Response of a Thin Airfoil Encountering a Strong Density Discontinuity

Airfoil theory for unsteady motion has been developed extensively assuming the undisturbed medium to be of uniform density, a restriction accurate for motion in the atmosphere, Glauert (1929), Burgers (1935), Theodorsen (1935), Kussner (1936), Karman and Sears (1938), Kinney and Sears (1975). In some instances, notably for airfoils comprising fan, compressor and turbine blade rows, the undisturbed medium may carry density variations or "spots," resulting from non-uniformaties in temperature or composition, of a size comparable to the blade chord. This condition existsfor turbine blades, Marble (1975), Giles and Krouthen (1988), immediately downstream of the main burner of a gas turbine engine where the density fluctuations of the order of 50 percent may occur. Disturbances of a somewhat smaller magnitude arise from the ingestion of hot boundary layers into fans, Wortman (1975), and exhaust into hovercraft. Because these regions of non-uniform density convect with the moving medium, the airfoil experiences a time varying load and moment which we propose to calculate

The Effect of Strain Rate on Diffusion Flames

Several steady state and time-dependent solutions to the compressible conservation laws describing direct one-step near-equilibrium irreversible exothermic burning of initially unmixed gaseous reactants, with Lewis-Semenov number unity, are presented. The quantitative investigation first establishes the Burke-Schumann thin-flame solution using the Shvab-Zeldovich formulation. Real flames do not have the indefinitely thin reaction zone associated with the Burke-Schumann solution. Singular perturbation analysis is used to provide a modification of the thin-flame solution which includes a more realistic reaction zone of small but finite thickness. The particular geometry emphasized is the un bounded counterflow such that there exists a spatially constant rate of strain along the flame. While the solutions for diffusion flames under a finite tangential strain rate may be of interest in and of themselves for laminar flow, the problems are motivated by the authors' belief that they are pertinent to the study of diffusion-flame burning in transitional and turbulent shear flows

The generation of noise by the fluctuations in gas temperature into a turbine

An actuator disc analysis is used to calculate the pressure fluctuations produced by the convection of temperature fluctuations (entropy waves) into one or more rows of blades. The perturbations in pressure and temperature must be small, but the mean flow deflection and acceleration are generally large. The calculations indicate that the small temperature fluctuations produced by combustion chambers are sufficient to produce large amounts of acoustic power. Although designed primarily to calculate the effect of entropy waves, the method is more general and is able to predict the pressure and vorticity waves generated by upstream or downstream going pressure waves or by vorticity waves impinging on blade rows

Shock enhancement and control of hypersonic mixing and combustion

The possibility that shock enhanced mixing can substantially increase the rate of mixing between coflowing streams of hydrogen and air has been studied in experimental and computational investigations. Early numerical computations indicated that the steady interaction between a weak shock in air with a coflowing hydrogen jet can be well approximated by the two-dimensional time-dependent interaction between a weak shock and an initially circular region filled with hydrogen imbedded in air. An experimental investigation of the latter process has been carned out in the Caltech 17 Inch Shock Tube in experiments in which the laser induced fluorescence of byacetyl dye is used as a tracer for the motion of the helium gas after shock waves have passed across the helium cylinder. The flow field has also been studied using an Euler code computation of the flow field. Both investigations show that the shock impinging process causes the light gas cylinder to split into two parts. One of these mixes rapidly with air and the other forms a stably stratified vortex pair which mixes more slowly; about 60% of the light gas mixes rapidly with the ambient fluid. The geometry of the flow field and the mixing process and scaling parameters are discussed here. The success of this program encouraged the exploration of a low drag injection system in which the basic concept of shock generated streamwise vorticity could be incorporated in an injector for a Scramjet combustor at Mach numbers between 5 and 8. The results of a substantial computational program and a description of the wind tunnel model and preliminary experimental results obtained in the High Reynolds Number Mach 6 Tunnel at NASA Langley Research Center are given here

Spacecraft Propulsion

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The Interaction of Entropy Fluctuations with Turbine Blade Rows; A Mechanism of Turbojet Engine Noise

The theory relating to the interaction of entropy fluctuations ('hot spots'), as well as vorticity and pressure, with blade rows is described. A basic feature of the model is that the blade rows have blades of sufficiently short chord that this is negligible in comparison with the wavelength of the disturbances. For the interaction of entropy with a blade row to be important, it is essential that the steady pressure change across the blade row should be large, although all unsteady perturbations are assumed small. A number of idealized examples have been calculated, beginning with isolated blade rows, progressing to single and then to several turbine stages. Finally, the model has been used to predict the low-frequency rearward-radiated acoustic power from a commercial turbojet engine. Following several assumptions, together with considerable empirical data, the correct trend and level are predicted, suggesting the mechanism to be important at low jet velocities

Research on Mechanisms of Exciting Pressure Oscillations in Ramjet Engines

An analytical and experimental study is being made of the role of combustion in large vortical structures in the mechanism of unsteady and unstable burning in air-breathing engines. A large body of experimental evidence supports the contention that these periodic fluctuations are themselves generated by the nonsteady flow over the flame holders and other surfaces. The mechanism itself is relatively independent of the acoustic configuration of the powerplant and its installation and hence constitutes the fundamental element of the combustion instability process. Whether or not the mechanism is excited does, however, depend upon the detailed acoustic properties of the combustion chamber and its environment and in many circumstances it is apparent that non-linear acoustics plays an essential role. As a consequence, the program includes detailed analytical studies of linear and non-linear acoustics in combustion configurations as a means of coupling the instability mechanism to a particular environment. The effective separation of the instability process into i) its mechanism and ii) its environment is aimed at eventually providing means of rational scaling of laboratory size experiments