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Browsing by Author "Ng, Wing F."

Now showing 1 - 7 of 7
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    High resolution algorithms for the Navier Stokes equations for generalized descretizations
    Mitchell, Curtis Randall (Virginia Tech, 1992)
    Accurate finite volume solution algorithms for the two dimensional Navier Stokes equations and the three dimensional Euler equations for both structured and unstructured grid topologies are presented. Results for two dimensional quadrilateral and triangular elements and three dimensional tetrahedral elements will be provided. Fundamental to the solution algorithm is a technique for generating multidimensional polynomials which model the spatial variation of the flow variables. Cell averaged data is used to reconstruct pointwise distributions of the dependent variables. The reconstruction errors are evaluated on triangular meshes. The implementation of the algorithm is unique in that three reconstructions are performed for each cell face in the domain. Two of the reconstructions are used to evaluate the inviscid fluxes and correspond to the right and left interface states needed for the solution of a Riemann problem. The third reconstruction is used to evaluate the viscous fluxes. The gradient terms that appear in the viscous fluxes are formed by simply differentiating the polynomial. By selecting the appropriate cell control volumes, centered, upwind and upwind-biased stencils are possible. Numerical calculations in two dimensions include solutions to elliptic boundary value problems, Ringleb’s flow, an inviscid shock reflection, a flat plate boundary layer, and a shock induced separation over a flat plate. Three dimensional results include the ONERA M6 wing. All of the unstructured grids were generated using an advancing front mesh generation procedure. Modifications to the three dimensional grid generator were necessary to discretize the surface grids for bodies with high curvature. In addition, mesh refinement algorithms were implemented to improve the surface grid integrity. Examples studied include a Glasair fuselage, High Speed Civil Transport, and the ONERA M6 wing. The role of reconstruction as applied to adaptive remeshing is discussed and a new first order error estimator is presented. Numerical examples of the remeshing procedure include both smooth and discontinuous flows.
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    An interferometric study of unsteady passing shock flow in a turbine cascade
    Wesner, Angela L. (Virginia Tech, 1996)
    A single-plate interferometric system was developed for the evaluation of unsteady flow generated by a passing shock in a linear transonic turbine cascade. A 15 mW Helium-Neon laser was used as the light source. An intensified charge-coupled device (CCD) was used to capture images, allowing the required 100 ns shutter times to “freeze” the passing shock. High frequency Kulite pressure transducers and miniature hot wire probes were used to capture the required known density field for the quantitative evaluation of the flowfield. Variations in the density field data as the shock passes upstream of the studied blade passage are presented, showing the capabilities of this system and detailing the unsteady flow field. The automation of the interferogram interpretation is presented. Image analysis techniques used include adaptive binarization and Hilditch line thinning. Further areas for improvement are also discussed. In addition to the interferometric study, a boundary layer transition study was also undertaken. Magnified boundary layer spark shadowgraphs were taken on both the pressure and suction surfaces of the turbine blade. Flow conditions including low (<1.0%) and high (5%) free stream turbulence values were investigated. The transition location was found to be dependent not only on the level of turbulence but also on the turbulence characteristics. Further, an investigation was made into the application of hydrodynamic stability theory to turbine blade flows. The results of this evaluation were used in the determination of appropriate length scales for tailoring free stream turbulence to more effectively induce suction surface transition. Little instability and no transition was calculated for the blade suction surface in agreement with the shadowgraph results. Free stream turbulence with frequencies near 40 kHz were found to most affect the instabilities in the boundary layer. Tailored free stream turbulence was found to produce transition on the blade suction surface.
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    Investigation of suction recirculation in rocket pump inducers
    Van Sintern, Wolff H. (Virginia Tech, 1996)
    Rocket pump inducers are studied with a Navier-Stokes solver, the Moore Elliptic Flow Program, MEFP. Two redesigns of the Rocketdyne ADP inducer are performed. The first redesign changes the hub-tip ratio from 0.3 to 0.4. The second redesign influences the leading edge tip region. This part of the blade is rounded off in a meridional view, resulting in an increased sweep angle over the outer 30 percent of the blade height. Inlet recirculation and cavitation performance are the main concern of this thesis. The literature review focuses on these two areas and on previously performed inducer calculations. The calculation grid of the original design was altered for the two redesigns using a C-program. Then the new designs were investigated at three different flow rates: 110%, 100%, and 89%. Computational results were available at the same flow rates for the original design. The comparison of the results showed that the performed design changes did not significantly influence the recirculation performance. In fact for the redesign with the rounded-off leading edge the backflow increased. The cavitation performance showed some interesting changes. For both redesigns the minimum pressure was raised significantly. The case of the rounded-off leading edge showed an increase in minimum pressure at the leading edge and a decrease of minimum pressure over the rest of the leading edge height, thus resulting in probably better cavitation performance. A theory was presented that gives a possible explanation for the occurrence of inlet recirculation. The calculation results for the design changes of the Rocketdyne inducer are consistent with the theory, but the theory does not correctly predict the different recirculation performance of an SEP inducer.
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    Large eddy simulation of subsonic mixing layers
    Sheen, Shaw-Ching (Virginia Tech, 1993)
    Large eddy simulation is used to study the large-scale structures in a low subsonic mixing layer and their breakdown to small scales. For 3-D simulations, different finite-difference and pseudo-spectral schemes are tested. The (2, 4) MacCormack Scheme developed by Gottlieb and Turkel (1976) shows the best overall performance. It is very fast and supplies enough but not excessive artificial dissipation. Though slower than MacCormack scheme, the pseudo-spectral method has its advantage: high resolution of the high-wavenumber range when adequate de-aliasing scheme is used. When efficient fast Fourier transform routines are available, this method can be a very good alternative to the MacCormack scheme. Most of the simulations use a modified Smagorinsky-type model (Erlabacher et al. 1992). The effect of different models and model constants is also studied. It is found that the two subgrid-scale (SGS) models, the Smagorinsky model and the linear combination model (Bardina et al. 1983), show significant difference even at the low wavenumber range of the spectra. In the study of three-dimensional subsonic temporal mixing layers, it is found that the streamwise vortex tubes play an important role in the transition process. The vortex interaction of the streamwise vortex tubes and undulated spanwise vortex structures proves to be the dominant mechanism in the development of three-dimensionality and the subsequent generation of small-scale motions. In the absence of pairing of the spanwise vortex tubes, this vortex interaction causes uneven distribution of vorticity along the span of the spanwise vortex tubes and the breaking of the large structures. Following the breaking of the spanwise vortex tubes, the secondary streamwise vortex tubes become the dominant vortex structures. In the case involving pairing, it is found that the relative motion of the spanwise vortex tubes in the pre-pairing process creates much stronger strain rate field between the pairing vortex tubes than the case without pairing. The stronger strain rate field leads to the formation of streamwise vortex tubes with very high vorticity and low induced pressure. This also leads to much stronger vortex interaction between the spanwise and streamwise vortex tubes due to the increased strength of the streamwise vortex tubes.
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    Laser Doppler anemometry measurements of a confined turbulent water jet with a uniform background flow
    Hsu, Cheng-Hsing (Virginia Polytechnic Institute and State University, 1989)
    An axisymmetric, turbulent water jet with several very slow, coflowing external streams was measured with a frequency shifted laser Doppler anemometer. The objective was to approximate a jet submerged in an ambient fluid of infinite domain by using a confined jet in a uniform coflow. The coflow prevents flow reversal outside the jet, but if the coflow velocity is not small compared to the jet velocity, the jet will no longer be self-preserving. Thus, the objective is reached in the limit as the coflow approaches zero, but in the absence of reverse flow. In the present study, a jet with several slow coflows was examined to investigate this behavior, the data was extrapolated to the limit (Us/Uj) to obtain the free jet results and reduce uncertainty in earlier data. The Reynolds number based on the jet diameter and exit velocity was 32100. Conservation of momentum of the jet was demonstrated up to the measurement limit of x/d = 100. Its distribution suggests that the near fleld axial pressure variation has significant effects on the momentum flux. The results also indicate that momentum flux measurements require accurate data to the edge of the jet. The similarity of mean and rms velocity profiles suggest the existence of a region of self-preservation. The entrainment rate, centerline velocity decay rate and spreading rate of the jet were determined and compared to previous measurements with and without a coflowing stream. The variation of these jet parameters with respect to the veIocity ratios was obtained. The limiting values of the jet parameters were determined by extrapolation to zero velocity ratio. This study indicates that a slow coflowing stream is an ideal way to eliminate the recirculating zone present outside jets without coflows. By reducing the coflow to a negligible velocity with constant Craya-Curtet number, researchers can greatly reduce the wide experimental variation in jet entrainment and spreading rates found in different facilities. The results also indicated that a confined jet with a very slow coflow without recirculation can asymptotically approach the conditions of a free jet. An estimate of the variation of the duct size versus the velocity ratio is obtained. It suggests that it is not possible to reduce the velocity ratio to an arbitrarily small value without backflow because the duct would become impractically large.
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    Oscillating shock impingement on low-angle gas injection into a supersonic flow
    Wood, Charles Wade (Virginia Tech, 1991)
    Experiments were performed to determine the effects of impinging oscillating shocks of different frequencies on a 15° downstream angled, underexpanded, sonic helium jet injected into a supersonic airflow. Information on mixing, penetration, total pressure loss and turbulence structure from these experiments was used to estimate mixing control achieved by adding an oscillating shock to the helium injection flow field. Tests were conducted at Mach 3.0, with a total pressure of 6.5 atm, a total temperature of 290 K and a Reynolds number of 51.0 x 10⁶ per meter. Oscillating shocks of three different frequencies were studied. The frequencies selected were designed to allow tuning of the shock frequency to the estimated frequency, about 100 - 150 kHz, of the largest eddies in the approach boundary layer. Visualization using nanoshadowgraph photography showed large turbulent structures in all cases. In addition, there were clear changes in eddy size with changing shock frequency visible on the nanoshadowgraphs. The primary measurement made for the mixing studies was the molar concentration of helium. Concentration data, as well as mean flow data, was collected at nine lateral positions at each of three axial stations downstream of the helium injector. The resulting data produced contours of helium concentration, total pressure, Mach number, velocity, mass flux and static flow properties. Additional tests were conducted to determine the shock oscillation frequency, the correlation between the oscillating shock and the turbulence in the shear layer and the angle of large-scale structures in the flow. Mixing and penetration rates were determined from the helium concentration data. The major result of this study was that impingement of an oscillating shock on a high-speed shear layer can be used to control the rate of mixing. Depending on the shock oscillation frequency, mixing enhancement or inhibition can be produced. It was found that increasing shock oscillation frequency resulted in more rapid injectant concentration decay and increased freestream air entrainment leading to a stoichiometric H₂-air mixture ratio while also reducing penetration of the helium injectant. A strong correlation was found between the highest frequency shock and changes in the mixing flow field. The maximum oscillation frequency was approximately 140 kHz, which was consistent with numerical estimates for the frequency necessary for mixing augmentation under these test conditions. It was concluded that oscillating shock impingement has promise as a means of controlling gaseous mixing in a high-speed cross-flow.
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    System and method of non-intrusive anemometry
    (United States Patent and Trademark Office, 2019-05-07)
    A system and method of non-intrusive anemometry. The system comprises an acoustic transmitter disposed at a boundary of fluid flow and first and second acoustic receivers adapted to receive transmissions from the acoustic transmitter. A processor is coupled to the acoustic receivers to determine the time of arrival of the transmission at the acoustic receivers. The acoustic transmitter and acoustic receivers are arranged such that the acoustic transmitter is upstream from the first acoustic receiver which is in turn upstream from the second acoustic transmitter.