What is Blunt Body

7/22/2019 What is Blunt Body

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ACKNOWLEDGEMENT

ABSTRACT

TABLE OF CONTENT

INTRODUCTION

HISTORICAL BACKGROUND

PROJECT METHODOLOGY

RESULT AND DISCUSSION

CONCLUSION


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Acknowledgments

First of all we would like to thank our family which supported us during all our studies,without them we would have had much more difficulties finishing our education.

It is a pleasure to express our thanks to ----------for the encouragement and guidancethroughout the course of this project. We would like to express our deep sense ofgratitude and admiration to Head Of Aeronautical Engineering Department. ourgratitude goes also to our teachers for their valuable advices and help.

We would also like to thank our team members ( Ankush ,Nitin sharma,Rohit kumar,

and Md. fahad khan) who supported our team and made us feel happy especially during

the difficult times.

Ankush

Nitin Sharma

Rohit Kumar

Md.Fahad khan


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Abstract

Now a days hypersonic flights have become a dominant feature of aerodynamics.

Aerodynamicists mostly use slender pointed body. Because more pointed and slender

the body, the shock wave attached to the nose will be weaker and the wave drag

associated with the body will also be less. But when the flights are in hypersonic

speeds, there will be sudden change in certain factors, like in intercontinental ballistic

missiles (ICBM).These vehicles are designed to cruise outside the earth atmosphere.

When these vehicles reenter the earth atmosphere, the cruise speeds will

approximately be of 20000 to 22000 ft/sec. At this hypersonic speeds, aerodynamic

heating is the severe problem and it will give quite influence over the design of the

proper nose cone and vehicle. So aerodynamicists prefer the blunt body concept for the

reentry vehicle.


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INTRODUCTION

Now a days hypersonic flights have become a dominant feature of aerodynamics.

Aerodynamicists mostly use slender pointed body. Because more pointed and slender

the body, the shock wave attached to the nose will be weaker and the wave drag

associated with the body will also be less. But when the flights are in hypersonic

speeds, there will be sudden change in certain factors, like in intercontinental ballistic

missiles (ICBM).These vehicles are designed to cruise outside the earth atmosphere.

When these vehicles reenter the earth atmosphere, the cruise speeds will

approximately be of 20000 to 22000 ft/sec. At this hypersonic speeds, aerodynamic

heating is the severe problem and it will give quite influence over the design of the

proper nose cone and vehicle. So aerodynamicists prefer the blunt body concept for the

reentry vehicle.

As we know that at the outer edge of the atmosphere, due to its velocity and altitude,

the reentry vehicles have high amount of kinetic and potential energies. But near the

earth surface, the reentry vehicles have less kinetic energy and approximately zero

potential energy. Here all the energy changes to heat energy. Some energy goes to

heat the airflow and some to reentry vehicle. We know that in the supersonic flow, the

shock waves are generated in the flow and a large temperature gradient is generated.

And at the same time, the body gets heated by the frictional dissipation in the boundary

layer adjacent to the body. But by the using some special type of bodies, we can dump

the energy into the airflow. These bodies are called blunt body which creates a stronger

shock wave. Studies show that the heating of airflow can be enlarged by creating a

stronger shock wave at the nose. So accordingly the shape of the nose should be suchthat a stronger shock is formed at the nose. Here comes the use of Blunt bodies.

Studies shows that the heat load experienced by any reentry vehicle is inversely

proportional to the drag coefficient experienced by it. Greater the drag, the less the heat

load. By the use of Blunt reentry vehicle, air can't "get out of the way" quickly enough.

And so vehicle acts as an air cushion to push the heated shock layer forward (away

from the vehicle). Since most of the hot gases are not in contact with the vehicle, the

heat energy will stay in the shocked gas and simply move around the vehicle to later

dissipate into the atmosphere.


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Historical Background

Contribution of Harry Julian Allen

Harry Julian Allen(1 April 191029 January 1977), also known as Harvey Allen, was

an aeronautical engineer and a Director of the NASAAmes Research Center, most

noted for his "Blunt Body Theory" of re-entry aerodynamics which permitted successful

recovery of orbiting spacecraft. His technique is still used to this day.

H. Julian Allen is best known for his "Blunt Body Theory" of aerodynamics, a design

technique for alleviating the severe re-entry heating problem which was then delaying

the development of ballistic missiles. His findings revolutionized the fundamental design

of ballistic missle re-entry shapes. Subsequently, applied research led to applications of

the "blunt" shape to ballistic missles and spacecraft which were intended to re-enter the

Earth's atmosphere. This application led to the design of ablative heat shields that

protected the Mercury, Gemini and Apollo astronauts as their space capsules re-

entered the Earth's atmosphere

Allen was interested in the full range of aerodynamics research, and made contributions

to the study of subsonic, transonic, supersonicand hypersonic flow. When the United

Statesbecame interested in the design of ballistic missiles, Allen began research in the

dynamics and thermodynamics of atmospheric reentry, as well as the effects

of radiation and meteoriteson space vehicles. His most significant contribution in this

area was the idea of using a blunt nose for reentry vehicles, otherwise known as his

"Blunt Body Theory". Earlier ballistic missiles, developed by both the United States and

the Soviet Union, featured long nose cones with very narrow tips, which had relatively

low dragwhen entering the atmosphere at high speeds. However, Allen demonstrated

that a blunt body, although it had greater drag, would have a detached shock

wavewhich would transfer far less heat to the vehicle than the traditional shape with its

attached shock wave. Excessive heating was the greatest concern in the design of

ballistic missiles and spacecraft, since it could melt their surface; the blunt body design

solved this problem. Allen's theory led to the design of ablative heat shieldsthatprotected the astronauts of the Mercury, GeminiandApollo programs as their space

capsules re-entered the atmosphere.


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Blunt Body reentry shapesThese four shadowgraph images represent early re-entry vehicle concepts. Ashadowgraph is a process that makes visible the disturbances that occur in a fluid flowat high velocity, in which light passing through a flowing fluid is refracted by the densitygradients in the fluid resulting in bright and dark areas on a screen placed behind thefluid. H. Julian Allen pioneered and developed the Blunt Body Theory which madepossible the heat shield designs that were embodied in the Mercury, Gemini and Apollospace capsules, enabling astronauts to survive the firey re-entry into Earth'satmosphere. A blunt body produces a shockwave in front of the vehicle--visible in thephoto--that actually shields the vehicle from excessive heating. As a result, blunt bodyvehicles can stay cooler than pointy, low drag vehicles.


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AERODYNAMIC HEATING PROBLEM

When an object cruise at very high suborbital speed (Hypersonic) , the shock waveattached to it is very strong. And consequently the temperature behind the shock willbe extraordinarily high. And this heat inputs directly to the body.

For example, during the suborbital speed 11.2 km/s the air temperature behind theshock wave reaches the 11,000K higher than the surface of the sun. At these hightemperatures, the air itself breaks down and the O2 and N2 molecules dissociate intoO and N atoms and ions respectively. The air becomes chemically reacting gas andthe heat inputs directly to the vehicle itself. As shown in the fig 1.1, the vehicle issheathed in a layer of hot air, first from the hot shock layer at the nose, then from thehot boundary layer on the forward and rearward surface .These hot gases then flow

downstream in the wake of the vehicle .So the major objective of entry vehicle design isto shield the vehicle from the severe aerodynamic heating.

The objective of full reentry vehicle design is to minimize the heat which goes into thevehicle and maximize that which go into the air. So properly designed reentry vehiclewill work as heat shield and reduces the aerodynamic drag.


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shock wave formation in front of a blunt body

What is blunt body?

With reference to aeronautics, a blunt body is a body in which the pressure drag is

large in comparison to the skin friction drag.

For example:

A blunt body fired from a gun against a supersonic flow in a wind tunnel, producing a

bow shock.

A bow shock, also called a detached shock, is a curved, stationary shock wave that is

found in supersonic flow past a finite body.

Unlike an oblique shock, the bow shock is not necessarily attached to the tip of the

body. Oblique shock angles are limited in formation and are based on the flowdeflection angle, upstream Mach number.

When these limitations are exceeded (greater deflection angle or lower Mach number),a detached bow shock forms instead of an oblique shock. As bow shocks form for highflow deflection angles, they are often seen forming around blunt objects.

Downstream of the shock, the flow-field is subsonic, and the boundary condition can be

respected at the stagnation point.

The bow shock significantly increases the drag in a vehicle traveling at a supersonic

speed. This property was utilized in the design of the return capsules during space

missions such as the Apollo program, which need a high amount of drag in order to

slow down during atmospheric reentry.


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FLOW FIELD AROUND THE BLUNT BODIES

Due to bluntness of the body, the deflection angle of the body will be large and thestrong bow shock will generate in the front of the body. This shock will produce a largegradient of flow property across itself. At the apex of the body the substantial portion of

the wave can be assumed as normal shock, and this portion of the wave will bestrongest. Due to this normal shock, temperature of extensive region of the air will behigh, and much of this high temperature air will simply flow past the body withoutencountering the surface. A blunt body will deposit much of its initial kinetic energy andpotential energies into heating the air, and little into heating the body .In this fashion, ablunt body tends to minimize the total heat input to the vehicle. After flow passingthrough nose, a thin hot boundary layer will generate adjacent to the surface and theflow will separate further and a hot wake region will be form as shown in figure.

Detached Shock in front of a Blunt Body


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Gas Dynamics

Before going further it is necessary to review some thermodynamic properties.We knowthat fluids are classified as Incompressibleand Compressiblefluids. Incompressiblefluids do not undergo significant changes in density as they flow. In general, liquids are

incompressible;water being an excellent example. In contrast compressible fluids doundergo density changes. Gases are generally compressible;air being the mostcommon compressible fluid we can find. Compressibility of gases leads to manyinteresting features such as shocks, which are absent for incompressible fluids.Gasdynamics is the discipline that studies the flow of compressible fluids and forms animportant branch of Fluid Mechanics.

Equations of Motion for a Compressible Flow

We now write the equations of motion for a compressible flow. Recall that for anincompressible flow one calculates velocity from continuity and other considerations.Pressure is obtained through the Bernoulli Equation. Such a simple approach is notpossible for a compressible flow where temperature is not a constant. One needs tosolve the energy equation in addition to the continuity and momentum equations. Thelatter equations have already been derived for incompressible flows. Of course, one hasto account for the changes in density. We focuss here on the energy equation andbriefly outline the other two. We restrict ourselves to an Integral Approach and write theequations for a control volume.

Equations are derived under the following assumptions.

Flow is one-dimensional. Viscosity and Heat Transfer are neglected. Behaviour of flow as a consequence of area changes only considered. The flow is steady.


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We consider a one-dimensional control volume as shown.

Control Volume for a Compressible Flow

Continuity Equation

For a steady flow it is obvious that the mass flow rates at entry (1) and exit (2) of thecontrol volume must be equal. Hence,

(1.1)

Written in a differential form the above equation becomes,

(1.2)


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At this stage, it is usual to consider some applications of the above equation. But wenote that this equation has always to be solved with momentum and energy equationswhile calculating any flow. Accordingly, we skip any worked example at this stage.

Momentum Equation

The derivation of Momentum Equation closely follows that for incompressible flows.Basically, it equates net force on the control volume to the rate of change of momentum.Defining pmas the average pressure between the entry (1) and exit (2),(see Fig.above),we have for a steady flow,

(1.3)

For a steady flow through a duct of constant area the momentum equation assumes asimple form,

(1.4)

It is to be noted that the above equations can be applied even for the cases wherefrictional and viscous effects prevail between (1) and (2). But it is necessary that theseeffects be absent at (1) and (2).

Energy Equation

From the first law of Thermodynamics it follows that, for a unit mass,

q + work done = increase in energy (1.5)

where qis the heat added. Work done is given by

p1v1- p2v2 (1.6)

We consider only internal and kinetic energies. Accordingly, we have,


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Change of energy =

(1.7)

Substituting Eqns. 1.6 and 1.7 into Eqn. 1.5 we have as the energy equation for a gasflow as,

(1.8)

Noting that enthalpy, h = e + p.v, we have

Considering an adiabatic process, q = 0, we have,

(1.9)

This equation demands that the states (1) and (2) be in equilibrium, but does allow non-

equilibrium conditions between (1) and (2).

If the flow is such that equilibrium exists all along the path from (1) to (2) then we have,at any location along 1-2,

= constant

(1.10)

Differentiating the above equation, we have,

(1.11)


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For a thermally perfect gas i.e., enthalpy, hdepends only on temperature, T(h =cpT) the

above equation becomes,

(1.12)

Further, for a calorically perfect gas, i.e., cpis constant, we have,

Stagnation Conditions

Stagnation conditions are reached when the flow is brought to rest,i.e., u = 0.Temperature, pressure, density, entropy and enthalpy become equal to "Stagnation

Temperature" , T0, "Stagnation Pressure", p0, "Stagnation Density", , "StagnationEntropy", s0and "Stagnation Enthalpy, s0. These are also known as "Total" conditions.


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Shock Wave Basics

The two main types of shock waves produced by a supersonic body are Normal Shocksand Oblique Shocks. These two types are produced by the same phenomena, but theshape of the supersonic object dictates which type of shock wave is experienced.

Normal Shock Waves

A Normal Shockis created by a blunt body in supersonic flow. The same body in asubsonic flow produces waves of sound that propagate ahead of the body, basicallywarning the approaching air stream of the approaching body. These sound wavescause the molecules in the air stream to begin to diverge around the body well inadvance of the actual body. When the object is traveling supersonically, however,these sound waves cannot outrun the object, and they pile up a short distance in frontof the object. This stacking of sound waves is a Normal shock wave, and it serves toinstantaneously force the air to change direction around the body. This effect is also

referred to as a Bow Shock, and is shown in the figure below, depicting a supersonicbullet.

As a unit of air passes through the Normal shock wave, its temperature, pressure,and density dramatically rise as its velocity falls. In the case of the Normal Shock, theair flow downstream of the shock (and therefore seen by the bullet)is always subsonic.

A Normal Shock, though, is generally a special case of a common Oblique Shock thattypically occurs on supersonic airplanes and rockets.

Oblique Shock Waves

An Oblique Shockis a sharp edged shock wave that is formed when supersonic flowis turned on itself. These shocks are weaker than Normal Shocks, and although thetemperature, pressure, density, and air stream velocity are reduced across the shocksimilar to the Normal Shock, the air stream behind the shock is not necessarilysubsonic. The Mach number behind the Oblique shock is calculated from theupstream Mach number, defined by the angle at which the flow is tuned.

The figure below shows a typical oblique shock formed by a sharp angle.


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The next figure shows the companion of oblique shocks, the Expansion Fan. Theexpansion fan is essentially an infinite number of Mach Waves, and has the oppositeeffects of an oblique shock. When the airflow is turned around a corner, thetemperature, pressure, and density fall as the Mach number rises.

These two typical shock waves formations are experienced in series on supersonicairplanes and rockets, and they dictate the air properties down the length of thevehicle.


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References

1. http://www.princeton.edu/~asmits/Bicycle_web/blunt.html2. 3. http://web.mit.edu/16.unified/www/FALL/fluids/Lectures/f05.pdf3. Fundamentals of Aerodynamics by J.D.Andreason

4. Wiki.answers.com about Blunt Body5. http://en.wikipedia.org/wiki/Atmospheric_entry 6. en.wikipedia.org/wiki/Harry_Julian_Allen 7. http://commons.wikimedia.org/wiki/File:Blunt_body_reentry_shapes.png8. http://www.aeromech.usyd.edu.au/aero/gasdyn/conststagn.html9. http://www.aeromech.usyd.edu.au/aero/gasdyn/node1.html10. http://www.asa-houston.org/technical/aerodynamics.htm

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