18 - Aerodynamics and Power Consumption Flashcards
Perspectives from which to consider aerodynamics of trains
Economic and environmental need to reduce aerodynamic drag, thereby controlling fuel consumption
Safety requirement to ensure aerodynamic effects do not result in dangerous conditions for train passengers and railway workers (e.g. stability at high speed)
Noise
When do aerodynamic effects become important?
Scale with square of train speed (i.e. kinetic energy)
Therefore become more important when train speeds become higher
Power requirements
Main aerodynamic issue has been reduction of aerodynamic drag (not noise or stability)
For electric trains, power consumption can be lowered at high speed if drag is reduced, and cost of infrastructure to supply power can also be reduced
For diesel trains (onboard power generation) power consumption has to be carefully managed for 200km/h to be viable
Higher power diesel engines become too heavy for speeds above 200km/h to be feasible
Power requirement reduction has obvious environmental benefits
Variables of Davis equation for train resistance
a - mechanical rolling resistance
b - mechanical resistance and momentum loss due to air intake to train (ventilation, engine)
c - aerodynamic drag (skin friction and pressure drag)
V - speed/velocity
Velocity squared term in aerodynamic drag begins to dominate as train speed increases
What does aerodynamic drag depend on?
CSA of train
Air density
Experimentally calculated drag along train
Train length
Front pressure and rear suction drag coefficient
Skin friction and sources of turbulence
Skin friction along sides and turbulence around bogies and underfloor equipment are much bigger factor than front/rear end design
Non-streamlined or shielded underfloor equipment, pantographs
Ventilated disc brakes - effectively radial fans, blowing out air and consuming energy
Bogies - not designed for good aerodynamic performance, lots of ‘dead zones’ behind structure in which recirculation of flow consumes energy
Gaps between vehicles
How contributions add up
Trains of reduced CSA have train body and skirt system to smooth structures underneath
Improve pantograph design and covers reduces drag
Total front facing CSA is proportional to aerodynamic drag
Testing and modelling train aerodynamics
CFD used
Flows around aeroplanes are mainly low turbulence attached flow and can be modelled in free air without surrounding surfaces
Trains have large wakes and are surrounded by very unsteady and turbulent flow - behaviour depends on proximity to ground, passing other vehicles, passing through narrow tunnels, and speeds and vehicle complexity are higher than for cars
Wind tunnels
Used to determine train drag
Use of moving ground plane can improve simulation
Cross wind forces can be determined
Turbulence of atmospheric wind cannot usually be modelled
Scaling can be a problem for small scale wind tunnels
Largest in the world (RTRI’s wind tunnel in Japan) can only take 1:5 scale train body
Simulation requirements
Perfect simulation would achieve: Reynolds number similarity; Mach number similarity and geometric similarity
Speeds and Mach numbers can be matched
Small scale testing needs conditions to raise Re: high pressure (increases air density); low temperature (reduces kinematic viscosity); bigger models
Non-drag aerodynamic issues
Issues of operational efficiency and safety
Pressure pulses caused by trains passing one another produce high structural loads on train and line side structures (e.g. tunnel linings)
Cross-winds, instability and train overturning
Slipstreams from trains can move people, wheelchairs, pushchairs etc. towards the train
High speed trains in tunnels can be subject to high transient pressures - uncomfortable or even dangerous to hearing
Noise
Pressure ahead and behind train
Highest flow velocities at end of train
Highest pressures at front of train - pressure pulse
Often, moving flow over stationary body is considered, and equivalent case of moving body in stationary flow can be considered
Looking at flow reaching front of train, flow velocity on central streamline must become equal to that of the train - stagnation point
What does pressure pulse depend on?
Train speed
Train front shape
Additional features - spoilers, snow plough etc.
Bernoulli’s equation for pressure ahead and behind train
Assuming flow to be steady, frictionless and incompressible
Ignore gravitational term
At stagnation point, velocity of fluid becomes zero relative to train, so pressure at this point must be equal to remote atmospheric pressure plus density-velocity term
Density-velocity term is part of stagnation pressure due to stopping fluid motion (called dynamic pressure)
Pressure coefficient
Pressures can be calculated ahead of and along train at range of heights
For fast moving train, pressure increases some distance ahed of front
Pressure coefficient is defined by dividing pressure rise by dynamic pressure
Cp = 1 at stagnation point
For trains up to 250km/h EU legislation sets maximum level of 720Pa peak to peak pressure change at position someone next to train would feel
