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
Flow around vehicle
High Re
Turbulent
Acceleration and deceleration
Recirculation and swirling
Surrounding Cp can be calculated to predict if its design will meet pressure limits fro surrounding structures and people
Pressure between passing trains
Doors and windows must withstand this pressure
Body must withstand this pressure, but this is a repeated load - structure must be designed to avoid fatigue
Air pressure on platforms and track maintenance staff
Slipstream at rear of passing high speed train has high flow velocities
Negative pressure behind train can pull in people/things
UK research - 1.5m from platform edge, slipstream velocities ranged from 3.5m/s to 19.4m/s (12-70km/h)
Cross-wind stability
Current aims: reduce train weight to cut fuel consumption; increase train speeds; keep service operating under al weather conditions
Result is that trains and cargo can become more vulnerable to overturning due to cross-winds
CFD and wind tunnel approaches can help understand how vehicle geometry affects cross-wind stability
Cross-wind needs to be included as an additional side load on vehicle
Cross-winds interact with suspension - predictions of safe running speed for a range of conditions can be made
Tunnel aerodynamics for general tunnels
Tunnels usually almost completely filled by CSA of train
Train acts as piston, driving air ahead and drawing in air behind
For short tunnels or those with ventilation shafts, pressure difference front to back is small
For longer tunnels, complex ventilation systems are needed to avoid extra drag
Drag relative to open air can be more than doubled
Tunnel aerodynamics on high speed lines
Have standard drag/piston effect problem
Also have noise generation problem
Compression wave formed as train enters tunnel and transmitted along tunnel at speed of sound
At tunnel exit, part of pressure waves reflected back, and superposition can lead to high/painful pressure for passengers
Pressure wave which isn’t reflected forms a micro-pressure wave/sonic boom
Radiates into surrounding area
Tunnel exits can be designed to dissipate energy of compression wave
Train nose design can reduce generation of compression wave
Train should be sealed to prevent passengers being aware of high superposition pressures in tunnel
What does cabin pressure depend on?
External pressure
Leakage area (pressure tightness)
Cabin volume
Cabin deformation
What does pressure tightness depend on?
Air inlets, toilet drainage
Bodyshell construction
Gangways and corridor connections
Door and window seals
Ducting and cabling
Intermodal freight
Transport of containers by road or rail
Rectangular shape containers generate huge drag because of gaps between adjacent containers
If loading leaves gaps on the train unfilled, the problem is even worse
Many technical solutions exist, but most effective thing is devising ways to load the trains without ‘missing’ containers
Efficiency of loading
Slot utilisation: traditional approach; % of slots on train filled by containers (irrespective of how completely the fill the slot); promotes efficient loading from operational perspective but not aerodynamically
Slot efficiency: more recent; % of length of containers carried relative to available space
Monitoring efficiency of loading
Slot efficiency only reveals how efficient current vehicles are, not how much better/worse a new/replacement vehicle may be
Wind tunnel tests have shown - smaller gaps are best (>1.8m and loads behave aerodynamically separately) and gaps cause greatest energy loss near front of train (keep smallest gaps at front)
US railways have developed an image analysis system to monitor and measure loading patterns and give operators advice on loading
Estimated fuel saving 2.25l/km/train
