Browsing by Author "Herbert, T."
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- Airfoil response to periodic disturbances: the unsteady Kutta conditionPoling, David R. (Virginia Polytechnic Institute and State University, 1985)Unsteady flow fields over a NACA 0012 at an angle of attack are investigated. The first is the classical pitching motion about the airfoil's quarter chord. The second is the flow over a fixed airfoil immersed in the wake of the pitching airfoil. Large reduced frequencies are considered. Measurements were obtained in a water tunnel by Laser-Doppler velocimetry. Ensemble-averaged velocity measurements were obtained in the vicinity of the trailing edges of both the pitching and the fixed airfoils. The flowfields in the wake and at the trailing edges of both airfoils were studied visually. The validity of the quasi-steady and an extension to an unsteady Kutta condition are examined. A new dynamic similarity parameter is proposed. An analytical method based on the dynamics of discrete vortices is employed. Numerical calculations of the flow over the fixed airfoil are compared with experimental results.
- Determination of the complex modulus of a solid propellant and random vibration analysis of the layered viscoelastic cylinders with finite element methodLee, Hsing-Juin (Virginia Polytechnic Institute and State University, 1987)Aeronautical structures, such as aircraft or missiles, are usually highly sophisticated systems often subjected to random vibration environment. Thus, in various design, development, and production stages, laboratory random vibration testing of sampled solid rocket motors on electromagnetic or hydraulic shakers are routinely performed as an important experiment-oriented quality control strategy. Nevertheless, it is crucial to understand the dynamic structural behavior of these layered viscoelastic cylinders such as solid rocket motors under random vibration tests analytically. In this study, a methodology has been developed to deal with the random vibration of a general class of composite structures with frequency-dependent viscoelastic material properties as represented by the example of solid rocket motors. The method combines the finite element method, structural dynamics, strain energy approach, and random vibration analysis concepts. The method is a more powerful technique capable of treating sophisticated random vibration problems with complicated geometry, nonhomogeneous materials, and frequency-dependent stiffness and damping properties. Before the random vibration analysis could proceed, a microcomputer-based dynamic mechanical analyzer system was used together with time-temperature superposition principle to obtain the frequency-dependent dynamic viscoelastic properties of the solid propellant. The strain energy approach has been used to calculate the frequency-dependent equivalent viscoelastic damping which is in turn judiciously represented by a combination of viscous damping and structural damping to accommodate this frequency dependent material property. Modal analysis data together with half power band width calculated at each natural frequency are highly useful guides in the harmonic analysis to achieve computational efficiency. On one hand, the technique used in this study has a hybrid taste in the sense that it makes use of best features and capabilities of both modal analysis and harmonic analysis to achieve the goal of random vibration analysis in addition to the power of finite element technique. The displacement, acceleration and stress power spectra have been obtained for significant points on the rocket model together with their root mean square values. These data can be used for various analyses, testing, design, and other purposes as discussed in later sections of this study.
- Experimental and finite-element investigation of flow past single and multiple cylindersDhaubhadel, Manoranjan N. (Virginia Polytechnic Institute and State University, 1986)Fluid flows past single and multiple cylinders in different configurations are investigated both experimentally and numerically. Three and five in-line cylinders and in-line and staggered bundles of cylinders with different pitch-to-diameter ratios are considered. Experimental work comprises of laser-Doppler velocimetry and flow visualization obtained in a water tunnel and skin friction, pressure, lift, drag and hot-wire measurements obtained in a wind tunnel. Both steady and pulsed flows are considered. Numerical work consists of finite element analysis of Navier-Stokes and energy equations governing viscous fluid flow past single and multiple cylinders. Detailed measurements of the fluid dynamic quantities for flow past cylinders reveal that flow pulsation at frequencies which induce lock-on increases the organization of the flow in gaps between cylinders. A new pattern of flow field is found for flow past a triad and a pentad of cylinders with a pitch-to-diameter ratio of l.8. The numerical analysis generates important integral characteristics like flow resistance and heat transfer. A staggered square arrangement of finite bundle of cylinders is found to have better heat transfer characteristics compared to the in-line or staggered equilateral-triangular arrangements.
- Experiments on the stability of the flat-plate boundary layer with suctionReynolds, Gregory A. (Virginia Polytechnic Institute and State University, 1982)The effects of boundary-layer suction in stabilizing the laminar boundary layer were considered in this experimental work. The experiments were conducted on a flat-plate wind-tunnel model in the V.P.I. & S.U. Stability Tunnel. The capabilities of the wind tunnel in terms of mean flow uniformity and low turbulence level were established and the basic flat-plate experiment was verified by comparison with the well known stability results for the Blasius boundary-layer. The two-dimensional Tollmein-Schlichting waves were introduced into the boundary layer using a vibrating ribbon. Suction was applied through the model surface vra two porous panels which could be located at various streamwise positions on the model. The porous Dynapore surface material of these panels was adapted by Douglas Aircraft Co. for possible use as a wing surface material on aircraft equipped with Laminar Flow Control. Detailed mean-flow measurements without suction showed that the Dynapore had no destabilizing effects in the laminar boundary layer. Using the porous panels, suction could be applied either continuously or in a discrete fashion through spanwise suction strips. With suction applied through a single spanwise strip, the mean-flow effects were determined in terms of the boundary-layer shape factor. Measurements of the disturbance behavior with suction were conducted to determine the relative merits of spatially continuous suction versus suction applied through various discrete suction strip configurations. These measurements were conducted in terms of the integral across the boundary layer of the streamwise disturbance amplitude, that is A = ∫ u' I dy. These results were compared with the theory of Reed & Nayfeh (1981). A method was also proposed by this theory for optimization of suction strip placement, and measurements were made which provided partial confirmation as to the validity of this optimization scheme. Experiments were also conducted without suction to investigate the weak nonlinear two- and three-wave interactions which occur at higher wave amplitudes. In these experiments, two-frequency Tollmein Schlichting waves were introduced simultaneously using a single vibrating ribbon, and initial disturbance amplitudes were controlled. In particular, these experiments established appropriate initial conditions for comparison with analytical models and considered the role of the difference frequencies which were generated through the nonlinear interaction of the two fundamental waves. This work also considered the behavior of the harmonic waves in the nonlinear regime.
- Secondary Instability of Plane Channel Flow to Subharmonic Three-Dimensional DisturbancesHerbert, T. (AIP Publishing, 1983)A linear secondary instability mechanism is presented that leads to the occurrence of subharmonic three‐dimensional disturbances in wall‐bounded shear flows. The instability originates from the periodic redistribution of vorticity in the shear flow by small but finite‐amplitude Tollmien–Schlichting waves. Low threshold amplitudes and other characteristics of this instability are consistent with experiments and may elucidate various obscure observations.
- Separation and wakes over three-dimensional bodiesCostis, Christopher E. (Virginia Polytechnic Institute and State University, 1985)The laminar flow over a prolate spheroid was investigated via flow visualization and Laser-Doppler Velocimetry. Experiments were conducted in a water tunnel and the flow was visualized with dyes. The measurement of three-dimensional boundary layers required a special design of the laser optics. Attention was focused in the neighborhood of three-dimensional separation. The Vortex-Lattice method was employed to calculate the inviscid flow and the development of separated vortex sheets over a prolate spheroid. An approximate-method based on the assumption of local similarity was used to solve the boundary layer equations and calculate the line of open separation. A condition of vortex shedding along separation is proposed. The two schemes, viscous and inviscid, interact through the line of separation which is allowed to displace as the wake grows. Results are compared with flow visualization data for laminar separation and pressure data for turbulent separation.
- Vorticity shedding over two-dimensional bodiesMathioulakis, Dimitri (Virginia Tech, 1985)The vorticity shedding characteristics in attached and separated regions were investigated over three configurations, namely a backward facing circular arc, an ellipse at an angle of attack and a pitching airfoil. A fully automated data acquisition system was developed, including a two-component Laser-Velocimetry system in backscatter mode, an accurately controlled traversing mechanism and a MINK-11 minicomputer. Two-component velocity measurements were obtained over the above mentioned bodies, with steady and unsteady free streams. Emphasis was concentrated on the separation region, the free-shear layers and the wake downstream of these bodies. Two inviscid vortex models were developed to predict two different flow phenomena, namely the separated flow over a circular cylinder started impulsively from rest and propagating stall over a linear stationary cascade.