December 13, 2010

 

Peter W Jones MInstP

Opinions expressed in this blog are mine and are not necessarily the same as those of the Institute of Physics.

INTRODUCTION

December 2011

I have copied the following from the internet to indicate what is happening concerning  F1 racing cars.

CFD fine tunes Benetton Formula 1 aerodynamics

 

 

 

.

Thanks to scalable parallel computing, large-scale simulations can typically be completed overnight.

By building up fine mesh layers with Star-CD from the CAD surface, an incredibly high flow resolution can be achieved.

For example the flow for the leading edge of a Benetton front wing can be accurately resolved to within 1 millimetre, giving designers a level of accuracy that was unimaginable just ten years ago.

This state of the art meshing flexibility is not only suited to the team’s needs for detailed geometrical modelling, but also allows F1 design engineers to perform whole car CFD simulation, leaving nothing to chance.

For instance, if you analysed the rear wing assembly on its own, the airflow approaching the wings would be far too uniform.

In this case, the upstream part of the car needs to be included to produce the correct velocity distribution at the rear wing, therefore leading to a more accurate prediction of downforce at the rear wing.

The tiniest adaptations can have enormous repercussions on the car’s performance.

Precision is always the main priority, even if a change seems minor the effect on the flow can be considerable.

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PWJ

On the UK’s roads accidents can normally only be investigated by the police;  until fairly recently their qualifications were awarded by the Institute  of Traffic Accident Investigators  (ITAI). Currently the police ( in the West Midlands and Devon and Cornwall) are now trying to ensure  that Road Traffic Accident Investigators (RTAI’s)  pass the City and Guilds Police exam in Forensic Collision Investigation. It would be preferable if police transferring to be RTAI’s were required to have 5 gcse’s at grade c or above (including Maths and Physics). The Police C&G Forensic exam could then be upgraded to C&G level 3 (equivalent to GCE “A” level and upper levels of  ONC Engineering), but as no entry requirement is made the police C&G exam must only be level 1 or 2 (and equivalent to about 3  GCSE subjects, in my opinion, due to the amount of work to be covered).

I have explained the above more fully in  my blog  www.westmidlandspoliceauthority.wordpress.com 

I wrote the folowing some time ago to reflect the stage we have reached with ordinary traffic on our roads.

Aerodynamics of Trailers.

I am seeking to show in this paper that although the Caravan Industry/ Clubs for many decades have blamed driver error for a considerable proportion of caravan accidents and burst caravan tyres for most of the remainder, the truth is that it is aerodynamic effects (if one includes the wind in this category) plus the poor over run brake actuating mechanism and stabilisers that do not stabilise that are the primary culprits.

As Professor Stephen Glaister of Imperial College, London University and now Director of the Charity RAC Foundation, pointed out so well about 18 months ago at a press conference, the causes of Road Traffic Accidents are hardly ever investigated in the UK so my speculations are as good as any.

However, a great many of my speculative comments are based on the excellent work that has been done in the Department of Mechanical Engineering at Bath University under the leadership of Dr Jos Darling.

When the first aircraft to fly in the UK left the ground the engineers were not aware of the fact that more lift could be obtained from the upper surface of the wings than the underside.
The upper surface of the wings had not been smoothed off. It is surprising that the aircraft actually managed to fly.
( From a lecture by Philip Jarett to the Birmingham, Wolverhampton and Cosford Branch of the RAeS. “Cody and Roe. Two Remarkable Men. Centenary of Powered Flight.”)

We would be paying a greater tribute to these two remarkable men if we learned from their initial mistakes.

I now propose to make some observations with regard to HGV’s and would first of all like readers to consider the following statistics selected from many hundreds from the same source concerning the same topics.

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www.dft.gov.uk/pgr/statistics/…/accidents/casualtiesgbar/…/flagchgvfsheet.

Due to the size and weight of HGVs, they tend to be involved in more severe accidents. In 2007 the rate of fatal accidents was higher for HGVs (1.6 per 100 million vkm) than for all motor vehicles (0.9 per 100 million vkm). LGVs however had the lowest fatal accident rate of all vehicle types (0.4 per million vkm).

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I think there must be an additional reason to “weight” that is a factor in so many HGV fatalities. The Police do not seem to have supplied different statistics for HGV’s without trailers, so I am assuming that all HGV’s tow a trailer, particularly as I only rarely see a HGV without a trailer.

The upper surface of most of our large road vehicles is very smooth; the front of the vehicles often has a “streamlined” shape to improve fuel consumption and consequently there will be a streamlined flow over the top surface which I think could produce enough lift to reduce road friction of the trailer wheels and enable strong cross winds to induce sideways drift which can lead to jack knifing.

My reasons for the aerodynamic lift taking place are “speculative,” but the existence of the lift should be well known to most observant road users and spectators at Formula 1 Racing events.
The latter have of course been developing reverse aerofoils for many years; initially they thought that the aerofoil needed to be some distance above the car and this gave rise to them looking similar to part of a World War One bi plane.

I first made a definite identification of the sideways drift hazard in about 1990. At the time I owned a Rover SDI 3500 V8 SE which had very little ground clearance.

My car possibly produced “ground effect” and should have had some extra resistance to the force exerted by a strong cross wind.
I was travelling on the M40 with two other adults plus a great deal of luggage and estimate that the total weight must have been close to two tonnes.
Due to the weight and the low profile I thought that I would be quite safe travelling at 70mph with a cross wind of about 44mph (gale force), but very soon began to experience a distinct sideways drift which I had to counter by slowing down.

A typical articulated lorry could be subject to the same wind strength; the weight of the tractor unit will be fairly constant, but the weight of the trailer will vary greatly depending on the nature and size of the load. In addition the area of the side of the trailer exposed to the wind is greatly in excess of that of the tractor unit so it is not surprising that when lightly loaded (and subject to strong cross winds) the trailer will drift sideways but the tractor unit will not be affected. This can result in a “jack knife” much dreaded by HGV drivers and the rest of us who might be in the wrong place at the wrong time.

The above assumptions on my part derive mainly from a similar argument by Standen (1999) in his Phd thesis (Bath University, Department of Mechanical Engineering) “Towed Vehicle Aerodynamics.” Standen experimented in the University Wind Tunnel with scale models of HGV’s and single axle caravans and managed to show that aerofoils could improve the stability of the caravan thus also in my view verifying the existence of the aerodynamic lift. In addition Standen’s experiments also showed that HGV’s produce waves and that these exerted sufficient force to destabilise the caravan as it was overtaken.

Furthermore, as Standen showed the existence of aerodynamic lift it must be assumed that the lift is directly proportional to the square of the air speed.

Ten years after the publication of Standen’s Phd thesis, the Caravan Club (sponsors of this wind tunnel research project), eventually acknowledged (in small print in the Club Members’ Handbook)) the general idea of aerodynamic lift and its effect on caravans.

I have been towing trailers, caravans and sailing cruisers on a recreational basis since 1976. The photographs in
http://www.aerodynamicsoftrailers2.wordpress.com

indicate the scope and variety of the items towed. I have formulated various ideas on improving road safety which are based on the fact that the air craft industry has similar procedures already in place.

Due to my long standing hobby of sailing in estuaries and coastal waters I have never taken an overseas holiday and the only air craft I have been a passenger in was a Tiger Moth Bi Plane when I was doing National Service in the RAF.

This gave me my first link with the aerodynamic lift of trailers/caravans when I recalled that the Tiger Moth had a take off speed of about 60 mph. Modern light aircraft weighing about 1000kg have a stalling speed of about 50 mph and this is the speed which (on the basis of road experience) I have decided is about the maximum safe air speed for my small caravan of 1000kg in weight.
By extrapolation and the use of data for larger aircraft this can give at least an indication of the vulnerability of trailers to aerodynamic forces and possibly a very rough estimate of the maximum safe air speed for specific larger trailers.

All air craft must have details in the pilot’s log book of the stalling speed and the maximum side wind component for a safe landing.
I hope that road safety will eventually be improved by trailer owners’ handbooks containing similar information.

As far as I can establish it is only the Japanese who have recognised the need for anemometers at the side of railway tracks, but even they are still suffering serious rail accidents due to trains being “blown off” the tracks.

I have discovered that a suitable anemometer for use on all road vehicles already exists. It only needs a software modification to enable it to give a read out of vehicle air speed and side wind component. The only unsuitable factor of the current design is that the maximum design wind strength of the instrument will rule out its use for high speed trains.

The introduction of this type of anemometer would bring about a big and instant increase in road safety as drivers would immediately begin to use their own experience of their vehicles and trailers, plus a knowledge of the air and side wind speeds, to drive more safely.

For relevant photographs see

www.aerodynamicsoftrailers2.wordpress.com

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December 17, 2008

Hello world!

November 28, 2008

 

 

 

 

 

Other blogs by the same writer

www.hgvaccidentstatistics.wordpress.com 

 
 
 
 
 
 
 
 
 
I have almost comleted writing some advice for Physics/Maths teachers (most of this has turned out to be for teaching at GCSE Physics level) on how to incoporate  teaching of items concerned with the  aerodynamics of towing a trailer with the School/University Mini Bus in to ordinary course work for exams.
Whether it is Maths or Physics being taught, examples of  the use made of theory in the exam syllabus is always being sort by teachers to keep their work in touch with new developments and to maintain students’ interest. 
 
  

Advice for Physics/Maths/Mechanical Engineering Teachers and Lecturers.

www.schoolminibustrailers.wordpress.com

Contains:-

(1) GCE “A” level; Anemometers ( Vector diagrams)

(2) To be completed

(3) GCE “A” level Maths/Physics; Stabilisers (Calculations involving the Principle of Moments. Levers.)
For single subject Physics/triple Science, both 1, 2 & 3 may be introduced at GCSE level.
(4) Further topics for GCE “A” level students:-
      Trailers with no brakes
       Resonance
.      Centrifugal Force
       Ventrui Effect
       Elementary Theory of Flight
.
contains:-
 
(1)  GCSE;  Over run Brakes
 
(2) GCSE; Caravan/trailer electric brakes. (Electro magnets; relays)
 
(3) GCSE; Wave theory applied to snaking caravans and trailers.
Constructive and Destructive Interference.