Values Of Roughness Parameters Research Articles

Some field measurements concerned with the behaviour of resistance coefficients in a tidal channel

Measurements of the individual terms in the one-dimensional unsteady flow momentum equation in a 1.2 km tidal reach of the Conwy estuary, N. Wales, have yielded values for the resistance coefficients commonly adopted in open channel flow hydraulics. The results indicate clearly that Manning's n or Darcy-Weisbach's f vary significantly with stage, and to a lesser extent with flow direction. The relative importance of the various terms in the one-dimensional momentum equation are illustrated, and the techniques employed in measuring them indicated. Values of Nikuradse's roughness parameter, k s , have also been computed and are shown to vary significantly with water level. Mean bed shear stresses have been evaluated, and the variation in bed shear velocity throughout different tidal cycles determined. The results indicate the inadvisability of selecting a single roughness parameter to represent the hydraulic characteristics of any individual element in a numerical tidal model.

An Error Analysis of the Thornthwaite-Holzman Equations for Estimating Sensible and Latent Heat Fluxes over Crop and Forest Canopies

Abstract Computations of sensible and latent heat fluxes over forest and crop canopies using the Thornthwaite-Holzman equations require an a priori knowledge of roughness and data displacement heights. If the values of these parameters are not representative of the wind structure above the canopy, then errors will be induced in the heat flux calculations. A detailed error transfer analysis is derived and tested against tower data taken in a tropical dry ever-green forest in Thailand. It is shown that significant errors may occur in heat flux computations if mean values of roughness parameter and datum displacement height are used during periods of significant variation of those parameters. Further, the sensitivity of a heat flux calculation to the discrepancy in the profile parameters used may be either suppressed or amplified. A simple model is derived to estimate this error transfer from estimates of the mean values and variances of the roughness parameter and data displacement height.

Urban-rural wind velocity differences

Wind speeds along a streamflow line through New York City are found to be decreased below (increased above) those at sites outside of the city during periods with regional wind speeds above (below) about 4 m/s. The decrease is attributed to increased values of the surface roughness parameter in the city, as compared to values in nearby non-urban regions. The increase is associated with accelerations produced by a well-developed urban heat island. Cyclonic turning of the flow along the streamflow line through the city during the high wind speed periods results from decreases in the value of the Coriolis force associated with the increased urban frictional drag. During periods of acceleration over the city, either cyclonic or anticyclonic turning was found, due to a combination of two effects. The origin of the flow was found to influence the direction of the turning over the city during night-time heat island hours. For easterly flows the origin is a highly stable rural region, while for westerly flows it is most likely one of the less stable, but aerodynamically smooth, water bodies around New York City.

Multiple scattering from a random interface

The problem of scattering sound from a random interface separating two fluids with different densities and sound speeds is considered. The interface height is Gaussian distributed. The mathematical approach is the construction of the Green's function G for the problem by applying Green's theorem in both fluids to G and a single free space Green's function. Mixed surface‐volume integral equations are derived in coordinate space using continuity conditions on G and its normal derivative. In Fourier transform space, these equations can be combined to yield a single three‐dimensional integral equation for a singularity‐free function Γ linearly related to G. Using cluster decomposition techniques yields the Dyson integral equation for the coherent part (mean, first moment) of Γ and the Bethe‐Salpeter integral equation for its second moment. The Born terms and kernels of both these equations can be represented as infinite series of Feynman‐like diagrams. The first diagram corresponds to a Kirchhoff approximation. Using it in the Dyson equation yields a one‐dimensional integral equation which can be solved numerically. This multiple‐scattering‐Kirchhoff solution, in the case of a Neumann boundary, yields more coherent specular intensity for large values of the Rayleigh roughness parameter than the single‐scattering‐Kirchhoff approximation, and can be used to account for diverse experimental data. This as well as further results are discussed, the main point being the necessity of taking multiple scattering into account for problems of this type.

The classification of binary eutectics

Values of the interface roughness parameter (α) have been calculated for the primary phases of the 12 simple binary eutectic systems for which sufficient thermodynamic data are available; namely, Ag−Bi, Ag−Cu, Ag−Pb, Al−Ge, Al−Sn, Bi−Cd, Bi−Sn, Cd−Pb, Cd−Zn, In−Zn, Pb−Sb and Sn−Zn. It is shown that in the presence of solute the values of α can be significantly different from those of the pure component elements. The calculated, values of α are correlated with eutectic morphologies on the basis of the Hunt and Jackson classification. If the broken lamellar structure is also in the faceted/nonfaceted group then the results are in agreement with Hunt and Jackson’s classification. The analysis is discussed in the light of recent interpretations of the theories of solid-liquid interfaces and resultant growth morphologies. It is concluded that thermodynamic calculations are useful in the prediction of eutectic structures even though they may not necessarily resolve arguments concerning the mechanisms of eutectic solidification.

Terminal Shape of a Shallow Liquid Front

Shallow liquid front attains terminal shape and velocity short time after liquid is introduced into open channel; equations for shape of such front; simplified differential equation for turbulent front is presented with numerical solutions obtained by high speed computer for various values of relative roughness parameter; experimental investigations with oil and water verify equations.