results confirm the theoretical model and indicate that for RB-SiC, different particle sizes correspond to distinct removal mechanisms, causing abrupt changes in surface roughness, while the layer under SiC acts as a buffer against the propagation of subsurface damage. These findings help optimize the manufacturing process, improve lapping efficiency, and enhance mirror performance.

Statistical properties of neutrally and stably stratified boundary layers in response to an abrupt change in surface roughness

We conducted experimental investigations on the effect of stable thermal conditions on rough-wall boundary layers, with a specific focus on their response to abrupt increases in surface roughness. For stably stratified boundary layers, a new analytical relation between the skin-friction coefficient, $C_f$ , and the displacement thickness was proposed. Following the sharp roughness change, the overshoot in $C_f$ is slightly enhanced in stably stratified layers when compared with that of neutral boundary layers. Regarding the velocity defect law, we found that the displacement thickness multiplied by $\sqrt{2/C_f}$ , performs better than the boundary layer thickness alone when describing the similarity within internal boundary layers for both neutral and stable cases. A non-adjusted region located just beneath the upper edge of the internal boundary layer was observed, with large magnitudes of skewness and kurtosis of streamwise and wall-normal velocity fluctuations for both neutral and stable cases. At a fixed wall-normal location, the greater the thermal stratification, the greater the magnitudes of skewness and kurtosis. Quadrant analysis revealed that the non-adjusted region is characterised by an enhancement/reduction of ejection/sweep events, particularly for stably stratified boundary layers. Spatially, these ejections correspond well with peaks of kurtosis, exhibit stronger intensity and occur more frequently following the abrupt change in surface conditions.

Large-eddy simulation and analytical modeling study of the wake of a wind turbine behind an abrupt rough-to-smooth surface roughness transition

The evolution of a wind turbine wake situated downstream of an abrupt change in surface roughness is investigated using large-eddy simulations (LES). The results are compared with the evolution of the wake of a turbine sited on a homogeneously rough surface, and with the flow over a surface undergoing an abrupt roughness transition without a turbine. The changed surface roughness affects the turbulent statistics such as streamwise velocity, turbulence intensity, and shear stress. Different velocity deficits can be constructed based on different definitions of “background” velocity. The usual definition, that is, the difference between the velocity upstream and downstream of a turbine, attains negative values over a significant portion of the turbine wake, rendering it difficult to model using the usual Gaussian radial shape-based framework. An alternative definition, that is, the difference between the velocity over a heterogeneous surface in the absence and in the presence of a turbine, has mostly positive values and is amenable to modeling. A new model accounting for streamwise and vertical variations of the background velocity profile is developed. The new model yields more accurate predictions of the LES results than the existing Gaussian wake-shape model, which is applicable only for turbines sited on homogeneously rough surfaces.

Coral Reef Coupling to the Atmospheric Boundary Layer Through Exchanges of Heat, Moisture, and Momentum: Case Studies From Tropical and Desert Fringing Coral Reefs

Coral reefs represent abrupt changes in surface roughness, temperature, and humidity in oceanic and coastal locations. This leads to formation of internal atmospheric boundary layers that grow vertically in response to turbulent mixing which conveys changes in surface properties into the prevailing wind. As a result, coral reefs under favourable conditions couple to the overlying atmosphere. Here we present rare observations of coral reef – atmospheric interactions during summer monsoon conditions on the Great Barrier Reef, Australia, and a desert fringing coral reef in the Gulf of Eilat, Israel. We show that in the hyper-arid location of the Gulf of Eilat where air temperatures are greater than water temperatures, a stable atmospheric boundary inhibits coupling of the reef to the atmosphere. In contrast, under monsoon conditions on the Great Barrier Reef, the coral reef is shown to couple to the overlying atmosphere leading to the formation of a convective internal boundary layer in which convective exchange of heat and moisture influences cloud and possibly precipitation. We conclude that understanding these processes is essential for determining the role of coral reefs in coastal meteorology, and whether through coupling with the overlying atmosphere coral reefs may regulate their meteorology by triggering formation of cloud and/or vertical exchange on aerosols and their precursor gases such as Dimethyl sulphide.

On the Flow Response to an Abrupt Change in Surface Roughness

On the Flow Response to an Abrupt Change in Surface Roughness

RANS Study of Flows across an Abrupt Change in Surface Roughness

RANS Study of Flows across an Abrupt Change in Surface Roughness

Experimental evaluation of flux footprint models

In this study we experimentally evaluate analytical flux footprint models, as well as models based on Lagrangian stochastic particle dispersion. For this purpose, we conducted tracer experiments at a grassland site in southern Germany. An artificial tracer was released continuously over a number of flux-averaging intervals from a surface source. The flux contributions from the tracer source were measured by eddy covariance and compared to those predicted by footprint models. Furthermore, an additional eddy covariance measurement tower was used to evaluate the along-wind distribution of footprint models, as well as to analyze to what extent a forest edge upwind of the measurement tower affects model performance. Additionally, we quantify footprint model uncertainty resulting from the random error of input parameters.Our measurements show that all evaluated models match observations roughly, but tend to underestimate the value of the footprint maximum, and overestimate its distance. The analysis of stability dependence of model performances indicates that one model, based on simulation outputs of a Lagrangian stochastic model, clearly underestimates observations for near neutral to stable conditions, while no clear stability dependence could be identified for the performance of the other models.As expected, model performance is sensitive to an abrupt change in surface roughness and sensible heat flux at a forest edge in the near upwind fetch of the measurement tower. Using the local apparent roughness length (derived from measured wind speed and friction velocity) only slightly or negligibly improved model performance compared to the use of a constant local roughness length (determined from local surface characteristics). Thus we confirm experimentally that footprint estimates and related data quality assessments should be handled with care at sites with inhomogeneities in surface roughness.

Dynamic subgrid‐scale models for momentum and scalar fluxes in large‐eddy simulations of neutrally stratified atmospheric boundary layers over heterogeneous terrain

The accuracy of large‐eddy simulations (LESs) of the atmospheric boundary layer (ABL) over complex terrain relies on the ability of the subgrid‐scale (SGS) models to capture the effect of subgrid turbulent fluxes on the resolved fields of velocity and scalars (e.g., heat, water vapor, and pollutants). A common approach consists of parameterizing the SGS stresses and fluxes using eddy viscosity and eddy diffusivity models, respectively. These models require the specification of two parameters: the Smagorinsky coefficient in the eddy viscosity model and, in addition, the SGS Schmidt/Prandtl number in the eddy diffusivity model. This is complicated by the dependence of the coefficients on local conditions such as distance to the ground, mean shear, and atmospheric stability. In this study, scale‐dependent dynamic SGS models are used in conjunction with Lagrangian averaging to compute both the Smagorinsky coefficient and the SGS Schmidt (or Prandtl) number dynamically as the flow evolves in both space and time based on the local dynamics of the resolved scales. These tuning‐free models are implemented in LES of both homogeneous and heterogeneous neutral atmospheric boundary layers with surface fluxes of a passive scalar. In the homogeneous simulations the models are shown to accurately predict the resolved flow statistics (mean profiles and spectra of velocity and scalar concentration) and spatial distributions of the SGS model coefficients and parameters. In simulations over heterogeneous surfaces both coefficients adjust in a self‐consistent way to horizontal flow inhomogeneities associated with changes in surface conditions. For smooth‐to‐rough (rough‐to‐smooth) abrupt changes in surface roughness the Smagorinsky coefficient decreases (increases) in response to increased (decreased) mean shear and flow anisotropy associated with these transitions. The SGS Schmidt number also adjusts to inhomogeneities in the scalar field associated with changes in surface scalar flux. This illustrates the need for local calculation of model coefficients and brings into question the common practice of using a constant SGS Schmidt/Prandtl number in LES of the ABL.

Evolution of Surface Topography of as-Grown Si Films near Amorphous-to-Polycrystalline Transition

The evolution of the surface topography and microstructure of silicon films deposited near the amorphous-to-polycrystalline transition was investigated, and was found to be accompanied by the appearance of crystalline aggregates whose density and size distribution strongly depend on deposition parameters. The films were deposited by low-pressure chemical vapor deposition, and the parameters varied were temperature, pressure, film thickness, and substrate material. Both temperature and pressure have a dramatic effect on the density and size of surface aggregates. A transition from amorphous to polycrystalline silicon at the critical temperature, is evident from the abrupt change in surface roughness. This effect was far more pronounced for thin (55 nm) silicon films than for their thicker (150 nm) counterparts and was generally larger for films grown directly on c-Si rather than on SiOx substrates. Evidence for nanocrystalline silicon particles embedded into the amorphous matrix below was found. In this study, it is suggested that the amorphous-polycrystalline silicon transition and consequently the surface topography evolution of the films is fluctuation assisted. The transition manifests itself in an enhanced population of Si nucleation centers, which are mostly formed at the upper surface of the deposited film. © 2004 The Electrochemical Society. All rights reserved.

Effects of Coastal Orography on Landfalling Cold Fronts. Part II: Effects of Surface Friction

Abstract The role of surface friction in modifying cold fronts as they make landfall in regions of steep coastal orography is examined by means of idealized simulations. Both the effects of the surface-roughness change at the coast in the absence of orography and the effects of orography are considered. Flow over a large and abrupt change in surface roughness generates an inertia–gravity wave above the boundary layer with characteristics similar to that associated with flow over a plateau. Deceleration of the cross-coast flow occurs over land, as well as for a short distance upstream, and causes retardation of frontal motion. A prescribed northerly postfrontal jet weakens rapidly after landfall. Maximum vertical motions are several centimeters per second; however, only small rainfall enhancement is expected since the updraft is very narrow and produces only small vertical displacements. Friction modifies the flow over the orography by increasing the upstream flow deceleration and reducing the magnitude of...

Surface length scales and shear stress: Implications for land‐atmosphere interaction over complex terrain

A large eddy simulation (LES) code of the atmospheric boundary layer (ABL) has been developed and applied to study the effect of spatially variable surface properties on the areally averaged surface shear stress at the land‐atmosphere interface. The LES code simulates the space and time evolution of the large‐scale turbulent eddies and their transport effects in the ABL. We report here on simulations of flow over spatially variable roughness fields. The dynamics are simulated, and the resulting space‐time fields are averaged to explore the effects of the surface variability length scales on the average surface shear stress, as used in large‐scale models to estimate scalar fluxes, such as evaporation. We observe asymmetrical response of the smooth‐to‐rough and rough‐to‐smooth transitions, such that the effects of the transitions accumulate rather than cancel. It is shown that the presence of abrupt changes in surface roughness and the atmosphere's response to these patches create a marked dependence of the statistical structure of surface shear stress on the length scale of the surface patches. An increase in regionally averaged surface stress for decreasing horizontal patch length scale is found.

Response of turbulent flow to abrupt changes in surface roughness and its relevance in horizontal annular flow

The response of turbulent flow to abrupt changes in pipe wall roughness is studied here. It is shown that a circumferential flow will not develop near the wall in a pipe with a short patch of circumferentially varying roughness. It is therefore concluded that the secondary flow mechanism would play no part in the liquid film distribution in horizontal annular flow.

Variation in wind velocity over water

Starting from the equations of motion and continuity, a theoretical model is deduced in this paper for the variation in wind velocity over water caused by abrupt changes in surface roughness and temperature when air flows from land to water, based on the consideration that the turbulent exchange coefficient varies with height and distance from the upwind edge. According to the computation of this model, the variation in wind velocity over water, as the drift of air is from land to water, occurs mainly in the first few kilometers from the upwind edge. The wind velocity over water increases to a maximum when the air over land is stable, it tends to moderate when neutral condition is reached, and least variation is shown in unstable condition. And when the air over land is unstable the wind velocity is less over water than over land in strong winds, but some-what greater in light winds.

A wind-tunnel boundary-layer study of the effects of a surface roughness change: Rough to smooth

The changes imposed on mean velocities and turbulence statistics in the lower atmosphere by an abrupt change in surface roughness, from very rough to smooth, were modelled in a wind tunnel. The influence of a change in the effective surface level, which often accompanies such a variation in surface roughness, was also studied. A deep, turbulent flow was generated upstream of the change, which had a logarithmic mean velocity profile and constant shear-stress for approximately 200 mm above the floor, except for a region near the surface which was influenced by the three-dimensional nature of the random rough surface.

Turbulent boundary layer wall-pressure fluctuations downstream from an abrupt change in surface roughness

Spectra of wall-pressure fluctuations beneath a turbulent boundary layer have been investigated immediately downstream from an abrupt change in surface roughness, from slightly rough to very rough. Over the measurable frequency band (400 Hz to 7 kHz), the spectra in the first 150 mm scale on wall variables. This indicates that the motions responsible for the pressure fluctuations measured in this region are very close to the wall. Farther downstream this scaling procedure breaks down. It is suggested that the initially thin internal layer behaves independently of the flow external to it until it has grown appreciably and only then interacts with the outer boundary layer. It was found that, for all frequencies measured, the dimensional spectral level decreases initially then rises to a maximum at 250 mm from the roughness change before decreasing once more. The position of the maximum is close to where the internal layer ceases to have a distinct edge.

Ocean Current Monitoring Employing a New Satellite Sensing Technique

The very-high-resolution radiometer on the NOAA-2 (National Oceanic and Atmospheric Administration) satellite has recently obtained imagery in the visible channel containing sunglint over a major portion of the coastal waters off the eastern seaboard of the United States. An abrupt change in surface roughness has been observed at the shoreward edge of the Gulf Stream Current from Florida to Cape Hatteras that results from the opposition of waves propagating against the flow of the Gulf Stream.

Wind profile and vertical motions above an abrupt change in surface roughness and temperature

A review is presented of some Russian-language papers published as early as 1952–1954 that are unknown to most English-speaking readers. In these papers the influence of an abrupt change in the surface friction velocityv*, surface roughnessz0 and surface temperature or heat flux on the wind velocity profile and vertical motions has been investigated analytically and numerically. Most of the theories are based on the exchange-coefficient approximation for momentum and heat. In terms of this approximation, further generalization and development of the problem is discussed.

Self-preserving flow inside a turbulent boundary layer

If a thick, turbulent boundary layer is disturbed near the rigid boundary, the flow changes are confined initially to a thin layer adjacent to the boundary. Elliott (1958) and Panofsky & Townsend (1964) have attempted to calculate the flow disturbance caused by an abrupt change in surface roughness by assuming special velocity distributions which are consistent with a logarithmic velocity variation near the boundary. Inspection of their distributions shows that the deviations from the upstream distribution are self-preserving in form, and it is shown that self-preserving development is dynamically possible if log l0/z0 (l0 being depth of modified flow, z0 roughness length) is fairly large and if l0 is small compared with the total thickness of the layer. Other kinds of surface disturbance may lead to self-preserving changes of the original flow and the theory is developed also for flow downstream of a line roughness, for the temperature distribution downstream of a boundary separating an upstream region of uniform roughness and heat-flux from a region of different or possibly varying roughness and heat-flux, and for the return of a complete boundary layer to self-preserving development after a disturbance. The requirement that the distributions of velocity and temperature should conform to the logarithmic, equilibrium forms near the surface makes the predictions of surface stress and surface flux nearly independent of the exact nature of the turbulent transfer process, and the profiles of velocity and temperature are determined within narrow limits by the surface fluxes. To provide explicit profiles, the mixing-length transfer relation is used. Its validity for the self-preserving flows is discussed in an appendix.