Control Volume Concept Research Articles

Vapor‐Budget Measurement of Irrigation Evaporative Losses

A vapor‐budget technique for measurement of short‐term evaporation from moist, rough surfaces (such as irrigated cropped areas) is developed based on the control volume concept. The control volume is specified so that the difference in the horizontal fluxes of outgoing and incoming atmospheric water vapor equals the evaporation rate. The fluxes are estimated by measuring vertical profiles of wind speed and water vapor. A height of 3 m is sufficient for the present field application. Simplifications used in actual application of the technique are discussed and evaluated. This approach is field tested (square test site, 30 m on each side) and compared with simultaneous water‐budget measurements of the evaporation. The vapor budget is within 12% of the water budget for steady wind direction so long as an average of five profile sets is used for each 15‐min evaporation value. One of the primary advantages of the control volume approach is that it requires measurements only at the periphery, thus providing integrated evaporation values for complex surfaces.

Measurement of various terms in the turbulent kinetic energy balance within a flame and comparison with theory

Direct measurements have been made of four of the six terms in the exact form of the turbulent kinetic energy (TKE) equation within a premixed turbulent flame, in order to understand how the combustion interacts with the turbulence. Results are compared to the predictions of the Bray-Moss-Libby (B-M-L) model. The mean reaction ( w ) also was estimated from the data, using a local control volume concept. The measurements were made possible by the simultaneous use of laser velocimetry and Rayleigh scattering. Increasing the heat release was found to increase the “flame generated” turbulence as well as the “apparent” turbulence and the turbulent flame speed. Part of this flame-generated turbulence is due to vorticity created by the curved flamelets, as evidenced by the unexpected existence of low speed products. The flame also is found to exert an upstream influence that causes an increase in the turbulence of the pure reactants before these reactants ever reach a flamefront. The measured diffusion term in the TKE balance indicates a countergradient diffusion of TKE toward the center of the flame where TKE is maximum; however, this is not true diffusion since the TKE balance includes apparent turbulence. The B-M-L model correctly predicts quantities which are dominated by intermittency due to flame motion such as the diffusion term in the TKE, and fluxes ϱu″c″ and ϱu″ 2c″ which change sign within the flame. This is because the model successfully accounts for intermittency by separating the physics of reactants and products. The model does not successfully predict even the trends associated with the more subtle “true turbulence” associated with products ( u′ p). It is felt that additional work is needed to remove “apparent turbulence” from the TKE balance, and eventually to replace one closure relation in the model to account for vorticity produced by curved flamelets.

Finite difference computation of the dynamic motion of cylindrical shells including the effect of rotatory inertia and transverse shear

AbstractThe accuracy of finite difference computations for the dynamic motion of cylindrical shells, including transverse shear and rotatory inertia, has been assessed by comparison with Fourier series solutions. The finite difference models were based upon either the differential equations or upon control volume concepts, with the results of the latter being less sensitive to mesh size and in better agreement with the Fourier series results. A frequency analysis of all of the finite difference algorithms and the Fourier series demonstrated that for shells with clamped ends the finite difference spectra had a very considerable gap which closed slowly as the mesh size decreased, with the spectra being most complete for the best algorithm. A new algorithm was based upon these spectra and is shown to yield good results while permitting increases in the computational speed by the factor of mesh size/shell thickness.

Control Volume Analysis of Elastic Wave Front in Composite Materials

A theory is proposed for the analysis of an elastic wave front in unidirectionally laminated as well as fiber reinforced composite mate rials. According to this theory, which is based on the concept of control volume, elastic wave front due to abrupt loadings propagates at a velocity which is bounded by the wave velocities of the fiber material and of the matrix material. The theory includes a nonlinear effect of area change; the wave front velocity is a function of the impact intensity. But for linearly elastic materials, this non-linear effect is small; if it is neglected, then wave front speed in the com posite becomes independent of the impact intensity. Consideration has also been given to the determination of the shear force along the interface between the reinforcing fiber and the matrix. It is found that for certain combination of fiber and matrix, the shear stress attains an extremum value which may cause debond ing of the composite under moderate impact loading.