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Hassan, H. A.

Publications and source records attributed to Hassan, H. A..

At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Mixing layer control for tangential slot injection in turbulent flows

Tangential injection into turbulent flows is one of the most promising methods of minimizing skin friction and providing thermal protection. The technique also has application to laser hardening. The effectiveness of the injected material can be increased if the spreading rate of the resulting mixing region can be reduced. Various techniques which have been shown to be effective in manipulating the rate of growth of mixing layers under certain conditions have been applied to a slot configuration having a thick external turbulent boundary layer. These include geometry modifications to the slot lip trailing edge and acoustic excitation of the slot exit plane over a wide range of frequencies. Neither of these approaches produced any noticeable effect on the downstream evolution of the mixing layer. This lack of effectiveness is attributed to the dominating influence of the well-developed incoming turbulent boundary layer. The placement of large-eddy breakup devices in this boundary layer upstream of the injection point did produce significantly lower velocities in the near-wall region of the flow downstream of the slot exit.

Mcinville, R. M.↗
NASA NTRS1985

Euler calculations for multielement airfoils using Cartesian grids

A finite volume formulation for the Euler equations using Cartesian grids is presented and used to study complex two-dimensional configurations. The formulation extends methods developed for the potential equation to the Euler equations. Results using this approach for single element airfoils are shown to be competitive with and as accurate as other methods that employ mapped grids. Further, it is demonstrated that this method provides a simple and accurate procedure for solving flow problems involving multielement airfoils.

Clarke, D. K.↗
NASA NTRS1985

Application of a variational method for generating adaptive grids

The application of variational methods for generating adaptive grids is not as straightforward as one is led to believe. Proper scaling, suitable weight functions and appropriate clustering on boundaries must be employed to obtain a satisfactory grid. This work, which is based on the framework developed by Brackbill and Saltzman, provides simple methods for determining scaling and investigates possible options for selecting the weight function and clustering points on the boundaries. The concepts developed here are applied to two two-dimensional problems: a model problem based on Burger's equation which contains two length scales and, transonic flow past airfoils using the Euler equations.

Kreis, R. I.↗
NASA NTRS1985

Assessment of wind tunnel corrections for multielement airfoils at transonic speeds

A finite volume formulation of the Euler equations using Cartesian grids is used to calculate the transonic flow over multielement airfoils and to use the resulting solutions to assess wall interference effects in wind tunnels. Available methods and recommendations for evaluating such effects, which are based on shifts in Mach number and angle of attack, are examined and the results are compared with measurements using the flapped supercritical SKF 1.1 airfoil. Based on the calculations, it is concluded that shifts in Mach number and angle of attack cannot by themselves account for viscous and wall effects on multielement airfoils at transonic speeds.

Gaffney, R. L., Jr.↗
NASA NTRS1984

Transonic flow calculations using a flux vector splitting method for the Euler equations

A study of the flux vector splitting method of Steger and Warming for the solution of the time dependent Euler equations in strong conservation law form for arbitrary two-dimensional geometries is presented. The procedure employed here differs from that of Buning and Steger in that it uses a different algorithm and employs implicit boundary conditions. Moreover, the method, as implemented here, does not contain any explicit smoothing or any adjustable parameters. Calculations were carried out for an NACA 0012 airfoil at various Mach numbers and angles of attack, and cylinders. Steady symmetric solutions were obtained for the full cylinder at a freestream Mach number of .5 without imposing a symmetry condition. In general, good agreement with other methods was obtained.

Seaford, C. M.↗
NASA NTRS1984

Embedded shear layer computations for increased drag reduction

One of the most promising methods of minimizing drag is the reduction of skin friction by injection of low momentum fluid into the near-wall region of turbulent boundary layer flows. This method could be made more effective by limiting the spread rate of the resulting mixing region. In order to achieve a better understanding of how this goal might be achieved, numerical investigations of the relevant fluid dynamic processes governing these regions have been conducted. A compact finite-difference algorithm has been applied to the complete form of the governing conservation equations for a two-dimensional laminar mixing layer. The ability of this computational approach to model successfully the formation and interaction of the large scale vortical structures which dominate such flow fields is verified in the present study. Parameters which affect the spread rate of the mixing region are also identified. In addition, the relative importance of viscous and momentum transport effects in the vortex interactions is determined.

Gatski, T. B.↗
NASA NTRS1984

A strongly implicit procedure for the compressible Navier-Stokes equations

The strongly implicit procedure of Stone is used to obtain numerical solutions of the compressible Navier-Stokes equations in conservative form. In contrast to the spatially split Douglas-Gunn type methods, the method is shown to be numerically stable for the three-dimensional wave equation. The method is applied to a variety of external and internal two-dimensional flow problems involving shock wave boundary-layer interaction for both laminar and turbulent flows. The results are in good agreement with other methods and/or experiments. The storage penalty associated with the method is discussed and a simple, yet effective, means of minimizing the problem is presented.

Walters, R. W.↗
NASA NTRS1984

Physics on aircraft wakes

The roll-up of a vortex sheet is analyzed by two approaches. The first is based on exact compressible Euler equations while the second is based on the exact incompressible Navier-Stokes equations. The inviscid calculations for the two-dimensional problem do not indicate any roll-up of the sheet. On the other hand, the viscous calculations capture the dynamics of the roll-up rather well. This suggests that the generally held views regarding the roll-up process of aircraft wakes, namely, that it be treated as an inviscid process, may not be completely accurate.

Hassan, H. A.↗
NASA NTRS1983

Heating analysis of bent-nose biconics at high angles of attack using the parabolized Navier-Stokes equations

A method based on the Parabolized Navier-Stokes equations is used to calculate the flow field and heat transfer of lifting entry vehicles. The method is based on the Bean and Warming implicit algorithm and uses a new procedure for preventing departure solutions. Calculations are carried out for blunt on-axis and bent biconics, assuming a perfect gas and laminar flow, and compared with available heat transfer, surface pressure and shock shape measurements for a range of Mach numbers and angles of attack. In all calculations presented here, the starting solution is obtained from available inviscid and boundary layer codes. Good agreement with experiment is indicated. Thus, the method provides an accurate and rather inexpensive procedure for calculating three-dimensional flows at supersonic Mach numbers.

Stephenson, B. L.↗
NASA NTRS1983

A new stream function formulation for the Euler equations

A new stream function formulation is developed for the solution of Euler's equations in the transonic flow region. The stream function and the density are the dependent variables in this method, while the governing equations for adiabatic flow are the momentum equations which are solved in the strong conservation law form. The application of this method does not require a knowledge of the vorticity. The algorithm is combined with the automatic grid solver (GRAPE) of Steger and Sorenson (1979) in order to study arbitrary geometries. Results of the application of this method are presented for the NACA 0012 airfoil at various Mach numbers and angles of attack, and cylinders. In addition, detailed comparisons are made with other solutions of the Euler equations.

Atkins, H. L.↗
NASA NTRS1982

Conversion of blackbody radiation into laser energy

By employing detailed kinetic models, three concepts which utilize a blackbody cavity for the conversion of solar energy into laser energy using a CO2 lasant are analyzed and compared. In the first, the blackbody radiation is used to excite flowing CO2 directly. The second and third employ a mixing laser concept with CO and N2 being the donor gases. The CO is optically pumped while thermal heating excites the N2. Blackbody temperatures ranging from 1500 deg K - 2500 deg K are considered. Based on calculated laser power output per unit flow rate of CO2, it appears that the N2-CO2 mixing laser is the most attractive system.

Mcinville, R. M.↗
NASA NTRS1982

Transonic flow calculations using Euler's equations

An implicit finite difference method with implicit boundary conditions is employed to solve the steady Euler's equations for flows past arbitrary geometries. The resulting code is used to investigate in a systematic way various aspects of flow past airfoils at transonic speeds such as, method of solution, boundary conditions, grid stretching and generation, shock and sonic point operators, the Kutta condition, and smoothing. Results obtained are in good agreement with results of other codes. Moreover, it appears that the method of solution employed is such that Kutta's condition need not be invoked. This statement appears to be valid for other existing schemes employed in the solution of Euler's equations.

Atkins, H. L.↗
NASA NTRS1980

Analysis of He-N2-CO2-UF6 laser system

Direct nuclear pumping of a CO2 laser system by the U-235(n,ff)FF reaction is examined. Calculations based on a kinetic model which takes into consideration all relevant kinetic processes indicate that dissociation of CO2 prevents the efficient operation of a nuclear pumped CO2 laser system.

Hassan, H. A.↗
NASA NTRS1980

Nuclear pumping by the U-235 /n,ff/ FF reaction

The quenching role of the UF6 in noble gas nuclear pumped lasers is examined. Detailed results are presented for He-UF6-Xe system. These results indicate that depletion of the atomic ion is the mechanism responsible for the observed behavior. Based on this it is concluded that UF6 is not compatible with noble gas lasers but should be compatible with molecular lasers.

Hassan, H. A.↗
NASA NTRS1979

Kinetics of a CO2 nuclear pumped laser

A detailed kinetic model is presented for the analysis of a nuclear pumped CO2-N2-He laser system. The model assumes that collisional recombination is the dominant pumping mechanism. The results show that, for mixtures typical of those employed in electric discharge systems, the gain coefficients are such that lasing is not expected to take place. On the other hand, concentrations of CO2 in the range 1/2 percent - 3 percent are optimal for direct nuclear pumping.

Hassan, H. A.↗
NASA NTRS1979

Kinetics of a CO2 nuclear pumped laser

A detailed kinetic model is presented for the analysis of a nuclear pumped CO2-N2-He laser system. The model assumes that collisional recombination is the dominant pumping mechanism. The results show that, for mixture typical of those employed in electric discharge systems, the gain coefficients are such that lasing is not expected to take place. On the other hand, concentrations of CO2 in the range 1/2%-3% are optimal for direct nuclear pumping.

Hassan, H. A.↗