Aerodynamic noise measurement in anechoic wind tunnel of rod-airfoil with leading edge serrations

Turbulent flow interaction with the leading edge of an airfoil generates a well-known leading edge noise. The present work is an experimental investigation of reducing the leading edge noise by adapting serrations on the leading edge of NACA0012 airfoil. A rod is placed upstream of the airfoil to magnify the turbulence flow-leading edge interaction. Locations and the size of the serrations are two parameters being investigated for the noise reduction. The study found that a noise reduction up to 3.5 dB is obtained when the size of the serration is wider. Additionally, more sound reduction can be obtained when the angle of attack is increased

OpenFOAM implementation for the study of streamwise vortex-induced vibration-based energy harvester for sensor networks

The study of streamwise vortex induced vibration has reached a level of maturity that allows it to be harnessed to generate power. However, studies have primarily concentrated on the variables that measured through point-based instruments. This severely limits our understanding of the fluid forcing mechanism that results in the vibration of the elastically supported bluff body. We proposed the usage of computational fluid dynamics: the open source C++ libraries of OpenFOAM. To implement this successfully to the streamwise vortex-induced vibration problem, which involves near-wall fluid-structure interaction, we explored the method of dynamic mesh handling in OpenFOAM for six degrees of freedom motion of a rigid body fully submerged in fluid. Finally, we argued for the usage of arbitrarily coupled mesh interface to overcome the problem of severely distorted mesh in tight gaps between two walls. We run a short simulation to test this setup and found that the case runs uninterrupted, unlike its former counterpart that relies solely on cell displacement diffusion, suggesting the potential success of a further converged solution of the setup when running on a more powerful machine

Survey of resident behaviour related to air conditioner operation in low-cost apartments of Kuala Lumpur

Predictions of air conditioner (AC) use can greatly facilitate efforts to increase energy efficiency in buildings and promote sustainability. In order to determine the statistical characteristics of occupant behaviour related to AC operation, a series of surveys were conducted between September 2013 and December 2015 in 63 dwellings within two apartments in Kuala Lumpur, Malaysia. Measurements were taken either in the bedroom or living room of each dwelling. The results indicate that occupants who use ACs more frequently tend to use them for longer periods. It was found that AC use in bedrooms is not affected by outdoor temperature but is controlled by the habitual behaviour of the occupant. AC usage in living rooms slightly increases with increases in outdoor temperature. AC usage predominantly occurs at night time, with a maximum of 50 % of occupants using ACs in bedrooms. The start time and duration of use vary for each dwelling. These findings on stochastic resident behaviour might be useful for various building energy simulations in predicting realistic AC loads, particularly in tropical climates

Microwatt energy harvesting by exploiting flow-induced vibration

The green technology approaches by harvesting energy from aerodynamic flowinduced vibrations using a flexible square cylinder is experimentally investigated. The practicability of flow-induced vibration system to supply a sufficient base excitation vibration in microwatt scale is evaluated through a series of wind tunnel tests with different velocities. Test are performed for high Reynolds number 3.9 × 103≤ Re 1.4 × 104 and damping ratio ζ = 0.0052. The experiment setup is able to replicate the pattern of vibration amplitude for isolated square cylinder with previous available study. Then, the experimental setup is used to study the effect of vibration cylinder in harvesting the fluid energy. A prototype of electromagnetic energy harvesting is invented and fabricated to test its performance in the wind tunnel test. Test results reveal that the harnessed power is corresponding to vibration amplitude flow pattern, but the power obtained is much lower than the vibration amplitude due to the power dissipation at the resistor. The best condition for harnessing power is identified at UR = 7.7 where the Karman Vortex-Induced Vibration (KVIV) is the largest

Thermal comfort and occupant adaptive behaviour in university offices with cooling and free running modes

This paper presents results of a small-scale field survey of occupant thermal comfort and adaptive behaviour, conducted in university office buildings in Fukuoka, Japan (August 2014). A comparison was made between offices with cooling (AC) and free running (FR) modes. Indoor environmental conditions were measured, simultaneously with administration of a questionnaire survey. Most Predicted Mean Vote (PMV) values were higher than Thermal Sensation Vote (TSV) values for both cases. This indicates that the PMV model over-predicted actual thermal response. The thermal response in FR offices was found to be more sensitive than in those with AC; additionally, respondents could tolerate a narrower range of variation in indoor operative temperature. Occupants’ adaptive behaviours in AC office were more limited compared to those of respondents with FR. This indicates amenable thermal conditions in A/C-controlled indoor environments, with occupants having no wish to make changes

Sound from high-Reynolds number flow over bluff bodies

Purpose - This paper aims to investigate the aerodynamic sound generated from flow over bluff bodies at a high Reynolds number. By taking circular and square cylinders as two representative geometries for the cross-section of bluff bodies, this study aims to clarify the difference in flow formation and sound generation between the two types of bluff bodies. Furthermore, the possibility for a downstream flat plate to be used as sound cancellation passive mechanism is also discussed in this study. Design/methodology/approach - Sound source from the near field is numerically solved by using the Unsteady Reynolds-Averaged Navier Stokes equations. While for the sound at far-field, the compact sound theory of Curle's analogy is used. Findings - Magnitude of the generated sound is dominant by the aerodynamic forcer fluctuations, i.e. lift and drag, where the lift fluctuation gives the strongest influence on the sound generation. The square cylinder emits 4.7 dB higher than the sound emitted from flow over the circular cylinder. This relates to the longer vortex formation length for the case of square cylinder that provides space for more vortex to dissipate. It is suggested that downstream flat plate is possible to be applied for a sound cancellation mechanism for the case of circular cylinder, but it would be more challenging for the case of square cylinder. Practical implications - This study include implications for the development of noise reduction study especially in high-speed vehicles such as the aircrafts and high-speed trains. Originality/value - This study identified that there is possible method for sound cancellation in flow over bluff body cases by using passive control method, even in flow at high Reynolds number

Benchmark on the Dynamics of Liquid Draining Inside a Tank

Immense information and details observation of flow physics inside a draining tank can be achieved by adopting reliable numerical simulations. Yet the accuracy of numerical results has been always debatable and it is mainly affected by the grid convergence error and computational modeling approaches. Hence, this study is divided into two stages. In the first stage, this paper determines a systematic method of refining a computational grid for a liquid draining inside a tank using OpenFOAM software. The sensitivity of the computed flow field on different mesh resolutions is also examined. In order to study the effect of grid dependency, three different grid refinements are investigated: fine, medium and coarse grids. By using a form of Richardson extrapolation and Grid Convergence Index (GCI), the level of grid independence is attained. In this paper, a monotonic convergence criteria is reached when the fine grid has the GCI value below 10% for each parameter. In the second stage, different computational modeling approaches (DNS, RANS k-ε, RANS k-ω and LES turbulence models) are investigated using the finer grid from the first stage. The results for the draining time and flow visualization of the generation of an air-core are in a good agreement with the available published data. The Direct Numerical Simulation (DNS) seems most reasonably satisfactory for VOF studies relating air-core compared to other different turbulence modeling approaches

Numerical study for flow over a realistic generic model, DrivAer, using URANS

The aim of this study is to present a systematic approach for the grid convergence analysis in simulating flow around a realistic generic model, DrivAer. The numerical simulation is modelled using unsteady Reynolds Averaged Navier-Stokes (URANS) equation. A minimum of three different grid resolutions are considered, which are fine, medium and coarse in order to investigate the grid independency. Richardson extrapolation and Grid Convergence Index (GCI) are introduced to quantitatively evaluate the grid independency. Based on the results between those three different grids, a monotonic convergence criteria has been achieved. The reduction in GCI value indicates that the grid convergence error has been significantly reduced, in which the fine grid has a GCI value less than 7%. Additionally, the result from the fine grid is only 6% difference if compared with previous experimental study

Wind noise from A-pillar and side view mirror of a realistic generic car model, DriAver

Interior noise of a production car is a total contribution mainly from engine, tyres and aerodynamics. At high speed, wind noise can dominate the total interior noise. Wind noise is associated with the unsteadiness of the flow. For most production cars, A-pillar and side view mirror are the regions where the highly separated and turbulent flows are observed. This study quantifies the wind noise contribution from A-pillar and side view mirror with respect to the interior noise of a generic realistic model, DrivAer. The noise sources are obtained numerically from the flow-structure interactions based on the unsteady Reynolds averaged Navier stokes (URANS) while the noise propagation is estimated using Curle's equation of Lighthill acoustic analogy. The sound pressure frequency spectrum of the interior noise is obtained by considering the sound transmission loss from the side glass by using the mass law for transmission loss. The study found that the noise from the A-pillar is higher than the noise from the side view mirror in the whole frequency range. Near the end of the A-pillar component contributes the highest radiated noise level with up to 20 dB louder than that at the front part of the A-pillar

Numerical study on air-core vortex inside draining tank using different computational modelling approaches

Accurate numerical simulation of liquid draining is important to study the physics fluid flow. However, liquid draining involves multiphase and rotational flows, where numerical simulation is expensive to accurately recreate these flow behaviors. The accuracy of numerical results has been also debatable and it is mainly affected by the computational modeling approaches. Therefore, this study evaluates different computational modelling approaches such as DNS, RANS k-ε, RANS k-ω and LES turbulence models. The results for the draining time and flow visualization of the generation of an air-core are in a good agreement with the available published data. The Direct Numerical Simulation (DNS) seems most reasonably satisfactory for VOF studies relating air-core compared to other different turbulence modeling approaches