György Paál

Budapest University of Technology and Economics · Department of Hydrodinamic Systems

Delaying the onset of laminar-turbulent transition is an attractive method in reducing skin friction drag, especially on streamlined bodies where Tollmien–Schlichting instabilities are the dominating mechanism for transition. Miniature Vortex Generators (MVGs) offer an effective approach to attenuate these instabilities by generating counter-rotati...

This paper presents an attempt to decrease the domain for the simulation of blood flow in certain types of bifurcation aneurysms. A reduced computational cost would be beneficial for the simulation of large datasets, to have statistical data for various hemodynamical parameters. The reduction is achieved by cutting the geometry in an objective mann...

Delaying the onset of laminar-turbulent transition is an attractive method in reducing skin friction drag, especially on streamlined bodies where Tollmien-Schlichting instabilities are the dominating mechanism for transition. Miniature vortex generators (MVGs) offer an effective approach to attenuate these instabilities by generating counter-rotati...

Miniature vortex generators (MVGs) have a large potential to reduce the friction drag by delaying laminar–turbulent transition in boundary layer flows with attenuating the growth of the Tollmien–Schlichting (TS) waves. This study is concerned with the modal stability analysis of the streamwise elongated streaks in the boundary layer on the suction...

Utilizing miniature vortex generators (MVGs) proves to be an effective approach to delay the laminar-turbulent transition in boundary layer flows and reduce the friction drag by attenuating Tollmien-Schlichting (TS) instabilities. This method is particularly beneficial for natural transition, notably in the context of streamlined bodies. The MVGs a...

The Common Carotid Artery plays a vital role in supplying the brain, and its bifurcation is susceptible to vascular diseases. It is often analyzed using computational fluid dynamics (CFD) simulations, but it is challenging to prescribe boundary conditions that approach patient-specific flow conditions. We examined six boundary condition (BC) groups...

Purpose The high efficacy of flow diverters (FD) in the case of wide-neck aneurysms is well demonstrated, yet new challenges have arisen because of reported posttreatment failures and the growing number of new generation of devices. Our aim is to present a measurement-supported in silico workflow that automates the virtual deployment and subsequent...

Delaying laminar-turbulent transition is an attractive method to reduce friction drag on streamlined bodies. In the case of natural transition, in which turbulence is triggered by the growth of two-dimensional Tollmien-Schlichting (TS) instabilities, this growth of TS instabilities can be attenuated by introducing steady streamwise streaks into the...

Computing the emerging flow in blood vessel sections by means of computational fluid dynamics is an often applied practice in hemodynamics research. One particular area for such investigations is related to the cerebral aneurysms, since their formation, pathogenesis, and the risk of a potential rupture may be flow-related. We present a study on the...

Aeroacoustic tonal noise, often caused by self-excited flow oscillations, represents a significant challenge in various engineering applications. One prominent example is the cavity tone, responsible for undesirable sound emissions from gaps found on the surfaces of vehicles, door gaps, and pantographs of trains. A similar configuration, known as t...

A large part of computational fluid dynamics (CFD) studies in hemodynamics concentrates on the berry-like bulgings on cerebral vessel walls, called intracranial aneurysms (IA). One technique is the calculation of particle paths, which can help understand important physiological processes like thrombus formation or drug propagation. The problem is t...

Intracranial aneurysms are nowadays treated with endovascular flow diverter devices to avoid sac rupture. This study explores how different linear and quadratic hydrodynamic resistance parameters reduce the flow in the sac for five patient-specific sidewall aneurysms.The 125 performed blood flow simulations included the stents using a Darcy-Forchei...

Aortic pressure can be estimated using one-dimensional arterial flow simulations. This study demonstrates that two peripheral pressure measurements can be used to acquire the central pressure curve through the patient-specific optimization of a set of system parameters. Radial and carotid pressure measurements and parameter optimization were perfor...

Low-dimensional (1D or 0D) models can describe the whole human blood circulation, for example, 1D distributed parameter model for the arterial network and 0D concentrated models for the heart or other organs. This paper presents a combined 1D-0D solver, called first_blood, that solves the governing equations of fluid dynamics to model low-dimension...

In this paper, CFD-based results are presented on the estimation of losses in bends with validated numerical models for single-phase, incompressible Bingham fluids. Five fittings were investigated: $$90^\circ$$ 90 ∘ bends of different R / D ratios between 1 and 10. Bingham plastic fluids were studied with Hedström numbers in the wide range from 1 t...

The Reynolds–Orr equation predicts the unconditional stability limit of a flow. Although this seems to be a desirable aim in engineering applications, the predicted critical Reynolds numbers are one magnitude below the experimental observations. In this paper, an attempt is made to reduce this gap for incompressible shear flows. It is known that th...

The parabolized stability equation (PSE) is a widely used, efficient method to calculate the evolution of streamwise traveling instabilities in spatially developing, weakly nonparallel flows. Although the PSE method is very economical, computational time can be significant due to the repeated solution of linear systems of equations in each marching...

Purpose To develop a method of coronary flow reserve (CFR) calculation derived from three-dimensional (3D) coronary angiographic parameters and intracoronary pressure data during fractional flow reserve (FFR) measurement. Methods Altogether 19 coronary arteries of 16 native and 3 stented vessels were reconstructed in 3D. The measured distal intrac...

A partial differential equation has usually a regular solution at the initial time if the initial condition is smooth in space, fulfills the governing equations and is compatible with the boundary condition. In the case of Navier-Stokes equation, the initial velocity field must also be divergence–free. It is common belief that the initial condition...

Miniature vortex generators (MVGs) are an attractive technology to delay laminar to turbulent transition or to postpone flow separation of laminar boundary layers. Despite the appearance of several recent studies, much more work still needs to be done to optimize these MVGS or to study their interaction with a boundary layer flow. Therefore, in thi...

Delaying laminar-turbulent transition is an attractive method to reduce friction drag on streamlined bodies. In the case of natural transition, in which turbulence is triggered by the growth of two-dimensional Tollmien-Schlichting (TS) instabilities, this growth of the TS instabilities can be attenuated by introducing steady streamwise streaks into...

Purpose Wall shear stress (WSS) distribution has been associated with aneurysm formation. Results of WSS studies are hampered by the subjective geometry creation for numerical studies. We analised the crossectional flow fields across aneurysm bearing locations, considering that the flow field is less susceptible to errors of geometry as the WSS its...

One-dimensional arterial flow simulations are suitable to estimate the aortic pressure from peripheral measurements in a patient-specific arterial network. This study introduces a reduction of the system parameters, and a novel calculation method to estimate the patient-specific set and the aortic curve based on radial applanation tonometry. Periph...

Purpose: To develop a method of coronary flow reserve (CFR) calculation derived from three-dimensional (3D) coronary angiographic parameters and intracoronary pressure data during fractional flow reserve (FFR) measurement. Methods: Altogether 19 coronary arteries of 16 native and 3 stented vessels were reconstructed in 3D. The measured distal in...

Compliant coatings that are capable of delaying laminar–turbulent transition have received much attention. Most examined compliant coatings exhibit dominantly wall-normal motion, and although they are able to delay laminar–turbulent transition, they also give rise to other instability mechanisms that advance transition. Previous research (e.g., Car...

Purpose Despite the high efficacy of flow diverters (FD) in treating sidewall intracranial aneurysms, failures are reported. One of the physical factors determining efficacy is the flow reducing capacity of the FD that is currently unknown to the operator. Our aim was to measure the flow reducing capacity expressed as the hydrodynamic resistance (H...

Purpose: To develop a method of coronary flow reserve (CFR) calculation derived from threedimensional (3D) coronary angiographic parameters and intracoronary pressure data during fractional flow reserve (FFR) measurement. Methods: Altogether 19 coronary arteries of 16 native and 3 stented vessels were reconstructed in 3D. The measured distal intrac...

Background: Measurements of fractional flow reserve (FFR) and/or coronary flow reserve (CFR) are widely used for hemodynamic characterization of coronary lesions. The frequent combination of the epicardial and microvascular disease may indicate a need for complex hemodynamic evaluation of coronary lesions. This study aims at validating the calcula...

This paper presents a novel method for the evaluation of three-dimensional blood-flow simulations based, on the decomposition of the velocity field into localized coordinate systems along the vessels centerline. The method is based on the computation of accurate centerlines with the Vascular Modeling Toolkit (VMTK) library, to calculate the localiz...

Background: The image reconstruction of stenotic carotid bifurcation can be managed by medical practitioners and non-medical investigators with semi-automatic or manual segmentation. The outcome of blood flow simulations may vary because of a single mean voxel difference along the examined section, possibly more in the stenotic lesions, which can l...

Knowing the pressure at all times in each node of a water distribution system (WDS) facilitates safe and efficient operation. Yet, complete measurement data cannot be collected due to the limited number of instruments in a real-life WDS. The data-driven methodology of reconstructing all the nodal pressures by observing only a limited number of node...

Real-time control of pumps can be an infeasible task in water distribution systems (WDSs) because the calculation to find the optimal pump speeds is resource-intensive. The computational need cannot be lowered even with the capabilities of smart water networks when conventional optimization techniques are used. Deep reinforcement learning (DRL) is...

This paper describes a study based on computational fluid dynamics (CFD) and deep neural networks that focusing on predicting the flow field in differently distorted U-shaped pipes. The main motivation of this work was to get an insight about the justification of the deep learning paradigm in hydrodynamic hull optimisation processes that heavily de...

This paper is concerned with the interaction of a two-dimensional plane jet with transverse plane acoustic waves, which occur, for example, in flue instruments in the vicinity of the nozzle exit. The acoustic excitation is modeled with fluctuating boundary conditions within the framework of an incompressible simulation. This method can be easily im...

Virtual flow diverter deployment techniques underwent significant development during the last couple of years. Each existing technique displays advantageous features, as well as significant limitations. One common drawback is the lack of quantitative validation of the mechanics of the device. In the following work we present a new spring‐mass‐based...

Purpose Intracranial aneurysms are malformations forming bulges on the walls of brain arteries. A flow diverter device is a fine braided wire structure used for the endovascular treatment of brain aneurysms. This work presents a rig and a protocol for the measurement of the hydrodynamic resistance of flow diverter stents. Hydrodynamic resistance is...

In this paper, the effect of a special active control on the Blasius boundary layer is investigated with the Orr-Sommerfeld and the Reynolds-Orr energy equations. The control moves the wall in the streamwise direction proportional to the fluctuating wall shear stress which was proposed by Józsa et al. [“Active and passive in-plane wall fluctuations...

A novel technique to accelerate optimization-driven aerodynamic shape design is presented in the paper. The methodology of optimization-driven design is based on the automated evaluation of many similar shapes which are generated according to the output of an optimization algorithm. The vast amount of numerical simulations makes this process slow b...

Purpose Assessing the rupture probability of intracranial aneurysms (IAs) remains challenging. Therefore, hemodynamic simulations are increasingly applied toward supporting physicians during treatment planning. However, due to several assumptions, the clinical acceptance of these methods remains limited. Methods To provide an overview of state-of-...

Aneurysm pathogenesis is thought to be strongly linked with hemodynamical effects. According to our current knowledge, the formation process is initiated by locally disturbed flow conditions. The aim of the current work is to provide a numerical investigation on the role of the flow field at the stage of the initiation, before the aneurysm formatio...

Purpose Advanced morphology analysis and image-based hemodynamic simulations are increasingly used to assess the rupture risk of intracranial aneurysms (IAs). However, the accuracy of those results strongly depends on the quality of the vessel wall segmentation. Methods To evaluate state-of-the-art segmentation approaches, the Multiple Aneurysms A...

The description of the feedback mechanism is one of the crucial points in the understanding of self-sustained flow oscillations, such as the edge tone or the cavity tone phenomenon. A technique is developed by which any incompressible flow can be excited with a known vortex field. An analytical approximation of the vortex field is suggested. As an...

The laminar-turbulent transition is investigated in a boundary layer flow over a coated wall. The coating consists of small elements which can move in streamwise and in some cases additionally in spanwise directions. The elements are mounted to the wall with dampers and springs in both directions. The growth rate of the disturbances is determined w...

Four methods of varying complexity to investigate jet instability were tested on a plane jet and compared with CFD results. A unified treatment that includes the acoustic excitation in an incompressible simulation was developed. It turned out that the more sophisticated methods have no real advantage over the well-established Orr-Sommerfeld and Ray...

Planar jets are widely researched flows. Many aspects of their instability have been well-known for a long time. One of them is the observation that the jet is more sensitive to disturbances near the orifice than elsewhere. This paper aims at giving an explanation to this hitherto unexplained phenomenon. To this end, linear stability investigations...

Computing the emerging flow in blood vessel sections by means of computational fluid dynamics is an often applied practice in hemodynamics research. One particular area for such investigations is related to the cerebral aneurysms, since their formation, pathogenesis and the risk of a potential rupture may be flow-related. We present a study on the...

The CFD simulation of the emergent flow field inside vessel malformations is a generally employed technique during research processes. The results of an accurate numeric calculation might be used for risk assessment or for predictive purposes, such as the analysis of the possible pathogenesis. Even though several generally accepted numerical method...

The purpose of this study was to investigate the effect of spreading the fingers in human swimming. Two-dimensional CFD simulations were carried out to gain insight into this problem, modelling the fingers by equidistant cylinders. One, two and then four cylinders were studied with varying distance. The effect of the thumb was neglected. The drag c...

A simplified automobile door gap model, defined at the Third Computational Aeroacoustics (CAA) Workshop on Benchmark Problems (Category 6) was investigated. After a thorough mesh study compressible and incompressible simulations were carried out and various turbulence models were tried. The influence of three dimensional effects and boundary layer...

A retrospective modeling was performed in a case with restenosis after 6 months of a DES implantation to investigate the effect of the step-down at the distal edge of the stent on the wall shear stress (WSS) using 3D coronary angiography reconstruction and the intracoronary pressure traces during a cardiac cycle. The kinetics of the WSS was calcula...

Abdominal aneurysms are pathological lesions on our largest artery, the abdominal aorta. These mal-formations, amongst other unfavourable physiological effects, carry a severe risk of rupture. Applying stent grafts in these arteries as part of the medical treatment is a common practice to exclude weakened arterial wall sections from the blood flow....

Planar jets belong to the most researched flows. Many aspects of their instability have been well-known for a long while. One of them is the observation that the jet is more sensitive to disturbances near the orifice exit than elsewhere. Linear stability investigations on various velocity profiles were carried out using the Orr-Sommerfeld (OS) equa...

The cavity-tone is one of the fundamental aeroacoustic flow configurations. In the past, cavities have been investigated especially at high flow speeds (high subsonic to supersonic) as e.g., occurring in military aircrafts. Nowadays, much research focuses on cavities at low speed as occurring in automobiles and trains. The investigated cavity is a...

The nonlinear dynamics of an acoustically excited spherical gas bubble in water is being investigated numerically. The applied model to describe the motion of the bubble radius is the Keller–Miksis equation, a second order ordinary differential equation, which takes into account the compressibility of the liquid. During the radial oscillations of t...

The behaviour of biological fluid flows is often investigated in medical practice to draw conclusions on the physiological or pathological conditions of the considered organs. One area where such investigations are proven to be useful is the flow-related formation and growth of different pathologic malformations of the cerebro vascular system. In t...

Thrombus formation is a thoroughly researched area, posing several unanswered questions. Quite many of the underlying processes are still not well understood. This uncertainty arises from the fact that the blood clotting mechanism in our bodies involves a rather complex reaction cascade with plenty of components. These underlying dynamic processes...

Flow diverters (FDs) have been successfully applied in the recent decade to the treatment of intracranial aneurysms by impairing the communication between the flows in the parent artery and the aneurysm and, thus, the blood within the aneurysm sac. It would be desirable to have a simple and accurate computational method to follow the changes in the...

Cerebral aneurysms are bulges appearing on brain arteries. The development, growth and rupture has been intensively studied using numerical tools. We present a series of studies covering many of the open questions related to the boundary conditions used in numerical simulations, namely the inlet, outlet and the wall. The effect of boundary conditio...

The unsteady fluid flow inside the complex geometry of an intracranial aneurysm during a cardiac cycle was simulated. The results obtained by several versions of a lattice Boltzmann based code were compared with the results of PIV and LDA measurements and with those of a commercial finite volume solver. The best version from the tested lattice Bolt...

Free oscillations of a single spherical gas bubble in glycerol have been examined numerically and experimentally at different ambient temperatures and pressures. The bubble was generated using a Q-switched Nd:YAG laser and the unsteady radius measurement was based on a shadowing technique of a He–Ne laser beam. The measurements were compared to com...

Resource requirements for the calculation of sound production in a planar flow are often unnecessarily high due to a full, three dimensional computational fluid dynamics (CFD) simulation. However, planar flows can be simulated in two dimensions thus reducing the time and memory requirement of the CFD part of the coupled aeroacoustic computation. A...

The present paper shows numerical simulations of the flow responsible for the sound generation in an organ pipe. Only the foot model of the organ pipe (i.e., with the resonator detached) is investigated by two-dimensional incompressible CFD simulations. It is shown that in spite of the moderately high Reynolds number (Re≈2350) no turbulence modelin...

Endovascular treatment of intracranial aneurysms is a routine medical practice. The most widely used technique is the packing the aneurysm sac with an embolic material. To gain deeper understanding in the effects of specific treatment methods, the intra-aneurysmal haemodynamics are studied with the help of patient-specific computational models. Num...

Incompressible flow computations have been performed on a cavity with a length to depth ratio of 2. Depending on the Reynolds number, three qualitatively different unsteady flow regimes have been identified. In the first (regular) regime the second Rossiter mode dominates the spectrum, in the second (transitional) regime two incommensurable frequen...

The edge tone flow is one of the most simple aero-acoustic flow formulation. Despite it has been researched for several decades it is not yet fully understood. The flow consists of a planar free jet and a wedge shaped object placed oppositely to the jet. Under certain circumstances the jet starts to oscillate as it impinges on the wedge. The jet ca...