Experimental study of wake steadiness of an airfoil in pitch–hold–return motion

2020 ◽  
Vol 92 (7) ◽  
pp. 1019-1025
Author(s):  
Mehran Masdari ◽  
Maryam Ghorbani ◽  
Arshia Tabrizian
Keyword(s):  
Flow Field ◽  
Time Lag ◽  
Wake Flow ◽  
Phase Duration ◽  
Specific Criterion ◽  
Content Type ◽  
Supercritical Airfoil ◽  
Reduced Frequency ◽  
Downstream Flow ◽  
And Control

Purpose The purpose of this paper is to analyze experimentally subsonic wake of a supercritical airfoil undergoing a pitch–hold–return motion. The focus of the investigation has been narrowed to concentrate on the steadiness of the flow field in the wake of the airfoil and the role of reduced frequency, amplitude and the hold phase duration. Design/methodology/approach All experiments were conducted in a low sub-sonic closed-circuit wind tunnel, at a Reynolds number of approximately 600,000. The model was a supercritical airfoil having 10% thickness and wall-to-wall in ground test facilities. To calculate the velocity distribution in the wake of the airfoil, total and static pressures were recorded at a distance of one chord far from the trailing edge, using pressure devices. The reduced frequency was set at 0.012, 0.03 and the motion pivot was selected at c/4. Findings Analysis of the steadiness of the wake flow field ascertains that an increase in reduced frequency leads to further flow time lag in the hold phase whereas decreases the time that the wake remains steady after the start of the return portion. Also, the roles of amplitude and stall condition are examined. Practical implications Examination of a pitch–hold–return motion is substantial in assessment of aerodynamics of maneuvers with a rapid increase in angle of attack. Moreover, study of aerodynamic behavior of downstream flow field and its steadiness in the wake of the airfoil is vital in drag reduction and control of flapping wings, dynamic stability and control of aircrafts. Originality/value In the present study, to discuss the steadiness of the flow field behind the airfoil some statistical methods and concept of histogram using an automatic algorithm were used and a specific criterion to characterize the steadiness of flow field was achieved.

The Effect of Boundary Proximity Upon the Wake Structure of Horizontal Axis Marine Current Turbines

2008 ◽  
Author(s):  
L. E. Myers ◽  
A. S. Bahaj ◽  
R. I. Rawlinson-Smith ◽  
M. Thomson
Keyword(s):  
Flow Field ◽  
Three Dimensional ◽  
Tidal Energy ◽  
Wake Flow ◽  
Small Scale ◽  
Energy Yield ◽  
Novel Approach ◽  
Marine Current ◽  
Downstream Flow ◽  
Hydraulics Laboratory

An experimental and theoretical investigation of the flow field around small-scale mesh disk rotor simulators is presented. The downstream wake flow field of the rotor simulators has been observed and measured in the 21m tilting flume at the Chilworth hydraulics laboratory, University of Southampton. The focus of this work is the proximity of flow boundaries (seabed and surface) to the rotor disks and the constrained nature of the flow. A three-dimensional Eddy-viscosity numerical model based on an established wind turbine wake model has been modified to account for the change in fluid and the presence of a bounding free surface. This work has shown that previous axi-symmetric modelling approaches may not hold for marine current energy technology and a novel approach is required for simulation of the downstream flow field. Such modelling solutions are discussed and resultant simulation results are given. This work has been conducted as part of a BERR-funded project to develop a numerical modelling tool which can predict the flow onto a marine current turbine within an array. The work presented in this paper feeds into this project and will eventually assist the layout design of arrays which are optimally spaced and arranged to achieve the maximum possible energy yield at a given tidal energy site.

Numerical analysis for wake flow field of Ahmed model based on a nonlinear-LRN/DES turbulence model

2021 ◽  
Vol ahead-of-print (ahead-of-print) ◽  
Author(s):  
Le Dian Zheng ◽  
Yi Yang ◽  
Guang Lin Qiang ◽  
Zhengqi Gu
Keyword(s):  
Experimental Data ◽  
Flow Field ◽  
Reynolds Stress ◽  
Turbulence Model ◽  
Turbulence Models ◽  
Aerodynamic Characteristics ◽  
Wake Flow ◽  
Field Analysis ◽  
Content Type ◽  
Des Model

Purpose This paper aims to propose a precise turbulence model for automobile aerodynamics simulation, which can predict flow separation and reattachment phenomena more accurately. Design/methodology/approach As the results of wake flow simulation with commonly used turbulence models are unsatisfactory, by introducing a nonlinear Reynolds stress term and combining the detached Eddy simulation (DES) model, this paper proposes a nonlinear-low-Reynolds number (LRN)/DES turbulence model. The turbulence model is verified in a backward-facing step case and applied in the flow field analysis of the Ahmed model. Several widely applied turbulence models are compared with the nonlinear-LRN/DES model and the experimental data of the above cases. Findings Compared with the experimental data and several turbulence models, the nonlinear-LRN/DES model gives better agreement with the experiment and can predict the automobile wake flow structures and aerodynamic characteristics more accurately. Research limitations/implications The nonlinear-LRN/DES model proposed in this paper suffers from separation delays when simulating the separation flows above the rear slant of the Ahmed body. Therefore, more factors need to be considered to further improve the accuracy of the model. Practical implications This paper proposes a turbulence model that can more accurately simulate the wake flow field structure of automobiles, which is valuable for improving the calculation accuracy of the aerodynamic characteristics of automobiles. Originality/value Based on the nonlinear eddy viscosity method and the scale resolved simulation, a nonlinear-LRN/DES turbulence model including the nonlinear Reynolds stress terms for separation and reattachment prediction, as well as the wake vortex structure prediction is first proposed.

Experimental investigation of the flow field of an oscillating airfoil and estimation of lift from wake surveys

1994 ◽  
Vol 265 ◽  
pp. 65-95 ◽  
Author(s):  
J. Panda ◽  
K. B. M. Q. Zaman
Keyword(s):  
Flow Field ◽  
Large Scale ◽  
Full Range ◽  
Hysteresis Loops ◽  
Phase Delay ◽  
Force Balance ◽  
Wake Flow ◽  
Naca0012 Airfoil ◽  
Reduced Frequency ◽  
Delay Phenomenon

The flow field of an airfoil oscillated periodically over a reduced frequency range, 0 ≤ k ≤ 1.6, is studied experimentally at chord Reynolds numbers of Rc = 22000 and 44000. For most of the data, the NACA0012 airfoil is pitched sinusoidally about one quarter chord between angles of attack α of 5° and 25°. The cyclic variation of the near wake flow field is documented through flow visualization and phase-averaged vorticity measurements. In addition to the familiar dynamic stall vortex (DSV), an intense vortex of opposite sign is observed to originate from the trailing edge just when the DSV is shed. The two together take the shape of the cross-section of a large ‘mushroom’ while being convected away from the airfoil. The phase delay in the shedding of the DSV with increasing k, as observed by previous researchers, is documented for the full range of k. It is observed that the sum of the absolute values of all vorticity convected into the wake over a cycle is nearly constant and is independent of the reduced frequency and amplitude of oscillation but dependent on the mean α. The time varying component of the lift is estimated in a novel way from the shed vorticity flux. The analytical foundation of the method and the various approximations are discussed. The estimated lift hysteresis loops are found to be in reasonable agreement with available data from the literature as well as with limited force balance measurements. Comparison of the lift hysteresis loops with the corresponding vorticity fields clearly shows that major features of the lift variation are directly linked to the evolution of the large-scale vortical structures and the phase delay phenomenon.

Antecedents and consequences of replacing international independent intermediaries

2014 ◽  
Vol 26 (3) ◽  
pp. 218-237
Author(s):  
Erik B. Nes
Keyword(s):  
Longitudinal Design ◽  
Operation Mode ◽  
Content Type ◽  
Performance Variables ◽  
Direct Sales ◽  
Home Based ◽  
Process Theories ◽  
Antecedents And Consequences ◽  
Communication Control ◽  
And Control

Purpose – What characterizes the relationships with intermediaries that are soon to be replaced, and are the replacements successful? The paper aims to discuss these issues. Design/methodology/approach – The study applies a longitudinal design. The paper conducted a survey among a sample of exporters concerning their relationships with independent intermediaries in terms of trust, commitment, communication, control and two performance variables; financial performance and strategic goal achievement. Five years thereafter the author contacted the same companies again and questioned which independent intermediaries had been replaced in the period. The author then compared the replaced intermediaries with the extended intermediaries before they were replaced and analysed changes in the evaluations of the relationships and of the performance after the replacement. Findings – The relationships with terminated intermediaries that were replaced by sales subsidiary or home-based direct sales were characterized having higher trust, communication and control than extended relationships before replacement. While it may be surprising that these quite successful relationships were terminated, this is in line with internationalization process theories. The replacements, both intermediaries that were replaced by other intermediaries and by sales subsidiary/home-based direct sales, were highly successful in terms of improvement in performance and behavioural relationship variables. Research limitations/implications – The empirical findings are limited by the sample and by data collection from the principal only in the dyadic relations. Practical implications – International independent intermediaries should analyse the likelihood of being replaced by the principal because the relationship or the performance is unsatisfactory, but also because of satisfactory evaluations. Terminations of satisfactory relationships tend to be accompanied by change in operation mode to internal organization. The independent intermediary should in such cases build a defence structure against unwanted termination and/or prepare for buyout. The success of the replacements suggests that international marketers benefit from being proactive in replacing intermediaries with new intermediaries or with a hierarchal entry mode. Originality/value – This is the first study that applies most of these variables from interorganizational relations theory in the study of international independent intermediary replacements. It is also the first to give insight into the consequences of intermediary terminations.

Dynamic Stall Simulation of Flow Over NACA0012 Airfoil at 1 Million Reynolds Number

2015 ◽  
Author(s):  
Venkata Ravishankar Kasibhotla ◽  
Danesh Tafti
Keyword(s):  
Reynolds Number ◽  
Flow Field ◽  
Lift Coefficient ◽  
Three Dimensional ◽  
Dynamic Stall ◽  
Sharp Drop ◽  
Airfoil Surface ◽  
Naca0012 Airfoil ◽  
Reduced Frequency ◽  
Edge Vortex

The paper is concerned with the prediction and analysis of dynamic stall of flow past a pitching NACA0012 airfoil at 1 million Reynolds number based on the chord length of the airfoil and at reduced frequency of 0.25 in a three dimensional flow field. The turbulence in the flow field is resolved using large eddy simulations with the dynamic Smagorinsky model at the sub grid scale. The development of dynamic stall vortex, shedding and reattachment as predicted by the present study are discussed in detail. This study has shown that the downstroke phase of the pitching motion is strongly three dimensional and is highly complex, whereas the flow is practically two dimensional during the upstroke. The lift coefficient agrees well with the measurements during the upstroke. However, there are differences during the downstroke. The computed lift coefficient undergoes a sharp drop during the start of the downstroke as the convected leading edge vortex moves away from the airfoil surface. This is followed by a recovery of the lift coefficient with the formation of a secondary trailing edge vortex. While these dynamics are clearly reflected in the predicted lift coefficient, the experimental evolution of lift during the downstroke maintains a fairly smooth and monotonic decrease in the lift coefficient with no lift recovery. The simulations also show that the reattachment process of the stalled airfoil is completed before the start of the upstroke in the subsequent cycle due to the high reduced frequency of the pitching cycle.

scholarly journals Impact of Substratum Surface on Microbial Community Structure and Treatment Performance in Biological Aerated Filters

2013 ◽  
Vol 80 (1) ◽  
pp. 177-183 ◽  
Author(s):  
Lavane Kim ◽  
Eulyn Pagaling ◽  
Yi Y. Zuo ◽  
Tao Yan
Keyword(s):  
Microbial Community ◽  
Microbial Communities ◽  
Bacterial Species ◽  
Community Analysis ◽  
Microbial Community Analysis ◽  
Rrna Gene ◽  
Content Type ◽  
Property Change ◽  
And Control ◽  
Start Ups

ABSTRACTThe impact of substratum surface property change on biofilm community structure was investigated using laboratory biological aerated filter (BAF) reactors and molecular microbial community analysis. Two substratum surfaces that differed in surface properties were created via surface coating and used to develop biofilms in test (modified surface) and control (original surface) BAF reactors. Microbial community analysis by 16S rRNA gene-based PCR-denaturing gradient gel electrophoresis (DGGE) showed that the surface property change consistently resulted in distinct profiles of microbial populations during replicate reactor start-ups. Pyrosequencing of the bar-coded 16S rRNA gene amplicons surveyed more than 90% of the microbial diversity in the microbial communities and identified 72 unique bacterial species within 19 bacterial orders. Among the 19 orders of bacteria detected,BurkholderialesandRhodocyclalesof theBetaproteobacteriaclass were numerically dominant and accounted for 90.5 to 97.4% of the sequence reads, and their relative abundances in the test and control BAF reactors were different in consistent patterns during the two reactor start-ups. Three of the five dominant bacterial species also showed consistent relative abundance changes between the test and control BAF reactors. The different biofilm microbial communities led to different treatment efficiencies, with consistently higher total organic carbon (TOC) removal in the test reactor than in the control reactor. Further understanding of how surface properties affect biofilm microbial communities and functional performance would enable the rational design of new generations of substrata for the improvement of biofilm-based biological treatment processes.

EFD and CFD Study of Forces, Ship Motions, and Flow Field for KRISO Container Ship Model in Waves

2020 ◽  
Vol 64 (01) ◽  
pp. 61-80
Author(s):  
Ping-Chen Wu ◽  
Md. Alfaz Hossain ◽  
Naoki Kawakami ◽  
Kento Tamaki ◽  
Htike Aung Kyaw ◽  
...  
Keyword(s):  
Fluid Dynamics ◽  
Flow Field ◽  
Flow Simulation ◽  
Wake Flow ◽  
Ship Motions ◽  
Container Ship ◽  
Added Resistance ◽  
Ocean Engineering ◽  
Ship Model ◽  
Calm Water

Ship motion responses and added resistance in waves have been predicted by a wide variety of computational tools. However, validation of the computational flow field still remains a challenge. In the previous study, the flow field around the Korea Research Institute for Ships and Ocean Engineering (KRISO) Very Large Crude-oil Carrier 2 tanker model with and without propeller condition and without rudder condition was measured by the authors, as well as the resistance and self-propulsion tests in waves. In this study, the KRISO container ship model appended with a rudder was used for the higher Froude number .26 and smaller block coefficient .65. The experiments were conducted in the Osaka University towing tank using a 3.2-m-long ship model for resistance and self-propulsion tests in waves. Viscous flow simulation was performed by using CFDShip-Iowa. The wave conditions proposed in Computational Fluid Dynamics (CFD) Workshop 2015 were considered, i.e., the wave-ship length ratio λ/L = .65, .85, 1.15, 1.37, 1.95, and calm water. The objective of this study was to validate CFD results by Experimental Fluid Dynamics (EFD) data for ship vertical motions, added resistance, and wake flow field. The detailed flow field for nominal wake and self-propulsion condition will be analyzed for λ/L = .65, 1.15, 1.37, and calm water. Furthermore, bilge vortex movement and boundary layer development on propeller plane, propeller thrust, and wake factor oscillation in waves will be studied.

Moving the needle: can training alter officer perceptions and use of de-escalation?

2021 ◽  
Vol ahead-of-print (ahead-of-print) ◽  
Author(s):  
Michael D. White ◽  
Victor J. Mora ◽  
Carlena Orosco ◽  
E. C. Hedberg
Keyword(s):  
Controlled Trial ◽  
Police Department ◽  
Content Type ◽  
Officer Safety ◽  
Frequent Use ◽  
Before And After ◽  
June July ◽  
Officer Attitudes ◽  
Excessive Use Of Force ◽  
And Control

PurposeDe-escalation training for police has received widespread attention as a method for reducing unnecessary and excessive use of force. There is virtually no research on de-escalation, and as a result, there is little understanding about what it is, what it includes and whether it is effective. The current study compares attitudes about the importance and use of de-escalation among officers who were randomly assigned to participate (or not) in de-escalation training.Design/methodology/approachThe current study draws from a larger randomized controlled trial of de-escalation training in the Tempe, Arizona Police Department (TPD). Approximately 100 officers completed a survey in June–July 2019 and again in June–July 2020. TPD delivered the de-escalation training to half the patrol force in February–March 2020. The authors compare treatment and control officers' attitudes about the importance of specific de-escalation tactics, how often they use those tactics and their sentiments de-escalation training. The authors employ an econometric random-effects model to examine between-group differences post-training while controlling for relevant officer attributes including age, race, sex, prior training and squad-level pretraining attitudes about de-escalation.FindingsTreatment and control officers reported positive perceptions of de-escalation tactics, frequent use of those tactics and favorable attitudes toward de-escalation before and after the training. After receiving the training, treatment officers placed greater importance on compromise, and reported more frequent use of several important tactics including compromise, knowing when to walk away and maintaining officer safety.Originality/valueOnly a few prior studies have has examined whether de-escalation training changes officer attitudes. The results from the current study represent an initial piece of evidence suggesting de-escalation training may lead to greater use of those tactics by officers during encounters with citizens.

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