AERODYNAMIC TEST FACILITIES Mechanical--Principle-of-Fluid-Dynamics- Notes | EduRev

: AERODYNAMIC TEST FACILITIES Mechanical--Principle-of-Fluid-Dynamics- Notes | EduRev

 Page 1


NPTEL – Mechanical – Principle of Fluid Dynamics 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                            Page 1 of 13 
Module 8 : Lecture 1 
AERODYNAMIC TEST FACILITIES 
(Part I) 
 
Overview 
With the advancement of aerospace vehicles, the human’s dream is to fly faster and 
higher. Speed of manned and unmanned flight vehicles has increased by some orders 
of magnitude over the last few decades. As the speed of the vehicle is increased, the 
aerodynamic environment becomes increasingly hostile. At speeds around the local 
speed of sound (Transonic) and higher (Supersonic), the aerodynamic loads increase 
and their distributions change. When the speed of the vehicle becomes several times 
higher than the speed of sound (Hypersonic), additional problem of aerodynamic 
heating demands the change in design geometry and materials. At still higher speeds 
(hypervelocity), the behavior of air begins to change significantly; both physically and 
chemically. There is a conventional ‘rule-of-thumb’ that defines the flow regimes 
based on the free stream Mach number (M
8
) i.e. 
Subsonic: 0 = M
8 
= 0.8; Transonic: 0.8 = M
8 
= 1.2 
Supersonic: 1.2 = M
8 
= 5; Hypersonic: M
8 
> 5 
  
 The past four decades have seen major flights cruising from subsonic to 
hypersonic speeds. The most routine flights made possible when American Jet 
Transport started its first flight (Boeing-707) on October 26, 1958 cruising at Mach 
0.7. Traveling at speeds faster than sound speed and thus breaking  ‘Sound Barrier’ 
became reality with the taste of supersonic travel from London to Bahrain in the 
aircraft ‘Concorde’ commenced by British airways on January 21, 1976. The first 
ever-fastest commercial passenger aircraft ‘Concorde’ with its cruising altitude 18 km 
at Mach 2, crossed Atlantic Ocean from London to New York little less than 3.5 hours 
as opposed to about eight hours for a subsonic flight. This mile stone was achieved on 
November 22, 1977. Now the age of ‘hypersonic flight’ is about to dawn with the 
evolution of orbital space crafts, hypersonic airliners and re-entry vehicles. The first 
such major landmark has been achieved after the launch of USSR satellite SPUTNIK-
II on November 3, 1957. It was the first man-made earth satellite that carried living 
organism (a dog named LAIKA) into the space and remained in orbit till April 13, 
Page 2


NPTEL – Mechanical – Principle of Fluid Dynamics 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                            Page 1 of 13 
Module 8 : Lecture 1 
AERODYNAMIC TEST FACILITIES 
(Part I) 
 
Overview 
With the advancement of aerospace vehicles, the human’s dream is to fly faster and 
higher. Speed of manned and unmanned flight vehicles has increased by some orders 
of magnitude over the last few decades. As the speed of the vehicle is increased, the 
aerodynamic environment becomes increasingly hostile. At speeds around the local 
speed of sound (Transonic) and higher (Supersonic), the aerodynamic loads increase 
and their distributions change. When the speed of the vehicle becomes several times 
higher than the speed of sound (Hypersonic), additional problem of aerodynamic 
heating demands the change in design geometry and materials. At still higher speeds 
(hypervelocity), the behavior of air begins to change significantly; both physically and 
chemically. There is a conventional ‘rule-of-thumb’ that defines the flow regimes 
based on the free stream Mach number (M
8
) i.e. 
Subsonic: 0 = M
8 
= 0.8; Transonic: 0.8 = M
8 
= 1.2 
Supersonic: 1.2 = M
8 
= 5; Hypersonic: M
8 
> 5 
  
 The past four decades have seen major flights cruising from subsonic to 
hypersonic speeds. The most routine flights made possible when American Jet 
Transport started its first flight (Boeing-707) on October 26, 1958 cruising at Mach 
0.7. Traveling at speeds faster than sound speed and thus breaking  ‘Sound Barrier’ 
became reality with the taste of supersonic travel from London to Bahrain in the 
aircraft ‘Concorde’ commenced by British airways on January 21, 1976. The first 
ever-fastest commercial passenger aircraft ‘Concorde’ with its cruising altitude 18 km 
at Mach 2, crossed Atlantic Ocean from London to New York little less than 3.5 hours 
as opposed to about eight hours for a subsonic flight. This mile stone was achieved on 
November 22, 1977. Now the age of ‘hypersonic flight’ is about to dawn with the 
evolution of orbital space crafts, hypersonic airliners and re-entry vehicles. The first 
such major landmark has been achieved after the launch of USSR satellite SPUTNIK-
II on November 3, 1957. It was the first man-made earth satellite that carried living 
organism (a dog named LAIKA) into the space and remained in orbit till April 13, 
NPTEL – Mechanical – Principle of Fluid Dynamics 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                            Page 2 of 13 
1958. In the bumper year of 1961, Yuri Gagarin (USSR) became the first man in the 
history to fly in space with an orbital space craft VOSTAK-I that entered the earth 
atmosphere at Mach 25 on April 12. His safe return from the space has inspired the 
future objectives of hypersonic flight. In the same year i.e. on June 23, U.S. air force 
test pilot Major R. White accomplished the concept of ‘miles per second’ flight in an 
X-15 airplane by flying at Mach 5.3. White again extended this record with same X-
15 flight at Mach 6. Since 1961, major space programs carried out by U.S. space 
agency NASA and Indian Space Research Organization (ISRO) have achieved 
milestones in the development of satellites and aero-assisted space transfer vehicles.  
 
The aerodynamic flow fields at very high Mach numbers experience a very high 
pressure, temperature and density. Even, the temperature rise can be so high that the 
gaseous medium gets decomposed and thereby the properties (specific heat, gas 
constant and specific heat ratio) can change as well. So, even at same free stream 
Mach number, different velocities can be obtained. Thus, the high Mach number 
flows where the behavior of the medium begins to change significantly, are normally 
classified in terms their velocities: suborbital velocities speed (4-7 km/s), super orbital 
speed (8-12 km/s) and escape velocities (>13 km/s). However, these flow conditions 
are normally achieved at different trajectories/altitudes of a flight vehicle in the earth 
atmosphere. So, the more realistic way is to simulate these conditions experimentally 
in the laboratory for entire range of operational speeds. Some of these aerodynamic 
test facilities are broadly summarized and discussed in this module.  
- Low speed wind tunnel (continuous type; up to 40 m/s) 
- High speed wind tunnel (intermittent/blow down type; Mach 3, 600m/s) 
- Shock tunnel (impulse type; Mach 7, 2km/s) 
- Free piston shock tunnel (impulse type, Mach 4-10, 5km/s) 
- Expansion tube (impulse type, Mach 10, 10km/s) 
 
 
 
 
 
 
 
Page 3


NPTEL – Mechanical – Principle of Fluid Dynamics 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                            Page 1 of 13 
Module 8 : Lecture 1 
AERODYNAMIC TEST FACILITIES 
(Part I) 
 
Overview 
With the advancement of aerospace vehicles, the human’s dream is to fly faster and 
higher. Speed of manned and unmanned flight vehicles has increased by some orders 
of magnitude over the last few decades. As the speed of the vehicle is increased, the 
aerodynamic environment becomes increasingly hostile. At speeds around the local 
speed of sound (Transonic) and higher (Supersonic), the aerodynamic loads increase 
and their distributions change. When the speed of the vehicle becomes several times 
higher than the speed of sound (Hypersonic), additional problem of aerodynamic 
heating demands the change in design geometry and materials. At still higher speeds 
(hypervelocity), the behavior of air begins to change significantly; both physically and 
chemically. There is a conventional ‘rule-of-thumb’ that defines the flow regimes 
based on the free stream Mach number (M
8
) i.e. 
Subsonic: 0 = M
8 
= 0.8; Transonic: 0.8 = M
8 
= 1.2 
Supersonic: 1.2 = M
8 
= 5; Hypersonic: M
8 
> 5 
  
 The past four decades have seen major flights cruising from subsonic to 
hypersonic speeds. The most routine flights made possible when American Jet 
Transport started its first flight (Boeing-707) on October 26, 1958 cruising at Mach 
0.7. Traveling at speeds faster than sound speed and thus breaking  ‘Sound Barrier’ 
became reality with the taste of supersonic travel from London to Bahrain in the 
aircraft ‘Concorde’ commenced by British airways on January 21, 1976. The first 
ever-fastest commercial passenger aircraft ‘Concorde’ with its cruising altitude 18 km 
at Mach 2, crossed Atlantic Ocean from London to New York little less than 3.5 hours 
as opposed to about eight hours for a subsonic flight. This mile stone was achieved on 
November 22, 1977. Now the age of ‘hypersonic flight’ is about to dawn with the 
evolution of orbital space crafts, hypersonic airliners and re-entry vehicles. The first 
such major landmark has been achieved after the launch of USSR satellite SPUTNIK-
II on November 3, 1957. It was the first man-made earth satellite that carried living 
organism (a dog named LAIKA) into the space and remained in orbit till April 13, 
NPTEL – Mechanical – Principle of Fluid Dynamics 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                            Page 2 of 13 
1958. In the bumper year of 1961, Yuri Gagarin (USSR) became the first man in the 
history to fly in space with an orbital space craft VOSTAK-I that entered the earth 
atmosphere at Mach 25 on April 12. His safe return from the space has inspired the 
future objectives of hypersonic flight. In the same year i.e. on June 23, U.S. air force 
test pilot Major R. White accomplished the concept of ‘miles per second’ flight in an 
X-15 airplane by flying at Mach 5.3. White again extended this record with same X-
15 flight at Mach 6. Since 1961, major space programs carried out by U.S. space 
agency NASA and Indian Space Research Organization (ISRO) have achieved 
milestones in the development of satellites and aero-assisted space transfer vehicles.  
 
The aerodynamic flow fields at very high Mach numbers experience a very high 
pressure, temperature and density. Even, the temperature rise can be so high that the 
gaseous medium gets decomposed and thereby the properties (specific heat, gas 
constant and specific heat ratio) can change as well. So, even at same free stream 
Mach number, different velocities can be obtained. Thus, the high Mach number 
flows where the behavior of the medium begins to change significantly, are normally 
classified in terms their velocities: suborbital velocities speed (4-7 km/s), super orbital 
speed (8-12 km/s) and escape velocities (>13 km/s). However, these flow conditions 
are normally achieved at different trajectories/altitudes of a flight vehicle in the earth 
atmosphere. So, the more realistic way is to simulate these conditions experimentally 
in the laboratory for entire range of operational speeds. Some of these aerodynamic 
test facilities are broadly summarized and discussed in this module.  
- Low speed wind tunnel (continuous type; up to 40 m/s) 
- High speed wind tunnel (intermittent/blow down type; Mach 3, 600m/s) 
- Shock tunnel (impulse type; Mach 7, 2km/s) 
- Free piston shock tunnel (impulse type, Mach 4-10, 5km/s) 
- Expansion tube (impulse type, Mach 10, 10km/s) 
 
 
 
 
 
 
 
NPTEL – Mechanical – Principle of Fluid Dynamics 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                            Page 3 of 13 
Low Speed Wind Tunnel 
In general, the wind tunnels are the devices which provide an airstream flowing under 
controlled conditions for external/internal flow simulations in the laboratory. The 
most fundamental experiments undertaken in the wind tunnel are the force/heat 
transfer measurement and flow visualization on aerodynamic models. The flows 
generated in the test section of the tunnel may be laminar/turbulent, steady/unsteady 
etc. The other features may be study of boundary layer separation, vortex flow 
generation etc. The low speed wind tunnels limit their speed to 50-60 m/s and based 
on the need, the tunnel may be designed.  Depending on the discharge of the air flow 
to atmosphere or recirculation of air, it is classified as open or closed circuit wind 
tunnels (Fig. 8.1.1).  
 
Fig. 8.1.1: Schematic representation of wind tunnel: (a) open circuit wind tunnel; 
(b) closed circuit wind tunnel. 
 
 
Page 4


NPTEL – Mechanical – Principle of Fluid Dynamics 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                            Page 1 of 13 
Module 8 : Lecture 1 
AERODYNAMIC TEST FACILITIES 
(Part I) 
 
Overview 
With the advancement of aerospace vehicles, the human’s dream is to fly faster and 
higher. Speed of manned and unmanned flight vehicles has increased by some orders 
of magnitude over the last few decades. As the speed of the vehicle is increased, the 
aerodynamic environment becomes increasingly hostile. At speeds around the local 
speed of sound (Transonic) and higher (Supersonic), the aerodynamic loads increase 
and their distributions change. When the speed of the vehicle becomes several times 
higher than the speed of sound (Hypersonic), additional problem of aerodynamic 
heating demands the change in design geometry and materials. At still higher speeds 
(hypervelocity), the behavior of air begins to change significantly; both physically and 
chemically. There is a conventional ‘rule-of-thumb’ that defines the flow regimes 
based on the free stream Mach number (M
8
) i.e. 
Subsonic: 0 = M
8 
= 0.8; Transonic: 0.8 = M
8 
= 1.2 
Supersonic: 1.2 = M
8 
= 5; Hypersonic: M
8 
> 5 
  
 The past four decades have seen major flights cruising from subsonic to 
hypersonic speeds. The most routine flights made possible when American Jet 
Transport started its first flight (Boeing-707) on October 26, 1958 cruising at Mach 
0.7. Traveling at speeds faster than sound speed and thus breaking  ‘Sound Barrier’ 
became reality with the taste of supersonic travel from London to Bahrain in the 
aircraft ‘Concorde’ commenced by British airways on January 21, 1976. The first 
ever-fastest commercial passenger aircraft ‘Concorde’ with its cruising altitude 18 km 
at Mach 2, crossed Atlantic Ocean from London to New York little less than 3.5 hours 
as opposed to about eight hours for a subsonic flight. This mile stone was achieved on 
November 22, 1977. Now the age of ‘hypersonic flight’ is about to dawn with the 
evolution of orbital space crafts, hypersonic airliners and re-entry vehicles. The first 
such major landmark has been achieved after the launch of USSR satellite SPUTNIK-
II on November 3, 1957. It was the first man-made earth satellite that carried living 
organism (a dog named LAIKA) into the space and remained in orbit till April 13, 
NPTEL – Mechanical – Principle of Fluid Dynamics 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                            Page 2 of 13 
1958. In the bumper year of 1961, Yuri Gagarin (USSR) became the first man in the 
history to fly in space with an orbital space craft VOSTAK-I that entered the earth 
atmosphere at Mach 25 on April 12. His safe return from the space has inspired the 
future objectives of hypersonic flight. In the same year i.e. on June 23, U.S. air force 
test pilot Major R. White accomplished the concept of ‘miles per second’ flight in an 
X-15 airplane by flying at Mach 5.3. White again extended this record with same X-
15 flight at Mach 6. Since 1961, major space programs carried out by U.S. space 
agency NASA and Indian Space Research Organization (ISRO) have achieved 
milestones in the development of satellites and aero-assisted space transfer vehicles.  
 
The aerodynamic flow fields at very high Mach numbers experience a very high 
pressure, temperature and density. Even, the temperature rise can be so high that the 
gaseous medium gets decomposed and thereby the properties (specific heat, gas 
constant and specific heat ratio) can change as well. So, even at same free stream 
Mach number, different velocities can be obtained. Thus, the high Mach number 
flows where the behavior of the medium begins to change significantly, are normally 
classified in terms their velocities: suborbital velocities speed (4-7 km/s), super orbital 
speed (8-12 km/s) and escape velocities (>13 km/s). However, these flow conditions 
are normally achieved at different trajectories/altitudes of a flight vehicle in the earth 
atmosphere. So, the more realistic way is to simulate these conditions experimentally 
in the laboratory for entire range of operational speeds. Some of these aerodynamic 
test facilities are broadly summarized and discussed in this module.  
- Low speed wind tunnel (continuous type; up to 40 m/s) 
- High speed wind tunnel (intermittent/blow down type; Mach 3, 600m/s) 
- Shock tunnel (impulse type; Mach 7, 2km/s) 
- Free piston shock tunnel (impulse type, Mach 4-10, 5km/s) 
- Expansion tube (impulse type, Mach 10, 10km/s) 
 
 
 
 
 
 
 
NPTEL – Mechanical – Principle of Fluid Dynamics 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                            Page 3 of 13 
Low Speed Wind Tunnel 
In general, the wind tunnels are the devices which provide an airstream flowing under 
controlled conditions for external/internal flow simulations in the laboratory. The 
most fundamental experiments undertaken in the wind tunnel are the force/heat 
transfer measurement and flow visualization on aerodynamic models. The flows 
generated in the test section of the tunnel may be laminar/turbulent, steady/unsteady 
etc. The other features may be study of boundary layer separation, vortex flow 
generation etc. The low speed wind tunnels limit their speed to 50-60 m/s and based 
on the need, the tunnel may be designed.  Depending on the discharge of the air flow 
to atmosphere or recirculation of air, it is classified as open or closed circuit wind 
tunnels (Fig. 8.1.1).  
 
Fig. 8.1.1: Schematic representation of wind tunnel: (a) open circuit wind tunnel; 
(b) closed circuit wind tunnel. 
 
 
NPTEL – Mechanical – Principle of Fluid Dynamics 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                            Page 4 of 13 
Remarks for open/closed circuit wind tunnel 
- In a closed circuit wind tunnel, the high quality flow can be assured in the test 
section and power requirement is less as compared to open circuit wind tunnel. 
However, it is not suitable for smoke flow visualization and incurs high 
capital/construction cost.  
- An open circuit wind tunnel is more adaptive for flow visualization 
experiments because due to its direct connection with the atmosphere. In order 
to assure the flow quality in the test section, one needs to install special 
devices such as flow straightener explicitly aligning the flow axially. Of 
course, it requires more power as compared to closed circuit wind tunnel.    
 
Wind tunnel components 
The important components of the wind tunnel are listed below;  
Motor/Fan Driven unit: This is the air supply unit that drives the air flow in the wind 
tunnel. Typically, the fan is axial/centrifugal type and the axial fan is a better choice 
in the closed circuit tunnels since it produces a static pressure rise necessary to 
compensate for the total pressure loss in the rest of the circuit. The fans with higher 
ratio of tip speed to axial velocity generally produce the required pressure rise in a 
small blade area. The wind tunnels fitted with blower are generally driven by a 
centrifugal impeller of squirrel-cage type. While in operation, the fan draws air from 
the atmosphere through the honeycomb/screen section.  
 
Fig. 8.1.2: Honeycomb structures for low speed wind tunnels. 
 
Page 5


NPTEL – Mechanical – Principle of Fluid Dynamics 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                            Page 1 of 13 
Module 8 : Lecture 1 
AERODYNAMIC TEST FACILITIES 
(Part I) 
 
Overview 
With the advancement of aerospace vehicles, the human’s dream is to fly faster and 
higher. Speed of manned and unmanned flight vehicles has increased by some orders 
of magnitude over the last few decades. As the speed of the vehicle is increased, the 
aerodynamic environment becomes increasingly hostile. At speeds around the local 
speed of sound (Transonic) and higher (Supersonic), the aerodynamic loads increase 
and their distributions change. When the speed of the vehicle becomes several times 
higher than the speed of sound (Hypersonic), additional problem of aerodynamic 
heating demands the change in design geometry and materials. At still higher speeds 
(hypervelocity), the behavior of air begins to change significantly; both physically and 
chemically. There is a conventional ‘rule-of-thumb’ that defines the flow regimes 
based on the free stream Mach number (M
8
) i.e. 
Subsonic: 0 = M
8 
= 0.8; Transonic: 0.8 = M
8 
= 1.2 
Supersonic: 1.2 = M
8 
= 5; Hypersonic: M
8 
> 5 
  
 The past four decades have seen major flights cruising from subsonic to 
hypersonic speeds. The most routine flights made possible when American Jet 
Transport started its first flight (Boeing-707) on October 26, 1958 cruising at Mach 
0.7. Traveling at speeds faster than sound speed and thus breaking  ‘Sound Barrier’ 
became reality with the taste of supersonic travel from London to Bahrain in the 
aircraft ‘Concorde’ commenced by British airways on January 21, 1976. The first 
ever-fastest commercial passenger aircraft ‘Concorde’ with its cruising altitude 18 km 
at Mach 2, crossed Atlantic Ocean from London to New York little less than 3.5 hours 
as opposed to about eight hours for a subsonic flight. This mile stone was achieved on 
November 22, 1977. Now the age of ‘hypersonic flight’ is about to dawn with the 
evolution of orbital space crafts, hypersonic airliners and re-entry vehicles. The first 
such major landmark has been achieved after the launch of USSR satellite SPUTNIK-
II on November 3, 1957. It was the first man-made earth satellite that carried living 
organism (a dog named LAIKA) into the space and remained in orbit till April 13, 
NPTEL – Mechanical – Principle of Fluid Dynamics 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                            Page 2 of 13 
1958. In the bumper year of 1961, Yuri Gagarin (USSR) became the first man in the 
history to fly in space with an orbital space craft VOSTAK-I that entered the earth 
atmosphere at Mach 25 on April 12. His safe return from the space has inspired the 
future objectives of hypersonic flight. In the same year i.e. on June 23, U.S. air force 
test pilot Major R. White accomplished the concept of ‘miles per second’ flight in an 
X-15 airplane by flying at Mach 5.3. White again extended this record with same X-
15 flight at Mach 6. Since 1961, major space programs carried out by U.S. space 
agency NASA and Indian Space Research Organization (ISRO) have achieved 
milestones in the development of satellites and aero-assisted space transfer vehicles.  
 
The aerodynamic flow fields at very high Mach numbers experience a very high 
pressure, temperature and density. Even, the temperature rise can be so high that the 
gaseous medium gets decomposed and thereby the properties (specific heat, gas 
constant and specific heat ratio) can change as well. So, even at same free stream 
Mach number, different velocities can be obtained. Thus, the high Mach number 
flows where the behavior of the medium begins to change significantly, are normally 
classified in terms their velocities: suborbital velocities speed (4-7 km/s), super orbital 
speed (8-12 km/s) and escape velocities (>13 km/s). However, these flow conditions 
are normally achieved at different trajectories/altitudes of a flight vehicle in the earth 
atmosphere. So, the more realistic way is to simulate these conditions experimentally 
in the laboratory for entire range of operational speeds. Some of these aerodynamic 
test facilities are broadly summarized and discussed in this module.  
- Low speed wind tunnel (continuous type; up to 40 m/s) 
- High speed wind tunnel (intermittent/blow down type; Mach 3, 600m/s) 
- Shock tunnel (impulse type; Mach 7, 2km/s) 
- Free piston shock tunnel (impulse type, Mach 4-10, 5km/s) 
- Expansion tube (impulse type, Mach 10, 10km/s) 
 
 
 
 
 
 
 
NPTEL – Mechanical – Principle of Fluid Dynamics 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                            Page 3 of 13 
Low Speed Wind Tunnel 
In general, the wind tunnels are the devices which provide an airstream flowing under 
controlled conditions for external/internal flow simulations in the laboratory. The 
most fundamental experiments undertaken in the wind tunnel are the force/heat 
transfer measurement and flow visualization on aerodynamic models. The flows 
generated in the test section of the tunnel may be laminar/turbulent, steady/unsteady 
etc. The other features may be study of boundary layer separation, vortex flow 
generation etc. The low speed wind tunnels limit their speed to 50-60 m/s and based 
on the need, the tunnel may be designed.  Depending on the discharge of the air flow 
to atmosphere or recirculation of air, it is classified as open or closed circuit wind 
tunnels (Fig. 8.1.1).  
 
Fig. 8.1.1: Schematic representation of wind tunnel: (a) open circuit wind tunnel; 
(b) closed circuit wind tunnel. 
 
 
NPTEL – Mechanical – Principle of Fluid Dynamics 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                            Page 4 of 13 
Remarks for open/closed circuit wind tunnel 
- In a closed circuit wind tunnel, the high quality flow can be assured in the test 
section and power requirement is less as compared to open circuit wind tunnel. 
However, it is not suitable for smoke flow visualization and incurs high 
capital/construction cost.  
- An open circuit wind tunnel is more adaptive for flow visualization 
experiments because due to its direct connection with the atmosphere. In order 
to assure the flow quality in the test section, one needs to install special 
devices such as flow straightener explicitly aligning the flow axially. Of 
course, it requires more power as compared to closed circuit wind tunnel.    
 
Wind tunnel components 
The important components of the wind tunnel are listed below;  
Motor/Fan Driven unit: This is the air supply unit that drives the air flow in the wind 
tunnel. Typically, the fan is axial/centrifugal type and the axial fan is a better choice 
in the closed circuit tunnels since it produces a static pressure rise necessary to 
compensate for the total pressure loss in the rest of the circuit. The fans with higher 
ratio of tip speed to axial velocity generally produce the required pressure rise in a 
small blade area. The wind tunnels fitted with blower are generally driven by a 
centrifugal impeller of squirrel-cage type. While in operation, the fan draws air from 
the atmosphere through the honeycomb/screen section.  
 
Fig. 8.1.2: Honeycomb structures for low speed wind tunnels. 
 
NPTEL – Mechanical – Principle of Fluid Dynamics 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                            Page 5 of 13 
Settling chamber and flow straightener: It mainly comprises of honeycomb and 
screens as combination. The main function is to reduce the turbulence and straighten 
the flow only in the axial direction. In principle, the air can enter to the tunnel from 
any directions. But, only the axial flow is desired in the test section. The main purpose 
of the screen is to reduce the turbulent intensity in the flow and not to allow any 
unwanted objects to enter the tunnels. The honeycomb can be made with cells or 
various shapes as shown in Fig. 8.1.2. These cells are aligned in the stream wise 
direction in the settling chamber thereby straightens the flow. The honeycomb has a 
longer length that reduces the transverse velocity components of the flow with 
minimal pressure drop in the stream wise direction. The minimum length required for 
this honeycomb is six to eight times the cell size. The number of screens required in 
the settling chamber depends on the flow quality requirement in the test section. 
Moreover, the power requirement is more when the number of screens is increased. 
The preferable length of the settling chamber is about 0.5 times the diameter of its 
inlet.  
Contraction: The prime objective is to accelerate incoming flow from the settling 
chamber and supplies it to the test section at desired velocity. This section essentially 
reduces the cross-sectional velocity variation and maintains the flow uniformity. In 
general, small radius of curvature is used at the entry to this section and curvature of 
large radius is considered at the exit of the contraction section. However, the 
boundary-layer separation should be avoided at both the ends of this section. The 
contraction length is expected to be small so that large contraction area ratios are 
preferred.  
Test Section: It is the basic element of wind tunnel on which all other designs are 
generally made. All the aerodynamic models are mounted in the test section when the 
tunnel is operated with desired flow velocity. Various shapes for the test section are 
considered for constructing the wind tunnel viz. hexagonal, octagonal, rectangle etc. 
The test section is generally designed on the basis of utility and aerodynamic 
considerations since cost of construction depends on the test section area. Length of 
the test section is mostly equal to major dimension of the cross-section of the same or 
twice of it. In addition, the test section should also be provided with facilities as per 
the testing requirement. The test section velocity is generally specified as percentage 
variation from the average of the cross-section. The ideal test section has steady 
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