Constant-pressure Boundary Layer Research Articles

An Alternative Reynolds Shear Stress Model and Wake Model for a Flat Plate Boundary Layer Flow

The well-known closure problem has led to many models for the Reynolds shear stress. But many of these models are based on the Boussinesq approximation involving the mean velocity gradient and an eddy viscosity, which, however, requires to be modeled. Here, for a steady, incompressible and constant-pressure boundary layer, the Reynolds shear stress is expressed as a product of the mean velocity and the local momentum-deficit. This closure model is then incorporated in the boundary layer equation for the outer layer to predict the streamwise velocity in the wake region.

A model for turbulent dissipation rate in a constant pressure boundary layer

Estimation of the turbulent dissipation rate in a boundary layer is a very involved process. Experimental determination of either the dissipation rate or the Taylor microscale, even in isotropic turbulence, which may occur in a portion of the turbulent boundary layer, is known to be a difficult task. For constant pressure boundary layers, a model for the turbulent dissipation rate is proposed here in terms of the local mean flow quantities. Comparable agreement between the estimated Taylor microscale and Kolmogorov length scale with other data in the logarithmic region suggests usefulness of this model in obtaining these quantities experimentally.

A new approach to modelling near-wall turbulence energy and stress dissipation

A new model for the transport equation for the turbulence energy dissipation rate ε and for the anisotropy of the dissipation rate tensor εij, consistent with the near-wall limits, is derived following the term-by-term approach and using results of direct numerical simulations (DNS) for several generic wall-bounded flows. Based on the two-point velocity covariance analysis of Jovanović, Ye & Durst (1995) and reinterpretation of the viscous term, the transport equation is derived in terms of the ‘homogeneous’ part εh of the energy dissipation rate. The algebraic expression for the components of εij was then reformulated in terms of εh, which makes it possible to satisfy the exact wall limits without using any wall-configuration parameters. Each term in the new equation is modelled separately using DNS information. The rational vorticity transport theory of Bernard (1990) was used to close the mean curvature term appearing in the dissipation equation. A priori evaluation of εij, as well as solving the new dissipation equation as a whole using DNS data for quantities other than εij, for flows in a pipe, plane channel, constant-pressure boundary layer, behind a backward-facing step and in an axially rotating pipe, all show good near-wall behaviour of all terms. Computations of the same flows with the full model in conjunction with the low-Reynolds number transport equation for (uiui) All Overbar, using εh instead of ε, agree well with the direct numerical simulations.

Linear instability of laterally strained constant pressure boundary-layer flows

Linear instability of laterally strained constant pressure boundary-layer flows

Characteristics of a young turbulent spot

A young turbulent spot develops in a spatially developing direct numerical simulation of an incompressible constant-pressure boundary layer exposed to a strong localized disturbance. The data are analyzed to determine both the gross characteristics of the spot and the substructures that develop inside the spot. The calculations confirm that hairpinlike vortices are added near the trailing edge of the spot. However, the computations also suggest that the importance of large spanwise vorticity structures may have been overestimated by previous experiments. The current simulation data reveal that streamwise vortices are more intense and more numerous. In addition, the streamwise vortices provide at least one route for the entrainment of near-wall fluid into the turbulent spot near the leading edge.

Entrainment and the structure of turbulent flow

Although the entrainment of non-turbulent fluid into a turbulent flow occurs across sharply defined boundaries, its rate is not determined solely by the turbulent motion adjacent to the interface but depends on overall properties of the flow, in particular, on those that control the energy balance. In the first place, attention is directed to the many observations which show that the motion in many turbulent shear flows has a structure closely resembling that produced by a rapid, finite, plane shearing of initially isotropic turbulence. The basic reasons for the similarity are the stability and permanence of turbulent eddies and the finite distortions undergone by fluid parcels in free turbulent flows. Next, the existence of eddy similarity and the condition of overall balance of energy are used to account for the variation of entrainment rates within groups of broadly similar flows, in particular mixing layers between streams of different velocities and wall jets on curved surfaces. For some flows which satisfy the ordinary conditions for self-preserving development, no entrainment rate is consistent with the energy balance and self-preserving development is not possible. Examples are the axisymmetric, small-deficit wake and the distorted wake. Finally, the implications of an entrainment rate controlled by the general motion are discussed. It is concluded that the relatively rapid entrainment in a plane wake depends on an active instability of the interface, not present in a constant-pressure boundary layer whose slow rate of entrainment is from ‘passive’ distortion of the bounding surface by eddies of the main turbulent motion. Available observations tend to support this conclusion.

Hypersonic viscous flow over slender bodies with sharp leading edges.

Abstract : The hypersonic viscous pressure interaction is treated by the development of a set of equations valid throughout the boundary layer, shock-wave structure and inviscid core. Primary interest is concerned with the nature of the leading edge continuum merged layer in which the shock wave and boundary layer are indistinguishable. Due to the parabolic nature of the equations, finite-difference solutions are attainable. The flow over a flat plate at zero incidence, as well as angle of attack, was considered. Velocity and state variable distributions across the viscous layer depict the formation of an outer shock wave and inner constant pressure boundary layer. The calculated values of surface pressure, heat transfer and shock jump conditions were at first significantly below the values predicted by strong interaction theory. Agreement was quite good downstream of the merged layer where Rankine-Hugoniot jump conditions were satisfied to within 8%. (Author)