scholarly journals Effects of roof configuration on natural ventilation for an isolated building

2021 ◽  
Vol 15 (3) ◽  
pp. 8379-8389
Author(s):  
Lip Kean Moey ◽  
Man Fai Kong ◽  
Vin Cent Tai ◽  
Tze Fong Go ◽  
Nor Mariah Adam
Keyword(s):  
Kinetic Energy ◽  
Natural Ventilation ◽  
Velocity Ratio ◽  
Mixing Layer ◽  
Flow Characteristics ◽  
Mean Velocity ◽  
Gable Roof ◽  
Building Models ◽  
Large Vortex ◽  
Field Information

Numerical analyses based on CFD steady RANS were conducted to investigate the effects of roof configuration on wind-induced natural ventilation for an isolated roofed building. Gable roof and saltbox roof building models were tested with 15˚, 25˚, 35˚ and 45˚ roof pitch in present study. The flow field information and flow characteristics were obtained from the contours and plots generated by CFD. In accordance to the increment of roof pitch, the turbulence kinetic energy and mean velocity ratio show vigorous response. The flow separated at the windward corner do not reattach onto the roof, thus induced higher velocity gradient and form a large vortex at the roof ridge. The vortices behind then building caused by the flow separation at the roof ridge extend along the mixing layer and spread up to the roof. The pressure differences mainly rely on the roof shapes. Greater pressure differences between the upstream, interior and downstream was observed in saltbox roof cases. This is due to the extended roof height which boosted the impinging effect caused by the incoming wind. Generally, the saltbox roof configuration exhibit better performance than gable roof in terms of the measured parameters.

scholarly journals Analysis of a Converging, Annular, Isothermal Melt Blowing Jet Using Computational Fluid Dynamics

2005 ◽  
Vol os-14 (3) ◽  
pp. 1558925005os-14
Author(s):  
Eric M. Moore ◽  
Dimitrios V. Papavassiliou ◽  
Robert L. Shambaugh
Keyword(s):  
Fluid Dynamics ◽  
Computational Fluid Dynamics ◽  
Kinetic Energy ◽  
Near Field ◽  
Flow Characteristics ◽  
Sectional Area ◽  
Melt Blowing ◽  
Mean Velocity ◽  
Cross Sectional ◽  
Computational Fluid Dynamics Cfd

An unconventional melt blowing die was analyzed using computational fluid dynamics (CFD). This die has an annular configuration wherein the jet inlet is tapered (the cross-sectional area decreases) as the air approaches the die face. It was found that the flow characteristics of this die are different from conventional slot and annular dies. In particular, for the tapered die the near-field normalized turbulent kinetic energy was found to be lower at shallow die angles. Also, it was found that the peak mean velocity behavior was intermediate between that of conventional annular and slot dies. The centerline turbulence profiles were found to be qualitatively similar to those of annular dies; quantitatively, higher values were present for tapered dies.

The stability of countercurrent mixing layers in circular jets

1991 ◽  
Vol 227 ◽  
pp. 309-343 ◽  
Author(s):  
P. J. Strykowski ◽  
D. L. Niccum
Keyword(s):  
Shear Layer ◽  
Critical Velocity ◽  
Velocity Ratio ◽  
Mixing Layer ◽  
Mixing Layers ◽  
Mean Velocity ◽  
Axisymmetric Vortex ◽  
Circular Jets ◽  
Axisymmetric Shear ◽  
The Stability

A spatially developing countercurrent mixing layer was established experimentally by applying suction to the periphery of an axisymmetric jet. A laminar mixing region was studied in detail for a velocity ratio R = ΔU/2U between 1 and 1.5, where ΔU describes the intensity of the shear across the layer and U is the average speed of the two streams. Above a critical velocity ratio Rr = 1.32 the shear layer displays energetic oscillations at a discrete frequency which are the result of very organized axisymmetric vortex structures in the mixing layer. The spatial order of the primary vortices inhibits the pairing process and dramatically alters the spatial development of the shear layer downstream. Consequently, the turbulence level in the jet core is significantly reduced, as is the decay rate of the mean velocity on the jet centreline. The response of the shear layer to controlled external forcing indicates that the shear layer oscillations at supercritical velocity ratios are self-excited. The experimentally determined critical velocity ratio of 1.32, established for very thin axisymmetric shear layers, compares favourably with the theoretically predicted value of 1.315 for the transition from convective to absolute instability in plane mixing layers (Huerre & Monkewitz 1985).

Flow Characteristics of Swirling Coaxial Jets From Divergent Nozzles

1987 ◽  
Vol 109 (3) ◽  
pp. 275-282 ◽  
Author(s):  
T. Mahmud ◽  
J. S. Truelove ◽  
T. F. Wall
Keyword(s):  
Bituminous Coal ◽  
Static Pressure ◽  
Reverse Flow ◽  
Velocity Ratio ◽  
Flow Characteristics ◽  
Aerodynamic Characteristics ◽  
Flow Zone ◽  
Mean Velocity ◽  
Coaxial Jets ◽  
Mass Flowrate

The aerodynamic characteristics of free, swirling, coaxial jets issuing from an air model of a typical burner for pulverized bituminous coal have been studied. Detailed measurements of mean velocity and static pressure have been obtained in the region near the nozzle exit. The boundary of the reverse-flow zone has been mapped and the recirculated-mass flowrate measured in order to quantify the effects of velocity ratio and swirl in the primary and secondary jets. The influence of burner geometry (divergent-nozzle length and centre-line blockage) has also been studied. The type of flow pattern is found to depend upon the level of swirl in the primary and secondary jets. The recirculated-mass flowrate is predominantly influenced by secondary swirl. The measurements have been compared with predictions obtained by numerical solution of the governing conservation equations in orthogonal curvilinear co-ordinates. The general features of the flows are adequately predicted although discrepancies in detail seem to indicate deficiencies in the turbulence model.

Turbulence in Wind Turbine Wake: Effect of Atmospheric Forcings

2014 ◽  
Author(s):  
Kiran Bhaganagar ◽  
Mithu Debnath
Keyword(s):  
Mixing Layer ◽  
Average Power ◽  
Flow Characteristics ◽  
Atmospheric Conditions ◽  
Wake Effect ◽  
Mean Velocity ◽  
Eddy Simulation ◽  
Vortex Merging ◽  
Horizontal Axis Wind Turbines ◽  
Atmospheric Forcings

Large eddy simulation (LES) is used as a tool to understand the near-wake effects of large 5-MW, 3-blade horizontal-axis wind turbines (WT) in convective atmospheric boundary layer (ABL). The simulations are performed for two inline WT separated by distance of 2.5D (D is diameter of the rotor) in unstable ABL so that the downstream WT is operated under the wake of the upstream WT. The flow characteristics are analyzed in the wake regions behind WT to understand the flow physics. Tip and root vortices undergo vortex merging due to instability. Turbulent mixing layer that develops in the wake region is stronger for the downstream WT. The rate of growth/decay of the mean velocity and turbulence is much higher for WT2 than WT1. The time evolution of the wake of WT1 and WT2 revealed additional wake induced shear that contributes to faster turbulence diffusion which results in shrinking of the shear layer (in height) downstream. The average power output of WT2 is 40% lower than WT1 during unstable stratified atmospheric conditions.

A K—ε—γ equation turbulence model

1992 ◽  
Vol 237 ◽  
pp. 301-322 ◽  
Author(s):  
Ji Ryong Cho ◽  
Myung Kyoon Chung
Keyword(s):  
Kinetic Energy ◽  
Turbulent Kinetic Energy ◽  
Turbulence Model ◽  
Mixing Layer ◽  
Spreading Rate ◽  
Reynolds Stresses ◽  
Mean Velocity ◽  
Model Equations ◽  
Entrainment Effect ◽  
Intermittency Factor

By considering the entrainment effect on the intermittency in the free boundary of shear layers, a set of turbulence model equations for the turbulent kinetic energy k, the dissipation rate ε, and the intermittency factor γ is proposed. This enables us to incorporate explicitly the intermittency effect in the conventional K–ε turbulence model equations. The eddy viscosity νt is estimated by a function of K, ε and γ. In contrast to the closure schemes of previous intermittency modelling which employ conditional zone averaged moments, the present model equations are based on the conventional Reynolds averaged moments. This method is more economical in the sense that it halves the number of partial differential equations to be solved. The proposed K–ε–γ model has been applied to compute a plane jet, a round jet, a plane far wake and a plane mixing layer. The computational results of the model show considerable improvement over previous models for all these shear flows. In particular, the spreading rate, the centreline mean velocity and the profiles of Reynolds stresses and turbulent kinetic energy are calculated with significantly improved accuracy.

Turbulent Flow Measurements and Visualizations of Supersonic Jets Impinging on a Flat Plate and an Inclined Plate

2020 ◽  
Author(s):  
Thien D. Nguyen ◽  
Blake Maher ◽  
Yassin A. Hassan
Keyword(s):  
Kinetic Energy ◽  
Flat Plate ◽  
Pressure Ratio ◽  
Flow Characteristics ◽  
Field Measurements ◽  
Central Plane ◽  
Inclined Plate ◽  
Mean Velocity ◽  
Flow Measurements ◽  
Full Field

Abstract This study experimentally investigates the flow characteristics of a high-pressure air jets impinging on a flat plate and an inclined plate with various nozzle-to-plane gaps of 10 mm, 20 mm, and 30 mm. Full-field measurements of flow characteristics in the central plane of the nozzle and near the impinging surface are performed using two-dimensional two-component (2D2C) particle image velocimetry (PIV) technique. This paper presents results from the nozzle pressure ratio (NPR) of 2.77, approximately yielding the sonic jet with Mach number of 1.2. Flow characteristics obtained from the 2D2C-PIV measurements with various spatial gaps are compared and presented. Results including the first- and second-order flow statistics, such as mean velocity and turbulent kinetic energy, and effects of the impinging surface to the flow patterns are investigated. Finally, proper orthogonal decomposition (POD) analysis is applied to reveal the statistically dominant flow structures that capture the highest amount flow kinetic energy and play important roles to the flow dynamics and heat transfers.

Experimental investigation of jets in a crossflow

1984 ◽  
Vol 138 ◽  
pp. 93-127 ◽  
Author(s):  
J. Andreopoulos ◽  
W. Rodi
Keyword(s):  
Kinetic Energy ◽  
Turbulent Kinetic Energy ◽  
Velocity Ratio ◽  
Three Dimensional ◽  
Turbulent Shear ◽  
Shear Stresses ◽  
Hot Wire ◽  
Mean Flow ◽  
Mean Velocity ◽  
Turbulent Kinetic Energy Equation

The paper reports on measurements in the flow generated by a jet issuing from a circular outlet in a wall into a cross-stream along this wall. For the jet-to-crossflow velocity ratios R of 0.5, 1 and 2, the mean and fluctuating velocity components were measured with a three-sensor hot-wire probe. The hot-wire signals were evaluated to yield the three mean-velocity components, the turbulent kinetic energy, the three turbulent shear stresses and, in the case of R = 0.5, the terms in the turbulent-kinetic-energy equation. The results give a quantitative picture of the complex three-dimensional mean flow and turbulence field, and the various phenomena as well as their dependence on the velocity ratio R are discussed in detail.

Experimental Study of Flow Structure Characteristics for a T-Junction Duct With Horizontal Vanes

2019 ◽  
Vol 141 (11) ◽  
Author(s):  
Shicong Li ◽  
Xiaoyu Wang ◽  
Jing He ◽  
Mei Lin ◽  
Hanbing Ke
Keyword(s):  
Experimental Study ◽  
Flow Field ◽  
Separation Bubble ◽  
Velocity Ratio ◽  
Flow Characteristics ◽  
Vortex Sheet ◽  
Trailing Edge ◽  
Branch Duct ◽  
Instantaneous Flow ◽  
Large Vortex

An experimental study is carried out to investigate the flow characteristics of the trailing edge of the horizontal vanes mounted at the branch entrance of a T-junction duct by means of particle image velocimetry (PIV). The measured region starts at the trailing edge of the vanes and ends at about 1.26D (hydraulic diameter) length at downstream of the branch duct. The velocity field is obtained across a number of vertical height planes (z/D = ±0.2, 0, and −0.4) under different flow conditions (cross velocity: uc = 30–50 m/s; velocity ratio: R = 0.08–0.18). The instantaneous flow results show that Kelvin-like vortices with counter-clockwise direction appear at the heights of z/D = ±0.2 and 0, and that a separation bubble is formed at the upper wall of the branch duct at the same heights, respectively. As for near wall z/D = −0.4, one large vortex is observed at the downstream channel, but the separation bubble vanishes as the branching Reynolds number is increased to 3.6 × 104. The time-average flow field is slightly different from that of instantaneous flow field. In addition, the vorticity distribution indicates that two significant vortex sheet layers with negative and positive values are found at the high velocity ratio or high cross velocity, and the normalized vorticity strength increases with increasing velocity ratio and decreases with increasing cross velocity except at z/D = −0.4.

Numerical Study of Counter Jet Formed by Impinging Jets in Cross-Flow and its Effect on Mixing

2017 ◽  
Author(s):  
Thomas A. Epalle ◽  
Fabien Gaugain ◽  
Vincent Melot ◽  
Nasser Darabiha ◽  
Olivier Gicquel
Keyword(s):  
Numerical Study ◽  
Velocity Ratio ◽  
Flow Characteristics ◽  
Cross Flow ◽  
Passive Scalar ◽  
Impinging Jets ◽  
Mean Velocity ◽  
Plane Flow ◽  
Flow Mixing ◽  
Mixing Chambers

In this paper we will numerically analyse flow mixing in multiple jets in a crossflow. The system comprises a row of six radially-distributed injectors around the main pipe. The configuration represents mixing zones in industrial systems where a counter jet can be formed in the injection plane. Flow mixing can be modified as a result of geometry and injection velocities. We propose a simple model to describe the counter jet length as a function of injection flow characteristics. We also develop empirical laws to help engineers design practical test facilities. We then vary the velocity ratio to obtain both impinging and non-impinging jets in the injection plane. The focus is mainly on flow characteristics around the radial injection plane in the case of impinging jets, examining the mixing quality and efficiency by introducing a passive scalar discharge in a nitrogen flow. The mean velocity and width of the counter jet are finally analyzed by changing the injection velocities. These results are compared to those of non-impinging jets. It is found that the non-impinging jet configurations are convenient for short length mixing chambers, while the impinging ones should be considered in the case of longer mixing chambers.

scholarly journals Effect of Mean Velocity-to-Critical Velocity Ratios on Bed Topography and Incipient Motion in a Meandering Channel: Experimental Investigation

Water ◽  
2021 ◽  
Vol 13 (7) ◽  
pp. 883
Author(s):  
Nargess Moghaddassi ◽  
Seyed Habib Musavi-Jahromi ◽  
Mohammad Vaghefi ◽  
Amir Khosrojerdi
Keyword(s):  
Experimental Investigation ◽  
Critical Velocity ◽  
Velocity Ratio ◽  
Scour Depth ◽  
Incipient Motion ◽  
Mean Velocity ◽  
Straight Path ◽  
Meandering Channel ◽  
Bed Topography ◽  
The Mean

As 180-degree meanders are observed in abundance in nature, a meandering channel with two consecutive 180-degree bends was designed and constructed to investigate bed topography variations. These two 180-degree mild bends are located between two upstream and downstream straight paths. In this study, different mean velocity-to-critical velocity ratios have been tested at the upstream straight path to determine the meander’s incipient motion. To this end, bed topography variations along the meander and the downstream straight path were addressed for different mean velocity-to-critical velocity ratios. In addition, the upstream bend’s effect on the downstream bend was investigated. Results indicated that the maximum scour depth at the downstream bend increased as a result of changing the mean velocity-to-critical velocity ratio from 0.8 to 0.84, 0.86, 0.89, 0.92, 0.95, and 0.98 by, respectively, 1.5, 2.5, 5, 10, 12, and 26 times. Moreover, increasing the ratio increased the maximum sedimentary height by 3, 10, 23, 48, 49, and 56 times. The upstream bend’s incipient motion was observed for the mean velocity-to-critical velocity ratio of 0.89, while the downstream bend’s incipient motion occurred for the ratio of 0.78.

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