Browsing by Subject "Boundary layer"
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Item A numerical method for the calculation of unsteady lifting potential flow problems(Texas Tech University, 1982-08) Im, Bong-jinA potential flow model for two-dimensional airfoils in unsteady motion with boundary layer separation is described. The airfoil and wake surfaces are represented by a finite set of uniform strength doublet panels. The doublet strengths on the airfoil surface are determined by applying a kinematic surface tangency condition to a Green's function representation of the potential field, while simultaneously enforcing the Kutta condition. Wake shedding is governed by a dynamic free surface condition and the characteristics of the flow near any boundary layer separation points. Wake deformation is predicted by applying a geometric free surface condition.Item A numerical model for two-dimensional unsteady flow calculations with a surface panel method(Texas Tech University, 1985-12) Im, Bong-jinA numerical model for two-dimensional airfoils in unsteady motion with boundary layer separation is described. The airfoil and wake surfaces are represented by a finite set of combined source and vortex panels. The source strengths are prescribed to have the same magnitude as the normal relative velocity on the surface due to the freestream and motion of the airfoil. The vertex strengths on the airfoil surface are determined by applying a kinematic surface tangency condition to a Green's function representation of the potential field, while simultaneously enforcing the Kutta condition. Wake shedding is governed by a dynamic free surface condition and the characteristics of the flow near any boundary layer separation points. Wake deformation is predicted by applying a geometric free surface condition. Calculation results are presented for steady motion, impulsively started rectilinear motion, harmonic pitch oscillations, and constant pitching motions. Experimental data and analytical solutions are also presented for comparison.Item Experimental measurements of conjugate heat transfer on a scaled-up gas turbine airfoil with realistic cooling configuration(2010-05) Dees, Jason Edward; Bogard, David G.; Clemens, Noel T.; da Silva, Alexandre K.; Ezekoye, Ofodike A.; Moser, Robert D.This study performed detailed measurements on and around scaled up conducting and adiabatic airfoils with and without film cooling. The conducting vane was a matched Bi airfoil, which accurately scaled the convective heat transfer and conduction through the solid, in order to produce non-dimensional surface temperatures and thermal boundary layers that were representative of an actual engine. Measurements made on all vane models included surface temperature measurements and thermal profiles above the walls. Separate measurements on non-film cooled and film cooled conducting models allowed for the individual contributions of the internal convective cooling and external film cooling to the overall cooling scheme to be quantified. Surface temperature and thermal field measurements above the wall were also performed on a film cooled adiabatic model. For the conducting model with internal cooling only, strong streamwise temperature variations were seen. The surface temperature variations were highly dependent on the local external and internal heat transfer coefficients. Spanwise temperature variations also existed, but were modest in comparison to streamwise variations. Comparing the thermal fields above the film cooled adiabatic and conducting walls allowed for the assumption that the conducting wall would not significantly affect the thermal field in the film cooling jet to be tested. Near the edge of the film cooling jet the developing thermal boundary layer had a clear effect on the overlying gas temperature, suggesting that the common assumption that the adiabatic wall temperature is the appropriate driving temperature for heat transfer to a film cooled wall was invalid. On the jet centerline thermal boundary layer effects were less influential, due to the development of a new, thin boundary layer. This suggested that the adiabatic wall temperature as driving temperature for heat transfer was a reasonable assumption on the jet centerline for most cases tested. As film cooling momentum flux ratio increase, thermal boundary layer effects became more influential on the jet centerline. Additionally, the high resolution surface temperature measurements and thermal field measurements above the wall presented in the current study represent a significant improvement in the data available for validation of computational simulations of conducting turbine airfoils.Item Ionic separation in electrodialysis : analyses of boundary layer, cationic partitioning, and overlimiting current(2010-08) Kim, Younggy; Lawler, Desmond F.; Liljestrand, Howard M.; Katz, Lynn E.; Meyers, Jeremy P.; Sepehrnoori, KamyElectrodialysis performance strongly depends on the boundary layer near ion exchange membranes. The thickness of the boundary layer has not been clearly evaluated due to its substantial fluctuation around the spacer geometry. In this study, the boundary layer thickness was defined with three statistical parameters: the mean, standard deviation, and correlation coefficient between the two boundary layers facing across the spacer. The relationship between the current and potential under conditions of the competitive transport between mono- and di-valent cations was used to estimate the statistical parameters. An uncertainty model was developed for the steady-state ionic transport in a two-dimensional cell pair. Faster ionic separations were achieved with smaller means, greater standard deviations, and more positive correlation coefficients. With the increasing flow velocity from 1.06 to 4.24 cm/s in the bench-scale electrodialyzer, the best fit values for the mean thickness reduced from 40 to less than 10 μm, and the standard deviation was in the same order of magnitude as the mean. For the partitioning of mono- and di-valent cations, a CMV membrane was examined in various KCl and CaCl₂ mixtures. The equivalent fraction correlation and separation factor responded sensitively to the composition of the mixture; however, the selectivity coefficient was consistent over the range of aqueous-phase ionic contents between 5 and 100 mN and the range of equivalent fractions of each cation between 0.2 and 0.8. It was shown that small analytic errors in measuring the concentration of the mono-valent cation are amplified when estimating the selectivity coefficient. To minimize the effects of such error propagation, a novel method employing the least square fitting was proposed to determine the selectivity coefficient. Each of thermodynamic factors, such as the aqueous- and membrane-phase activity coefficients, water activity, and standard state, was found to affect the magnitude of the selectivity coefficient. The overlimiting current, occurring beyond the electroneutral limit, has not been clearly explained because of the difficulty in solving the singularly perturbed Nernst-Planck-Poisson equations. The steady-state Nernst-Planck-Poisson equations were converted into the Painlevé equation of the second kind (P[subscript II] equation). The converted model domain is explicitly divided into the space charge and electroneutral regions. Given this property, two mathematical formulae were proposed for the limiting current and the width of the space charge region. The Airy solution of the P[subscript II] equation described the ionic transport in the space charge region. By using a hybrid numerical scheme including the fixed point iteration and Newton Raphson methods, the P[subscript II] equation was successfully solved for the ionic transport in the space charge and electroneutral regions as well as their transition zone. Above the limiting current, the sum of the ionic charge in the aqueous-phase electric double layer and in the space charge region remains stationary. Thus, growth of the space charge region involves shrinkage of the aqueous-phase electric double layer. Based on this observation, a repetitive mechanism of expansion and shrinkage of the aqueous-phase electric double layer was suggested to explain additional current above the limiting current.Item Large eddy simulations (LES) of boundary layer flashback in wall-bounded flows(2014-12) Hassanaly, Malik; Clemens, Noel T.In the design of high-hydrogen content gas turbines for power generation, flashback of the turbulent flame by propagation through the low velocity boundary layers in the premixing region is an operationally dangerous event. The high reactivity of hydrogen combined with enhanced flammability lim- its (compared to natural gas) promotes flame propagation along low-speed boundary layers adjoining the combustion walls. This work focuses on the simulation of boundary layer flashback using large-eddy simulations (LES). A canonical channel configuration is studied to assess the capabilities of LES and determine the modeling requirements for boundary layer flashback simulations. To extend this work to complex geometries, a new reactive low-Mach number solver has been written in an unstructured code.Item The effects of boundary layer development and turbulence on the performance of a gas-blown spark gap switch(Texas Tech University, 1984-12) Glass, Bradley LyleThe effects of boundary layer development and turbulence on a gas-blown spark gap switch operated in the self breakdown mode are presented. The switch consisted of two electrodes mounted flush in the diverging walls of a wind tunnel. The electrode spacing was 2.54 mm at the minimum separation distance and the wind tunnel had an aspect ratio based on the minimum separation distance of 40:1 to insure two-dimensional flow. To obtain various stages of boundary layer development for laminar flow, entry lengths of 0, 6.19, 17.46, 22.78, and 57.3 cm were used. These were tested at divergence angles of 5.0 and 10.0 degrees. Turbulence was investigated by placing a coarse, plastic grid 7.62 mm" upstream of the minimum electrode spacing using the 22.78 cm entry length and a 10.0 degree divergence angle. The gas used was instrument grade dry air (< 10 ppm H2O), and the electrode material was 304 stainless steel. It was found that switch performance at low flow velocities was improved with entry lengths up to 17.46 cm and then deteriorated with increasing length. This suggests that there is a stage of boundary layer development that produces optimum switch operation at minimum flowrates. The results also suggest that there is an average gas density level that must be attained at the minimum separation point before the maximum voltage can again be applied to the switch. For the only turbulence test conducted, 10.0 degree divergence angle with an entry length of 27.78 cm, switch performance was improved as compared with the laminar case.Item The hp-finite element method for problems with boundary layers(Texas Tech University, 1999-05) Olsen, Jon HowardThis thesis studies the convergence of a singularly perturbed two-point boundary value problem of the convection-diffusion type. The problem is solved using a hp finite element method on a piecewise uniform fitted mesh. Results will show exponential convergenceof the solution independent of the singular perturbation parameter.Item Three dimensional viscous/inviscid interactive method and its application to propeller blades(2012-08) Yu, Xiangming, 1987-; Kinnas, Spyros A.; Liljestrand, HowardA three dimensional viscous/inviscid interactive boundary layer method for predicting the effects of fluid viscosity on the performance of fully wetted propellers is presented. This method is developed by coupling a three dimensional low-order potential based panel method and a two dimensional integral boundary layer analysis method. To simplify the solution procedures, this method applies a reasonable assumption that the effects of the boundary layer along the span wise direction (radially outward for propeller blades) could be negligible compared with those along the stream wise direction (constant radius for propeller blades). One significant development of this method, compared with previous work, is to completely consider the effects of the added sources on the whole blades and wakes rather than evaluate the boundary layer effects along each strip, without interaction among strips. This method is applied to Propeller DTMB4119, Propeller NSRDC4381 and DTMB Duct II for validation. The results show good correlation with experimental measurements or RANS (ANSYS/FLUENT) results. The method is further used to develop a viscous image model for the cases of three dimensional wing blades between two parallel slip walls. An improved method for hydrofoils and propeller blades with non-zero thickness or open trailing edges is presented as well. The method in this thesis follows the idea of Pan (2009, 2011), but applies a new extension scheme, which uses second order polynomials to describe the extension edges. A improved simplified search scheme is also used to find the correct shape of the extension automatically to ensure the two conditions are satisfied.