Boundary Layer Wind Tunnel Experiments Research Articles

Active learning applied to automated physical systems increases the rate of discovery

Active machine learning is widely used in computational studies where repeated numerical simulations can be conducted on high performance computers without human intervention. But translation of these active learning methods to physical systems has proven more difficult and the accelerated pace of discoveries aided by these methods remains as yet unrealized. Through the presentation of a general active learning framework and its application to large-scale boundary layer wind tunnel experiments, we demonstrate that the active learning framework used so successfully in computational studies is directly applicable to the investigation of physical experimental systems and the corresponding improvements in the rate of discovery can be transformative. We specifically show that, for our wind tunnel experiments, we are able to achieve in approximately 300 experiments a learning objective that would be impossible using traditional methods.

Boundary layer wind tunnel modeling experiments on pumping ventilation through a three-story reduce-scaled building with two openings

Pumping ventilation is a special single-sided wind driven building ventilation induced by periodic vortex shedding at the building wake. This work aimed to investigate the oscillating frequency at the building façade opening and ventilation rate of pumping ventilation. The effects of opening separations on different floors of a three-story reduce-scaled building with two openings were considered using boundary-layer wind tunnel experiments. The wind velocity of the surrounding flow field and the center of the opening were measured. The ventilation rate was obtained by tracer gas method with constant and continuous injection rate. The results showed that the oscillating frequency was independent of the opening separation except on the third floor probably because of the disturbance of the rooftop shedding frequency. The oscillating frequency on the first floor was the lowest due to the resistance of the ground to the vortex shedding. Pumping ventilation indicated that its ventilation rate is greater than that of general ventilation across the single opening with the same total opening area. The promotion of ventilation rate was up to about 123% on the first floor, about 65% on the second floor and about 44% on the third floor. Expansion of opening separations could boost the ventilation rate. Meanwhile, the fluctuation of opening center wind velocity and indoor species concentration were not positively correlated with the ventilation rate of pumping ventilation. Conclusions of this research could provide some useful reference to the design of natural ventilation for buildings.

Prediction of long-range infrasound propagation from tornadoes based on new atmospheric boundary layer wind tunnel experiments

Tornadoes generate infrasonic noise that propagates over long distances. NOAA is developing tornado early warning systems via infrasound microphone arrays. Accurate detection must be coupled with a propagation algorithm that accounts for infrasound distortion due to the effects of atmospheric turbulence. Numerical solutions of the generalized Burgers' equation and the acoustic ray equations are used to predict changes to infrasonic tornado noise within the turbulent atmosphere. The effects of nonlinearity, refraction, attenuation, dispersion, and scattering are captured by the solver. We define a new parameter to model the alteration of infrasound that depends on atmospheric turbulence. Turbulent kinetic energy, integral length scale, integral time scale, and local speed of sound are arguments of the model and vary along each ray path. A series of tests are designed and conducted at the University of Florida Boundary Layer Wind Tunnel to calibrate and validate the model at scale. Predictions show satisfactory agreement with measurements with varying wind speed, turbulence intensity, humidity, and temperature. Finally, we present a parametric study for propagation of sinusoidal signals at 90 dB and 7 Hz in regions of the United States using realistic weather.

Generation and characterization of a non-stationary flow field in a small-scale wind tunnel using a multi-blade flow device

Physical experimentation for non-stationary wind phenomena has been the focal point of much research. Replication of transient, non-synoptic wind events such as thunderstorm downbursts and tornadoes require innovative means that break from traditional boundary-layer wind tunnel experiments. This paper examines the feasibility of a multi-blade flow device capable of reproducing the gust front of a non-stationary outflow. The device is inserted into the test chamber of the small-scale, closed-circuit wind tunnel at Northeastern University. Replication of spatial and temporal features of a non-stationary gust front (slowly-varying flow and turbulence fluctuations) is investigated through velocity measurements at various elevations. The wind velocity record from the Andrews Air Force Base downburst is employed for verification of the experimental procedure; its physical effects on a tall building model are recorded. The results suggest that this device may be used to simulate the main features of simplified non-stationary gust fronts in a conventional wind tunnel without significant modifications to the test chamber.

Snow particle characteristics in the saltation layer

AbstractShadowgraphy was employed to study snow saltation in boundary-layer wind tunnel experiments with fresh, naturally deposited snow. The shadowgraphy method allowed for a temporally and spatially high-resolution investigation of snow particle characteristics within a measurement area of up to 50 mm × 50 mm. Snow particle size and number characteristics, and their variation with height in the saltation layer, were analysed. The following observations and findings were made for the saltation layer: (1) the particle number decreases exponentially with height, (2) the mean particle diameter is fairly constant, with a very slight tendency to decrease with height, (3) the maximum particle diameter decreases linearly with height, and (4) the snow particle size distribution can be adequately described by gamma probability density functions. The shape and scale parameters of the gamma distribution were found to vary systematically, though only slightly, with height over ground and between experiments with different snowpack characteristics.

Experimental study on contamination of building exhaust emissions in urban environment under changes of stack locations and atmospheric stability

Contamination on exhaust emissions from short stacks on a building is the major concern in the urban environment. Dispersion of exhaust emissions around the building was investigated experimentally in a boundary layer wind-tunnel experiments under changes of stack locations and atmospheric stability conditions. Experiments with different conditions of atmospheric stability were conducted using an isolated cubical building model. Tracer gas (C2H4) was released from different stack locations on the building roof. Flow, temperature and dispersion fields were analyzed and measured using a split fiber probe, a cold-wire thermometer, and high-response flame ionization detector. Atmospheric diffusion was examined at different distances downwind of the leeward side of the cubical building. Flow and dispersion characteristics studied included mean velocity, turbulent kinetic energy, mean and fluctuation concentrations around the building. The results show that the stable condition increased the vertical velocity around the building. The unstable condition increased the longitudinal velocity around the building. The concentration distributions increased for the right stack and decreased for the lift stack on the building. There is an increase in the concentrations with stable conditions, and a decrease in the concentrations with unstable conditions.

Issues with validation of urban flow and dispersion CFD models

The paper describes difficulties in the proper evaluation of obstacle-resolving urban CFD models. After a brief description of the evaluation methodology suggested by the European COST action 732, focus is laid on the question of how to obtain validation data that can be regarded as a reliable standard. Data from an entire year of measurements at an urban monitoring station are analyzed, which show a large amount of scattering for seemingly identical cases. The atmospheric variability issue is thoroughly discussed, and a concept for the provision of validation data based on a combination of field and boundary layer wind tunnel experiments is presented.

Experimental simulation of wind flow over the ridge topography

Atmospheric boundary layer that flows over a complex topography is an important feature in the analysis of the wind energy systems, dispersion of pollutants in the atmosphere, and many meteorological and engineering applications. In this study, wind flow measurements have been made over a three-dimensional ridge crossing hill model with an obstacle fence in a simulated neutrally stratified boundary layer. It was investigated by the boundary layer wind tunnel experiments using laser Doppler anemometer. Two configurations of the obstacle fence were tested for two flow directions, 0o and 45o. Analysis of the wind flow characteristics included the mean velocity, turbulence intensity, turbulent energy, and Reynolds stress for the mean flow components. Results indicated that the dynamic large structure of separation and reattachment occurred over the ridge with a fence. Strong inverse flow direction was observed for the flow with a fence. High velocities were found at the ridge crest. The maximum velocity was reduced to about 50% to 70% of the velocity on the ridge. The maximum changes of the turbulence intensity was found to occur in the ridge leeside.

Impinging jet simulation of stationary downburst flow over topography

A non-translating, long duration thunderstorm downburst has been simulated experimentally and numerically by modelling a spatially stationary steady flow impinging air jet. Velocity profiles were shown to compare well with an upper-bound of velocity measurements reported for full-scale microbursts. Velocity speed-up over a range of topographic features in simulated downburst flow was also tested with comparisons made to previous work in a similar flow, and also boundary layer wind tunnel experiments. It was found that the amplification measured above the crest of topographic features in simulated downburst flow was up to 35% less than that observed in boundary layer flow for all shapes tested. From the computational standpoint we conclude that the Shear Stress Transport (SST) model performs the best from amongst a range of eddy-viscosity and second moment closures tested for modelling the impinging jet flow.

Wind loads on rainscreen walls: Boundary-layer wind tunnel experiments

This paper presents the results of the wind tunnel experiments and compares them with full-scale data obtained at the Technical University of Eindhoven (TUE) building in Eindhoven, The Netherlands. Difficulties with the wind tunnel testing are discussed. The objective of the study is to quantify the parameters involved in the design of pressure-equalized rainscreen (PER) walls in order to establish specific design guidelines for standards and codes of practice. Comparisons with the currently available limited provisions show clearly their inadequacy.

Wind‐Induced Internal Pressures in Buildings

Wind‐induced internal pressures in buildings can be analytically evaluated by using external pressure coefficients appropriate for each surface of the building envelope. The evaluation considers the overall building porosity and the specific (intentional or unintentional) wall (or roof) openings. Analytically‐evaluated internal pressure coefficients show reasonable agreement with experimentally‐determined values in either full‐scale or boundary layer wind tunnel experiments. Comparison of internal pressure coefficients with those suggested in the North‐American Standards indicate the need to consider further the influence of overall porosity and relative size of openings for better estimation of internal pressures. ANSI Standard in particular seems to underestimate wind‐induced internal pressures for specific leakage configurations.

Scale effects in wind tunnel testing of low buildings

Further analysis of wind loads derived from boundary layer wind tunnel experiments on low buildings sheds light on the question of scaling. Results are available for building models tested at three geometric scales under two different terrain exposures representing open country and suburban areas. Both mean and fluctuating pressure coefficients appropriate for both local and area loads have been examined. The errors introduced by using larger models are discussed and evidence is presented that a small relaxation of scale (up to a factor of 2) leads to errors of the order of 10% or less for all cases examined, except for the particular case of local pressure coefficients acting on building walls, which may be subject to non-conservative errors of the order of 25%.