Jung Hee Seo

Linearized perturbed compressible equations for low Mach number aeroacoustics

For efficient aeroacoustic computation at low Mach numbers, the linearized perturbed compressible equations (LPCE) are proposed. The derivation is based on investigation of the perturbed vorticity transport equations. In the original hydrodynamic/acoustic splitting method, perturbed vorticity is generated by a coupling effect between the hydrodynamic vorticity and the perturbed velocities. At low Mach numbers, the effect of perturbed vorticity on sound generation is not significant. However, the perturbed vorticity easily becomes unstable, and causes inconsistent acoustic solutions, based on grid dependence. The present LPCE ensures grid-independent acoustic solutions by suppressing the generation of perturbed vorticity in the formulation. The present method is validated for various dipole and quadruple vortex-sound problems at low Mach numbers: (i) laminar dipole tone from a circular cylinder at Reynolds number based on the cylinder diameter, ReD = 150 and free stream Mach number, M∞ = 0.1, (ii) quadruple sound of Kirchhoff vortex at Mach number based on the rotating speed, MΘ = 0.1, and (iii) temporal mixing layer noise at Reynolds number based on the shear layer thickness, Reδ = 10000 and Mach number based on the shear rate, Ms = 0.1.

Effect of diastolic flow patterns on the function of the left ventricle

Direct numerical simulations are used to study the effect of intraventricular flow patterns on the pumping efficiency and the blood mixing and transport characteristics of the left ventricle. The simulations employ a geometric model of the left ventricle which is derived from contrast computed tomography. A variety of diastolic flow conditions are generated for a fixed ejection fraction in order to delineate the effect of flow patterns on ventricular performance. The simulations indicate that the effect of intraventricular blood flow pattern on the pumping power is physiologically insignificant. However, diastolic flow patterns have a noticeable effect on the blood mixing as well as the residence time of blood cells in the ventricle. The implications of these findings on ventricular function are discussed.

Computational modeling of cardiac hemodynamics

The proliferation of four-dimensional imaging technologies, increasing computational speeds, improved simulation algorithms, and the widespread availability of powerful computing platforms is enabling simulations of cardiac hemodynamics with unprecedented speed and fidelity. Since cardiovascular disease is intimately linked to cardiovascular hemodynamics, accurate assessment of the patients hemodynamic state is critical for the diagnosis and treatment of heart disease. Unfortunately, while a variety of invasive and non-invasive approaches for measuring cardiac hemodynamics are in widespread use, they still only provide an incomplete picture of the hemodynamic state of a patient. In this context, computational modeling of cardiac hemodynamics presents as a powerful non-invasive modality that can fill this information gap, and significantly impact the diagnosis as well as the treatment of cardiac disease. This article reviews the current status of this field as well as the emerging trends and challenges in cardiovascular health, computing, modeling and simulation and that are expected to play a key role in its future development. Some recent advances in modeling and simulations of cardiac flow are described by using examples from our own work as well as the research of other groups.

Prediction of cavitating flow noise by direct numerical simulation

In this study, a direct numerical simulation procedure for the cavitating flow noise is presented. The compressible Navier-Stokes equations are written for the two-phase fluid, employing a density-based homogeneous equilibrium model with a linearly-combined equation of state. To resolve the linear and non-linear waves in the cavitating flow, a sixth-order compact central scheme is utilized with the selective spatial filtering technique. The present cavitation model and numerical methods are validated for two benchmark problems: linear wave convection and acoustic saturation in a bubbly flow. The cavitating flow noise is then computed for a 2D circular cylinder flow at Reynolds number based on a cylinder diameter, 200 and cavitation numbers, @s=0.7-2. It is observed that, at cavitation numbers @s=1 and 0.7, the cavitating flow and noise characteristics are significantly changed by the shock waves due to the coherent collapse of the cloud cavitation in the wake. To verify the present direct simulation and further analyze the sources of cavitation noise, an acoustic analogy based on a classical theory of Fitzpatrik and Strasberg is derived. The far-field noise predicted by direct simulation is well compared with that of acoustic analogy, and it also confirms the f^-^2 decaying rate in the spectrum, as predicted by the model of Fitzpatrik and Strasberg with the Rayleigh-Plesset equation.

Effect of the mitral valve on diastolic flow patterns

The leaflets of the mitral valve interact with the mitral jet and significantly impact diastolic flow patterns, but the effect of mitral valve morphology and kinematics on diastolic flow and its implications for left ventricular function have not been clearly delineated. In the present study, we employ computational hemodynamic simulations to understand the effect of mitral valve leaflets on diastolic flow. A computational model of the left ventricle is constructed based on a high-resolution contrast computed-tomography scan, and a physiological inspired model of the mitral valve leaflets is synthesized from morphological and echocardiographic data. Simulations are performed with a diode type valve model as well as the physiological mitral valve model in order to delineate the effect of mitral-valve leaflets on the intraventricular flow. The study suggests that a normal physiological mitral valve promotes the formation of a circulatory (or “looped”) flow pattern in the ventricle. The mitral valve leaflets also increase the strength of the apical flow, thereby enhancing apical washout and mixing of ventricular blood. The implications of these findings on ventricular function as well as ventricular flow models are discussed.

Toward a simulation-based tool for the treatment of vocal fold paralysis.

Advances in high-performance computing are enabling a new generation of software tools that employ computational modeling for surgical planning. Surgical management of laryngeal paralysis is one area where such computational tools could have a significant impact. The current paper describes a comprehensive effort to develop a software tool for planning medialization laryngoplasty where a prosthetic implant is inserted into the larynx in order to medialize the paralyzed vocal fold (VF). While this is one of the most common procedures used to restore voice in patients with VF paralysis, it has a relatively high revision rate, and the tool being developed is expected to improve surgical outcomes. This software tool models the biomechanics of airflow-induced vibration in the human larynx and incorporates sophisticated approaches for modeling the turbulent laryngeal flow, the complex dynamics of the VFs, as well as the production of voiced sound. The current paper describes the key elements of the modeling approach, presents computational results that demonstrate the utility of the approach and also describes some of the limitations and challenges.

Investigation and modeling of bubble-bubble interaction effect in homogeneous bubbly flows

The effect of bubble-bubble interaction in homogeneous bubbly flow is investigated by direct numerical simulation and a bubbly mixture model for bubbly shock flows at void fraction 0.4%–13%. It is found that the bubble-bubble interaction effect is significant at void fraction higher than O(1)% and decreases the amplitude and wavelength of the macroscale oscillations in the dispersive shock structure. For the modeling of bubble-bubble interaction effect, the locally volume averaged Rayleigh–Plesset (LVARP) equation, which is an extended version of the original Rayleigh–Plesset equation, is proposed in the present study. The results of bubbly mixture model using LVARP agree well with the direct simulation results for bubbly shock flows at void fraction up to 13%. The bubble-bubble interaction in nonuniform bubbly flows is also investigated in bubbly flows with randomized initial bubble positions. It is found that the LVARP model predicts the ensemble averaged behavior with reasonable accuracy.

A computational model of blast loading on the human eye

Ocular injuries from blast have increased in recent wars, but the injury mechanism associated with the primary blast wave is unknown. We employ a three-dimensional fluid–structure interaction computational model to understand the stresses and deformations incurred by the globe due to blast overpressure. Our numerical results demonstrate that the blast wave reflections off the facial features around the eye increase the pressure loading on and around the eye. The blast wave produces asymmetric loading on the eye, which causes globe distortion. The deformation response of the globe under blast loading was evaluated, and regions of high stresses and strains inside the globe were identified. Our numerical results show that the blast loading results in globe distortion and large deviatoric stresses in the sclera. These large deviatoric stresses may be indicator for the risk of interfacial failure between the tissues of the sclera and the orbit.

Prediction of flat plate self-noise

In this study, the accuracy of the computational methodology developed for prediction of turbulent flow noise at low Mach numbers is assessed for the flat plate self-noise. The far-field self-noise and the wall-pressure field over the flat plate (chord=10cm, thickness=3mm, and span=30cm) are measured at Ecole Centrale de Lyon for a flow speed UO=20m/s at zero angle of attack. The three-dimensional turbulent flow over the plate is computed by incompressible large eddy simulation (LES), while the near- and far-field acoustics are calculated by the linearized perturbed compressible equations (LPCE), coupled with the LES solutions. Comparisons are made for the wall pressure PSD spectra near the trailing-edge, the spanwise coherence function of the surface pressure, and the farfield sound pressure level spectrum. The computations agree well with the experiment. The tonal and broadband noise characteristics of the flat plate are also discussed.

Prediction of Low Mach Number Turbulent Flow Noise Using the Splitting Method

A new methodology is proposed for the low Mach number turbulent flow noise prediction. Based on the hydrodynamic/acoustic splitting method, turbulent flow field is computed by an incompressible large eddy simulation(LES), while its acoustic field is predicted by the filtered perturbed-compressible-equations(FPCE). The FPCE which is derived from the incompressible and compressible LES equations does not require any particular modeling of the source term for turbulence. The present method is applied to turbulent noise prediction of flow past a circular cylinder at ReD=46000 and M=0.21. The predicted sound pressure level spectrum agrees fairly well with the experimental data of Boudet et al(AIAA Paper 2003-3217).