Aerodynamic admittance of a 6:1 rectangular cylinder: A computational study on the role of turbulence intensity and integral length scale

2021 ◽  
Vol 218 ◽  
pp. 104738
Author(s):  
Weilin Li ◽  
L. Patruno ◽  
Huawei Niu ◽  
Stefano de Miranda ◽  
Xugang Hua
Keyword(s):  
Turbulence Intensity ◽  
Computational Study ◽  
Integral Length Scale ◽  
Length Scale ◽  
Rectangular Cylinder

Lateral dispersion in random cylinder arrays at high Reynolds number

2008 ◽  
Vol 600 ◽  
pp. 339-371 ◽  
Author(s):  
YUKIE TANINO ◽  
HEIDI M. NEPF
Keyword(s):  
Turbulence Intensity ◽  
Turbulent Diffusion ◽  
Volume Fraction ◽  
Solid Volume Fraction ◽  
Laboratory Data ◽  
Integral Length Scale ◽  
Length Scale ◽  
Linear Superposition ◽  
Cylinder Diameter ◽  
The Mean

Laser-induced fluorescence was used to measure the lateral dispersion of passive solute in random arrays of rigid, emergent cylinders of solid volume fraction φ=0.010–0.35. Such densities correspond to those observed in aquatic plant canopies and complement those in packed beds of spheres, where φ≥0.5. This paper focuses on pore Reynolds numbers greater than Res=250, for which our laboratory experiments demonstrate that the spatially averaged turbulence intensity and Kyy/(Upd), the lateral dispersion coefficient normalized by the mean velocity in the fluid volume, Up, and the cylinder diameter, d, are independent of Res. First, Kyy/(Upd) increases rapidly with φ from φ =0 to φ=0.031. Then, Kyy/(Upd) decreases from φ=0.031 to φ=0.20. Finally, Kyy/(Upd) increases again, more gradually, from φ=0.20 to φ=0.35. These observations are accurately described by the linear superposition of the proposed model of turbulent diffusion and existing models of dispersion due to the spatially heterogeneous velocity field that arises from the presence of the cylinders. The contribution from turbulent diffusion scales with the mean turbulence intensity, the characteristic length scale of turbulent mixing and the effective porosity. From a balance between the production of turbulent kinetic energy by the cylinder wakes and its viscous dissipation, the mean turbulence intensity for a given cylinder diameter and cylinder density is predicted to be a function of the form drag coefficient and the integral length scale lt. We propose and experimentally verify that lt=min{d, 〈sn〉A}, where 〈sn〉A is the average surface-to-surface distance between a cylinder in the array and its nearest neighbour. We farther propose that only turbulent eddies with mixing length scale greater than d contribute significantly to net lateral dispersion, and that neighbouring cylinder centres must be farther than r* from each other for the pore space between them to contain such eddies. If the integral length scale and the length scale for mixing are equal, then r*=2d. Our laboratory data agree well with predictions based on this definition of r*.

scholarly journals Turbulence Intensity and Length Scale Effects on Premixed Turbulent Flame Propagation

2021 ◽  
Author(s):  
Shrey Trivedi ◽  
R. S. Cant
Keyword(s):  
Strain Rate ◽  
Flame Propagation ◽  
Turbulence Intensity ◽  
Turbulent Flame ◽  
Integral Length Scale ◽  
Length Scale ◽  
Tangential Strain ◽  
Turbulent Flame Speed ◽  
Turbulence Length ◽  
Premixed Turbulent Flame

AbstractThe effects of varying turbulence intensity and turbulence length scale on premixed turbulent flame propagation are investigated using Direct Numerical Simulation (DNS). The DNS dataset contains the results of a set of turbulent flame simulations based on separate and systematic changes in either turbulence intensity or turbulence integral length scale while keeping all other parameters constant. All flames considered are in the thin reaction zones regime. Several aspects of flame behaviour are analysed and compared, either by varying the turbulence intensity at constant integral length scale, or by varying the integral length scale at constant turbulence intensity. The turbulent flame speed is found to increase with increasing turbulence intensity and also with increasing integral length scale. Changes in the turbulent flame speed are generally accounted for by changes in the flame surface area, but some deviation is observed at high values of turbulence intensity. The probability density functions (pdfs) of tangential strain rate and mean flame curvature are found to broaden with increasing turbulence intensity and also with decreasing integral length scale. The response of the correlation between tangential strain rate and mean flame curvature is also investigated. The statistics of displacement speed and its components are analysed, and the findings indicate that changes in response to decreasing integral length scale are broadly similar to those observed for increasing turbulence intensity, although there are some interesting differences. These findings serve to improve current understanding of the role of turbulence length scales in flame propagation.

Whole Process Wind Characteristics Field Measurements of a Strong Wind

2011 ◽  
Vol 243-249 ◽  
pp. 5094-5100 ◽  
Author(s):  
Ke Yang ◽  
Wen Hai Shi ◽  
Zheng Nong Li
Keyword(s):  
Wind Speed ◽  
Turbulence Intensity ◽  
Field Measurements ◽  
Integral Length Scale ◽  
Length Scale ◽  
Strong Wind ◽  
Gust Factor ◽  
Mean Wind Speed ◽  
Wind Characteristics ◽  
The Mean

This paper presents field measurement results of boundary layer wind characteristics over typical open country during the passages of typhoon Fung-wong passed by Wenzhou in July 2008. The field data such as wind speed and wind direction were measured from two propeller anemometers placed at the height of about 30m. The measured wind data are analyzed to obtain the information on mean wind speed and direction, turbulence intensity, gust factor, turbulence integral length scale and spectra of wind speed fluctuations. The results clearly demonstrate that the turbulence intensity and gust factor of typhoon Fung-wong are larger than normal, and there is a tendency for the turbulence intensities to decrease with the increase of the mean wind speed, however, there is another tendency for the turbulence integral length scale to increase with the increase of the mean wind speed. The power spectral densities of fluctuating wind speed in longitudinal and lateral directions obtained from the measured wind speed data roughly fit with Von Karman spectra. The results presented in this paper are expected to be of use to researchers and engineers involved in design of low-rise buildings.

Experiments on the Physical Mechanism of Heat Transfer Augmentation by Freestream Turbulence at a Cylinder Stagnation Point

2005 ◽  
Author(s):  
A. C. Nix ◽  
T. E. Diller
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Stagnation Point ◽  
Turbulence Intensity ◽  
Integral Length Scale ◽  
Length Scale ◽  
Freestream Turbulence ◽  
Hot Wire ◽  
Velocity Signal ◽  
Heat Transfer Augmentation

Detailed time records of velocity and heat flux were measured near the stagnation point of a cylinder in low-speed air flow. The freestream turbulence was controlled using five different grids positioned to match the characteristics from previous heat flux experiments at NASA Glenn using the same wind tunnel. A hot wire was used to measure the cross-flow velocity at a range of positions in front of the stagnation point. This gave the average velocity and fluctuating component including the turbulence intensity and integral length scale. The heat flux was measured with a Heat Flux Microsensor located on the stagnation line underneath the hot-wire probe. This gave the average heat flux and the fluctuating component simultaneous with the velocity signal, including the heat flux turbulence intensity and the coherence with the velocity. The coherence between the signals allowed identification of the crucial positions for measurement of the integral length scale and turbulence intensity for prediction of the time average surface heat flux. The frequencies corresponded to the most energetic frequencies of the turbulence, indicating the importance of the penetration of the turbulent eddies from the freestream through the boundary layer to the surface. The distance from the surface was slightly less than the local value of length scale, indicating the crucial role of the turbulence in augmenting the heat flux. The resulting predictions of the analytical model matched well with the measured heat transfer augmentation.

Experiments on the Physical Mechanism of Heat Transfer Augmentation by Freestream Turbulence at a Cylinder Stagnation Point

2009 ◽  
Vol 131 (2) ◽  
Author(s):  
A. C. Nix ◽  
T. E. Diller
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Stagnation Point ◽  
Turbulence Intensity ◽  
Integral Length Scale ◽  
Length Scale ◽  
Freestream Turbulence ◽  
Hot Wire ◽  
Velocity Signal ◽  
Heat Transfer Augmentation

Detailed time records of velocity and heat flux were measured near the stagnation point of a cylinder in low-speed airflow. The freestream turbulence was controlled using five different grids positioned to match the characteristics from previous heat flux experiments at NASA Glenn using the same wind tunnel. A hot wire was used to measure the cross-flow velocity at a range of positions in front of the stagnation point. This gave the average velocity and fluctuating component including the turbulence intensity and integral length scale. The heat flux was measured with a heat flux microsensor located on the stagnation line underneath the hot-wire probe. This gave the average heat flux and the fluctuating component simultaneous with the velocity signal, including the heat flux turbulence intensity and the coherence with the velocity. The coherence between the signals allowed identification of the crucial positions for measurement of the integral length scale and turbulence intensity for prediction of the time-averaged surface heat flux. The frequencies corresponded to the most energetic frequencies of the turbulence, indicating the importance of the penetration of the turbulent eddies from the freestream through the boundary layer to the surface. The distance from the surface was slightly less than the local value of length scale, indicating the crucial role of the turbulence in augmenting the heat flux. The resulting predictions of the analytical model matched well with the measured heat transfer augmentation.

Free-Stream Turbulence Effects on Film Cooling With Shaped Holes

2003 ◽  
Vol 125 (1) ◽  
pp. 65-73 ◽  
Author(s):  
Christian Saumweber ◽  
Achmed Schulz ◽  
Sigmar Wittig
Keyword(s):  
Turbulence Intensity ◽  
Film Cooling ◽  
Free Stream ◽  
Operating Conditions ◽  
Integral Length Scale ◽  
Length Scale ◽  
Free Stream Turbulence ◽  
Blowing Ratio ◽  
Cylindrical Holes ◽  
The Impact

A comprehensive set of generic experiments has been conducted to investigate the effect of elevated free-stream turbulence on film cooling performance of shaped holes. A row of three cylindrical holes as a reference case, and two rows of holes with expanded exits, a fanshaped (expanded in lateral direction), and a laidback fanshaped hole (expanded in lateral and streamwise direction) have been employed. With an external (hot gas) Mach number of Mam=0.3 operating conditions are varied in terms of free-stream turbulence intensity (up to 11%), integral length scale at constant turbulence intensity (up to 3.5 hole inlet diameters), and blowing ratio. The temperature ratio is fixed at 0.59 leading to an enginelike density ratio of 1.7. The results indicate that shaped and cylindrical holes exhibit very different reactions to elevated free-stream turbulence levels. For cylindrical holes film cooling effectiveness is reduced with increased turbulence level at low blowing ratios whereas a small gain in effectiveness can be observed at high blowing ratios. For shaped holes, increased turbulence intensity is detrimental even for the largest blowing ratio M=2.5. In comparison to the impact of turbulence intensity the effect of varying the integral length scale is found to be of minor importance. Finally, the effect of elevated free-stream turbulence in terms of heat transfer coefficients was found to be much more pronounced for the shaped holes.

Free-Stream Turbulence Effects on Film Cooling With Shaped Holes

2002 ◽  
Author(s):  
Christian Saumweber ◽  
Achmed Schulz ◽  
Sigmar Wittig
Keyword(s):  
Turbulence Intensity ◽  
Film Cooling ◽  
Free Stream ◽  
Operating Conditions ◽  
Integral Length Scale ◽  
Length Scale ◽  
Free Stream Turbulence ◽  
Blowing Ratio ◽  
Cylindrical Holes ◽  
The Impact

A comprehensive set of generic experiments has been conducted to investigate the effect of elevated free-stream turbulence on film cooling performance of shaped holes. A row of three cylindrical holes as a reference case, and two rows of holes with expanded exits, a fanshaped (expanded in lateral direction), and a laidback fanshaped hole (expanded in lateral and streamwise direction) have been employed. With an external (hot gas) Mach number of Mam = 0.3 operating conditions are varied in terms of free-stream turbulence intensity (up to 11%), integral length scale at constant turbulence intensity (up to 3.5 hole inlet diameters), and blowing ratio. The temperature ratio is fixed at 0.59 leading to an engine-like density ratio of 1.7. The results indicate that shaped and cylindrical holes exhibit very different reactions to elevated free-stream turbulence levels. For cylindrical holes film cooling effectiveness is reduced with increased turbulence level at low blowing ratios whereas a small gain in effectiveness can be observed at high blowing ratios. For shaped holes, increased turbulence intensity is detrimental even for the largest blowing ratio (M = 2.5). In comparison to the impact of turbulence intensity the effect of varying the integral length scale is found to be of minor importance. Finally the effect of elevated free-stream turbulence in terms of heat transfer coefficients was found to be much more pronounced for the shaped holes.

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