György Paál

György Paál
Budapest University of Technology and Economics · Department of Hydrodinamic Systems

PhD

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70
Publications
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Introduction
Skills and Expertise

Publications

Publications (70)
Article
Full-text available
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...
Article
Full-text available
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...
Article
Full-text available
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...
Preprint
Full-text available
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...
Article
Full-text available
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...
Article
Full-text available
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...
Article
Full-text available
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...
Preprint
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...
Preprint
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...
Preprint
Full-text available
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...
Article
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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...
Article
Full-text available
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...
Article
Full-text available
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...
Article
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...
Article
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...
Article
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-...
Article
Full-text available
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...
Article
Full-text available
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...
Conference Paper
Full-text available
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...
Conference Paper
Full-text available
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...
Article
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...
Article
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...
Chapter
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...
Conference Paper
Full-text available
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...
Article
Full-text available
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...
Article
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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...
Article
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...
Conference Paper
Full-text available
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....
Conference Paper
Full-text available
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...
Conference Paper
Full-text available
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...
Article
Full-text available
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...
Article
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...
Article
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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...
Article
Full-text available
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...
Article
Full-text available
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...
Article
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...
Article
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...
Article
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...
Article
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...
Article
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...
Article
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...
Article
Full-text available
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...
Article
Full-text available
Cerebral aneurysms are preferentially located at arterial curvatures and bifurcations that are exposed to major hemodynamic forces, increasingly implicated in the life cycle of aneurysms. By observing the natural history of aneurysm formation from its preaneurysm state, we aimed to examine the hemodynamic microenvironment related to aneurysm initia...
Article
The initiation, growth and rupture of intracranial aneurysms are intensively studied by computational fluid dynamics. To gain confidence in the results of numerical simulations, validation of the results is necessary. To this end the unsteady flow was measured in a silicone phantom of a realistic intracranial aneurysm. A flow circuit was built with...
Article
A novel method is presented to simulate densely woven stents used in the parent artery reconstruction of aneurysms appearing on the large arteries of the brain. Using computational fluid dynamics in combination with measurements the effects of stenting on the hemodynamic processes are studied. The hydraulic properties of the applied stents are firs...
Article
To understand the acoustics of the flowinduced noise, it is essential to reveal the attributes of the flow which generates the noise. This paper throws lite on some very interesting nonlinear phenomena of the well known yet not fully understood low Mach number edge tone flow. The flow can take qualitatively different looks. Called the stages. Highe...
Article
Computational one way coupled 2D simulations were performed on the cavity tone. First a thorough physical parameter study was made focusing only on the flow. The fluid part of the simulation was performed by ANSYS 11.0 commercial software. Qualitatively different ranges were obtained both at the velocity parameter study and the cavity length study....
Article
Extensive experimental and computational work was performed to characterize the edge tone phenomenon. The dependence of the oscillating frequency on some of the main parameters of the configuration including the nozzle-wedge distance and the profile and the mean velocity of the jet was measured. The frequency was obtained from the FFT of a pressure...
Article
Full-text available
This study was performed to assess the effect of aneurysm geometry on parameters that may have an impact on the natural history of intracranial aneurysms, such as intraaneurysmal flow pressure and shear stress. Flow was simulated in 21 randomly selected aneurysms using finite volume modeling. Ten aneurysms were classified as side-wall aneurysms, wi...
Article
Numerical flow simulations have been performed on simplified artificial intracranial aneurysm models, as well as on real models obtained from rotational digital subtraction angiography images. The simplified geometries provide a basic understanding of the flow and allow the study of the influence of certain geometrical parameters with high accuracy...
Article
Despite its geometrical simplicity, the edge tone displays a remarkably complex behaviour. A plane jet oscillates around the wedge-shaped object with a relatively stable frequency and under certain circumstances emits an audible tone. This configuration plays a central role in the sound production of several wind instruments but occurs in industria...
Article
The reduction of noise in our everyday life requires the understanding of its generation and propagation. Aeroacoustic sources are among the most complicated noise sources and their simulation still presents a challenge. In this paper recent results of the developed numerical aero-acoustic simulation algorithm are presented. A commercial CFD code w...
Article
Temporally resolved schlieren flow visualization and Laser-Doppler Anemometry (LDA) measurements have been performed on real flue organ pipes during the stationary state as well as during the attack transient. The techniques prove highly efficient in providing information on the details of the flow. The measurements reveal several details of the fl...
Article
Full-text available
An extensive numerical parametric investigation of the turbulent flow in 90° T- junctions with sharp and rounded corners is under way aimed at quantifying the energy losses and the size and strength of the recirculating region in the branch pipe. The parameter that is being investigated is the radius of curvature of the corner of the junction and s...
Article
Full-text available
This paper focuses on the analysis of the haemodynamic pattern and biophysical properties of cerebral aneurysms, diagnosed and delineated in living human individuals. An aneurysm is a bulging out of part of the wall of a blood vessel. The aim of the research is to delineate flow patterns inside the aneurysm and its parent artery, to estimate stress...
Article
Full-text available
High precision unsteady numerical simulations have been carried out on the well-known low Mach number edge tone configuration playing a central role in many wind instruments. Detailed parametric investigations have been performed (velocity, inlet profile, jet-edge distance). Several interesting physical phenomena known from experimental investigati...
Article
Full-text available
Predictions of the turbulent flow in a 90° tee junction were carried ou t with the commercial code CFX 5.5 and compared with experimental results. Two turbulence models were used in the numerical simulations: a full Reynolds s tress model (RSM) and a k- model (SST). The simulations predicted well the pressure drop in the tee junction and the mean v...
Article
The influence of the wall vibrations on the timbre of flue organ pipes is one of the most disputed questions of organ building. Wall vibrations of metal and wooden labial pipes have been investigated by using miniature piezoelectric accelerometers and optical noncontact vibrometers. Both free and forced vibrations have been studied. Two sets of dia...
Conference Paper
Diesel fuel was injected through a pintle nozzle into quiescent ambient air and the transient characteristics of the spray were examined as a function of injection pump speed. The laser-based techniques characterized the spray in terms of its transient structure, tip penetration, droplet axial mean and rms velocities and average droplet size. The r...
Article
A kutatás az élhang (edge tone) áramlási konfigurációra koncentrált. Az élhang egyszerű rendszer, ami egy sík szabadsugárból, és egy azzal szembe fordított ékből áll, és mégis meglepően komplex viselkedést mutat. Stacionárius peremfeltételek mellett is nemlineáris, instacionárius jelenségeket produkál. A szabadsugár többé-kevésbé periodikusan leng...
Article
Koponyaűri aneurysmák áramlástani elemzését végeztük idealizált 2D és 3D geometriai modelleken valamint valós aneurysmákról készült rotációs angiográfia 3D rekonstrukciójából készült véges elemes modelleken, számítógépes áramlás szimulációval. Idealizált modelleken összefüggést találtunk az aneurysma bemeneti nyílásának ("nyak") átmérője illetve te...
Article
Szabályozó szelep fojtóelemének nyílásában megfigyelt kavitációs zóna és CFD számítás eredményeinek összehasonlítása. A BME Hidrodinamikai Rendszerek Tanszék laboratóriumában lévő kavitációs csatorna újjáépítése. A mérőszakasz Re-száma eléria 800000-et. Négyzetes hasáb Reynolds, Strouhal, Thoma-féle kavitációs száma és a zónahossz mérése és a függv...

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Projects (2)
Project
https://www.mdpi.com/journal/jpm/special_issues/Invasive_Coronary_Artery This Special Issue intends to cover the following important aspects: Coronary physiology and coronary flow, indices characterizing epicardial coronary arteries and microcirculation; New methodologies for the assessment of epicardial or microvascular physiology; Clinical application of coronary physiology modalities in specific patient populations, including ACS, CCS, MINOCA, CAD with comorbidities, COVID-19 cases and complications, and post-COVID-19 and long-COVID-19 syndromes; Personalized treatment strategies, guided by invasive physiologic assessment; Image-based physiological assessment.