Unsteady Airflow Research Articles

Aerodynamic drag of radiator in unsteady air flow

Nonuniform distribution of velocity of cooling air over the front surface of a radiator increases its drag. The paper analyzes the factors affecting the increase of aerodynamic drag under the influence of unsteady air flow. The degree of drag change depends on aerodynamic properties of radiator core, namely, the curvature of the graph of aerodynamic characteristics of a radiator and nonuniform air velocity field over the front of a radiator. In actual use, increase of drag of a radiator can be more than 20%.

Experimental Research on the Vibration and Noise Characteristics of the Marine Diesel Engine Turbocharger

An experimental study on the spectrum analysis and sound intensity identification of vibration and noise source was carried out. The study result represents that the primary reasons leading to the volute vibration of turbocharger, whose vibration energy focus on the frequency range of 0~8Kz, are the volute internal rotor shaft frequency of compressor and its harmonic vibration excitation and rotating unsteady air flow excitation. The volute radiation noise is primarily produced by the structural vibration caused by tip clearance noise and rotor shaft frequency and its harmonics. The noise energy of the turbocharger distributes at a wide range of 1k~8kHz and the sound power almost increases linearly with its rotating speed, which increases by about 20dB as the rotating speed increased from 50% to rated speed.

A Flapping of Wings

Robot aircraft that fly like birds could open new vistas in maneuverability, if designers can forge a productive partnership with an old enemy: unsteady airflow.

A numerical simulation of train-induced unsteady airflow in a tunnel of Seoul subway

This study has been conducted to investigate numerically the characteristics of train-induced unsteady airflow in a subway tunnel. A three-dimensional numerical model using the dynamic layering method for the moving boundary of a train is applied. The validation of the present study has been carried out against the experimental data obtained by Kim and Kim [1] in a model tunnel. After this, for the geometries of the tunnel and subway train which are very similar to those of the Seoul subway, a three-dimensional unsteady tunnel flow is simulated. The predicted distributions of pressure and air velocity in the tunnel as well as the time series of mass flow rate at natural ventilation ducts reveal that the maximum exhaust mass flow rate of air through the duct occurs just before the frontal face of a train reaches the ventilation duct, while the suction mass flow rate through the duct reaches the maximum value just after the rear face of a train passes the ventilation duct. The results of this study can be utilized as basic data for optimizing the design of tunnel ventilation systems.

Experimental and Numerical Study of High-Altitude Ignition of a Turbojet Combustor

This paper aims at contributing to the methodology used for the numerical prediction of ignition inside a combustion chamber. For this purpose, experiments are carried out in a model combustor with improved optical access. Laser tomography and high-speed video give a first insight into the unsteady airflow and the flame structure. Laser Doppler anemometry is used to measure the gas flow velocity field, and the nonreactive two-phase flow is studied in detail using particle Doppler analysis. The velocity field of the burning spray is measured using particle image velocimetry. Ignition tests are performed to evaluate the minimum global equivalence ratio. This in-depth database is used to validate RANS simulations conducted in parallel using the ONERA computational fluid dynamics (CFD) code CEDRE. The numerical model for transient, spherical kernel ignition, proposed in previous work, has been improved and fully implemented in CEDRE. A first parametric study has been conducted on a basic configuration consisting of three validation cases: a gaseous mixture, a monodisperse spray, and a polydisperse spray. These validation cases are inspired from previous studies found in the literature and give a better understanding of the basic phenomena involved in the first stages of flame propagation. This model is then used in combination with CEDRE to estimate the ignition probability of given spark-plug positions in a more realistic configuration: the MERCATO combustor.

Calculation Domain and Boundary Conditions for Push-Pull Ventilation

CFD simulation is a useful tool for studying. However, in reality there are often complex, unsteady air flow patterns and large geometry domain and complex boundary conditions which are very difficult to totally take into consideration in the simulations. So sometimes we made the calculation domain not the same with geometry domain and simplified the boundary conditions. In this paper, five cases were made to study the calculation domain and boundary conditions for push-pull ventilation. According to the analyses and calculations the walls with windows and door closed setting for wall boundary conditions were not correct. On that basis cracks added, and the boundary conditions were pressure-inlet, and the pressure was zero. The calculation domain was reduced, the result was some different: the tendency was the same, but the difference of specific point values was some big. The new boundaries of the reduce calculation domain were set for pressure-inlet, and the pressure was zero. Under this condition, the cracks could be simplified to wall boundary conditions.

Heave-Pitch Motions of a Platform Flying in Extreme Ground Effect

High-payload platforms flying in extreme-ground-effect represent a promising concept for high-speed amphibious transportation. Their dynamics is governed, to a large extent, by unsteady airflow under the platform. A quasi-one-dimensional model for the underplatform channel-flow is applied to predict heave and pitch motions of a vehicle in the presence of external disturbances, such as nonuniformities on the underlying ground-surface. It is found that a complete nonlinear theory should be used instead of a linear model to accurately predict motions when ground nonuniformities are relatively short. A flat platform with a flap moving in extreme-ground-effect is intrinsically unstable. An aft-wing out of ground-effect can be added to the system to achieve stability. Movable control surfaces, such as a platform flap, can be also modeled by the nonlinear theory. It is demonstrated that an oscillating flap can reduce heave and pitch amplitudes in flight over a wavy ground-surface.

Clarifications of the mechanism of nano-fluctuation of aerostatic thrust bearing with surface restriction

Fluctuations of objects supported by aerostatic bearings are a severe problem for ultraprecise positioning technology. The cause of small fluctuations, called nano-fluctuations of objects supported by the thrust bearing was investigated. An aerostatic thrust bearing with surface restriction by T-shaped grooves was the focus of this study. A computational fluid analysis of the airflow around the bearing was conducted and compared with experimental photographs. In addition, the amount of nano-fluctuations was measured by changing the bearing clearance and inlet pressure of the bearing. The nano-fluctuation was induced by the fluctuation of atmospheric pressure due to the unsteady airflow around the bearing edge, and the amount of the fluctuation depended on the Reynolds number corresponding to the flow velocity.

AirDyn: an instrumented model-scale helicopter for measuring unsteady aerodynamic loading in airwakes

This paper describes the design, calibration and application of an instrument that measures the effects of unsteady air flow (airwake) on a helicopter in flight. The instrument is a 1/54th-scale model helicopter that is mounted on a six-component dynamic force balance to measure the forces and moments that an airwake imposes onto the helicopter; it is therefore an ‘Airwake Dynamometer’ to which we have given the name AirDyn. The AirDyn has been designed, in particular, to measure the effect of a ship airwake on a helicopter translating over the ship's landing deck. The AirDyn, which has been implemented in a water tunnel, in preference to a wind tunnel, senses the integrated effect of a turbulent airwake on the helicopter, and the resulting unsteady forces and moments are an indication of the workload the pilot would need to exert to counteract these effects in a real helicopter. Binocular sensing elements and semiconductor strain gauges have been adopted to achieve high sensitivity and relatively high stiffness. The compact strain gauge balance is fitted into the helicopter fuselage, and protective coatings and a flexible bellows are used to seal the balance and protect it from the water. The coefficient matrix of the AirDyn has been obtained by static calibrations, while impulse excitation tests have confirmed that its frequency response is suitable for the measurements of unsteady loads. The application of the instrument is illustrated by using it to quantify the effect that a bulky ship mast has on the airwake and thus on a helicopter as it lands onto a simplified ship in a scaled 50 knot headwind.

RESPONSE OF METAL CORE PIEZOELECTRIC FIBERS TO UNSTEADY AIRFLOWS

In the previous study, possible applications of metal core piezoelectric fibers with a diameter of 200 to 250 µm as bionic airflow sensors mimicking the flow sensitive receptor hairs of crickets have been proposed. This study aims to investigate the dynamic responses of the metal core piezoelectric fibers to unsteady airflow. The metal core piezoelectric fiber is half coated on the outer surface and is used in the bending mode. Wind tunnel tests were carried out and the output voltage of the fiber under the excitation of the unsteady aerodynamic force during flow acceleration and deceleration was measured when the wind tunnel was suddenly closed or opened by a shutter. The relationship between the maximum voltage and the steady-state velocity and that between the voltage and the acceleration of flow were also obtained.

A numerical study of the train-induced unsteady airflow in a subway tunnel with natural ventilation ducts using the dynamic layering method

The objective of this study is to investigate numerically the characteristics of train-induced unsteady airflow in a subway tunnel with natural ventilation ducts. A three-dimensional numerical model using the dynamic layering method for the moving boundary of a train is first developed, and then it is validated against the model tunnel experimental data. With the tunnel and subway train geometries in the numerical model exactly the same as those in the model tunnel experimental test, but with the ventilation ducts being connected to the tunnel ceiling and a barrier placed at the tunnel outlet, the three-dimensional train-induced unsteady tunnel flows are numerically simulated. The computed distributions of the pressure and the air velocity in the tunnel as well as the time series of the mass flow rate at the ventilation ducts reveal the impact of the train motion on the exhaust and suction of the air through ventilation ducts and the effects of a barrier placed at the tunnel outlet on the duct ventilation performance. As the train approaches a ventilation duct, the air is pushed out of the tunnel through the duct. As the train passes the ventilation duct, the exhaust flow in the duct is changed rapidly to the suction flow. After the train passes the duct, the suction mass flow rate at the duct decreases with time since the air pressure at the opening of the duct is gradually recovered with time. A drastic change in the mass flow rate at a ventilation duct while a train passes the corresponding ventilation duct, causes a change in the exhaust mass flow rate at other ventilation ducts. Also, when a barrier is placed at the tunnel outlet, the air volume discharge rate at each ventilation duct is greatly increased, i.e., the barrier placed at the tunnel outlet can improve remarkably the ventilation performance through each duct.

Aerodynamic vibrations of a maglev vehicle running on flexible guideways under oncoming wind actions

This paper intends to present a computational framework of aerodynamic analysis for a maglev (magnetically levitated) vehicle traveling over flexible guideways under oncoming wind loads. The guideway unit is simulated as a series of simple beams with identical span and the maglev vehicle as a rigid car body supported by levitation forces. To carry out the interaction dynamics of maglev vehicle/guideway system, this study adopts an onboard PID (proportional-integral-derivative) controller based on Ziegler–Nicholas (Z–N) method to control the levitation forces. Interaction of wind with high-speed train is a complicated situation arising from unsteady airflow around the train. In this study, the oncoming wind loads acting on the running maglev vehicle are generated in temporal/spatial domain using digital simulation techniques that can account for the moving effect of vehicle's speed and the spatial correlation of stochastic airflow velocity field. Considering the motion-dependent nature of levitation forces and the non-conservative characteristics of turbulent airflows, an iterative approach is used to compute the interaction response of the maglev vehicle/guideway coupling system under wind actions. For the purpose of numerical simulation, this paper employs Galerkin's method to convert the governing equations containing a maglev vehicle into a set of differential equations in generalized systems, and then solve the two sets of differential equations using an iterative approach with the Newmark method. From the present investigation, the aerodynamic forces may result in a significant amplification on acceleration amplitude of the running maglev vehicle at higher speeds. For this problem, a PID+LQR (linear quadratic regulator) controller is proposed to reduce the vehicle's acceleration response for the ride comfort of passengers.

Natural convection simulations and numerical determination of critical tilt angles for a parallel plate channel

Abstract Three dimensional laminar unsteady natural convection flow of incompressible air between two inclined parallel plates is analyzed by using spectral methods. The channel that is periodical in streamwise and spanwise directions is heated from below. The motivation behind the study is to perform simulations for a domain representing a bottom heated, open ended and inclined enclosure that appears mostly in solar energy applications. The governing equations are solved using a pseudospectral solver in order to obtain velocity and temperature fields in the channel, accurately. Fourier series and Chebyshev polynomial expansions are used for the variables. The standard Boussinesq approximation is used to include density variation. Modified Adams–Bashforth/Crank–Nicolson semi-implicit time stepping is used for stability. By using the obtained temperature distribution between the plates, the local and the average Nusselt numbers (Nu) are calculated. The Nu values are correlated with the tilt angle for fixed Rayleigh numbers (Ra) or for a fixed Ra times cosine of the tilt angle. By observing the changes in flow structures, the critical tilt angles for air at which the maximum and the minimum heat fluxes occur are determined. There is a very good agreement with the present results and the available empirical correlations in the literature. Moreover, the present study also explores the instabilities beyond the lower and upper tilt angle limits of the experimental studies in the literature.

Application of MI Simulation Using a Turbulent Model for Unsteady Orifice Flow

Measurement-integrated (MI) simulation is a numerical simulation in which experimental results are fed back to the simulation. The calculated values become closer to the experimental values. In the present paper, MI simulation using a turbulent model is proposed and applied to steady and unsteady oscillatory airflows passing an orifice plate in a pipeline. Velocity and pressure feedbacks are conducted and both feedback methods showed good agreement with the experimental results. Moreover, the calculation times between the MI simulation and ordinary simulation were compared in steady and unsteady conditions. The calculation time was demonstrated to be significantly reduced compared with ordinary simulation.

Indicators for the correct usage of intranasal medications: A computational fluid dynamics study

Intranasal medications are commonly used in treating nasal diseases. However, technical details of their correct administration are unclear. A three-dimensional model of nasal cavity was constructed from the magnetic resonance imaging scans of a healthy human subject. Nasal cavities corresponding to moderate and severe nasal obstruction were derived from the healthy nose by enlarging the inferior turbinate geometrically, which can be documented by approximately one third reduction of the minimum cross-sectional area in the moderate and two thirds in the severe obstruction. The discrete phase model based on computational fluid dynamics was used to study the gas particle flow. The percentage of discrete particles that pass through the minimum cross-sectional area in the nasal valve (NV) region is computed as the percentage of NV penetration rate. The percentage of NV penetration is 10 to 20 times higher when nasal spray is accompanied by an inspiratory airflow than no flow, which can be as low as 4.69% to 8.81% in the healthy model, and 0% in moderate and severe blockage models. In the presence of inspiratory airflow, there is no significant difference in the percentage of NV penetration (80.97%-82.13%) among different head tilt angles (0 degrees -90 degrees ). When using an intranasal medication, it is advisable to have an inspiratory flow to improve drug penetration. Various suggested head positions do not improve drug penetration significantly, even in the presence of uniform quiet breathing. Further studies that consider turbulence and unsteady airflow are needed to investigate the deposition distribution of discrete particles inside various nasal cavities.

Experimental and numerical study of patterns in laryngeal flow

Unsteady airflow is investigated in a channel with a geometry approximating that of the human larynx. The laryngeal flow is simulated by solving the Navier-Stokes equations for an incompressible two-dimensional viscous fluid, and visualized using the Schlieren technique in an experimental setup consisting of a rigid replica of the larynx, with and without ventricular bands. This study shows the spontaneous formation of vortex couples in several regions of the laryngeal profile, and at different stages of the evolution of the starting glottal jet.

Simulation of a freight train brake system with 120 valves

A freight car air brake system simulation model that is based on air flow dynamics and function principle of the ‘120’ control valve is established. Functions of unsteady air flows in the air brake pipes, the boundary conditions used in the air brake system, and the calculation method for pressures in the brake pipe, brake cylinder, and auxiliary air reservoir are discussed. A comparison of the simulation results with the experiment test results is made to verify the validity of the simulation. The results of the simulations, including long and short train with large and small pressure reduction service brake application, release, and emergency brake application, show good consistence with the experiment results on pressure change and its timing in the brake pipe, brake cylinder, and auxiliary reservoir, with the exception of long train at small pressure reduction application. This indicates that the simulation model can correctly predict the pressure behaviour of the unsteady air flows in the air brake system and component movements in the ‘120’ control valve. The model can simulate braking characteristics of a freight train consisting of up to four locomotives. The simulation results can be used to successfully predict the braking distances and to assist longitudinal, dynamic, and impact force analyses of a train. The simulation is useful in the design, research, and development of brake systems and can provide insight into how components of the ‘120’ air brake control valve work.

Theoretical evaluation of the influence of geometric parameters and materials on the behavior of the airflow in a solar chimney

An analytical and numerical study of the unsteady airflow inside a solar chimney was performed. The conservation and transport equations that describe the flow were modeled and solved numerically using the finite volumes technique in generalized coordinates. The numerical results were physically validated through comparison with the experimental data. The developed model was used for airflow simulation in solar chimneys with operational and geometric configurations different from those found in the experimental prototype. Analysis showed that the height and diameter of the tower are the most important physical variables for solar chimney design.

Unsteady-State Airflow and Particle Deposition in a Three-Generation Human Lung Geometry

The study of particle transport and deposition in the human lung is critical in health risk assessment of air pollutants and in pharmaceutical drug delivery. Several computational fluid dynamics (CFD) studies have investigated particle deposition in the lung for simplified airflow scenarios. A shortcoming with most CFD studies is uncertainty regarding flow boundary conditions, which directly impacts airflow and particle deposition. The influence of inlet and outlet conditions on airflow and particle deposition in lung common airways was assessed here. Common airways consisted of nine airways of the human lung ahead of lobes: the trachea, main, and lobar bronchi connected as a network of cylindrical tubes with dimensions based on morphometric measurements. Three different boundary conditions were used: (1) prescribed constant flow rate at the trachea entrance and atmospheric pressure at terminal branch exits, (2) atmospheric pressure at the trachea inlet and prescribed outlet flow rates corresponding to uniform lobar expansion, and (3) the same as case (2) with exit flow rates according to nonuniform lobar expansion. Unsteady airflow fields were numerically solved for a 2-s inhalation. Spherical particles of 1 nm to 10 μm diameter were injected at the trachea inlet, and particle deposition patterns during inhalation were evaluated. A Lagrangian particle tracking method was used that included particle inertia, gravity, and Brownian motion. Predicted flows showed similar trends but with a notable difference in magnitude. Lower particle deposition was found in case (1) for all particle sizes. The differences among these cases indicated the significance of realistic boundary conditions for accurate assessment of the flow field and particle deposition.

Solving for unsteady airflow in a glottal model with immersed moving boundaries

This work builds on previous efforts to characterize the dynamic development of the airflow in the glottis from a fluid mechanical point of view. A multigrid finite-difference method with immersed boundaries is implemented to solve the Navier–Stokes equations in a channel constricted by a vibrating rigid structure with a shape conforming to the human vocal folds. For the dynamically evolving boundaries we apply a forced oscillation glottal model. The large scale deformations of the boundaries are handled without regridding and tracheal input velocity is either set to a constant value or synchronized with wall motion. Particular attention is paid to the mobility of the point where the airflow detaches from the flapping walls. Results illustrate the relevance and the diversity of flow separation dynamics within the constriction standing for the glottis, while flow instabilities past the constriction are not found to affect flow behavior between the moving walls significantly. A comparison between static and dynamic numerical experiments show that mobility of the flow separation point is nontrivial in general and only rarely quasi-static.