Roughness Transition Research Articles

The flow in a turbulent boundary layer upstream of a change in surface roughness

Modification of a turbulent flow upstream of a change in surface roughness has been studied by means of a stream function-vorticity model. A flow reduction is found upstream of a step change in surface roughness when a fluid flows from a smooth onto a rough surface. Above that layer and above the region of flow reduction downstream of a smooth-rough transition, a flow acceleration is observed. Similar flow modification can be seen at a rough-smooth transition with the exception that flow reduction and flow acceleration are reversed. Within a fetch of −500 < x/z 0< + 500 (z 0 is the maximum roughness length, the roughness transition is located at x/z 0 = 0), flow reduction (flow acceleration) upstream of a roughness transition is one order of magnitude smaller than the flow reduction (flow acceleration) downstream of a smooth-rough (rough-smooth) transition. The flow acceleration (flow reduction) above that layer is two orders of magnitude. The internal boundary layer (IBL) for horizontal mean velocity extends to roughly 300z 0 upstream of a roughness transition, whereas the IBL for turbulent shear stress as well as the distortion of flow equilibrium extend almost twice as far. For the friction velocity, an undershooting (overshooting) with respect to upstream equilibrium is predicted which precedes overshooting (undershooting) over new equilibrium just behind a roughness transition. The flow modification over a finite fetch of modified roughness is weaker than over a corresponding fetch downstream of a single step change in roughness and the flow stays closer to upstream equilibrium. Even in front of the first roughness change of a finite fetch of modified roughness, a distortion of flow equilibrium due to the second, downwind roughness change can be observed.

A Yersinia enterocolitica serotype 0:3 lipopolysaccharide-specific monoclonal antibody reacts more strongly with bacteria cultured at room temperature than those cultured at 37 degrees C.

It has been suggested that the O-side chains of the lipopolysaccharide (LPS) of serotype 0:3 strains of Yersinia enterocolitica vary quantitatively and qualitatively depending on whether they are cultured at 37 degrees C or 25 degrees C. It is uncertain whether this affects the expression of the serotype-specific antigens that are probably carried on the LPS. We studied this question with a serotype 0:3-specific monoclonal antibody, 2C1. This monoclonal antibody immunoprecipitated a 39K major protein from detergent-solubilized 125I-labeled Yersinia preparation. However, results of Western blot experiments demonstrated that the antigens reactive with 2C1 were not actually the 39K protein but the O-side chains of the LPS. Significantly, reactivity could be detected whether the bacteria were cultured at room temperature or at 37 degrees C. However, absorption experiments confirmed that there were less accessible antigenic determinants on the 37 degrees C-LPS. The LPS preparations were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and silver staining. These SDS-PAGE profiles showed that less O-side chains were present in the 37 degrees C-LPS. Hence, the most likely explanation for our observation is that the 37 degrees C incubation condition induced a partial smooth to rough transition of the Yersinia LPS with a decrease in the amount of 2C1-reactive antigen.

Aerosol depletion and deposition in forests—A model analysis

A system of partial differential equations is developed to describe an aerosol entering from an open field into a downwind forest, where deposition and turbulent transfer affect particle concentration. The model considers the interplay between forest structure, open field and forest aerodynamics and aerosol characteristics. The modelling problem is broken down into a set of simpler submodels which can be refined or simplified individually. Model results suggest, basically, significant horizontal (vertical) depletion of super-μm particles downwind from the forest edge (downwards from the canopy top). Distinct edge effects in deposition rates are common. In certain cases vertical exchange processes predominate, leading to enrichment of the aerosol. Sub-μm particles (unless extremely small) are subject to almost negligible depletion and edge effects. Deposition of super-μm particle oriented aerosols are governed by a complex interaction between particle size, forest structure and aerodynamics whereas deposition of sub-μm particles is simply controlled by particle concentration and forest structure. Under certain conditions (dense homogenous forests) aerosol particle concentration across the open field to forest roughness transition increases again after initial depletion.

Depth profiling and interface analysis using spectroscopic ellipsometry

The use of variable energy (1.5 to 5.5 eV) and variable angle of incidence (45° to 85°) ellipsometry makes possible a nondestructive analysis of thin films and interfaces, described in multilayer models. This is demonstrated for the following selected cases: depth profiling of B and Se implanted in GaAs with the influence of thermal annealing, transition region between a crystalline Si substrate and a CVD deposited Si3N4 film. In this latter case, thermal nitridation of Si is examined in situ by attaching the ellipsometer to the CVD chamber and a rough transition between c‐Si and Si3N4 is confirmed.

Real structure of the solid surface at high temperatures

Our aim in this paper is to show that existing models of the boundary interface in front of a growing or melting crystal should be modified in the light of new experimental data and theoretical investigations. As a result of the strong vibration of surface atoms, the number of vacancies generated at the boundary of a crystal at a high temperature can be large and their condensation into large voids can occur very readily. Voids growing at a constant rate at high temperatures cause (1) great deformation of the crystalline lattice, (2) such high stress in the crystal that slip bands occur and (3) extensive generation of dislocations. These transformations can be viewed as a “preparation” of the solid surface before transition into the liquid phase. Thus it can be assumed that the boundary of the crystal at high temperatures is amorphous or has lost long-range ordering. The measured increase in the surface specific heat for highly dispersed particles supports the idea of a roughness transition at a crystal surface at high temperatures. It can also be shown theoretically that long-range ordering in a crystal vanishes at a temperature far below its melting point. Thus a good correlation exists between experimental result and theoretical prediction.

Vapour cloud dispersion and the evaporation of volatile liquids in atmospheric wind fields—II. Wind tunnel experiments

Rates of evaporation from free surfaces of toluene and pentane have been measured in a wind tunnel as a function of wind velocity, wind tunnel surface roughness and liquid temperature. For smooth boundary layer flows, the experimental data are found to be in very good agreement with predictions from the diffusion model of the preceding article. The earlier proposed semi-empirical procedure for taking transitions in surface roughness into account turns out to work satisfactorily for describing measured evaporation data at different upwind surface roughnesses. Neglect of the roughness transition from land to liquid is shown to yield very erroneous results. The influence of the liquid temperature on the evaporation turns out to be quite important, especially near the boiling point of the liquid. An adequate description of the observed effects is given by the effective diffusivity approach, described in Part I.

Surface drag coefficient dependence on the aerodynamic roughness of the sSea

Analysis of logarithmic wind profiles measured to a height of 10 m on the south shore of Long Island indicates three distinct classes of aerodynamic roughness of the sea surface. On the basis of the roughness Reynolds number, defined as u/sub */z/sub 0//v, where u/sub */ is the surface friction velocity, z/sub 0/ the roughness length, and v the kinematic viscosity, the sea surface is aerodynamically smooth for u/sub */z/sub 0//v less than or equal to 0.15. There is a moderately rough transition regime for 0.15 < u/sub */z/sub 0//v < 4, the surface becoming fully rough for the roughness Reynolds numbers exceeding 4. The surface drag coefficient C/sub D/sub 6// obtained from the mean wind speed at a height of 6 m is found to vary from 0.75 x 10/sup -3/ for an aerodynamically smooth sea surface to 1.9 x 10/sup -3/ for a fully rough surface. Moderately rough flow conditions yielded a C/sub D/sub 6// of 1.15 x 10/sup -3/. Wind profile measurements from two other offshore or coastal sites showed good agreement. Mean wind speeds ranged from 3.5 to 10 m s/sup -1/. The drag coefficient did not vary with mean wind speed but was weakly dependentmore » on the atmospheric stability. Charnock's constant ..cap alpha.. in z/sub 0/ = au/sub *//sup 2//g, where g is the gravitational acceleration, was found to have a mean value of 0.016 for moderately rough conditions and 0.072 for fully rough conditions.« less

Generalized Flow in Gaps and Slots Including the Effects of Ablation

An analytical method of predicting the flow properties in gap regions about control surfaces of re-entry vehicles has been developed. The method is particularly applicable to regions containing ablating surfaces. The analysis is based on a quasi-one-dimensional compressible flow solution including the effects of area change, friction, heat transfer, mass addition, and shocks. Wall roughness and boundary-layer transition effects also are included. The boundary conditions match the local upstream and downstream pressure, when appropriate, determine the upstream total temperature, and employ a steady-state mass and energy balance at the wall-gas interface. Equilibrium thermochemistry is assumed. The method is compared to static pressure and aerodynamic heating measurements obtained inside gap regions on nonablating models in hypersonic flow and shows reasonably good agreement. Additional solutions are shown to illustrate the effects of various flight parameters, wall roughness, and area change on ablating gap flow conditions.

Stratified flow over non-uniform surface conditions: Mixing-length model

The response of a neutrally stratified atmospheric surface layer to sudden changes in surface roughness and temperature is investigated by solving the steady-state boundary-layer equations. Near the discontinuities, the distributions of velocity and shear stress are found to be critically dependent upon whether or not the pressure and buoyancy terms are retained in the governing equations. Computed distributions of velocity and shear stress are found to be in good agreement with windtunnel and field data. The growth of the internal boundary layer under neutral conditions for smooth to rough transitions follows the 0.8 power law; however, rough to smooth transitions are associated with a 0.7 power law.