Turbulent transfer control in a boundary layer:

mechanisms and models

N.N.Kovalnogov

Ulyanovsk State Technical University

Ulyanovsk, Russia

nnk@ulstu.ru

The problem of control by turbulent transfer in a boundary layer is actual for effective affect on intensity of a convective heat and mass transfer, and resistance of a moving stream of a propulsive mass in energy devices, transport systems etc. In the literature a number of methods and devices both to an intensification heat transfer and resistance, and their easing is offered (see, for example, [1-7]). Such as, for example, imposing on a moving stream of an acoustic field or ultrasonic fluctuations of pressure; use of channels with alternating narrowed and extending sites; use of the punched surfaces with deaf damping cavities, application of heat exchanging surfaces with cross gofers or elements of a discrete roughness on which changing of a longitudinal gradient of pressure at length of a streamline surface, curvature of a surface and so forth is realized.

Search and practical realization of optimum methods of management by turbulent transfer restrain insufficient development of the theory of turbulence. It concerns to a problem of the adequate account of various managing influences on intensity of turbulent transfer in a boundary layer, and also to the mathematical description of processes of interaction of managing elements and turbulent formations with much various scales. Development of corresponding theoretical generalizations on turbulent transfer in near-the-wall the boundary layer under managing influences allows to prove a basic opportunity and limits of such management, to establish the list of managing influences and to establish their interrelation with characteristics of turbulent transfer in a boundary layer.

Within the framework of the accepted approach the direct and indirect influence of managing influences on mean parameters of flow in a boundary layer, and, hence, on intensity of heat transfer, friction, efficiency of a veil, are allocated. Thus it means, that the system of the differential equations of a turbulent boundary layer together with initial and boundary conditions correctly reflects only direct influence of various influences. Indirect influence of the mentioned influences is shown through change of factors of turbulent transfer of heat and quantity of movement.

The influence of different effects on coefficients of turbulent transport was studied experimentally with use of original thermo-anemometer complex. This complex has allowed in particular to execute examination of turbulent transport in dynamically non-stationary (pulsatory) streams in channels of the different shape, and also to reveal features stipulated by redistribution of the contribution different component pulsation velocity in a kinetic energy, of a component of to pulsation velocities, with a longitudinal lapse rate of pressure.

The net effect of different effects on heat exchange and abrasion in a boundary layer was studied on the basis of numerical examination of a boundary layer (stationary and non-stationary). Thus in the equations the actual performances of turbulent transfer established on the basis of experimental examination of a turbulent pattern of streams were included. For affirming adequacy of results of a numerical analysis were used available in the literature and experimental data, obtained by the author, on a convective heat exchange and resistance of stationary and non-stationary streams in channels of the different shape.

Systematization and generalization of results executed have allowed to modify two-parameter differential model of turbulent transfer (with reference to steady flows), and also Prandtl model of a path of mixing. Last is extended on boundary-layer flows with intensive time-space reorganization of a velocity profile and with an inhomogeneous field of pressure (at monotone and alternation of parameters), and also on flows about a punched surface with deaf damping vacuities. The model of a boundary layer with interior radiants is offered which has allowed as a first approximation to reveal the basic mechanism of essential intensification of a convective heat exchange of streams of a gas mixture in requirements of slugged shedding of condensed particles on a wall.

Thus, in operation the effects(results) of complex examination of regularities of turbulent transport, convective heat exchange and abrasion in streams with control actions of a longitudinal lapse rate of pressure, heightened exterior turbulence, thermal and dynamic non-stationarity, interior radiants of heat and momentum, deaf damping vacuities on a punched surface are shown. The models and procedures of calculation adequately reflecting detected features are offered.