385 research outputs found

    Numerical investigation of V/STOL jet induced interactions

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    Direct numerical simulation using the full three dimensional, time dependent Navier-Stokes equations is used to investigate V/STOL jet induced interactions. The objective of this numerical simulation is to compute accurately the details of the flow field and to achieve a better understanding of the physics of the flow, including the role of initial turbulence in the jet, the influence of forward motion on hover aerodynamics, the collision zone and fountain characteristics. Preliminary results are presented

    A new unsteady mixing model to predict NO(x) production during rapid mixing in a dual-stage combustor

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    An advanced gas turbine engine to power supersonic transport aircraft is currently under study. In addition to high combustion efficiency requirements, environmental concerns have placed stringent restrictions on the pollutant emissions from these engines. A combustor design with the potential for minimizing pollutants such as NO(x) emissions is undergoing experimental evaluation. A major technical issue in the design of this combustor is how to rapidly mix the hot, fuel-rich primary zone product with the secondary diluent air to obtain a fuel-lean mixture for combustion in the second stage. Numerical predictions using steady-state methods cannot account for the unsteady phenomena in the mixing region. Therefore, to evaluate the effect of unsteady mixing and combustion processes, a novel unsteady mixing model is demonstrated here. This model has been used to study multispecies mixing as well as propane-air and hydrogen-air jet nonpremixed flames, and has been used to predict NO(x) production in the mixing region. Comparison with available experimental data show good agreement, thereby providing validation of the mixing model. With this demonstration, this mixing model is ready to be implemented in conjunction with steady-state prediction methods and provide an improved engineering design analysis tool

    Turbulent Premixed Flames in Microgravity

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    The experimental cold-flow facility is now full operational and is currently being used to obtain baseline turbulence data in a Couette flow. The baseline turbulence data is necessary to confirm the capability of the chosen device to generate and maintain the required turbulence intensity. Subsequent reacting flow studies will assume that a similar turbulent flow field exists ahead of the premixed flame. Some modifications and refinements had to be made to enable accurate measurements. It consists of two rollers, one (driven by a motor) which drives a continuous belt and four smaller rollers used to set the belt spacing and tension to minimize belt flutter. The entire assemble is enclosed in a structure that has the dimensions to enable future drop tower experiments of the hot facility. All critical dimensions are the same as the original plans except for the pulley ratio which has been changed to enable a wider operating regime in terms of the Reynolds number. With the current setup, Reynolds numbers as low as 100 and as high as 14,000 can be achieved. This is because the in-between belt spacing can be varied from 1 cm to 7.6 cm, and the belt speed can be accurately varied from .15 m/sec to 3.1 m/sec

    Parallel Simulation of Unsteady Turbulent Flames

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    Time-accurate simulation of turbulent flames in high Reynolds number flows is a challenging task since both fluid dynamics and combustion must be modeled accurately. To numerically simulate this phenomenon, very large computer resources (both time and memory) are required. Although current vector supercomputers are capable of providing adequate resources for simulations of this nature, the high cost and their limited availability, makes practical use of such machines less than satisfactory. At the same time, the explicit time integration algorithms used in unsteady flow simulations often possess a very high degree of parallelism, making them very amenable to efficient implementation on large-scale parallel computers. Under these circumstances, distributed memory parallel computers offer an excellent near-term solution for greatly increased computational speed and memory, at a cost that may render the unsteady simulations of the type discussed above more feasible and affordable.This paper discusses the study of unsteady turbulent flames using a simulation algorithm that is capable of retaining high parallel efficiency on distributed memory parallel architectures. Numerical studies are carried out using large-eddy simulation (LES). In LES, the scales larger than the grid are computed using a time- and space-accurate scheme, while the unresolved small scales are modeled using eddy viscosity based subgrid models. This is acceptable for the moment/energy closure since the small scales primarily provide a dissipative mechanism for the energy transferred from the large scales. However, for combustion to occur, the species must first undergo mixing at the small scales and then come into molecular contact. Therefore, global models cannot be used. Recently, a new model for turbulent combustion was developed, in which the combustion is modeled, within the subgrid (small-scales) using a methodology that simulates the mixing and the molecular transport and the chemical kinetics within each LES grid cell. Finite-rate kinetics can be included without any closure and this approach actually provides a means to predict the turbulent rates and the turbulent flame speed. The subgrid combustion model requires resolution of the local time scales associated with small-scale mixing, molecular diffusion and chemical kinetics and, therefore, within each grid cell, a significant amount of computations must be carried out before the large-scale (LES resolved) effects are incorporated. Therefore, this approach is uniquely suited for parallel processing and has been implemented on various systems such as: Intel Paragon, IBM SP-2, Cray T3D and SGI Power Challenge (PC) using the system independent Message Passing Interface (MPI) compiler. In this paper, timing data on these machines is reported along with some characteristic results

    A seven-equation diffused interface method for resolved multiphase flows

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    The seven-equation model is a compressible multiphase formulation that allows for phasic velocity and pressure disequilibrium. These equations are solved using a diffused interface method that models resolved multiphase flows. Novel extensions are proposed for including the effects of surface tension, viscosity, multi-species, and reactions. The allowed non-equilibrium of pressure in the seven-equation model provides numerical stability in strong shocks and allows for arbitrary and independent equations of states. A discrete equations method (DEM) models the fluxes. We show that even though stiff pressure- and velocity-relaxation solvers have been used, they are not needed for the DEM because the non-conservative fluxes are accurately modeled. An interface compression scheme controls the numerical diffusion of the interface, and its effects on the solution are discussed. Test cases are used to validate the computational method and demonstrate its applicability. They include multiphase shock tubes, shock propagation through a material interface, a surface-tension-driven oscillating droplet, an accelerating droplet in a viscous medium, and shock-detonation interacting with a deforming droplet. Simulation results are compared against exact solutions and experiments when possible

    The importance of feature preprocessing for differentially private linear optimization

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    Training machine learning models with differential privacy (DP) has received increasing interest in recent years. One of the most popular algorithms for training differentially private models is differentially private stochastic gradient descent (DPSGD) and its variants, where at each step gradients are clipped and combined with some noise. Given the increasing usage of DPSGD, we ask the question: is DPSGD alone sufficient to find a good minimizer for every dataset under privacy constraints? As a first step towards answering this question, we show that even for the simple case of linear classification, unlike non-private optimization, (private) feature preprocessing is vital for differentially private optimization. In detail, we first show theoretically that there exists an example where without feature preprocessing, DPSGD incurs a privacy error proportional to the maximum norm of features over all samples. We then propose an algorithm called DPSGD-F, which combines DPSGD with feature preprocessing and prove that for classification tasks, it incurs a privacy error proportional to the diameter of the features maxx,xDxx2\max_{x, x' \in D} \|x - x'\|_2. We then demonstrate the practicality of our algorithm on image classification benchmarks

    Avoidable Benign Kidney Tumor Resections—Data from a Tertiary Care Cancer Institute

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    Enhancing renal masses are conventionally treated as malignant unless proven otherwise due to the difficulty distinguishing between malignant and benign tumors based on imaging. Data from the Western registries suggests overtreatment of renal tumors with a Benign Kidney Tumor Resection Rate (BKTRR) ranging from 10 to 33%, with an increasing trend. Since robust, population-based data from India was unavailable, we sought to determine BKTRR in an apex cancer institute, which would provide insight into the rates in the community. The institutional kidney tumor database was queried for all patients aged ≥18 years with renal neoplasms between January 2000 and December 2022. Patients who underwent surgery, either radical or partial nephrectomy, with intent to cure were analyzed and the BKTRR during the study period was evaluated. A total of 330 patients underwent surgery for renal tumors presumed to be malignant. A final pathologic diagnosis of the benign tumor was made in 16 (4.8%) patients, comprising 7.2, 7.2, and 3.7% of resections with LTD ≤4, 4–7, and >7 cm, respectively. Asymptomatic benign tumors ≤7 cm comprised 3.0% of all resections, and these were potentially unnecessary surgeries. A multivariable analysis suggested that no patient or imaging characteristic could predict a final benign extirpative pathology. Our study suggests a lower rate of BKTRR compared to the published international literature but is likely to be the lower limit of that in the community. Population-based studies are required to determine the true BKTRR and the quantum of potentially unnecessary surgeries for benign kidney tumors

    COMPUTATIONAL ISSUES FOR SIMULATING FINITE-RATE KINETICS IN LES

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    Abstract At present, large-eddy simulations (LES) of turbulent flames with multi-species finite-rate kinetics is computationally infeasible due to the enormous cost associated with computation of reaction kinetics in 3D flows. In a recent study, In-Situ Adaptive Tabulation (ISAT) and Artificial Neural Network (ANN) methodologies were developed for computing finiterate kinetics in a cost effective manner. Although ISAT reduces the cost of direct integration considerably, the ISAT tables require significant on-line storage in memory and can continue to grow over multiple flowthrough times (an essential feature in LES). Hence, direct use of ISAT in LES may not be practical, especially in parallel solvers. In this study, a storageefficient Artificial Neural Network (ANN) is investigated for LES application. Preliminary studies using ANN to predict freely propagating turbulent premixed flames over a range of operational parameters are described and issues regarding the implementation of such ANNs for engineering LES are discussed

    Domestic fish marketing in India - changing structure, conduct, performance and policies

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    This study has been conducted in all the major coastal states and some selected inland states to understand the domestic marketing of fish in India. The total marketing costs of auctioneer, wholesaler, retailer, vendor, marine fishermen cooperative society and contractor/freshwater fishermen cooperative society have been found to be Re 0.98, Rs 8.89, Rs 6.61, Rs 4.50, Rs 6.00 and Rs 3.51, respectively. The marketing efficiencies for Indian major carps (IMC), sardine and seer fish have been found to vary from 34 per cent to 74 per cent, depending on the length of market channel. The marketing efficiency has been found more in the case of marine species than freshwater species, since the latter travel longer distances from the point of production to consumption centre, passing many intermediaries as compared to the former. The fisherman’s share in consumer’s rupee has shown variations across species, marketing channels and markets. The infrastructure facilities at most of the surveyed landing centres, fishing harbours and wholesale and retail markets have been found grossly inadequate and poorly maintained. The study has highlighted the need for formulating a uniform market policy for fishes for easy operation and regulation so that the country’s fish production is efficiently managed and delivered to the consuming population, ensuring at the same time remunerative prices to the fishers.Marketing,

    A case of neurofibromatosis type 1 with clival chordoma

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    Neurofibromatosis type-1 is an autosomal dominant tumour syndrome with a high clinical susceptibility to malignancies, especially nervous system malignancies. Here, we reported a case of neurofibromatosis type-1 in a male in his 50s, who presented with generalised weakness, headache associated with ear pain, a feeling of heaviness of the head, and giddiness for a duration of 1 week. On examination, he had axillary freckling and multiple neurofibromas over his body. Nystagmus and dysdiadokokinesia were present. MRI brain revealed an enhancing lesion in the sphenoid and clivus, extending into the sellar and supra-sellar region. The possibility of pituitary adenoma and clival chordoma were considered. He was referred to Neurosurgery and underwent Trans-nasal Trans-sphenoidal near-total resection of the tumour. Biopsy of the lesion was indicative of conventional clival chordoma, which is rarely reported with NF-1. The post-operative period was uneventful and the patient is planned for regular follow-up to detect recurrence
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