Turbulator
Encyclopedia
A turbulator is a device that turns a laminar flow
into a turbulent flow
. Turbulent flow can be desired on parts of the surface of an aircraft wing (airfoil) or in industrial applications such as heat exchanger
s and the mixing of fluids.
(an airfoil
) of an aircraft
, there is a layer of air called the boundary layer
between the wing's surface and where the air is undisturbed. Depending on the profile of the wing, the air will often flow smoothly in a thin boundary layer across much of the wing's surface. The boundary layer will be laminar near the leading edge and will become turbulent a certain distance from the leading edge depending on surface roughness and Reynolds Number (speed). However there comes a point, the separation point
, in which the boundary layer breaks away from the surface of the wing due to the magnitude of the positive pressure gradient. Beneath the separated layer, bubbles of stagnant air form, creating additional drag
because of the lower pressure in the wake behind the separation point.
These bubbles can be reduced or even eliminated by shaping the airfoil
to move the separation point downstream or by adding a device, a turbulator that trips the boundary layer into turbulence. The turbulent boundary layer contains more energy, so will delay separation until a greater magnitude of negative pressure gradient is reached, effectively moving the separation point further aft on the airfoil and possibly eliminating separation completely. A consequence of the turbulent boundary layer is increased skin friction relative to a laminar boundary layer, but this is very small compared to the increase in drag associated with separation.
In gliders
the turbulator is often a thin zig-zag strip that is placed on the upper side of the wing and sometimes on the vertical stabilizer
. The DG 300
glider used small holes in the wing surface to blow air into the boundary layer, but there is a risk that these holes will become blocked by polish, dirt and moisture.
For the aircraft with low Reynolds numbers (i.e. where minimizing turbulence and drag is a major concern) such as gliders, the small increase in drag from the turbulator at higher speeds is minor compared with the larger improvements at best glide speed, at which the glider can fly the farthest for a given height.
close to the pipe wall that is stagnant or moves at slow speed, thus acting as an insulating layer. Such heat exchangers are, for example, in domestic central heating
systems. This boundary layer can be broken or reduced in thickness if turbulators are placed in the pipe, which create a turbulent flow that reduces the boundary-layer thickness and thereby increase the heat-transfer coefficient.
Examples of turbulators for pipe flow are:
Laminar flow
Laminar flow, sometimes known as streamline flow, occurs when a fluid flows in parallel layers, with no disruption between the layers. At low velocities the fluid tends to flow without lateral mixing, and adjacent layers slide past one another like playing cards. There are no cross currents...
into a turbulent flow
Turbulence
In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic and stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time...
. Turbulent flow can be desired on parts of the surface of an aircraft wing (airfoil) or in industrial applications such as heat exchanger
Heat exchanger
A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...
s and the mixing of fluids.
Airfoil turbulators
When air flows over the wingWing
A wing is an appendage with a surface that produces lift for flight or propulsion through the atmosphere, or through another gaseous or liquid fluid...
(an airfoil
Airfoil
An airfoil or aerofoil is the shape of a wing or blade or sail as seen in cross-section....
) of an aircraft
Aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...
, there is a layer of air called the boundary layer
Boundary layer
In physics and fluid mechanics, a boundary layer is that layer of fluid in the immediate vicinity of a bounding surface where effects of viscosity of the fluid are considered in detail. In the Earth's atmosphere, the planetary boundary layer is the air layer near the ground affected by diurnal...
between the wing's surface and where the air is undisturbed. Depending on the profile of the wing, the air will often flow smoothly in a thin boundary layer across much of the wing's surface. The boundary layer will be laminar near the leading edge and will become turbulent a certain distance from the leading edge depending on surface roughness and Reynolds Number (speed). However there comes a point, the separation point
Flow separation
All solid objects travelling through a fluid acquire a boundary layer of fluid around them where viscous forces occur in the layer of fluid close to the solid surface. Boundary layers can be either laminar or turbulent...
, in which the boundary layer breaks away from the surface of the wing due to the magnitude of the positive pressure gradient. Beneath the separated layer, bubbles of stagnant air form, creating additional drag
Drag (physics)
In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...
because of the lower pressure in the wake behind the separation point.
These bubbles can be reduced or even eliminated by shaping the airfoil
Airfoil
An airfoil or aerofoil is the shape of a wing or blade or sail as seen in cross-section....
to move the separation point downstream or by adding a device, a turbulator that trips the boundary layer into turbulence. The turbulent boundary layer contains more energy, so will delay separation until a greater magnitude of negative pressure gradient is reached, effectively moving the separation point further aft on the airfoil and possibly eliminating separation completely. A consequence of the turbulent boundary layer is increased skin friction relative to a laminar boundary layer, but this is very small compared to the increase in drag associated with separation.
In gliders
Glider (sailplane)
A glider or sailplane is a type of glider aircraft used in the sport of gliding. Some gliders, known as motor gliders are used for gliding and soaring as well, but have engines which can, in some cases, be used for take-off or for extending a flight...
the turbulator is often a thin zig-zag strip that is placed on the upper side of the wing and sometimes on the vertical stabilizer
Vertical stabilizer
The vertical stabilizers, vertical stabilisers, or fins, of aircraft, missiles or bombs are typically found on the aft end of the fuselage or body, and are intended to reduce aerodynamic side slip. It is analogical to a skeg on boats and ships.On aircraft, vertical stabilizers generally point upwards...
. The DG 300
Glaser-Dirks DG-300
The DG-300 is a Standard Class single-seat high-performance glider built of glass-reinforced plastic. The DG-300 was designed by Wilhelm Dirks and manufactured by Glaser-Dirks Flugzeugbau's Slovenian partner company Elan . A total of 511 of all versions were built since production started in 1983...
glider used small holes in the wing surface to blow air into the boundary layer, but there is a risk that these holes will become blocked by polish, dirt and moisture.
For the aircraft with low Reynolds numbers (i.e. where minimizing turbulence and drag is a major concern) such as gliders, the small increase in drag from the turbulator at higher speeds is minor compared with the larger improvements at best glide speed, at which the glider can fly the farthest for a given height.
In pipe flow
The heat transfer coefficient for liquids and gases flowing through pipes in heat exchangers tends to be limited due to a fluid boundary layerBoundary layer
In physics and fluid mechanics, a boundary layer is that layer of fluid in the immediate vicinity of a bounding surface where effects of viscosity of the fluid are considered in detail. In the Earth's atmosphere, the planetary boundary layer is the air layer near the ground affected by diurnal...
close to the pipe wall that is stagnant or moves at slow speed, thus acting as an insulating layer. Such heat exchangers are, for example, in domestic central heating
Central heating
A central heating system provides warmth to the whole interior of a building from one point to multiple rooms. When combined with other systems in order to control the building climate, the whole system may be a HVAC system.Central heating differs from local heating in that the heat generation...
systems. This boundary layer can be broken or reduced in thickness if turbulators are placed in the pipe, which create a turbulent flow that reduces the boundary-layer thickness and thereby increase the heat-transfer coefficient.
Examples of turbulators for pipe flow are:
- Twisted-tape turbulators, a twisted ribbon that forces the fluid to move in a helicoidalHelixA helix is a type of smooth space curve, i.e. a curve in three-dimensional space. It has the property that the tangent line at any point makes a constant angle with a fixed line called the axis. Examples of helixes are coil springs and the handrails of spiral staircases. A "filled-in" helix – for...
path rather than in a straight line; - Brock turbulators, a zig-zag folded ribbon;
- Wire turbulators, typically an open structure of looped and entangled wires that extends over the entire pipe length.
See also
- Vortex generatorVortex generatorA vortex generator is an aerodynamic surface, consisting of a small vane or bump that creates a vortex. Vortex generators can be found on many devices, but the term is most often used in aircraft design....
- aerodynamicsAerodynamicsAerodynamics is a branch of dynamics concerned with studying the motion of air, particularly when it interacts with a moving object. Aerodynamics is a subfield of fluid dynamics and gas dynamics, with much theory shared between them. Aerodynamics is often used synonymously with gas dynamics, with...
- Boundary layer controlBoundary layer controlBoundary layer control refers to methods of controlling the behaviour of fluid flow boundary layers. This holds particular interest in aeronautical engineering because drag may be reduced whilst achieving high lift ....
- Circulation control wingCirculation control wingA circulation control wing is a form of high-lift device for use on the main wing of an aircraft to increase the lift coefficient. CCW technology has been in the research and development phase for over sixty years, and the early models were called blown flaps.The CCW works by increasing the...