Turbulent flow in a stably stratified atmosphere

1958 ◽  
Vol 3 (4) ◽  
pp. 361-372 ◽  
Author(s):  
A. A. Townsend
Keyword(s):  
Turbulent Flow ◽  
Richardson Number ◽  
Velocity Fields ◽  
Turbulent Intensity ◽  
Turbulent Motion ◽  
Mean Square ◽  
Upper Limit ◽  
Developing Flow ◽  
The Mean ◽  
Temperature And Velocity Fields

Fluctuations of velocity and temperature which occur in a turbulent flow in a stably-stratified atmosphere far from restraining boundaries are discussed using the equations for the turbulent intensity and for the mean square temperature fluctuation. From these, an equation is derived for the flux Richardson number in terms of the ordinary Richardson number and some non-dimensional ratios connected with the turbulent motion. It is shown that the interaction between the temperature and velocity fields imposes on the flux Richardson number an upper limit of 0·5, and on the ordinary Richardson number a limit of about 0·08. If these values are exceeded, no equilibrium value of the turbulent intensity can exist and a collapse of the turbulent motion would occur. Although the analysis applies strictly only to a homogeneous non-developing flow, it should have approximate validity for effectively homogeneous, developing flows, and the predictions are compared with some recent observations of these flows.

Study on the Characteristics of Turbulent Flow of Viscoelastic Fluid Through a Planar Sudden Expansion by PIV System

2015 ◽  
Author(s):  
Lu Wang ◽  
Jia-Qi Bao ◽  
Tong-Zhou Wei ◽  
Wei-Hua Cai ◽  
Feng-Chen Li
Keyword(s):  
Turbulent Flow ◽  
Particle Image ◽  
Reynolds Shear Stress ◽  
Streamwise Velocity ◽  
Velocity Fields ◽  
Sudden Expansion ◽  
Image Velocimetry ◽  
The Mean ◽  
Cetyltrimethyl Ammonium ◽  
Component Particle

The influences of drag-reducing surfactant additives on the characteristics of a turbulent flow over a planar sudden expansion with expansion ration R = D/d = 3 and aspect ratio A = w/h = 30 were experimentally investigated by a 2D-2C (two dimensional-two component) particle image velocimetry (PIV) system. The 2D-2C velocity fields in the streamwise-wall-normal planes (x-y planes) at three spanwise locations are measured for the flows of water and 50ppm aqueous solution of CTAC/NaSal (CTAC: cetyltrimethyl ammonium chloride; NaSal: sodium salicylate) under the Reynolds number of 1.85 × 104. From the streamline in the x-y plane, it is observed that the reattachment lengths of the vortices in CTAC/NaSal solution are longer. Then the mean streamwise velocity fields and the apparent flow rate at three spanwise locations show that the flow fields in the other two x-y planes are practically symmetrical about the x-y centreplane in CTAC/NaSal solution, as compared with that in water flow. Finally, it is perceived that the Reynolds shear stress for three spanwise locations in CTAC/NaSal solution are obviously decreased.

Numerical Simulations of Turbulent Flow Through a 90° Elbow

2019 ◽  
Author(s):  
Ravon Venters ◽  
Brian Helenbrook ◽  
Goodarz Ahmadi
Keyword(s):  
Turbulent Flow ◽  
Cross Section ◽  
Navier Stokes ◽  
Mean Square ◽  
Reynolds Stresses ◽  
Streamline Curvature ◽  
Secondary Motion ◽  
Flow Through ◽  
The Mean ◽  
Flow Transitions

Abstract Turbulent flow in an elbow has been numerically investigated. The flow was modeled using two approaches; Reynolds Averaged Navier-Stokes (RANS) and Direct Numerical Simulation (DNS) methods. The DNS allows for all the scales of turbulence to be evaluated, providing a detailed depiction of the flow. The RANS simulation, which is typically used in industry, evaluates time-averaged components of the flow. The numerical results are accompanied by experimental data, which was used to validate the two methods. Profiles of the mean and root-mean-square (RMS) fluctuating components were compared at various points along the midplane of the elbow. Upstream of the elbow, the predicted mean and RMS velocities from the RANS and DNS simulations compared well with the experiment, differing slightly near the walls. However, downstream of the elbow, the RANS deviated from the experiment and DNS, showing a longer region of flow re-circulation. This caused the mean and RMS velocities to significantly differ. Examining the cross-section flow field, secondary motion was clearly present. Upstream secondary motion of the first kind was observed which is caused by anisotropy of the reynolds stresses in the turbulent flow. Downstream of the bend, the flow transitions to secondary motion of the second kind which is caused by streamline curvature. Qualitatively, the RANS and DNS showed similar results upstream of the bend, however downstream, the magnitude of the secondary motion differed significantly.

scholarly journals Thermal Comfort Analysis in Naturally-Ventilated Handball Arena Utilizing CFD Techniques

2019 ◽  
Vol 111 ◽  
pp. 01040
Author(s):  
Ahmed A. Masoud ◽  
Essam E. Khalil ◽  
Abdelmaged H. Ibrahim ◽  
Esmail M. ElBialy
Keyword(s):  
Fluid Dynamics ◽  
Thermal Comfort ◽  
Natural Ventilation ◽  
Velocity Fields ◽  
Prevailing Wind ◽  
Training Time ◽  
Wind Speeds ◽  
Upper Limit ◽  
Full Capacity ◽  
Temperature And Velocity Fields

This work investigates the feasibility and thermal comfort of using natural ventilation in order to achieve thermal comfort in a handball arena with realistic dimensions and a full occupation of 4300 persons in the Gulf area. The work numerically simulates the temperature and velocity fields inside the full arena using computational fluid dynamics techniques at different internal loads, prevailing wind speeds, prevailing wind temperatures and prevailing wind angles. The work generates certain air opening configuration to be used for natural ventilation and the results show that natural ventilation is feasible if the following conditions are met simultaneously: the occupation density is 25% or less, sitting in the prevailing wind side, the lighting load does not exceed 50% of its full capacity, the prevailing wind temperature does not exceed 30 °C and the prevailing wind velocity is in range 3-4 m/s, where the upper limit arises from the requirement to avoid high velocities in the playing area. These conditions can be met during the training time and during parts of the day and over parts of the year hours making this method conditionally feasible.

Measurements with a pulsed-wire and a hot-wire anemometer in the highly turbulent wake of a normal flat plate

1976 ◽  
Vol 77 (3) ◽  
pp. 473-497 ◽  
Author(s):  
L. J. S. Bradbury
Keyword(s):  
Turbulent Flow ◽  
Flat Plate ◽  
Flow Direction ◽  
Operating Conditions ◽  
Turbulent Intensity ◽  
Hot Wire ◽  
Mean Flow ◽  
Mean Velocity ◽  
The Mean ◽  
Better Than

This paper describes an investigation into the response of both the pulsed-wire anemometer and the hot-wire anemometer in a highly turbulent flow. The first part of the paper is concerned with a theoretical study of some aspects of the response of these instruments in a highly turbulent flow. It is shown that, under normal operating conditions, the pulsed-wire anemometer should give mean velocity and longitudinal turbulent intensity estimates to an accuracy of better than 10% without any restriction on turbulence level. However, to attain this accuracy in measurements of turbulent intensities normal to the mean flow direction, there is a lower limit on the turbulent intensity of about 50%. An analysis is then carried out of the behaviour of the hot-wire anemometer in a highly turbulent flow. It is found that the large errors that are known to develop are very sensitive to the precise structure of the turbulence, so that even qualitative use of hot-wire data in such flows is not feasible. Some brief comments on the possibility of improving the accuracy of the hot-wire anemometer are then given.The second half of the paper describes some comparative measurements in the highly turbulent flow immediately downstream of a normal flat plate. It is shown that, although it is not possible to interpret the hot-wire results on their own, it is possible to calculate the hot-wire response with a surprising degree of accuracy using the results from the pulsed-wire anemometer. This provides a rather indirect but none the less welcome check on the accuracy of the pulsed-wire results, which, in this very highly turbulent flow, have a certain interest in their own right.

Effect of the Wall on Two-Phase Turbulent Motion

1960 ◽  
Vol 27 (1) ◽  
pp. 5-15 ◽  
Author(s):  
S. L. Soo ◽  
C. L. Tien
Keyword(s):  
Stationary Solution ◽  
Turbulence Intensity ◽  
Solid Particles ◽  
Turbulent Intensity ◽  
Turbulent Motion ◽  
Two Phase ◽  
The Mean

Stationary solution on the effect of a wall on two-phase (solid particles in gas) turbulent motion shows that the intensity of motion of solid particles is affected by the presence of the wall and the distribution of turbulent intensity of the stream near the wall. The intensity of motion of solid particles can be significantly higher than the turbulence intensity of the mean stream. These modifications are consequences of Bernoulli force acting between the wall and the particle.

Non-invasive turbulent mixing across a density interface in a turbulent Taylor–Couette flow

2010 ◽  
Vol 663 ◽  
pp. 347-357 ◽  
Author(s):  
ANDREW W. WOODS ◽  
C. P. CAULFIELD ◽  
J. R. LANDEL ◽  
A. KUESTERS
Keyword(s):  
Root Mean Square ◽  
Richardson Number ◽  
Outer Cylinder ◽  
Salt Transport ◽  
Time Dependent ◽  
Mean Square ◽  
Mean Flow ◽  
Flow Measurements ◽  
Cylindrical Annulus ◽  
The Mean

In this paper, we present new experimental measurements of the turbulent transport of salt across an interface between two layers of fluid of equal depth but different salinities. The fluid is confined to a cylindrical annulus with a vertical axis. The outer cylinder is stationary and the inner cylinder rotates to produce a turbulent flow field consisting of an approximately irrotational mean azimuthal flow, with narrow boundary layers on the inner and outer cylinders. We focus on the limit of high-Richardson-number flow, defined as Ri = gΔρH/(ρ0u2rms), where ρ0 is a reference density, Δρ is the time-dependent difference of the layers' mean densities, urms is the root mean square of the turbulent velocity fluctuations and H is the layer depth. The mean flow has Reynolds number of the order of 104−105, and the turbulent fluctuations in the azimuthal and radial directions have root-mean-square speed of order 10% of the mean azimuthal flow. Measurements based on our experimental system show that when the Richardson number is in the range 7 < Ri < 200, the interface between the two layers remains sharp, each layer remains well mixed, and the vertical flux of salt between the layers, Fs ~(1.15 ± 0.15)Ri−1𝒜(H/ΔR)urmsΔS, where ΔS is the spatially-averaged time-dependent salinity difference between the layers and in general 𝒜(H/ΔR) is a dimensionless function of the tank aspect ratio, here taken to be unity, with ΔR being the gap width of the annulus. The salt transport appears to be caused by turbulent eddies scouring and sharpening the interface and implies a constant rate of conversion of the turbulent kinetic energy to potential energy, independent of the density contrast between the layers. For smaller values of Ri, the flow regime changes qualitatively, with eddies penetrating the interface, causing fluid in the two layers to co-mingle and rapidly homogenize.

Hot-Wire Measurements in a Radial Turbulent Jet

1966 ◽  
Vol 33 (2) ◽  
pp. 417-424 ◽  
Author(s):  
Gunnar Heskestad
Keyword(s):  
Energy Balance ◽  
Velocity Profiles ◽  
Turbulent Jet ◽  
Turbulent Velocity ◽  
Turbulent Motion ◽  
Mean Square ◽  
Hot Wire ◽  
Downstream Location ◽  
Mean Velocity ◽  
The Mean

Results from hot-wire measurements in an approximate radial turbulent incompressible jet of air are reported. In the range of measurements, the jet did not attain a self-preserving form. The mean velocity profiles at various radial locations were quite similar, while the mean-square turbulent velocity profiles were similar only away from the jet center plane, and the lateral intermittency distributions were highly dissimilar. Data for the energy balance of the turbulent motion were obtained at a convenient downstream location.

Turbulent flow over large-amplitude wavy surfaces

1984 ◽  
Vol 140 ◽  
pp. 27-44 ◽  
Author(s):  
Jeffrey Buckles ◽  
Thomas J. Hanratty ◽  
Ronald J. Adrian
Keyword(s):  
Boundary Layer ◽  
Turbulent Flow ◽  
Velocity Fields ◽  
Laser Doppler Velocimeter ◽  
Free Shear Layer ◽  
Wave Surface ◽  
Velocity Measurements ◽  
Mean Velocity ◽  
Wavy Surfaces ◽  
The Mean

The laser-Doppler velocimeter is used to measure the mean and the fluctuating velocity for turbulent flow over a solid sinusoidal wave surface having a wavelength λ of 50.8 mm and a wave amplitude of 5.08 mm. For this flow, a large separated region exists, extending from x/λ = 0.14 to 0.69. From the mean velocity measurements, the time-averaged streamlines and therefore the extent of the separated region are calculated. Three flow elements are identified: the separated region, an attached boundary layer, and a free shear layer formed by the detachment of the boundary layer from the wave surface. The characteristics of these flow elements are discussed in terms of the properties of the mean and fluctuating velocity fields.

Analysis of the thermal plumes in turbulent Rayleigh–Bénard convection based on well-resolved numerical simulations

2009 ◽  
Vol 618 ◽  
pp. 89-112 ◽  
Author(s):  
M. KACZOROWSKI ◽  
C. WAGNER
Keyword(s):  
Passive Scalar ◽  
Velocity Fields ◽  
Core Region ◽  
Thermal Dissipation ◽  
Turbulent Fluctuations ◽  
The Core ◽  
The Mean ◽  
Temperature And Velocity Fields ◽  
Rayleigh Benard Convection ◽  
Rayleigh Benard

In this study, direct numerical simulations and high-resolved large eddy simulations of turbulent Rayleigh–Bénard convection were conducted with a fluid of Prandtl number Pr = 0.7 in a long rectangular cell of aspect ratio unity in the cross-section and periodic boundaries in a horizontal longitudinal direction. The analysis of the thermal and kinetic energy spectra suggests that temperature and velocity fields are correlated within the thermal boundary layers and tend to be uncorrelated in the core region of the flow. A tendency of decorrelation of the temperature and velocity fields is also observed for increasing Ra when the flow has become fully turbulent, which is thought to characterize this regime. This argument is also supported by the analysis of the correlation of the turbulent fluctuations |u|′ and θ′. The plume and mixing layer dominated region is found to be separated from the thermal dissipation rates of the bulk and conductive sublayer by the inflection points of the probability density function (PDF). In order to analyse the contributions of bulk, boundary layers and plumes to the mean heat transfer, the thermal dissipation rate PDFs of four different Ra are integrated over these three regions. Hence, it is shown that the core region is dominated by the turbulent fluctuations of the thermal dissipation rate throughout the range of simulated Ra, whereas the contributions from the conductive sublayer due to turbulent fluctuations increase rapidly with Ra. The latter contradicts results by He, Tong & Xia (Phys. Rev. Lett., vol. 98, 2007). The results also show that the plumes and mixing layers are increasingly dominated by the mean gradient contributions. The PDFs of the core region are compared to an analytical scaling law for passive scalar turbulence which is found to be in good agreement with the results of the present study. It is noted that the core region scaling seems to approach the behaviour of a passive scalar as Ra increases, i.e. it changes from pure exponential to a stretched exponential scaling.

The Bordeaux Automatic Meridian Circle

1978 ◽  
Vol 48 ◽  
pp. 227-228
Author(s):  
Y. Requième
Keyword(s):  
Mean Square Error ◽  
Mean Square ◽  
Meridian Circle ◽  
The Mean ◽  
Full Automation

In spite of important delays in the initial planning, the full automation of the Bordeaux meridian circle is progressing well and will be ready for regular observations by the middle of the next year. It is expected that the mean square error for one observation will be about ±0.”10 in the two coordinates for declinations up to 87°.

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