Nonlinear centrifugal instabilities in curved free shear layers

Curved free shear layers exist in many engineering problems involving complex flow geometries, such as the backward facing step flow, flows with wall injection, the flow inside side-dump combustors, or flows generated by vertical axis wind turbines, among others. Most of the studies involving centrifugal instabilities have been focused on wall flows where Taylor instabilities between two rotating concentric cylinders or Görtler vortices in boundary layers resulting from the imbalance between centrifugal effects and radial pressure gradients, are generated. Curved free shear layers, however, did not receive sufficient attention. An examination of the stability characteristics and the flow structures associated with curved free shear flows should provide a better understanding of these complex flow problems. In this work, we study the development of Görtler vortices inside a curved shear layer in both the incompressible and compressible regimes using a numerical solution to a parabolized form of the Navier-Stokes equations, in the assumption that the streamwise wavenumber associated with the vortex flow is much smaller than the crossstream wavenumbers. Various results consisting of contour plots of centrifugal instabilites in crossflow planes, and energy and streak amplitude distributions along the streamwise direction are reported and discussed. In addition, we conduct a biglobal stability analysis to study the growth rates and the eigenmodes associated with these flows

Comparative study of the repair of cracked plates with two different composite patches

The purpose of this study is to analyze the behavior of a crack with and without reinforcement by a composite patch of an aluminum plate in mode I using the finite element method. The repair patch is boron / epoxy and Carbon / epoxy, which are used with great success by many researchers For the distribution of the stresses according to the various loadings, we can conclude that the effect of repair by patch in composite is very distinct, considering the intensities of stresses which decrease for each repair corresponding to the plate not repaired, therefore, the patch in composite dampens the stress field induced at the crack tip and causes a reduction in stresses. The repair with the Bore / epoxy composite patch is more effective than the Carbon/Epoxy patch, this is due to the mechanical properties and the various characteristics specific to boron/Epoxy which gives very significant and very effective results for the repai

Study of mechanical behavior by fatigue of a cracked plate repaired by different composite patches

This research is based on the study of the fatigue behavior of an aluminum alloy plate with a central crack. The plate is subjected to a tensile loading on its lower and upper parts. Several parameters were highlighted, such as the loading effect with a load ratio R = 0. The effects of the load ratio on both the repaired and not repaired plates, by two composite patches, which are boron/epoxy and graphite/epoxy, were investigated, as well as the effect of the plate material on plate life, comparing different material

Numerical Analysis of the Crack Growth Path in the Cement of Hip Spacers

The use of temporary hip prosthesis made of orthopedic cement (spacer) in conjunction with antibiotics became a prevalent method used for prosthetic infections remedy; consequently, this method makes bone cement (PMMA) more fragile. Hence, the necessity of reinforcement incorporation is crucial to strengthen the bone cement. In this study, the finite element (FE) method was used to analyze the spacers behavior. FE model using an implicit integration method was used to simulate the mechanical behavior of the spacer under static loading. In addition, the extended finite element method (XFEM) was also used to investigate the fracture behavior of the non-reinforced and reinforced spacers. The results of this numerical analysis showed that the simulated crack initiation and propagation were in a good accordance with in vivo radiography and in vitro experimental observations. The full-stem reinforcement of 8 mm using reduce significantly the stress intensity factor and, consequently prevent the spacer fracture effectively. The FE models developed in this study contribute to help mechanical designers and engineers for prostheses’ quality and durability improvement. Abstract must be 200 words maximum, without figures or refs.             &nbsp

Investigation of Görtler vortices in high-speed boundary layers via an efficient numerical solution to the non-linear boundary region equations

Streamwise vortices and the associated streaks evolve in boundary layers over flat or concave surfaces due to disturbances initiated upstream or triggered by the wall surface. Following the transient growth phase, the fully developed vortex structures become susceptible to inviscid secondary instabilities resulting in early transition to turbulence via ‘bursting’ processes. In high-speed boundary layers, more complications arise due to compressibility and thermal effects, which become more significant for higher Mach numbers. In this paper, we study Görtler vortices developing in high-speed boundary layers using the boundary region equations (BRE) formalism, which we solve using an efficient numerical algorithm. Streaks are excited using a small transpiration velocity at the wall. Our BRE-based algorithm is found to be superior to direct numerical simulation (DNS) and ad hoc nonlinear parabolized stability equation (PSE) models. BRE solutions are less computationally costly than a full DNS and have a more rigorous theoretical foundation than PSE-based models. For example, the full development of a Görtler vortex system in high-speed boundary layers can be predicted in a matter of minutes using a single processor via the BRE approach. This substantial reduction in calculation time is one of the major achievements of this work. We show, among other things, that it allows investigation into feedback control in reasonable total computational times. We investigate the development of the Görtler vortex system via the BRE solution with feedback control parametrically at various freestream Mach numbers M∞ and spanwise separations λ of the inflow disturbances

Simulating and investigating compressible flows interaction with fractal structures

Previous experimental and numerical studies have investigated incompressible flow interactions with multi-scale fractal structures with the objective of generating turbulence at multiple scales. Depending on various flow conditions, it was found that these fractal structures are able to enhance mixing and scalar transport, and in some cases to contribute to the reduction of flow generated sound in certain frequency ranges. The interaction of compressible flows with multi-scale fractal structures did not receive much attention as the focus was entirely on the incompressible regime. The objective of this study is to conduct large eddy simulations of flow interactions with various fractal structures in the compressible regime and to extract and analyze different flow statistics in an attempt to determine the effect of compressibility. Immersed boundary methods will be employed to overcome the difficulty of modeling the fractal structures, with adequate mesh resolution around small features of the fractal shapes

Numerical simulation of the femur fracture for different cemented hip femoral prosthesis under forces during stumbling

Total hip prosthesis was used for the patients who has the hip fracture and unable to recover naturally. To design highly durable prostheses one has to take into account the natural processes occurring in the bone. In this paper, the static load analysis is based, by selecting the peak load during the stumbling activity. Two different implant materials have been selected to study appropriate material. The results showed the difference of maximum von Misses stress and detected the fracture of the femur shaft for different model (Charnley and Osteal) implant with the extended finite element method (X-FEM), and after the results of the numerical simulation of X-FEM for different was used in determining the stress intensity factors (SIF) to identify the crack behavior implant materials for different crack length. It has been shown that the maximum stress intensity factors were observed in the model of Charnley

Investigation, Analysis and Optimization of PEMFC Channel Cross-Section Shape

In this study, a three-dimensional (CFD) model is employed to simulate and optimize the CCS (Channel Cross-Section) shape of the single straight channel PEMFC. Four CCS shapes, namely trapeze, inverted trapeze, half of ellipse and inverted half of ellipse, are investigated using ANSYS-FLUENT software and compared to the rectangular and triangular CCS shapes. The results obtained from the simulation are compared to the experimental results of the literature. A good agreement is observed between the numerical and experimental results. From the obtained results, it appears that the best delivered power density is reported by the trapeze CCS configuration, whereas, the worst delivered power density is obtained by the inverted half of ellipse CCS configuration. The highest pressure-drop and pumping power are obtained with the triangular CCS configuration and the smallest are resulted by the rectangular CCS configuration. Finally, the highest net power output is reported by the trapeze channel cross-section configuration, while, the lowest one is yielded by the inverted half of ellipse CCS configuration

Control of streamwise vortices developing in compressible boundary layers

We derive and test an optimal control algorithm in the context of compressible boundary layers, in an attempt to suppress or at least limit the growth of streamwise vortices caused by high-amplitude freestream disturbances. We aim to reduce the vortex energy and ultimately delay the transition to turbulent flow. We introduce flow instabilities to the flow either through roughness elements equally separated in the spanwise direction or via freestream disturbances. We analytically reduce the compressible Navier-Stokes equations to the compressible boundary region equations (CBRE) in a high Reynolds number asymptotic framework, based on the assumption that the streamwise wavenumber of the streaks is much smaller than the cross-flow wavenumbers. We employ Lagrange multipliers to derive the adjoint compressible boundary region equations, and the associated optimality conditions. The wall transpiration velocity represents the control variable, whereas the wall shear stress or the vortex energy designates the cost functional. We report and discuss results for different Mach numbers, wall conditions, and spanwise separations

Control of Görtler vortices in high-speed boundary layer flows using nonlinear boundary region equations

We formulate a mathematical framework for the optimal control of compressible boundary layers to suppress the growth rate of the streamwise vortex system before breakdown occurs. We introduce flow instabilities to the flow either through roughness elements equally separated in the spanwise direction or via freestream disturbances. We reduce the compressible Navier-Stokes equations to the boundary region equations (BRE) in a high Reynolds number asymptotic framework wherein the streamwise wavelengths of the disturbances are assumed to be much larger than the spanwise and wall-normal counterparts. We apply the method of Lagrange multipliers to derive the adjoint compressible boundary region equations and the associated optimality conditions. The wall transpiration velocity represents the control variable while the wall shear stress or the vortex energy designates the cost functional. The control approach induces a significant reduction in the kinetic energy and wall shear stress of the boundary layer flow. Contour plots visually demonstrate how the primary instabilities gradually flatten out as more control iterations are applied