### Browsing by Author "Korhonen, Marko"

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Item The association between chronotype and wages at mid-age(Elsevier, 2023-08) Conlin, Andrew; Nerg, Iiro; Ala-Mursula, Leena; Räihä, Tapio; Korhonen, Marko; Department of Finance; University of OuluSleep has been shown to affect economic outcomes, including wages. The mechanisms by which sleep affects wages remain unclear. We examine the relationship between chronotype – morning larks, evening owls – and wages at mid-age. We propose a novel model relating chronotype to wages in consideration of human, social, and health capital constructs. Empirically, we explore the effects of chronotype mediated through life course choices, such as work experience, trust, and health behaviour. The data come from the 46-year-old follow-up study of the Northern Finland Birth Cohort (1966) and from registers of the Finnish Tax Administration. We find evening chronotype to have a significant indirect negative effect on wages, which occurs through accumulating less work experience and through poor health outcomes. The effect is largest for male workers, with a total indirect effect on average wages of − 4%. We also provide evidence that chronotype has a long-term association with wages between 29 and 50 years of age. We conclude that evening-type workers are less suited to typical working hours and accumulate less human, social and health capital which in turn negatively affects their wages. Our findings are of great socio-economic importance because evening chronotypes make up a significant part of the population.Item Constriction Flow of Cellulose Laden Air-Aqueous Foam(NORTH CAROLINA STATE UNIV DEPT WOOD & PAPER SCI, 2019) Viitanen, Leevi; Halonen, Alisa; Fristrom, Eira; Koivisto, Juha; Korhonen, Marko; Puisto, Antti; Alava, Mikko; Department of Applied Physics; Department of Computer Science; Complex Systems and Materials; Department of Applied Physics; Department of Computer ScienceFoams are encountered in everyday life across wide applications, e.g., in foods and cleaning products. They have also been widely used in different industries in processes such as flotation and oil recovery. The application of bio-based materials is a novel interest, and foam forming enables these materials to be used more flexibly. For efficient industrial usage, the flow of such materials must be well understood and characterized. This work measured the velocity field of nanocellulose laden foam in a two-dimensional Hele-Shaw cell with a constriction, using optical imaging and particle image velocimetry. The measurements showed that the addition of cellulose increased the effective viscosity of the liquid films. In a numerical simulation using the Giesekus polymer model, the experimental trend was reproduced through increasing the fluid's viscosity. Adding highly viscoelastic nanofibrillated cellulose suspension to foam affected only the viscous component of the foam. The delayed elastic response did not change.Item Development of flexible mould for polyurethane cushion components(2001) Korhonen, Marko; Konetekniikan osasto; Teknillinen korkeakoulu; Helsinki University of Technology; Ekman, KaleviTyön tavoitteena oli tutkia superplastisesti muovattavan levyn soveltuvuutta polyuretaanipehmusteiden valmistamisessa käytettäviin muotteihin. Toisena tavoitteena oli tutkia MDF:stä valmistettujen mallien käyttämistä superplastisessa muovauksessa. Työn kirjallisuusosassa tarkastellaan polyuretaanipehmusteiden valmistamista ja työvälineitä sekä superplastisten levyosien muovausmenetelmiä ja ominaisuuksia. Työn kirjallisuusosassa on myös käsitelty kehitetyn muotin rakennetta ja toimintaperiaatetta. Työn aikana kehitettiin ja valmistettiin koemuotti, jolla voitiin testata muuttuva muotti -idean toimivuutta. Työn kokeellisessa osassa esitetään koemuotilla tehtyjä muovauskokeita ja niiden tuloksia. Tulosten perusteella voidaan päätellä muottiratkaisun toimivan superplastisen muovauksen osalta. Lisäksi muovauskokeissa käytetyn mallin todettiin soveltuvan superplastiseen muovaukseen. Varsinaisia pehmusteiden valamiskokeita ei ehditty tämän työn aikana tekemään, joten lopullisia johtopäätöksiä muottirakenteen soveltumisesta itse tuotantoon ei voi vielä tehdä.Item Friction controls even submerged granular flows(ROYAL SOC CHEMISTRY, 2017-09-15) Koivisto, Juha; Korhonen, Marko; Alava, Mikko; Ortiz, Carlos P.; Durian, Douglas J.; Puisto, Antti; Department of Applied Physics; Complex Systems and Materials; University of PennsylvaniaWe investigate the coupling between the interstitial medium and granular particles by studying the hopper flow of dry and submerged systems experimentally and numerically. In accordance with earlier studies, we find that the dry hopper empties at a constant rate. However, in the submerged system we observe the surging of the flow rate. We model both systems using the discrete element method, which we couple with computational fluid dynamics in the case of a submerged hopper. We are able to match the simulations and the experiments with good accuracy by fitting the particle–particle contact friction for each system separately. Submerging the hopper changes the particle–particle contact friction from µvacuum = 0.15 to µsub = 0.13, while all the other simulation parameters remain the same.Item A GPU-accelerated computational fluid dynamics solver for assessing shear-driven indoor airflow and virus transmission by scale-resolved simulations(Elsevier Science B.V., 2024-06) Korhonen, Marko; Laitinen, Alpo; Isitman, Gizem Ersavas; Jimenez, Jose L.; Vuorinen, Ville; Department of Energy and Mechanical Engineering; Energy Conversion and Systems; University of Colorado BoulderWe explore the applicability of MATLAB for 3D computational fluid dynamics (CFD) of shear-driven indoor airflows. A new scale-resolving, large-eddy simulation (LES) solver titled DNSLABIB is proposed for MATLAB utilizing graphics processing units (GPUs). In DNSLABIB, the finite difference method is applied for the convection and diffusion terms while a Poisson equation solver based on the fast Fourier transform (FFT) is employed for the pressure. The immersed boundary method (IBM) for Cartesian grids is proposed to model solid walls and objects, doorways, and air ducts by binary masking of the solid/fluid domains. The solver is validated in two canonical reference cases and against experimental data. Then, we demonstrate the validity of DNSLABIB in a room geometry by comparing the results against another CFD software (OpenFOAM). Next, we demonstrate the solver performance in several isothermal indoor ventilation configurations and the implications of the results are discussed in the context of airborne transmission of COVID-19. The novel numerical findings using the new CFD solver are as follows. First, a linear scaling of DNSLABIB is demonstrated and a speed-up by a factor of 3-4 is also noted in comparison to similar OpenFOAM simulations. Second, ventilation in three different indoor geometries are studied at both low (0.1 m/s) and high (1 m/s) airflow rates corresponding to Re=5000 and Re=50000. An analysis of the indoor CO2 concentration is carried out as the room is emptied from stale, high CO2 content air. We estimate the air changes per hour (ACH) values for three different room geometries and show that the numerical estimates from 3D CFD simulations differ by 80%–150% (Re=50000) and 75%–140% (Re=5000) from the theoretical ACH value based on the perfect mixing assumption. Third, the analysis of the CO2 probability distributions (PDFs) indicates a relatively non-uniform distribution of fresh air indoors. Fourth, utilizing a time-dependent Wells-Riley analysis, an example is provided on the growth of the cumulative infection risk which is shown to reduce rapidly after the ventilation is started. The average infection risk is shown to reduce by a factor of 2 for lower ventilation rates (ACH=3.4-6.3) and 10 for the higher ventilation rates (ACH=37-64). Finally, we utilize the new solver to comment on respiratory particle transport indoors. A key contribution of the paper is to provide an efficient, GPU compatible CFD solver environment enabling scale-resolved simulations (LES/DNS) of airflow in large indoor geometries on a single GPU designed for high-performance computing. The demonstrated efficacy of MATLAB for GPU computing indicates a high potential of DNSLABIB for various future developments on airflow prediction.Item Large-eddy simulation of buoyant airflow in an airborne pathogen transmission scenario(Elsevier BV, 2023-08-01) Laitinen, Alpo; Korhonen, Marko; Keskinen, Karri; Kaario, Ossi; Vuorinen, Ville; Department of Energy and Mechanical Engineering; Energy Conversion and SystemsIndoor airflow patterns and the spreading of respiratory air were studied using the large-eddy simulation (LES) computational fluid dynamics (CFD) approach. A large model room with mixing ventilation was investigated. The model setup was motivated by super-spreading of the SARS-CoV-2 virus with a particular focus on a known choir practice setup where one singer infected all the other choir members. The room was heated with radiators at two opposite walls in the cold winter time. The singers produced further heat generating buoyancy in the room. The Reynolds number of the inflow air jets was set to Re=2750, corresponding to an air-changes-per-hour (ACH) value of approximately 3.5. The CFD solver was first validated after which a thorough grid convergence study was performed for the full numerical model room with heat sources. The simulations were then executed over a time of t=20 min to account for slightly more than one air change timescale for three model cases: (1) full setup with heat sources (radiators+singers) in the winter scenario, (2) setup without radiators in a summer scenario, and (3) theoretical setup without buoyancy (uniform temperature). The main findings of the paper are as follows. First, the buoyant flow structures were noted to be significant. This was observed by comparing cases 1/2 with case 3. Second, the dispersion of the respiratory aerosol concentration, modeled as a passive scalar, was noted to be significantly affected by the buoyant flow structures in cases 1–2. In particular, the aerosol cloud was noted to either span the whole room (cases 1–2) or accumulate in the vicinity of the infected singer (case 3). Turbulence was clearly promoted by the interaction of the upward/downward moving warmer/cooler air currents which significantly affected the dispersion of the respiratory aerosols in the room. The study highlights the benefits of high-resolution, unsteady airflow modeling (e.g. LES) for interior design which may consequently also impact predictions on exposure to potentially infectious respiratory aerosols.Item Modeling structure-flow coupling in complex fluids(Aalto University, 2020) Korhonen, Marko; Puisto, Antti, Dr., Aalto University,Finland; Teknillisen fysiikan laitos; Department of Applied Physics; Complex systems and materials; Perustieteiden korkeakoulu; School of Science; Alava, Mikko, Prof., Aalto University, Department of Applied Physics ,FinlandComplex fluids encompass a wide variety of natural and manufactured substances which are utilized in ever larger quantities as our technical understanding of them develops. Landslides, foams, micellar solutions, polymer melts and granular suspensions all display the characteristic non-Newtonian behavior typical of complex fluids, which implies that their resistance to imposed flow (viscosity) does not remain constant over time and/or the intensity of the flow. This non-Newtonian behavior can be largely attributed to the internal, structure forming constituents and their interactions. Under imposed flow,this structure responds to the shear, for instance, by breaking up or forming new aggregates, which can macroscopically manifest itself as variations in viscosity. Modeling such complex fluid structure and its coupling to flow dynamics is accomplished here both by a phenomenological coarse-grained kinetic model and a diffusion model coupled to flow mechanics via the Navier-Stokes equations in the continuum limit. These models are explored then in the context of shear banding, localized shear and dewatering events involving a complex fluid. Additionally, submerged granular flows are described employing multi-scale modeling involving interactions at the microscopic level using the Discrete Element Method (DEM) and the macroscopic multiphase Navier-Stokes equations in the continuum limit. The results concerning shear banding and shear localization indicate the importance of the finite fluid inertia, occasionally neglected in rheological modeling, that is present in e.g. start-up flows and Large Amplitude Oscillatory Shears (LAOS). Contrary to the assumptions utilized in the literature, this inertia is sufficient to trigger significant shear localization and shear banding without any elastic response considerations, which is distinguishable due to a viscosity contrast between congested and fluidized regions in the sample. Similar regions are also observed in dewatering events of fibrous suspensions and an optimized dewatering scheme is developed based on homogenizing the highly uneven viscosity profile by pressure pulsing. Finally, the issue of a surging flow rate reported in submerged granular hopper flows, lacking a theoretical explanation at the time this thesis is written, is also displayed to result from similar transitions, mediated by the force chains formed in the structure of the granular material.Item Modeling the orientation of elongated particles in channel flows(2021-03-16) Haapalehto, Matias; Puisto, Antti; Korhonen, Marko; Perustieteiden korkeakoulu; Alava, MikkoCellulose nanofibrils (CNF) are a promising new material to produce bio-compatible fibers with advantageous mechanical characteristics. Recently, remarkably tough CNF fibers have been experimentally prepared by controlling the alignment of the nanofibrils in an extrusion capillary. In this Thesis, the flow of the CNF suspension is computationally simulated using OpenFOAM implementing the Fokker-Planck orientation model. In the Fokker-Planck model, CNF behavior is represented in probabilistic manner using a probability distribution function (PDF). The present OpenFOAM simulation is based on (a) a model implemented previously by Michael Mohtaschemi, which solves the Fokker-Planck orientation model in simple shear, and (b) the standard OpenFOAM icoFoam which solves the incompressible Navier-Stokes equation using the PISO algorithm. Validation is performed by comparing the results of the new simulation with the original model. Agreement between the two models is rather good. The simulation is run on a two-dimensional tapered channel representing a section of the extrusion capillary. The resulting PDFs are plotted at three different location in the computational mesh. The results show that the degree of alignment of the CNF was roughly proportional to the shear rate. In addition, physical fields, such as velocity, kinematic pressure, magnitude of the strain rate tensor and effective kinematic viscosity are plotted. The effective viscosity profile shows two regions of maximal viscosity spanning from the edges of the inlet to the center of the outlet of the channel. Stability conditions are presented which impose bounds for the PDF discretization and time step parameters. Stability considerations introduce feasibility issues due to an increase in computational cost. To improve the feasibility of the application, a hybrid parallel programming model is implemented and tested.Item Modelling aerosol transport and virus exposure with numerical simulations in relation to SARS-CoV-2 transmission by inhalation indoors(Elsevier, 2020-10) Vuorinen, Ville; Aarnio, Mia; Alava, Mikko; Alopaeus, Ville; Atanasova, Nina; Auvinen, Mikko; Balasubramanian, Nallannan; Bordbar, Hadi; Erästö, Panu; Grande, Rafael; Hayward, Nick; Hellsten, Antti; Hostikka, Simo; Hokkanen, Jyrki; Kaario, Ossi; Karvinen, Aku; Kivistö, Ilkka; Korhonen, Marko; Kosonen, Risto; Kuusela, Janne; Lestinen, Sami; Laurila, Erkki; Nieminen, Heikki J.; Peltonen, Petteri; Pokki, Juho; Puisto, Antti; Råback, Peter; Salmenjoki, Henri; Sironen, Tarja; Österberg, Monika; Department of Energy and Mechanical Engineering; Department of Applied Physics; Department of Chemical and Metallurgical Engineering; Department of Neuroscience and Biomedical Engineering; Department of Civil Engineering; Department of Information and Service Management; Department of Bioproducts and Biosystems; Department of Electrical Engineering and Automation; Energy Conversion; Complex Systems and Materials; Chemical engineering; Structures – Structural Engineering, Mechanics and Computation; Biohybrid Materials; Energy efficiency and systems; Hydrometallurgy and Corrosion; Bioproduct Chemistry; Finnish Meteorological Institute; CSC - IT Center for Science Ltd.; VTT Technical Research Centre of Finland; Mikkeli Central Hospital; Hydrometallurgy and Corrosion; University of HelsinkiWe provide research findings on the physics of aerosol and droplet dispersion relevant to the hypothesized aerosol transmission of SARS-CoV-2 during the current pandemic. We utilize physics-based modeling at different levels of complexity, along with previous literature on coronaviruses, to investigate the possibility of airborne transmission. The previous literature, our 0D-3D simulations by various physics-based models, and theoretical calculations, indicate that the typical size range of speech and cough originated droplets (d⩽20μm) allows lingering in the air for O(1h) so that they could be inhaled. Consistent with the previous literature, numerical evidence on the rapid drying process of even large droplets, up to sizes O(100μm), into droplet nuclei/aerosols is provided. Based on the literature and the public media sources, we provide evidence that the individuals, who have been tested positive on COVID-19, could have been exposed to aerosols/droplet nuclei by inhaling them in significant numbers e.g. O(100). By 3D scale-resolving computational fluid dynamics (CFD) simulations, we give various examples on the transport and dilution of aerosols (d⩽20μm) over distances O(10m) in generic environments. We study susceptible and infected individuals in generic public places by Monte-Carlo modelling. The developed model takes into account the locally varying aerosol concentration levels which the susceptible accumulate via inhalation. The introduced concept, ’exposure time’ to virus containing aerosols is proposed to complement the traditional ’safety distance’ thinking. We show that the exposure time to inhale O(100) aerosols could range from O(1s) to O(1min) or even to O(1h) depending on the situation. The Monte-Carlo simulations, along with the theory, provide clear quantitative insight to the exposure time in different public indoor environments.Item Modelling aerosol-based exposure to SARSCoV-2 by an agent based Monte Carlo method: Risk estimates in a shop and bar(Public Library of Science, 2021-11-22) Salmenjoki, Henri; Korhonen, Marko; Puisto, Antti; Vuorinen, Ville; Alava, Mikko J.; Department of Applied Physics; Department of Energy and Mechanical Engineering; Complex Systems and Materials; Energy ConversionPresent day risk assessment on the spreading of airborne viruses is often based on the classical Wells-Riley model assuming immediate mixing of the aerosol into the studied environment. Here, we improve on this approach and the underlying assumptions by modeling the space-time dependency of the aerosol concentration via a transport equation with a dynamic source term introduced by the infected individual(s). In the present agent-based methodology, we study the viral aerosol inhalation exposure risk in two scenarios including a low/high risk scenario of a “supermarket”/“bar”. The model takes into account typical behavioral patterns for determining the rules of motion for the agents. We solve a diffusion model for aerosol concentration in the prescribed environments in order to account for local exposure to aerosol inhalation. We assess the infection risk using the Wells-Riley model formula using a space-time dependent aerosol concentration. The results are compared against the classical Wells-Riley model. The results indicate features that explain individual cases of high risk with repeated sampling of a heterogeneous environment occupied by non-equilibrium concentration clouds. An example is the relative frequency of cases that might be called superspreading events depending on the model parameters. A simple interpretation is that averages of infection risk are often misleading. They also point out and explain the qualitative and quantitative difference between the two cases—shopping is typically safer for a single individual person.Item Models for fibre aggregation and orientation in nanocellulose suspensions(2014-06-27) Korhonen, Marko; Mohtaschemi, Mikael; Perustieteiden korkeakoulu; Alava, MikkoItem Multiphase CFD modeling of front propagation in a Hele-Shaw cell featuring a localized constriction(American Physical Society, 2021-08-17) Mac Intyre, Jonatan; Puisto, Antti; Korhonen, Marko; Alava, Mikko; Ortín, Jordi; Department of Applied Physics; Complex Systems and Materials; Universitat de BarcelonaWe study a liquid-gas front propagation in a modulated Hele-Shaw cell by means of multiphase computational fluid mechanics based on the three-dimensional Navier-Stokes equations. In the simulations an obstacle that partially fills the gap is placed at the center of the cell, and the liquid-gas interface is driven at a constant velocity. We study the morphological differences between imbibition and drainage for a wide range of capillary numbers, and explore how the wetting properties of the constriction affect the amount of liquid that remains trapped in the draining process. We observe increasing remaining volumes with increasing capillary number and decreasing contact angle. The present CFD implementation for a single mesa defect provides insight into a wide number of practical applications.Item Shear localization in large amplitude oscillatory shear (LAOS) flows of particulate suspensions(American Physical Society, 2021-03-15) Korhonen, Marko; Wallgren, Kristian; Puisto, Antti; Alava, Mikko; Vuorinen, Ville; Department of Applied Physics; Department of Energy and Mechanical Engineering; Complex Systems and Materials; Energy Conversion; Department of Applied PhysicsStrong shear localization effects are observed in large amplitude oscillatory shear (LAOS) simulations of a particulate suspension. Here, the structural response of this complex fluid is completely viscous and governed by the general shear-driven diffusion model by Phillips et al. [Phys. Fluids 4, 30 (1992)10.1063/1.858498]. When coupled to oscillatory shear in LAOS, this model is shown to produce concentration gradients, which imply the existence of regions of disparate viscosities across the simulated measurement gap. This suggests the presence of strong shear localization which is conceived even though the intrinsic flow curve of the model is monotonic, and the simulated geometry is a planar Couette setup, expected to display simple shear flow characteristics. This shear localization is generated due to the oscillatory shear at the shearing plate, which, therefore, induces accelerating motion. The subsequent inertial effects act as perturbations in the nonlinear response of the fluid structure to shear and are sufficient to trigger significant localization in the flow. Due to the ubiquitous nature of shear-driven diffusive mechanisms in complex fluids, these results suggest shear localization to be an integral feature of a LAOS measurement of many complex fluids.Item Start-up inertia as an origin for heterogeneous flow(2017-02-23) Korhonen, Marko; Mohtaschemi, Mikael; Puisto, Antti; Illa Tortos, Xavier; Alava, Mikko J.; Department of Applied Physics; Complex Systems and MaterialsFor quite some time nonmonotonic flow curve was thought to be a requirement for shear banded flows in complex fluids. Thus, in simple yield stress fluids shear banding was considered to be absent. Recent spatially resolved rheological experiments have found simple yield stress fluids to exhibit shear banded flow profiles. One proposed mechanism for the initiation of such transient shear banding process has been a small stress heterogeneity rising from the experimental device geometry. Here, using computational fluid dynamics methods, we show that transient shear banding can be initialized even under homogeneous stress conditions by the fluid start-up inertia, and that such mechanism indeed is present in realistic experimental conditions.Item Superspreading of SARS-CoV-2 at a choir rehearsal in Finland—A computational fluid dynamics view on aerosol transmission and patient interviews(Public Library of Science, 2024-09) Matvejeff, Anna Tuhkuri; Laitinen, Alpo; Korhonen, Marko; Oksanen, Lotta Maria; Geneid, Ahmed; Sanmark, Enni; Vuorinen, Ville; Department of Energy and Mechanical Engineering; Energy Conversion and Systems; University of HelsinkiIntroduction COVID-19 pandemic has highlighted the role of aerosol transmission and the importance of superspreading events. We analyzed a choir rehearsal in November 2020, where all participants, except one who had recently earlier recovered from COVID-19, were infected. We explore the risk factors for severe disease in this event and model the aerosol dispersion in the rehearsal room. Materials and methods Characteristics of participants were collected by interviews and supplemented with patient records. A computational simulation of aerosol distribution in the rehearsal room and the efficacy of potential safety measures was conducted using the Large-Eddy Simulation approach. Infection risk was studied by analyzing quanta emission and exposure with the Wells-Riley equation. Results The simulation showed that airborne transmission likely explains this mass contagion event. Every singer was exposed to the virus in only 5 min from the beginning of the rehearsal, and maximum concentration levels were reached at 20 min the concentration levels started to approach a steady state after 20 min. Although concentration differences existed in the room, risk levels near (1 m) and far (5 m) from the aerosol source were similar for certain singers. Modeling indicated infection risk levels of 70–100% after one hour; the risk would have been considerably reduced by wearing high-filtration respirators. Age and pre-existing comorbidities predicted more severe disease. The high incidence of illness may be partly attributed to the relatively high median age of individuals. Additionally, those admitted to the hospital had multiple underlying health conditions that predispose them to more severe disease. Conclusions Airborne transmission and indoor space can explain this mass exposure event. High-filtration respirators could have prevented some infections. The importance of safety distances diminishes the longer the indoor event. The concept of safety distance is challenging, as our study suggests that long range airborne transmission may occur in indoor events with extended duration. We encourage informing the public, especially persons at risk, of safety measures during epidemics.Item Theoretical modeling of granular-fluid hopper flows(2017-11-07) Korhonen, Marko; Puisto, Antti; Perustieteiden korkeakoulu; Alava, MikkoSands, slurries and powders represent systems that consist of granular particles and are ubiquitous both in nature and in the industrial environment. Their flow behavior is of particular interest, since these materials are able to both bear external stress as solids and flow like fluids. These attributes are particularly conspicuous in a hopper flow, which is regularly encountered in industrial settings, where granular media is stored and tranported in these hoppers/silos. In this type of flow, the orifice of a hopper filled with granular particles is opened, and the granular particles exit the hopper via the orifice under the stress imposed by gravitation. As a result, the hopper empties. Theoretically, these flows have been primarily modeled in dry conditions, and typically, any interactions between the granular particles and the surrounding air are ignored. In such a setting, the Beverloo equation has great predictive power over the granular discharge rate. However, in recent experimental work involving hoppers submerged in a liquid, the Beverloo equation is unable to account for the observed results. Therefore, this work aims to provide theoretical results matching the ones produced in these experiments and offer a coherent description of the surge (acceleration) of the granular discharge rate that is retrieved in the experimental work. The approach adopted here is to model the submerged, filled hopper as a two-phase system, in which the liquid phase is modeled on a continuum level using Computational Fluid Dynamics (CFD), while the granular phase is described as discrete particles by the Discrete Element Method (DEM). By modifying and applying a fluid-particle interaction term to the underlying equations in these methods, the experimental results are well matched by the simulations. Additionally, it is demonstrated that the surge is a result of an effective pumping induced by the exiting granular particles, which is a direct consequence of the condition imposed by the first Navier-Stokes equation on the flow fields.