Camille A. Issa

Effects of heat generation/absorption and thermophoresis on hydromagnetic flow with heat and mass transfer over a flat surface

The problem of steady, two‐dimensional, laminar, hydromagnetic flow with heat and mass transfer over a semi‐infinite, permeable flat surface in the presence of such effects as thermophoresis and heat generation or absorption is considered. A similarity transformation is used to reduce the governing partial differential equations into ordinary ones. The obtained self‐similar equations are then solved numerically by an implicit, tri‐diagonal, finite‐difference scheme. Favourable comparison with previously published work is performed. Numerical results for the velocity, temperature and concentration profiles as well as for the skin‐friction coefficient, wall heat transfer and particle deposition rate are obtained and reported graphically for various parametric conditions to show interesting aspects of the solution.

Natural convection from an inclined plate embedded in a variable porosity porous medium due to solar radiation

Natural convection boundary-layer flow of an absorbing and electrically-conducting fluid over a semi-infinite, ideally transparent, inclined flat plate embedded in a porous medium with variable porosity due to solar radiation is considered. The governing equations are derived using the usual boundary-layer and Boussinesq approximations and accounting for the presence of an applied magnetic field and an applied incident radiation flux. To account for the heat loss from the plate surface, a convective-type boundary condition is employed there. These equations and boundary conditions are non-dimensionalized and transformed using a non-similarity transformation. The resulting non-linear partial differential equations are then solved numerically subject to the transformed boundary conditions by an implicit iterative finite-difference scheme. Graphical results for the velocity and temperature fields as well as the boundary friction and Nusselt number are presented and discussed for various parametric conditions.

Mixed convection effects on unsteady flow and heat transfer over a stretched surface

Abstract This work focuses on the effects of mixed convection currents on the problem of unsteady, laminar, boundary-layer flow and heat transfer of an electrically-conducting and heat generating or absorbing fluid over a semi-infinite vertical stretched surface in the presence of a uniform magnetic field. The surface is assumed to be permeable so that to account for possible fluid wall suction or injection and that it is maintained at a variable power-law temperature and is being stretched with a linear velocity with the distance along the surface. The governing equations are derived based on the boundary-layer theory and using the Boussinesq approximation. An appropriate transformation is employed and the transformed equations are solved numerically using the finite-difference method. Comparisons with previously published work are performed and the results are found to be in excellent agreement. A comprehensive parametric study is conducted and a representative set of graphical results for the velocity and temperature profiles as well as the time development of the skin-friction and wall heat transfer coefficients are reported and discussed.

Adaptive hp-finite element for transient analysis

Abstract Even though a tremendous effort has been devoted to the area of transient analysis, many of these models fall short of achieving the overall desired objective due to oversights and rigid simplifying assumptions. Some of those shortfalls are summarized as follows: 1. (i) The grid used is too fine or too coarse and is usually fixed in size; 2. (ii) No error estimates are being used to trigger a grid size change; 3. (iii) A fixed polynomial degree is being used to interpolate the approximate solution of the differential equation at hand (finite element approach); 4. (iv) An approximate differential equation is used to model the physical phenomena rather than the exact one (finite difference approach). In our study, a self-adaptive grid refinement technique coupled with the p -version of the finite element method is investigated. This type of approach is called the hp -version of the finite element. In the modeling process, the approximate solution to the exact differential equation achieves convergence by applying two distinct solution enrichment strategies. One strategy is to solve the problem using a very coarse grid and to enrich the quality of the approximation by increasing the degree p of the interpolating polynomial shape functions for those elements where it is needed. The other strategy involves local grid h refinements for those elements where it is needed. Both strategies would interact synergistically to produce an optimal computational model that produces an accurate simulation of the transient phenomena addressed with minimal computational effort. The triggering for the increase or decrease in polynomial degree p and placement or removal of local mesh h refinements is based on the calculation of a reliable a posteriori error estimate over each element at the end of each time-step. Acase study of particular interest is the simulation of wave propagation in a semi-infinite media.

Effect of Washout Loss on Bond Behavior of Steel Embedded in Underwater Concrete

Limited studies have been undertaken to investigate the bond properties of reinforcing steel bars embedded in underwater concrete (UWC). Approximately 60 pullout tests were carried out to evaluate the effect of washout loss (W) on residual compressive and bond strengths. Washout was determined using the CRD C61 test and by simulation using a newly developed air-pressurized tube. Reference mixtures sampled in dry conditions were also tested. Test results showed that bond between steel and UWC is affected by a combination of parameters that complement those documented in the literature for concrete cast and consolidated above water. These include the level of W, degree of segregation, hydrostatic water head, and interfacial concrete-water velocity. The bond-stress-versus-slip behavior of UWC is remarkably different from the one obtained using reference mixtures. Initially, the linear response is less stiff due to a coupled effect related to lower strength and increase in the relative coarse aggregate concentration. The slip at ultimate bond strength was found to decrease for UWC mixtures exhibiting higher levels of W.

Structural effects of perlite compaction on cryogenic storage vessels subjected to thermal cycles and vibrations

Abstract Double-wall cryogenic storage vessels containing perlite insulation are used to store LH 2 for test firing of NASA rockets. The vessels have been designed using the assumption that the perlite offers negligible structural resistance thus permitting the vessel to freely expand or contract under thermal cycles. Observation of perlite compaction has led to concern that after many loading cycles the perlite would become compacted enough to cause significant load transfer between the inner and outer tank, possibly causing structural damage. This study describes a detailed analysis of the potential for perlite compaction and assesses the potential for structural damage as a result of compaction. A finite element stress analysis program coded in FORTRAN was developed and implemented for this investigation. The vessel model was examined with dynamic and thermal loads to determine the effects by the given loads. The endochronic theory of plasticity was used to model the perlite insulation. Various tests were performed on the perlite to determine the parameters required by the model constitutive equations. These constitutive equations were then entered into the FEA program and the results compared with the test results.

Effect of Recycled Acrylic-Based Polymers on Bond Stress-Slip Behavior in Reinforced Concrete Structures

AbstractLimited studies investigated the effect of recycled polymers obtained from the paint industry on concrete bond stress-slip with embedded steel bars, including whether such behavior would be similar to that imparted by virgin polymeric latexes. Around 50 concrete mixtures containing different vinyl acrylic-based polymer concentrations are tested by direct bond and beam-end methods. Test results have shown that the concrete-bar interfacial bond stresses occurring in the elastic region substantially improved with recycled and virgin polymers. At a similar polymer-to-cement ratio, concrete incorporating recycled polymers exhibited improved bond properties than mixtures prepared with virgin ones. This was indirectly related to the pigment and extender powders in the waste latex paints, thus reducing porosity and improving denseness of cement paste that strengthen the transition zone adjacent to reinforcing bars. The effect of reducing the water-to-cement ratio while adding superplasticizer to compensat...

Numerical modelling of ultrasonic wave propagation using the efficient p-version finite element method

Abstract In this paper, the new p-version of the finite element method has been used for the numerical modelling of ultrasonic wave propagation in solid media. The p-method involves increasing the polynomial order of the interpolation functions to achieve convergence. This is unlike the conventional h-version in which the number of elements is increased to achieve the same goal. Illustrative examples of wave propagation through an isotropic aluminium block, including a case with a built-in defect in the form of a crack, have been discussed. Also, a convergence criterion has been developed for dynamic problems. The potential to extend this model to other applications such as medical ultrasonics or to a more complex anisotropic medium is significant.

Use of Locked-Cycle Protocol to Assess Cement Fineness and Properties in Laboratory Grinding Mills

Laboratory grinding mills operated for a given time interval do not consider the effect of circulating load (CL), thus generating excessively wide cement particle size distribution (PSD) curves that are not representative of what is normally obtained from real-scale mills. The main objective of this paper is to develop a locked-cycle protocol that takes CL into account and mimics the clinker grinding operations encountered in closed-circuit industrial ball mills. The protocol consisted of screening the laboratory mill content after each grinding run to remove undersize, which is then replenished with an equal mass of new clinker until CL becomes constant. For given Blaine fineness, test results have shown that cement ground in the laboratory for a fixed time interval possesses wide PSD curves characterized by spread factor n less than 0.96. Conversely, the locked-cycled protocol led to narrower PSD, with n varying from 1.025 to 1.175. Cement ground using the locked-cycle protocol was found to require increased water demand as compared to that ground for a fixed time interval. Also, this protocol was found to generate a reduced fraction of particles finer than 1 μm, which led to relatively delayed setting times and reduced 1-day compressive strength. The locked-cycle approach led to higher 28-day compressive strength, given the reduced fraction of particles larger than 50 μm.

Numerical Modeling of Plain Concrete Beams Strengthened with Externally Bonded CFRP

A detailed finite element model for plain concrete beams strengthened with externally bonded CFRP sheets was developed using ADINA a commercially available code and specific materials behavior models. The beams are modeled in full length in twodimensions and all the thicknesses are included in the FEM input data. 2D-solid, plane stress eight noded quadrilaterals are used to model the concrete and CFRP warps elements, and the reinforcement bars and CFRP sheets are modeled as three node truss elements. Comparisons are made for load-deflection plots at mid span; loads at failure and crack patterns at failure. Modeling simplifications and assumptions developed during this research are presented. Conclusions from current research efforts and recommendations for further studies are included.