Vehicles

Question 1

What is the characteristic wind curve?

Answer 1

The aim of the CWC is to define the wind condition Vwind that leads to overturning. The threshold varies with the train speed Vtrain with the wind turbulence characteristics and with the railway line characteristics.

Question 2

What is the procedure for defining the characteristic wind curves (CWC)?

Answer 2

1) First, the static aerodynamic coefficients are calculated. A lot of factors have to be considered for the tests of static aerodynamic coefficients such as the vehicle geometry effect, the infrastructure scenario effect (ground, ground and single ballast rail, embankment, viaducts etc), the reynolds number effect, the first vs second vehicle effect and the static vs moving model test.

2) The wind field is created, by either using a deterministic or stochastic method.

3) The aerodynamic forces are calculated

4) Dynamic simulation and CWC
i) The multi body model of the train is defined
ii) A time-space wind field is defined according to specific turbulence conditions and a defined mean wind speed
iii) If the parameter is below the threshold for wheel unloading, the simulation is repeated with an increased wind speed, otherwise if the threshold is overcome the simulation is repeated with a reduced mean wind speed
iv) the procedure is repeated several times decreasing at each iteration the step of variation of the wind (bisection method)
v) A convergence is checked comparing the variation of wind speedwith a defined tolerance

5) Risk Analysis
6) Alleviation Method (wind break barriers/train speed regulation)
At the end of an iterative loop, a single point of the CWC is defined.

Question 3

Why is there a splitter plate used in train testing in wind tunnel?

Answer 3

It cuts off the boundary layer of the wind tunnel flow developing on the flow, resulting in more uniform flow.

Question 4

What are the problem related to wind engineering and vehicles?

Answer 4

1) Tunnel exit (step input of force)
2) Tower wake crossing
3) Barrier gap

Question 5

When is superelevation dangerous for trains?

Answer 5

Superelevation is used to balance the centrifugal force when turning.

When the wind direction is in a direction from the outside to inside of the rail, oveturning is more dangerous.

Question 6

Why are modern trains more dangerous for overturning, regarding power distribution.

Answer 6

Old trains had the power in the first and last wagons, and therefore the wagons were heaviers. Now, trains use distributed power through boogeys, and the first and last wagons are lighters, making them more dangerous for overturning.

Question 7

How can the aerodynamic coefficients be calculated?

Answer 7

1) Full scale tests
No control of wind
Needs time
Valuable by Reynolds number point of view

2) Wind tunnel tests
Size down
Reynolds important parameter
Not real relative speed

3) CFD simulations
Doesnt allow to reach Reynolds number –> computationally expensive

Question 8

What can be observed from the Aerodynamic Force coefficients?

Answer 8

At around 40-50 degrees, the CFy, CMy coefficients start decreasing, in what seems as a “wing stall”. This happens because the flow passing from the nose, the roof of the train, and the gap between train/track created a vortex. The vortex stays aligned with the vehicle but separates at some point.

Question 9

Define Cmx,lee

Answer 9

A combination of overturning moment and vertical force can be given by transferring the moment to the leeward wheel. Cmx,lee = Cmx - b/2 CFz/H.

Question 10

Discuss the first and second car of the train.

Answer 10

The first car has higher overturning moments and that’s why it is used for analysis. However, if the first and second car are connected by boogey, both should be safe.

Question 11

How can coverings help alleviate overturning moments?

Answer 11

Covering the roof of the train, can reduce Cmxlee.

Question 12

Discuss different scenario effects.

Answer 12

Different scenario effects can be true flat ground, single track ballast and rail, embankments (6m) and viaducts.

1) Between flat ground and single track ballast and rail, there is no difference in Cmx, CFY is higher and stalls earlier, because of the retardment of airspeed in the gap. CFx and CFz are also increased.

2) Single track ballast and rail and 6m high embarkment:
The embankment reaches lower values at almost all coefficients (except CFx) but stalls earlier.

Question 13

How does the embankment affect the wind flow?

Answer 13

The embankment speeds up the normal component of speed, by a parameter femb. The parallerl vector is unchanged. This results in a different AoA. Usually it is between 1:1.3

Question 14

How does Reynolds number affect the aerodynamic force and pressure coefficients?

Answer 14

There are Reynold number dependancies at large AoA for CFy (>50) and CFz (>30).

Regarding the pressure coefficients, in the leeward surface prssure coefficients there is no Reynolds dependancy, but in the windwards there is a large dependancy at 40-50 deg.

However, increasing surface rougness, the Reynolds number dependancy can dissapear, at least to up to high AoA.

Question 15

Should a static vs moving Vehicle test be considered?

Answer 15

In general, static tests can produce more reliable results. The trend obtained by the static tests is similar to those of the moving models. The dynamic model is characterized by large fluctuations, maybe related to a not completed depuration of the inertial effects.

An estimation of the error introduced by the aerodynamic coefficients by the lack of relative motion between train and infrastructure is computed using CFD simulations.

The errors become larger in general as the infrastructure gets larger.

Question 16

How are the wind characteristics of the site considered?

Answer 16

1) Metereological conditions at parts of the lines
2) More dangerous when line is close to sea

Question 17

Which are the wind characteristics that influence the problem?

Answer 17

1) Turbulence intensity
2) Integral length scale
3) PSD
4) Coherence function

Question 18

Describe two ways to define the wind speed?

Answer 18

1) Deterministic Methodology
It models the worst condition. It works well for a single even but sometimes the sequence of events is important. It requires the filtering of the wind speed time history by a spatial average filter with a window size equal to the vehicle length.

2) Stochastic methodology
It required a large number of wind field reconstruction. With the same statistical properties, different times histories are created, but the worst case scenario is not known. It required an admittance function

Question 19

What is the train admittance function?

Answer 19

At low frequencies, quasi steady theory can be used, with a mean Speed U. At high frequencies, a corrected quasi steady theory is needed, with a fluctuating speed U(t,x). For corrected quasi steady theory, the modified relative wind speed is calculated after weighting the U absolute wind speed in the frequency domain through a train admittance function. This allows to acound for the spatial correlation of the wind pressures at any two points of the vehicle surface in conditions of turbulent wind.

It can be defined by experimental wind tests or numerical models.

Question 20

How are vehicle dynamics calculated?

Answer 20

Multi body dynamic vehicle models are employed. When the windward wheel force is decreased by 90% (safety margin), then train is considered to be overturned.

ΔQ/Q0 = 1- Qd1+Qd2/2Qd0

Question 21

How are the characteristic wind curves created with mean CWC and spead band?

Answer 21

Using same statistical wind properties and different wind time histories, for each speed, the condition of overturning is calculated for all the time histories, creating a spread and mean value,

Question 22

Does the curve affect the CWC?

Answer 22

Yes, in general the accepted wind speeds are lower for the same train speeds.

Question 23

What is the risk analysis?

Answer 23

By using the probability of exceedence of a given wind speed depending on the wind direction, we can compare the wind properties and the CWC with respect to the AoA. The CWC should be higher than the wind curves for each given probability. If there is a local probability of CWC exceedence, then measures can be used to alleviate the method, for example wind barriers or reducing train speed, or cutting level of track below ground.