ABSTRACT

I love aerodynamics! It makes an airplane fly like a bird and keeps a sports car stuck to the track!

Since my childhood, I’ve been fascinated by racecars aerodynamics, and I learn to draw by filling my schoolbooks with Formula 1 car drawings.

In the last decades aerodynamics has had a major role on cars development.

Unlike motorsport, the underside of road cars has been forgotten in this evolution, and, nowadays, the use of a smooth underfloor or even of a rear diffuser is almost limited to high performance sports cars.

The use of an optimized flat and smooth underbody, together or not with an optimized rear diffuser, can result in significant improvements in road and race cars’ aerodynamics, and expected enhancements in handling, performance and fuel economy will be matched with significant ecological footprint reduction.

Laminar-flow control is a technical concept that has been tested for nearly a century to improve airplanes aerodynamics and has already been in use in aeronautics for more than a decade.

In 2001 I had the idea to develop and use this concept to improve aerodynamics not only of the most extreme sports cars, but also of the most common family cars.

I now propose its use in the performance and economy enhancing of road vehicles, from sports to family cars or even other transport vehicles, a concept also applicable to race cars, as the technical regulations allow its use. (Figures 1 and 2)

INTRODUCTION

From the 1930’s there has been huge research on Laminar-flow controlby the great aerospace industry. Its main goal is to reduce shear stress and fuel consumption with the development of aerodynamic solutions to delay or even prevent transition from laminar to turbulent and separation of the airflow over airplane’s wings and tail.

(Figure 3)

In 2001, I was at the beginning of my aerodynamic studies and had the idea to develop this technical concept to be used on motorsport race car’s rear diffuser.

Twenty years later, further development of this original concept led to an innovative technical solution that can be adapted to any sports or family cars or other transport vehicles. While its use in motorsport and sports cars can be motivated for the competitiveness advantage of a performance improvement, in any road vehicle this goal will be matched with economic and ecological improvements.

THE AIRFLOW PROBLEMS

"When a vehicle starts moving, still air is also set into motion" (Katz).

A vehicle in movement displaces the air that surrounds it. This airflow suffers a continuous change on its behavior during its travel along the vehicle’s bodywork. By Newton third law, the vehicle will experience forces and moments from the displaced airflow (White). (Figure 4)

If we consider the vehicle at rest and imagine the air flowing around it at the same real speed, we can better understand the airflow behavior and the forces and moments exerted over its bodywork.

As they flow, air particles react to each other and over the surface they flow over and develop normal or pressure forces and tangential or shear forces. The pressure forces in the smooth free airflow around a vehicle can be computed by Bernoulli’s Equation, except on separated flow. The shear forces develop in the thin boundary layer next to the surface (Figure 5) and depend on the fluid viscosity (Milliken).

If we consider the longitudinal axis of the vehicle as parallel to the free stream, and positive downstream, these forces and moments, that greatly influence the road vehicle’s handling and performance, will act about the three-coordinate axis (White; Milliken) (Figure 6):

The drag is the force on the vehicle along its longitudinal x-axis, results from the integration of all the local shear forces on the vehicle (skin friction drag) plus the integration of all the local pressure force components in the x-direction, mainly due to airflow separation (pressure drag) and acts in the same direction but in opposite sense of the vehicle’s displacement. The drag influences the forward acceleration and braking deceleration at higher speeds, and also the maximum speed and fuel consumption.

The moment about this axis is the rolling moment.

The lift is the force perpendicular to the drag (z-axis) and results from pressure differential on the airflow over and under the vehicle. As it acts normal to the vehicle’s displacement, the lift is responsible for the loads over the wheels that are not vehicle’s mass dependent. The lift influences the longitudinal and lateral acceleration and braking, the directional balance of the car and tires friction coefficient.

The moment about this axis is called yaw.

The side force is perpendicular to both lift and drag (y-axis), results from pressure differential on the airflow over the lateral sides of the vehicle and influences the directional stability and control.

The pitching moment is about this axis.

When the air flows over a concave surface, it slows down, and its static pressure will rise. When the airflow turns around a convex surface, it will accelerate, and its static pressure will decrease. Therefore, most road cars will develop positive lift, a result of the airflow behavior over their rounded upper bodywork that generates instability, mainly felt over the rear wheels (Katz).

To reduce this positive lift, and to generate some downforce or negative lift, we must enhance the flow under the vehicle, thus accelerating the airflow over its undersurface. By properly doing this, significant levels of downforcecan be generated and the increased loads on the tires will improve the car’s performance (Katz).

The vehicle must have a clean and smooth underbody, and there will also be a pressure drag reduction by this faster and less turbulent airflow from under the bodywork, with better rear wake control.

So, if we pay as much attention to the entire underside, as to the upper surface, and properly conceive a smooth undersurface with diffusers, in any road vehicle, we can reduce the natural positive lift and have a good fuel economy, with low drag coefficient and medium and high-speed stability (Katz).

For the best understanding of the airflow behavior over the vehicle, we must consider it separated in two layers (Andersson) (Figure 7):

· An outer air stream or inviscid layer, with particles traveling parallel to each other and along streamlines, without turbulence, separated from the vehicle’s bodywork by the inner layer.

· An inner viscous boundary layer flows close to the bodywork, with the particles being progressively retarded by the shear stressdeveloped in contact with the solid surface. Due to this viscous stress, the velocity of the airflow progressively slow, from the boundary layer outer limit, where the air flows at the outer free air velocity, until zero velocity (the no-slip condition) of the airflow adjacent to the surface.

This inner viscous boundary layerwill grow from zero thickness at the front of the vehicle and large shearing gradients and stresses will develop downstream and over the solid bodywork surface, because of the momentum difference between the no-slip condition at the surface and the large velocities in the main flow (Carpenter).

As a result of this shearing action, the fluid layers adjacent to the surface will suffer a retard. From the surface, where the shearing action is greatest, the retarded boundary layer will grow farther and in thickness.

This friction and viscous shearing stress strongly depend on the slope of the boundary layer velocity profile. Under this stress, the laminar boundary layer will grow to a point where a sudden transition to turbulent flow will occur (Figure 8) (Carpenter).

The transition occurs because of the growth of small disturbances, the Tollmien-Schlichting waves or instabilities, inside theboundary layer. Transition depends on some free-stream turbulence, surface vibrations, sound waves, but mainly on the roughness of the surface and the Reynolds number.

The Reynolds number (Re) is a dimensionless measure of the relationship between the inertial and viscous forces of a flow, and it is commonly accepted that transition from laminar to turbulent airflow will occur, over any aerodynamic surface, for a Re equal or superior to 5 x 10e5.

From this transition point, where the boundary layer suffers a sudden and relevant enlargement, a turbulent boundary layer will grow slowly than a laminar one, but its greater thickness and shear stress will result in an increased skin friction drag (Carpenter). (Figure 8)

As the airflow surrounds the vehicle, it can reach a point where the retarded boundary layer separates form the bodywork. A turbulent and more energetic boundary layer will resist better to the separation than a laminar one, but usually it is only a matter of time and pressure gradients.

Flow separation results in stalled turbulent airflow, with a sudden fall in the lift and an abrupt rise in the pressure drag over any aerodynamic device. (Figure 9)

Considering Bernoulli’s Equation for the incompressible flows outside the boundary layer:

p + (1/2) x ρ x V2 = p0 = constant

p – is the static pressure.

(1/2) x ρ x V2 – is the dynamic pressure.

p0 – is the total pressure.

From this equation, when the airflow velocity increases, as it happens in a convergent duct, its static pressure drops. This is a decreasing or favorable pressure gradient, and the accelerated flow is naturally attached to the duct walls.

On the other hand, when the airflow velocity decreases, as in a diffuser or divergent duct, its static pressure rises. This is called an increasing or adverse pressure gradient, and the retarded airflow will tend to separate from the diffuser’s walls (Carpenter).

In an attached flow the streamlines near the solid surface follow exactly the shape of the solid body. In a separated airflow this doesn’t happen (Katz).

The maximum lift developed by any aerodynamic device is limited by the boundary layer separation or stall over its walls (Carpenter).

The separation of a boundary layerfrom its adjacent solid surface depends on:

· The quality and magnitude of the adverse pressure gradient the flow must overcome.

· The kinetic energy defect in the boundary layerat the start of the adverse pressure gradient, measured by the difference between the kinetic energy of the mainstream flow velocity and the kinetic energy of the boundary layer flow near the wall.

The thinner the boundary layers at the start of the adverse pressure gradient, the farther it can develop before separating. So, it is of the utmost relevance to keep the boundary layer both laminar and as thin as possible, with the smallest kinetic energy defect, to present at the inlet of the diffuser (Carpenter).

The most used technique to better control the boundary layer, in almost a century of investigation on Laminar-flow control, is to remove the low energy airflow layer adjacent to the bodywork, through suction slits or small holes on a flat surface. This will delay the laminar to turbulent flow transition and, further on the airflow pathway, will prevent the boundary layer separation from the aerodynamic surface.

The minimum suction flow velocity, which is necessary to keep the boundary layer laminar, even for large Re, can be calculated as:

Vsuction=1,2 x 10e-4 x Vfree airflow

Behind the suction slit or hole, a thinner and laminar new boundary layer will progress further over and along the solid surface, properly attached and overcoming the adverse pressure gradient (Schlichtingand Truckenbrodt).

“Boundary layer removal by suction can be extremely effective in eliminating separation, delaying stall, increasing circulation, and generally augmenting lift on airfoils.” (Milliken)

A well-conceived suction distribution can delay or even suppress separation and laminar to turbulent transition, making the boundary layer more stable and able to overcome the small disturbances growth (Carpenter):

· By reducing the boundary layer thickness, as for a fixed negative pressure gradient, a critical Reynolds number, based on the boundary layer thickness, must be reached before transition can occur.

· By creating a much fuller velocity profile within the boundary layer, like in a favorable pressure gradient.

In a road vehicle with a flat underbody:

· The proper use of a Laminar-flow control suction technique, over this smooth underbody, can have enormous benefits, both reducing viscous and pressure drag and the free stream airflow static pressure, thus generating some level of downforce.

· If the lower rear undersurface is scanted, with two side plates we can create a diffuser and simulate an underbody tunnel Venturi. This geometry can generate very large values of negative lift (Katz).

A diffuser is an expansion intended to reduce velocity to recover the pressure head of the flow (White). When used in the aft underfloor on a sports or race car, its function is to drain as much air as possible, to increase the airflow velocity under the bodywork, thus increasing the downforce generated there. (Figure 10)

When a free airflow enters a diffuser, its velocity drops, and its static pressure rises. To avoid the flow separation over the walls, on this adverse pressure gradient, the best geometry for a diffuser is in the range 5 degrees <2θ<15 degrees (White).

2Θ is the angle of divergence of the diffuser.

The efficiency of a diffuser is measured by its Cp or “pressure-recovery coefficient”:

Cp = 📷

Subscript e – for diffuser exit.

Subscript t – for diffuser throat or inlet.

The higher the Cp the most efficient a diffuser is, and the most important parameters that influence this coefficient are (White):

· The area ratio:

AR=📷

· The divergence angle: 2θ

· The inlet boundary layer blockage factor:

Bt=📷

A1 – is the diffuser inlet or throat area.

A2 – is the diffuser exit area.

Abl – is the diffuser area of the inlet or throat blocked by the retarded boundary layer.

So, if we can increase the divergence angle, and consequently the area ratio, and decrease the inlet boundary layer blockage factor, there will result in improved diffuser efficiency, with a much higher Cp and an increased airflow draining capacity.

The free stream airflow under the bodywork would then be further accelerated, with an additional drop in its static pressure and the development of higher downforce levels. Simultaneously both viscous and pressure drag would be reduced, with, respectively, the laminar to turbulent transition and separation of the airflow both delayed.

MY INNOVATIVE TECHNICAL SOLUTION

The use of the exhaust gases kinetic energy in race cars began in 1983, with the exhaust pipe outlets opening and blowing inside the rear diffusers, as an evolution of the “Upper-surface-blowing” technical concept, previously and successfully used, since the seventies, in aviation. (Figure 11)

In this concept, the kinetic energy of the exhaust gases increases the airflow velocity over the airplane’s wings and under the car’s bodywork, to generate greater levels of lift and downforce, respectively. Additionally, the unavoidable increase in flow turbulence that this blown diffuser technique generates will create more friction and loss of energy and efficiency.

In the last years some constructors have made use of this concept in the rear diffusers of their road sports cars, like the Bugatti Veyron. In the more recently presented Ferrari 815 Competizione there has been used of an outside blowing diffuser.

The research on Laminar-flow control, made by the great aviation industry during the last century, aims to develop aerodynamic surfaces and technical solutions to delay or even prevent the flow separation and transition from laminar to turbulent. Its main goal is to reduce both the pressure and skin friction, thus increasing the lift and reducing viscous drag, respectively, improving fuel consumption economy, but also to stabilize the airflow and avoid the increased noise and vibrations of turbulence and separation.

There are two main classic concepts on this research:

Natural Laminar-flow control (NLFC) with the development of aerodynamic shapes that naturally delay the transition. This investigation led to the development of notorious series of low drag airfoil shapes and wings.
Hybrid Laminar-flow control (HLFC) in which the low drag wings make use of active mechanical devices to remove the low-speed retarded boundary layer through micro perforations on their surfaces.

The development of this last concept (HLFC) that I’ve made led to a technical innovation that can be used on sports cars, race cars and motorbikes or road vehicles, with improved fuel economy and ecological benefits and an increase in handling and performance.

This innovative technical solution of mine not only removes, by aspiration, the slow retarded boundary layer and delays or even prevents the airflow transition and separation over its underside and inside its rear diffuser, but also increases the downforce generated, with the free air stream additional suction. (Figure 12)

This innovation makes use of the usually lost kinetic energy of the gas escape from the vehicle’s thermal engine.

MY TECHNICAL SOLUTION DETAILS

The technical solution I’ve developed can be used in sports and in family cars or on any other road or track vehicle. As shared advantages there is an improvement in handling and fuel economy and the reduced carbon footprint. With sports or race cars and motorbikes we can also expect an increase in global performance and competitivity. (Figure 13)

The technical device consists of the underfloor of the vehicle and/or its rear diffuser, for one side, and the gas escape exhausts final internal tunnel for the other side.

The underside of the vehicle can be all flat, eventually have venturi tunnels and a rear diffuser.

The gas escape exhausts, after passing the silencer and the catalyst, will rest horizontally over the inside of the underfloor. From this point until its exit, at the vehicle’s rear, the exhaust will form a final internal tunnel positioned over the underfloor and/or extended over the rear diffuser. (Figure 14)

The technical device must have very small holes or transverse slits open on the underfloor, on the venturi tunnels and/or on the rear diffuser and go through the over positioned gas escape exhaust final internal tunnel. These holes or slits put in communication the outer air that flows under the underbody and/or inside the venturi tunnels or the rear diffuser and the escape gas from the vehicle’s thermal engine, flowing on the final internal exhaust tunnel.

The small holes or slits will promote the outer airflow to be aspirated to the final internal tunnel exhaust by the high velocity gas escape from the engine and must be designed to prevent the gas escape from flowing out through them. (Figure 15)

So, these holes or slits must be vertical or, ideally, develop from down to up and from the front to the rear, making a 450 to 600angle open, up and to the aft, along the longitudinal axle of the vehicle. Its outer borders and inside aft should be made in smooth chamfer and the inside front ones in sharp edges with the same angle of the holes or slits. (Figure 16)

Due to the high gas escape temperatures, from 600 Celsius degrees in road cars up to 900 Celsius degrees in high-performance sports cars and motorbikes engines, an appropriate thermal insulation must be taken care of all around the final exhaust internal tunnel, namely in its underfloor interface.

BUSINESS BENEFITS

The main advantages for the car or motorbike manufacturer that uses this innovative solution in its models will depend on the kind of vehicles where the device will be applied.

· In a sports or competition car or motorbike (Figure 17) we can expect:

o Additional performance with better handling

o Competitiveness improvement

o Carbon footprint reduction

· When applying in a familiar or other transport vehicle (Figure 17) we can expect:

o Better handling

o Improved the fuel economy

o Carbon footprint reduction

There can be expected several prestigious marketing and image advantages to any company that consider using this solution on its models:

· The exclusive use of the most advanced diffuser ever made.

· The exclusive use of innovative technology that relates high performance with ecology sensitivity.

· The planet friendly image for the carbon footprint reduction.

SUMMARY

The long-term research on Hybrid Laminar-flow control led to the development of technical solutions that already have been applied to commercial airplanes. Its main goal is the fuel economy and consequent reduction in costs and carbon footprint.

From this aerospace concept, I developed a technical solution that recovers the usually lost kinetic energy of the gas escape from the car or motorbike thermal engine to improve road and race vehicle aerodynamics.

My innovative technical solution delays the laminar to turbulent airflow transition, prevents flow separation and increases the flux over the underfloor, on the venturi tunnels and/or the rear diffuser, consequently reducing the drag and increasing the downforce generated.

This solution makes use of small holes or slits made on the underfloor, on the venturi tunnels and/or on the rear diffuser, that put in communication the outer air that flows under the vehicle, with the gas escape flow on the final internal tunnel exhausts.

I propose its use in all kinds of track and road vehicles, from sports to family cars or other transport or competition vehicles, applied to a flat underfloor, to Venturi tunnels and/or to a rear diffuser.

The use of this technological innovation will improve the vehicle´s handling, performance and competitivity, with better fuel economy and carbon footprint reduction.

CALL ME!

If you want to be in the first line of technological development, and your models to be considered state-of-the-art in modern aerodynamics, contact me!

And do it fast…

I intend to keep my technology as an exclusive of my few prestigious clients!

BIBLIOGRAPHY

· Anderson, J., “Fundamentals of Aerodynamics” - McGraw Hill, 1991.

· Carpenter, P.W.; Houghton, E.L., “Aerodynamics for Engineering Students” -BH, 2003.

· Katz, J., “Race Cars Aerodynamics: Designing for Speed” – B, 1997.

· Milliken, W.F.; Milliken, D.L., “Race Car Vehicle Dynamics” – SAE International, 1995.

· Schlichting, H.; Truckenbrodt, E., “Aerodynamics of the Airplane”, McGraw-Hill, 1979.

· White, F.M., “Fluid Mechanics”, McGraw Hill, 2007.

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