Windward Face Research Articles

Simultaneous measurement of the distributed longitudinal strain and velocity field for a cantilevered cylinder exposed to turbulent cross flow

The structural response of a cantilevered cylinder under free-stream conditions both with low and high turbulence intensity generated by turbulence-generating grids is analysed with concurrent measurements of the velocity field and the distributed strain. The thin-walled cylinder, with a diameter (D) of 50 mm, is mounted in a water flume as a cantilevered beam supported at one end, with 95% of its body submerged and exposed to cross flow yielding a Reynolds number of Re = 25000. We employ a novel combination of simultaneous particle image velocimetry and distributed strain measurements using Rayleigh backscattering fibre optic sensors. These sensors are embedded onto the surface of the cylinder to measure the experienced strain (ε\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\varepsilon $$\\end{document}) of the structure along its spanwise direction, covering both the windward and leeward faces of the cylinder. The sensors fine spatial resolution allows us to discern the influence of the flow on the structural response of the cylinder in two distinct regions of the structure: upstream and downstream of the mean separation location. This differentiation allows us to isolate the local effects introduced by the free-stream conditions on the loading events over the body from the global force generated by the vortex shedding and other coherent motions present within the flow. Distinguishing between these direct and indirect effects helps determine which is more relevant for fatigue-life cycle analysis. The cross power spectral density between the fluctuating velocity field and the strain reveals that the load is dominated by the vortex shedding, and this relationship is intensified with the introduction of free-stream turbulence. It also helps to discern the different dynamics imposed by the two free-stream turbulence conditions.

Predicting Wildfire Ember Hot-Spots on Gable Roofs via Deep Learning

Ember accumulation on and around homes can lead to spot fires and home ignition. Post wildland fire assessments suggest that this mechanism is one of the leading causes of home destruction in wildland urban interface (WUI) fires. However, the process of ember deposition and accumulation on and around houses remains poorly understood. Herein, we develop a deep learning (DL) model to analyze data from a series of ember-related wind tunnel experiments for a range of wind conditions and roof slopes. The developed model is designed to identify building roof regions where embers will remain in contact with the rooftop. Our results show that the DL model is capable of accurately predicting the position and fraction of the roof on which embers remain in place as a function of the wind speed, wind direction, roof slope, and location on the windward and leeward faces of the rooftop. The DL model was augmented with explainable AI (XAI) measures to examine the extent of the influence of these parameters on the rooftop ember coverage and potential ignition.

Wind Effects on Re-Entrant Wing Faces of Plus Plan-Shaped Building

The wind flow patterns and pressure distribution around a plus-shaped building are significantly influenced by re-entrant corner dimensions and building height. The present study aims to find the wind effects and study the pressure distribution with different flow patterns. The building has a plan area of approximately 300 square meters and a height of 50 m, maintaining equal lengths for the re-entrant corners. The research study is performed using the ANSYS CFX academic version. The study employs diverse visualizations, featuring the pressure coefficient (CPe), vertical and specific surface streamlines, and pressure contours. Wind incidence angles for the study are varied from 0° to 90° at an interval of 15°. Building dimensions are scaled down as per the ASCE wind tunnel test manual, and the length scale is kept at 1:100 for CFD investigation. This approach yields crucial insights into the circulation of wind patterns and the distribution of pressure across a wide range of wind incidence angles. The influence of streamlines in the bottom portion of the building model is relatively minimal. In contrast to the upper portion, the bottom exhibits robust vortices, particularly for the wind angles varying from 0° to 30°. Positive pressure is observed on the windward face, and the leeward face and side face exhibit negative pressure. This study furnishes valuable insights into flow patterns and pressure distribution across a wide range of wind incidence angles from 0° to 90° at an interval of 15°. These findings contribute significantly to a thorough comprehension of the wind flow patterns and pressure distribution around the plus-shaped building model.

Pore-scale hydrodynamics of non-Newtonian power-law fluids across a partially blocked porous medium in a confined channel

Transport of non-Newtonian fluids in porous media is pervasive in many natural and industrial applications. However, capturing the rheological behaviors of fluids by direct experimental techniques is challenging at the pore-scale. This paper outlines the pore-scale hydrodynamic interactions of non-Newtonian power-law fluids across a partially blocked porous medium in the laminar flow regime by computational fluid dynamics. The porous medium consists of an array of uniformly arranged square pillars. We explore the complex interplay of power-law rheology and Reynolds number on the microscopic flow field at the pore-scale. We capture the momentum transfer at the permeable interface between the porous and non-porous regions through stream-wise and span-wise velocity components and average volumetric flow rates at each pore-throat. Our results unveil a significant augmentation in stream-wise momentum by shear-thinning behavior and a diminution in momentum by shear-thickening behavior of the fluid through the porous medium. Further, the flow-leakage at the top interface purely depends on the combined effects of Reynolds number and power-law index. The channel pressure drop between the windward and leeward faces of the porous medium increases with the power-law index at low Reynolds number, while it decreases at high Reynolds number. Moreover, we provide a simple numerical framework to comprehend how the power-law behavior of the fluid dynamically regulates the flow field at the pore-scale.

Cyclic Appearance and Disappearance of Aerosol Nucleation in the Boundary Layer of Drops of Volatile Liquid

The cyclic appearance and disappearance of nucleation was observed in the boundary layer of drops of 1,3-propanediol, 1,2-propanediol, and glycerol, close to the boiling point and exposed to a cooler airflow. Although continuous nucleation has previously been widely observed, the cyclic nature of the phenomenon observed here is unusual. It was observed in experiments with free-falling drops and fixed drops in an upflow of air. To investigate this unexpected phenomenon further, the phenomenon was reproduced in two finite volume models. The first model used 1D potential flow solutions to approximate the airflow around the spherical windward face of the droplet. The second model used CFD to model the airflow. Both models used classical nucleation theory, the Stefan–Fuchs model of droplet growth by condensation, mass transfer by evaporation, diffusion, convection, and heat transfer by diffusion and convection. Despite several simplifications, the most important being the assumption that the drop has a uniform temperature, both models predict the frequency of nucleation to be better than the order of magnitude. These models also predict the experimentally observed power law dependence of nucleation frequency on air speed. It is proposed that the cyclic nature of the phenomenon is caused by the following process: the depletion of condensable vapour around the freshly nucleated aerosol due to condensation onto the aerosol results in reduced supersaturation, which stops further nucleation, and then the replenishment of this vapour by diffusion and convection from the parent drop, with nucleation of aerosol recommencing when the supersaturation has recovered sufficiently—then, the repetition of these steps in a cycle. It is proposed that the process depends mostly on the maximum saturation ratio in the boundary layer, which itself is determined by four key dimensionless numbers: the Lewis number, the Peclet number, the Reynolds number, and the ratio of the vapour pressure of the condensable compound at drop surface temperature to the vapour pressure of the same species at ambient temperature. A practical application of the phenomenon may be as a means of validation of thermo-fluid models, which include nucleation.

Experimental investigation of the wind loads on the solar parabolic trough collectors for structural stability

Solar parabolic trough collectors consist of reflectors that concentrate the solar radiation to the absorber tube located at its focal length. The shape accuracy of the reflectors is essential for the performance of the solar parabolic trough collectors. One of the important factors affecting the shape accuracy of the reflector is the wind load acting on the surface of the reflector. Hence, the pressure distribution study over the surface of the reflector is important, which influences the shape accuracy of the reflector. In the present study, wind tunnel tests are conducted to quantify the pressure distribution for different configurations of the solar parabolic trough. The test is conducted for the reflectors at 0°, 30°, 60°, −30° and −60° pitch angles, and 0°, 30°, 60° yaw angles for the wind speed of 5 ms−1. At 0° pitch angles, the fluctuating pressures are observed at either edge of the reflector, and 30° and 60° yaw angles result in higher pressure acting over the windward face for 5 ms−1 wind speed and the maximum pressure coefficients are observed to be located around the edges of the reflector and hence the stiffness of the reflectors must be increased at the edges.

A free-streamline boundary-layer model for small-amplitude oscillation regime of square cylinder under vortex-induced rotation

Numerical studies about vortex-induced rotation (VIR) have shown that six regimes exist for the dynamics of square cylinder under VIR at small Reynolds numbers; however, theoretical explanations are still rare. Aiming at stepping a little further, we select one of these regimes, that is, small-amplitude oscillation regime, to develop an analytical model. In the present model, we expand the symmetric free streamline theory to an inviscid flow pasting an inclined plate and introduce the Blasius boundary layer theory in the calculation of viscous torque on a square cylinder. This model is supported by comparing with numerical results using the immersed boundary method. Results explain the mechanism of small-amplitude oscillation regime: the main sources of the out-of-phase torque come from the imbalanced distribution of pressure on the windward face and the asymmetric distribution of pressure and viscous force on the top/bottom face. An alternative way to determine the local flow parameters by global dynamic quantities is also proposed based on this model.

Evaluating the impact of wind effects on tall buildings with various plan shapes

Tall building is playing important role in the growth of urbanization and has resulted in developing to the vertical rise of the structure. The data available for the designing of the tall building is not sufficient or need some modification up to very limited extent in the present scenario. In this study a rectangular shape having L: B ratio as 1.15 and octagonal plan shape are selected to investigate the wind loads on building. The area of both models is nearly equal for the comparison of the results. Pressure effects are the main concern of considered building model. When it comes to predicting wind-related events on buildings or other structures, computational fluid dynamics (CFD) is a reliable and quicker option. CFD analysis is carried out using a software program called (ANSYS-CFX). This paper presents pressure contours that are calculated at the front, back & adjacent sides of the individual tall buildings. The results clearly show that based upon the building shape, the pressures generated can be either positive pressure or negative which can be either beneficial or can have adverse effects. The result on both the building model shows that the pressure on wind ward face is more or less the same and it does not depend on the shape of the building. In the case of a symmetrical building, the leeward face will always be subjected to the suction effect, while the side faces will always exhibit an equal pressure distribution.

Firebrand deposition on cubic structures in tandem and parallel arrangements

Spot fires created by wind–carrying firebrands are common in wildland–urban interface (WUI) fires. Firebrand deposition over cubic blocks in tandem and parallel arrangements representing simplified nearby small structures in WUI was studied. The flow turbulence was modeled by large eddy simulation, and the firebrands were simulated in the Lagrangian framework. Both translational and rotational motions, and the thermal degradation of firebrands were accounted for. Various separation distances between blocks and wind speeds were considered. Simulations revealed that the blocks shielded their immediate leeward areas on the ground from firebrand deposition. These areas were distinguished by having no deposited firebrands, and were hence called safe zones, which were nearly in the shape of an isosceles trapezoid. The longer base of the trapezoid coincided with the bottom edge of the block’s leeward face, and the shorter base and the legs of the trapezoid constituted the borderlines of the safe zone. The safe zone length (i.e. the trapezoid height) in the wind direction increased with the wind speeds without a significant sensitivity to the block position in either arrangement. For the blocks in the tandem arrangement with a smaller separation distance and/or higher wind speeds, the safe zone encroaches on the windward face of the downwind block. Therefore, an upwind block also shielded the lower part of the windward face of the downwind block from firebrand deposition in these cases. In none of the simulations, firebrands were deposited on the leeward faces. In all simulations, the firebrands deposited on the lateral faces that were parallel to the wind direction were much less than those on the windward faces.

Aerodynamic characteristics of a high-rise building in a steady thunderstorm outflow-like flow field

This paper investigates the aerodynamic characteristics of a high-rise building loaded by steady wind flows with different boundary layer profile shapes. The multiple-fan wind tunnel (MFWT) at Tamkang University was used to simulate a mean velocity field with a nose-shaped boundary layer profile, similar to that observed in thunderstorm outflows, and a standard boundary layer flow, both with 3–5% turbulence over the depth of the profile. A regular ‘open terrain’ boundary layer flow was also generated using spires and roughness blocks to enable comparison. Aerodynamic characteristics of the Commonwealth Aeronautical Advisory Research Council (CAARC) building loaded by each of these flow fields through a range of incidence angles were measured, analyzed, and compared. Results showed that pressures on the windward face of the building largely reflected the dynamic pressure in the incident wind field. Pressures on faces within the separated wake region were less easily explained. Incident mean velocity and turbulence profiles, as well as building orientation, were all found to influence measured wind loads and forcing spectra on these faces. Of particular interest was a significant reduction in cross-wind forcing observed in the simulated outflow-like winds, largely believed to occur because the lower elevation of strong winds in this profile limits the development of a separation vortex up the height of the building. It was also found that the estimated building response was almost universally lower in the outflow-like flow field when compared with the boundary layer cases.

Numerical simulation of wind-induced load on evacuated tube solar water heaters

Solar Water Heater (SWH) has been used as a traditional device in many countries and regions for collecting solar energy. Unfortunately, there are many wind-induced accidents of SWHs, owing to insufficient consideration of wind-resistant design and installation. To better promote the safe use of SWHs, characteristics of wind load acting on them should be clarified thoroughly as far as possible. In this article, a systematic study is carried out by numerical simulation based on computational fluid dynamics (CFD) to analyze the effects of the tilt angle and density of evacuated tube collectors (ETC) as well as wind direction on the aerodynamic characteristics of SWH. Results show that the wind-resistance performance of SWH decreases significantly for wind flows blowing from 0° and 180° due to the influence of aerodynamic characteristics of its main components. When the ETC is located upstream of the water tank, the arrangement form of the tubes will have an impact on the aerodynamic characteristics of the SHW. A lower porosity of ETC can increase wind load on the weak region of water tank and the windward face of vertical supports, it can also increase the bending moment of inclined supports significantly. By contrast, lower tilt angles of ETC are more beneficial to the wind resistance of SHW, although they can result in an increase of the local wind suction on water tank.

Comparing a 3-d printed hemispherical-head and Rankine-body probe shapes for very low speed flush air data system (FADS) measurements

This study investigates the feasibility of using Flush Air Data Sensing (FADS) System technology for air data measurements at the very low-airspeeds, where many Unmanned Aerial Vehicles (UAVs) operate. FADS is a non-intrusive alternative to pitot probes, where the vehicle nosecone, wing leading edge, or other aerodynamic surfaces are configured with multiple pressure-ports distributed along the windward face. Although FADS technology has been used for a variety of high-speed aircraft, FADS has never been applied to very low-airspeed flight regimes. This study reports on wind tunnel tests of two 3-D printed shapes: 1) a cylindrical body with a hemispherical head, and 2) a Rankine-Body. These body shapes can act as a vehicle analog to a wide range of three-dimensional shapes and account for both blunt leading edge and trailing after body flow characteristics. For this study the "probes" were printed with 5 pressure ports and the associated flow channels aligned at 0o, +22.5o and +45o direction-angles along the vertical centerlines of the models. Sensed pressure data were curve-fit, developing quasi-potential flow calibration models for each probe, with coefficients compiled as a function of geometric angle-of-attack and tunnel airspeed. The calibration models account for end-to-end systematic effects, including the mounting sting flow compression, up wash, and tunnel blockage. Using the derived calibration models and the sensed pressure data, the effective angles-of-attack were re-calculated using the well-known "Triples" algorithm. The associated airspeed and dynamic pressure are estimated from the sensed pressure data using non-linear regression. The resulting estimates are compared to the tunnel reference conditions. Generally, both probe shapes performed well, with the redundant 5-port arrangement allowing for significant noise rejection. Both probes achieved RMS airspeed errors of less than 5%, angle-of-attack errors less than 1 deg., and dynamic pressure errors of less than 12 pascals, across airspeeds ranging from 5 to 25 m/sec. The sensed Airdata measurements at the lowest airspeeds (5 m/sec), exhibited similar accuracy to those sensed at the highest airspeeds (25 m/sec), verifying the applicability of FADS technology to very low airspeed flight regimes.

Global stability analyses of Mack mode on the windward face of a hypersonic yawed cone

What is the natural transition scenario for the windward boundary layer over a hypersonic inclined cone? Based on comprehensive linear and nonlinear global stability analyses, this work shows that the K-type breakdown triggered by the Mack-mode instability is responsible for the windward boundary-layer transition.

Closed‐loop adaptive model predictive control of a bluff body wake

AbstractA machine learning methodology is outlined to achieve robust closed‐loop feedback control of a bluff body turbulent wake. A Long Short‐Term Memory (LSTM) Neural Network is implemented with Model Predictive Control (MPC) to achieve closed‐loop flow control. The LSTM model is trained using actuation and pressure sensor data to forecast future pressure states. The candidate system is a square cross‐sectional cylinder with two modulated moving surface actuators embedded in the windward face leading corners. The controller performance is tested experimentally for three objective functions: recovery of mean‐base pressure set‐point after perturbation; and minimization of drag or wake fluctuation intensity. An adaptive learning strategy is implemented to adjust the model to new Reynolds number (Re) conditions without user intervention, thereby extending the controller performance and achieving more robust control. The identified minimum drag and wake fluctuation cases are analysed using velocity field data measured with particle image velocimetry.

Evaluation of Wind Effects on High-Rise Buildings Using Ansys CFX

Tall building is the growing pattern of cities, these buildings are highly vulnerable to wind loads. Wind load effects on the high-rise building can be found using wind tunnel testing and CFD tools. Wind tunnel facilities are limited all around the world, so CFD tools can be incorporated. In this study, ANSYS CFX is used to investigate the effects of wind on different shapes of high-rise buildings. The results of numerical simulation are compared with the international standards of various countries. Streamline patterns and pressure contours on different faces are discussed in this paper. It is found that building shape and size are having a direct effect on the pressure distribution on various surfaces of the building model. Windward face pressure will always be positive in nature. In this study, the comparison is presented for the variation in the cross-sectional shape of the building model. For this purpose Model C and Model I are having considerable variations and Model X is considered for validation purposes only. It is well established that the results are within the allowable limit. This paper aims to discuss these windgenerated effects in the tall building model.

Evaluation of aerodynamic effects on a tall building with various cross-section shapes having equal area

When wind hits a high-rise building, it has a big impact. Due to a lack of available land and an increase in population, the demand for high-rise buildings is growing daily. This necessitates the design of high-rise buildings with an effective wind load. Utilizing pressure and force coefficients, wind loads are calculated. There are numerous international standards that provide the coefficients for common regular shapes. However, in an expanding and competitive world, the architect seeks out complicated and unique building shapes that are aesthetically pleasing and able to endure wind loads that are applied to buildings. Our research focuses on determining how wind affects high-rise structures of seven different regular shapes that have the same base area and height. The ANSYS CFX for Computation Fluid Dynamics studies is used to compute the wind pressure on each face of building models. Scaled at 1:350, each building is represented as a model. For each face, the mean pressure coefficient CP is determined. Additionally, the wind flow pattern is also established using ANSYS CFX. Due to the direct head-on impact, pressure contours indicated positive pressure on the front or windward face. Faces turned sideways and backward displayed suction or negative pressure. On the reverse face, vortices were seen. According to the vertical flow pattern, the wind was blowing in two different directions: upward to a height of one-third of the building model from the top and downward to a height of two-thirds of the building model from the bottom.

Comparative study between field measurements of wind pressures on a 600-m-high skyscraper during Super Typhoon Mangkhut and wind tunnel test

Wind pressures on cladding of a 600-m-high skyscraper during Super Typhoon Mangkhut were monitored by a synchronous monitoring system including 48 pressure transducers. Then, simultaneous wind pressure measurements on a 1:500 scale model of the skyscraper were conducted in wind tunnel test. The field measurements of wind pressures are further analyzed to investigate the wind pressure characteristics on the skyscraper under extreme windstorm condition and utilized to validate the wind tunnel test results. The full-scale and model-scale wind pressure coefficients (mean, root mean square, maximum and minimum) are compared with each other and analyzed in detail. Moreover, to explore the deviations between the full-scale and model-scale results, the Reynolds number effect (5.5 × 104 to 1.8 × 108) on the wind pressure coefficients, probability distributions, power spectral densities, spatial correlations and coherences of wind pressures are investigated. The comparative study shows that the wind tunnel test can reproduce the wind pressures on the windward face. While for the other faces, the results from the wind tunnel test generally deviate from those from the field measurements. The reasons that caused the discrepancies, especially the Reynolds number effect, are discussed. The objectives of this paper are to examine the accuracy of wind tunnel testing for predicting wind pressures on high-rise buildings under severe wind condition and enhance the understanding of wind pressures on high-rise buildings during strong windstorms, and therefore provide useful information for the wind-resistant design of skyscrapers.

An experimental Investigation of stagnant air pollution dispersion around a building in a turbulent flow

This paper investigates air pollution dispersion and exposure time around a building in a turbulent flow via tracer gas techniques. In the wind tunnel experiments, the tracer gas as the air pollutant model was trapped around the building model and exposed to a boundary layer wind flow. First, flow visualisation was performed, illustrating the highly transient features of wind flow and air pollution dispersion process dominated by underlying wind-structure interaction mechanisms. Next, the concentration field was normalised, analysed and presented as a function of time around the building. Evidently, the stagnant air pollution shields from the wind flow in the building's wake, causing a much longer air pollutant exposure time compared to the regions near the windward and side faces. The air pollutant exposure time in the side region was longer than in the windward region, influenced by the separation-reattachment and bifurcating flow near the side and windward faces, respectively. These results are compiled in order to provide a validation database for similar CFD studies on exposure time. Also, the normalised air pollutant exposure time was scaled up and integrated with air pollutant concentration in the form of two new indices to establish a methodology for the interpretation of air pollutant exposure time in full-scale built-environment scenarios.

CFD of roughness effects on laminar heat transfer applied to additive manufactured minichannels

AbstractAdditive manufacturing has received significant interest in the fabrication of functional channels for heat transfer; however, the inherent rough surface finish of the additively manufactured channels can influence thermal performance. This study investigates the impact of roughness on the thermo-fluid characteristics of laminar forced convection in rough minichannels. A numerical model was developed to create 3D Gaussian roughness with specified root-mean-square height. The finite volume method was used to solve the conjugate heat transfer of developed laminar flow in square minichannels. For Reynolds numbers ranging from 200 to 1600, the simulation results indicated enhanced heat transfer and increased flow resistance as Reynolds number increases, compared to a smooth minichannel, where effects on heat transfer and flow friction were negligible. For channels with relative roughness (root-mean-square height to channel hydraulic diameter) of 0.0068, 0.0113, and 0.0167, increasing the Reynolds number led to increased friction factor by 1.56, 1.71, and 2.91%, while the Nusselt number was enhanced up to 0.03%, 32.74%, and 46.05%, respectively. Heat transfer reduced in roughness valleys due to the presence of local low-velocity fluid in these regions; however, recirculation regions can occur in deep valleys of high roughness, increasing heat transfer and flow friction. Heat transfer was enhanced over roughness peaks due to flow impingement on the windward face of roughness as well as intensified energy transfer to the channel wall from roughness. Moreover, surfaces with higher roughness have a greater number of high peaks providing a thermal-flow path of a larger area and a thermal conductivity greater than that of the fluid.

CFD simulations for evaluating the wind effects on high-rise buildings having varying cross-sectional shape

PurposeIn this study the comparison is presented for the variation in cross-sectional shape along the height of the building model. For this purpose Model B and Model C are having the considerable variation and Model A result can be easily predicted on the basis of the result of Model B and C while Model X is considered for the validation purposes only and it is well established that the results are within the allowable limit. This paper aims to discuss these wind generated effects in the tall building model.Design/methodology/approachComputational Fluid Dynamics (CFD) in ANSYS: CFX is used to investigate the wind effects on varying cross-sectional shape along the height of the building model.FindingsFrom pressure contours, it was observed that shape and size of the face is independent of the pressure distribution. It is also observed that pressure distribution for the windward face (A) was less than the magnitude of the leeward face for both models. The leeward face and lateral faces had similar pressure distribution. Also slight changes in pressure distribution were observed at the periphery of the models.Originality/valueThis study has been performed to analyse and compare the wind effect on tall buildings having varying cross sections with variation of different cross sections along the height. Most of the studies done in the field of tall buildings are concentrated to one particular cross-sectional shape while the present study investigates wind effects for combination of two types of cross sections along the height. This analysis is performed for wind incidence angles ranging from 0° to 90° at an interval of 30°. Analysis of wind flow characteristics of two models, Models B and C will be computed using CFD. These two models are the variation of Model A which is a combination of two types of cross section that is square and plus. Square and plus cross-sectional heights for Model B are 48 m and 144 m, respectively. Similarly, square and plus cross-sectional heights for Model C are 144 m and 48 m, respectively. The results are interpreted using pressure contours and streamlines, and comparative graphs of drag and lift forces are presented.