Recent Volume
Vol13 , No 6 (in press)
Blood Flow in Prefractal Media: Rheological Model Approaches and Sensitivity Analysis
Pages : 1675-1682
Authors : A. Miguel, 
The development of microfluidic media supporting blood flow is significant for many applications. Prefractal models have considerable potential for contributing to the study of flow in these media, since information about scale complexity is captured by a small number of parameters. Flows of power law fluids, Bingham fluids and described by the marginal zone theory are considered. In this study, physically based models for estimating the permeability of a microfluidic porous materials are presented. Models are derived assuming that media are represented by a bundle of tortuous capillary tubes with fractal pore-size distributions. They are expressed in terms of porosity, microstructural parameters and fluid characteristics. Expressions for the flow resistance through single tortuous tubes, and the relationship between fluid velocity through tortuous tubes and through straight tubes, in terms of fractal dimensions, are also obtained.

Self-starting Simulation of a Hypersonic Inlet with Variable Free Stream Condition
Pages : 1683-1691
Authors : Jie WU,  An-yuan Yu,  Yuan-yuan He,  Jia-ling Le, 
The flow structure transition of a hypersonic inlet from unstart to start during accelerating trajectory is studied by numerical simulation. The results of pressure distribution along the inlet wall, mass flow rate, total pressure recovery coefficient and aerodynamic forces simulated by the quasi-steady and unsteady two dimensional and three-dimensional quasi-steady methods are compared. Analysis indicates that the two-dimensional quasi-steady and unsteady simulations can get the consistent inlet self-starting Mach number, and unsteady simulation can capture the periodical change of aerodynamic forces while this phenomena is not found in quasi-steady simulation. Due to the consistency between the quasi-steady and unsteady in self-starting Mach number in our strategy, for the self-starting process of hypersonic inlet with variable free stream condition, three-dimensional quasi-steady method can be used to approximate the real self-starting process during the acceleration of the aircraft climb, which can greatly save the computational time and improve simulation efficiency.

Numerical Investigation of the Influence of Axial Overlap of Blade Angle Slots on an Axial Flow Compressor Stability
Pages : 1693-1705
Authors : Haoguang Zhang,  Wenhao Liu,  Enhao Wang,  W. L. Chu,  Jingjing Yang,  Weiguang Zhao, 
In this paper, a numerical investigation was conducted on a subsonic compressor rotor with blade angle slot casing treatments. The purpose of the investigation is to reveal the influence of axial overlap of blade angle slot on the compressor stability. Six kinds of blade angle slot casing treatments with different axial overlap rates were investigated in this paper. The results show that with the increasing of axial overlap rates, the stall margin improvement firstly increased and then decreased. And the optimal blade angle slot can obtain 62.51% improvement of stall margin. The flow field analyses show that the bleeding flows formed inside slots can restrain the adverse tip leakage flow, which is the critical factor making the onset of the rotor stall. With the increasing of axial overlap rate of the slots, the relative position between bleeding flows in the slots and tip leakage flow plays an important role in the stall margin improvement.

Electrical Characteristics and Electrohydrodynamic Flows in Electrostatic Precipitator of Six Shaped Discharge Electrodes
Pages : 1707-1718
Authors : Heng Shen,  Hongwei Jia,  Yanming Kang, 
The electrical characteristics and electrohydrodynamic (EHD) flows in a wire-plate electrostatic precipitator with six shaped discharge electrodes are analyzed by employing the commercial software ANSYS FLUENT with the aid of User Defined Function (UDF). The results show that the corona position of the discharge electrode plays important roles in generating space charge distribution. When the inlet velocity is relatively low, the vortexes induced by the secondary flow exist not only in the downstream of the discharge electrode, but also in the vicinity of the collecting plate. The vortex near the collecting surface in the Knife-shaped system produces the highest recirculation velocity and turbulence intensity, and covers the widest region among the six configurations. For the high inlet velocity, the local recirculation and high turbulence by the secondary flow near the collecting plate in the all six channels disappear but still remain in the downstream of the discharge electrodes, and the airflow in the central region of the six channels would be accelerated. The highest vortex strength and turbulence intensity and the strongest speed-up effect occur in the center of the channel with the Knife-shaped discharge electrode compared with the cases of the other five systems.

Effect of Circumferential Casing Treatment on Low-Speed Contra-Rotating Fans
Pages : 1719-1726
Authors : Martin Heinrich,  Hossein Khaleghi,  Christian Friebe, 
The benefits of circumferential groove casing treatment on the performance of a low-speed contra-rotating fan are investigated. Three-dimensional, time-dependent simulations are carried out with the k-ω SST-SAS hybrid turbulence model using the open-source CFD library OpenFOAM. The numerical model is validated with experimental data from the contra-rotating fan with a smooth casing. This comparison showed a very good agreement. Then, two casing treatment variations are analyzed: 1) circumferential grooves on top of the front rotor, and 2) circumferential grooves on top of the rear rotor. Simulating the performance curve at design speed reveals an increase in pressure rise for casing treatment at the front rotor of up to 4 %. This results from significantly reduced blade pressure fluctuations and weakened blade tip vortices at the front rotor. A weaker tip leakage vortex leads to a less disturbed inflow for the rear rotor and thus pressure fluctuations. In contrast, grooves on top of the rear rotor offer no positive effect since there is no rotor downstream to benefit from reduced fluctuations or weakened tip leakage vortex. Pressure probes downstream of the rear rotor were evaluated using FFT. Grooves reduce the magnitude of blade passing frequencies and their harmonics while partly increasing lower frequencies.

Numerical Simulation of Ventilated Cavitation Evolution with an Insight on How Ventilation Influences Pressure Fluctuation and Cavitation Noise
Pages : 1727-1741
Authors : An YU,  Zhaohui Qian,  Jingjing Ji,  Qinghong Tang,  Huixiang Chen,  Daqing Zhou, 
Cavitation occurred in hydraulic machines can generate severe pressure fluctuations and induce high-pitched noise. Ventilated cavitation (inject air into the cavitating flow) is one of the most effective ways to control cavitation and then alleviate the noise and fluctuations. Thus, the evolution of ventilation cavitation around a NACA0015 hydrofoil was numerically investigated with a modified model. The results indicated that the ventilated cavitation consists of two parts: the attached cavity which attached to the leading edge of the hydrofoil and the detached cavity which detached from the hydrofoil surface. With the air injection increased, the detached cavity becomes larger. Besides, the ventilated cavity evolves periodically along with two opposite vortexes which fall off in turn near the tailing edge of the hydrofoil. Among three ventilation volumes, an air injection of 250 L/min presents the best alleviation on pressure fluctuation induced by cloud cavitation. The acoustic analysis indicated that air injection is an effective way to alleviate the cavitation induced noise. With air injected into the flow, two new types noises induced by the ventilated cavitation has been detected by monitoring points along the upper side and behind of the hydrofoil: the lower frequency noise induced by the waving of attached cavity and the higher frequency noise induced by the shedding of the detached cavity. While with the air injection increased, both of the two types noises increased. The acoustic and dynamic patterns under different air injection conditions are able to provide guidance in engineering application.

Feasibility of Rotor Fault Detection from a Fluid Dynamics Perspective
Pages : 1743-1758
Authors : Shane Robbins,  Stephan Heyns,  Johan Heyns, 
The majority of condition monitoring techniques employed today consider the acquisitioning and analysis of structural responses as a means of profiling machine condition and performing fault detection. Modern research and newer technologies are driving towards non-contact and non-invasive methods for better machine characterisation. Yet current literature lacks investigations into the monitoring and detection of anomalous conditions using fluid dynamic behaviour. If one considers unshrouded rotors which are exposed to a full field of fluid interaction such as helicopter rotors and wind turbines amongst others, such an approach could potentially be beneficial. In this work, time-dependent fluid dynamic data is numerically simulated around a helicopter tail rotor blade using URANS CFD with the Open FOAM software package. Pressures are probed at locations in the field of the rotor and compared to results attained in an experimental investigation where good correlation is seen between the results. A blade is modelled with a seeded fault in the form of a single blade out of plane by 4°. Comparisons are drawn between the blade in its ‘healthy’ and ‘faulty’ configurations. It is observed that the fault can be detected by deviations in the amplitudes of the pressure signals for a single revolution at the probed locations in the field. These deviations manifest as increases in the frequency spectrum at frequencies equivalent to the rotational rate (1 per revolution frequencies). The results described are assessed for their fidelity when the pressure is probed at different locations in the domain of the rotor. Deviations in the pressure profiles over the surface of the blades are also seen for the asymmetric rotor configuration, but may prove too sensitive for practical application.

Radial Basis Function Differential Quadrature for Hydrodynamic Pressure on Dams with Arbitrary Reservoir and Face Shapes Affected by Earthquake
Pages : 1759-1768
Authors : abdollmahdi behroozi,  M. Vaghefi, 
A mesh-free numerical model based on the Radial Basis Function Differential Quadrature is introduced to simulate the hydrodynamic response of the dam-reservoirs-foundation system affected by earthquake acceleration. The governing equation of the hydrodynamic pressure of reservoir-dam system was discretized using the presented model in an arbitrary shape, semi-infinite domain which truncated by different far-end boundary condition. For this purpose, the effects of fluid compressibility and energy absorption at the reservoir boundaries were considered simultaneously. The presented model was used to calculate the hydrodynamic pressure distribution on dam face caused by earthquake acceleration in several practical examples and the obtained results were compared with available well-known analytical solutions. The comparison demonstrates that the accuracy and efficiency of the presented model are quite satisfactory.

Experimental Study on Time Evolution of Shock Wave and Turbulent Boundary Layer Interactions
Pages : 1769-1780
Authors : Xiaoge Lu,  Shihe Yi,  Lin He,  xiaolin liu,  Feng Zhang, 
In this paper, a test system based on the Nano-tracer Planar Laser Scattering (NPLS) technique for studying time evolution of unsteady flow structures was finished. Based on this system, the experimental study on the interactions between the incident shock wave and the turbulent boundary layer of the incoming wall was performed. The experiments were performed in a Mach 3.4 supersonic low-noise wind tunnel at the unit Reynolds number of 6.30 × 106/m-1. For the first time, five frames of temporal-correlated fine structure images of transient flow field with shock wave and the turbulent boundary layer interactions (SWTBLI) were obtained under the experimental conditions, and the spatiotemporal evolution characteristics of the flow structure were analyzed. At the same time, the flow field characteristics of temporal-correlated images were studied when the density boundary layer thickness of incoming turbulent layer is δ1 = 0.55δ, δ2 = 0.72δ, δ3 = 0.87δ respectively. During the development of vortex structure in the boundary layer from turbulent boundary layer to separation bubble, the oscillation interval distribution law of induced shock wave was summarized, and the group velocity of vortex structure development in the boundary layer and the relationship between boundary layer thickness and physical space size growth law of separation bubble under different incoming turbulent boundary layer thicknesses were obtained. The results also show that with the increase of the incoming boundary layer thickness, the group velocity in the development process of vortex structure in the turbulent boundary layer does not change significantly. As the thickness of the boundary layer entering the separation bubble increases, the overall growth height of the separation bubble also increases.

Continuous Wavelet Transform Analysis of Turbulent Wall Jet Evolving Through a Backward Facing Step
Pages : 1781-1793
Authors : Kenza Zaibak,  Nora Nait Bouda,  Samira Biskri,  Fazia Mekideche-Chafa,  Youcef Chibani, 
A continuous wavelet transform (CWT) is used to detect the most important scales governing the dynamics of a turbulent wall jet flow that evolves through a backward facing step. Our particular interest is the region downstream of the step. The fluctuating velocity signals obtained experimentally by a laser Doppler anemometer at different heights from the wall are first analyzed in Fourier space by performing the density energy spectra (PSD). In the recirculation zone, we noticed that the flow loses its equilibrium when we approach the wall. This is obviously due to the complex nature of flow dynamics which exhibits a complex structure with various scales. Then, we applied the CWT with two wavelet functions: the eighth derivative of a Gaussian which is selected on the basis of the wavelet entropy measures and a Morlet wavelet. The first one is used to locate the more energetic structures and the second to detect the dominant frequencies of the high energy structures. It turns out that, in the external zone characterized by the presence of intermittent eddies; most of the energy is concentrated in the large scale structures. In the shear layer, different scales of structures are observed. We can also observe the physical phenomena such as extension or breakup of structures. In addition, the relative wavelet energy is applied to give the energy distribution at each scale. On the other hand, the Morlet wavelet is used in order to monitor the dragging of large structures characterized by a low frequency (large scale) originating from the wall-jet's external region towards the reattachment region. It is shown that the energy of these eddy structures decreases along their dragging.

Flow Field Study of Mixed Compression Supersonic Air Intake with Cowl Ventilation
Pages : 1795-1805
Authors : N. K. Gahlot,  N. K. Singh, 
The present investigation is focused on the effect of cowl porosity on the performance of supersonic mixed compression air intake. Four different cases (namely 4.4 %, 5 %, 5.5 % and 7.2% of total cowl area) of cowl porosity at three contraction ratios of air intake have been studied. The pattern of the cowl porosity (Square shape) is chosen symmetrically along the span in the longitudinal direction from the cowl tip. Commercially available software Ansys is used in the computational studies to solve the RANS equations with the k-ω STD turbulence model. Various performance parameters of supersonic air intake are obtained and discussed. Excess amount of flow spillage appears near the cowl tip, which is responsible for the standing strong bow shock wave just before the throat for the uncontrolled case (Clean Model). The minimum energy losses and starting behavior of supersonic air intake are captured at 7.2 % cowl porosity for the contraction ratio of 1.25, which reveals the overall improvement in the flow physics and performance parameters. An increase of 32.73 % in the total pressure recovery is observed for 7.2 % cowl porosity at design contraction ratio of 1.25. All the simulations are performed at three contraction ratios of 1.22, 1.25 & 1.31.

Study of Non-Newtonian Blood Flow of Jeffrey Fluid in an Elastic Tube
Pages : 1807-1817
Authors : S. Sreenadh,  Baina Sumalatha,  A. N. S. Srinivas, 
Most of the biological ducts which considered to be elastic in nature are layered and possess different fluid properties from that of pumped fluids Best & Taylor (1958). A mathematical model is presented according to the two-layered blood flow in an artery. Such flow demands a two-fluid model with elastic boundary. In addition, biofluids such as blood can be described well using two-fluid models rather than single fluid model. In the present paper, the flow of a Jeffrey fluid in contact with a Newtonian fluid is considered. The expressions for velocity of core and peripheral fluids and the flux flow rate are derived. The effect of the peripheral layer on the fluid motion and pumping characteristics is presented. The core and peripheral fluid velocities along with interface velocity are obtained in terms of inlet, outlet and external pressure; Jeffrey parameter, ratio of viscosity and elastic properties. The results obtained from the present analytical study of flux variation considering elastic properties are in good comparison with the published literature. It is concluded that the elastic parameters significantly affect velocity and flux. Further, it is also found that flux reduces with increase in viscosity ratio. The analysis of the interface velocity on various physical parameters may be useful in understanding the behavior of the blood flow in normal and pathological states.

Passive Flow Control over an Airfoil by Control Rod at Low Reynolds Number
Pages : 1819-1833
Authors : Tahir Durhasan, 
In the present study, the flow control mechanism of SD7062 airfoil by a rod illustrated using Particle image velocimetry (PIV) technique at pre-stall angles of attack at Reynolds number of Re = 30000. The rod was installed on the suction surface of the airfoil at different chordwise locations. Diameter of the rod was normalized with the chord length of the airfoil and three diameter ratios (d / c = 0.017, 0.033 and 0.044) were examined at angles of attack of α = 6°, 8° and 10°. Formation of laminar separation bubble for the baseline airfoil and the effect of rod on the laminar separation bubble were investigated in detail. It is observed that the height of boundary layer was reduced up to 22% by proper rod location and diameter ratio. Moreover, the rod suppressed the unsteady vortices over the suction surface of airfoil significantly. Therefore, the peak magnitudes of turbulent statistics were also decreased up to 30% by the rod.

Experimental Investigation and Numerical Simulation of Gas Flow Through Wastegated Turbine of Gasoline Turbocharger
Pages : 1835-1845
Authors : Seyed Shahabeddin Alaviyoun,  M. Ziabasharhagh,  Mojtaba Farajpoor, 
Turbochargers are generally used in the combustion engine due to their capability to increase the specific power. This paper investigates the performance of the turbocharger turbine, which is mounted in a gasoline engine. Different working points, including close and open wastegate positions, are studied in a steady-state condition. The experimental test of this article has been performed on an engine test cell. The movement of the wastegate linkage in different working conditions of the engine was measured in the test cell. Furthermore, the static pressure was measured at the different positions of the turbine housing. Simulation results show that as the wastegate starts to open, maximum loading happens. However, increasing the waste-gate opening angle will decrease the force, which is caused by the passing gas. It was found that at 2 degrees opening angle of the wastegate, there is an 80 kPa pressure difference between two sides of the wastegate valve. When the wastegate has a small opening angle, the pressure distribution on the flat surface of the valve is not symmetric, which means the gas does not provide a 360° flow distribution around the valve. Moreover, streamlines show that the high-speed flow passing bypass passage disturbs the flow exiting the turbine blades. Results show that the opening of the wastegate flap can reduce the turbine’s power. The 6 degrees opening of the wastegate causes a 30 percent power reduction. Moreover, the simulation results of the turbine’s map show that at the constant mass flow rate, the opening of the wastegate from the closed position to 6 degrees causes the pressure ratio to decrease 26 percent.

Numerical Analysis of Flow Geometry in I-Shaped Viscous Micropumps using LB-IBM
Pages : 1847-1858
Authors : Amir Alimoradi,  seyed ali mirbozorgi, 
In viscous micropumps one of the main reasons for a flow rate reduction is vortices which are located at the top of the rotating rotor. In this paper, we have tried to add proper additional walls in the micropump channel, to eliminate or decrease the size of these vortices. Among the all investigated new models, only one, the I-Shaped micropump with an extra step above the rotor, could reduce the size of the vortices and also increase the outlet flow rate. In this paper, the numerical simulations were conducted by using the Lattice Boltzmann Method and by exploiting the Immersed Boundary method and the Blocking technique in order to overcome the LBM drawbacks. The results show that at the channel height H^*=3.7, this new model can produce a flow rate of 150% more than the normal I-Shaped micropumps. Also, one can tune the maximum produced pressure by adjusting the height of this step and micropump with higher channel height can be much more efficient and usable. In addition, by using this new structure for micropump, the designers can also use bigger channel heights which were not efficient in the original design.

Performance Analysis of Flapping Foil Flow Energy Harvester Mounted on Piezoelectric Transducer using Meshfree Particle Method
Pages : 1859-1872
Authors : Muhammad Jamil,  A. Javed,  Irtiza Ali Shah,  Mohtashim Mansoor,  Asad Hameed,  K. Djidjeli, 
Performance of a semi-active flapping foil flow energy harvester, coupled with a piezoelectric transducer has been analyzed in this work. The airfoil is mounted on a spring, damper and piezoelectric transducer arrangement in its translational mode. External excitation is imparted in pitch mode and system is allowed to oscillate in its translational mode as a result of unsteady fluid forces. A piezoelectric transducer is used as an electrical power converter. Flow around moving airfoil surface is solved on a meshfree nodal cloud using Radial Basis Function in Finite Difference Mode (RBF-FD). Fourth order Runge-Kutta Method is used for time marching solution of solid equations. Before the solution of complex Fluid-Structure Interaction problem, a parametric study is proposed to identify the values of kinematic, mechanical and geometric variables which could offer an improved energy harvesting performance. For this purpose, the problem is modelled as a coupled electromechanical system using Lagrange energy equations. Airfoil lift and pitching moment are formulated through Theodorson’s two dimensional thin-plate model and a parametric analysis is conducted to work out the optimized values of pivot location, pitch amplitude, spring stiffness and damping constant. The subsequent computational analysis resulted in an enhanced performance compared to the potential flow model with an efficiency of up to 27% based on total power extraction through the flow. Higher efficiency is obtained when the pitch axis is located aft of mid chord. However, this setting does not correspond to the maximum power output. Interestingly, power is maximized at much lower efficiency values.

On the Aerodynamic and Acoustic Behavior of Double Outlet Squirrel Cage Fans
Pages : 1873-1883
Authors : Mostafa Mahmoodi,  Nader Montazerin, 
Investigations and observations on fluid flow and performance characteristics (numerically and experimentally) and sound generation (experimentally) of single and double outlet squirrel cage fans are performed in this study. The main objective is to survey the performance of double outlet fans and the effect of two volute tongues as main sound sources. Fan performance and sound experiments are conducted using an in-duct experimental setup. The efficiency and pressure curves show that each outlet channels of the double outlet fan operates similar to a single outlet one. As a criterion for evaluating the sound generation, total sound pressure level (SPL) and the noise component at blade passing frequency (BPF) in the power spectrum are considered. A comparison between the total sound pressure levels of the fans shows that in both of them the BPF noise increases with flow rate, while higher SPL is found for the double outlet one. An exactly-higher velocity jet/wake flow from the rotor in double outlet fan is responsible for the higher BPF noise.

Experimental and Numerical Investigation of the Influence of Leading Edge Tubercles on S823 Airfoil Behavior
Pages : 1885-1899
Authors : Supreeth R,  A Arokkiyaswamy,  Anirudh K,  Pradyumna R Koushik,  Pramod P Kashyap,  Sana Rahmath Kodakeri, 
Investigating the role of leading edge tubercles on the aerodynamic behavior of S823 airfoil tailored for wind turbine applications has been the forefront of the study. The aerodynamic characteristics of S823 airfoil effectuated by leading edge tubercles are ascertained at Reynolds number Re=200000 which is the usual operating range of most of the small-scale wind turbines. Firstly, the study elucidates the numerical investigation of baseline airfoil and later modified airfoils exhibiting different amplitude A and wavelength λ of the sinusoidal leading edge tubercles represented as A07W50, A12W50, and A07W25. The aerodynamic characteristics of the airfoils at Re=200000 and angles-of-attack ranging from 00 to 200 are evaluated numerically through k-ω SST turbulence model using ANSYS FLUENT® software package. A preliminary comparison of the computational data shows that the coefficient of lift Cl of all the modified airfoils was visibly superior to the baseline model across the angles tested. A07W50, A12W50 and A07W25 registered 20.6%, 26.2%, and 8.7% increase in the Cl values as compared to the baseline model. Contrasting to the Cl values, the aerodynamic efficiency Cl/Cd of the baseline model was slightly better but only across the pre-stall regime and later culminated with a sudden hard stall. Promisingly, this type of hard stall was not true for the tubercled models that demonstrated a more gradual and restrained stalling characteristic, thus showcasing superior performance in the post stall envelope that was never observed for the baseline model. Based on the outcomes, A07W50 model that displayed better aerodynamic characteristics was eventually fabricated and experimentally tested for its performance in a low speed wind tunnel. The numerical results of A07W50 were in good agreement with the experimental results. The overall results of the study prove beyond any point of doubt that tubercles indeed aid in improving the aerodynamic characteristics by enhancing the lift Coefficient Cl, rendering soft stalling nature and extending the scope of operation for the airfoil under study. Finally, the study positively confirms that leading edge tubercles very much play a significant role in passively augmenting the fluid dynamic characteristics of S823 airfoil and also qualify them to be a competitive passive flow control device.

Elastic and Viscoelastic Modeling of Cell Deformation in Acoustically Driven Microchannel
Pages : 1901-1909
Authors : Hamid Reza Aghaie,  M. Saghafian,  Davood Saeidi, 
Application of acoustic waves in cell manipulation and cell separation is very usual these days but considering that the acoustic force can cause what kind of changes in cell shape, is a question right now. Under the influence of the ultrasound field in specific circumstances, cell deformation can occur. In order to model this deformation, elastic and shell models are usually used for simulation. In the current study, we present a numerical procedure to investigate the cell deformation based on the viscoelastic model while the cell is exposed to a bulk acoustic wave. Second-order acoustic pressure in the resonance frequency of 8 MHz is applied to cell boundary as an acoustic force and cell deformation is determined by solving the fluid-solid interaction (FSI) physics. Results show that the viscoelastic model predicts the cell deformation closer to experimental data relative to the elastic deformation model. Kelvin, Maxwell and SLS models are used to approximate a viscoelastic behavior. The present study shows that the Kelvin viscoelastic model is more compatible with experimental data compared with previous elastic and other viscoelastic models. By applying the Kelvin model, the root mean square error (RMSE) is obtained about 0.064 at 980kpa pressure amplitude. The effect of stiffness on aspect ratio is also investigated and it’s observed that the cell deformation decreases gradually by increasing Young’s modulus. Results also show that in the cases with stiffness up to the 600 pa in Young’s modulus, there’s a sharp drop in cell deformation.

Analytical and Numerical Investigation of the Airflow in Face Masks used for Protection against COVID-19 Virus –Implications for Mask Design and Usage
Pages : 1911-1923
Authors : Robinson Peric,  Milovan Peric, 
The use of face masks for the general public has been suggested in literature as a means to decrease virus transmission during the global COVID-19 pandemic. However, literature findings indicate that most mask designs do not provide reliable protection. This paper investigates the hypothesis that the impaired protection is mainly due to imperfect fitting of the masks, so that airflow, which contains virus-transporting droplets, can leak through gaps into or out of the mask. The fluid dynamics of face masks are investigated via analytical and numerical computations. The results demonstrate that the flow can be satisfactorily predicted by simplified analytical 1D-flow models, by efficient 2D-flow simulations and by 3D-flow simulations. The present results show that already gap heights larger than 0.1mm can result in the mask not fulfilling FFP2 or FFP3 standards, and for gap heights of ca. 1mm most of the airflow and droplets may pass through the gap. The implications of these findings are discussed and improvements to existing mask designs are suggested.

Receptivity to Freestream Periodic Vorticity Disturbance on a Flat Plate with an Elliptic Leading Edge
Pages : 1925-1935
Authors : S. Izawa,  Hiroko Isawa,  Yu Nishio,  Y. Fukunishi, 
The receptivity of the Blasius boundary layer over a semi-infinite flat plate with an elliptic leading edge and of aspect ratio five was investigated using a direct numerical solution of two-dimensional Navier-Stokes equations. The result of the computation where the slip condition is applied to the fluctuating component of velocity at the wall surface is compared with that of an ordinary computation using a nonslip condition. Another numerical experiment is performed where no vorticity fluctuation is supplied from a freestream while prerecorded values of vorticities at the wall in response to the passage of convecting fluctuations are used as the wall vorticity boundary condition. It is shown that vorticity fluctuations in the boundary layer can be classified according to their wavelengths. Waves with longer wavelengths originate from the freestream, whereas waves with shorter wavelengths close to T-S waves originate from the surface of the plate. In another numerical experiment, the slip boundary condition against the fluctuation component of vorticity is applied to the limited area of the wall surface. The aim of the study is to determine the part of the elliptic leading edge or flat plate that induces vorticity fluctuations, thereby resulting in the creation of T-S waves. The numerical results show that the contribution of vorticity fluctuations originating from the juncture is the most crucial, whereas the vorticities supplied in the elliptic leading-edge surface negatively affect the amplitude of vorticity fluctuations inside the boundary layer. And, the stagnation section did not show positive contribution.