BLUFF BODY AERODYNAMICS

Why bluff bodies?

• One of the most obvious features of the flow around bluff bodies is the formation of strong large vortices in their wakes.

• These have a large impact on the wind loading of tall buildings and bridges, particularly the loading in the across-wind direction.

• However, measures can be taken to reduce the effects of vortex shedding, including shape changes and supplemental damping devices.

• Wind turbulence is also a parameter that affects vortex shedding strongly and this can be sensitive to the terrain around the structure.

• Another feature of bluff body flows is that some of them are Reynolds number sensitive.

• Aerodynamic bodies those characterized by thin boundary layers completely attached over their whole surface, which leave behind them thin and generally steady wakes containing vorticity. The aerodynamic forces acting on these bodies may be evaluated through simplified potential flow-boundary layer procedure.

• bluff bodies are characterized by a more or less precocious separation of the boundary layer from their surface, and by wakes having significant lateral dimensions and normally unsteady velocity fields.

No mathematical solutions

Solution throughout Complete Navier-Stokes equation, Experiments

• A bluff body is one in which the length in the flow direction is close or equal to the length perpendicular to the flow direction.

Buildings, Stadiums, Bridges etc.

• A great deal of aircraft aerodynamics is in smooth flow, and is two dimensional.

• The atmosphere in which buildings are placed is turbulent and three dimensional.

• The discussion will be taken in two parts, in the first the flow will be assumed to be two dimensional, and study the effects of turbulence.

Aerodynamics-smooth flow, 2D, no flow separation

Wind Engineering/Building aerodynamics-turbulent flow, 3D

BOUNDARY LAYER

• When a real fluid flows past a solid body or a solid wall the fluid particles adhere to the boundary

• Condition of no slip occurs

• Velocity of the fluid particle will be same as that of the boundary

• If the boundary is stationary then u=0, v=0.

• Further away from the boundary the velocity will be higher and as a result the velocity gradient exists.

• A very thin region of the fluid where viscous forces are dominant is called boundary layer.

BOUNDARY LAYER SEPARATION

• Consider the inviscid region

• Euler’s equations

• Acceleration and deceleration in inviscid flow is the pressure term, ∇p.

• Pressure gradient dp/dx < 0 to accelerate the fluid

• Pressure gradient dp/dx > 0 to decelerate the fluid

• Bernouilli’s equation

• Pressure energy=kinetic energy

• Kinetic energy=pressure energy

• Since no dissipation in an inviscid fluid/energy loss.

BOUNDARY LAYER SEPARATION

• Adverse pressure gradient, dp/dx > 0, must exist round the back of a blunt obstacle.

• Energy is dissipated by viscous drag: the fluid can therefore be left with insufficient energy to reach F so that it halts at some “separation” point S, beyond which it reverses and peels away from the surface.

• Once separation has occurred, the flow behind the separation point typically comprises a vortex filled wake that differs drastically from the predictions of inviscid theory.

• In particular, it exerts a suction drag on the body that is typically much larger than any viscous drag exerted by the boundary layer itself.

• The boundary layer is a thin layer of air adjacent to the surface in which the air speed increases from zero on the surface to its value outside. All the shear is contained within the boundary layer, so the Potential Flow is free of shear.

• Within the boundary layer the flow experiences a frictional force at the surface which dissipates energy, and the boundary layer is the region containing all the air which has lost energy.

• This loss in energy is transmitted through the boundary layer by viscosity.

• In the case of turbulent flow, by the movement of elements of fluid from a region of one air speed to another, and so a turbulent boundary layer is thicker than a laminar one but is better able to transmit energy from the potential flow outside to the surface.

• If the flow outside the boundary layer is accelerating, then the kinetic energy of the air there is increasing, and can usually supply the loss of energy at the surface.

• If, however, the flow outside is decelerating, then there is less energy to feed back to the surface, and the energy of the air over a finite thickness very close to the surface will ultimately fall to zero.

• In the laminar condition, in which the fluid particles move one over the other in steady layers.

• However, when the Reynolds number exceeds a certain critical value, then the laminar flow becomes unstable to perturbations, and, after a transition region, a new condition is developed in which the fluid particles have random fluctuations superimposed on their mean velocity.

• As a result of the larger mixing between the various layers of fluid that is produced by the macroscopic migration of fluid particles turbulent boundary layers are thicker than the laminar ones, and are characterized by an increased value of the tangential stresses at the surface.

• Turbulent boundary layers give rise to higher friction forces on the bodies.

• However, the larger energy content inside turbulent boundary layers has also the consequence that they are much more resistant to separation than laminar boundary layers, i.e. they are capable of remaining attached to the surface for larger adverse pressure gradients or for a greater surface extension for the same value of the pressure gradient.

• In laminar flow, as the separation is before the maximum width, the separated flow continues to expand and the wake is wider than the cylinder.

• But in the fully turbulent case, the separation is after the point of maximum width, so the wake continues to contract and the wake is narrow.

• Pressure distribution on a circular cylinder shows that the pressures and suctions on the front face almost balance, and that most of the drag of a circular cylinder comes from pressures on the rear face, so a wide wake means high drag, and visa versa.

• Laminar flow on one side, and turbulent flow on the other can only happen together in special circumstances.

• when they do, the flow downstream is deflected sideways-produces a cross wind force on the cylinder

VORTEX SHEDDING

• When the aspect ratio increases, the wake opens, and the former boundary layer on the front face separates and travels downstream as two shear layers, each containing vortices of a sign appropriate to their generation, that is to say, the vortices in the two shear layers are of different sign.

• Thompson's Theorem states that, if there is no circulation around a field containing a member in an airstream, then there can never be a circulation around that field.

• So, if the member sheds a vortex of strength K, then there must develop a circulation of -K around the member, so that the circulation round the field containing the member and the shed vortex shall still be zero

• Every time a vortex is shed from the member, a circulation is generated around the member. The vortex sheet breaks up into vortices alternatively from each side, and these are of opposite sign to each other. This is called a Karman Street.

• If there is a circulation of strength K around a member in an airstream of density r and velocity V, there will be a cross-wind force per unit span (L) exerted upon the body given by

L=KVr

• Over time therefore, there will be a cross-wind force of ± L/2 exerted upon the member, changing every time a vortex is shed.

• If the frequency of shedding is n, then the Strouhal Number is given by where L is the cross-wind dimension and V is the wind speed.

St=n L/V

• The frequency of excitation is dependent on wind speed.

• If the member is flexible, or can move, then a matching of the frequency of excitation to the natural frequency of the member has a dramatic effect on the behavior of the member.

Avoid vortex shedding?

The prevention of vortex shedding comes from the understanding of its generation. prevent the interaction of the two shear layers

• By placing a solid sheet between the two shear layers

• Bleeding air into the wake between the layers

Wind direction, so that the position of the sheet can be located.

In the building context, when wind can come from any direction, the prevention is impractical.

• It can also be achieved by wrapping a porous shroud of slightly larger diameter round the member, so that air can enter the annulus between the shroud and the member where the

pressure is positive, leaving where the pressure is negative, i.e. in the wake, this is almost omnidirectional. This shroud can sometimes also reduce the drag of the cylinder

FLOW SWITCHING

• When a set of similar buildings are built in a regular pattern and wind blows between them, a stable pattern usually develops, which means that the flow pattern round each building is the same.

• In some instances a stable pattern can occur with different patterns round each, and the flow pattern about pairs of buildings can "switch" from one to the other.

• The switching can either be regular with a switching frequency, or it can be random.

• The best known example of flow switching is flow around an isolated circular cylinder in laminar and turbulent flow.

• When there is a line of identical cylinders, the flow round all should be the same.

• However, providing the Reynolds Number is within a small range of values, it only takes a slight misalignment in either direction for there to develop wide wakes and narrow wakes as the Coanda effect

• This pattern can become oscillatory with flexible cylinders, because the regions with wider wakes have a higher drag, and this can cause the tube to deflect further, whilst the ones with the narrow wake will have lower drag and will deflect less.

COANDA EFFECT

• It describes how a flow impinging on the curved surface of a body will stick to the surface and enter the wake of the body.

• An example of this is when a finger is moved sideways into a stream of water flowing from a tap. A s soon as the finger touches the water, it sticks to the finger and runs behind the finger.

STRANDED CABLES

• The characteristic which makes stranded cables special is the helical form of the outer strands. The rough cross section of the cable behaves as surface roughness

• If the wind direction is inclined at a small angle to the length of the wire, the flow on one side of the cable is more normal to the strands than on the other.

• Over a small range of Reynolds Numbers the additional effective surface roughness of the strands more normal to the flow can cause transition, whilst the reduced effective roughness of the strands less normal to the flow does not cause transition.

• It is then possible to have laminar flow over one side of the cable, whilst having turbulent flow over the other.

• The net result is that the wake is deflected, and this produces a cross wind force on the cable.

• As the cable is flexible, it is able to move under the wind loads; increase and decrease of drag will allow it to move in a fore and aft movement.

• To avoid the critical value of Reynolds Number occurring in the practical range of wind speeds, a cable was produced in which the strands were machined so that they fitted together leaving a surface which had only a 0.15 mm depression between strands, and the outside surface of the strands followed the circular curvature of the whole cable.

• When made with the same helix angle, the critical wind speed was over 100 m/s, and no problem occurred in practice.

AERODYNAMICS OF TRAIN

• The importance for developing energy efficient rail vehicles is increasing with rising energy prices and the vital necessity to reduce the drag.

• Comparison of different train types like regional and high-speed trains and provides estimation for improvements of the aerodynamic drag coefficient.

• Traditionally, aerodynamic improvements of high-speed trains were in the focus of the engineering community as the resistance to motion is increasing with the square of the velocity.

• The influence of unsteady flow phenomena as well as the impact of the train’s induced flow field on humans and infrastructure has to be investigated.

• For the investigation of viscous flow effects like separation and other boundary layer phenomena, a pressurized wind tunnel can realize high Reynolds numbers without entering the compressible flow regime.

• The possibility to increase the Reynolds number in a wind tunnel is to cool down the working fluid, thus increasing the Reynolds number.

• The main advantage of cryogenic wind tunnels is the possibility of independent adjustment of the Mach and Reynolds number.

• Another possibility to increase the Reynolds number is to change the working fluid of the medium by using water instead of air.

• Human-Train Interference.

• A substantial portion of the energy provided by the substation to the line is already lost on the way to the train.

• The energy demanded by the train can be subdivided into so many parts; the main losses are due to aerodynamic drag which is the driving losses.

R = a+b* v +c * v2 + d+ e

R is the total running resistance

V is the train speed

a,b,c coefficients

SKIN FRICTION AND THE PRESSURE DRAG

• head and tail loss

• skin friction

• bogies

• protruding objects

• pantographs

• inter-car gaps

• ventilated brake

• underbelly friction

ROLLING RESISTANCE

• The force which is necessary to roll the train at zero speed on a track.

• The wheel and rail surfaces deform due to the axle load at the point of contact.

• Accordingly, the superstructure of the track deforms in a similar way.

• The magnitude of this deformation depends predominantly on the axle load and the properties of the involved material of the track and the wheel.

MOMENTUM RESISTANCE

• The mechanical resistance scales linearly with the train velocity. The term is separated into two different parts; i.e the actual mechanical resistance (b1) and the air momentum drag (b2).

b=b1+b2

• The b1-term supplements the rolling resistance with respect to modeling the wheel/track sliding friction including brake and transmission drag, the resistance of friction bearings, friction inside gear units and suspension-drag contributions. b1 depends on the specific track design and details of the suspension and transmission.

• The air-momentum drag coefficient b2 is dependent on the following parameters: brake drag, transmission losses and air momentum drag. The air momentum drag is the energy required to accelerate the mass of air intake, e.g. cooling, air conditioning, etc. to the speed of the running train.

AERODYNAMIC RESISTANCE

• The coefficient for the aerodynamic drag is calculated as follows

• The ratio between the surface of the train perpendicular to the flow direction, i.e. side walls, roof and underbelly, and the cross section area determines the relative importance of the skin friction.

• Thus, for high speed trains with approximately 200 m length it is the skin friction which dominates the drag.

• The drag of the regional train is dominated by the front and tail drag and the drag generated by the unshielded roof equipment.

• The drag value of one specific part of a train is highly influenced by the incoming flow and thus dependent on the surrounding flow which is influenced by nearby objects.

REGIONAL TRAFFIC

• To quantify the effect of reducing the aerodynamic coefficient on the energy consumption for regional traffic a driving cycle has been taken into account.

• Relatively short time where the train is running at maximum speed of 160 km/h which diminishes the importance of the aerodynamic drag and increases the relative importance of mass slightly related to the energy demand.

• A typical 70 m long regional train exhibits drag values around cd =1.1.

• The traction energy herein stands for the energy that can be metered between the line and the train.

• The regenerated energy is the energy that can be fed back to the line while the train is braking. Therefore the train has to be equipped with electrical brakes.

HIGH-SPEED TRAFFIC

• The speed profile shows a relatively short time where the train is running at maximum speed of 300 km/h.

• This is particularly important as the relative importance of the aerodynamic performance of the train related to energy demand is determined by the speed of the train and by the frequency of stops.

• The higher the top speed and the lower the number of stops the more important becomes the aerodynamic drag.

PANTOGRAPH

• A pantograph for rail lines is a hinged electric-rod device that collects electric current from overhead lines for electric trains or trams. The pantograph typically connects to a one-wire line, with the track acting as the ground wire. The term stems from the resemblance to pantograph lever-rod devices for copying handwriting and drawings.

BOGIE FAIRINGS ON THE AERODYNAMIC DRAG

• ETR 500 (Elettro Treno Rapido 500) is a family of Italian high-speed trains introduced in 1993.

• Capacity 650 passengers, maximum speed 350 km/h.

• Experiments were carried on reduced and full-scale tests carried out on the new ETR 500 high speed train at speeds up to 300 km/h.

• Reduce drag for such trains is to cover the bogie areas with smooth and streamlined surfaces.

• Measurements carried out on the first trailer without fairing and with different fairing geometries have shown that optimized fairings can decrease drag of about 20% while retrofittable fairings allow a reduction higher than 10%.

FAIRINGS

• Fairings are layers/sheets used to cover the outer layer of the body, so that the body is streamlined and will avoid the interference of the air flow with the moving bodies.

• An accurate model of the ETR 500 was built at a scale factor 1:7.5 (frontal area 0.174 m2). The ETR 500 model was composed by the locomotive and two trailer cars.

• The moving belt in the test section could drive in rotation the wheels of the model. The trailer car next to the locomotive was instrumented to measure the longitudinal force.

• Tests were carried out both with the moving belt off and working. Finally, the turbulent conditions on the tunnel boundary layer could be modified to simulate the flow conditions when the trailer car is at the rear of the train.

• A Configuration without fairings and three different fairing configurations were tested. The first fairing type is optimized for drag.

• The second fairing shape can be retrofitted on the existing ETR 500 bogies and includes the openings needed to avoid interferences.

• Finally, the third fairing configuration is the retrofittable type with taped openings.

• The fairing geometry was designed to respect the gauge limits and to avoid interference between bogie and car elements such as yaw dampers, doors, etc.

• In addition, openings on the fairings were necessary to allow, for instance, the view of the brake indicators. As a result, it was not possible to design a fairing surface optimized to reduce the aerodynamic drag.

• Different fairing geometries were realized for the bogies of the locomotives and of the trailer cars.

THE FULL SCALE TESTS AND THE MEASUREMENTS

• The maximum allowed speed in operations is 250 km/h, but speeds up to 300 km/h can be achieved under test conditions.

• Drag measurements have been undertaken on a test section which has a length of about 10 km. The maximum slope within the test section does not exceed 7.5‰ and the minimum curve radius is about 3000 m.

• Three different train configurations have been tested for a total of 95 test-runs.

• The first configuration is the standard train of 2 locomotives and 8 trailer cars without bogie fairings.

• A total of 32 test-runs has been undertaken on both directions of the line at speeds covering the range from 120 km/h to 300 km/h.

• The second configuration is the same train of 2 locomotives and 8 trailer cars with bogie fairings.

• A total of 37 test-runs has been undertaken on both directions of the line at speeds covering the range from 140 km/h to 300 km/h.

• The second configuration is the same train of 2 locomotives and 8 trailer cars with bogie fairings.

• A total of 37 test-runs has been undertaken on both directions of the line at speeds covering the range from 140 km/h to 300 km/h.

• The train speed, the train location and the wind speed have been measured to examine the train drag. The train speed and the train location have been measured.

• The wind speed has been measured by two different locations along the test section.

• All the drag measurements have been carried out according to the coasting method.

• In this method the train is accelerated as far as it achieves the target speed.

• This speed is maintained as far as the train enters the test section; then the train coasts all along the test section.

• When the target speed is not reached at the entering of the test section, the train continues to accelerate up to the target speed and then the train coats as far as the end of the test section.

• In this case the track length where the drag data are available is lower than the test section.

DRAG CALCULATION

• The train drag is generated by the friction in the wheel rail contact, the friction in curve riding; the aerodynamic forces and the gravity force when there is a gradient on the track.

• The train drag R is defined as the sum of wheel rail forces and aerodynamic forces. It is usually described by the following function of the train speed:

• where A, B and C are constant coefficients.

• When the train coasts on a track section with slope i , the train speed changes from Vn at the initial location of the section to Vn+1 at the final location. On the straight track the energy equation of the train can be approximated as below:

Where,

i slope of the track section (positive if the track climbs);

g gravity acceleration;

l length of the track section;

m train mass;

Met translating mass equivalent to the rotating masses of the train;

T traction force;

Vn train speed at the initial location of the track section;

Vn+1 train speed at the final location of the track section.

Since coasting T=0

Defining rotating mass coefficient k as