825 research outputs found
Jet pumps for thermoacoustic applications: design guidelines based on a numerical parameter study
The oscillatory flow through tapered cylindrical tube sections (jet pumps) is
characterized by a numerical parameter study. The shape of a jet pump results
in asymmetric hydrodynamic end effects which cause a time-averaged pressure
drop to occur under oscillatory flow conditions. Hence, jet pumps are used as
streaming suppressors in closed-loop thermoacoustic devices. A two-dimensional
axisymmetric computational fluid dynamics model is used to calculate the
performance of a large number of conical jet pump geometries in terms of
time-averaged pressure drop and acoustic power dissipation. The investigated
geometrical parameters include the jet pump length, taper angle, waist diameter
and waist curvature. In correspondence with previous work, four flow regimes
are observed which characterize the jet pump performance and dimensionless
parameters are introduced to scale the performance of the various jet pump
geometries. The simulation results are compared to an existing quasi-steady
theory and it is shown that this theory is only applicable in a small operation
region. Based on the scaling parameters, an optimum operation region is defined
and design guidelines are proposed which can be directly used for future jet
pump design.Comment: The following article has been accepted by the Journal of the
Acoustical Society of America. After it is published, it will be found at
http://scitation.aip.org/JAS
A numerical investigation on the vortex formation and flow separation of the oscillatory flow in jet pumps
A two-dimensional computational fluid dynamics model is used to predict the
oscillatory flow through a tapered cylindrical tube section (jet pump) placed
in a larger outer tube. Due to the shape of the jet pump, there will exist an
asymmetry in the hydrodynamic end effects which will cause a time-averaged
pressure drop to occur that can be used to cancel Gedeon streaming in a
closed-loop thermoacoustic device. The performance of two jet pump geometries
with different taper angles is investigated. A specific time-domain impedance
boundary condition is implemented in order to simulate traveling acoustic wave
conditions. It is shown that by scaling the acoustic displacement amplitude to
the jet pump dimensions, similar minor losses are observed independent of the
jet pump geometry. Four different flow regimes are distinguished and the
observed flow phenomena are related to the jet pump performance. The simulated
jet pump performance is compared to an existing quasi-steady approximation
which is shown to only be valid for small displacement amplitudes compared to
the jet pump length.Comment: The following article has been accepted by the Journal of the
Acoustical Society of America. After it is published, it will be found at:
http://scitation.aip.org/JAS
It Is Different When Items Are Older: Debiasing Recommendations When Selection Bias and User Preferences Are Dynamic
User interactions with recommender systems (RSs) are affected by user
selection bias, e.g., users are more likely to rate popular items (popularity
bias) or items that they expect to enjoy beforehand (positivity bias). Methods
exist for mitigating the effects of selection bias in user ratings on the
evaluation and optimization of RSs. However, these methods treat selection bias
as static, despite the fact that the popularity of an item may change
drastically over time and the fact that user preferences may also change over
time. We focus on the age of an item and its effect on selection bias and user
preferences. Our experimental analysis reveals that the rating behavior of
users on the MovieLens dataset is better captured by methods that consider
effects from the age of item on bias and preferences. We theoretically show
that in a dynamic scenario in which both the selection bias and user
preferences are dynamic, existing debiasing methods are no longer unbiased. To
address this limitation, we introduce DebiAsing in the dyNamiC scEnaRio
(DANCER), a novel debiasing method that extends the inverse propensity scoring
debiasing method to account for dynamic selection bias and user preferences.
Our experimental results indicate that DANCER improves rating prediction
performance compared to debiasing methods that incorrectly assume that
selection bias is static in a dynamic scenario. To the best of our knowledge,
DANCER is the first debiasing method that accounts for dynamic selection bias
and user preferences in RSs.Comment: WSDM 202
Recent Advances in the Foundations and Applications of Unbiased Learning to Rank
Since its inception, the field of unbiased learning to rank (ULTR) has remained very active and has seen several impactful advancements in recent years. This tutorial provides both an introduction to the core concepts of the field and an overview of recent advancements in its foundations along with several applications of its methods.The tutorial is divided into four parts: Firstly, we give an overview of the different forms of bias that can be addressed with ULTR methods. Secondly, we present a comprehensive discussion of the latest estimation techniques in the ULTR field. Thirdly, we survey published results of ULTR in real-world applications. Fourthly, we discuss the connection between ULTR and fairness in ranking. We end by briefly reflecting on the future of ULTR research and its applications.This tutorial is intended to benefit both researchers and industry practitioners who are interested in developing new ULTR solutions or utilizing them in real-world applications
The treatment of primary tumors of the femur with chemotherapy (if indicated), resection and reconstruction with an endoprosthesis
The treatment protocol of 15 patients with a primary tumor of the femur, including osteosarcoma, malignant fibrous histiocytoma and chondrosar-coma is presented. All patients had been selected for resection and reconstruction with an endoprosthesis. An endoprothesis was implanted in 12 patients. \ud
The results of this type of treatment appear to be satisfactory. In eight osteosarcoma cases resection and reconstruction with an endoprosthesis combined with preoperative and postoperative chemotherapy, according to Rosen, were performed. Follow-up in all 15 patients, varying from 1.4 to 6.0 years, showed no evidence of disease in 12 patients. Three patients had died. Function of the involved leg was satisfactory in most cases. \ud
The advantage and disadvantages of the use of an endoprosthesis are discussed as well as complications in this series of patients
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