Software package for computational aero- and hydrodynamics Flow Vision designed for virtual aerodynamic blowdowns of various technical or natural objects. Transport products, energy facilities, military-industrial products and others can serve as objects. Flow Vision makes it possible to simulate the flow around at different velocities of the oncoming flow and at different degrees of its disturbance (degree of turbulence).
The modeling process is carried out strictly in a three-dimensional spatial formulation of the problem and proceeds according to the "as is" principle, which implies the possibility of studying a full-fledged geometric model of the user's object without any simplifications. The created system for processing imported three-dimensional geometry allows you to work painlessly with models of any complexity, where the user, in fact, chooses the level of detail of his object - whether he wants to blow through a simplified smoothed model of external contours or a full-fledged model with the presence of all structural elements, down to the bolt heads on the wheel disks and the manufacturer's logo in the form of a figurine on the nose of the car.
Velocity distribution in the vicinity of the body of a racing car.
All the details are taken into account - the spokes of the wheels, the effect of the asymmetry of the spokes of the steering wheel on the flow pattern.
Flow Vision created Russian team developers (TESIS, Russia) more than 10 years ago and is based on the developments of the domestic fundamental and mathematical school. The system was created in the expectation that users of very different qualifications will work with it - students, teachers, designers and scientists. You can equally effectively solve both simple and complex problems.
The product is used in various industries, science and education - aviation, cosmonautics, energy, shipbuilding, automotive, ecology, mechanical engineering, processing and chemical industry, medicine, nuclear industry and the defense sector and has the largest installation base in Russia.
In 2001, by decision of the Main Council of the Ministry Russian Federation, FlowVision was recommended for inclusion in the curriculum of teaching fluid and gas mechanics in Russian universities. Currently, FlowVision is used as an integral part of the educational process of the leading Russian universities - Moscow Institute of Physics and Technology, MPEI, St. Petersburg State Technical University, Vladimir University, UNN and others.
In 2005, FlowVision was tested and received a certificate of conformity from the State Standard of the Russian Federation.
Key features
At the core Flow Vision the principle of the law of conservation of mass lies - the amount of substance entering the filled closed calculated volume is equal to the amount of substance decreasing from it (see Fig. 1).
Rice. 1 Principle of the law of conservation of mass
The solution for such a problem occurs by finding the average value of a quantity in a given volume based on data at the boundaries (the Ostrogradsky-Gauss theorem).
Rice. 2 Volume integration based on boundary values
To obtain a more accurate solution, the original calculated volume is divided into smaller volumes.
Rice. 3 Thickening of the computational grid
The procedure for dividing the original volume into smaller volumes is called CONSTRUCTION OF THE COMPUTATIONAL GRID , and the array of resulting volumes is COMPUTATION GRID . Each volume obtained in the process of constructing the computational grid is called CALCULATED CELL , in each of which the balance of the incoming and outgoing mass is also observed. The closed volume in which the calculation grid is built is called COMPUTATION AREA .
Architecture
Ideology Flow Vision based on a distributed architecture, where program block, which performs arithmetic calculations, can be located on any computer in the network - on a high-performance cluster or laptop. The architecture of the software package is modular, which allows painless improvements and new functionality to be introduced into it. The main modules are the PrePostProcessor and the solver block, as well as several auxiliary blocks that perform various operations for monitoring and tuning.
Pressure distribution over the body of a sports car
The functional purpose of the Preprocessor includes importing the geometry of the computational domain from geometric modeling systems, setting the environment model, setting the initial and boundary conditions, editing or importing the computational grid and setting the convergence criteria, after which control is transferred to the Solver, which starts the process of constructing the computational grid and performs the calculation according to given parameters. During the calculation process, the user has the opportunity to conduct visual and quantitative monitoring of the calculation and evaluate the process of solution development using the Postprocessor tools. When the required value of the convergence criterion is reached, the counting process can be stopped, after which the result becomes fully available to the user, who, using the Postprocessor tools, can process the data - visualize the results and quantify with subsequent saving to external data formats.
Calculation grid
IN Flow Vision a rectangular computational grid is used, which automatically adapts to the boundaries of the computational domain and the solution. Approximation of curvilinear boundaries with a high degree accuracy is ensured by using the subgrid geometry resolution method. This approach allows you to work with geometric models consisting of surfaces of any degree of complexity.
Initial computational domain
Orthogonal grid overlaid on the area
Cropping the initial grid by the borders of the region
Final computational grid
Automatic construction of the computational grid, taking into account the curvature of the surface
If it is necessary to refine the solution at the boundary or in the right place of the computational volume, it is possible to dynamically adapt the computational grid. Adaptation is the fragmentation of cells lower level into smaller cells. Adaptation can be by boundary condition, by volume and by solution. Grid adaptation is performed at the specified boundary, in specified place computational domain or by solution, taking into account the change in the variable and the gradient. Adaptation is carried out both in the direction of mesh refinement, and in reverse side- merging small cells into larger ones, up to the entry-level grid.
Grid adaptation technology
Movable bodies
The moving body technology makes it possible to place a body of arbitrary geometric shape inside the computational domain and give it translational and/or rotational motion. The law of motion may be constant or variable in time and space. Body motion is defined in three main ways:
Explicitly through setting the speed of the body;
- through setting the force acting on the body and shifting it from the starting point
Through the influence of the environment in which the body is placed.
All three methods can be combined with each other.
Dropping a rocket in an unsteady flow under the action of gravity
Reproduction of the Mach experience: the movement of the ball at a speed of 800 m / s
Parallel Computing
One of key features software package Flow Vision parallel computing technologies, when several processors or processor cores are used to solve one problem, which allows to speed up the calculation in proportion to their number.
Acceleration of task calculation, depending on the number of involved cores
The launch procedure in parallel mode is fully automated. The user only needs to specify the number of cores or processors on which the task will run. All further actions on splitting the computational domain into parts and exchanging data between them will be carried out by the algorithm independently, choosing the best parameters.
Decomposition of near-surface cells into 16 processors for two-car problems
Team Flow Vision maintains close ties with representatives of the domestic and foreign HPC (High Performance Computing) community and participates in joint projects aimed at achieving new opportunities in the field of improving performance in parallel computing.
In 2007, FlowVision, together with the Research and Development Center of Moscow State University, became a participant in the federal program to create a national teraflop parallel settlement system. As part of the program, the development team is adapting FlowVision to perform large-scale computing on the very modern technology. The SKIF-Chebyshev cluster installed at the Research and Development Center of Moscow State University is used as a test hardware platform.
Cluster SKIF-Chebyshev installed at the Research and Development Center of Moscow State University
In close cooperation with specialists from the Research and Development Center of Moscow State University (under the guidance of Corresponding Member of the Russian Academy of Sciences, Doctor of Physical Mathematics Vl.V.Voevodin), the software and hardware complex SKIF- Flow Vision to improve the efficiency of parallel computing. In June 2008, the first practical calculations were carried out at 256 settlement nodes in parallel mode.
In 2009, the FlowVision team, together with the Research and Development Center of Moscow State University, Sigma Technology and the state scientific center TsAGI became participants in the federal targeted program to create algorithms for solving problems of parallel optimization in problems of aero- and hydrodynamics.
text, illustrations: TESIS company
Why do you need aerodynamics for a car, everyone knows. The more streamlined its body, the less resistance to movement and fuel consumption. Such a car will not only save you money, but also in environment Throw out less rubbish. The answer is simple, but far from complete. Aerodynamics specialists, finishing the body of the new model, also:
- calculate the distribution along the axes of the lifting force, which is very important given the considerable speeds of modern cars,
- provide air access for cooling the engine and brake mechanisms,
- think over the places of air intake and outlet for the interior ventilation system,
- seek to reduce the level of noise in the cabin,
- optimize the shape of body parts to reduce pollution of glass, mirrors and lighting equipment.
Moreover, the solution of one task often contradicts the implementation of another. For example, reducing the drag coefficient improves streamlining, but at the same time worsens the car's resistance to crosswind gusts. Therefore, experts must seek a reasonable compromise.
drag reduction
What determines the drag force? Two parameters have a decisive influence on it - the aerodynamic drag coefficient Cx and the cross-sectional area of \u200b\u200bthe car (midship). You can reduce the midsection by making the body lower and narrower, but it is unlikely that there will be many buyers for such a car. Therefore, the main direction of improving the aerodynamics of the car is to optimize the flow around the body, in other words, to reduce Cx. The aerodynamic drag coefficient Cx is a dimensionless quantity, which is determined experimentally. For modern cars, it lies in the range of 0.26-0.38. In foreign sources, the drag coefficient is sometimes referred to as Cd (drag coefficient). A drop-shaped body has ideal streamlining, Cx of which is equal to 0.04. When moving, it smoothly cuts through the air currents, which then seamlessly, without breaks, close in its “tail”.
Air masses behave differently when the car is moving. Here, air resistance consists of three components:
- internal resistance during the passage of air through the engine compartment and interior,
- frictional resistance of air flows on the outer surfaces of the body and
- form resistance.
The third component has the greatest impact on the aerodynamics of the car. Moving, the car compresses the air masses in front of it, creating an area high blood pressure. Air flows flow around the body, and where it ends, the air flow is separated, turbulences are created and an area reduced pressure. So the area high pressure in front prevents the car from moving forward, and the area of low pressure in the back "sucks" it back. The strength of the turbulence and the size of the area of low pressure is determined by the shape of the rear of the body.
The best streamlining performance is demonstrated by cars with a stepped rear end - sedans and coupes. The explanation is simple - the air flow that has escaped from the roof immediately hits the trunk lid, where it normalizes and then finally breaks off its edge. Side streams also fall on the trunk, which prevents harmful eddies from arising behind the car. Therefore, the higher and longer the trunk lid, the better the aerodynamic performance. On big sedans and the coupe sometimes even manages to achieve a seamless flow around the body. A slight narrowing of the rear also helps to reduce Cx. The edge of the trunk is made sharp or in the form of a small protrusion - this ensures the separation of the air flow without turbulence. As a result, the discharge area behind the vehicle is small.
The bottom of the car also has an impact on its aerodynamics. The protruding parts of the suspension and exhaust system increase drag. To reduce it, they try to smooth the bottom as much as possible or cover everything that “sticks out” below the bumper with shields. Sometimes a small front spoiler is installed. The spoiler reduces airflow under the vehicle. But here it is important to know the measure. A large spoiler will significantly increase the resistance, but the car will be better "snuggle" to the road. But more on that in the next section.
Downforce
When the car is moving, the air flow under its bottom goes in a straight line, and the upper part of the flow goes around the body, that is, it travels a longer distance. Therefore, the speed of the upper stream is higher than the lower one. And according to the laws of physics, the higher the air speed, the lower the pressure. Consequently, an area of increased pressure is created under the bottom, and a lower one is created above. This creates a lifting force. And although its value is small, the trouble is that it is unevenly distributed along the axes. If the front axle is loaded by a stream that presses on the hood and Windshield, then the rear is additionally unloaded by the discharge zone formed behind the car. Therefore, as speed increases, stability decreases and the car becomes prone to skidding.
There is no need to invent any special measures to combat this phenomenon, since what is done to improve streamlining at the same time increases downforce. For example, optimizing the rear reduces the vacuum zone behind the car, and therefore reduces lift. Leveling the bottom not only reduces air resistance, but also increases the flow rate and therefore reduces the pressure under the vehicle. And this, in turn, leads to a decrease in lift. Similarly, two tasks are performed by and rear spoiler. It not only reduces vortex formation, improving Cx, but also simultaneously presses the car to the road due to the air flow repelled from it. Sometimes a rear spoiler is designed solely to increase downforce. In this case, it has large dimensions and a slope or is made retractable, entering into work only on high speeds.
For sports and racing models the measures described will, of course, be ineffective. To keep them on the road, you need to create a lot of downforce. For this, a large front spoiler, side skirts and rear wings are used. But installed on production cars, these elements will play only a decorative role, amusing the pride of the owner. They will not give any practical benefit, but on the contrary, they will increase the resistance to movement. Many motorists, by the way, confuse a spoiler with a wing, although it is quite easy to distinguish between them. The spoiler is always pressed to the body, making up a single whole with it. The wing is installed at some distance from the body.
Practical aerodynamics
Following a few simple rules will allow you to get savings from the air by reducing fuel consumption. However, these tips will be useful only to those who often and a lot of driving on the track.
When driving, a significant part of the engine power is spent on overcoming air resistance. The higher the speed, the higher the resistance (and hence the fuel consumption). So if you slow down even by 10 km/h, you save up to 1 liter per 100 km. In this case, the loss of time will be insignificant. However, this truth is known to most drivers. But other "aerodynamic" subtleties are not known to everyone.
Fuel consumption depends on the drag coefficient and the cross-sectional area of the vehicle. If you think that these parameters are set at the factory, and the car owner cannot change them, then you are mistaken! Changing them is not difficult at all, and you can achieve both positive and negative effects.
What increases consumption? Unreasonably "eats" the fuel load on the roof. And even a streamlined box will take at least a liter per hundred. It is irrational to burn fuel when windows and sunroofs are open while driving. If you are transporting a long cargo with the trunk ajar, you will also get an overrun. Various decorative elements such as a fairing on the hood (“fly swatter”), “kenguryatnik”, a wing and other elements of home-grown tuning, although they will bring aesthetic pleasure, they will make you fork out extra. Look under the bottom - for everything that sags and looks below the threshold line, you will have to pay extra. Even something as small as the absence plastic caps on steel discs, increases consumption. Each listed factor or detail individually increases the consumption by a small amount - from 50 to 500 g per 100 km. But if you summarize everything, it will “run in” again, about a liter per hundred. These calculations are valid for small cars at a speed of 90 km/h. Owners of large cars and lovers of high speeds, make an adjustment towards increasing consumption.
If all of the above conditions are met, we can avoid unnecessary spending. Is it possible to further reduce losses? Can! But this will require a small external tuning (we are talking, of course, about professionally made elements). Front aerodynamic kit does not allow the air flow to “break in” under the bottom of the car, the sill covers the protruding part of the wheels, the spoiler prevents the formation of turbulences behind the “stern” of the car. Although the spoiler, as a rule, is already included in the body structure of a modern car.
So getting savings out of thin air is quite realistic.
No car will pass through brick wall, but daily passes through the walls from the air, which also has a density.
Nobody perceives air or wind as a wall. On low speeds, in calm weather, it is difficult to see how the air flow interacts with the vehicle. But at high speeds, in strong winds, air resistance (the force on an object moving through the air - also referred to as drag) greatly affects how the car accelerates, how much it handles, how it uses fuel.
This is where the science of aerodynamics comes into play, studying the forces generated as a result of the movement of objects in the air. Modern cars are designed with aerodynamics in mind. A well-aerodynamic car cuts through a wall of air like a knife through butter.
Due low resistance airflow, such a car accelerates better and consumes fuel better, since the engine does not have to spend extra power to "push" the car through the air wall.
To improve the aerodynamics of the car, the shape of the body is rounded so that the air channel flows around the car with the least resistance. In sports cars, the body shape is designed to direct the air flow predominantly along the lower part, you will see why below. They also put a wing or spoiler on the trunk of the car. Wing presses back car preventing lift rear wheels, due to the strong air flow when it moves at high speed, which makes the car more stable. Not all rear wings are the same and not all are used for their intended purpose, some serve only as an element of automotive decor that does not perform a direct function of aerodynamics.
The science of aerodynamics
Before talking about automotive aerodynamics, let's go over the basics of physics.
As an object moves through the atmosphere, it displaces the surrounding air. The object is also subject to gravity and resistance. Resistance is generated when a solid object moves in a liquid medium - water or air. Resistance increases with the speed of an object - the faster it moves through space, the more resistance it experiences.
We measure the motion of an object by the factors described in Newton's laws - mass, velocity, weight, external force, and acceleration.
Resistance directly affects acceleration. The acceleration (a) of an object = its weight (W) minus its drag (D) divided by its mass (m). Recall that weight is the product of the mass of the body and the acceleration of free fall. For example, on the Moon, a person's weight will change due to the lack of gravity, but the mass will remain the same. Simply put:
As an object accelerates, the speed and drag increase until the end point where the drag becomes equal to the weight - the object won't accelerate any more. Let's imagine that our object in the equation is a car. As the car moves faster and faster, more and more air opposes its movement, limiting the car to maximum acceleration at a certain speed.
We approach the most important number - the coefficient of aerodynamic drag. This is one of the main factors that determines how easily an object moves through the air. The drag coefficient (Cd) is calculated using the following formula:
Cd = D / (A * r * V/2)
Where D is resistance, A is area, r is density, V is speed.
Drag coefficient in a car
We figured out that the drag coefficient (Cd) is a value that measures the force of air resistance applied to an object, such as a car. Now imagine that the force of the air is pushing against the car as it travels down the road. At a speed of 110 km / h, a force four times greater acts on it than at a speed of 55 km / h.
The aerodynamic capabilities of a car are measured by the drag coefficient. The lower the Cd value, the better the aerodynamics of the car, and the easier it will pass through the wall of air that presses on it from different sides.
Let's consider indicators Cd. Remember the angular boxy Volvos from the 1970s, 80s? The old Volvo 960 sedan has a drag coefficient of 0.36. New Volvo body smooth and smooth, due to this the coefficient reaches 0.28. Smoother and more streamlined shapes show better aerodynamics than angular and square ones.
Reasons Aerodynamics Loves Sleek Shapes
Let's remember the most aerodynamic thing in nature - a tear. The tear is round and smooth on all sides, and tapers at the top. When the tear drops down, the air flows around it easily and smoothly. Also with cars, on a smooth, rounded surface, air flows freely, reducing air resistance to the movement of an object.
Today, most models have an average drag coefficient of 0.30. SUVs have a drag coefficient of 0.30 to 0.40 or more. The reason for the high coefficient in the dimensions. Land Cruisers and Gelendvagens accommodate more passengers, they have more cargo space, large grilles to cool the engine, hence the square-like design. Pickup trucks designed with a purposefully square Cd greater than 0.40.
The body design is debatable, but the car has a revealing aerodynamic shape. Drag coefficient Toyota Prius 0.24, so the fuel consumption of the car is low, not only because of the hybrid power plant. Remember, every minus 0.01 in the coefficient reduces fuel consumption by 0.1 liters per 100 kilometers.
Models with poor aerodynamic drag:
Models with good aerodynamic drag:
Methods for improving aerodynamics have been known for a long time, but it took a long time for automakers to start using them when creating new vehicles.
The models of the first cars that appeared have nothing to do with the concept of aerodynamics. Take a look at Ford's Model T - the car looks more like a horse-drawn carriage without a horse - the winner of a boxy design contest. To tell the truth, most models were pioneers and did not need aerodynamic design, as they drove slowly, there was nothing to resist at such a speed. However, the racing cars of the early 1900s began to narrow down a little in order to win competitions at the expense of aerodynamics.
In 1921, the German inventor Edmund Rumpler created the Rumpler-Tropfenauto, which means "tear car" in German. Modeled after the most aerodynamic shape in nature, the teardrop shape, this model had a drag coefficient of 0.27. The Rumpler-Tropfenauto design never found acceptance. Rumpler managed to create only 100 Rumpler-Tropfenauto units.
In America, a leap in aerodynamic design was made in 1930, when the Chrysler model airflow. Inspired by the flight of birds, the engineers made Airflow with aerodynamics in mind. To improve handling, the weight of the car was evenly distributed between the front and rear axles - 50/50. Society, tired of the Great Depression, did not accept the unconventional appearance of the Chrysler Airflow. The model was considered a failure, although the streamlined design of the Chrysler Airflow was far ahead of its time.
The 1950s and 60s saw the biggest advances in automotive aerodynamics that came from the racing world. Engineers began to experiment with different body shapes, knowing that a streamlined shape would speed up cars. Thus was born the shape of the racing car, which has survived to this day. The front and rear spoilers, spade noses, and aero kits all served the same purpose, directing airflow over the roof and generating the necessary downforce to the front and rear wheels.
The wind tunnel contributed to the success of the experiments. In the next part of our article, we will tell you why it is needed and why it is important in car design.
Measuring drag in a wind tunnel
To measure the aerodynamic efficiency of a car, engineers borrowed a tool from the aviation industry - the wind tunnel.
A wind tunnel is a tunnel with powerful fans that create airflow over an object inside. A car, plane, or something else whose air resistance is measured by engineers. From a room behind the tunnel, scientists observe how air interacts with the object and how air currents behave on different surfaces.
The car or plane inside the wind tunnel does not move, but to simulate real conditions, fans supply airflow with different speed. Sometimes real cars not even driven down the pipe - designers often rely on exact models created from clay or other raw materials. The wind blows over the car in the wind tunnel, and computers calculate the drag coefficient.
Wind tunnels have been used since the late 1800s, when they were trying to create an airplane and measured the effect of air flow in the wind tunnels. Even the Wright brothers had such a trumpet. After World War II, engineers racing cars, in search of an advantage over competitors, began to use wind tunnels to evaluate the effectiveness of the aerodynamic elements of the models being developed. Later, this technology made its way into the world of passenger cars and trucks.
Over the past 10 years, large wind tunnels costing several million US dollars have been used less and less. Computer modeling is gradually replacing this way of testing the aerodynamics of a car (more). The wind tunnels are only run to make sure there are no miscalculations in the computer simulations.
There are more concepts in aerodynamics than air resistance alone - there are also factors of lift and downforce. Lift (or lift) is the force that works against the weight of an object, lifting and holding the object in the air. Downforce, the opposite of an elevator, is the force that pushes an object to the ground.
Anyone who thinks that the drag coefficient of 320 km/h Formula 1 racing cars is low is wrong. A typical Formula 1 racing car has a drag coefficient of about 0.70.
The reason for the high air resistance coefficient racing cars Formula 1 is that these cars are designed to create as much downforce as possible. With the speed at which the fireballs move, with their extremely light weight, they begin to experience lift for high speeds- physics makes them rise into the air like an airplane. Cars are not designed to fly (although the article - a flying transformer car claims otherwise), and if the vehicle starts to rise into the air, then you can only expect one thing - a devastating accident. That's why, downforce must be maximum to keep the car on the ground at high speeds, which means that the drag coefficient must be large.
Formula 1 cars achieve high downforce with the help of front and rear parts of the vehicle. These wings direct the flow of air so that they press the car to the ground - the same downforce. Now you can safely increase the speed and not lose it when cornering. At the same time, the downforce must be carefully balanced with the lift in order for the car to gain the desired straight-line speed.
Many production cars have aerodynamic additions to create downforce. the press criticized for the appearance. Controversial design. And all because all GT-R body designed to direct airflow over the car and back through the oval rear spoiler, creating more downforce. No one thought about the beauty of the car.
Off the Formula 1 circuit, rear wings are often found on stock cars, such as sedans. Toyota companies and Honda. Sometimes these design elements add a bit of stability at high speeds. For example, on first Audi TT didn't originally have a spoiler, but Audi I had to add it when it turned out that the TT's rounded shape and light weight created too much lift, which made the car unstable at speeds above 150 km / h.
But if the car is not an Audi TT, not a sports car, not a sports car, but an ordinary family sedan or hatchback, there is no need to install a spoiler. A spoiler will not improve handling on such a car, since the “family car” already has high downforce due to high Cx, and you cannot squeeze speeds above 180 on it. Spoiler on regular car can cause oversteer or, conversely, reluctance to enter corners. However, if you too think that the giant Honda Civic spoiler is in place, don't let anyone convince you otherwise.
The current regulations allow teams to test in a wind tunnel car models that do not exceed 60% of the scale. In an interview with F1Racing, former Renault team technical director Pat Symonds spoke about the specifics of this job…
Pat Symonds: “Today, all teams work with models of 50% or 60% scale, but this was not always the case. The first aerodynamic tests in the 80s were carried out with mock-ups of 25% of the real value - the power of the wind tunnels at the University of Southampton and Imperial College in London did not allow more - only there it was possible to install models on a movable base. Then wind tunnels appeared, in which it was possible to work with models at 33% and 50%, and now, due to the need to limit costs, the teams agreed to test models no more than 60% at an air flow speed of no more than 50 meters per second.
When choosing the scale of the model, the teams proceed from the capabilities of the available wind tunnel. To obtain accurate results, the dimensions of the model should not exceed 5% of the working area of the pipe. The production of smaller scale models is cheaper, but than smaller model, the more difficult it is to maintain the required accuracy. As with many other issues in the development of Formula 1 cars, here you need to look for the best compromise.
In the past, models were made from the wood of the Diera tree, which grows in Malaysia, which has a low density, now equipment for laser stereolithography is used - an infrared laser beam polymerizes a composite material, resulting in a part with specified characteristics. This method allows you to test the effectiveness of a new engineering idea in a wind tunnel in a few hours.
The more accurately the model is made, the more reliable the information obtained during its blowing. Every little thing counts here, even through exhaust pipes the flow of gases must pass at the same speed as in a real machine. Teams are trying to achieve the highest possible accuracy for the existing equipment in the simulation.
For many years tires have been replaced with scaled-up nylon or carbon-fibre replicas, but significant progress has been made when Michelin has made exact scaled-down replicas of their tires. racing tires. The car model is equipped with many sensors for measuring air pressure and a system that allows you to change the balance.
Models, including the measuring equipment installed on them, are slightly inferior in cost real cars For example, they are more expensive than real cars GP2. This is actually an ultra-complex solution. A basic frame with sensors costs about $800,000 and can be used for several years, but usually teams have two sets to keep the work going.
Every revision body elements or suspension leads to the need to manufacture new version body kit, which costs another quarter of a million. At the same time, the operation of the wind tunnel itself costs about a thousand dollars per hour and requires the presence of 90 employees. Serious teams spend about 18 million dollars per season on these studies.
The costs pay off. An increase in downforce by 1% allows you to win back one tenth of a second on a real track. With a stable schedule, engineers play about that much a month, so in the modeling department alone, every tenth costs the team one and a half million dollars.
Since the first man fixed a sharpened stone on the end of a spear, people have always been trying to find best form objects moving in the air. But the car turned out to be a very difficult aerodynamic puzzle.
The fundamentals of road traction calculations provide us with four basic forces acting on a vehicle while it is in motion: air resistance, rolling resistance, climbing resistance, and inertial forces. It is noted that only the first two are the main ones. Rolling resistance force car wheel mainly depends on the deformation of the tire and the road in the contact zone. But already at a speed of 50-60 km / h, the air resistance force exceeds any other, and at speeds over 70-100 km / h it surpasses them all combined. In order to prove this statement, it is necessary to give the following approximate formula: Px=Cx*F*v2, where: Px – air resistance force; v – vehicle speed (m/s); F is the area of the projection of the car onto a plane perpendicular to the longitudinal axis of the car, or the area of the largest cross-section of the car, i.e. frontal area (m2); Cx is the air resistance coefficient (streamlining coefficient). Note. The speed in the formula is squared, and this means that if it is doubled, for example, the air resistance force is quadrupled.
At the same time, the power costs required to overcome it grow eight times! In Nascar races, where speeds go over 300 km / h, it has been experimentally found that to increase top speed for only 8 km/h, you need to increase the engine power by 62 kW (83 hp) or reduce Cx by 15%. There is another way - to reduce the frontal area of the car. Many high-speed supercars are significantly lower ordinary cars. This is just a sign of work to reduce the frontal area. However, this procedure can be performed up to certain limits, otherwise it will be impossible to use such a car. For this and other reasons, streamlining is one of the main issues that arise when designing a car. Of course, the resistance force is influenced not only by the speed of the car and its geometric parameters. For example, the higher the airflow density, the greater the resistance. In turn, the density of air directly depends on its temperature and height above sea level. As the temperature rises, the air density (and hence its viscosity) increases, while high in the mountains the air is thinner and its density is lower, and so on. There are many such nuances.
But back to the shape of the car. Which item has the best flow? The answer to this question is known to almost any student (who did not sleep in physics lessons). A drop of water falling down takes on a shape that is most acceptable from the point of view of aerodynamics. That is, a rounded front surface and a smoothly tapering long back (the best ratio is 6 times the length of the width). The drag coefficient is an experimental value. Numerically he equal to strength air resistance in newtons created when it moves at a speed of 1 m/s per 1 m2 of frontal area. It is customary to consider Cx of a flat plate = 1 as a unit of reference. So, for a drop of water, Cx = 0.04. Now imagine a car like this. Nonsense, isn't it? Not only will such a contraption on wheels look somewhat caricatured, it will not be very convenient to use this car for its intended purpose. Therefore, designers are forced to find a compromise between the aerodynamics of the car and the convenience of its use. Constant attempts to reduce the air resistance coefficient have led to the fact that some modern cars have Cx = 0.28-0.25. Well, fast record cars boast Cx = 0.2-0.15.
Resistance forces
Now we need to talk a little about the properties of air. As you know, any gas consists of molecules. They are in constant motion and interaction with each other. There are so-called van der Waals forces - forces of mutual attraction of molecules that prevent their movement relative to each other. Some of them begin to stick more strongly to the others. And with an increase in the chaotic movement of molecules, the effectiveness of the impact of one layer of air on another increases, and the viscosity increases. And this happens due to an increase in air temperature, and this can be caused both by direct heating from the sun, and indirectly from the friction of air on any surface or simply its layers among themselves. This is where speed comes into play. In order to understand how this affects the car, just try to wave your hand with an open palm. If you do it slowly, nothing happens, but if you wave your hand harder, the palm already clearly perceives some resistance. But this is only one component.
When air moves over some fixed surface (for example, a car body), the same van der Waals forces cause the nearest layer of molecules to begin to stick to it. And this "stuck" layer slows down the next one. And so layer by layer, and the faster the air molecules move, the farther they are from a stationary surface. In the end, their speed is equalized with the speed of the main air flow. A layer in which particles move slowly is called a boundary layer, and it appears on any surface. The higher the surface energy value of the vehicle coating material, the stronger its surface interacts at the molecular level with the surrounding air, and the more energy must be expended to destroy these forces. Now, based on the above theoretical calculations, we can say that air resistance is not just wind beating on the windshield. This process has more components.
Shape resistance
This is the most significant part - up to 60% of all aerodynamic losses. It is often referred to as pressure drag or drag. When driving, the car compresses the air flow on it and overcomes the effort to push the air molecules apart. The result is a zone of high pressure. Then the air flows around the surface of the car. In the process, the air jets break off with the formation of turbulences. The final separation of the air flow at the rear of the vehicle creates a zone of low pressure. The drag at the front and the suction effect at the rear of the car create a very strong reaction. This fact obliges designers and designers to look for ways to give the body. Arrange on shelves.
Now you need to consider the shape of the car, as they say, "from bumper to bumper." Which of the parts and elements have a greater impact on the overall aerodynamics of the machine. The front of the body. Experiments in a wind tunnel have established that for the best aerodynamics, the front part of the body should be low, wide and not have sharp corners. In this case, there is no separation of the air flow, which has a very beneficial effect on the streamlining of the car. The radiator grille is often not only a functional element, but also a decorative one. After all, the radiator and engine must have effective airflow, so this element is very important. Some automakers are studying ergonomics and airflow distribution in engine compartment as serious as the overall aerodynamics of the car. Incline windshield- a very striking example of a compromise of streamlining, ergonomics and performance. Its insufficient slope creates excessive resistance, and its excessive slope increases dustiness and the mass of the glass itself, visibility drops sharply at dusk, it is necessary to increase the size of the wiper, etc. The transition from glass to sidewall should be carried out smoothly.
But you should not get carried away with excessive curvature of the glass - this can increase distortion and worsen visibility. The influence of the windshield pillar on aerodynamic drag depends very much on the position and shape of the windshield, as well as on the shape of the front end. But, while working on the shape of the rack, we must not forget about protecting the front side windows from rainwater and dirt blown off the windshield, maintaining an acceptable level of external aerodynamic noise, etc. Roof. Increasing the camber of the roof can lead to a decrease in the drag coefficient. But a significant increase in bulge can conflict with the overall design of the car. In addition, if the increase in bulge is accompanied by a simultaneous increase in the area of drag, then the force of air resistance increases. And on the other hand, if you try to maintain the original height, then the windshield and rear windows will have to be introduced into the roofs, since visibility should not deteriorate. This will lead to an increase in the cost of glasses, while the decrease in the force of air resistance in this case is not so significant.
side surfaces. In terms of vehicle aerodynamics side surfaces have little effect on the creation of an irrotational flow. But you can't round them too much. Otherwise, it will be difficult to get into such a car. The glass should, if possible, form a single whole with the side surface and be located in line with the outer contour of the car. Any steps and lintels create additional obstacles for the passage of air, unwanted turbulences appear. You may notice that the gutters, which were previously present on almost any car, are no longer used. Other Constructive decisions, which do not have such a big impact on the aerodynamics of the car.
The rear of the car has perhaps the greatest influence on the streamlining coefficient. It is explained simply. At the rear, the air flow breaks off and forms swirls. It is almost impossible to make the rear of a car as streamlined as an airship (the length is 6 times the width). Therefore, they work more carefully on its form. One of the main parameters is the angle of inclination of the rear of the car. The example has already become a textbook Russian car"Moskvich-2141", where it was the unfortunate solution of the rear that significantly worsened the overall aerodynamics of the car. But in other way, rear glass"Moskvich" has always remained clean. Again a compromise. That is why so many additional attachments are made specifically for the rear of the car: rear wings, spoilers, etc. Along with the angle of inclination of the rear, the design and shape of the side edge of the rear of the car greatly affects the drag coefficient. For example, if you look at almost any modern car from above, you can immediately see that the front body is wider than the rear. This is also aerodynamics. The bottom of the car.
As it may seem at first, this part of the body cannot affect the aerodynamics. But then there is such an aspect as downforce. The stability of the car depends on it and how correctly the air flow under the bottom of the car is organized, as a result, the strength of its "sticking" to the road depends. That is, if the air under the car does not linger, but flows quickly, then the reduced pressure that occurs there will press the car to the roadway. This is especially important for ordinary cars. The fact is that for racing cars that compete on high-quality, even surfaces, you can set the clearance so low that the effect of the "earth cushion" begins to appear, in which downforce increases and drag decreases. For normal cars, low ground clearance is unacceptable. Therefore, designers have recently been trying to smooth the bottom of the car as much as possible, to cover such uneven elements as exhaust pipes, suspension arms, etc. with shields. By the way, wheel arches have a very large impact on the aerodynamics of the car. Incorrectly designed niches can create additional lift.
And again the wind
Needless to say, the required engine power depends on the streamlining of the car, and therefore the fuel consumption (i.e., the wallet). However, aerodynamics does not only affect speed and economy. Not last place occupy the tasks of ensuring good directional stability, vehicle controllability and reducing noise during its movement. With noise, everything is clear: the better the streamlining of the car, the quality of the surfaces, the smaller the size of the gaps and the number of protruding elements, etc., the less noise. Designers have to think about such an aspect as the turning moment. This effect is well known to most drivers. Anyone who has ever driven past a “truck” at high speed or simply drove in a strong side wind should have felt the appearance of a roll or even a slight turn of the car. It makes no sense to explain this effect, but this is precisely the problem of aerodynamics.
That is why the coefficient Cx is not unique. After all, air can affect the car not only "on the forehead", but also at different angles and in different directions. And all this has an impact on handling and safety. These are just a few of the main aspects that affect overall strength air resistance. It is impossible to calculate all parameters. Existing formulas do not give a complete picture. Therefore, designers study the aerodynamics of the car and correct its shape with the help of such an expensive tool as a wind tunnel. Western firms do not spare money for their construction. The cost of such research centers can run into the millions of dollars. For example: the Daimler-Chrysler concern invested $37.5 million in the creation of a specialized complex to improve the aerodynamics of its cars. Currently, the wind tunnel is the most significant tool for studying the forces of air resistance that affect the car.
