Ahmed Abd El-Rahman

Plasmodium Falciparum (pf) Malaria is one of the life-threatening infections for human red blood cells (RBCs), which deteriorates the topology of the corresponding bi-layer membranes and causes a 10-fold increase in their respective shear modulus during the well-distinguished Ring, Trophozoite and Schizont stages of infection progression. Previous efforts to characterize the bulk shear stiffness of pf-iRBC membranes include both in-vitro stretching tests and few simulations that enabled partial description of the membrane elasticity while assuming a uniform shear modulus. Although these results provided good insights into the axial-deformation of pf-iRBCs, the computed transverse diameter did not show similar agreement with the experimental values. The aim of the present work is to build a computational model that simulates the stretching tests of healthy and infected RBCs to better understand the mechanics of disease progression and its influence on the elastic properties of RBC membranes. For this purpose, a new patching technique is developed to mimic the infection progression through the decomposition of the cell membrane into infected pair patches and quasi-normal membrane segments. The incompressible membranes are modeled using the non-linear hyper-elastic Skalak constitutive model implemented through a VUMAT subroutine within the framework of a 3-D ABAQUS/Explicit finite-element model. In the advanced Schizont stage, a spheroidal-like geometry with uniform shear modulus is assumed, whereas in the other two stages, sizable circular patches of 2.4 and 4 µm in diameters, respectively, with adaptive shear moduli are implemented to replicate the stiffer pair-patches. The Skalak model indicates an 8-fold increase in the bulk shear modulus of the Schizont-cell beside showing better agreement with the published experimental results. Interestingly, for all other intermediate stages of infection, the bulk shear modulus is found to increase linearly with the percent area-infection, whereas nearly-constant shear modulus in the range of 14 µm is obtained for the quasi-normal membrane segments.

The geometry of airfoil sections in small wind turbine blades affect the boundary layer separation, promote the flow transition to turbulence and consequently influence the performance of many types of small wind turbines. However, few optimization results are available for typical small wind turbine blades. Here, we report a particular shape optimization study of the standard SG6043 airfoil that applies the genetic algorithm along with the low-Re XFOIL solver. The numerical model is first validated against available experimental values for the SG6043 airfoil at a Reynolds number of 100, 000. The predicted lift and drag profiles are qualitatively found in good agreement with the experimental values, particularly in the pre-stall regime, provided that the XFOIL’s transition exponent is adjusted to 11. The simulation is then extended to a lower Reynolds number of 60, 000 that is consistent with an average tip-speed-ratio λ = 4.0. Four specific scenarios seeking different objective functions are proposed for optimization. While the first two scenarios focus on typical optimum configurations at which the maximum lift-to-drag ratios are realized, the objective function of the latter scenarios considers the maximum arithmetic-mean over multiple consecutive degrees of the corresponding angle of attacks. The preliminary results show remarkable improvement in the range from 24% to 10% in the predicted lift-to-drag ratios in comparison with the original configuration at the expense of the corresponding airfoils’ stiffness. This is followed by an elaboration on the entire blade design along with the specification of the resulting power coefficient and annual energy production.

The design, the manufacture and the testing of a novel refrigeration system that is based on the thermoacoustic energy-conversion technology is reported. A 1-D linear acoustic model is specifically developed and carefully implemented in the present device. The system consists of two similar harmonically-oscillating pistons driven by a commercial 1-HP rotary drive mechanism operating at a frequency of 42 Hz -hereby, replacing typical expensive acoustic drivers-, and a thermoacoustic stack within which the energy conversion of sound into heat is taken place. Air at ambient conditions is used as the working gas while the amplitude of the driver's displacement reaches 19 mm. The 30-cm-long stack is a simple porous ceramic material having 100 square channels per square inch. Oscillating-gas pressure and temperature measurements show a maximum temperature span between the stack hot and cold ends of about 27° with an estimated Carnot COP around 11. A corresponding dynamic pressure of 7-kPa-amplitude is recorded (drive ratio of 7%) and found in a good agreement with theoretical prediction. The measured cooling effects at various phase shifts between the motions of the two opposite pistons show qualitative agreement with available theoretical values. However, the system behavior is noted to be clearly non-linear with significant energy loss mechanisms. This work helps understanding the operation principles of thermoacoustic refrigerators and represents a keystone towards developing commercial thermoacoustic refrigerators.

Abstract The thermoviscous behavior of the oscillating gas within the porous medium of a thermoacoustic refrigerator enables the conversion of sound into heat in the process of typical standing-wave thermoacoustic refrigeration systems. Several nonlinear mechanisms, such as harmonics generation, self-induced streaming and possible turbulence, cause complicated flow behavior and influence the performance of such devices. Few analytical and numerical approximations (Cao et al., 1996; Worlikar and Knio, 1996; Worlikar et al., 1998) describe the flow field and the energy flux density in standing devices comprising stacks of parallel plates, but almost no 3-D simulation has been developed that models the large-amplitude excitations and enables the prediction of the oscillating-flow behavior within a porous medium made of square channels. Here, we build on existing effort (Abd El-Rahman and Abdel-Rahman, 2014) and report a computational fluid dynamics analysis of a Helium-filled half-wavelength thermoacoustic refrigerator. The finite volume method is used, and the solid and gas domains are represented by large numbers of hexahedral elements. The calculations assume a 3-D periodic cell structure to reduce the computational cost and apply the dynamic mesh technique to account for the adiabatically oscillating wall boundaries. The simulation uses an implicit time integration of the full unsteady compressible flow equations along with the conjugate heat transfer algorithm. For the sake of simulation, the geometry and operating conditions closely follow the experimental setup of Lotton et al. (2009) [5]. A typical run involves about 200,000 computational cells, whereas the working frequency and the applied drive ratio are 592.5 Hz and 2.0%, respectively. The stationary system response is predicted and the numerical values are compared to theory. The results show good agreement with theoretical behavior in the linear regime of drive ratios up to 2.0% while a maximum cooling effect of 10 degrees is captured between the stack ends. This simulation provides an interesting tool for understanding the transient flow behavior, mean energy-flux density and flow streaming, and helps building 3-D computational fluid dynamics models for thermoacoustic devices.

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