Unsteady Airflow Research Articles

Train-Induced Unsteady Airflow in a Metro Tunnel with a Ventilation Shaft

Train-Induced Unsteady Airflow in a Metro Tunnel with a Ventilation Shaft

A simplified model for unsteady airflow analysis in ultra-long tunnels based on the resistance compensation method

A simplified model for unsteady airflow analysis in ultra-long tunnels based on the resistance compensation method

Evaluating the effect of frigate hangar shape modifications on helicopter recovery using piloted flight simulation

Turbulent ship airwakes can present a major challenge for a pilot landing a helicopter to the ship. A recent study has proposed modifications to the hangar of a simple ship, the SFS2, to improve the air flow over the deck. To assess the effect of the proposed hangar modification on the helicopter and pilot, the unsteady air flow over the modified ship has been computed using time-accurate CFD, and then integrated with a full-motion flight simulator for a pilot to conduct deck landings to the original and modified ship geometries in wind speeds from 30 kt to 50 kt. The effectiveness of the proposed modification was assessed through pilot workload ratings for the landing task, and by recording pilot control inputs and helicopter states. The study has shown that there are some benefits from the hangar modifications. In the headwind the helicopter was deemed to be at the safe limit at 50 kt when operating to the original SFS2, while the limit was not reached in the 50 kt wind for the modified ship. In an oblique wind, the safe wind speed limit was found to be 40 kt for the original ship and 50 kt for the modified version. Although the improvements are not substantial, they do represent a positive outcome.

Rapid prediction of transient particle transport under periodic ventilation using a non-uniform state Markov chain model

Predicting transient particle transport is crucial to address the risks posed to human health by indoor contaminants and to improve the design and control of ventilation systems. The Markov chain model with a coarse matrix has proven to be efficient in this regard, demonstrating faster speeds than traditional methods and higher computational accuracy when based on non-uniform states. However, its application under unsteady airflow conditions is limited, and discussions of how to determine non-uniform Markov states and their corresponding transient transition probabilities when the flow field changes are limited. Therefore, this study developed an application method of Markov chain model based on fixed non-uniform states in unsteady airflow fields. First, this model rapidly provides various Markov state schemes based on airflow field velocity and clustering methods. It then selects the optimal scheme through frequency and finally obtains the transition matrix through Lagrangian tracking for particle transport prediction. This model reduces the computational cost of transient particle transport when the wind speed changes while ensuring high computational accuracy. The proposed method was validated using published flow field experiments and simulation data. The computational speed of the proposed method was compared with computational fluid dynamics (CFD), and in the case studies considered in this research, the normalized root-mean-square deviation of the model was less than 10% and the total computation time was more than 75% faster than that of CFD.

Experimental study on the periodic pulsating ventilation by fluidic oscillator on pollutant dispersion and ventilation performance in enclosed environment

Ventilation plays an essential role in creating healthy and comfortable indoor environment, while adopting unsteady airflow field has the potential to improve the indoor air mixing effect, which is conducive to the dispersion and eventually the discharge of indoor pollutants. A fluidic oscillator capable of producing periodic pulsating airflow is employed to conduct a ventilation experiment to test the performance of such unsteady flow regime. The distribution of indoor pollutants at different source locations is compared under both steady and unsteady ventilation mode. The assessment of the fluidic oscillator ventilation system performance in eliminating pollutants involves the concentration exceedance rate, the decay of indoor pollutant concentration, and the resultant ventilation efficiency. At a relatively lower air supply rate, the fluidic oscillator device can enhance indoor air mixing, leading to a reduction of the peak indoor pollutant concentration. Additionally, the fluidic oscillator air terminal significantly prolongs the time needed to reach the pollutant concentration threshold, increases ventilation efficiency, and improves indoor air quality.

Investigation on the unsteady ventilation performance of oscillating jet air supply through both spontaneous vortex street effect and active modulation

Ventilation is one of the most vital measures to create a healthy and comfortable indoor environment, significantly influencing indoor air quality and building energy consumption. The conventional steady-state airflow field of a ventilated room, if not reasonably configured, is prone to cause air stagnation in certain regions, whereas the unsteady airflow improves the mixing effect, which favors the dispersion and purging of indoor air pollutants. This paper proposes a method to induce an unstable air flow field utilizing the spontaneous Karman vortex street effect on the air-supply jet and swinging modulation. The quasi-periodicity of the airflow field and its influence on the indoor pollutant purging effect were investigated. The effects of air velocity oscillation and pollutant concentration decay in a ventilated room were studied through experimental measurements and computational fluid dynamics analysis. The results show that the transverse oscillation jet has a dominant period of 1.6 s and the longitudinal oscillation jet has a dominant period of 2.987 s, and both jets have a width about twice that of the free jet. The airborne pollutant discharge efficiency of the oscillating jet was higher than that of the free jet at the occupancy level, and the longitudinal oscillating jet purged pollutants faster than the transverse oscillating jet. This study reveals that oscillating jets can provide an option to create periodic flow fields and improve the efficiency of ventilation.

On the role of unsteadiness in impulsive-flow-driven shear instabilities: precursors of fragmentation

Shear instabilities at the interface of two fluids, such as classical Kelvin–Helmholtz instability (KHI), is the precursor of interface destabilization, leading to fluid fragmentation critical in a wide range of applications. While many insights into such instabilities are derived for steady background forcing flow, unsteady impulse flows are ubiquitous in environmental and physiological processes. Yet, little is understood on how unsteadiness shapes the initial interface amplification necessary for the onset of its topological change enabling subsequent fragmentation. In this combined theoretical, numerical and experimental study, we focus on an air-on-liquid interface exposed to canonical unsteady shear flow profiles. Evolution of the perturbed interface is formulated theoretically as an impulse-driven initial value problem using both linearized potential flow and nonlinear boundary integral methods. We show that the unsteady airflow forcing can amplify the interface's inherent gravity–capillary wave, up to wave-breaking transition, even if the configuration is classically KH stable. For impulses much shorter than the gravity–capillary wave period, it is the cumulative action, akin to total energy, that determines amplification, independent of the details of the impulse profile. However, for longer impulses, the details of the impulse profile become important. In this limit, akin to a resonance, it is the entangled history of the interaction of the forcing, i.e. the impulse, that changes rapidly in amplitude, and the response of the oscillating interface that matters. The insights gained are discussed and experimentally illustrated in the context of interface distortion and destabilization relevant for upper respiratory mucosalivary fluid fragmentation in violent exhalations.

Investigating unsteady airflow characteristics in the human upper airway based on the clinical inspiration data

To enhance understanding of the airflow characteristics in the human respiratory system during realistic inspiration, we investigated the airflow field in a human upper airway model using large eddy simulation and the dynamic grid method, taking into account clinically measured inspiratory characteristics. The results reveal the following novel findings: (1) The laryngeal jet and recirculation zone exhibit significant unsteadiness, with their dynamic characteristics primarily influenced by the transient inspiration flow rate and glottis motion. This pattern holds true for other airflow characteristics as well. (2) Glottis expansion reduces the energy consumed during inhalation for both steady and unsteady inspiratory flow rates, with the degree of expansion being directly related to the reduction in energy. We can accurately predict power loss by considering the glottis area and inspiratory flow rate. (3) Analysis of spectral entropy clearly demonstrates that the flow transitions from the laminar to turbulence earlier when using clinical inspiration data. Turbulence intensity in the trachea increases when either glottis motion or the transient inspiratory is ignored. In conclusion, the airflow dynamics are significantly more unsteady compared to cases where we ignore either glottis motion or the transient inspiratory flow rate. A precise understanding of realistic respiratory airflow cannot be achieved by assuming either a rigid glottis or a steady inspiration pattern. Therefore, it is crucial to use accurate inspiratory data when studying the properties of airflow structures in the human respiratory system. Moreover, incorporating more physiological data is also essential to obtain realistic respiratory airflow characteristics.

Wind-tunnel analysis of turbulent flow in cross-ventilated buildings with windcatchers: Impact of surrounding buildings

This paper presents a detailed experimental analysis of ventilation flows in a building with a windcatcher surrounded by building arrays. The surrounding building arrangements were in two forms: inline and staggered, representing an abstract variety of suburban morphology. Ventilation measurements were performed for five approaching wind directions: two normal to the building opening(s) at 0° and 180° orientations, two oblique at 40° and 140° orientations, and one parallel to the building opening(s) at 90° orientation. Time-averaged and instantaneous data analyses were carried out. The time-averaged approach showed the overall ventilation flow interaction with the building. Meanwhile, the instantaneous data analysis helped investigate the unsteady airflow characteristics. Hence, the room pressure fluctuations were considered in the frequency domain. Additionally, the coefficient of variance and probability density functions were obtained to identify the nature of ventilation flow fluctuations.The results showed that the maximum reductions in ventilation rate were 30% and 42% for the inline and staggered arrays, respectively, compared to an isolated building case. The highest obstructions of the onset flow to the openings occurred at direct wind incidences for the inline array and oblique wind incidences for the staggered array. The nearby buildings enhanced the fluctuating ventilation flows at 0° and 180° incidences. However, the isolated building case showed the highest fluctuations at 90° incidence since the turbulent flow was mostly dissipated along the surrounding street canyons for the arrayed cases. The room interior pressure fluctuations and spectra showed prominent peaks associated with the Helmholtz frequency in all case studies.

Large-eddy simulation of buoyant airflow in an airborne pathogen transmission scenario

Indoor 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.

Investigation of the impact of unsteady air flows on the mechanical and operational properties of aircraft structural elements

The use of pulsating subsonic gas flow treatment (gas pulse treatment) in the process of maintenance and repair for a duration not exceeding a certain value contributes to the restoration and improvement of mechanical and operational properties of aeronautical equipment structural elements. This article presents the results of a study to determine the optimal duration of gaspulse processing of aircraft parts, as which, the duration of processing was adopted, providing the maximum increase in the properties of material viscosity, which prevents crack development without reducing the strength properties. As a result of the study, the influence of various factors, such as the subsonic airflow velocity and the frequency of oscillations, the material and geometric parameters of the processed product, on both the optimal and leading to a decrease in mechanical properties duration of processing by pulsating airflow of aircraft structural elements during maintenance and repair, was evaluated. It has been established that mechanical waves generated by gas flow pulsations can have a significant impact on the structural strength of aircraft structural elements, which makes it possible to increase their reliability as well as the accuracy of forecasting the technical condition. The harmonic nature of the attenuation of the ratio of residual stresses to their initial values has been experimentally established, depending on the duration of gas pulse treatment, which allows us to control their magnitude and sign. An empirical formula has been obtained to determine the optimal processing time for products of various materials. As an indicator of the duration of gas pulse treatment, an increase in the viscosity of the material was adopted without reducing the strength properties. Graphical dependences of the relative time of gas pulse processing, which provides an increase in the mechanical properties of the relative frequency of the gas flow oscillations, have been constructed.

Evaluating the effect of unsteady air flow on a slotted aerofoil of wind turbines

Evaluating the effect of unsteady air flow on a slotted aerofoil of wind turbines

Dynamic Response of Outer Windshield Structure in Different Schemes under Aerodynamic Load

With the increase in high-speed train (HST) operation speed, the light-weight design of the train body and component structure is pursued to reduce energy consumption during operation, but this seriously deteriorates the aerodynamic performance of the light-weight structure outside the train body under the effect of strong unsteady airflow, and the more obvious case is the frequently occurring problem of vibration, large deformation, and damage to the rubber exterior windshield at the connection position of HST carriages. We investigate the fluid–structure coupling mechanism of the interaction between the rubber external windshield and aerodynamic force, and compare the dynamic characteristics of windshield structure under different design parameters. A numerical simulation of three rubber outer windshield structure parameters (sidewall distance of U-shaped capsule, sidewall thickness, sidewall inclination angle) is carried out using FSI simulation of the two-way coupling method. The aerodynamic load, airflow dynamics around the windshield, and the nonlinear vibration and deformation form of the windshield is analyzed in detail. The results show that the aerodynamic response of the HST rubber external windshield analyzed by the FSI method is in good agreement with the full-scale test results. Additionally, the stiffness of the windshield can be improved by increasing the thickness of the windshield sidewall. When the distance between the sidewall of the windshield is increased, an insufficient thickness at the top of the arc causes a large local deformation at the top of the arc of the windshield. The method established and relevant research results can provide good support for the aerodynamic stability evaluation of HST windshields.

Effects of Train-Induced Unsteady Airflow on Air Quality and Air Distribution in a Subway Station

Using train-induced unsteady airflow in transitional seasons can greatly reduce the energy consumption of a subway station’s mechanical ventilation. However, the unsteady airflow carries the particles from tunnels into subway platforms, and airflow distribution at controllable vents is uneven. To determine the variation in PM2.5 (particulate matter of 2.5 micrograms or less) concentration and the air velocity distribution at the vents, field tests were conducted. The results showed that the PM2.5 concentrations at the vents and platform were positively correlated with the air velocity. The average ratio of the PM2.5 concentration at the vents to that on the platform was 71.15%. The maximum PM2.5 concentration and average PM2.5 concentration on the platform were 0.067 mg/m3 and 0.037 mg/m3, respectively. The air velocity distribution along the length of the platform conformed to logistic regression. Adjustment of the opening height of the vents was proposed to improve the air velocity distribution. Through numerical simulation, the ratio of the minimum airflow rate to the maximum airflow rate was increased from 5.9% to 37.5% after adjustment. This method is helpful for the uniform distribution of the airflow rate. This study will provide references for the design and operation of the vents of subway stations.

Rapid prediction of airborne gaseous pollutant transport in aircraft cabins based on proper orthogonal decomposition and the Markov chain method

Rapid prediction of airborne gaseous pollutant transport is important for designing a safe indoor environment. Current models generally solve the airflow fields by CFD first and then predict the transport of a pollutant in fixed airflow patterns. Every time the air-supply parameters are adjusted, the airflow field must be re-solved by CFD, which is time-consuming. This study proposed a model to improve the prediction efficiency. The model first applies proper orthogonal decomposition to the sampled airflow fields, to construct a database related to all the airflow fields in the sample ranges, and then uses the Markov chain method to obtain the airflow field with the desired air-supply parameters for construction of a transport probability matrix. Finally, the airborne gaseous pollutant transport can be predicted quickly in the fixed airflow pattern. The proposed model was applied to an aircraft cabin model, first with a single gaseous pollutant source and then with two sources, for validation of the proposed model. The results show that the proposed model can predict both the airflow field and the transport of a gaseous pollutant with outcomes similar to those obtained by the conventional CFD method, but with a much shorter computing time. When the database has been prepared in advance, the use of the model reduces the computing time by more than 90%. Further improvement of the proposed model in terms of accuracy and extension of the model to prediction of pollutant transport within unsteady airflow fields will be the main objectives of future work.

Unsteady air flow measurement method based on adaptive tracking differentiator for pneumatic system

When using a flowmeter to measure unsteady air flow in the pneumatic control system, problems such as high cost and pressure loss are often encountered. It is an available method to measure the unsteady flow rate using isothermal chamber through direct differentiation of pressure signal with a traditional fixed parameter differentiator, but the adaptability of this method is insufficient due to its narrow dynamic range. To this end, an improved adaptive tracking differentiator which can adaptively adjust the control parameters according to the changes of pressure signal and pressure tracking error was proposed for unsteady air flow measurement. The algorithm structure of the adaptive tracking differentiator was designed. Moreover, theoretical simulation and flow measurement experiments under multiple working conditions were carried out. The simulation and experiment results showed that the flow measurement method based on improved adaptive tracking differentiator effectively measured the unsteady flow in a wide dynamic range, and had the advantages of more simplified process and lower cost than traditional method.

Numerical model for cough-generated droplet dispersion on moving escalator with multiple passengers.

To investigate the motion of virus-laden droplets between moving passengers in line, we performed numerical simulations of the distribution of airborne droplets within a geometrically detailed model similar to an actual escalator. The left and right sides and the ceiling of the escalator model were surrounded by walls, assuming a subway used by many people every day with concern to virus-laden droplets. Steps and handrails were incorporated in the model to faithfully compute the escalator-specific flow field. The ascending and descending movements of the escalator were performed with 10 or 5 passengers standing at different boarding intervals. To resolve the unsteady airflow that is excited by a moving boundary consisting of passengers, steps, and handrails, the moving computational domain method based on the moving-grid finite-volume method was applied. On the basis of the consideration that the droplets were small enough, droplet dispersion was computed by solving the equation of virus-laden droplet motion using a pre-computed velocity field, in which the flow rate of a cough, diameter distribution, and evaporation of droplets are incorporated. The simulation resolved the detailed motion of droplets in flow, and therefore, we were able to evaluate the risk of viral adhesion to following passengers. As a result, we found that the ascending escalator had a higher risk of being exposed to virus-laden droplets than the descending escalator. We also reported that the chance of viral droplet adhesion decreases as the distance from the infected person increases, emphasizing the importance of social distancing.

Airborne transmission of exhaled pollutants during short-term events: Quantitatively assessing inhalation monitor points

The infection risk assessment associated with the contaminant inhalation can provide a scientific basis for formulating mitigation measures. Previous studies on the breathing zone are primarily based on the assumption of the steady formation and homogeneous property, while it might not be applicable for short-term events. Large-eddy simulation (LES) is employed in the present study, as well as two computational thermal manikins with detailed facial features and transient breathing conditions. Exposure risks in eight commonly used monitor points are compared in short-term events and under steady-state conditions. Three representative physical distances between room occupants are investigated, namely 0.35 m, 1.0 m, and 1.5 m. Based on the statistical difference in the contaminant distribution at a short physical distance, the breathing zone could be identified from the time-averaged concentration field. The results highlight that the previously defined breathing zone ignores unsteady airflow characteristics, significantly impacting the exposure risk estimation in short-term events. Owing to the substantial temporal variation of the contaminant in the identified breathing zone, the instant exposure risk analysis in short-term events should consider its turbulence intensity and concentration fluctuation characteristic. Overall, instead of using the identified breathing zone. Point_A, Point_B, and Point_C should be employed to evaluate infection risk in short-term events. The localized method with direct interference on the respiratory airflow should be recommended in short-term events.

Estimation of airflow distribution in cubic building group model using POD-LSE and limited sensors

Urban airflow has strong turbulence characteristics, and its distribution varies with time. Urban airflow distribution can be measured with sensors or simulated using Computational Fluid Dynamics (CFD). However, CFD is considered still expensive for the rapid calculation of unsteady airflow distribution and the measurement value of sensors is often inferior in terms of spatial resolution. In this study, the high spatial resolution airflow distribution in the cubic building group model was rapidly estimated from limited sensors by combining Proper Orthogonal Decomposition (POD) with Linear Stochastic Estimation (LSE). Validated airflow data simulated by Large-Eddy Simulation (LES) in a cubic building group model were selected as the airflow database. This study was conducted with two main objectives: 1) to verify the basic performance of the POD-LSE; and 2) to analyze the effects of changing the number of sensors and POD analysis regions on the estimation accuracy. By comparing the results of the POD-LSE estimations with the LES data as the true value in the reference region, the following conclusions were obtained: 1) The airflow distribution in the cubic building group model can be reconstructed and predicted by the POD-LSE; 2) nine sensors can provide acceptable estimation accuracy; 3) the velocity value of a point located in the wake region is difficult to estimate; 4) the accuracy does not increase as the POD region expands (without changing the number of sensors); and 5) with an increasing number of sensors, the accuracy of the velocity estimation is improved in reconstruction.

CFD investigation of the unsteady airflow and pressure characteristics in the underground highway station with the train passing

ABSTRACT A numerical research is conducted to clarify the unsteady airflow and pressure characteristics in the full-scale underground highway station to ensure the passenger safety and energy saving in Heating, ventilation and air-conditioning design. Results show pressure characteristics in the tunnel change greatly due to the propagations of compression and expansion waves. The propagations of compression and expansion waves determine the local pressures together. For the first two locations X = −900 m and X = −800 m, the pressure climbs to the peak when the head of the train reaches. However, for the other locations, the pressure is not the maximum when the train reaches, due to the effect of Initial Expansion Wave (IEW). With the raising of train speeds, the positive maximum velocities are increased by 32.60% and 47.58%, respectively. It should be noted that there are two peak values of horizontal velocities successively, with the train speed of 250 km/h and 300 km/h.