A new viscous-inviscid semi-inverse (VISI) interaction method has been developed for predicting the flow field arising from a combination of inviscid potential flow and viscous flow. The technique involves matching the bounding velocities for each region by iteratively solving for the displacement thickness, δ*(x). The formula used to update δ*(x) after each iteration is generated by linearly perturbing the governing equations. Application of the VISI procedure to predict the unseparated flow past a flat plate gives excellent results, producing numerical solutions essentially indistinguishable from the appropriate analytical solution in less than 0.5 seconds of CPU time on an Amdahl 5850 computer. Application of the technique to external flow over a backward facing step (BFS) indicates that the region of strong interaction between the viscous and inviscid flows is much larger than reported for internal flow. Calculated reattachment lengths, LR, are clearly influenced by the thickness of the boundary layer upstream of the step, thicker boundary layers producing longer reattachment lengths. Good accuracy is achieved for a relatively coarse distribution of control points, and rapid convergence (< 2 seconds on the Amdahl 5850) usually occurs. Finite-difference predictions using an elliptic code (TEACH-T), modified at the outer boundary to simulate external flow, have also been made for the BFS, largely as a basis of comparison for the VISI results. Comparison of results for the two models (VISI and TEACH) gives similar trends in LR as a function of Rh and x₈, (a measure of the displacement thickness at the step). The values of LR obtained with the VISI method, however, are 15-80% longer than those from TEACH. Direct comparison with experiments is difficult because the experimental data does not clearly identify the effects of x₈, in the resulting values of LR. Trends appear to be the same for all computed and observed cases however. Disagreement between the VISI and TEACH results is thought to be due to a combination of neglecting velocities in the recirculation region in the VISI model, and numerical error and inaccurate boundary conditions in the TEACH code. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

Thermal and fingering convection in a horizontal porous layer underlying a fluid layer was studied using linear stability analysis, experiment (for the thermal convection case only), and nonlinear simulation. For the thermal convection case, the linear analysis shows that when the fluid layer is thin, convection is largely confined to the porous layer. When the fluid layer thickness exceeds 15% of the porous layer thickness, convection is localized in the fluid layer and the critical wavelength is dramatically reduced. Experimental investigations were then conducted in a test box 24 cm x 12 cm x 4 cm high to substantiate the predictions. The ratio of the thickness of the fluid layer to that of the porous layer, d, varied from 0 to 1. The results were in good agreement with predictions. To investigate supercritical convection, a nonlinear computational study was carried out. It was found that for d ≤ 0.13, the Nusselt number increases sharply with the thermal Rayleigh number, whereas at larger values of d, the increase is more moderate. Heat transfer rates predicted for d = 0.1 and 0.2 are in good agreement with the experimental results. For salt-finger convection at R(m) ≤ 1, the critical value of the solute Rayleigh number R(sm) decreases as d increases; the convection is unicellular. For 5 ≤ R(m) ≤ 10, the critical R(sm) initially decreases with d, and then remains almost constant for larger values of d; multicellular convection prevails at high d. For 20 ≤ R(m) ≤ 50, the critical R(sm) first decreases and then increases as d increases from 0 to 0.1. When d > 0.1, the critical R(sm) decreases slowly with d and remains almost constant for d ≥ 0.4. In the nonlinear computations for R(m) = 1, periodic convection sets in at a value of R(sm) between ten and eleven times the critical value. For the case of R(m) = 50, an aperiodic oscillation occurs when R(sm) is between four and five times the critical value. For the superposed layer cases d = 1 and 0.5, the convection characteristics are similar to those of thermal convection when R(m) = 0.01. For R(m) = 1, it was found that the onset of salt-finger convection is oscillatory. For R(m) = 50, the nonlinear code failed to obtain satisfactory results.

A numerical scheme has been developed to solve the quasilinear form of the transonic stream function equation. The method is applied to compute steady two-dimensional axisymmetric solar wind-type problems. A single, perfect, non-dissipative, homentropic and polytropic gas-dynamics is assumed. The four equations governing mass and momentum conservation are reduced to a single nonlinear second order partial differential equation for the stream function. Bernoulli's equation is used to obtain a nonlinear algebraic relation for the density in terms of stream function derivatives. The vorticity includes the effects of azimuthal rotation and Bernoulli's function and is determined from quantities specified on boundaries. The approach is efficient. The number of equations and independent variables has been reduced and a rapid relaxation technique developed for the transonic full potential equation is used. Second order accurate central differences are used in elliptic regions. In hyperbolic regions a dissipation term motivated by the rotated differencing scheme of Jameson is added for stability. A successive-line-overrelaxation technique also introduced by Jameson is used to solve the equations. The nonlinear equationfor the density is a double valued function of the stream function derivatives. The velocities are extrapolated from upwind points to determine the proper branch and Newton's method is used to iteratively compute the density. This allows accurate solutions with few grid points. The applications first illustrate solutins to solar wind models. The equations predict that the effects of vorticity must be confined near the surface and far away the streamlines must resemble the spherically symmetric solution. Irrotational and rotational flows show this behavior. The streamlines bend toward the rotation axis for rapidly rotating models because the coriolis force is much larger than the centrifugal force. Models of galactic winds are computed by considering the flow exterior to a surface which surrounds a uniform density oblate spheroid. Irrotational results with uniform outward mass flux show streamlines bent toward the equator and nearly spherical sonic surfaces. Rotating models for which Bernoulli's function is not constant show the sonic surface is deformed consistent with the one-dimensional theory.

Two unique spatial discretizations employing generalized indexing strategies suitable for use with space-marching algorithms are presented for the numerical solution of the equations of fluid dynamics. Both discretizations attempt to improve geometric flexibility as compared to structured indexing strategies and have been formulated while considering the current and future availability of unstructured grid generation techniques. The first discretization employs a generalized indexing strategy utilizing triangular elements in the two dimensions normal to the streamwise direction, while maintaining structure within the streamwise direction. The second discretization subdivides the domain into a collection of computational blocks. Each block has inflow and outflow boundaries suitable for space marching. A completely generalized indexing strategy utilizing tetrahedra is used within each computational block. The solution to the flow in each block is found independently in a fashion similar to the cross-flow planes of a structured discretization. Numerical algorithms have been developed for the solution of the governing equations on each of the two proposed discretizations. These spatial discretizations are obtained by applying a characteristic-based, upwind, finite volume scheme for the solution of the Euler equations. First-order and higher spatial accuracy is achieved with these implementations. A time dependent, space-marching algorithm is employed, with explicit time integration for convergence of individual computational blocks. Grid generation techniques suitable for the proposed discretizations are discussed. Applications of these discretization techniques include the high speed flow about a 5° cone, an analytic forebody, and a model SR71 aircraft. / Ph. D.