DRAG REDUCTION METHODS
Four main types of drag are encountered in aerodynamics-Namely
- skin-friction drag,
- form drag,
- wave drag,
- induced drag.
The methods in use for reduction for the reduction of each type of drag are discussed in turn
REDUCTION OF SKIN- FRICTION DRAG
In broad terms, skin-friction drag can be reduced in one of the two ways. Either laminar flow is maintained by postponing transition, this is so called laminar- flow technology, or ways to found to reduce the surface shear stress generated by the turbulent boundary layer.The laminar-flow can be maintained passively by prolonging the favourable constant-pressure region over the wing surface. Active control of transition requires the use of suction, either distributed or through discrete spanwise slots. Often the suction is used in conjunction with the favourable pressure distributions. The basic principle of maintaining laminar flow by means of suction has been known for at least thirty-five to forty years. However, the problems with the practical implementation on aircraft, either real or perceptual, have prevented the widespread of the technology. It seems increasingly likely, however, that the considerable gains in efficiency which would result from the use of laminar -flow technology, will ensure that it will be much more widely exploited on commercial aircraft in the near future.Other methods for maintaining laminar-flow have been developed, but as yet, have not been seriously considered seriously for practical application in aviation.
A moderately effective method for reducing turbulent flow friction which has been developed in the recent years involves minute modification of the surfaces so that they are covered with riblets.Riblets can take many forms but essentially consist of streamwise ridges and valleys, as shown. These triangularly shaped riblets are available in the form of the film.
This corresponds to the actual spacing of 0.025 to 0.075 mm for flight conditions. This 3M riblet film has been tested on an in-service Airbus a 300-600 and in other aircrafts. The skin friction drag reduction observed as of the order of 5 to 8 percent and the skin-friction drag 30 to 40% of the total aircraft drag.Thus the overall drag reduction is modest but, nevertheless, represents a very considerable economic benefit with little in the way of a penalty.It is very likely, therefore, that riblets will be widely used on commercial aircraft In the future.The basic concept behind riblets had many origins, but it is probably the work at NASA Langley in the USA, which led to the present developments .he concept, was also discovered independently in Germany through a study of hydrodynamics of riblet –like formations on shark scales. A plausible explanation for the effect of riblets is that they interfere with the rear wall structure of the turbulent boundary layer in the region where the turbulence is mainly generated.The flow field in the turbulent boundary layer is highly complex, but the form of near wall structures has been elucidated in the seminal study of Kline et al at Stanford university.It appeared from this work that ‘hairpin’ vortices form near wall as shown these vortices then continue to grow until a point is reached when the head is violently ejected away from the wall and, simultaneously, the contra-rotating streamwise oriented branches of the vortices come together, inducing a powerful downwash of high-momentum fluid between the vortices towards the wall. This sequence of events constitutes what is termed a ‘near =wall burst’. The riblets act as barriers which prevent the free spanwise movement of the hairpin vortices.It is thought that owing to this the vortices cannot approach each other closely thereby weakening the near wall bursting process. It should be noted, however, that this explanation is not universally accepted
2. REDUCTION OF FORM DRAG
Form drag is kept to the minimum by preventing boundary-layer separation. Streamlining is vitally important for reducing form drag. It is worth noting that at high Reynolds’s numbers a circular cylinder has roughly the same overall drag as a streamlined aerofoil with a chord length equal to 100 cylinder radii. Form drag is overwhelmingly the main contribution to the overall drag for bluff bodies like the cylinder, whereas the predominant contribution in the case of the streamlined body is skin-friction drag, form drag being less than ten percent of the overall drag.For bluff bodies even minimal streamlining can be very effective.
3.REDUCTION OF INDUCED DRAG
Induced drag falls as the aspect ratio of the wing is increased.It was also shown that for a given aspect ratio elliptic-shaped wings have the lowest induced drag. Over the past fifteen years, the winglet has been developed as a device for reducing induced drag without increasing aspect ratio.Typical example is illustrated in the figure .winglets of this type have now been fitted too much type of commercial aircraft.The physical principle behind the winglet is illustrated in the figure. On all sub-sonic wings, there is a tendency for a secondary flow to develop from the high-pressure region below the wing around the wing tip to the relatively low-pressure region on the upper surface. This is the part of the process of forming the trailing vortices if a winglet of the appropriate design and orientation is fitted to the wing-tip, the secondary flow causes the winglet to be at an effective angle of incidence, giving rise to lift and drag components Lw and Dw relative to the winglet, as shown in the figure. Both Lw and Dw have components in the direction of the drag of the aircraft as a whole.Lw provides a component counter to the aircraft drag, while Dw provides a component which augments the aircraft drag.for a well-designed winglet, the contribution of Lw predominates, resulting in a net eduction in overall drag, or thrust, equal to delta T.
4.REDUCTION OF WAVE DRAG
To some extent in the discussion of a supercritical aerofoil, it was found that keeping the pressure uniform over the upper wing surface minimized the shock strength, thereby reducing the wave drag. A somewhat similar principle holds for the wing-body combination of transonic aircraft. This was encapsulated in the area rule formulated by Whitcomb. It was known that as a wing-body combination passed through the speed of sound of sound, the conventional straight fuselage, shown in the figure., experienced a sharp rise in wave drag. Whitcomb showed that this rise in drag could be considered reduced if the fuselage was Waisted as shown in the figure, in such a way as to keep the total cross-sectional area of the wing-body combination constant. Waisted fuselages of this type are now a common feature of aircraft designed for transonic operation.
Fundamentals of turbomachines -The Euler’s equation:
Rotating machines are called as turbomachines.
Turbomachines are a device, in which the rotating fluid interacts with the rotor which rotates about an axis.
When a fluid interacts with a rotor, two things take place
The energy transfer and
The energy transformation
The energy transfer takes place in the rotating parts( i.e) it may be transferred from the fluid to rotor or vice versa
In case if the energy is transferred from fluid to rotor it is called as a turbine
If the energy is transferred from rotor to fluid it is called as a compressor.
Energy transfer takes place only in the rotating parts
Whereas the energy transformation takes place both in the rotating and stationary parts.
Energy transformation means the change of kinetic energy to the pressure energy.
The forces developed in the turbomachines is because of Newton’s second law of motion or the combined action of both newtons second and third law.
Newton’s second law states that the rate of change of momentum is directly proportional to the force applied.
In case of turbomachines, the angular momentum takes place,
Therefore the rate of change of angular momentum is directly proportional to the applied torque.
Consider a fluid of mass m and interacts with the rotor with a tangential velocity C1 at a distance r1 from the axis of the rotor and leaves the rotor with a tangential velocity c2 at a distance r2 from the rotor. as this fluid (gas) interacts with the rotor which rotates about the axis A-A, there comes an angular momentum which is directly proportional to the torque applied.
The angular momentum at the inlet is mc1r1
The angular momentum at the exit is mc2r2
Applying Newton’s second law,
The applied torque = mc2r2 -mc1r1
The energy transfer E = T×ω(energy = torque * angular momentum)
E=m(c2u2 -c1u1) (u=rω )
this is the Euler’s energy equation.
where u2 and u1 is the tangential velocity of the rotoe and c2 and c1 is the tangential velocity of the gas.
c2u2 >c1u1 the energy transfer is positive and is a Turbine
If c2u2 < c1u1 the energy transfer is negative and is a compressor
Get the video lesson here,
How to study effectively for GATE AEROSPACE 2018?
As the big day is nearing, all those GATE aspirants are Working hard to step into a great future.All your dreams come true when you work smart rather than hard for your GATE AEROSPACE 2018. There are few effective strategies, I teach my students, When you follow with this method, you can increase your success rate.
Here is my complete guide for GATE AEROSPACE 2018 preparation. these are my best-known ways to prepare for an exam.
I have got 100% success rate with this methodology, even I use to teach using this methodology.
Planning your study time effectively for GATE AEROSPACE 2018:
So, when you come to the point of “time management “.You frequently hear the boring proverb “time and tide waits for none “.However, I believe “time is just an illusion” all you need to do is concentrate. Even if you read for half an hour, just try to understand the context. A deeper understanding of the context helps you learn the application techniques.
Therefore, it is not that how many hours you read; it is how well you read that matters.
I call this technique as “O-BAD”.
Yup, it’s really bad, because it gives you big bang results when you follow it seriously.
- Outline the” big Chunks”
- Break down and list the subtasks
- Analyze the tasks and subtasks
- Decide when each subtask has to be completed.
Outline the” big Chunks”:
Start with analysing in which subject you are strong. The subject that you choose will be your Big Chunk.Check for the major topics in that subject, these will be TASKS now.
For example, Big chunks in GATE AEROSPACE are Aerodynamics, Aircraft Propulsion, Aircraft Structures, Mathematics.
If you are good at Aerodynamics, it will be your Big chunk now. you have to sort down the topics in that, to find out your tasks.
In Aerodynamics, tasks will be
- Task No:1 -Fundamentals
- Task No:2-Inviscid Incompressible Flow,
- Task No:3-Inviscid Compressible Flow,
- Task No:4-Viscous flow.
now you are done with the first step of this technique. Let’s move to the next step.
Breakdown and list the subtasks :
Identify and list the steps that need to be taken in order to completely Cover each task.
Example: if aerodynamics is the Big chunk, and choosing task 2 -Inviscid Incompressible Flow as your task, now you have to list down the subtasks in that particular chapter and analyse the importance of each subtask with the previous year questions asked in that topic.and list down all the subtopics.
so for the task 2 Inviscid Incompressible flow, Sub tasks are
- Introduction to Bernoulli’s equation
- Introduction to Pitot tube
- Introduction to Laplace equation
- Types of Flows
- Kutta-Joukowski theorem
- d’Alembert’s Paradox
- Kutta condition
- Kelvin’s circulation theorem
- Classical Thin Airfoil theory
- Induced drag and Downwash
- The vortex filament
- The biot savart law
- Helmholtz theorem
- Prandtl’s lifting line theory
- Three-dimensional flows.
Hope you have understood.
Analyze the tasks and subtasks:
Take out and dust the previous year questions for it is much needed to analyze and Segregate the important topics.
Ask: which section-or concept has got more problems?
And which section you struggle the most?
(highlight and remember these topics. Since you need more attention )
Example: when you analyze the previous year question papers, on the Subject of aerodynamics, boundary layer problems have got a subsequent place.
If you are weak in this you need to have deeper learning in this boundary layer topic.
Decide when each subtask has to be completed :
Follow the seven-day rule for GATE AEROSPACE 2018 Preparation:
Day 1: organize your study material, that is you have to allow for a particular subject.Aerodynamics what are the materials you Have. Textbook or class lecture notes or some other, Downloaded from the web.
Day 2: Study with lecture notes
Day 3: Prepare with textbook
Day 4: Solve if you have got problems in class
Day 5: Try solving problems in book
Day 6: Go with previous year questions on the subtask chosen
Day 7: Review everything. If it is a highlighted topic all you have to do is to allot extra time on it.
Example: if you have chosen Classical thin airfoil theory as a subtask in Aerodynamics
You have to follow the seven-day rule allotting half an hour each day. If it is a Highlighted topic ( your weaker area) you have to allot one hour each day.
Make your own study timetable with this technique and follow it.This method will give you 100% success in GATE AEROSPACE
The books that I recommend for aerodynamics are
The method I have followed to prepare the study materials for GATE AEROSPACE 2018 propulsion.
5 Quick reference Points about Atmosphere:
- The atmosphere is the term given to the layer of air, which surrounds the Earth and extends upwards from the surface to about 500 miles.
- The Earth’s atmosphere can be said to consist of four concentric gaseous layers.
- Pressure falls steadily with height, temperature falls steadily in the troposphere, and constant in the stratosphere and decreases in the mesosphere and thereafter it increases. Density also falls steadily with height.
- The composition of Air.
- Nitrogen- 78.08
- Oxygen- 20.94
- Argon- 0.93
- Carbon dioxide- 0.03
Are you an aeronautical engineer and tired of finding free study materials for your gate aerospace 2018 preparation?
Then you are in the right place.
according to me best way to kick-start your GATE preparation is, to begin with, the best of what you know.
I have started with Aircraft propulsion since it is one among the major streams in aeronautical.
- Basics Laws of Thermodynamics
- Fundamentals of rotating machines
- introduction to axial flow compressors
- velocity triangle for axial flow compressors
Here is a set of AIRCRAFT PROPULSION Exclusive handwritten materials
Share this link to the wanted.
And I know sharing is caring, so I share my materials !!!!