The presentation summarizes the work on the thesis for investigating different dynamic stall models and their utilization for modeling unsteady aerodynamics for wind turbines.
Thesis: https://www.researchgate.net/profile/Muhammad_Arsalan_Khan2/publication/337466116_Dynamic_Stall_Modelling_for_Wind_Turbines/links/5dd92037a6fdccdb445cb860/Dynamic-Stall-Modelling-for-Wind-Turbines.pdf
A reluctance motor is a type of electric motor that induces non-permanent magnetic poles on the ferromagnetic rotor. The rotor does not have any windings. It generates torque through magnetic reluctance.
Reluctance motor sub types include synchronous, variable, switched and variable stepping.
Reluctance motors can deliver high power density at low cost, making them attractive for many applications. Disadvantages include high torque ripple (the difference between maximum and minimum torque during one revolution) when operated at low speed, and noise due to torque ripple.
Fluid Mechanics Chapter 4. Differential relations for a fluid flowAddisu Dagne Zegeye
Introduction, Acceleration field, Conservation of mass equation, Linear momentum equation, Energy equation, Boundary condition, Stream function, Vorticity and Irrotationality
A reluctance motor is a type of electric motor that induces non-permanent magnetic poles on the ferromagnetic rotor. The rotor does not have any windings. It generates torque through magnetic reluctance.
Reluctance motor sub types include synchronous, variable, switched and variable stepping.
Reluctance motors can deliver high power density at low cost, making them attractive for many applications. Disadvantages include high torque ripple (the difference between maximum and minimum torque during one revolution) when operated at low speed, and noise due to torque ripple.
Fluid Mechanics Chapter 4. Differential relations for a fluid flowAddisu Dagne Zegeye
Introduction, Acceleration field, Conservation of mass equation, Linear momentum equation, Energy equation, Boundary condition, Stream function, Vorticity and Irrotationality
This report is a simulation for a flow over an airfoil "NACA 0009" at Reynolds number equals 1 million for four angles of attack using three different turbulence models and of cause a grid independence solution.
The goal of this study is to apply the knowledge obtained from studying in the university and self-learning in order to solve a specific task of finding the coefficient of drag and lift for the airfoil.
A youtube video made by me explaining how to simulate a flow over an airfoil: https://goo.gl/9VYRFM
Team members:
Ahmed Kamal Shalaby
Ahmed Gaber Ahmed
Esraa Mahmoud Saleh
EMF EQUATION OF SYNCHRONOUS GENERATOR,ALTERNATOR|DAY16|SHORT&FULL PITCH WINDI...Prasant Kumar
#EMF EQUATION OF SYNCHRONOUS GENERATOR
#SHORT PTCH WINDING OF ALTERNATOR
#DISTRIBUTED WINDING OF ALTERNATOR
#ALTERNATOR EMF EQUATION IN HINDI
#DERIVATION OF ALTERNATOR EMF EQUATION
#FARADAY LAW OF ELECTROMAGNETIC INDUCTION
FOR OTHER TOPICS OF ELECTRICA MACHINE MUST VISIT BELOW VIDEOS
#Topic - ELECTRICAL TRANSFORMER
DAY 1 (Need/Definition)
https://youtu.be/BvaykFJ_NoE
DAY 2 (Working principle and Construction)
https://youtu.be/06rgxocihaM
DAY 3 (EMF equation and Turns Ratio)
https://youtu.be/g7e5xBPmv3Y
DAY 4 (Classification of Transformer)
https://youtu.be/6NP5L4MlvY4
DAY 5 ( Ideal and practical transformer on no load)
(Equivalent Transformer)
https://youtu.be/6LCLQC1p3lg
DAY 6 ( Losses in Transformer)
https://youtu.be/ObYNiGgd3hA
DAY 7 (O.C. and S.C. test)
https://youtu.be/8WiJRawHiTce/6LCLQC1p3lg
DAY 8 (Voltage Regulation & Efficiency)
https://youtu.be/6LCLQC1p3lg
DAY 9 (Zero Lecture)
https://youtu.be/N4xWOwgi8I4
DAY 10 (Classification of machine)
https://youtu.be/bmxnU5rC5m4
Construction of Machine
https://youtu.be/34mpphDk3gg
Working Principle of Synchronous Generator & Synchronous Motor
https://youtu.be/bkgf72M8BCY
Working Principle of Induction Motor
https://youtu.be/Lj_iQBoRiK0
Emf equation of Dc machine
https://youtu.be/RRSy-LTK4bI
The Powerpoint presentation discusses about the Introduction to CFD and its Applications in various fields as an Introductory topic for Mechanical Engg. Students in General.
The mechanism is an assembly of machine components (Kinematic Links) designed to obtain the desired motion from an available motion while transmitting appropriate forces and moments.
Four bar linkage is a simple planer mechanism which has four bar shaped members. Usually it has one fixed link and three moving links and four pin joints.
Fluid Dynamics describes the physics of fluids at level of Undergraduate in science (physics, math, engineering). For comments or improvements please contact solo.hermelin@gmail.com. Thanks.
For more presentations on different subjects visit my website at http://www.solohermelin.com.
A short introduction on the device GYROSCOPE and a brief description on its properties, history, applications, types and future work.
Source:-
1. Theory of Machines by R.S.Khurmi and J.K.Gupta
2. www.google.co.in
2. www.wikipedia.org
this is the ppt on application of integrals, which includes-area between the two curves , volume by slicing , disk method , washer method, and volume by cylindrical shells,.
this is made by dhrumil patel and harshid panchal.
This report is a simulation for a flow over an airfoil "NACA 0009" at Reynolds number equals 1 million for four angles of attack using three different turbulence models and of cause a grid independence solution.
The goal of this study is to apply the knowledge obtained from studying in the university and self-learning in order to solve a specific task of finding the coefficient of drag and lift for the airfoil.
A youtube video made by me explaining how to simulate a flow over an airfoil: https://goo.gl/9VYRFM
Team members:
Ahmed Kamal Shalaby
Ahmed Gaber Ahmed
Esraa Mahmoud Saleh
EMF EQUATION OF SYNCHRONOUS GENERATOR,ALTERNATOR|DAY16|SHORT&FULL PITCH WINDI...Prasant Kumar
#EMF EQUATION OF SYNCHRONOUS GENERATOR
#SHORT PTCH WINDING OF ALTERNATOR
#DISTRIBUTED WINDING OF ALTERNATOR
#ALTERNATOR EMF EQUATION IN HINDI
#DERIVATION OF ALTERNATOR EMF EQUATION
#FARADAY LAW OF ELECTROMAGNETIC INDUCTION
FOR OTHER TOPICS OF ELECTRICA MACHINE MUST VISIT BELOW VIDEOS
#Topic - ELECTRICAL TRANSFORMER
DAY 1 (Need/Definition)
https://youtu.be/BvaykFJ_NoE
DAY 2 (Working principle and Construction)
https://youtu.be/06rgxocihaM
DAY 3 (EMF equation and Turns Ratio)
https://youtu.be/g7e5xBPmv3Y
DAY 4 (Classification of Transformer)
https://youtu.be/6NP5L4MlvY4
DAY 5 ( Ideal and practical transformer on no load)
(Equivalent Transformer)
https://youtu.be/6LCLQC1p3lg
DAY 6 ( Losses in Transformer)
https://youtu.be/ObYNiGgd3hA
DAY 7 (O.C. and S.C. test)
https://youtu.be/8WiJRawHiTce/6LCLQC1p3lg
DAY 8 (Voltage Regulation & Efficiency)
https://youtu.be/6LCLQC1p3lg
DAY 9 (Zero Lecture)
https://youtu.be/N4xWOwgi8I4
DAY 10 (Classification of machine)
https://youtu.be/bmxnU5rC5m4
Construction of Machine
https://youtu.be/34mpphDk3gg
Working Principle of Synchronous Generator & Synchronous Motor
https://youtu.be/bkgf72M8BCY
Working Principle of Induction Motor
https://youtu.be/Lj_iQBoRiK0
Emf equation of Dc machine
https://youtu.be/RRSy-LTK4bI
The Powerpoint presentation discusses about the Introduction to CFD and its Applications in various fields as an Introductory topic for Mechanical Engg. Students in General.
The mechanism is an assembly of machine components (Kinematic Links) designed to obtain the desired motion from an available motion while transmitting appropriate forces and moments.
Four bar linkage is a simple planer mechanism which has four bar shaped members. Usually it has one fixed link and three moving links and four pin joints.
Fluid Dynamics describes the physics of fluids at level of Undergraduate in science (physics, math, engineering). For comments or improvements please contact solo.hermelin@gmail.com. Thanks.
For more presentations on different subjects visit my website at http://www.solohermelin.com.
A short introduction on the device GYROSCOPE and a brief description on its properties, history, applications, types and future work.
Source:-
1. Theory of Machines by R.S.Khurmi and J.K.Gupta
2. www.google.co.in
2. www.wikipedia.org
this is the ppt on application of integrals, which includes-area between the two curves , volume by slicing , disk method , washer method, and volume by cylindrical shells,.
this is made by dhrumil patel and harshid panchal.
Bubble columns are widely used in the chemical and biochemical process industries. In order to develop design tools for engineering purposes, a large amount of research has been carried out in the area of CFD of gas-liquid flows. In this paper a transient Euler-Lagrange solver developed using the open source Caelus library is used to simulate the gas-liquid flow in a 3D square cross-sectioned bubble column. The turbulence is modelled using large eddy simulation (LES). The results of the simulations are compared to published PIV measurements. It is found that, good quantitative agreement with experimental data is obtained when drag, lift and virtual mass forces are used.
Computational and experimental investigation of aerodynamics of flapping aero...Lahiru Dilshan
Renewal interest on the exploitation of flapping flight motions to attain high propulsion efficiency of air vehicles is inspired by the aerodynamics of birds’ and insects’ flights. The flapping characteristics can be majorly used to develop micro aerial vehicles (MAV) as this is a lucrative method to generate lift and thrust simultaneously. In this project, the variation of the flow properties and the thrust generation of an airfoil in a flapping (plunging) motion, is evaluated using both computational and experimental methods. The NACA 2412 airfoil was selected for the study and, the computational method was carried out using an inviscid flow model and computational fluid dynamics (CFD) simulations, simultaneously to obtain and compare the variation of properties.
The inviscid model was developed using conformal mapping and potential flow theories, and it is capable of producing results for any arbitrary aerofoil. Steady-state results were compared and validated in both CFD and inviscid flow modelling as the computational framework along with flow visualisation and force sensing as the experimental framework. The validated CFD and inviscid models have been developed to produce a plunging motion to the aerofoil and obtain the variation of drag and lift coefficients with time. The experimental setup was designed to obtain the forces acting on the airfoil, and the flow characteristics were visually observed using a flow visualization technique. The force calculations were done through a developed and optimized load cell arrangement. The developed smoke flow visualisation technique is capable of successfully capturing streamline patterns, flow separation regions. These results were compared along with wake development between computational and experimental models. The Level of agreement and limitations of each method have been discussed in this report.
Metaheuristics-based Optimal Reactive Power Management in Offshore Wind Farms...Aimilia-Myrsini Theologi
The aim of the thesis is to optimally coordinate the reactive power sources in offshore wind farms in a predictive manner based to the principle of minimizing the wind farm power losses, as well the variations of the transformers tap positions. First, an accurate Neural Network-based wind speed forecasting algorithm was developed in order to counteract the uncertainty of the wind and finally, the optimal management of the available reactive sources is tackled by a metaheuristics-based method. Two different cases were investigated: a far-offshore wind farm with HVDC interconnection link and the AC connected Dutch wind farm BORSSELE.
DSD-INT 2019 Delft3D FM model for Hong Kong-GroenenboomDeltares
Presentation by Julien Groenenboom, Deltares, at the Delft3D - User Days (Day 2: Hydrodynamics), during Delft Software Days - Edition 2019. Tuesday, 12 November 2019, Delft.
Validation of a Low-Cost Transitional Turbulence Model for Low-Reynolds-Numb...counse
This is a slide show I presented at the 2011 AIAA CFD Conference in Honolulu, Hawaii. The research herein was published and presented in 2 journal articles, 6 conferences, and an award-nominated thesis. The presentation focuses on the use of the SST gamma-Re_theta turbulence model and its applicability to the low-Reynolds-number regime.
Introduction
• Got attention after the failure of Ariane 5 flight 501in July 1996
• Apart from the main cause, which was the control software bug, heavy side loads around the EPC (Vulcain) nozzle were also observed at the time of failure
• At launch, the Vulcain engine is responsible to produce an enormous amount of thrust. The transonic free-stream flow from the outside of the main engine interacts with the high velocity supersonic flow forming a shear layer
• The complex flow physics is expensive experimentally and numerical computation is helpful
Read| The latest issue of The Challenger is here! We are thrilled to announce that our school paper has qualified for the NATIONAL SCHOOLS PRESS CONFERENCE (NSPC) 2024. Thank you for your unwavering support and trust. Dive into the stories that made us stand out!
June 3, 2024 Anti-Semitism Letter Sent to MIT President Kornbluth and MIT Cor...Levi Shapiro
Letter from the Congress of the United States regarding Anti-Semitism sent June 3rd to MIT President Sally Kornbluth, MIT Corp Chair, Mark Gorenberg
Dear Dr. Kornbluth and Mr. Gorenberg,
The US House of Representatives is deeply concerned by ongoing and pervasive acts of antisemitic
harassment and intimidation at the Massachusetts Institute of Technology (MIT). Failing to act decisively to ensure a safe learning environment for all students would be a grave dereliction of your responsibilities as President of MIT and Chair of the MIT Corporation.
This Congress will not stand idly by and allow an environment hostile to Jewish students to persist. The House believes that your institution is in violation of Title VI of the Civil Rights Act, and the inability or
unwillingness to rectify this violation through action requires accountability.
Postsecondary education is a unique opportunity for students to learn and have their ideas and beliefs challenged. However, universities receiving hundreds of millions of federal funds annually have denied
students that opportunity and have been hijacked to become venues for the promotion of terrorism, antisemitic harassment and intimidation, unlawful encampments, and in some cases, assaults and riots.
The House of Representatives will not countenance the use of federal funds to indoctrinate students into hateful, antisemitic, anti-American supporters of terrorism. Investigations into campus antisemitism by the Committee on Education and the Workforce and the Committee on Ways and Means have been expanded into a Congress-wide probe across all relevant jurisdictions to address this national crisis. The undersigned Committees will conduct oversight into the use of federal funds at MIT and its learning environment under authorities granted to each Committee.
• The Committee on Education and the Workforce has been investigating your institution since December 7, 2023. The Committee has broad jurisdiction over postsecondary education, including its compliance with Title VI of the Civil Rights Act, campus safety concerns over disruptions to the learning environment, and the awarding of federal student aid under the Higher Education Act.
• The Committee on Oversight and Accountability is investigating the sources of funding and other support flowing to groups espousing pro-Hamas propaganda and engaged in antisemitic harassment and intimidation of students. The Committee on Oversight and Accountability is the principal oversight committee of the US House of Representatives and has broad authority to investigate “any matter” at “any time” under House Rule X.
• The Committee on Ways and Means has been investigating several universities since November 15, 2023, when the Committee held a hearing entitled From Ivory Towers to Dark Corners: Investigating the Nexus Between Antisemitism, Tax-Exempt Universities, and Terror Financing. The Committee followed the hearing with letters to those institutions on January 10, 202
Safalta Digital marketing institute in Noida, provide complete applications that encompass a huge range of virtual advertising and marketing additives, which includes search engine optimization, virtual communication advertising, pay-per-click on marketing, content material advertising, internet analytics, and greater. These university courses are designed for students who possess a comprehensive understanding of virtual marketing strategies and attributes.Safalta Digital Marketing Institute in Noida is a first choice for young individuals or students who are looking to start their careers in the field of digital advertising. The institute gives specialized courses designed and certification.
for beginners, providing thorough training in areas such as SEO, digital communication marketing, and PPC training in Noida. After finishing the program, students receive the certifications recognised by top different universitie, setting a strong foundation for a successful career in digital marketing.
Exploiting Artificial Intelligence for Empowering Researchers and Faculty, In...Dr. Vinod Kumar Kanvaria
Exploiting Artificial Intelligence for Empowering Researchers and Faculty,
International FDP on Fundamentals of Research in Social Sciences
at Integral University, Lucknow, 06.06.2024
By Dr. Vinod Kumar Kanvaria
Embracing GenAI - A Strategic ImperativePeter Windle
Artificial Intelligence (AI) technologies such as Generative AI, Image Generators and Large Language Models have had a dramatic impact on teaching, learning and assessment over the past 18 months. The most immediate threat AI posed was to Academic Integrity with Higher Education Institutes (HEIs) focusing their efforts on combating the use of GenAI in assessment. Guidelines were developed for staff and students, policies put in place too. Innovative educators have forged paths in the use of Generative AI for teaching, learning and assessments leading to pockets of transformation springing up across HEIs, often with little or no top-down guidance, support or direction.
This Gasta posits a strategic approach to integrating AI into HEIs to prepare staff, students and the curriculum for an evolving world and workplace. We will highlight the advantages of working with these technologies beyond the realm of teaching, learning and assessment by considering prompt engineering skills, industry impact, curriculum changes, and the need for staff upskilling. In contrast, not engaging strategically with Generative AI poses risks, including falling behind peers, missed opportunities and failing to ensure our graduates remain employable. The rapid evolution of AI technologies necessitates a proactive and strategic approach if we are to remain relevant.
Francesca Gottschalk - How can education support child empowerment.pptxEduSkills OECD
Francesca Gottschalk from the OECD’s Centre for Educational Research and Innovation presents at the Ask an Expert Webinar: How can education support child empowerment?
The French Revolution, which began in 1789, was a period of radical social and political upheaval in France. It marked the decline of absolute monarchies, the rise of secular and democratic republics, and the eventual rise of Napoleon Bonaparte. This revolutionary period is crucial in understanding the transition from feudalism to modernity in Europe.
For more information, visit-www.vavaclasses.com
Normal Labour/ Stages of Labour/ Mechanism of LabourWasim Ak
Normal labor is also termed spontaneous labor, defined as the natural physiological process through which the fetus, placenta, and membranes are expelled from the uterus through the birth canal at term (37 to 42 weeks
Operation “Blue Star” is the only event in the history of Independent India where the state went into war with its own people. Even after about 40 years it is not clear if it was culmination of states anger over people of the region, a political game of power or start of dictatorial chapter in the democratic setup.
The people of Punjab felt alienated from main stream due to denial of their just demands during a long democratic struggle since independence. As it happen all over the word, it led to militant struggle with great loss of lives of military, police and civilian personnel. Killing of Indira Gandhi and massacre of innocent Sikhs in Delhi and other India cities was also associated with this movement.
A Strategic Approach: GenAI in EducationPeter Windle
Artificial Intelligence (AI) technologies such as Generative AI, Image Generators and Large Language Models have had a dramatic impact on teaching, learning and assessment over the past 18 months. The most immediate threat AI posed was to Academic Integrity with Higher Education Institutes (HEIs) focusing their efforts on combating the use of GenAI in assessment. Guidelines were developed for staff and students, policies put in place too. Innovative educators have forged paths in the use of Generative AI for teaching, learning and assessments leading to pockets of transformation springing up across HEIs, often with little or no top-down guidance, support or direction.
This Gasta posits a strategic approach to integrating AI into HEIs to prepare staff, students and the curriculum for an evolving world and workplace. We will highlight the advantages of working with these technologies beyond the realm of teaching, learning and assessment by considering prompt engineering skills, industry impact, curriculum changes, and the need for staff upskilling. In contrast, not engaging strategically with Generative AI poses risks, including falling behind peers, missed opportunities and failing to ensure our graduates remain employable. The rapid evolution of AI technologies necessitates a proactive and strategic approach if we are to remain relevant.
Thesis Statement for students diagnonsed withADHD.ppt
Dynamic stall modelling for wind turbines
1. 1
Dynamic Stall Modeling for
Wind Turbines
Muhammad Arsalan Khan
European Wind Energy Master
Rotor Design – Aerodynamics
Supervisor ECN: Dr. Gerard J. Schepers
Supervisor TUD: Dr. Carlos S. Ferreira
Supervisor DTU: Prof. Niels N. Sorensen
2. 2
Outline
• Motivation
• Objective
• Methodology
• Code Description
• Results
– New MEXICO
– Standstill Conditions
– Improvement of Models
– Rotating Yawed Conditions
– AVATAR Case Study
• Conclusions
3. 3
What is Dynamic Stall ?
U∞
U∞
Angle of Attack
Lift
Stall
Disclaimer: Animations are exaggerated
4. 4
Motivation
• Dynamic Stall (DS)
• Relevance for Wind Turbines
– High Freq. DS
• Turbulent environment
• Storm Conditions
– 1P DS
• Yaw misalignment
• Wind shear
• Implications:
• Fatigue
• Instabilities – Aero. Damping
• Cost of Energy
Graphics adapted from Leishman, “Principle of Helicopter Aerodynamics”
5. 5
Objective
• Comparing the performance of different
dynamic stall models in yawed and standstill
conditions by using New MEXICO experimental
data for validation.
• Dynamic Stall Models used:
– Snel Model
– Beddoes Leishman Model
– ONERA Model
6. 6
Methodology
New MEXICO
Data Analysis
• Data Quality
Assessment
• 3D Airfoil Polars
Standstill
Conditions
• Validation of BEM + DS
Models
• Cross-Flow Principle
• AVATAR Case Study
Yawed Flow
Conditions
• Stall delay models
• Validation of BEM + DS
Model
• Optimum BL Model
Parameters
ECN Aero-Module
7. 7
Code Description
Graphics taken from Boorsma, “ECN Aero-Module User’s Manual v238”
• ECN Aero-Module
– Aero-BEM
• Classical BEM Theory
• Annulas Average Approach
• Based on implementation in PHATAS
– Aero-AWSM
• Lifting Line Model
• Vortex Wake Model
8. 8
Code Description
• Dynamic Stall Models
– Snel’s Model
• 1st Order Model
• 2nd Order Model
– Beddoes-Leishman Model
• Attached Flow Module
• Separated Flow Module
• Leading Edge Separation Module
• Vortex Lift Module
– ONERA Model
• Linear attached flow
• Non-linear separated flow
12. 12
Results – New MEXICO
• Strouhal Number
Structured
Vortex Shedding
x - Tip to Root flow
- Root to Tip flow
- no span-flow
Low
Strouhal Freq.
, where
14. 14
Results – New MEXICO
• 3D Steady Airfoil Polars
– Comparison with Flat Plate Theory in Deep Stall
15. 15
Results – Standstill Conditions (New MEXICO)
Axial Flow
AOA Range Pitch Range Observation
Low , ~ -15o to +15o ~ 75o to 90o Good Agreement
High , ~ 45o to 90o ~30o to ~-2.3o Bad Agreement
Pitch = 30o
Wind Speed = 30 m/s
Yaw = 0o
16. 16
Results – Standstill Conditions (New MEXICO)
Yawed Flow
AOA Range Yaw Angles Observation
Low 15o & 30o Good Agreement
High -30o, -45o, -60o, & -90o Bad Agreement
Pitch = 90o
Wind Speed = 30 m/s
Yaw = -45o
Non-axisymmetric Flow
18. 18
Results – Improvement of Models
• Beddoes-Leishman Flow Separation Model
– Øye Model
– FFA (Bjørck) Model
– Larsen Model
Graphics taken from Larsen et al., Journal of Fluids and Structures 2007.
29. 29
Conclusions
• Limited Bluff Body Vortex Shedding for Finite Aspect Ratio
Blade in Deep Stall
• Improvement of DS Models in Aero-Module
– Snel relocation of fader function
– B-L Larsen Separation Model + Effect of Varying
Incoming Velocity
– ONERA Piece-wise function for Stiffness Term
• Typically, a 10% reduction in Error with DS models in yawed
conditions
• Edgewise Instability effected by shape and rotation of DS
hysteresis loop
36. 36
Backup – Cross Flow Model
• Cross-Flow Error Ratio*
* Cross-Flow Model by Gaunaa et al., Journal of Physics, 2016.
θcf = +60 deg. θcf = -60 deg.
TE
LE
37. 37
Backup – Cross Flow Model
• Cross-Flow Model*
* Cross-Flow Model by Gaunaa et al., Journal of Physics, 2016.
Yaw = -60o
Pitch = 90o
49. 49
Results – Standstill Conditions (New MEXICO)
• Mean of Standard Deviation of Normal Force
Axial Flow Yawed Flow
Pitch: 90o -2.3o Yaw: -90o 30o
54. 54
Results – New MEXICO
• Frequency Domain Analysis
x - Tip to Root flow
- Root to Tip flow
- no span-flow
Editor's Notes
What is Dynamic Stall?
Before we try to answer this question lets look at what is stall.
Lets say we have an airfoil and we slowly increase the AoA of this airfoil and observe the lift response.
What we see is the lift starts decreasing after a certain AoA, which we call as the stall angle of the airfoil.
Now, imagine we have another similar airfoil. We repeat the process for this airfoil
But this time instead of doing it slowly we do it really fast.
What happens is that not only do we see an increase in lift but also a delay in stall angle.
This behavior is what is known as dynamic stall.
The reason why it happen is because flow over the airfoil is unable to react to changes in AoA instantaneously.
What is Dynamic Stall?
The classical definition of dynamic stall characterizes it into 5 distinct stages from lag in separation to development of LEV to lag in re-attachment of flow.
Principally, dynamic stall is an unsteady aerodynamic phenomenon characterized by five distinct stages.
Stage 1 Airfoil exceed static stall angle and flow reversal starts taking place in the boundary near the TE
Stage 2 Onset of formation of LE vortex
Stage 3 Convection of LE vortex
Stage 4 LEV has crossed TE and complete separation happens
Stage 5 Flow re-attachment at lower AoA than static stall AoA
Turbulence can be from the general atmospheric conditions or due to operation in the wake of another wind turbine in a wind farm
High AoA in storm operation when the turbine has no control and wind can hit it at high speed from any direction. Wind turbine operating in deep stall …. Where the stall is extremely dynamic in nature.
Yaw misalignment is very common on wind turbines. The response of yaw system is not fast enough for changing wind directions
Horizontal or vertical wind shear introduced as fluctuation in AoA causing DS
Turbulent Environment High Freq. DS
Storm Conditions High Freq. DS
Yaw Misalignment 1P DS
Wind Shear 1P DS
IN the end it all boils down to reducing the cost of energy by accurately predicting dynamic stall
Behavior of stall at very large AoA b/c that is still a big unknown in the scientific community due to lack of experimental data.
Model are semi-empirical and require back-bone curves and parameters
Three different dynamic stall models are implemented in Aero-Module
Of these models only Snel’s first order model has been extensively validated before.
Dynamic Stall Models
Snel’s Model
1st Order Model
2nd Order Model
Beddoes-Leishman Model
Attached Flow Module (A1,A2,b1,b2,TI)
Separated Flow Module (Tp and Tf)
Leading Edge Separation Module (Cn1)
Vortex Lift Module ( Tv and Tvl)
ONERA Model
Flat plate parameters
Mean airfoil parameters
A nice animation showing the dynamic stall phenomenon in action.
-Sharp rise in leading edge suction pressure due to creation of LEV.
The suction peaks drops abruptly but the lift coefficient still increases b/c LEV is convecting over upper surface
The lift coefficient suffers an abrupt stall once the LEV has passed the trailing edge.
A nice animation showing the dynamic stall phenomenon in action.
-Sharp rise in leading edge suction pressure due to creation of LEV.
The suction peaks drops abruptly but the lift coefficient still increases b/c LEV is convecting over upper surface
The lift coefficient suffers an abrupt stall once the LEV has passed the trailing edge.
What sort of dynamic phenomenon is observed in deep stall conditions. Is there any structured vortex shedding?
Self-induced dynamic stall/turbulence
Two cluster of points were identified.
One of them corresponding to bluff body vortex shedding Stourhal Numbers
While the other corresponded to low Strouhal Freq. shedding that was also observed in NREL UAE experiment in NASA Ames Wind tunnel. The cause of these low freq. strouhal number has been associated to periodic switching between attached and separated flow near the leading edge.
What was surprising to see was that no structured vortex shedding freq. was observed at very large AoA beyond 50 degrees. Lindenburg had postulated that finite aspect ratio blades can only have partially structured vortex shedding.
Early break-up of shed vorticity due to suction of air into the vortex core.
Mainly large difference b/w 2D and 3D polars in deep stall. Mainly because of base drag as 2D profile has higher base drag. But also according to Linderburg due to absence of structured vortex shedding in 3D blade of finite aspect ratio the drag will be lower than 2D blade sections because 2D blade sections have structured vortex shedding. The reason for this is the fact ‘’’’mention’’’ the experiment with and without end blades on airfoils.
The Standstill conditions for Rigid New MEXICO rotor were validated with Simulations and It was found that simulations show good agreement with measurements at Low AoA. While show bad agreement at Large AoA.
One example case of bad agreement is presented in the figure below. Particularly important to note is the strange behavior predicted by B-L and ONERA model for this case.
Now looking at standstill conditions but with yawed flow. Again good agreement was seen for test cases with low AoA. An example test case of bad agreement between simulations and experiments is shown in the figure below.
Here again ONERA model and B-L model were seen to give strange results.
Therefore, the cause of this discrepancy was investigated and three major causes were found:
Implementation of fader functions in both models
Kirchoff Flow separation model in B-L model
One of the forcing terms in non-linear second order differential equation of the ONERA model
Before proceeding with further validation the model were improved. Firstly the B-L model was improved.
This model is based on Kirchoff flow separation model which itself is based on small angle approximation. Therefore, this model was giving problems in re-constructing the static polar at very large AoA as shown in the figure.
Also, by default Aero-Module was using fader functions to circumvent this problem but they did not seem to remedy the problem.
To fix the problem the following three different flow separation model were programmed in Aero-Module
The Oye Model and FFA model used a different formulations of same Kirchoff model to fix the problems at large AoA.
While Larsen used a conformal mapping approach by mapping the f parameter onto a circle in the complex plane.
The main advantage of this model is that it avoids an intrinsic singularity in the kirchoff model.
Here you can see a comparison between the default model in Aero-Module and the improved model.
The results show simulated hysteresis loops at different mean AoAs.
The default model gives unrealistic results at large AoA which was also why the standstill validation cases with New MEXICO data were showing bad results.
Airfoil: DU 91-W2-250
In case of ONERA model two main improvement were made.
Firstly, the second forcing term on RHS of second order equation was replaced with alpha dot instead of rate of change of delta Cl, based on the recommendation from Petot.
Secondly, a piece-wise function was introduced to model the stiffness term in the second order equation.
This function increase the stiffness term for a downstroke cycle resulting in better comparison with experimental results during the re-attachment phase of dynamic stall.
The reason why alpha dot (pitch rate) was used is because the lift coefficient was found to be more sensitive to alpha dot in comparison to rate of change of delta Cl
This figure shows a comparison between default model and the improved ONERA model.
It was seen that even with the corrections the model showed increasing size of hystersis loops at large AoA.
Hence a fader function called “cdelPot” from PHATAS was applied to the dynamic lift coefficient to switch off the ONERA model at very large AOA to prevent queer results.
Airfoil: DU 91-W2-250
A through error analysis was performed for different yaw angles and different spanwise sections.
Dynamics stall models did not show any reduction in error for outboard blade sections in comparison to quasi-steady model
For the inboard sections all the model showed large improvement in error except the ONERA model which was having trouble in modeling the re-attachment phase.
In simple words this case study is as simple as taking a blade and hitting with a hammer to see how the vibrations are damped over time
In simple words the purpose of this case study was to excite one of the blades and see how the vibrations on the blade decay over time with and without using Dynamic Stall Models
The last part of the research involved an aero-elastic case study of the AVATAR rotor.
This case study mainly follows the work of test case no. 2 from Aeroelastic Workout in Polimi in 2015.
Simulation time = 120 seconds
For the aeroelastic simulations Aero-Module was coupled with PHATAS using windows command line.
The case study mainly involved analyzing the tip displacements response and root bending moments.
** flapwise results independent of time step size.
** Edgewise results dependent on time step size
A time step sensitivity was performed.
The figures show peak to peak tip displacements for in-plane and out-of-plane deformations for different time step sizes with quasi-steady aerodynamic model
No monotonic trend was observed with reducing time step size. However, the range of pitch angles for which very large peak to peak response was observed, possibly due to instability, was consistent for different time step sizes.
This slide shows the variation of damping ratio versus pitch angle for different dynamic stall models.
It can be clearly seen from figure that dynamic stall model are adding aerodynamic damping to the system in comparison to quasi-steady aerodynamic model in the stall regime.
ONERA model shows a positive damping ratio for the complete range of AoA
While BL and Snel model show negative damping for a small range of AoA. This behavior of these two model was further investigated to identify the cause.
First we look at Snel model at 30 deg. pitch setting
In this animation the top row of figures present the tip trace while the bottom row presents the hysteresis loops.
On observing hysteresis loop for Snel model a counter-clockwise hysteresis loop was observed in stark contrast to typically observed clockwise hysteresis loop that give positive damping.
It was found that a fader function called “cdelPot” in Aero-Module was causing this problem. If this fader function is removed then the Snel model gives positive aerodynamic damping over the entire range of pitch settings
For the test case where B-L model shows instability a different reason was identified.
Observing the hysteresis loop for B-L model a sort of half hysteresis loop was identified causing positive damping in upstroke while negative damping similar to quasi-steady aerodynamic model in downstroke.
The reason why B-L model gives this hysteresis loop behavior in deep stall is due to the fact that flow is completely separated over the upper surface and only Vortex Lift Module is causing the hysteresis effects.
Now I present a few results from the New MEXICO measurements.
As an example here the pressure distributions for 30 degree yaw case are shown.
Large STD in pressure for Low TSR cases as AoA is higher and so is the advancing and retreating blade effect regime at Low TSR
- Low TSR has higher AoA Stagnation point indicated by -1 further aft than High TSR curve
Low TSR has more STD than High TSR cases large hyteresis in normal force. dynamic stall
Kinks in pressure data at inboard stations was normally seen for High TSR
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The cross-flow error ratio was determined based on CFD measurements on DTU 10 MW reference turbine.
A first order correction was fitted to the obtained error ratios to obtain the cross flow model to be used as an add-on on the regular cross-flow principle.
Lift and drag error ratios were identical therefore a the same error ratio was used for both
Testing this model does not directly have to do anything with dynamic stall. However, the main purpose of testing this model was to see if it improves the overall prediction of loads in standstill condition.
It seems to accurately predict results for the 8 o’clock blade.
The model completely misrepresents the trend for the 4 o’ clock blade.
Model still needs development and validation to be used in standstill conditions. Hence, it was not pursued further as it was out of the scope of the current research.
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2 problems with optimization:
Explores unrealistic solution space.
Does not give any knowledge about the domain
Snels and Lindenburg model are very similar. Lindenburg infact builds on Snels model by adding a local speed ratio dependency.
Schepers mode is a tuned version of Hansens model.
The mean was subtracted from the time series to remove energy from the DC component of the time signal as it contains the most energy and could potentially mask any dominant vortex shedding frequencies.
Additionally, signals were divided by their standard deviation to standardize the spectrum output and facilitate comparison between normal and tangential force on the same plot.
PSD Welch’s Method MATLAB Windowing function applied to mitigate spectral noise by averaging segments of time series.
Hanning Window --- 50% overlap
Testing this model does not directly have to do anything with dynamic stall. However, the main purpose of testing this model was to see if it improves the overall prediction of loads in standstill condition.
It seems to accurately predict results for the 8 o’clock blade.
The model completely misrepresents the trend for the 4 o’ clock blade.
Model still needs development and validation to be used in standstill conditions. Hence, it was not pursued further as it was out of the scope of the current research.
Snels and Lindenburg model are very similar. Lindenburg infact builds on Snels model by adding a local speed ratio dependency.
Schepers mode is a tuned version of Hansens model.
Snels and Lindenburg model are very similar. Lindenburg infact builds on Snels model by adding a local speed ratio dependency.
Schepers mode is a tuned version of Hansens model.
A FFT of the non-linear time response from all simulation revealed that the edgewise mode was causing instability.
Therefore, to determine damping ratio a bandpass filter was developed to isolate the edgewise frequency component in the time signal.
Once isolating the signal peaks were extracted and plotted against number of cycles between amplitudes to fit a linear regression line. The slope of this line gave the logarithmic decrement
From which damping ratio was computed using a simple formula.
Before simulating dynamic stall in standstill and yawed conditions, standalone dynamic stall models were validated using OSU wind tunnel experiments.
S814 airfoil profile, which is a wind turbine specific airfoil with relative thickness comparable to DU91 profile on the MEXICO blade.
Snels and Lindenburg model are very similar. Lindenburg infact builds on Snels model by adding a local speed ratio dependency.
Schepers mode is a tuned version of Hansens model.
A through error analysis was performed for different yaw angles and different spanwise sections.
Dynamics stall models did not show any reduction in error for outboard blade sections.
For the inboard sections all the model showed large improvement in error except the ONERA model which was having trouble in modeling the re-attachment phase.
The frequencies below the dashed line will not be considered for calculating Strouhal number for two main reasons:
The frequencies below the – – line give very low Strouhal Numbers
All the erroneous frequencies were observed to lie below this level
For 25% section dominant frequencies seen even in the attached flow AoA ranges. However, on observing the standard deviation of pressure distribution no large transients could be identified at these low AoA. Hence these frequencies were not considered for computing the Strouhal numbers.