Mechanical Engineering - The University of Texas at Austin

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TEXSTAN is a finite-difference computer code that solves the two-dimensional boundary layer formulated equations of heat, mass, and momentum transfer. Currently within the code the momentum sources include pressure gradient, free convection body force, and a generalized body force. For the stagnation enthalpy equation (or temperature, if low speed or constant property), the sources include viscous dissipation, body force work, and a generalized volumetric source. For convective mass transfer, there are currently no source terms implemented for combustion, but in principle the appropriate source terms can be defined. A large variety of flow geometries can be analyzed by TEXSTAN, including the external wall shear flow family of flat plates, airfoil-shaped surfaces, two-dimensional and axisymmetric converging-diverging nozzles, and bodies of revolution of the missile shape. The internal wall shear flow family include circular pipes, planar ducts, and annular ducts. Turbulence models in TEXSTAN include three levels of mean field closure via the eddy viscosity model. These include the Prandtl mixing length with Van Driest damping; a one-equation turbulence kinetic energy model, and numerous low-turbulence Reynolds number two-equation (k-ε) models. The turbulent heat flux closure is via a turbulent Prandtl number concept. TEXSTAN can handle a variety of different boundary conditions. For the energy equation the flow-direction variation of either the wall heat flux or the wall temperature may be specified. The mass flux at a wall may also vary in the flow direction for certain geometries. For geometries with two different walls, such as annuli or planar ducts, asymmetric thermal boundary conditions can be accommodated provided they are both the same type (temperature or heat flux) and not mixed. For external wall shear flows, the free-stream velocity, rather than the pressure, is treated as a variable boundary condition, while the free-stream stagnation enthalpy is held constant. The user may specify the initial profiles of the dependent variables, or an automatic initial profile generator may be used. Fluid properties in TEXSTAN may be treated as constant or variable. Variable properties within TEXSTAN include air, water, nitrogen, helium, and products of combustion, and constant fluid properties are user-supplied via the input dataset.

Convective Flows in TEXSTAN - The types of flows that can be solved by the TEXSTAN boundary layer code are classified depending on how the flow is driven:

- Forced convective flows typically come from fluid machinery or vehicle motion, with or without heat transfer
- Natural convective flows generally involve heat transfer and a component of the gravitational body force aligned with the flow surface
- Mixed convective flows are typically low-Re forced convective flows with heat transfer and a gravitational body force component that can be aligned either in the direction of forced flow (aided flow) or against the direction of flow (opposed flow)

Limitations to TEXSTAN - There are several limitations to TEXSTAN. The transport equations are parabolic, meaning reverse flow is not permitted (nor is there a scheme to calculate reverse flow). Free shear flows of the jet and wake class wall jet flows have received limited checking and are not currently validated for TEXSTAN. Internal flows with cross-sectional area change and/or wall transpiration are not permitted. The boundary layer transport equations do not explicitly have a swirl velocity component, and they do not have axial flow radius of curvature terms or centripetal terms. Thus, the pressure field is one-dimensional. Flow-direction curvature effects for external flows (for example over a turbine blade) are modeled primarily by the user-supplied free stream velocity variation and to some degree by one choice of the mixing length turbulence model.

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