Engineering topics
Publications and source records attributed to Moretti, G..
A three-dimensional microelectronic device (3DANN-R) capable of performing general image convolution at the speed of 10***sup12*** operations/second (ops) in a volume of less than 1.5 cubic centimeter has been successfully built under the BMDO/JPL VIGILANTE program.
A formulation of the lambda scheme for the analysis of two dimensional inviscid, compressible, unsteady transonic flows is presented. The scheme uses generalized Riemann variables to determine the appropriate two point, one sided finite difference approximation for each derivative in the unsteady Euler equations. These finite differences are applied at the predictor and corrector levels with shock updating at each level. The weaker oblique shocks are captured, but strong near normal shocks are fitted into the flow using the Rankine-Hugoniot relations. This code is demonstrated with a numerical example of a duct flow problem with developing normal and oblique shock waves. The technique is implemented in a code which has been made efficient by streamlining to a minimal number of operations and by eliminating branch statements. The scheme is shown to provide an accurate analysis of the flow, including formation, motions, and interactions of shocks; the results obtained on a relatively coarse mesh are comparable to those obtained by other methods on much finer meshes.
A code for the calculation of one-dimensional flows is presented, which combines a simple and efficient version of the lambda-scheme with tracking of discontinuities. The latter is needed to identify points where minor departures from the basic integration scheme are applied to prevent infiltration of numerical errors. Such a tracking is obtained via a systematic application of Boolean algebra. It is, therefore, very efficient. Fifteen examples are presented and discussed in detail. The results are exceptionally good. All discontinuites are captured within one mesh interval.
A numerical technique to solve the Euler equations for steady, one dimensional flows is presented. The technique is essentially implicit, but is structured as a sequence of explicit solutions for each Riemann variable separately. Each solution is obtained by integrating in the direction prescribed by the propagation of the Riemann variables. The technique is second-order accurate. It requires very few steps for convergence, and each step requires a minimal number of operations. Therefore, it is three orders of magnitude more efficient than a standard time-dependent technique. The technique works very well for transonic flows and provides shock fitting with errors as small as 0.001. Results are presented for subsonic problems. Errors are evaluated by comparison with exact solutions.
A numerical technique to solve the Euler equations for steady, two-dimensional flows is presented. The technique extends to two-dimensional problems a formulation which was found to be extremely efficient for one-dimensional flows. Generalized Riemann variables are defined along two families of orthogonal coordinates, and integrated separately, sweeping back and forth alternatively along coordinate lines. The technique is second-order accurate and converges very rapidly. In addition, each step requires a minimal number of operations. Preliminary results for subsonic and transonic shockless flows are presented and discussed.
A numerical technique is described for the calculation of three dimensional, inviscid, supersonic, steady flows over wing-body configurations. A high degree of accuracy without increasing the number of computational nodes is obtained by means of a powerful conformal mapping technique. Results are presented for some simple body configurations and for a more complex arrow wing airframe. The numerical results show good agreement with experimental measurements.
A detailed description of a computational program for the evaluation of three dimensional supersonic, inviscid, steady flow past airplanes is presented. Emphasis was put on how a powerful, automatic mapping technique is coupled to the fluid mechanical analysis. Each of the three constituents of the analysis (body geometry, mapping technique, and gas dynamical effects) was carefully coded and described. Results of computations based on sample geometrics and discussions are also presented.
Two types of boundaries are considered: rigid walls, and artificial (open) boundaries which were arbitrarily drawn somewhere across a wider flow field. A set of partial differential equations (typically, the Euler equations) has an infinite number of solutions, each one defined by a set of initial and boundary conditions. The initial conditions remaining the same, any change in the boundary conditions will produce a new solution. To pose the problem well, a necessary and sufficient number of boundary conditions are prescribed.
The physical implications of modeling a subsonic boundary in an internal flow with a bump are discussed with reference to the effects of three different models. A numerical treatment of the corners is presented.
Effects of three different models for the treatment of subsonic boundary conditions, applied to the problem of flow in a channel with a bump, are discussed. A preliminary discussion of the numerical treatment of the corners is presented.
A brief historical review of conformal mapping and its applications to problems in fluid mechanics and electromagnetism is presented. The use of conformal mapping as a grid generator is described. The philosophy of the 'closed form' approach and its application to a Neumann problem is discussed. Karman-Trefftz mappings and grids for ablated, three dimensional bodies are also discussed.
A method for integrating the Euler equations of gas dynamics for compressible flows in any hyperbolic case is presented. This method is applied to the Mach number distribution over a stretch of an infinite duct having a variable cross section, and to the distribution in a channel opening into a vacuum with the Mach number equalling 1.04. An example of the ability of this method to handle two-dimensional unsteady flows is shown using the steady shock-and-isobars pattern reached asymptotically about an ablated blunt body with a free stream Mach number equalling 12. A final example is presented where the technique is applied to a three-dimensional steady supersonic flow, with a Mach number of 2 and an angle of attack of 5 deg.
A scheme for integrating the Euler equations of compressible flow in any hyperbolic case is presented. The scheme relies on the concept of characteristics but is strictly a finite difference scheme. Improvements in accuracy and physical consistence due to the scheme are discussed and results of its application to complex flows are shown.
A numerical approach is presented for the computation of flows in which the component of velocity in the selected marching direction is subsonic although the total velocity is supersonic. A local coordinate rotation procedure is employed together with an implicit differencing scheme. Complex coordinate transformations and time-consuming iterations are avoided. The implementation of the described approach is illustrated with the aid of a two-dimensional problem. An application in the case of three-dimensional flows is also discussed.
A detailed description of the procedure used to compute three dimensional, supersonic, inviscid, steady flows past airframes is given. No limitations are imposed on the geometry of the airplane. Suitable computational grids are generated by automatic conformal mappings. The equations of motion, with pressure, entropy, and velocity direction as basic unknowns, are written and discretized in the computational space. Special rules to approximate derivatives are given. Boundary points are treated by a modified method of characteristics.
A gasdynamical analysis is presented for a three-dimensional, supersonic, inviscid, steady, shockless flow past an arbitrary airframe, using computational grids. The analysis, which includes special treatments for body points and bow-shock points, relies on the equations of motion written in terms of logarithm of pressure entropy and two angles reprising the velocity vector. A FORTRAN code was employed (sample geometry is shown). The flow analysis is considered reliable for aircraft cross-sections that are elliptical in shape.
The evaluation of steady, supersonic, three-dimensional inviscid flows is considered, taking into account the construction of suitable grids on the basis of a simple conformal mapping. A description is presented of an analytical mapping technique. The technique is illustrated with the aid of examples of mappings, involving nontrivial geometries. Attention is given to a wing-fuselage cross section, a FORTRAN program for the basic mapping, the coupling of mappings, and the equations of motion.
A computer program to predict the inviscid, transonic flow field about isolated nacelles was developed. The problem was to be formulated to solve Euler's equations without any approximation (such as small disturbances) and hence the terminology exact solution. The flow field was complicated by the presence of imbedded shock waves, an engine-inlet interface, and exhaust plumes. Furthermore, the transonic nacelles of interest had a very slender but blunt cowl lip. This created two distinct length scales, the length of the nacelle and the cowl lip radius that can differ by several orders of magnitude. These aspects of the flow field presented many numerical difficulties. The approach to the problem was to calculate the nacelle flow field using the method of time-dependent computations (TDC). Although at the time of the issuance of this contract, other approaches to transonic flow calculations existed, it was felt that TDC offered the most effective means of meeting the goals of the contract.