Mechanical Engineering
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Item Impact of Polymeric Drops on Drops and Films of a Different but Miscible Polymer(2024) Bera, Arka; Das, Siddhartha; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The fluid mechanics of a liquid drop impacting on another stationery (or spreading) liquid drop or on a liquid film (of thickness comparable, or smaller, or larger than the impacting drop) has attracted significant attention over the past several years. Such problems represent interesting deviations from the more widely studied problems of liquid drops impacting on solid surfaces having different wettabilities with respect to the impacting drops. These deviations stem from the fact that the resting liquid (in the form of the drop or the film) itself undergoes deformation on account of the drop impact and can significantly affect the overall combined drop-drop or drop-film dynamics. The problem becomes even more intriguing depending on the rheology of the drop(s) and the film as well as the (im)miscibility of the impacting drop with the underlying drop or the film. Interestingly, the majority of such drop-impact-on-drop or drop-impact-on-film studies have considered Newtonian drop(s) and films, with little attention to polymeric drop(s) and films. This thesis aims to bridge that void by studying, using Direct Numerical Simulation (DNS) based computational methods, the impact-driven dynamics of one polymeric drop on another (different but miscible) polymeric drop or film. As specific examples, we consider two separate problems. In the first problem, we consider the impact of a PMMA (poly-methyl methacrylate) drop on a resting PVAc (polyvinyl acetate) drop as well as the impact of a PVAc drop on a resting PMMA drop. In the second problem, we consider the impact of a PMMA drop on a PVAc film as well as the impact of a PVAc drop on a PMMA film. For the first problem, the wettability of the resting drop (on the resting surface), the Weber number of the impacting drop, the relative surface tension values of the two polymeric liquids (PVAc and PMMA), and the miscibility (or how fast the two liquids mix) dictate the overall dynamics. PVAc has a large wettability on silicon (considered as the underlying solid substrate); as a result, during the problem of the PMMA drop impacting on the PVAc drop, the PVAc drop spreads significantly and the slow mixing of the two liquids ensures that the PMMA drop spreads as a thin film on top of the PVAc film (formed as the PVAc drop spreads quickly on silicon). Depending on the Weber number, such a scenario leads to the formation of transient liquid films (of multitudes of shapes) with stratified layers of PMMA (on top) and PVAc (on bottom) liquids. On the other hand, for the case of the PVAc drop impacting on the PMMA drop, a combination of the weaker spreading of the PMMA drop on silicon and the “engulfing” of the PMMA drop by the PVAc drop (stemming from the PVAc having a smaller surface tension than PMMA) ensures that the impacting PVAc drop covers the entire PMMA drop and itself interacts with the substrate giving rise to highly intriguing transient and stratified multi-polymeric liquid-liquid structures (such as core-shell structure with PMMA core and PVAc shell). For both these cases, we thoroughly discuss the dynamics by studying the velocity field, the concentration profiles (characterizing the mixing), the progression of the mixing front, and the capillary waves (resulting from the impact-driven imposition of the disturbance). In the second problem, we consider a drop of the PMMA (PVAc) impacting on a film of the PVAc (PMMA). In addition to the factors dictating the previous problem, the film thickness (considered to be either identical or smaller than the drop diameter) also governs the overall droplet-impact-driven dynamics. Here, the impact, being on the film, the dynamics is governed by the formation of crown (signifying the pre-splashing stage) and a deep cavity (the depth of which is dictated by the film thickness) on the resting film. In addition to quantifying these facets, we further quantify the problem by studying the velocity and the concentration fields, the capillary waves, and the progression of the mixing front. For the PMMA drop impacting on the thin film, a noticeable effect is the quick thinning of the PMMA drop on the PVAc film (or the impact-driven cavity formed on the PVAc film), which gives rise to a situation similar to the previous study (development of transient multi-polymeric-liquid structures with stratified polymeric liquid layers). For the case of the PVAc drop impacting on the PMMA film, the PVAc liquid “engulfs” the deforming PMMA film, and this in turn, reduces the depth of the cavity formed, the extent of thinning, and the amplitude of the generated capillary waves. All these fascinating phenomena get captured through the detailed DNS results that are provided. The specific problems considered in this thesis have been motivated by the situations often experienced during the droplet-based 3D printing processes (e.g., Aerosol jet printing or inkjet printing). In such printing applications, it is commonplace to find one polymeric drop interacting with an already deposited polymeric drop or a polymeric film (e.g., through the co-deposition of multiple materials during multi-material printing). The scientific background for explaining these specific scenarios routinely encountered in 3D printing problems, unfortunately, has been very limited. Our study aims to fill this gap. Also, the prospect of rapidly solidifying these polymeric systems (via methods such as in-situ curing) can enable us to visualize the formation of solidified multi-polymeric structures of different shapes (by rapidly solidifying the different transient multi-polymeric-liquid structures described above). Specifically, both PMMA and PVAc are polymers well-known to be curable using in-situ ultraviolet curing, thereby establishing the case where the present thesis also raises the potential of developing PMMA-PVAc multi-polymeric solid structures of various shapes and morphologies.Item A CAUSAL INFORMATION FUSION MODEL FOR ASSESSING PIPELINE INTEGRITY IN THE PRESENCE OF GROUND MOVEMENT(2024) Schell, Colin Andrew; Groth, Katrina M; Reliability Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Pipelines are the primary transportation method for natural gas and oil in the United States making them critical infrastructure to maintain. However, ground movement hazards, such as landslides and ground subsidence, can deform pipelines and potentially lead to the release of hazardous materials. According to the Pipeline and Hazardous Materials Safety Administration (PHMSA), from 2004 to 2023, ground movement related pipeline failures resulted in $413M USD in damages. The dynamic nature of ground movement makes it necessary to collect pipeline and ground monitoring data and to actively model and predict pipeline integrity. Conventional stress-based methods struggle to predict pipeline failure in the presence of large longitudinal strains that result from ground movement. This has prompted many industry analysts to use strain-based design and assessment (SBDA) methods to manage pipeline integrity in the presence of ground movement. However, due to the complexity of ground movement hazards and their variable effects on pipeline deformation, current strain-based pipeline integrity models are only applicable in specific ground movement scenarios and cannot synthesize complementary data sources. This makes it costly and time-consuming for pipeline companies to protect their pipeline network from ground movement hazards. To close these gaps, this research made significant steps towards the development of a causal information fusion model for assessing pipeline integrity in a variety of ground movement scenarios that result in permanent ground deformation. We developed a causal framework that categorizes and describes how different risk-influencing factors (RIFs) affect pipeline reliability using academic literature, joint industry projects, PHMSA projects, pipeline data, and input from engineering experts. This framework was the foundation of the information fusion model which leverages SBDA methods, Bayesian network (BN) models, pipeline monitoring data, and ground monitoring data to calculate the probability of failure and the additional longitudinal strain needed to fail the pipeline. The information fusion model was then applied to several case studies with different contexts and data to compare model-based recommendations to the actions taken by decision makers. In these case studies, the proposed model leveraged the full extent of data available at each site and produced similar conclusions to those made by decision makers. These results demonstrate that the model could be used in a variety of ground movement scenarios that result in permanent ground deformation and exemplified the comprehensive insights that come from using an information fusion approach for assessing pipeline integrity. The proposed model lays the foundation for the development of advanced decision making tools that can enable operators to identify at-risk pipeline segments that require site specific integrity assessments and efficiently manage the reliability of their pipelines in the presence of ground movement.Item LENGTH-SCALE DEPENDENCE OF VISCOPLASTIC PROPERTIES OF SILVER SINTER REVEALED BY INDENTATION TESTING AND MODELING(2024) Leslie, David; Dasgupta, Abhijit; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This doctoral dissertation research focuses on using a combination of indentation testing and modeling to characterize the creep behavior of heterogeneous silver sinter at different temperatures, using multiple indenter sizes to interrogate length-scale effects. The measured steady-state creep deformation is characterized with three different modeling approaches, that rely on: (i) conventional deviatoric creep potential; (ii) pressure-sensitive Drucker-Prager creep potential; and (iii) length-scale dependent deviatoric creep potential. The creep flow rule for all three cases is Norton’s power-law creep. The materials in this study are from a family of sintered silver materials used for interconnects and die-attach in high-temperature electronics and for conductor traces in printed electronics. The dissertation focuses on identifying and quantifying the length-scale dependence presented by sintered materials due to their non-homogeneous morphology. Testing consists of constant-force indentations using spherical indenters of two different radii at three different temperatures: 25°C, 75°C, and 125°C. The indentation results were first analyzed using two different post-processing methods: an empirical approach with closed-form models (CFM) and a computational FEA approach based on classical continuum mechanics. Differences found between the CFM and numerical (FEA) analyses, while significant at room temperature, reduce with temperature. Both models reveal that indenters of different radii cause significantly different viscoplastic behavior. This dependence on tip radius increases with temperature The research was extended to examine two second-order influences of the metallic agglomerated phase and the discontinuous compliant phase of the microstructure of sintered silver on its viscoplastic behavior: (i) dependence on hydrostatic stress; (ii) dependence on microstructural length-scale. The aim of incorporating the pressure-sensitive modeling was to investigate what effect the intrinsic compressive hydrostatic stress in indentation tests might have on the measured viscoplastic properties. Results from using the Drucker-Prager creep model further confirmed the increasing dependence on length-scale with temperature. The length-scale dependence seen in all the above results is investigated and quantified with the help of a simplified strain-gradient viscoplastic model. This modeling approach is motivated by the conventional mechanism-based strain gradient (CMSG) model that is widely used in plasticity theory to quantify length-scale effects. The characteristic length-scale metric in this problem is presented by the agglomerate size distribution in the sintered material and is quantified in this study with ‘watershed analysis’ of cross-sectional features observed via electron microscopy. This discrete length scale is believed to cause the variations in the observed creep response when queried with indenters of different radii, because of the different strain gradients produced by the two different indenters. The length-scale dependence is incorporated in a strain-gradient viscoplastic constitutive model suitable for finite element modeling of deformation fields containing strong strain-gradients (e.g. in the die attach layer in microelectronics chip assembly). Finally, a procedure is proposed, to incorporate the scale-dependence in the empirical closed-form approach, currently available in the literature, for extracting viscoplastic properties from indentation tests. This approach provides corrected model constants for the strain-gradient viscoplastic model, using simple closed-form equations instead of expensive finite element modeling.Item Phonon Transport and Nonequilibrium Kinetics with Stimulation Modeling in Molecular Crystals(2024) Liu, Zhiyu; Chung, Peter W.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)An important family of materials known as molecular crystals has been used extensively in fields such as organic semiconductors, energy, optoelectronics, and batteries. Due to their periodic crystal structure, phonons are the predominant heat and energy carriers. Phonons and their transport behaviors are crucial to the performance of semiconductors, the figure of merit of thermoelectrics, shock-induced properties of molecular crystals, and light-matter interactions of materials. Recent decades have seen significant advancements in the understanding of the phonon transport behaviors in inorganic crystals. However, a comprehensive understanding of phonon properties in molecular crystals is still lacking. While various theoretical models and computational simulations have been developed to study vibrational energy transfer in molecular crystals and to correlate vibrational structure with the stability of materials, these approaches often suffer from limitations. Many of these studies either neglect anharmonic scattering entirely or rely on simplified representations of phonon scattering. In this dissertation, we focus on investigating the phonon transport and nonequilibrium kinetics in molecular crystals. In the first work, we study the harmonic phonon properties of cellulose Iβ using tapered reactive force fields (ReaxFF). While geometry optimization with the original ReaxFF potential often results in structures with negative eigenvalues, indicating structural instability, the modified potential with a tapering function yields structures with no associated negative eigenvalues. Three ReaxFF parameterizations are evaluated by comparing lattice properties, elastic constants, phonon dispersion, temperature-dependent entropy, and heat capacity with experimental results from the literature. In the second study, we study the phonon transport behavior of Si, Cs2PbI2Cl2, cellulose Iβ, and α-RDX by calculating the thermal conductivity using different thermal transport models including the Phonon gas model, Cahill-Watson-Pohl, and the Allen-Feldman model and the Wigner formulation. By comparing the calculated thermal conductivity with experimental values, we highlight the significant contributions of wave-like heat carriers in cellulose Iβ and α-RDX. We show how different phonon properties influence particle-like and wave-like behavior in various materials and reveal unusual mechanisms present in molecular crystals. Lastly, we investigate nonequilibrium phonon kinetics resulting from direct vibrational excitations by employing the phonon Boltzmann transport equations. The results of our mid-IR pump-probe spectroscopy simulations align closely with experimental data from the literature. Additionally, by exciting different phonon modes at varying frequencies, we uncover distinct stages and pathways of vibrational energy transfer. To gain insights into the decomposition mechanism of RDX under excitation, we further calculate the bond activities of the N-N and N-O bonds, identifying possible stimuli that could trigger bond cleavage.Item Analyzing Inverse Design Problems from a Topological Perspective(2024) Chen, Qiuyi; Fuge, Mark; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Inverse design (ID) problems are inverse problems that aim to rapidly retrieve the subset of valid designs having the desired performances and properties under the given conditions. In practice, this can be solved by training generative models to approximate and sample the posterior distributions of designs. However, little has been done to understand their mechanisms and limitations from a theoretical perspective. This dissertation leverages theoretical tools from general and differential topology to answer these three questions of inverse design: what does a set of valid designs look like? How helpful are the data-driven generative models for retrieving the desired designs from this set? What topological properties affect the subset of desired designs? The dissertation proceeds by dismantling inverse (design) problems into two major subjects: that is, the representing and probing of a given set of valid designs (or data), and the retrieval of the desired designs (or data) from this given set. It draws inspiration from topology and geometry to investigate them and makes the main contributions below: 1. Chapter 3 details a novel representation learning method called Least Volume, which has properties similar to nonlinear PCA for representing datasets. It can minimize the representation's dimension automatically and, as shown in Chapter 4, conducts contrastive learning when applied to labeled datasets. 2. Two conditional generative models are developed to generate performant 2-D airfoils and 3-D heat sinks in Chapter 5 and 6 respectively. They can produce realistic designs to warm-start further optimization, with the relevant chapters detailing their acceleration effects. 3. Lastly, Chapter 7 describes how to use Least volume to solve high-dimensional inverse problems efficiently. Specifically, using examples from physic system identification, the chapter uncovers the correlation between the inverse problem's uncertainty and its intrinsic dimensions.Item Acoustic Black Hole with Functionally Graded Perforated Rings(2024) Petrover, Kayla; Baz, Amr; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis investigates a novel class of acoustic black hole waveguides (ABH) that harnesses the functionality of an array of optimally designed Functionally Graded Perforated Rings (FGPR). Through this approach, the developed ABH exhibits inherent energy dissipation characteristics derived from the flow through the perforations, which enhances its acoustic absorption behavior, resulting in rapid attenuation of the propagating waves as it traverses the length of the waveguide. Furthermore, the proposed ABH structure facilitates the incorporation of additional porous absorbing layers sandwiching the rings to further enhance its absorption characteristics. Consequently, the operational mechanism of this new class of ABH waveguides diverges significantly from that of the conventional ABH waveguide, which generates the black hole effect by employing sequential solid-flat rings of decreasing inner radius to create the necessary virtual power law taper. Instead, the new class of ABH generates the black hole effect through reactive means rather than the effective dissipative means of the conventional ABH. Therefore, this thesis develops a transfer matrix modeling (TMM) approach and a finite element method (FEM) approach to model the absorption and reflection characteristics of the novel class of ABH, aiming to predict its behavior and, more importantly, demonstrate its merits as effective means for controlling sound propagation. The interior-point method for optimization was employed to select optimal geometric design parameters for the FGPR inside the proposed ABH. Accordingly, the ABH with FGPR is manufacturable, unlike the conventional, and its acoustic properties are tuned to minimize the reflection of incoming acoustic waves across the frequency range 0-5 kHz. This optimization process is then repeated for the ABH with FGPR sandwiched by absorbing layers. From the pool of optimal designs generated, those that offer manufacturing advantages are chosen for further testing and evaluation. Numerical simulations are conducted to showcase the advantages and behavior of the proposed ABH configurations. The predictions of the TMM and FEM are compared and validated against experimental results which are collected with the ACUPRO impedance tube. Furthermore, comparisons between the ABH with FGPR and conventional ABH are made to elucidate and distinguish their respective behaviors and underlying principles of operation.Item SOCIOTECHNICAL NETWORK MODELING TO QUANTITATIVELY ANALYZE THE IMPACT OF BLOCKCHAIN ON THE RISK OF PROCURING COUNTERFEIT ELECTRONICS(2024) Akhavantaheri, Hirbod; Sandborn, Peter PS; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation develops sociotechnical agent-based network modeling to quantitatively analyze the impact of blockchain and other related policies on the supply-chain risk associated with the procurement of counterfeit electronics in critical systems. Safety-critical, mission-critical, and infrastructure-critical systems (e.g., aerospace, transportation, defense, and power generation) are forced to source parts from a supply chain that they do not control over exceptionally long periods of time. Critical systems are exposed to the dual risks of the impacts of system failure and the exposure to the vagaries of the marketplace over decades. Therefore, critical-systems operators, manufacturers, and sustainers, must implement policies and technologies to reduce the risk of obtaining counterfeit parts. Several policies, ranging from debarment and claw back to “hop counting,” have been considered and used to mitigate such risks. One technology that critical-system operators, manufacturers, and sustainers could adopt is distributed digital ledger (i.e., blockchain) for the supply chain. This dissertation does not focus on how such a blockchain could be implemented but rather on how (and if) blockchain for the supply chain can provide value for verifying the authenticity of parts when prolonged periods of time (decades) elapse between part manufacturing and part sourcing. Additionally, during a part’s ownership changes, supply-chain actors may choose to participate in the distributed ledger based on individual incentives and can recuse themselves from such participation later. The lack of complete participation may affect the designed functionality, and the consequences of lack of participation need to be understood. Using a comprehensive supply-chain model, it can be shown that blockchain for supply chain can reduce the prevalence of counterfeit electronics in the supply chain of critical systems by up to 70%. However, such a reduction requires near complete participation by all supply-chain stakeholders, which is not likely. Due to the relatively high cost of ownership transfer on a blockchain, and the indirect cost of supply-chain information disclosure, a high participation rate is not anticipated. Although blockchain can have benefits in other aspects of supply chain, it may not be a viable solution to combat the counterfeit electronics problem.Item IMPROVING THE PROCESS OF SUPERCRITICAL CARBON-DIOXIDE-ASSISTED LIQUEFACTION OF BIOMASS FOR THE PRODUCTION OF BIOFUELS(2024) Murray, Cameron; Gupta, Ashwani; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The United States has been looking for alternative energy sources to combat energy dependence and carbon emissions. There exists a multitude of methods attempting to achieve this effort while making use of existing infrastructure. One such novel method is the supercritical carbon-dioxide-assisted liquefaction of biomass. This method seeks to exploit a large supply of biomass waste in the US through the use of carbon dioxide, a readily available and nontoxic gas. This paper investigated two potential improvements for liquid yields in the supercritical carbon-dioxide-assisted liquefaction system. Those improvements were the effects of heating on the solid and liquid yields and the efficacy of supercritical carbon dioxide extraction of liquid products. Three specific aspects of heating were investigated: resident time, heating rate, and total time. Resident times of 10 minutes 20 minutes and 60 minutes were tested. High heating rates were achieved via the use of induction heating. Heating rates of 6, 12, and 250 ℃/min were tested. The effects of total reaction time were also investigated; however, this was dependent on the heating rate and resident time, thus it could not be independently controlled. The investigation found that neither resident time nor total reaction time has a significant impact on the solid or liquid yields. The heating rate, on the other hand, showed a good correlation with a proposed relationship of L = 13.01 · H0.1687 and S = 58.57 · H-0.08348, where L is the liquid yield in wt%, S is the solids yield in wt%, and H is the heating rate in ℃/min. This investigation had a stated goal of achieving a liquid yield of over 30% in under 45 minutes while maintaining a solids yield of less than 50%. It achieved this goal with a particular test having a liquid yield of 32% and a solids yield of 40% in under 12 minutes. Supercritical carbon dioxide extraction was proven to be effective at recovering liquid yields. It was not as successful as acetone-aided extraction; however, it shows promise, especially given its potential for overall process integration in the future. sCO2 extraction was seen to be most effective when conducted in conjunction with sCO2 liquefaction.Item FATIGUE DEGRADATION SENSING WITH SURFACE MOUNTED CONJUGATE-STRESS (CS) SENSOR(2024) Bascolo, Manuel; Dasgupta, Abhijit AD; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This study advances a unique dual-stiffness mechanical sensor concept in the literature (termed Conjugate-stress sensor), to evaluate and validate the effectiveness of surface-mounted versions of the Conjugate Stress (CS) sensor in detecting cyclic fatigue progression under both quasi-static axial cycling and dynamic flexural loading conditions. The CS sensor’s capability to recognize fatigue was examined by observing the correlation between its readings and the host material's stiffness. Low carbon steel dog bone coupons and welded cruciform specimens were subjected to quasi-static cyclic fatigue testing. Additionally, dynamic flexural tests were performed on low carbon steel cantilever beams and welded cruciform specimens, which underwent random vibration fatigue testing. The results demonstrated that CS sensors consistently track fatigue damage, offering a promising potential for in-situ structural health monitoring and for providing continuous, real-time estimations of the remaining useful life (RUL) of materialsItem Heat Transfer Augmentation of Microencapsulated Phase Change Material Slurry in Herringbone microchannel heat sink(2024) Dave, Anagh; Agonafer, Damena; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The advancement of technologies such as artificial intelligence (AI), the Internet of Things(IoT), and cloud computing has driven the miniaturization of electronic devices while significantly increasing their power densities and compactness. This evolution has led to higher heat fluxes that must be effectively managed to ensure device reliability and optimal performance. Effective thermal management is crucial, as inadequate cooling can result in thermal stresses, reduced efficiency, and potential component failure. While air-cooled systems were traditionally sufficient, the growing power demands of modern electronics have necessitated the adoption of more advanced cooling strategies. Microchannel heat sinks(MCHS), introduced in 1981, have been extensively studied for their ability to reduce junction temperatures. These heat sinks are part of various thermal management solutions, including single-phase liquid cooling, two-phase flow boiling, and jet impingement cooling in microchannels. Studies have shown that single-phase liquid cooling can effectively dissipate high heat fluxes of up to 1000 W/cm2. However, despite their effectiveness in heat dissipation, single-phase liquid cooling systems in microchannel heat sinks experience diminishing returns in efficiency due to high-pressure drops at higher volumetric flow rates. Two-phase or multiphase cooling strategies have also been studied to overcome these limitations. These approaches rely on the phase change of the coolant and leverage the high heat capacity to improve heat transfer efficiency while maintaining a high thermal-hydraulic performance. However, they face challenges related to the flow instabilities during boiling and diminished heat removal rate at critical heat fluxes, which can undermine reliability. A promising alternative involves suspending microencapsulated phase change material (MEPCM) particles in a thermal fluid to create a slurry. MEPCM slurries typically consist of a base fluid, like water, mixed with MEPCM particles that enhance effective heat capacity through their high specific and latent heat. These particles absorb and release heat during phase transitions, significantly improving heat dissipation and storage in thermal-fluid systems. However, despite their high thermal capacity, the relatively low thermal conductivity of MEPCM particles can hinder their ability to melt uniformly and transfer heat effectively. To address these challenges, this study investigates a novel heat transfer enhancement approach by incorporating herringbone microstructures within microchannels to induce helical mixing. The herringbone design facilitates out-of-plane mixing, which promotes the effective utilization of MEPCM particles and enhances heat transfer without the instabilities associated with traditional two-phase boiling. This creates a pseudo ’two-phase’ flow within the microchannel heat sink, allowing MEPCM slurries to achieve high thermal performance, balancing effective heat transfer with reduced flow instabilities and manageable pressure drops.