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Open Access Publications from the University of California

Scholarly Works (16 results)

  • Article
  • Peer Reviewed
UC Berkeley Previously Published Works (2019)
A numerical procedure for conjugate heat transfer (CHT) analysis based on an immersed boundary (IB) method for large eddy simulation (LES) has been developed. The dynamic subgrid-scale (SGS) stress and heat flux models are implemented in the solver. Since discrepancy between the computational grid and solid-fluid interface generally exists, interpolation of thermal properties at the interface is necessary for the current method. Accordingly, an efficient interpolation scheme using the modified flood-fill algorithm is applied. In order to validate the capability of the developed method, three different CHT cases with turbulent flow were examined: channel flow between heated slabs, thermally-driven flow in a closed cavity, and crossflow around a heated circular cylinder. Compared to cases that just utilized an isothermal or constant-heat-flux boundary condition, when the CHT method was properly implemented, results more closely matched experimental data and existing correlations.
    Cover page: Development of an efficient immersed-boundary method with subgrid-scale models for conjugate heat transfer analysis using large eddy simulationCreative Commons 'BY-NC-ND' version 4.0 license
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2023)
    A spectral method using associated Legendre functions with algebraic mapping is developed for a linear stability analysis of wake vortices. These functions serve as Galerkin basis functions, capturing correct analyticity and boundary conditions for vortices in an unbounded domain. The incompressible Euler or Navier-Stokes equations linearised on a swirling flow are transformed into a standard matrix eigenvalue problem of toroidal and poloidal streamfunctions, solving perturbation velocity eigenmodes with their complex growth rate as eigenvalues. This reduces the problem size for computation and distributes collocation points adjustably clustered around the vortex core. Based on this method, strong swirling vortices with linear perturbation wavenumbers of order unity are examined. Without viscosity, neutrally stable eigenmodes associated with the continuous eigenvalue spectrum having critical-layer singularities are successfully resolved. The inviscid critical-layer eigenmodes numerically tend to appear in pairs, implying their singular degeneracy. With viscosity, the spectra pertaining to physical regularisation of critical layers stretch out toward an area, referring to potential eigenmodes with wavepackets found by Mao & Sherwin (J. Fluid Mech., vol. 681, 2011, pp. 1-23). However, the potential eigenmodes exhibit no spatial similarity to the inviscid critical-layer eigenmodes, doubting that they truly represent the viscous remnants of the inviscid critical-layer eigenmodes. Instead, two distinct continuous curves in the numerical spectra are identified for the first time, named the viscous critical-layer spectrum, where the similarity is noticeable. Moreover, the viscous critical-layer eigenmodes are resolved in conformity with the scaling law. The onset of the two curves is believed to be caused by viscosity breaking the singular degeneracy.
      Creative Commons 'BY' version 4.0 license
      • Thesis
      • Peer Reviewed
      UC Berkeley Electronic Theses and Dissertations (2010)

      Existing reinforced concrete (RC) buildings designed prior to 1970s are vulnerable to shear failure in beam-column joints under earthquake loads because of insufficient transverse reinforcement in the joint region. To assess the seismic risk of old RC buildings, the accurate prediction of shear strength and deformability for these unreinforced beam-column joints is essential. Several joint shear strength models are available in the literature but they have been originally developed to predict the shear strength of reinforced beam-column joints. Due to the different shear force transfer mechanism between reinforced and unreinforced beam-column joints, the existing models have little success to assess the shear strength of unreinforced beam-column joints. The ASCE/SEI 41-06 provisions specify shear strengths and backbone curves for unreinforced beam-column joints but the predictions using these provisions are usually conservative compared with many experimental test data collected from the literature. This study is focusing on developing accurate shear strength models and backbone relationships for unreinforced exterior and corner beam-column joints.

      This study proposes two shear strength models, semi-empirical and analytical, for unreinforced exterior and corner beam-column joints to reflect the influence of two key parameters: (1) joint aspect ratio which is defined as the ratio of beam to column cross-section heights, and (2) beam reinforcement index which is related to the amount of beam longitudinal reinforcement in tension. These key parameters are determined from a parametric study using a large experimental data set of unreinforced exterior and corner beam-column joints from the published literature. The proposed models are validated by accurate predictions of the shear strength for the database specimens. Besides the accuracy of the proposed models, the semi-empirical model has the advantage of straightforward extension to other types of beam-column joints. An important advantage of the analytical model is that for the case of beam yielding followed by joint failure, the analytical model can predict the reduced shear strength without the need for the complexity of ductility consideration.

      The experimental phase of this study includes testing four full-scale unreinforced corner beam-column joint specimens. These four specimens are designed to investigate the effect of the joint aspect ratio and the beam longitudinal reinforcement ratio. The test results show that the joint shear strengths are reduced with increase of the joint aspect ratio and for each of the joint aspect ratio, the joint shear strengths are proportional to the beam longitudinal reinforcement ratio within the range provided in the test specimens. The shear strengths of the four specimens are accurately predicted by the two proposed models, while the ASCE/SEI 41-06 provisions for shear strength produce conservative estimates of the strengths for the test specimens.

      Based on the measured joint shear stress-rotation and visual observation of the tested corner beam-column joint specimens, a multi-linear backbone relationship is proposed in this study to reflect the following beam-column joint responses: (1) initial joint cracking, (2) either beam reinforcement yielding or significant opening of existing joint cracks, (3) either existing joint cracks further propagation or additional joint cracks opening at the peak load, and (4) residual joint shear stress and rotation after severe damage in the joint. Corresponding parameters in the backbone relationship are defined from the comparison with test results. The proposed backbone relationship is verified by the simulations for beam-column subassemblies of the tested four specimens and other four planar exterior beam-column joint specimens from the literature. To investigate the effect of beam-column joint flexibility on the lateral response in a structural system level, nonlinear static and dynamic simulations are performed. These simulations indicate that beam-column joint flexibility is essential for older-type RC buildings characterized by having unreinforced beam-column joints. As an extension of this study, progressive collapse analysis for older-type RC buildings will be pursued with the proposed beam-column joint backbone relationships.

        Cover page: Experimental and Analytical Studies on Old Reinforced Concrete Buildings with Seismically Vulnerable Beam-Column Joints
        • Article
        UC Berkeley Previously Published Works (2024)
        The transient dynamics of a wake vortex, modelled by a strong swirling $q$-vortex, are examined with an emphasis on exploring optimal transient growth constructed by continuous eigenmodes associated with continuous spectra. The pivotal contribution of the viscous critical-layer eigenmodes (Lee & Marcus, J. Fluid Mech., vol. 967) amongst the entire eigenmode families to optimal perturbations is numerically confirmed, based on a spectral collocation method for a radially unbounded domain that ensures correct analyticity and far-field behaviour. The consistence of the numerical method against numerical sensitivity provides reliability of results as well as flexibility for tuning. Both axisymmetric and helical perturbation cases with axial wavenumbers of order unity or less are considered in the study through both linearised theory and non-linear simulations, yielding results that align with literature on both energy growth curves and optimal perturbation structures. Additionally, the initiation process of transient growth is discussed to reveal its practicability. Inspired by ice crystals in contrails, the role of backward influences of inertial particles on the carrier vortex flow, especially via particle drag, is underscored. In the pursuit of optimal transient growth, the particles are initially distributed at the periphery of the vortex core to disturb the vortex. Two-way coupled vortex-particle simulations conclusively demonstrate clear indications of optimal transient growth during continual vortex-particle interactions, reinforcing the robustness and significance of the transient growth process in the original non-linear vortex system over finite time periods.
          • Article
          UC Berkeley Previously Published Works (2022)
          A spectral method using associated Legendre functions with algebraic mapping is developed for a linear stability analysis of wake vortices. These functions serve as Galerkin basis functions, capturing correct analyticity and boundary conditions for vortices in an unbounded domain. The incompressible Euler or Navier-Stokes equations linearised on a swirling flow are transformed into a standard matrix eigenvalue problem of toroidal and poloidal streamfunctions, solving perturbation velocity eigenmodes with their complex growth rate as eigenvalues. This reduces the problem size for computation and distributes collocation points adjustably clustered around the vortex core. Based on this method, strong swirling $q$-vortices with linear perturbation wavenumbers of order unity are examined. Without viscosity, neutrally stable eigenmodes associated with the continuous eigenvalue spectrum having critical-layer singularities are successfully resolved. The inviscid critical-layer eigenmodes numerically tend to appear in pairs, implying their singular degeneracy. With viscosity, the spectra pertaining to physical regularisation of critical layers stretch out toward an area, referring to potential eigenmodes with wavepackets found by Mao & Sherwin (2011). However, the potential eigenmodes exhibit no spatial similarity to the inviscid critical-layer eigenmodes, doubting that they truly represent the viscous remnants of the inviscid critical-layer eigenmodes. Instead, two distinct continuous curves in the numerical spectra are identified for the first time, named the viscous critical-layer spectrum, where the similarity is noticeable. Moreover, the viscous critical-layer eigenmodes are resolved in conformity with the $Re^{-1/3}$ scaling law. The onset of the two curves is believed to be caused by viscosity breaking the singular degeneracy.
            Creative Commons 'BY' version 4.0 license
            • Thesis
            • Peer Reviewed
            UC Berkeley Electronic Theses and Dissertations (2024)

            The early-time growth of an individual vortex in a wake vortex pair is explored, focusing on its linear stability and transient growth, which are meaningful for understanding the lifespan of vortices and mitigating hazards from wake turbulence. This study synthesises two parts: linear stability analysis and transient growth analysis. The linear stability of wake vortices is examined in both inviscid and viscous contexts, using the Batchelor or Lamb-Oseen vortex as a base vortex model. Linear stability analysis determines the growth rates of infinitesimal perturbations in the vortex flow by linearising the incompressible Navier-Stokes or Euler equations around the base vortex, leading to an eigenvalue problem that discloses the vortex's stability properties. Inviscid analysis, with zero viscosity, successfully reveals a continuous spectrum associated with critical-layer singularities, causing discontinuity in perturbation velocity in the inviscid limit and requiring careful numerical parameter tuning to avoid under-resolved, or so-called spurious, solutions. Non-zero viscosity alters both the discrete and continuous spectra, necessitating numerical resolution conforming to the Reynolds number to the one-third power scaling law to resolve the newly discovered viscous critical-layer eigenmode family. The transient growth phenomenon is then explored through non-modal analysis to understand how initial perturbations, as combinations of numerous eigenmodes, can experience significant transient growth even in linearly stable vortices. The transient growth formalism is applied to the linearised Navier-Stokes equations. Optimal perturbations that maximise energy amplification over finite time intervals are found, to which the viscous critical-layer eigenmode family serves as the main contributor. Numerical simulations quantify the growth of perturbations considering nonlinearity, showing that while nonlinearity may slightly alter transient dynamics, overall trends align with formalism predictions. Lastly, motivated by contrails around wake vortices, the research investigates particle-initiated transient growth, where perturbations evolve through particle-vortex interactions. Two-way coupled particle-vortex simulations demonstrate continual perturbation development, presenting primary growth patterns as captured in the transient growth formalism. This investigation implies the practicability of the transient growth phenomenon.

              Cover page: Linear Stability of a Wake Vortex and its Transient Growth: Numerical Analysis in Light of Critical-Layer Eigenmodes and Spectra
              • Article
              UC Berkeley Previously Published Works (2024)
              Employing a poloidal-toroidal projection technique, a multi-scale analysis of resonant wave triads in columnar vortices is performed to obtain the governing equations of the triadically coupled wave amplitudes. For inviscid flows, we establish that resonance between neutral, smooth waves is conservative, and the temporal evolution of wave amplitudes is either bounded or explosively unstable based on the signs of the triad's interaction coefficients. Assessing the onset of weakly nonlinear instabilities through the pseudoenergy criterion introduced by Cairns (1979, J. Fluid Mech., vol. 92), we use the large-axial-wavenumber asymptotic approach by Le Dizes and Lacaze (2005, J. Fluid Mech., vol. 542) to evaluate each triad member's pseudoenergy and argue against the possibility of explosive conservative three-wave resonance involving only regular Kelvin waves. Additionally, extending our investigation to specific vortices, such as the Lamb-Oseen vortex and the Batchelor vortex, we find that triadic resonance among their neutral modes consistently results in bounded behaviour.
                • Article
                • Peer Reviewed
                UC Berkeley Previously Published Works (2023)
                Design-by-Morphing (DbM) is a novel design methodology that creates a search space for topology optimization. Traditional design techniques often impose geometric constraints and, sometimes, the designer’s biases on the design space, which restricts the novelty of the designs and allows for only small local changes. On the contrary, we show in this paper that DbM does not impose such restrictions on the design space, thus allowing for a radical and expansive search space with only a few design parameters. We compare DbM with other methods in the case of design space generation for 2D airfoils and find that DbM can reconstruct the entire UIUC database with >99.5% accuracy. Furthermore, using a bi-objective genetic algorithm, we optimize the airfoil designs created by DbM to maximize both the lift-over-drag ratio, CLDmax, and stall angle tolerance, ∆α, which results in a Pareto-front of innovative airfoils that exhibit substantial improvements in both objectives.
                  Creative Commons 'BY' version 4.0 license
                  • Article
                  UC Berkeley Previously Published Works (2022)
                  We present Design-by-Morphing (DbM), a novel design methodology applicable to creating a search space for topology optimization of 2D airfoils. Most design techniques impose geometric constraints and sometimes designers' bias on the design space itself, thus restricting the novelty of the designs created, and only allowing for small local changes. We show that DbM methodology does not impose any such restrictions on the design space and allows for extrapolation from the search space, thus granting truly radical and large search space with a few design parameters. In comparison to other shape design methodologies, we apply DbM to create a search space for 2D airfoils. We optimize this airfoil shape design space for maximizing the lift-over-drag ratio, $CLD_{max}$, and stall angle tolerance, $\Delta \alpha$. Using a bi-objective genetic algorithm to optimize the DbM space, it is found that we create a Pareto-front of radical airfoils exhibiting remarkable properties for both objectives.
                    Creative Commons 'BY-NC' version 4.0 license
                    • Article
                    • Peer Reviewed
                    UC Berkeley Previously Published Works (2018)
                    The flow field in a ribbed triangular channel representing the trailing edge internal cooling passage of a gas turbine high pressure turbine blade is investigated via Magnetic Resonance Velocimetry (MRV) and Large Eddy Simulation (LES). Results are compared to a baseline channel with no ribs. LES predictions of the mean velocity fields are validated by the MRV results. In the case of the baseline triangular channel with no ribs, the mean flow and turbulence level at the sharp corner are small, which would correspond to poor heat transfer in an actual trailing edge. For the staggered ribbed channel, turbulent mixing is enhanced, and flow velocity and turbulence intensity at the sharp edge increase. This is due to secondary flow induced by the ribs moving toward the sharp edge in the center of the channel. This effect is expected to enhance internal convective heat transfer for the turbine blade trailing edge.
                      Cover page: Experimental and Numerical Investigation of the Flow in a Trailing Edge Ribbed Internal Cooling PassageCreative Commons 'BY-NC-ND' version 4.0 license
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