boundary layer


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Related to boundary layer: Atmospheric boundary layer

boundary layer

[′bau̇n·drē ‚lā·ər]
(meteorology)
The lower portion of the atmosphere, extending to a height of approximately 1.2 miles (2 kilometers).

Boundary Layer

 

in a viscous fluid the flow region that forms at the surface of a body past which the fluid moves or at the interface of two streams of fluids with different velocities, temperatures, or chemical compositions; its transverse thickness is much smaller than its longitudinal dimensions. The boundary layer is characterized by an abrupt change in the transverse direction of velocity (a hydrodynamic boundary layer), temperature (a thermal boundary layer), or concentrations of individual chemical components (a diffusion boundary layer). The viscosity, thermal conductivity, and diffusivity of the fluid are the principal influences on the formation of the flow in a boundary layer. Within a dynamic boundary layer a smooth change in velocity occurs from the velocity of the external stream to zero at the wall as a result of the adhesion of the viscous fluid to the solid surface. Similarly, the temperature and concentration vary smoothly within a boundary layer.

The state of flow in a dynamic boundary layer depends on the Reynolds number Re and may be laminar or turbulent. In laminar flow, individual particles of the fluid follow trajectories whose shape is close to that of the body the fluid is flowing past or of the interface between the two liquid or gaseous media. In turbulent flow, the random, pulsating motion of individual particles of the fluid is superimposed on the average motion of the fluid particles in the direction of the main stream. The rate of transfer of momentum and the intensity of the processes of heat and mass transfer consequently are sharply increased, and an increase in skin friction and heat and mass transfer results. The value of the critical Reynolds number, at which the transition from laminar flow to turbulent flow occurs in the boundary layer, depends on several factors, including the degree of roughness of the surface past which the fluid is moving, the level of turbulence of the external stream, and the Mach number M. Moreover, the transition from laminar to turbulent flow with increasing Re in the boundary layer does not occur abruptly; there is a transition region where laminar and turbulent flows alternate.

The thickness δ of a hydrodynamic boundary layer is defined as the distance from the surface of the body (or from the interface of fluids) at which the velocity in the boundary layer can be considered essentially equal to the velocity in the external stream. The value of δ depends chiefly on the Reynolds number. For laminar flow, δ ~ lRe-0.5, and for turbulent flow, δ ~ IRe~-0.2, where l is a reference length of the body.

The development of a thermal boundary layer is determined not only by the Reynolds number but also by the Prandtl number, which characterizes the relationship between the thicknesses of the dynamic and the thermal boundary layers. The Schmidt number correspondingly has an additional effect on the development of a diffusion boundary layer.

When the external stream of gas has a high velocity, a conversion of the kinetic energy of molecules in the boundary layer into thermal energy occurs. As a result, the local temperature of the gas rises. In the case of a thermally insulated surface the temperature of the gas in a boundary layer may approach the stagnation temperature

where Te is the gas temperature outside the boundary layer and k = Cp/cv is the ratio of the specific heats at constant pressure and volume.

The character of the flow in a boundary layer has a decisive effect on the separation of the flow from the surface of the body past which the fluid is moving. The reason for this is that when a sufficiently great positive longitudinal pressure gradient is present, the kinetic energy of the fluid particles that are slowed down in the boundary layer becomes insufficient to overcome the pressure forces, the flow in the boundary layer loses stability, and separation of flow occurs.

When the Reynolds numbers are very large, the thickness of the boundary layer is very small in comparison with the characteristic dimensions of the body. Throughout nearly the entire flow region, with the exception of the thin boundary layer, the effect of viscous forces is therefore insignificant in comparison with inertial forces, and the fluid in this region can be regarded as perfect. At the same time, because of the small thickness of the boundary layer, the pressure in it in a transverse direction can be considered essentially constant. As a result, a highly effective method of studying the flow of a fluid stream past a body is the division of the entire flow field into two parts—the flow region of the perfect fluid and the thin boundary layer at the surface of the body. The flow in the first region is studied by means of the equations of motion of a perfect fluid; this approach permits determination of the pressure distribution along the surface of the body. The pressure in the boundary layer is also thereby determined. The flow within the boundary layer is then calculated, taking into account viscosity, thermal conductivity, and diffusion. As a result, the surface friction and the coefficients of heat and mass transfer can be determined. This approach, however, is inapplicable in its explicit form in the case of flow separation from the surface of the body. It also is inapplicable for small Re, when the effect of viscosity extends to quite large distances from the surface of the body.

REFERENCES

Loitsianskii, L. G. Mekhanika zhidkosti i gaza, 4th ed. Moscow, 1973.
Schlichting, H. Teoriia pogranichnogo sloia. Moscow, 1974. (Translated from German.)
Osnovy teploperedachi v aviatsionnoi i raketnoi tekhnike. Moscow, 1960.
Kutateladze, S. S., and A. I. Leont’ev. Teplomassoobmen i trente v turbulentnom pogranichnom sloe. Moscow, 1972.

N. A. ANFIMOV

boundary layer

boundary layer
The layer of fluid, close to the surface of a body placed in a moving stream, in which the impact pressure is reduced as a result of the viscosity of the fluid. A velocity gradient exists through the boundary layer, ranging from the velocity of the body to the velocity of the free airstream. The nature of the boundary layer determines the maximum lift coefficient, the stalling characteristics of a wing, the value of form drag, and to an extent the high-speed characteristics. In aerodynamics, the boundary-layer thickness is measured from the surface to an arbitrarily chosen point (e.g., where the velocity is 99% of the stream velocity). Thus, in aerodynamics, the boundary layer, by selection of the reference point, can include only the laminar boundary layer or the laminar boundary layer plus all, or a portion of, the turbulent boundary layer.
References in periodicals archive ?
The thickness of the boundary layer was quantified using the displacement thickness [delta]*, defined as
With this assumption and the application of the Oberbeck-Boussinesq approximation, the governing conservation equations for laminar boundary layer free convection flow can be written as:
Generally, the boundary layer and surface exchange treatment in these models is based on the boundary layer approximation, which essentially corresponds to a one-dimensional approach.
53m measured from direction of hot fluid entry towards hot fluid exit the temperature and thermal boundary layer remains constant, hence the length of thermal entrance region and fully developed flow region obtained from experimental measurement of temperature distribution along the thermal boundary layer are 0.
Pop, "Analytic series solution for unsteady mixed convection boundary layer flow near the stagnation point on a vertical surface in a porous medium," Transport in Porous Media, vol.
The non-linearity parameter s causes to reduce the flow and the boundary layer thickness increases as shown in fig.
The results show that the due to the high density of Cu, adding this nanoparticles to water generates more thinner boundary layer in contrast to other nanoparticles in a process.
Robertson of University of Colorado, Boulder, Colorado, USA, and his team showed that the apparent lack of charcoal in the K-Pg boundary layer resulted from changes in sedimentation rates: When the charcoal data are corrected for the known changes in sedimentation rates, they exhibit an excess of charcoal, not a deficiency.
In many engineering processes, boundary layer behavior occurs for a flow over a moving continuous solid surface.
Since the aerodynamic performance of compressor grids is mostly defined by the processes that occur at the boundary layers on blades, there is great interest in considering the influence of periodic flow non-uniformity in front of grids on the compressor grids streamlining and flow at the boundary layer on blades (Schlichting 1969; Liblein, Randebuseh 1956).
In the Sandia Cooler, the boundary layer of air that surrounds the cooling fins is subjected to a powerful centrifugal (to an observer in the rotating frame, where the boundary layer resides) pumping effect, providing an order-of-magnitude reduction in boundary layer thickness at a speed of a few thousand revolutions per minute.

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