Unsteady Airflow Research Articles

Calculation of the speed characteristics of an upward swirling gas flow under conditions of side wind

The paper presents the results of numerical modeling of lateral wind effects on three-dimensional unsteady air flows in an upward swirling stream of an artificially created tornado in a stationary mode of operation. The mathematical model is the complete system of Navier-Stokes equations taking into account the viscosity and thermal conductivity of a moving gas, as well as the action of gravity and Coriolis. Using an explicit difference scheme and an appropriate choice of initial and boundary conditions, solutions are obtained numerically for the complete system of Navier-Stokes equations in the computational domain in the form of a rectangular parallelepiped. All components of the gas flow velocity were calculated at fixed time instants and
 instantaneous streamlines were constructed with constant horizontal wind direction taken into account. The calculations showed that the result of the wind effect on the upward swirling flow is an asymmetric change in peripheral speed, its uneven
 deformation in opposite sections, and the total displacement of the vortex in the direction of the wind. In addition, the displacement and curvature of the vertical part of the vortex in the direction of the wind was established, a “detachment” of some instantaneous streamlines from the vertical rotating part and the appearance in the center of the vortex of an area that is free of streamlines increasing in diameter were recorded. In the studied time variation range, the stability of the air vortex to wind action and the stable operation of the used computational scheme are observed.

Direct Measurements of the Volume Flow Rate and Emissions in a Large Naturally Ventilated Building.

The direct measurement of emissions from naturally ventilated dairy barns is challenging due to their large openings and the turbulent and unsteady airflow at the inlets and outlets. The aim of this study was to quantify the impacts of the number and positions of sensors on the estimation of volume flow rate and emissions. High resolution measurements of a naturally ventilated scaled building model in an atmospheric boundary layer wind tunnel were done. Tracer gas was released inside the model and measured at the outlet area, using a fast flame ionization detector (FFID). Additionally, the normal velocity on the area was measured using laser Doppler anemometry (LDA). In total, for a matrix of 65 × 4 sensor positions, the mean normal velocities and the mean concentrations were measured and used to calculate the volume flow rate and the emissions. This dataset was used as a reference to assess the accuracy while systematically reducing the number of sensors and varying the positions of them. The results showed systematic errors in the emission estimation up to +97%, when measurements of concentration and velocity were done at one constant height. This error could be lowered under 5%, when the concentrations were measured as a vertical composite sample.

Calculation of the thermodynamic characteristics of an upward swirling gas flow under conditions of side wind

The paper presents the results of numerical modeling of lateral wind effects on three-dimensional unsteady air flows in an upward swirling stream of an artificially created tornado after entering a stationary mode. Under the corresponding initial and boundary conditions, the solutions of the complete system of Navier-Stokes equations in the computational domain in the form of a rectangular parallelepiped are numerically obtained using the explicit difference scheme. At fixed points in time, the gas-dynamic characteristics of air flows are calculated under the action of gravity and Coriolis, as well as taking into account the constant horizontal directional wind speed. Numerical calculations showed that the resulting density, temperature, and pressure gradients lead to significant and asymmetric changes in air flows at different points in time and at different points in the computational domain. The total displacement of the vortex in the direction of the wind, the deformation and increase in the diameter of the vortex, which begin in the bottom part and pass into its vertical part, are established. In the considered range of time changes, the stability of the air vortex under study against wind action and the stable operation of the computational scheme used are noted.

Dynamics of aerosol generation and release – Dry powder inhaler performance considerations

The main objective of the work was to investigate how the dynamics of inhalation affect aerosol release process in real-time, with an unsteady airflow through passive dry powder inhalers (DPIs) of comparable airflow resistance, but different principle of powder release. It consists of two complementary studies, concerning dynamics of controlled aerosol release (emission kinetics) and dynamics of aerosol generation (aerosol size characteristics in time series).Aerosol behavior of the powder was studied experimentally at the outlet of the inhalers: RS01 inhaler (Plastiape Spa, Osnago LC, Lombardy, Italy) and Turbospin (Ph&T, Milano, Lombardy, Italy). Two breathing profiles were analyzed throughout the study. Timing of aerosol emission was done by light scattering. Aerosol characterization was done by laser diffraction. It resulted in time evolution of volume concentration and volumetric particle size distribution.Both the design of the passive DPI inhaler and the dynamics of the inhalation (along with its intensification) influence the course of the aerosol generation and emission. The process of aerosol emission occurs only in the initial part of the inhalation process, before the peak inspiratory flow rate is established. This part is strongly responsible for the quality of the aerosol emitted from the device and consequently will potentially influence the drug delivery.

Linear Vibration of the Rotary Plate Under Combined Excitations in Subsonic Airflow

Due to strong nonlinear, unsteady characteristics and the fluid–structure interaction effect, vibration analysis of blades under the excitation of the airflow is still one of the technical difficulties. In this paper, the accurate subsonic aerodynamic force is obtained through numerical simulation, and the aerodynamic coupling model of the rotary blade is established. The distribution of the aerodynamic force of the compressor blade under the unsteady airflow is focused on. The blade is modeled as presetting a presetting pre-twisted rotary cantilever plate. Dynamic frequencies of the plate, calculated by Chebyshev–Ritz method, are compared with frequencies calculated using the finite element method (FEM). Effects of different parameters on natural frequencies of the rotary plate are discussed. Based on von-Karman nonlinear geometric relation and the first-order shear deformation theory, nonlinear dynamic equations of the pre-twisted rotary plate under the combination of the centrifugal force and the aerodynamic are derived by utilizing Hamilton’s principle. Second-order ordinary differential equations are derived by applying the Galerkin method. Analytical solution of the dynamic deformation of the plate is presented and is compared with that produced by FEM. Results indicate the accuracy of the explicit presentation of the aerodynamic of the low-pressure compressor blade. Effects of the rotary speed, the thickness, the pre-twisted angle and the presetting angle on vibration characteristics of the warping blade are studied. Mode shape shift and frequency loci veering are discussed.

Study on the train-induced unsteady airflow in a metro tunnel with multi-trains

Super-long metro tunnels, which allow two trains to simultaneously drive in the same tunnel direction, have become increasingly important in urban areas where construction is occurring at a rapid pace. The tunnel environment is significantly affected by the phenomenon of piston wind. Based on the unsteady flow theory of Bernoulli equation, the piston effect of metro tunnel caused by single train and multiple trains passing through the tunnel was mathematically modeled in this study. The accuracy and feasibility of the proposed theoretical method is verified by establishing a theoretical model based on a highly cited small-scale experimental study. Based on the verified theoretical method, the influence of various factors on the development law of the piston wind is analysed in detail. The results indicate that the maximum piston wind speed, vmax2, generated by double trains is greater than the maximum piston wind speed, vmax1, generated by a single train, and vmax2 is approximately 1.24 times the value of vmax1. According to the fitting results of five parameters (i.e., the train speed, train length, tunnel length, blockage rate, and frictional resistance coefficient) on the maximum piston wind speed, a unified formula for predicting the maximum piston wind speed with the consideration of various influencing factors is proposed.

Piloted Flight Simulation of Helicopter Recovery to the Queen Elizabeth Class Aircraft Carrier

This Paper describes how flight simulation has been used to investigate helicopter recovery operations to the deck of HMS Queen Elizabeth, the United Kingdom’s new aircraft carrier. A helicopter flight simulation environment has been developed in which the unsteady airflow over the ship has been created using full-scale computational fluid dynamics. A six-degree-of-freedom motion flight simulator has been used to conduct real-time piloted deck landings where a helicopter flight dynamics model representative of a Sikorsky SH-60B Seahawk helicopter was recovered to the designated rotorcraft landing spots toward the stern of the ship. A test pilot was instructed to land the helicopter and to give workload ratings for the difficulty of the task when flying in relative winds from Ahead, and 45 deg and 90 deg from starboard. The workload ratings, along with the corresponding pilot control activity and helicopter positional accuracy, are discussed in relation to the airflow to which the helicopter was subjected. The Paper demonstrates how flight simulation could be used to support flight trials and helicopter clearance activities but also notes that real-world trials data are needed to compare with the simulations before the techniques can be beneficially deployed.

Investigation on the stability of natural convection in an annular cavity with non-isothermal walls

The onset and nature of unsteady natural convection in a tall, closed annular cavity with non-isothermal walls is investigated experimentally. In the first of a sequence of experiments, thermal boundary conditions imposed on the inner and outer cylinders promote steady bi-cellular natural convection airflow inside the cavity. As the buoyant potential of the upper convection cell is increased, steady flow inside the cavity first bifurcates into mono-periodic flow and then follows a quasi-periodic route to chaotic flow. Similarly, when the buoyant potential of the lower convection cell is increased, steady flow bifurcates into mono-periodic flow and then follows an intermittent route to chaos. All of the observed unsteady airflows inside the annular cavity were three-dimensional, with all of the periodic flows exhibiting a gentle swirling motion in the azimuthal direction.

Numerical investigation of the unsteady coupling airflow impact of a full-scale warship with a helicopter during shipboard landing

In this paper, a comprehensive computational modeling study of the unsteady aerodynamic environment around a warship with a helicopter is performed. An experimental validation exercise is also conducted, comparing computational fluid dynamics (CFD) results of the airwake calculated for a reduced-scale model of the isolated Landing Helicopter Assault (LHA) model with high-quality particle image velocimetry experimental data provided by the NASA AMES Research Center. Comparisons of the results generally obtain agreement, indicating that the CFD numerical method is able to resolve the large-scale turbulent airflow. Building on this, a numerical simulation of a real Robin helicopter, immersed in the unsteady airwakes of a full-scale Amphibious Assault Ship (AAS), is performed. The aerodynamic simulation of the influence on the coupled airflow of warship–helicopter is explored and compared with that of the solitary ship airflow field and the superposition airwakes, where the vortex patterns and pressure on the ship surface, as well as the velocity distribution, are circumvented. As a further step, dynamic landing analysis of the airflow field for a shipborne helicopter is implemented at an important location through the landing path for headwind. The aerodynamic characteristics of a helicopter during a flight deck landing are also explored for the unsteady ship airwakes impacting on rotor force during shipboard landings. In addition, different shipboard landing paths of the helicopter are comparatively investigated for obtaining an optimal landing path decision. The present study demonstrates an effective aerodynamic analysis and robust numerical approach, which creates a solid foundation supporting further alternative evaluations of ship airflow fields.

Transient numerical simulation of airflow and fibrous particles in a human upper airway model

In this study, the unsteady airflow field in a realistic model of human upper airways during the breathing cycle and the fibrous particle transport and deposition were investigated using the CFD method. An anatomically realistic model of airway passage including vestibule to the end of the trachea was constructed from the CT images of a 24-year-old healthy woman. Several user-defined functions (UDFs) were developed and incorporated into the discrete phase model (DPM) of ANSYS Fluent code and were used for the evaluation of ellipsoidal particle motion in transient airflows. The developed UDF was used to solve the coupled translational and rotational equations of motion of ellipsoidal fibers and to analyze their dispersion and deposition in the upper airways. The total and regional depositions for a range of fiber sizes were evaluated. The transient particle deposition fraction was compared with those obtained for the equivalent steady flow condition. In addition, the fraction of ellipsoidal fibers that penetrate the lower respiratory tract under cyclic breathing was calculated and compared with the steady flow simulation. The presented results showed that the steady simulation with an appropriate equivalent airflow rate can predict the total fibrous particle deposition during cyclic breathing with reasonable accuracy. The steady simulations, however, cannot properly predict the regional particle deposition under transient flows. In particular, the comparison between steady and transient penetration fraction reveals that the steady breathing model fails to accurately predict the penetration fraction of ellipsoidal fibers into the lower respiratory tract.

Topological analysis of particle transport in lung airways: Predicting particle source and destination

Particle transport in lung airways can induce respiratory disease and play a vital role in aerosol drug delivery. Herein, we present dynamical systems features that influence airflow and particle transport in the tracheobronchial trees. Computational fluid dynamics (CFD) was used to solve for unsteady airflow in a patient-specific model. Particle tracking simulations were performed for micron-size particles. The destination map that connects the particle final location to the initial location and injection time was constructed. Finite-time Lyapunov exponent (FTLE) fields were calculated to identify inertial Lagrangian coherent structures (ILCS), topological features that act as separatrices. Our results demonstrated that these topological features control the destination map at the trachea. The temporal evolution of ILCS influenced the sensitivity of particle transport fate to injection time, whereas the emergence of new ILCS with an increased integration time controlled transport to different generations of airways. Additionally, particles starting at the ILCS were shown to mostly deposit at the airway walls. Finally, an innovative source inversion strategy was introduced to integrate the Maxey–Riley equation backward in time and identify the origin of dispersed particles. Our study explores novel dynamical systems tools that improve our understanding of particle transport and deposition in the airways and could be used to guide future targeted drug delivery studies.

Universal modeling of unsteady airflow in different hoistway structures and analysis of the effect of ventilation hole parameters

Accounting for the strong flow field in a hoistway during the operation of an ultra-high-speed elevator and reasonable ventilation in engineering practice, installation of ventilation holes (VH) is important. First, based on Bernoulli’s principle for unsteady flow, the unsteady airflow in hoistways without VH, with one pair of VH, and with two pairs of VH were modeled theoretically. Based on these three models, a universal theoretical model of unsteady airflow in a hoistway with multiple VH was deduced. The validity of the theoretical model was verified by comparing the theoretical velocity coefficients with those obtained experimentally in an elevator test tower. Finally, the influence of different VH parameters on the airflow were analyzed. The results showed that the weakening trend of the airflow movement in the hoistway decreased gradually when the cross-sectional areas of the VH increased. The change in the VH length had little effect on the airflow movement in the hoistway, and the change in the opening position of the VH had a significant effect on the airflow in the hoistway. With the increase in the number of VH with the same cross-sectional areas, the weakening trend of the airflow in the hoistway was gradually reduced.

Evaluating the influence of unsteady air density to the aerodynamic performance of a fixed wing aircraft at different angle of attack using computational fluid dynamics

Abstract Recent studies have focused on unsteady aerodynamics of an aircraft using computational fluid dynamics. It was found that a change in the density of air within the flight domain influences the aerodynamic performance of aircrafts in flight. This is as a result of the occurrence of turbulence and unsteadiness in the air velocity vectors. Limited work has focused on the impact of transient change in the density of air to lift, drag and moment coefficient acting on the aircraft fuselage during flight. This paper is aimed at determining how the unsteadiness in the density of air influences the aerodynamic performance of an aircraft with time. To determine the aerodynamic performance of an aircraft within this unsteady airflow condition, lift, drag coefficient and the wing design efficiency was used as the deterministic parameter for the investigation. A compressible simulation was performed for transonic flows with Mach number 0.84. The wing geometry was designed using SOLIDWORKS and the CFD simulation was performed using ANSYS Fluent version 18.1 software. A User Defined Function (UDF) was developed using a MATLAB code. This was aimed at, to introduce a time-dependent change in the density of air, in the Fluent environment. Results obtained showed that a 0.24 average change in air density caused a drop in lift coefficient by an amount 0.01014. The wing design efficiency achieved in this study was 56% and this value is low. Therefore, a change in the density of air will cause the aircraft to observe a poor aerodynamic behavior and eventually stall.

Experimental characterization of the impact of unsteady airflows on tracer gas measurement

Natural ventilation induces several issues when trying to measure airflows. Low pressure differences that drive natural airflows are very sensitive. A small pressure drop introduced by an airflow-meter would significantly affect the flow pattern. Furthermore, for window aired buildings or rooms, the implementation of a protocol to characterize the flow through a window is practically hard to implement and often poorly reliable. For these reasons, tracer gas methods are widely used to measure natural airflows. They do not interfere with the flow path, as they do not require any other instruments than concentration sensors. The principle is to inject a tracer gas inside a chamber and to observe the evolution of the concentration of the tracer gas. The knowledge of the emission rate of the gas, as well as its concentration evolution allows describing the airflow.Unfortunately, tracer gas methods are highly sensitive to the measurement noise, which is likely to be significant for concentration sensors. Most used methods, in the literature, use regression techniques to reduce the measurement noise. However, the regression implies steady airflows during the measurement, which should theoretically discard natural airflows. For lack of better methods, these techniques are often used in natural conditions, regardless of the variation of airflows. The present paper experimentally assess the impact of variable airflows on the accuracy of most common tracer gas methods, namely the constant injection and the concentration decay methods. The experiment was implemented in a laboratory cell, taking advantage of a controllable extract fan to simulate natural airflows, while allowing the direct measurement of the airflow in the extract duct. This value provides the reference value for the comparison of tracer gas methods results. Insights are also given, using signal-processing techniques to improve the reliability of tracer gas methods under variable airflows.

Analysis of unsteady airflow in a subway station influenced by train movement

The piston effect has a significant effect on ventilation systems in subway stations. In this article, experimental data and computational fluid dynamics (CFD) methods are used to examine the behavior of the unsteady airflow generated by train movement. The experimental data recorded in an actual station in the Tehran subway and a full-scale model were used for the numerical simulation. The simulation was conducted using layering method to simulate the train movement through the tunnels and station according to the actual subway train velocity trend. Air curtains were used to reduce the undesired influence of the piston effect in the station and increase the efficiency of the ventilation system. The results indicated that maximum amplitude of air velocity at the subway entrance caused by the train movement can be reduced 67% by using an air curtain on the subway platform.

Study on Amplitude and Flatness Characteristics of Elastic Thin Strip under Fluid–Structure Interaction Vibration Excited by Unsteady Airflow

Based on unsteady airflow excitation and elastic thin strip vibration theory, a SI-FLAT flatness meter was taken as the research object, and an amplitude–residual stress simulation analysis model of the cold rolling strip under aerodynamic loads was established by using ANSYS Workbench. First, the influences of fluid–structure interaction on the strip amplitude distribution and the flatness calculation deviation were analyzed. It was found that the analysis with fluid–structure interaction matched the actual measurement of the flatness meter better. Then, the influences of different aerodynamic loads and tensions on the strip midpoint amplitude and the flatness calculation deviation were analyzed. It was found that when alternating aerodynamic loads increased, the strip amplitude increased in the form of a quadratic polynomial. However, when the tensions decreased, the strip amplitude decreased exponentially. The strip dimensions also influenced the amplitude of vibration: The wider and thinner the strip, the larger the amplitude. Finally, the influences of different flatness defects on the strip amplitude distribution and the flatness calculation deviation were analyzed. The deviation was serious on the strip edge, and the fluctuation characteristics of the deviation were opposite to those of the initial flatness defects.

Corona Poling Conditions for Barium Titanate/Epoxy Composites and their Unsteady Wind Energy Harvesting Potential

The objective of this paper is to design and fabricate barium titanate (BTO)/epoxy composites, investigate the effect of poling conditions on their piezoelectric properties, and obtain their unsteady energy harvesting potential. The poling condition test deals with the effect of the poling conditions on the piezoelectricity of BTO/epoxy composites. The specimens are polarized by corona poling under different conditions, and their piezoelectric coefficient d33 is measured. The distribution of the piezoelectric particles in the matrix is studied using scanning electron microscopy (SEM), and the microstructure of the particle is investigated using X‐ray diffraction (XRD). A wind harvesting test is then performed, and the output voltage is measured for the composite sheets under unsteady air flow. This study opens the way for the development of flexible energy harvesting devices.

Theoretical modeling and sensitivity analysis of the car-induced unsteady airflow in super high-speed elevator

The high-speed airflow generated by super high-speed elevators during operation causes significant aerodynamic noise, and research on the airflow characteristics in hoistways is necessary to effectively attenuate this noise. At present, the research on the airflow characteristics of super high-speed elevators lacks a systematic theoretical analysis of the entire elevator movement process. Therefore, the goal of this study was to develop a theoretical model of the entire process and conduct a parameter impact analysis of the car-induced airflow. First, Bernoulli's principle for unsteady flow is used to theoretically model the car-induced unsteady airflow, after which the validity of the model is verified by comparing the velocity coefficients of the theoretical results and the test data. Second, based on the theoretical model, the effects of four key parameters on the car-induced airflow and hoistway ventilation rate are analyzed, and a universal predictive formula for the hoistway ventilation rate accounting for the four parameters is proposed. Finally, the model of super high-speed elevators is simplified, theoretical modeling is carried out based on Bernoulli's principle for unsteady flow, and the influence of the ventilating holes at different positions on the car-induced airflow is analyzed.

A numerical study of the train-induced unsteady airflow in a tunnel and its effects on the performance of jet fans

Analyzing the airflow generated by a moving train in tunnels ventilated by jet fans is of significant importance because of the piston effects and the alternating pressure and velocity fields inside the tunnel. Moreover, high blockage ratio and train velocity in these tunnels have caused serious concerns about the impact of train-induced airflow on the performance of jet fans. This study aims at simulating the train movement in such tunnels and investigating the above-mentioned effects. To this purpose, a finite-volume numerical approach with dynamic mesh technique is used to perform several simulations of a large tunnel with eight rows of jet fans and different train velocities. The results demonstrate that for the case in which the train moves in the opposite direction of the jet fans’ flow, their volume flow rates increase and decrease sharply, as the train head and tail reach the jet fans, respectively. However, the trend is opposite when the train travels along the direction of jet fans flow. From an electric perspective, it is shown that the generated flow field does not increase the motor torque and the current of the jet fans to a degree that causes the motors to shut down.

Ventilation for subway stations with adjustable platform doors created by train-induced unsteady airflow

At present, subways (rail transit) are undergoing rapid developments worldwide accompanied by massive energy consumption. Train-induced unsteady airflow (TIUA) generates airflow through the entrances and platform doors, which affects the ventilation and thermal environment of subway stations. Adjustable platform door systems are increasingly being installed in subway stations to reduce the energy consumption by utilising TIUA. In this study, numerical modelling was performed by IDA Tunnel software to analyse the air inflow rate (AIR) through the entrances and adjustable platform doors (APDs). Four key factors, namely, the open and closed systems, bypass ducts, train intervals, and adjustable air vents, were analysed. The results demonstrate that for subway stations with adjustable air vents open, the average AIR through entrances with a closed system was 1.99–4.17 times that of the open system. Moreover, the variations of the average AIR through entrances caused by bypass ducts were less than 9.20%, and turning off the bypass ducts increased the average AIR via the APDs from 35.58% to 65.58%, when the adjustable air vents were open. In addition, the results show that the AIR via the APDs was decreased from 32.48% to 80.45% by shutting the adjustable air vents. Therefore, an innovative energy-saving strategy of environmental control system (ECS) was proposed based on the utilisation of TIUA. A winter case to optimize the ventilation control modes of TIUA is discussed. Compared with the worst mode, the heating requirements of subway stations with optimal ventilation control modes of TIUA can be reduced by 62.85%.