An Experimental Study of Vigorous Transient Natural Convection

1970 ◽  
Vol 92 (4) ◽  
pp. 628-634 ◽  
Author(s):  
J. C. Mollendorf ◽  
B. Gebhart
Keyword(s):  
Boundary Layer ◽  
Natural Convection ◽  
Moving Boundary ◽  
Leading Edge ◽  
Linear Stability Theory ◽  
Extreme Condition ◽  
Transient Effects ◽  
Theory Analysis ◽  
Main Observation ◽  
The Stability

External natural convection transient response leading to transition and established turbulent flow is determined experimentally and compared with the laminar double-integral theory predictions for processes wherein all transient effects are important. The theory is shown to give very accurate predictions during the laminar portion of the transient, and temperature overshool is not observed experimentally. In addition, several unexpected and very interesting observations were made concerning the stability of the flow as it proceeds to turbulence. The first main observation is that the propagating leading edge effect serves as a very effective moving boundary layer trip and triggers the resulting turbulence. Also for the less extreme condition (less vigorous transient) there is a relaminarization of the boundary layer. Explanations of these observations are proposed in the light of recently acquired results of linear stability theory analysis for small disturbances.

Experimental and Theoretical Investigation of the Supersonic Boundary Layer Stability on the Swept Wing

2008 ◽  
Vol 3 (3) ◽  
pp. 34-38
Author(s):  
Sergey A. Gaponov ◽  
Yuri G. Yermolaev ◽  
Aleksandr D. Kosinov ◽  
Nikolay V. Semionov ◽  
Boris V. Smorodsky
Keyword(s):  
Boundary Layer ◽  
Three Dimensional ◽  
Leading Edge ◽  
Supersonic Boundary Layer ◽  
Mean Flow ◽  
Swept Wing ◽  
Wing Model ◽  
Dimensional Boundary ◽  
The Stability ◽  
Good Agreement

Theoretical and an experimental research results of the disturbances development in a swept wing boundary layer are presented at Mach number М = 2. In experiments development of natural and small amplitude controllable disturbances downstream was studied. Experiments were carried out on a swept wing model with a lenticular profile at a zero attack angle. The swept angle of a leading edge was 40°. Wave parameters of moving disturbances were determined. In frames of the linear theory and an approach of the local self-similar mean flow the stability of a compressible three-dimensional boundary layer is studied. Good agreement of the theory with experimental results for transversal scales of unstable vertices of the secondary flow was obtained. However the calculated amplification rates differ from measured values considerably. This disagreement is explained by the nonlinear processes observed in experiment

Spatio–temporal instability in mixed convection boundary layers

2000 ◽  
Vol 402 ◽  
pp. 89-107 ◽  
Author(s):  
P. MORESCO ◽  
J. J. HEALEY
Keyword(s):  
Boundary Layer ◽  
Reverse Flow ◽  
Temporal Stability ◽  
Leading Edge ◽  
Boundary Layer Separation ◽  
Absolute Instability ◽  
Limiting Similarity ◽  
Buoyancy Forces ◽  
Spatio Temporal ◽  
The Stability

In this work we analyse the stability properties of the flow over an isothermal, semi-infinite vertical plate, placed at zero incidence to an otherwise uniform stream at a different temperature. Near the leading edge the boundary layer resembles Blasius flow, but further downstream it approaches that of pure buoyancy-driven flow. A coordinate transformation that describes in a smooth way the evolution between these two limiting similarity states, where the viscous and buoyancy forces are respectively dominant, is used to calculate the basic flow. The stability of this flow has been investigated by making the parallel flow approximation, and using an accurate spectral method on the resulting stability equations. We show how the stability modes discussed by other authors can be followed continuously between the forced and free convection limits; in addition, new instability modes not previously reported in the literature have been found. A spatio–temporal stability analysis of these modes has been carried out to distinguish between absolute and convective instabilities. It seems that absolute instability can only occur when buoyancy forces are opposed to the free stream and when there is a region of reverse flow. Model profiles have been used in this latter case beyond the point of boundary layer separation to estimate the range of reverse flows that support absolute instability. Analysis of the Rayleigh equations for this problem suggests that the absolute instability has an inviscid origin.

Non-Newtonian Natural Convection Along a Vertical Heated Wavy Surface Using a Modified Power-Law Viscosity Model

2008 ◽  
Vol 131 (1) ◽  
Author(s):  
M. M. Molla ◽  
L. S. Yao
Keyword(s):  
Boundary Layer ◽  
Natural Convection ◽  
Shear Rate ◽  
Power Law ◽  
Leading Edge ◽  
Numerical Results ◽  
Wavy Surface ◽  
Viscosity Model ◽  
Length Scale ◽  
Modified Power Law

Natural convection of non-Newtonian fluids along a vertical wavy surface with uniform surface temperature has been investigated using a modified power-law viscosity model. An important parameter of the problem is the ratio of the length scale introduced by the power-law and the wavelength of the wavy surface. In this model there are no physically unrealistic limits in the boundary-layer formulation for power-law, non-Newtonian fluids. The governing equations are transformed into parabolic coordinates and the singularity of the leading edge removed; hence, the boundary-layer equations can be solved straightforwardly by marching downstream from the leading edge. Numerical results are presented for the case of shear-thinning as well as shear-thickening fluid in terms of the viscosity, velocity, and temperature distribution, and for important physical properties, namely, the wall shear stress and heat transfer rates in terms of the local skin-friction coefficient and the local Nusselt number, respectively. Also results are presented for the variation in surface amplitude and the ratio of length scale to surface wavelength. The numerical results demonstrate that a Newtonian-like solution for natural convection exists near the leading edge where the shear-rate is not large enough to trigger non-Newtonian effects. After the shear-rate increases beyond a threshold value, non-Newtonian effects start to develop.

On The Influence Of Porous Coating Thickness On Supersonic Boundary Layer Stability

2015 ◽  
Vol 10 (3) ◽  
pp. 41-47
Author(s):  
Vladimir Lysenko ◽  
Sergey Gaponov ◽  
Boris Smorodsky ◽  
Yuri Yermolaev ◽  
Aleksandr Kosinov ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Coating Thickness ◽  
Free Stream ◽  
Plate Boundary ◽  
Quantitative Agreement ◽  
Supersonic Boundary Layer ◽  
Linear Stability Theory ◽  
Porous Coating ◽  
Free Stream Mach Number ◽  
The Stability

Theoretical and experimental investigation of the influence of porous-coating thickness on the stability of the supersonic flat-plate boundary layer at free-stream Mach number M = 2 have been performed. Good quantitative agreement of experimental data obtained with artificially generated disturbances performed on models with various porous inserts and calculations based on the linear stability theory has been achieved. It is shown that the increase of the porous-coating thickness leads to the boundary layer destabilization.

Combined influence of coating permeability and roughness on supersonic boundary layer stability and transition

2016 ◽  
Vol 798 ◽  
pp. 751-773 ◽  
Author(s):  
V. I. Lysenko ◽  
S. A. Gaponov ◽  
B. V. Smorodsky ◽  
Yu. G. Yermolaev ◽  
A. D. Kosinov ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Plate Boundary ◽  
Leading Edge ◽  
Quantitative Agreement ◽  
Supersonic Boundary Layer ◽  
Linear Stability Theory ◽  
Porous Coating ◽  
Boundary Layer Stability ◽  
Turbulent Transition ◽  
Laminar Turbulent Transition

A joint theoretical and experimental investigation of the influence of the surface permeability and roughness on the stability and laminar–turbulent transition of a supersonic flat-plate boundary layer at a free-stream Mach number of $M_{\infty }=2$ has been performed. Good quantitative agreement of the experimental data obtained with artificially generated disturbances performed on models with various porous inserts and calculations based on linear stability theory has been achieved. An increase of the pore size and porous-coating thickness leads to a boundary layer destabilization that accelerates the laminar–turbulent transition. It is shown that as a certain (critical) roughness value is reached, with an increase in the thickness of the rough and porous coating, the boundary layer stability diminishes and the laminar–turbulent transition is displaced towards the leading edge of the model.

Natural Convection Heat Transfer from the Upper Plate of a Colinear, Separated Pair of Vertical Plates

1980 ◽  
Vol 102 (4) ◽  
pp. 623-629 ◽  
Author(s):  
E. M. Sparrow ◽  
M. Faghri
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Natural Convection ◽  
Vertical Plate ◽  
Convection Heat Transfer ◽  
Leading Edge ◽  
Lower Plate ◽  
Natural Convection Heat Transfer ◽  
Convection Heat ◽  
Plate Heat

The effect of a buoyant boundary layer spawned by a heated vertical plate on the natural convection heat transfer from an upper colinear vertical plate has been determined analytically. The interplate spacing was varied parametrically, as were the relative temperatures and relative lengths of the two plates; the Prandtl number was equal to 0.7 for all cases. Heat transfer at the upper plate was found to be affected both by the preheating and by the finite velocity imparted to the fluid by the first plate, respectively tending to degrade and to enhance the heat transfer. The upper-plate heat transfer was compared to that of an otherwise identical vertical plate, but with the lower plate absent. When the temperatures of the upper and lower plates are the same, the overall upper-plate heat transfer is less than that of its single-plate counterpart for small interplate spacings, with the opposite relationship at larger spacings. If the temperature of the upper plate is substantially below that of the lower plate, the overall heat transfer is degraded. On the other hand, heat transfer enhancement generally occurs when the upper plate is relatively hot. In general, the heat transfer from relatively short upper plates is very sensitive to the presence of the lower plate, with a lessening sensitivity with increasing plate length. The computed temperature and velocity profiles demonstrated that near the leading edge of the upper plate, a new boundary layer develops within the already existing boundary layer spawned by the first plate.

The long-time behaviour of incompressible swept-wing boundary layers subject to impulsive forcing

1998 ◽  
Vol 355 ◽  
pp. 359-381 ◽  
Author(s):  
M. J. TAYLOR ◽  
N. PEAKE
Keyword(s):  
Boundary Layer ◽  
Flow Direction ◽  
Three Dimensional ◽  
Cross Flow ◽  
Leading Edge ◽  
Rotating Disk ◽  
Linear Stability Theory ◽  
Swept Wing ◽  
Long Time ◽  
Layer Flow

The long-time limit of the response of incompressible three-dimensional boundary layer flows on infinite swept wedges and infinite swept wings to impulsive forcing is examined using causal linear stability theory. Following the discovery by Lingwood (1995) of the presence of absolute instabilities caused by pinch points occurring in the radial direction in the boundary layer flow of a rotating disk, we search for pinch points in the cross flow direction for both the model Falkner–Skan–Cooke profile of a swept wedge and for a genuine swept-wing configuration. It is shown in both cases that, within a particular range of the parameter space, the boundary layer does indeed support pinch points in the wavenumber plane corresponding to the crossflow direction. These crossflow-induced pinch points do not constitute an absolute instability, as there is no simultaneous pinch occurring in the streamwise wavenumber plane, but nevertheless we show here how they can be used to find the maximum local growth rate contained in a wavepacket travelling in any given direction. Lingwood (1997) also found pinch points in the chordwise wavenumber plane in the boundary layer of the leading-edge region of a swept wing (i.e. at very high flow angles). The results presented in this paper, however, demonstrate the presence of pinch points for a much larger range of flow angles and pressure gradients than was found by Lingwood, and indeed describe the flow over a much greater, and practically significant, portion of the wing.

The stability of the subsonic boundary layer during heating of the surface of a flat plate near the leading edge

1985 ◽  
Vol 20 (3) ◽  
pp. 394-398 ◽  
Author(s):  
A. V. Kazakov ◽  
M. N. Kogan ◽  
V. A. Kuparev
Keyword(s):  
Boundary Layer ◽  
Flat Plate ◽  
Leading Edge ◽  
The Stability

Onset of natural convection in layered aquifers

2015 ◽  
Vol 767 ◽  
pp. 763-781 ◽  
Author(s):  
Don Daniel ◽  
Amir Riaz ◽  
Hamdi A. Tchelepi
Keyword(s):  
Boundary Layer ◽  
Natural Convection ◽  
Layer Thickness ◽  
Boundary Layer Thickness ◽  
Onset Of Convection ◽  
Transient Nature ◽  
Gradient Of Permeability ◽  
Permeability Variation ◽  
Vorticity Production ◽  
The Stability

AbstractThe stability of gravitationally unstable, transient boundary layers in heterogeneous saline aquifers is examined with respect to the onset of natural convection. Permeability is assumed to vary periodically across the thickness of the aquifer. We study the interaction between permeability variation and concentration perturbations within the boundary layer. We observe that the instability decreases with an increase in the permeability variance if the boundary layer thickness is large compared with the permeability wavelength. On the other hand, when the boundary layer thickness is smaller than the permeability wavelength, the behaviour of instability as a function of variance depends on the phase of permeability variation. Such behaviours are shown to result from the interaction of two modes of vorticity production related to the coupling of concentration and velocity perturbations with the magnitude and gradient of permeability variation, respectively. We show that these two modes of vorticity production, when coupled with the transient nature of the boundary layer, determine the evolutionary paths followed by the most amplified perturbations that trigger the onset of convection. When the permeability variance is large, we find that small changes in the permeability field can lead to large changes in the onset times for convection.

Sign in / Sign up

Export Citation Format

Share Document