Seongwon Kang

Prediction of wall-pressure fluctuation in turbulent flows with an immersed boundary method

The objective of this paper is to assess the accuracy and efficiency of the immersed boundary (IB) method to predict the wall pressure fluctuations in turbulent flows, where the flow dynamics in the near-wall region is fundamental to correctly predict the overall flow. The present approach achieves sufficient accuracy at the immersed boundary and overcomes deficiencies in previous IB methods by introducing additional constraints - a compatibility for the interpolated velocity boundary condition related to mass conservation and the formal decoupling of the pressure on this surfaces. The immersed boundary-approximated domain method (IB-ADM) developed in the present study satisfies these conditions with an inexpensive computational overhead. The IB-ADM correctly predicts the near-wall velocity, pressure and scalar fields in several example problems, including flows around a very thin solid object for which incorrect results were obtained with previous IB methods. In order to have sufficient near-wall mesh resolution for LES and DNS computations, the present approach uses local mesh refinement. The present method has been also successfully applied to computation of the wall-pressure space-time correlation in DNS of turbulent channel flow on grids not aligned with the boundaries. When applied to a turbulent flow around an airfoil, the computed flow statistics - the mean/RMS flow field and power spectra of the wall pressure - are in good agreement with experiment.

DNS of buoyancy-dominated turbulent flows on a bluff body using the immersed boundary method

A novel immersed boundary (IB) method has been developed for simulating multi-material heat transfer problem - a cylinder in a channel heated from below with mixed convection. The method is based on a second-order velocity/scalar reconstruction near the IB. A novel algorithm has been developed for the IB method to handle conjugate heat transfer. The fluid-solid interface is constructed as a collection of disjoint faces of control volumes associated to different material zones. Coupling conditions for the material zones have been developed such that continuity and conservation of the scalar flux are satisfied by a second-order interpolation. Predictions of the local Nusselt number on the cylinder surface show good agreement with the experimental data. The effect of the Boussinesq approximation on this problem was also investigated. Comparison with the variable density formulation suggests that, in spite of a small thermal expansion coefficient of water, the variable density formulation in a transitional flow with mixed convection is preferable.

Accurate Immersed-Boundary Reconstructions for Viscous Flow Simulations

cells crossed by the immersed boundary is treated. A novel interpolation method based on momentum balance and massconservationbasedon finitevolumemethodareintroducedtocorrectlypredictthevelocityandpressure fieldin the vicinity of the boundaries. The assessment of the solution quality is based on calculations of Taylor decaying vortices. Examples of laminar and turbulent flow calculations are also reported.

A numerical study of a torque converter with various methods for the accuracy improvement of performance prediction

A comparative study was carried out on numerical methods for simulating a flow inside a torque converter. To investigate the effect of different methods for handling the relative motion of the parts, three methods were considered – the frozen rotor, sliding mesh and mixing plane methods. To improve the accuracy of performance prediction, the influence of viscosity variation with the temperature was studied by a thermo-fluid analysis. From parametric studies on the numerical scheme and the mesh resolution, it is observed that the results with the frozen rotor and sliding mesh methods agree well with the experimental data, whereas the mixing plane method induces a larger difference. The effect of viscosity variation on the accuracy of simulation is also investigated.

On a consistent high-order finite difference scheme with kinetic energy conservation for simulating turbulent reacting flows

Abstract The main objective of this study is to present an accurate and consistent numerical framework for turbulent reacting flows based on a high-order finite difference (HOFD) scheme. It was shown previously by Desjardins et al. (2008) [4] that a centered finite difference scheme discretely conserving the kinetic energy and an upwind-biased scheme for the scalar transport can be combined into a useful scheme for turbulent reacting flows. With a high-order spatial accuracy, however, an inconsistency among discretization schemes for different conservation laws is identified, which can disturb a scalar field spuriously under non-uniform density distribution. Various theoretical and numerical analyses are performed on the sources of the unphysical error. From this, the derivative of the mass-conserving velocity and the local Peclet number are identified as the primary factors affecting the error. As a solution, an HOFD stencil for the mass conservation is reformulated into a flux-based form that can be used consistently with an upwind-biased scheme for the scalar transport. The effectiveness of the proposed formulation is verified using two-dimensional laminar flows such as a scalar transport problem and a laminar premixed flame, where unphysical oscillations in the scalar fields are removed. The applicability of the proposed scheme is demonstrated in an LES of a turbulent stratified premixed flame.

An accurate and efficient method for automatic deformation of unstructured polyhedral grids to simulate the flow induced by the motion of solid objects

In the present study, an efficient numerical method to deform an unstructured polyhedral grid for accurate simulations of a flow induced by a moving solid object is proposed. In order to overcome inefficiency of the existing methods based on the vertices of a computational cell, the present approach deforms the polyhedral grids adaptively using a network based on the centre of each computational cell. It is shown that the proposed method provides with an improved efficiency for an arbitrary Lagrangian-Eulerian (ALE) simulation of a flow with a moving solid object compared to the existing methods based on the vertices. It is also shown that a polyhedral grid results in a better numerical accuracy than a triangular grid. The present numerical methods were applied to flows around a fixed and oscillating circular cylinder with various Reynolds numbers and oscillating frequencies. The produced numerical results were verified against aerodynamic characteristics of the previous numerical studies.

On a robust ALE method with discrete primary and secondary conservation

It is widely accepted that the non-linear convective/advective terms are the major sources of numerical instabilities in the Navier–Stokes and scalar transport equations. The lack of numerical stability is an issue often encountered in DNS/LES of flows with small values of the viscosity or diffusivity. Among numerous stabilization techniques developed so far, the idea of secondary conservation is attractive, particularly because it is based on a conservation law derived from the primary conservation law (e.g., transport equations) and not related to an artificial stabilization technique. As mentioned in previous studies ([1,2], etc.), there exists a group of conserved secondary quantities referred to as entropy functions. In the present study, however, we focus on conservation of the squared variable such as the kinetic energy for the Navier–Stokes equation. For a conserved variable ρφ, the transport equation in the limit of zero diffusivity is written as

DEVELOPMENT OF A CENTRIFUGAL BLOOD PUMP FOR ECMO AND VAD OPERATIONS

Using cardiopulmonary circulatory assist devices has been increased in the recent years as more models are available in the market. These devices can be employed in the situation during which both cardiac and respiratory support to a patient’s heart and lungs have to be provided, either during or after surgeries, for short time or even in the case of severe disease, for a period of weeks. Hence, it is critical to know the details of the phenomena happen inside a blood pump from both mechanical performances (such as pressure head and mechanical efficiency) and biomedical factors (such as hemolysis and thrombosis) and to design an optimum pump from both aspects. This paper investigates development of centrifugal blood pump impeller, specifically with focusing on the performances during ECMO condition. The baseline model is designed by investigating existing commercial pumps and considering results of recirculation, pressure heads and mechanical efficiencies together with their biomechanical performance via Modified Indices of Hemolysis (MIH). Afterword, two more modified models are designed and simulated. Overall, a comprehensive comparison between the results of all three case demonstrate that when impeller radius and prime volume is smaller, recirculation is reduced at impeller and MIH value becomes lower. Additionally, high scalar shear stress is observed near the volute and impeller walls and inside the top cavity gap.

A Numerical Study on Mechanical Performance and Hemolysis for Different Types of Centrifugal Blood Pumps

Centrifugal blood pumps have to be considered from both mechanical and biomechanical aspects. While, evaluations of mechanical factors, such as performance curve, are straightforward, biomechanical parameters, such as hemolysis indices, are still indistinct. Hence, different mathematical models and computational methods have been employed for the evaluation of hemolysis indices. This article aims to investigate four different types of centrifugal blood pumps from both mechanical and biomechanical aspects. The pumps are cone-type impeller (Type-A), channel-type impeller with shroud (Type-B), open impeller without shroud (Type-C) and shrouded impeller-type (Type-D). The CFD simulations are conducted using standard k-e turbulence model in multiple reference frame (MRF) method. Various values for rotational speed and flow rate are studied. The streamlines clearly show the effects of impeller geometry on flow patterns. It is also demonstrated that in all of the models, the areas of the recirculation have high value of von Mises stress. In addition, the effect of the volute in the Type-D on the pressure distribution and streamline smoothness is clearly observed. In another part, the modified index of hemolysis (MIH) calculated based on Eulerian approach is investigated for three predefined conditions of extracorporeal membrane oxygenation (ECMO), ventricular assist device (VAD), and full-load. The results reveal that the Type-A and Type-D have the highest and lowest MIH values, respectively in all of the predefined conditions. In addition, all of the pumps generate lower amount of hemolysis when they are operated in VAD condition.Copyright

An Approach on Fluid-Structure Interaction for the Prediction of Blood Flow in Aneurysms With Various Shapes

Recently, the rapid evolution of numerical methodologies for CFD and structural analyses has made it possible to predict the arterial hemodynamics closely related to vascular disease. In the present study, a framework for fluid-structure interaction (FSI) analysis was developed to accurately predict the arterial hemodynamics. The numerical results from the FSI analysis of the hemodynamics inside aneurysms of various shapes were compared to the results without FSI analysis. The results showed that FSI analysis needs to be performed in order to accurately predict the blood flow affected by the wall motion of compliant arteries. FSI analysis is essential to predict the hemodynamics in a saccular aneurysm because the arterial wall’s movement, which is a result of the variation of blood pressure in the aneurysmal sac, mainly produces the blood flow to a saccular aneurysm.Copyright