A deterministic model for the sublayer streaks in turbulent boundary layers for application to flow control

2007 ◽  
Vol 365 (1859) ◽  
pp. 2419-2441 ◽  
Author(s):  
Peter W Carpenter ◽  
Karen L Kudar ◽  
Reza Ali ◽  
Pradeep K Sen ◽  
Christopher Davies
Keyword(s):  
Theoretical Model ◽  
Body Force ◽  
Deterministic Model ◽  
Plane Waves ◽  
Streamwise Vortices ◽  
Oblique Waves ◽  
Theoretical Predictions ◽  
Wall Compliance ◽  
Streamwise Pressure Gradient ◽  
Vorticity Source

We present a relatively simple, deterministic, theoretical model for the sublayer streaks in a turbulent boundary layer based on an analogy with Klebanoff modes. Our approach is to generate the streamwise vortices found in the buffer layer by means of a vorticity source in the form of a fictitious body force. It is found that the strongest streaks correspond to a spanwise wavelength that lies within the range of the experimentally observed values for the statistical mean streak spacing. We also present results showing the effect of streamwise pressure gradient, Reynolds number and wall compliance on the sublayer streaks. The theoretical predictions for the effects of wall compliance on the streak characteristics agree well with experimental data. Our proposed theoretical model for the quasi-periodic bursting cycle is also described, which places the streak modelling in context. The proposed bursting process is as follows: (i) streamwise vortices generate sublayer streaks and other vortical elements generate propagating plane waves, (ii) when the streaks reach a sufficient amplitude, they interact nonlinearly with the plane waves to produce oblique waves that exhibit transient growth, and (iii) the oblique waves interact nonlinearly with the plane wave to generate streamwise vortices; these in turn generate the sublayer streaks and so the cycle is renewed.

scholarly journals Consumption Responses to In-Kind Transfers: Evidence from the Introduction of the Food Stamp Program

2009 ◽  
Vol 1 (4) ◽  
pp. 109-139 ◽  
Author(s):  
Hilary W Hoynes ◽  
Diane Whitmore Schanzenbach
Keyword(s):  
Theoretical Model ◽  
Empirical Work ◽  
Food Stamps ◽  
Cash Income ◽  
Food Stamp Program ◽  
Difference In Difference ◽  
Food Expenditures ◽  
Theoretical Predictions ◽  
Difference Methods ◽  
The Impact

Economists have strong theoretical predictions about how in-kind transfers, such as providing vouchers for food, impact consumption. Despite the prominence of the theory, there is little empirical work on responses to in-kind transfers, and most existing work fails to support the canonical theoretical model. We employ difference-in-difference methods to estimate the impact of program introduction on food spending. Consistent with predictions, we find that food stamps reduce out-of-pocket food spending and increase overall food expenditures. We also find that households are inframarginal and respond similarly to one dollar in cash income and one dollar in food stamps. (JEL D12, H23, I38)

A clinical study which relates to a theoretical simulation of the glucose transport in the human placenta under various diabetic conditions

2016 ◽  
Vol 44 (4) ◽  
Author(s):  
Efrath Barta ◽  
Arieh Drugan
Keyword(s):  
Clinical Study ◽  
Theoretical Model ◽  
Gestational Diabetes ◽  
Human Placenta ◽  
Major Effect ◽  
Fetal Weight ◽  
Blood Levels ◽  
Theoretical Simulation ◽  
Theoretical Predictions ◽  
Glucose Supply

AbstractTo characterize placental glucose delivery under normoglycemic conditions, gestational and pre-gestational diabetes and to relate the clinical data to theoretical predictions.Data from 125 pregnancies: 50 normal gestations and 75 ones with various types of diabetes were collected. In parallel, we formulated a theoretical model for the transport of glucose under various diabetic conditions. Measured glucose blood levels were fed into the theoretical model that predicts glucose supply to the fetus and the results were confronted with measured fetal weights.Measured fetal weight and computed glucose delivery in gestational diabetic parturients resemble the situation in normal pregnancies. However, pre-gestational diabetes has a major effect as it involves heavier fetuses and enhanced computed glucose fluxesFetal weight (increased in pre-gestational and unaltered in gestational diabetes) correlates with the predicted rate of glucose delivery through the placenta.

Chasing Diamonds and Crowns: Consumer Limited Attention and Seller Response

2021 ◽  
Author(s):  
Zemin (Zachary) Zhong
Keyword(s):  
Theoretical Model ◽  
Regression Discontinuity ◽  
Limited Attention ◽  
Supply Side ◽  
Demand Side ◽  
Demand Shock ◽  
Online Platforms ◽  
Theoretical Predictions ◽  
Shaped Pattern ◽  
Sales Increase

Online platforms often assign sellers summary symbols based on whether their ratings pass certain thresholds. Consumers may focus on the symbols and pay limited attention to the ratings. This bias leads to discontinuously increased demand at the thresholds. I use a theoretical model to illustrate that sellers will lower the prices before their ratings reach the thresholds and increase their prices afterward due to the positive demand shock. I collect data from Taobao to test the theoretical predictions. Using regression discontinuity, I find that on the demand side, the hourly sales increase significantly when a seller passes the threshold, even conditional on the same item. On the supply side, the prices indeed exhibit a V-shaped pattern with respect to the ratings. Furthermore, sellers preemptively increase prices shortly before reaching thresholds, supporting the theoretical predictions. This paper was accepted by Juanjuan Zhang, marketing.

Experimental Analysis of a Waveguide Pressure Measuring System

2010 ◽  
Vol 132 (4) ◽  
Author(s):  
Matthew A. White ◽  
Manuj Dhingra ◽  
J. V. R. Prasad
Keyword(s):  
Theoretical Model ◽  
Measuring System ◽  
Pressure Waves ◽  
Unique Contribution ◽  
Pressure Oscillations ◽  
Fluctuating Pressure ◽  
Response Data ◽  
Theoretical Predictions ◽  
Infinite Line ◽  
M 12

An infinite-line probe is commonly used to measure unsteady pressure in high-temperature environments while protecting the pressure transducer. In this study, an existing theoretical model is used to derive the response of a waveguide pressure measuring system. An ambient temperature centrifugal compressor rig acts as an experimental source of fluctuating pressure. The compressor is operated at different discrete rotational speeds, and the blade-passing frequencies are used to obtain frequency response data. In the experiments, pressure waves attenuated at a rate faster than that predicted by the theoretical model for a 0.322 m (12 in.) sensor offset. Furthermore, the decay in the magnitude of the pressure oscillations accelerated at blade-passing frequencies above 9 kHz. A unique contribution of this study is to show that whereas the experimentally observed overall attenuation is broadly consistent with the theoretical predictions, pressure oscillations corresponding to individual blade passages may be disproportionally attenuated.

The elastochrone: the descent time of a sphere on a flexible beam

2009 ◽  
Vol 465 (2107) ◽  
pp. 2293-2311 ◽  
Author(s):  
Jeffrey M. Aristoff ◽  
Christophe Clanet ◽  
John W.M. Bush
Keyword(s):  
Gravitational Field ◽  
Theoretical Model ◽  
Elastic Beam ◽  
Beam Theory ◽  
Flexible Beam ◽  
Bernoulli Beam ◽  
Horizontal Beam ◽  
Theoretical Predictions ◽  
Static Regime ◽  
Euler Bernoulli Beam

We present the results of a combined experimental and theoretical investigation of the motion of a sphere on an inclined flexible beam. A theoretical model based on Euler–Bernoulli beam theory is developed to describe the dynamics, and in the limit where the beam reacts instantaneously to the loading, we obtain exact solutions for the load trajectory and descent time. For the case of an initially horizontal beam, we calculate the period of the resulting oscillations. Theoretical predictions compare favourably with our experimental observations in this quasi-static regime. The time taken for descent along an elastic beam, the elastochrone, is shown to exceed the classical brachistochrone, the shortest time between two points in a gravitational field.

Fuel Injection Scheme for a Compact Afterburner Without Flameholders

2007 ◽  
Author(s):  
Shai Birmaher ◽  
Philipp W. Zeller ◽  
Peter Wirfalt ◽  
Yedidia Neumeier ◽  
Ben T. Zinn
Keyword(s):  
Theoretical Model ◽  
Gas Turbine ◽  
Fuel Injection ◽  
State Of The Art ◽  
Combustion Process ◽  
Operating Conditions ◽  
Experimental Setup ◽  
Turbine Engine ◽  
Theoretical Predictions ◽  
Significant Length

State of the art afterburner combustion employs spray bars and flameholders in a long cavity, which adds significant length and weight to the engine and increases its observability. This paper presents a feasibility study for the development of a compact “prime and trigger” afterburner that eliminates the flameholders and reduces the length of the engine. In this concept, fuel is injected just upstream or in between the turbine stages in such a manner that upon exiting the turbine the fuel has evaporated and premixed with the flow without significant combustion, a process referred to as “priming”. Downstream of the turbine, combustion is initiated either through autoignition or by using a low power plasma radical generator being developed in a parallel investigation to “trigger” the combustion process. The prime and trigger injection and ignition scheme has been investigated using an experimental setup that simulates the operating conditions in a typical gas turbine engine. For this investigation, a trigger is not used, and combustion of the fuel occurs through autoignition. A physics-based theoretical model was developed to predict the location of autoignition for given flow and spray properties and injection locations. The theoretical predictions and the experimental results obtained using thermocouple measurements and CH* chemiluminescence confirm the feasibility of the prime and trigger concept by demonstrating the predictable and controlled autoignition of the afterburner fuel.

Non-Linear Diffusion - Non-linear diffusion I. Diffusion and flow of mixtures of fluids

1963 ◽  
Vol 255 (1064) ◽  
pp. 607-633 ◽  
Keyword(s):  
Mechanical Properties ◽  
Steady State ◽  
Body Force ◽  
Equations Of Motion ◽  
Plane Waves ◽  
Viscous Fluids ◽  
Rigid Plate ◽  
Linear Diffusion ◽  
Diffusion Effects ◽  
Non Linear

A theory for the flow and non-linear diffusion effects in mixtures of fluids is formulated based upon hydrodynamical considerations. It is assumed that each point of the mixture is occupied simultaneously by all constituents in given portions. The motion of each constituent is governed by the usual equations of motion and continuity. The mechanical properties of each component are specified by means of constitutive equations for the stresses; diffusion effects are accounted for by means of a body force acting on each constituent which depends upon the composition and relative motion of the substances in the mixture. The theory is extended to deal with the diffusion of a mixture of fluids through a rigid solid. The theory is applied to a number of steady-state problems involving non-Newtonian fluids including the diffusion of a fluid through a rigid plate, the laminar flow of a mixture and the flow of a mixture between rotating cylinders. The propagation of plane waves through a homogeneous mixture of viscous fluids at rest is also examined.

A Theoretical and Experimental Investigation of a Gas-Operated Bearing Damper for Turbomachinery: Part I—Theoretical Model and Predictions

1995 ◽  
Vol 117 (4) ◽  
pp. 742-749 ◽  
Author(s):  
P. Sundararajan ◽  
J. M. Vance
Keyword(s):  
Theoretical Model ◽  
Squeeze Film ◽  
Working Fluid ◽  
Previous Theory ◽  
Damping Characteristics ◽  
Squirrel Cage ◽  
Theoretical Predictions ◽  
Current Design ◽  
Gas Damper ◽  
Hardware Designs

This is the first (Part I) of two papers describing recent results of the research program directed at developing a vibration damper suitable for high-temperature turbomachinery applications. It is expected that such dampers will replace squeeze-film dampers, which use oil as the working fluid and have limitations at higher temperatures. A novel gas-operated bearing damper has been evaluated analytically and experimentally for its damping characteristics. A theory based on the isentropic assumptions predicts the damper performance characteristics reasonably well. A maximum damping level of 2311 N-m/s (13.2 lb-s/in.) at a frequency of 100 Hz was measured with a single actuator of the gas damper. Since many such actuators could be placed circumferentially around the squirrel cage, considerable damping levels can be realized. The study also shows that significantly higher damping levels can be achieved by modifying the current design. Part I describes the theoretical model that has been developed based on isentropic assumptions. This model is an improved version of the previous theory (Vance et al., 1991) and includes the supply groove effects, dynamic area changes of the inlet feeding holes, and the effects of flow choking on damper behavior. The governing equations are derived and theoretical predictions using these equations have been made for two hardware designs that were experimentally investigated (see Part II for experimental results).

scholarly journals REINFORCED AND PRESTRESSED CONCRETE HOLLOW BEAMS UNDER TORSION

2014 ◽  
Vol 19 (Supplement_1) ◽  
pp. S141-S152 ◽  
Author(s):  
Luis F. A. Bernardo ◽  
Jorge M. A. Andrade ◽  
Luiz A. Pereira-De-Oliveira
Keyword(s):  
Theoretical Model ◽  
Prestressed Concrete ◽  
Experimental Results ◽  
Membrane Model ◽  
Global Behaviour ◽  
Theoretical Predictions ◽  
Hollow Beams

Recently a new rational theoretical model for beams under torsion has been proposed. This model, called Softened Membrane Model for Torsion (SMMT), is able to predict well the global behaviour of reinforced and prestressed concrete solid beams under torsion. This paper presents a study in order to check the SMMT for reinforced and prestressed concrete hollow beams under torsion. Theoretical predictions from SMMT are compared to some experimental results of hollow beams available in the literature and also with the predictions of another theoretical model previously proposed from the authors. It is shown that SMMT, with some corrections, is able to predict satisfactorily the overall behaviour of reinforced and prestressed concrete hollow beams under torsion.

Leakage analysis of bolted flange joints considering surface roughness: A theoretical model

2017 ◽  
Vol 232 (2) ◽  
pp. 203-233 ◽  
Author(s):  
Benben Ma ◽  
Fan Jin ◽  
Zhi Sun ◽  
Xu Guo
Keyword(s):  
Surface Roughness ◽  
Theoretical Model ◽  
Three Dimensional ◽  
Element Analysis ◽  
Low Leakage ◽  
Theoretical Predictions ◽  
Dimensional Finite Element ◽  
Three Dimensional Finite Element ◽  
Bolted Flange ◽  
Bolt Spacing

In the present paper, a theoretical model for leakage analysis of bolted flange joints without gaskets, which can take the surface roughness into consideration, is proposed based on percolation theory. In this model, Persson’s rough contact theory is employed to predict the height of the critical constriction along the percolating path. Based on this model, a criterion of maximum allowable bolt spacing is also suggested to guarantee a low leakage rate. The reasonable agreement between the theoretical predictions and detailed three-dimensional finite element analysis results verifies the validity and usefulness of the proposed theoretical model.

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