Experimental and Numerical Studies of Cold Inflow at the Exit of Buoyant Channel Flows

1987 ◽  
Vol 109 (2) ◽  
pp. 392-399 ◽  
Author(s):  
Vijay Modi ◽  
K. E. Torrance
Keyword(s):  
Boundary Layer ◽  
Numerical Study ◽  
Reynolds Numbers ◽  
Critical Value ◽  
Ambient Fluid ◽  
Buoyant Jet ◽  
Wall Boundary Layer ◽  
Numerical Studies ◽  
Turbulent Flow Regime ◽  
Neutrally Buoyant

Experimental and numerical studies of the separation of a smooth attached buoyant flow from the inner wall of a duct, as the duct discharges into a quiescent environment, are reported. The associated penetration of neutrally buoyant ambient fluid into the duct is called cold inflow. The experimental study was carried out for air flows over ranges of Reynolds and Froude numbers, based on duct radius, of Re = 2400 to 3300 and Fr = 0.68 to 2.69. The experiments provide information on the onset and extent of cold inflow in a turbulent flow regime. Spatial profiles of fluctuating temperature reveal a wedge-shaped cold inflow region at the wall near the exit when Fr is decreased below a critical value. The numerical study examines the influence of Re and Fr on the structure of the cold inflow phenomenon at moderate Reynolds numbers (Re = 200 to 500 and Fr = 1 to 5). Steady-state, two-dimensional, laminar flow solutions reveal a region of downward-flowing cold air near the wall of the duct which leads to premature separation of the wall boundary layer. The separated boundary layer merges into the buoyant jet above the duct exit.

scholarly journals Heat Transfer Enhancement and Hydrodynamic Characteristics of Nanofluid in Turbulent Flow Regime

2015 ◽  
Vol 2015 ◽  
pp. 1-6 ◽  
Author(s):  
Mohammad Nasiri-lohesara
Keyword(s):  
Heat Transfer ◽  
Numerical Study ◽  
Pure Water ◽  
Reynolds Numbers ◽  
Hydrodynamic Characteristics ◽  
Two Phase ◽  
Tube Heat Exchanger ◽  
Turbulent Flow Regime ◽  
Lower Slope ◽  
Good Agreement

Turbulent forced convection ofγ-Al2O3/water nanofluid in a concentric double tube heat exchanger has been investigated numerically using mixture two-phase model. Nanofluids are used as coolants flowing in the inner tube while hot pure water flows in outer tube. The studies are conducted for Reynolds numbers ranging from 20,000 to 50,000 and nanoparticle volume fractions of 2, 3, 4, and 6 percent. Results showed that nanofluid has no effects on fully developed length and average heat transfer coefficient enhances with lower slope than wall shear stress. Comparisons with experimental correlation in literature are conducted and good agreement with present numerical study is achieved.

Viscous flow past a flat plate with uniform injection

1965 ◽  
Vol 284 (1398) ◽  
pp. 370-396 ◽  
Keyword(s):  
Boundary Layer ◽  
Flat Plate ◽  
Numerical Study ◽  
Central Zone ◽  
Outer Zone ◽  
Analytic Study ◽  
Boundary Layer Equations ◽  
Numerical Studies ◽  
High Degree ◽  
Uniform Injection

The boundary-layer equations for an incompressible fluid in motion past a flat plate are examined, numerically and analytically, in the special case when the pressure gradient vanishes and there is a uniform injection of fluid from the plate. In the numerical study the principal properties of the boundary layer are computed as far as separation ( x ═ x δ ≑ 0.7456) with a high degree of accuracy. In the analytic study the structure of the singularity at separation is determined. It is of a new kind in boundary layer theory and its elucidation requires the division of the boundary layer into three zones—an outer zone in which the non-dimensional velocity u is much larger than x * (the non-dimensional distance from separation), a central zone in which u ~ x * and an inner zone in which u ≪ x *. A match is effected between solutions in the central and inner zones from which it is inferred that the skin friction τ 0 ~ ( x * / In (1/ x *) 2 as x * → 0. A completely satisfactory agreement between the numerical and analytic studies was not possible. The reason is that the analytic study is only valid when ln ( 1 / x *) ≫ 1 which means that for the analytic and numerical studies to have a common region of validity, the numerical integration must be extended to much smaller values of x * than is possible at present. It was also not possible to effect a match between the central and outer zones in the analytic solution due to the difficulty of finding the properties of the stress τ in the central zone as u / x * →∞.

Transition in Pressure-Surface Boundary Layers

1987 ◽  
Vol 109 (2) ◽  
pp. 296-302 ◽  
Author(s):  
R. I. Crane ◽  
G. Leoutsakos ◽  
J. Sabzvari
Keyword(s):  
Boundary Layer ◽  
Water Channel ◽  
Outer Wall ◽  
Reynolds Numbers ◽  
Surface Boundary ◽  
Mean Velocity ◽  
Pressure Surface ◽  
Wall Boundary Layer ◽  
Transition Criteria ◽  
Dimensional Boundary

Laminar-to-turbulent transition in the presence of Go¨rtler vortices has been investigated experimentally, in the outer wall boundary layer of a curved water channel. Ratios of boundary layer thickness at the start of curvature to wall radius were around 0.05 and core flow turbulence intensities were between 1 and 3 percent. Measurements of intermittency factor were made by hot film probe and of mean and rms velocity by laser anemometer. At Reynolds numbers low enough to allow considerable nonlinear vortex amplification in the laminar region, transition was found to begin sooner and progress faster at a vortex upwash position than at a spanwise-adjacent downwash position. Measured Go¨rtler numbers at transition onset bore little relationship to those often used as transition criteria in two-dimensional boundary layer prediction codes. Little spanwise variation in intermittency occurred at higher Reynolds numbers, where mean velocity profiles at upwash were much less inflected. Toward the end of curvature, favorable pressure gradients estimated to exceed the Launder relaminarization value corresponded with cases of incomplete transition.

A numerical study of the dynamics of a particle settling at moderate Reynolds numbers in a linearly stratified fluid

2014 ◽  
Vol 750 ◽  
pp. 5-32 ◽  
Author(s):  
A. Doostmohammadi ◽  
S. Dabiri ◽  
A. M. Ardekani
Keyword(s):  
Settling Velocity ◽  
Peak Velocity ◽  
Numerical Study ◽  
Maximum Velocity ◽  
Reynolds Numbers ◽  
Stratified Fluid ◽  
Settling Particle ◽  
Settling Behaviour ◽  
Prandtl Numbers ◽  
Neutrally Buoyant

AbstractIn this paper, the transient settling dynamics of a spherical particle sedimenting in a linearly stratified fluid is investigated by performing fully resolved direct numerical simulations. The settling behaviour is quantified for different values of Reynolds, Froude and Prandtl numbers. It is demonstrated that the transient settling dynamics is correlated to the induced Lagrangian drift of flow around the settling particle. A simplified model is provided to predict the maximum velocity of the settling particle in linearly stratified fluids. The peak velocity can be followed by the oscillation of the settling velocity and the particle can even reverse its direction of motion before reaching to its neutrally buoyant level. The frequency of oscillation of settling velocity scales with the Brunt–Väisälä frequency and the motion of the particle can lead to the formation of secondary and tertiary vortices following the primary vortex.

scholarly journals Vortical structure in the wake of a transverse jet

1994 ◽  
Vol 279 ◽  
pp. 1-47 ◽  
Author(s):  
T. F. Fric ◽  
A. Roshko
Keyword(s):  
Boundary Layer ◽  
Velocity Ratio ◽  
Adverse Pressure Gradient ◽  
Reynolds Numbers ◽  
Structural Features ◽  
Bluff Bodies ◽  
Wake Vortices ◽  
Transverse Jet ◽  
Wall Boundary Layer ◽  
Crossflow Velocity

Structural features resulting from the interaction of a turbulent jet issuing transversely into a uniform stream are described with the help of flow visualization and hot-wire anemometry. Jet-to-crossflow velocity ratios from 2 to 10 were investigated at crossflow Reynolds numbers from 3800 to 11400. In particular, the origin and formation of the vortices in the wake are described and shown to be fundamentally different from the well-known phenomenon of vortex shedding from solid bluff bodies. The flow around a transverse jet does not separate from the jet and does not shed vorticity into the wake. Instead, the wake vortices have their origins in the laminar boundary layer of the wall from which the jet issues. It is argued that the closed flow around the jet imposes an adverse pressure gradient on the wall, on the downstream lateral sides of the jet, provoking 'separation events’ in the wall boundary layer on each side. These result in eruptions of boundary-layer fluid and formation of wake vortices that are convected downstream. The measured wake Strouhal frequencies, which depend on the jet-crossflow velocity ratio, match the measured frequencies of the separation events. The wake structure is most orderly and the corresponding wake Strouhal number (0.13) is most sharply defined for velocity ratios near the value 4. Measured wake profiles show deficits of both momentum and total pressure.

A numerical study of the temporal eigenvalue spectrum of the Blasius boundary layer

1976 ◽  
Vol 73 (3) ◽  
pp. 497-520 ◽  
Author(s):  
Leslie M. Mack
Keyword(s):  
Boundary Layer ◽  
Numerical Study ◽  
Mathematical Proof ◽  
Reynolds Numbers ◽  
Contour Method ◽  
Eigenvalue Spectrum ◽  
Contour Lines ◽  
Wide Range ◽  
Blasius Boundary Layer ◽  
Sommerfeld Equation

A numerical study is made of the temporal eigenvalue spectrum of the Orr-Sommerfeld equation for the Blasius boundary layer. Unlike channel flows, there is no mathematical proof that this flow has an infinite spectrum of discrete eigenvalues. The Orr-Sommerfeld equation is integrated numerically, and the eigenvalues located by tracing out the contour lines in the complex wave velocity (c = cr + ici) plane on which the real and imaginary parts of the secular determinant are zero. This method gives only a finite and small number of discrete eigenvalues for a wide range of Reynolds numbers and wavenumbers. The spectrum of plane Poiseuille flow is used as a guide to study the spectrum of an artificial two wall flow which consists of two Blasius boundary layers. As the upper boundary of this flow moves to infinity, it is found that the portion of the spectrum with an infinite number of eigenvalues moves towards cr = 1 and the spacing between eigenvalues goes to zero. It is concluded, on the basis of this result and the contour method, that the original few eigenvalues found are the only discrete eigenvalues that exist for Blasius flow over a wide portion of the c plane for cr < 1 and cr > 1. It is suggested that the discrete spectrum is supplemented by a continuous spectrum which lies along the cr = 1 axis for ci < −α/R.

Momentum Thickness Measurements for Thick Axisymmetric Turbulent Boundary Layers

2002 ◽  
Author(s):  
Kimberly M. Cipolla ◽  
William L. Keith
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
High Speed ◽  
Large Scale ◽  
Control Volume ◽  
Reynolds Numbers ◽  
Momentum Thickness ◽  
Laser Measurements ◽  
Langley Research Center ◽  
Neutrally Buoyant

Experimental measurements of the mean wall shear stress and boundary layer momentum thickness on long, thin cylindrical bodies are presented. To date, the spatial growth of the boundary layer and the related boundary layer parameters have not been measured for cases where δ/a (a = cylinder radius) is of order one or greater. Moderate Reynolds numbers (104 &lt; Reθ &lt; 105) encountered in hydrodynamic applications, are considered. Tow tests of cylinders with diameters of 0.89 mm and 2.5 mm and lengths ranging from approximately 30 meters to 150 meters were performed using the High-Speed Seawater Tow Tank at NASA Langley Research Center. The total drag (axial force) was measured at tow speeds ranging from 2.4 to 17.4 m/sec. These data were used to determine the tangential drag coefficients on each test specimen, which were found to be two to three times greater than the values for the corresponding hypothetical flat-plate cases. Using the drag measurements, the turbulent boundary layer momentum thickness at the end of the cylindrical bodies is determined, using a control volume analysis. The results show that for the smallest diameter cylinders, there is no indication of relaminarization, and a fully developed turbulent boundary layer exists. In addition, laser measurements showed no large scale transverse motions (snaking) existed during the tows, and the tow angle was less than 1 degree for all cases, confirming that the cylinders were neutrally buoyant.

scholarly journals Numerical study of an asymmetrically heated rectangular duct with suspended cylinders

2018 ◽  
Vol 240 ◽  
pp. 04002 ◽  
Author(s):  
Gaurav Kumar Chhaparwal ◽  
Ankur Srivastava ◽  
Ram Dayal
Keyword(s):  
Boundary Layer ◽  
Vortex Flow ◽  
Numerical Study ◽  
Solar Air Heater ◽  
Rectangular Duct ◽  
Enhance Heat Transfer ◽  
Absorber Plate ◽  
Turbulent Flow Regime ◽  
Cold Stream ◽  
Stream Lines

An asymmetrically heated (only one side of the duct is heated) solar air heater duct is numerically investigated using openFOAM to study the effect of passive turbulators (suspended cylinders) installed within the boundary layer close to the absorber plate to enhance heat transfer from it. Vortex flow is created behind the suspended cylinders, which disturbs the boundary layer and causes mixing of hot stream lines near the surface with relatively cold stream away from the absorber plate. Diameter of the suspended cylinders and its distance from the absorber plate is varied to study its effect on wall temperature of absorber plate in the turbulent flow regime.

Oscillatory Flow Phenomena in Diffusers at Low Reynolds Numbers

1973 ◽  
Vol 95 (3) ◽  
pp. 401-407 ◽  
Author(s):  
A. H. Stenning ◽  
A. A. Schachenmann
Keyword(s):  
Boundary Layer ◽  
Oscillatory Flow ◽  
Travelling Waves ◽  
Reynolds Numbers ◽  
Low Reynolds Numbers ◽  
Inlet Velocity ◽  
Wall Boundary Layer ◽  
Displacement Thickness ◽  
Flow Phenomena ◽  
Flow Oscillations

In studies of a diffuser operating with inlet flow oscillations, it has been found that large amplification of the inlet velocity oscillations occurs within the diffuser when the throat Reynolds number lies in the range 103 to 104. This phenomenon is caused by travelling waves in the wall boundary layer which are initiated in the laminar portion of the boundary layer and propagate into the turbulent boundary layer, causing large variations in the displacement thickness.

The development of three-dimensional wave packets in a boundary layer

1968 ◽  
Vol 32 (1) ◽  
pp. 173-184 ◽  
Author(s):  
M. Gaster
Keyword(s):  
Boundary Layer ◽  
Linear Problem ◽  
Asymptotic Form ◽  
Wave Packets ◽  
Three Dimensional ◽  
Reynolds Numbers ◽  
Critical Value ◽  
Lateral Spread ◽  
Disturbed Region ◽  
Dimensional Wave

The formation and growth of three-dimensional wave packets in a laminar boundary layer is treated as a linear problem. The asymptotic form of the disturbed region developing from a point source is obtained in terms of parameters describing two-dimensional instabilities of the flow. It is shown that a wave caustic forms and limits the lateral spread of growing disturbances whenever the Reynolds number is √2 times the critical value. The analysis is applied to the boundary layer on a flat plate and shapes of the wave-envelope are calculated for various Reynolds numbers. These show that all growing disturbances are contained within a wedge-shaped region of approximately 10° semi-angle.

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