{"id":359,"date":"2009-05-03T19:00:31","date_gmt":"2009-05-03T19:00:31","guid":{"rendered":"http:\/\/refraction.co.uk\/blog\/?p=359"},"modified":"2009-05-03T19:29:07","modified_gmt":"2009-05-03T19:29:07","slug":"so-what-is-a-diffuser-and-what-does-it-do-for-an-f1-car","status":"publish","type":"post","link":"https:\/\/refraction.co.uk\/blog\/2009\/05\/03\/so-what-is-a-diffuser-and-what-does-it-do-for-an-f1-car\/","title":{"rendered":"So what is a diffuser and what does it do for an F1 car?"},"content":{"rendered":"

Since the 2009 F1 season started in Australia, there\u2019s been a new order on the grid.\u00a0 This year the sports technical regulations were changed, particularly those dealing with aerodynamics.\u00a0 A few teams have got the march on the others by interpreting the rules in a different way, and have developed faster, better cars.\u00a0 The old guard of McLaren<\/a> and Ferrari<\/a> are on their back foot, whilst teams like Brawn <\/a>find themselves leading the championship.<\/p>\n

If you believe the press this change of form has been down to the cars diffusers.\u00a0 But what are they?\u00a0 Most reports simply say that diffusers generate downforce, but strictly speaking that\u2019s not really what they\u2019re for.<\/p>\n

Having had a few conversations about this recently I thought I\u2019d attempt to write an explanation of what a diffuser does.\u00a0 I\u2019m not exactly an aerodynamicist, but hopefully this will make sense!\u00a0 Right\u2026 so a few basics\u2026 sorry if they\u2019re obvious!\u00a0<\/p>\n

Ultimately a cars performance is dictated by it\u2019s tyres.\u00a0 The more friction or grip they have with the road, the faster a car can corner, accelerate and brake.\u00a0 There are many weird and wonderful ways to get the tyres themselves to grip better, from compounds of rubber to their internal construction.\u00a0 One simple way to increase grip though is just to push them onto the road more.\u00a0 Sounds obvious eh?\u00a0 Of course you could do this by adding weight, but that would compromise other aspects of the car\u2019s performance.\u00a0 The more mass a car\u00a0has the more energy that is needed to accelerate it and keep it on the road, so really you want a way to push the car down, but without adding mass.\u00a0 That\u2019s where downforce comes in, using the airflow passing over \u2013 and under \u2013 the car to push it down into the road.<\/p>\n

An important idea to understand when thinking about downforce is the Bernouilli Principle<\/a> for fluid dynamics.\u00a0 In short all this says is that the faster a fluid flows the lower its pressure.\u00a0 In our case, the fast that air is flowing over the surface of a car, the lower it\u2019s pressure.\u00a0 (This is a vast simplification, but in general true.)<\/p>\n

An example of this in practice is an aeroplane wing<\/a>.\u00a0<\/p>\n

\"Airflow<\/a>
Airflow over a wing<\/figcaption><\/figure>\n

A wing is shaped so that the air flowing over the top of it is faster than the air flowing under it.\u00a0 The faster moving air has a lower pressure than the slower air, which generates an upward force that makes a plane fly.<\/p>\n

In many forms of motorsport wings are used to generate a downward force to push the car onto the road.\u00a0 In doing this huge amounts of downforce can be created.\u00a0 But wings aren\u2019t the\u00a0 only way of achieving this.\u00a0<\/p>\n

How about using the other surfaces on the car?\u00a0 If you can make the air under the car flow faster than the air over it, the car itself would generate downforce.\u00a0\u00a0 Of course the airflow around a car should be as smooth as possible to avoid drag that would slow the car down, so the best route to achieving our goal is to accelerate the air under the rather than slow down the air over the top.\u00a0<\/p>\n

The next principles to consider are the Conservation of Mass and Venturi Effect.\u00a0 These principles are quite hard to explain (for me at least!) but can be demonstrated using the venturi tube below.<\/p>\n

\"VenturiTube\"<\/a><\/p>\n

As the air passes through the tube it meets a narrow throat through which it must pass.\u00a0 The amount of air that enters through the inlet must be the same as the amount that exits at the end, so to pass through the throat the airflow must accelerate, and by increasing in velocity it reduces in pressure.\u00a0 Once through the throat the diffuser increases the diameter of the tube back to that of the inlet, and therefore slows the airflow back to its original velocity.<\/p>\n

A practical example of a venturi tube in use is a carburettor choke, where the low pressure is used to suck fuel into the airflow and then into the engine cylinders.<\/p>\n

So what has this got to do with downforce?\u00a0 Well imagine that half of the tube was flat (as below).<\/p>\n

\"VenturiTubeHalf\"<\/a>\u00a0<\/p>\n

The same still applies, the air in the throat accelerates and slows.\u00a0 So if the flat surface was a road and the throat the underside of a car you would get low pressure under the car \u2013 just what we want.<\/p>\n

For this to work the car has to have a flat underside, any interference in the airflow will slow it down.\u00a0<\/p>\n

The closer the car is to the road the faster the air will have to travel, hence the desire to run cars as low as possible.\u00a0 One thing to consider here though is that whilst the airflow will go faster the lower the car, air still has to be able to get under it.\u00a0 At some point the car will be so low that not enough air will pass under the front lip and the effect will stall, leading to a drop in downforce.\u00a0 This is a particular problem under braking when the front of the car will dip towards the road.\u00a0 If it drops too far the floor will stall and the driver will experience a sudden drop in grip just when he needs it.\u00a0 This was what the active suspension<\/a> of a few years ago was intended to avoid, it actively managed the suspension to keep the ride height at an optimum level.<\/p>\n

Another effect of having lower than ambient pressure under the car is that air will leak in from the sides reducing it\u2019s effect.\u00a0 This is a real problem and one that on the old Lotus 78<\/a> and 79<\/a> race cars was solved by running skirts along the side of the car to stop air getting in.\u00a0 Skirts are now banned, but other techniques have been adopted over the years such as creating vortex\u2019s along the sides \u2013 but that\u2019s all a bit off topic.<\/p>\n

So if the flat floor generates downforce why have the diffuser at the rear of the car?\u00a0 After all the more flat area you have the more downforce you\u2019ll get.\u00a0<\/p>\n

The diffusers purpose is to control the way the airflow at the back of the car decelerates back to its normal velocity.\u00a0 So it less about creating downforce, it\u2019s actually an essential part of the venturi whose purpose is to slow the airflow by increasing the space between the road and the underside of the car.\u00a0 This allows the pressure to rise back up to ambient so that the airflow leaves the underside of the car as smoothly as possible decreasing drag that would slow the car down.\u00a0 Having said that, the pressure in the diffuser will on average be lower than ambient, so it will create some downforce, but as a side effect.<\/p>\n

A common misconception, and one that I made before I read a book about aerodynamics, is that the diffuser expands the air to produce low pressure.\u00a0 This isn\u2019t the case as it would require the airs density to change, and in an open system like the underside of a car a diffuser wouldn\u2019t be able to do.<\/p>\n

So how are Brawn and the other teams getting such an advantage?\u00a0 Well by interpreting the aero rules differently they have been able to build diffusers that are much more efficient than their rivals.\u00a0 By being able to start their diffusers earlier they are able to run them up closer to the upper bodywork of the car, and importantly closer to the rear wing elements.\u00a0 The low pressures of the underside of the rear wing and diffuser interact and increase the effectiveness of each other.\u00a0 There\u2019s some really good information on the specific differences between the Brawn<\/a>\/Toyota<\/a>\/Williams<\/a> cars and the others in the FIA\u2019s<\/a> explanation of their appeal ruling here<\/a>.<\/p>\n

A good book on this subject is <\/em>Competition Car Aerodynamics: A Practical Handbook<\/em><\/a>\"\" by Simon McBeath.\u00a0\u00a0 <\/em><\/p>\n","protected":false},"excerpt":{"rendered":"

In 2009 Formula 1’s aerodynic rules have been changed and teams such as Brawn, Williams and Toyota find themselves with more efficient diffusers that have brought them to the top of the grid. but what are diffusers and what do they do?<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[8,6],"tags":[256,264,259,525,258,68,156,159,265,145,261,262,260],"class_list":["post-359","post","type-post","status-publish","format-standard","hentry","category-cars","category-tech","tag-aerodynamics","tag-bernoulli-principle","tag-brawn","tag-cars","tag-diffuser","tag-f1","tag-ferrari","tag-fia","tag-mclaren","tag-racing","tag-toyota","tag-venturi","tag-williams","entry"],"_links":{"self":[{"href":"https:\/\/refraction.co.uk\/blog\/wp-json\/wp\/v2\/posts\/359","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/refraction.co.uk\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/refraction.co.uk\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/refraction.co.uk\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/refraction.co.uk\/blog\/wp-json\/wp\/v2\/comments?post=359"}],"version-history":[{"count":5,"href":"https:\/\/refraction.co.uk\/blog\/wp-json\/wp\/v2\/posts\/359\/revisions"}],"predecessor-version":[{"id":364,"href":"https:\/\/refraction.co.uk\/blog\/wp-json\/wp\/v2\/posts\/359\/revisions\/364"}],"wp:attachment":[{"href":"https:\/\/refraction.co.uk\/blog\/wp-json\/wp\/v2\/media?parent=359"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/refraction.co.uk\/blog\/wp-json\/wp\/v2\/categories?post=359"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/refraction.co.uk\/blog\/wp-json\/wp\/v2\/tags?post=359"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}