Ali Cemal Benim

A segregated formulation of Navier-Stokes equations with finite elements

Abstract A finite element procedure is presented for the solution of the Navier-Stokes equations, adopting a segregated velocity-pressure formulation. The procedure is based on the derivation of an approximate pressure equation, which allows the uncoupling of pressure and velocity solutions. This equation is derived from the discretized continuity equation by considering the relationships established between velocity and pressure in the discretized momentum equations. The numerical performance of the proposed method is demonstrated by investigating several examples. The results are compared with finite difference predictions.

Laminar flamelet modelling of turbulent premixed combustion

Abstract Laminar flamelet method (LFM) based prediction procedures for turbulent premixed combustion are presented. Two different approaches are investigated. In one case, the standard eddy dissipation concept (EDC) is used as the turbulent combustion model and the laminar flamelet model is applied as a post-processor for subsequent nitrogen oxide predictions. In the second approach, however, a higher predictive potential is achieved by employing the LFM as the turbulent combustion model. Predictions are compared with experiments for two different turbulent premixed flame configurations, namely for an essentially parabolic, laboratory flame, and a strongly swirling, recirculating flame of an industrial gas turbine burner. Results show that a substantial increase of predictive capability compared to more traditional methods is achieved by the flamelet method, not only for laboratory flames, but also for practical gas turbine applications. For the latter, the classical order of magnitude analysis suggests that the combustion occurs outside the laminar flamelet regime. Despite this, laminar flamelet predictions show relatively good agreement with experimental data, supporting arguments that such modelling is approppriate beyond the classical laminar flamelet combustion limits defined in the Borghi diagram.

Computational investigation of turbulent jet impinging onto rotating disk

Purpose – The main purpose of the paper is the validation of a broad range of RANS turbulence models, for the prediction of flow and heat transfer, for a broad range of boundary conditions and geometrical configurations, for this class of problems.Design/methodology/approach – Two‐ and three‐dimensional computations are performed using a general‐purpose CFD code based on a finite volume method and a pressure‐correction formulation. Special attention is paid to achieve a high numerical accuracy by applying second order discretization schemes and stringent convergence criteria, as well as performing sensitivity studies with respect to the grid resolution, computational domain size and boundary conditions. Results are assessed by comparing the predictions with the measurements available in the literature.Findings – A rather unsatisfactory performance of the Reynolds stress model is observed, in general, although the contrary has been expected in this rotating flow, exhibiting a predominantly non‐isotropic tu...

Pulsatile extracorporeal circulation during on-pump cardiac surgery enhances aortic wall shear stress.

Controversy on superiority of pulsatile versus non-pulsatile extracorporeal circulation in cardiac surgery still continues. Stroke as one of the major adverse events during cardiopulmonary bypass is, in the majority of cases, caused by mobilization of aortic arteriosclerotic plaques that is inducible by pathologically elevated wall shear stress values. The present study employs computational fluid dynamics to evaluate the aortic blood flow and wall shear stress profiles under the influence of antegrade or retrograde perfusion with pulsatile versus non-pulsatile extracorporeal circulation. While, compared to physiological flow, a non-pulsatile perfusion resulted in generally decreased blood velocities and only moderately increased shear forces (48 Pa versus 20 Pa antegradely and 127 Pa versus 30 Pa retrogradely), a pulsatile perfusion extensively enhanced the occurrence of turbulences, maximum blood flow speed and maximum wall shear stress (1020 Pa versus 20 Pa antegradely and 1178 Pa versus 30 Pa retrogradely). Under these circumstances arteriosclerotic embolism has to be considered. Further simulations and experimental work are necessary to elucidate the impact of our findings on the scientific discourse of pulsatile versus non-pulsatile extracorporeal circulation.

URANS and LES analysis of turbulent swirling flows

URANS and LES analysis of confined, incompressible turbulent swirling flows exhibiting vortex breakdown are presented, employing a RSM closure in conjunction with the former. Emphasis is placed upon the predictability of the sub-and supercritical vortex core states. Results are assessed by comparisons with experiments. It is observed that significantly better results are obtained by the RSM based URANS approach, compared to the RANS or turbulent viscosity based URANS. This indicates the importance of transient phenomena such as the precessing vortex core and vortex shedding, along with the unsuitability of the turbulent viscosity based approaches in this bracket of flows. LES did not perform as well as expected. This is possibly due to inaccuracies in formulating the boundary conditions, or to too coarse a filter width for the simple subgrid model adopted. Although the present work shows that remarkable improvements can be achieved over RANS approaches, comparisons with measurements reveal that further research and model improvements are still necessary.

A finite element solution of radiative heat transfer in participating media utilizing the moment method

Abstract A finite element solution of the radiative heat transfer in participating media is presented. The expensive direct numerical treatment of the integrodifferential equations of radiation is avoided by employing the moment method, which allows the derivation of an approximate local field equation for the radiation intensity. This makes the procedure convenient for use in combined-mode heat transfer problems, where the coupling of the radiation model with the iterative convective flow solution is required. The approach is assessed by investigating several numerical examples.

Experimental and numerical investigation of isothermal flow in an idealized swirl combustor

– The main purpose of the paper is the validation of different modelling strategies for turbulent swirling flow of an incompressible fluid in an idealized swirl combustor., – Experiments have been performed and computations carried out for a water test rig, for a Reynolds number of 4,600 based on combustor inlet mean axial velocity and diameter. Two cases have been investigated, one low swirl and the other high swirl intensity. Measurements of time‐averaged velocity components and corresponding rms turbulence intensities were measured using laser Doppler anemometer, along radial traverses at different axial locations. In the three‐dimensional, unsteady computations, large eddy simulation (LES) and URANS (Unsteady Reynolds Averaged Navier‐Stokes Equations or Reynolds Averaged Numerical Simulations) RSMs (Reynolds‐stress models) are basically employed as modelling strategies for turbulence. To model subgrid‐scale turbulence for LES, the models due to Smagorinsky and Voke are used. No‐model LES and coarse‐grid direct numerical simulation computations are also performed for one of the cases., – The predictions are compared with the measurements and reveal that LES provided the best overall accuracy for all of the cases, whereas no significant difference between the Smagorinsky and Voke models are observed for the time‐averaged velocity components., – This paper provides additional valuable information on the performance of various modelling strategies for turbulent swirling flows.

Simulation of heat transfer enhancement in tube flow with twisted tape insert

Heat transfer behaviour in a tube with inserted twisted tape swirl generator is investigated numerically, for different values of the twist ratio and diameter ratio and for Reynolds numbers within the range 100-20,000. The transition-SST model is used as the turbulence model. The computational model is validated, first, on a plain tube, where a good agreement is achieved with the correlations. The subsequent analysis of the tube with twisted tape indicates that the use of twist tape enhances heat transfer generally, which is accompanied by a higher pressure drop. It is observed that an improvement of the thermal-hydraulic performance can only be observed for certain configurations and Reynolds numbers.

Dispersive Aortic Cannulas Reduce Aortic Wall Shear Stress Affecting Atherosclerotic Plaque Embolization

Neurologic complications during on-pump cardiovascular surgery are often induced by mobilization of atherosclerotic plaques, which is directly related to enhanced wall shear stress. In the present study, we numerically evaluated the impact of dispersive aortic cannulas on aortic blood flow characteristics, with special regard to the resulting wall shear stress profiles. An idealized numerical model of the human aorta and its branches was created and used to model straight as well as bent dispersive aortic cannulas with meshlike tips inserted in the distal ascending aorta. Standard cannulas with straight beveled or bent tips served as controls. Using a recently optimized computing method, simulations of pulsatile and nonpulsatile extracorporeal circulation were performed. Dispersive aortic cannulas reduced the maximum and average aortic wall shear stress values to approximately 50% of those with control cannulas, while the difference in local values was even larger. Moreover, under pulsatile circulation, dispersive cannulas shortened the time period during which wall shear stress values were increased. The turbulent kinetic energy was also diminished by utilizing dispersive cannulas, reducing the risk of hemolysis. In summary, dispersive aortic cannulas decrease aortic wall shear stress and turbulence during extracorporeal circulation and may therefore reduce the risk of endothelial and blood cell damage as well as that of neurologic complications caused by atherosclerotic plaque mobilization.

The primitive variables formulation of the Navier-Stokes equations using the finite analytic method

Abstract The finite analytic method is employed to solve the Navier-Stokes equations with the velocity-pressure formulation. The Chorins artificial compressibility method and the method of pressure corrections are employed in order to be able to compute the pressure explicitly. The results are compared with the stream function-vorticity ones, and those of finite element and finite difference methods.