540 research outputs found
Search in the Product Market and the Real Business Cycle
Empirical evidence suggests that most firms operate in imperfectly competitivemarkets. We develop a search-matching model between wholesalers and retailers. Firms face search costs and form long-term relationships. Price bargain results in both wholesaler and retailer markups, depending on firms’ relative bargaining power. We simulate the model to explore the role of product market search frictions in business cycles. We show that the way search costs are modelled is crucial to provide a realistic picture of firms’ business environment and improve the cyclical properties of an otherwise standard real business cycle model.Business cycle, Frictions, Product market, Price bargain
On the Benefits of Edge Caching for MIMO Interference Alignment
In this contribution, we jointly investigate the benefits of caching and
interference alignment (IA) in multiple-input multiple-output (MIMO)
interference channel under limited backhaul capacity. In particular, total
average transmission rate is derived as a function of various system parameters
such as backhaul link capacity, cache size, number of active
transmitter-receiver pairs as well as the quantization bits for channel state
information (CSI). Given the fact that base stations are equipped both with
caching and IA capabilities and have knowledge of content popularity profile,
we then characterize an operational regime where the caching is beneficial.
Subsequently, we find the optimal number of transmitter-receiver pairs that
maximizes the total average transmission rate. When the popularity profile of
requested contents falls into the operational regime, it turns out that caching
substantially improves the throughput as it mitigates the backhaul usage and
allows IA methods to take benefit of such limited backhaul.Comment: 20 pages, 5 figures. A shorter version is to be presented at 16th
IEEE International Workshop on Signal Processing Advances in Wireless
Communications (SPAWC'2015), Stockholm, Swede
NCAA Enforcement Staff and Member Institutions Protecting Mutual Interests: Binding Arbitration in Coaching Contracts
Article published in the Michigan State University School of Law Student Scholarship Collection
Physics-constrained Random Forests for Turbulence Model Uncertainty Estimation
To achieve virtual certification for industrial design, quantifying the
uncertainties in simulation-driven processes is crucial. We discuss a
physics-constrained approach to account for epistemic uncertainty of turbulence
models. In order to eliminate user input, we incorporate a data-driven machine
learning strategy. In addition to it, our study focuses on developing an a
priori estimation of prediction confidence when accurate data is scarce.Comment: Workshop on Synergy of Scientific and Machine Learning Modeling, SynS
& ML ICM
Physically constrained eigenspace perturbation for turbulence model uncertainty estimation
Aerospace design is increasingly incorporating Design Under Uncertainty based
approaches to lead to more robust and reliable optimal designs. These
approaches require dependable estimates of uncertainty in simulations for their
success. The key contributor of predictive uncertainty in Computational Fluid
Dynamics (CFD) simulations of turbulent flows are the structural limitations of
Reynolds-averaged Navier-Stokes models, termed model-form uncertainty.
Currently, the common procedure to estimate turbulence model-form uncertainty
is the Eigenspace Perturbation Framework (EPF), involving perturbations to the
modeled Reynolds Stress tensor within physical limits. The EPF has been applied
with success in design and analysis tasks in numerous prior works from the
industry and academia. Owing to its rapid success and adoption in several
commercial and open-source CFD solvers, in depth Verification and Validation of
the EPF is critical. In this work, we show that under certain conditions, the
perturbations in the EPF can lead to Reynolds stress dynamics that are not
physically realizable. This analysis enables us to propose a set of necessary
physics-based constraints, leading to a realizable EPF. We apply this
constrained procedure to the illustrative test case of a converging-diverging
channel, and we demonstrate that these constraints limit physically implausible
dynamics of the Reynolds stress tensor, while enhancing the accuracy and
stability of the uncertainty estimation procedure.Comment: The following article has been submitted to Physics of Fluid
Improved self-consistency of the Reynolds stress tensor eigenspace perturbation for Uncertainty Quantification
The limitations of turbulence closure models in the context of
Reynolds-averaged NavierStokes (RANS) simulations play a significant part in
contributing to the uncertainty of Computational Fluid Dynamics (CFD).
Perturbing the spectral representation of the Reynolds stress tensor within
physical limits is common practice in several commercial and open-source CFD
solvers, in order to obtain estimates for the epistemic uncertainties of RANS
turbulence models. Recent research revealed, that there is a need for
moderating the amount of perturbed Reynolds stress tensor tensor to be
considered due to upcoming stability issues of the solver. In this paper we
point out that the consequent common implementation can lead to unintended
states of the resulting perturbed Reynolds stress tensor. The combination of
eigenvector perturbation and moderation factor may actually result in moderated
eigenvalues, which are not linearly dependent on the originally unperturbed and
fully perturbed eigenvalues anymore. Hence, the computational implementation is
no longer in accordance with the conceptual idea of the Eigenspace Perturbation
Framework. We verify the implementation of the conceptual description with
respect to its self-consistency. Adequately representing the basic concept
results in formulating a computational implementation to improve
self-consistency of the Reynolds stress tensor perturbationComment: The following article has been submitted to AIP/Physics of Fluid
Efficient preliminary floating offshore wind turbine design and testing methodologies and application to a concrete spar design
The current key challenge in the floating offshore wind turbine industry and research is on designing economic floating systems that can compete with fixed-bottom offshore turbines in terms of levelized cost of energy. The preliminary platform design, as well as early experimental design assessments, are critical elements in the overall design process. In this contribution, a brief review of current floating offshore wind turbine platform pre-design and scaled testing methodologies is provided, with a focus on their ability to accommodate the coupled dynamic behaviour of floating offshore wind systems. The exemplary design and testing methodology for a monolithic concrete spar platform as performed within the European KIC AFOSP project is presented. Results from the experimental tests compared to numerical simulations are presented and analysed and show very good agreement for relevant basic dynamic platform properties. Extreme and fatigue loads and cost analysis of the AFOSP system confirm the viability of the presented design process. In summary, the exemplary application of the reduced design and testing methodology for AFOSP confirms that it represents a viable procedure during pre-design of floating offshore wind turbine platforms.Peer ReviewedPostprint (author’s final draft
Evaluation of physics constrained data-driven methods for turbulence model uncertainty quantification
In order to achieve a virtual certification process and robust designs for
turbomachinery, the uncertainty bounds for Computational Fluid Dynamics have to
be known. The formulation of turbulence closure models implies a major source
of the overall uncertainty of Reynolds-averaged Navier-Stokes simulations. We
discuss the common practice of applying a physics constrained eigenspace
perturbation of the Reynolds stress tensor in order to account for the model
form uncertainty of turbulence models. Since the basic methodology often leads
to overly generous uncertainty estimates, we extend a recent approach of adding
a machine learning strategy. The application of a data-driven method is
motivated by striving for the detection of flow regions, which are prone to
suffer from a lack of turbulence model prediction accuracy. In this way any
user input related to choosing the degree of uncertainty is supposed to become
obsolete. This work especially investigates an approach, which tries to
determine an a priori estimation of prediction confidence, when there is no
accurate data available to judge the prediction. The flow around the NACA 4412
airfoil at near-stall conditions demonstrates the successful application of the
data-driven eigenspace perturbation framework. Furthermore, we especially
highlight the objectives and limitations of the underlying methodology
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