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sm aerodynamics

Citroën SM windtunnel

The SM's body was developed in the wind tunnel and "sculptured" by the wind. Its steel and glass lines are free from sharp angles and flat surfaces. Designed by the styling division the profile of the SM is not only based on requirements such as air penetration coefficient and frontal area but also lift data resulting in a tapering rear end.

I - Generalities on the aerodynamic studies for a high-performance vehicle

The incessant increase in cars' top speed urges design and research centers to get involved in solving always more important aerodynamic problems. Performances, safety, comfort and economy may be considered as parameters widely depending on the seriousness put in facing the studies of aerodynamic phenomena. Among bigger-displacement European Granturismos today in production, there are many capable of easily going beyond the longed for goal of 200 Km/h, thus reaching the cruising speeds of planes built around the thirties. The history of aeronautics reminds us that it is right at that time, that tunnel testing acquired a decisive importance due to the aggravation of the problems set by aerodynamics with the increase in performances. At this moment, car designers have thus to cope with a similar situation, specially if they wish to achieve a perfect tuning of very fast vehicles, whose behavior and driving should not be exclusive appendage of a small elite of specialists. The field covered by these researches is very wide, going beyond the typical drag, of course conditioning absolute performances and remained over a long period of time the main worry of a minority of manufacturers. The problems of trajectory stability are bound to the distribution of aerodynamic loads of lift and side thrust on the vehicle's two axles: when the mobile moves within a field of velocity varying in intensity and direction (as it happens in presence of wind, either transversal or longitudinal) ,the system of forces to which it is subjected varies considerably. Delivery of interior air circulation, either for engine and brake cooling or for passenger compartment ventilation, are bound to the field of the pressures settling on the car's body. Among the various phenomena - from aerodynamic noise engendered, for instance, by drips or eaves to the effects of windshield wipers detachment at high speeds - numberless are the problems of which an effective and often rapid solution can be foreseen through serious preventive studies, consisting of tunnel tests either on scale models or on the full-size vehicle.

2 - Wind tunnel testing

It is at the origin of a shape study for a new model that the specialist in aerodynamics makes his entry: namely, it will be the first plaster model to provide - well ahead of the vehicle's formal finalization - the data needed to evaluate its potential performances and their comparison with the specifications of the product guidelines. These results are achieved starting from tunnel testing, where wholly particular precautions are necessary in order to obtain a satisfactory road/wind-tunnel correlation.

2.1 - Ground simulation.

One of the difficulties encountered in the aerodynamic 'lab' study of a road vehicle, comes up when one wishes to reproduce - in order to insure the similarity among the respective fluid flows (ecoulements) -the phenomena connected with the body proximity to the ground. On the road and in absence of wind the air is still as compared to the ground, while in the tunnel the model integral with the ground is swept by a wind equivalent in absolute value, but of opposite direction to that of the car's. This arrangement entails the engendering, on the walls bordering the experience flow, of a boundary layer (fluid whose energy is degraded by the viscous surface friction) which deeply modifies the phenomena bound to the ground-effect. Besides, this boundary Iayer tends to detach from the wall 1f stressed by positive pressure gradients due to the presence of the scale model. On the road the absence of this sort of phenomenon calls therefore for the adoption of experimental devices, suitable for the carrying out of either its entire suppression or its minimization as compared to the model's size. A first simulating device may be obtained by means of a rollaway running in the same direction and at the same speed of the overall air flow. Though obtaining under similar conditions the respect of the phenomena connected with the ground presence, this device entails various complications at the stage of its practical implementation as to the fixing of the model on the band and, what is worse, considerable difficulties of measurement bound to interaction phenomena between the model and its support. The solution adopted by CitroŽn (particularly visible in the largest color picture on page 47) consists of representing the road by means of a counter-floor composed of a very smooth flat surface located above the vein floor and dipped in the constant-speed flow, in such a way as to eliminate the boundary layer of the latter. Several experiments performed in English and American aerodynamic labs confirmed the validity of this method for moderate-lift bodies, as in the case of automobiles. To maintain an air delivery under the car identical in scale to that existing on the road, the clearance under the model's wheels is increased by the thickness of the boundary layer developing on the surface simulating the road. On the other hand, this value is reduced to the minimum (2 mm in tests on 1:5 scale models with the CitroŽn device) when a thorough surface 'finishing' of the counter-floor is insured. This preliminary stage is extremely important and requires the maximum accuracy as the validity of subsequent experiments depends on it.

2.2 - Body shape model.

The model scale in use at CitroŽn is 1:5, which permits a satisfactory compromise between the accurate reproduction of certain details, execution rapidity (It is necessary that the model be readied prior to selecting the final shape) and ease of handling. A similar scale further permits to obtain in wind tunnel testing a sufficiently high number of Reynolds (parameter, whose importance will be hinted at later). Initially modeled in plaster on an underbody conforming to that foreseen for the future vehicle, the body shape can be repeatedly modified until determination of the nearly final one in plastic material. The opposite picture shows the appearance of such a model served for the CitroŽn SM development: though made in the configuration without interior flow, it does already entail an accurate representation of the underbody particulars. A certain amount of minor details, engendering flow conditions differing from the real size, are purposely omitted just because of their reduced dimensions. Simulation of under-hood air circulation - needed to obtain exact values of the aerodynamic characteristics - brings to a thorough layout of air entries and exits as well as the execution of particulars under the hood, as shown by the pictures on this page referring to the same model. The losses of aerodynamic bad of the radiators are simulated by means of adequately permeable grillings.

2.3 - Checking on the shape model of the body aerodynamic efficiency.

The two main measurements carried out on the scale model are on one side the determination of the aerodynamic resultant and on the other the measurement of pressure distribution on the body walls. This information will be used together as regards the aerodynamic aspect of the problem while the pressure diagram will be precious help to engineers responsible for interior air circulation (both in engine and passenger compartment air conditioning and ventilation).

2.3.1 Measurement of the six aerodynamic components.

In the most common case, the ensemble of air reactions on the model can be reduced to a resultant and a moment which break up into three forces and three moments according to a reference trihedron we would assume tied to the vehicle. Lift, drag, side thrust and their respective pitch, roll and yaw moments. The model is set on an aerodynamic balance (buried into the thickness of the counter-floor) that will provide the 6 components sought for, to which it is anchored by means of 4 pins (integral with the weighed part of the balance) fixing the model under the wheels in order not to introduce any interference in the flow. The support of the balance rotates around a vertical axis, allowing the setting in aerodynamic drift oft model being tested: this arrangement permits to observe apart from the aerodynamic behavior in presence offside wind, all the configurations in which the relative speed is not parallel to the vehicle's plane of longitudinal symmetry (as it happens when turning, sliding athwart etc... where a transversal component exists in absence of actual wind). Generally overlooked in tunnel tests touring car models is the influence the Reynolds' number, adimensional parameter usually indicating the nature of the flow and playing a very important role in the similarity of the two ťcoulements (in test and in reality). For every new model. however, Citroën performs a test with a variable number of Reynolds to check the constancy of the aerodynamic coefficients. which is obtained with Re above 2.iO. The measuring results of the longitudinal force called drag provide the data for calculation of the performances, which will become always more precise as the model will acquire a formal definition and will complete its equipment (air circulation inside the engine compartment, exhaust details). The drag absorbs almost 50 per cent of the driving power at 60 Km/h, 750/o at 160 Km/h: these figures underline the interest in the search for the most favorable shape for vehicles going beyond 200 Km/h. On this page at top we are presenting for example (picture 1) the drag evolution R~ of a 1:5 scale model full (that is without inner ducting) of the CitroŽn SM for different values of the yew angle J0 at a 160 Km/h speed. This graph shows a moderate increase in the adimensional shape coefficient CX with the varying of J0, indicating a feeble action of the cross wind on the drag. This curve, obtained after numerous experiments, meets the specs posed by the product guidelines. Once the final shape allows the achievement of the expected performances, It is better to check that the latter be attained under satisfactory safety conditions, with particular reference to direction stability. The examination of the lift (Rz) and side thrust (Ry) curves as well as their respective moments, will permit to check that no disquieting phenomenon of aerodynamic origin comes up to abruptly modify the dynamic behavior of the vehicle (which is usually translated into curves free from discontinuities under yawing). Lift measurements on each axle inform on the variations of the vertical loads set on the wheels and play an important role in case of yawing. The effect of a cross wind on the lift entails a considerable increase in its coefficient CZ, due to the wakes (shape resistance) of the uncovered lower parts of the wheels. This component decreases the vertical bad on the wheels at the same moment when a transversal force comes up, which tends to stray the car laterally; the thrust is zero if the yaw angle is nulI and varies linearly with It. The point of intersection (called center of side thrust) of this force with the symmetry plane is located on most vehicles before the center of gravity, which determines a destabilizing moment - namely static aerodynamics instability and It is conceivable that ~beyond a certain speed the directional power of front wheels is insufficient to insure maintenance of a trajectory. The tests done in the CitroŽn wind tunnel on a SM scale model have shown no discontinuity in the lift (picture 2 on previous page) and side thrust ~picture 3) curves. up to the yaw angle tested j 20:. To fix one's mmd, this angle corresponds to a transversal wind of 63 Km/h for a vehicle's running speed of 200 Km/h The comparative behavior of the three lift curves shows for example how at j 15: (that is with a transversal wind of 54 Km/h associated with a longitudinal speed of 200 Km/h) the lift Iightens the front axle about twice as much as compared to that induced by motion at the same speed but in still air. The calculation does however indicate that under this latter circumstance. the front lifting force of aerodynamic origin does not exceed a value ∑'n the order of 45 daN, by virtue of a correct shaping of the front end; quantities;so 50 reduced may thus be neglected even in presence of a strong side wind. The aerodynamic study of the CitroŽn SM has been completed by a series of tests. destined to emphasize the influence of the longitudinal trim on the different components. Although the vehicles:cles provided with hydro-pneumatic suspensions maintain a constant trim, the knowledge of the efforts to its varying is advisable inasmuch as configurations of this kind may occur casually on the road (high speed braking. 'humping and ditching' etc.). It is further advisable to get interested in the reaction of a model invested for example by strong wind blasts with random character: even though researches in this field have not yet reached a stage of immediate practical utilization. they are no doubt destined to bring new knowledge in the study of directional stability at high speeds.

2.3.2- Pressure measurements.

The testing of scale models in the wind tunnel is completed by the measurement of pressure distribution on the body, which usually requires the taking of 600 to 700 pressures. As we have already said, the graph of the isobaric curves is indispensable to successfully solve the problems of under-hood air circulation, of interior ventilation, of air entry and exit positioning. The layout on page 46 (at bottom) shows the distribution of static pressures measured on the wall in the symmetry plane of the vehicle.

2.3.3 - Flow visualization.

Usually associated with the above said measurements are several visualization tests of the fluid flow on the vehicle's walls, by means of>smoke or wool tufts; the use of one of the two systems is complementary and not substitutive of the other and together allow a direct understanding of the physical phenomena, particularly useful in case the model is set 'athwart' to simulate a side wind. The quality of the flow may thus be evaluated, visualizing its possible detachments and get sure (together with the examination of the curves of the aerodynamic coefficients) of the absence of abrupt discontinuities in its evolution which might engender unstable forces and therefore start reactions impredictable at high speeds. Astride of these two pages we have grouped a few pictures regarding experiments of this kind made on the 1:5 scale model of a SM in the CitroŽn 'soufflerie'. The two photographs at top (as well as the color picture on cover) of the visualization by smoke show how on the vehicle's symmetry plane the external flow is just a little disturbed by the presence of the scale model and how the turbulent wake downstream of same is of feeble intensity. The state of the flow on the model's dorsal and lateral walls is instead shown by the photographs of wool tuft visualization (at center and below) presenting - from left to right, respectively - the flow behavior obtained at zero, moderate (j = 150) and considerable ci = 400) yaw. From the comparative examination between the first two pairs of pictures, it gives that the flow is scarcely influenced by the presence of moderate side wind (to give an idea j = 150 corresponds to a translation speed of 200 Km/h associated with a 50 Km/h wind). The third pair of pictures emphasizes instead the turbulent state involving the leeward side and the starting of a small area of detachment on the backlight surface. The extension of the transition zones from the laminar state to the turbulent one may be checked, thus permitting to get sure of the variation continuity in the aerodynamic efforts. The last two photographs of this group illustrate the flows setting on the two sides of a vehicle moving in presence of a strong side wind: better than all comments they serve to explain the appearance of the violent transversal thrusts of aerodynamic origin. Here, the yaw angle is 350, corresponding to a 150 km/h speed of the vehicle submitted to a wind of about 85 Km/h.

2.4 - Full-size tests.

The actual testing of the vehicle (final and probatory stage of its design) is done on the road. It entails, however, a certain amount of drawbacks above all when it has to be dealt with a prototype to keep secret. Full-size tunnel testing does than present several advantages: first of all being practically[ly reproducible, they enable to carry out a systematic research impossible to make on the road, due to the considerable changes in the parameters connected with weather conditions, road state and altitude etc.; the automated control of the tests and data processing by means of a computer speed up the acquisition of results: it is easier to keep the secret in the laboratory than on the road; in case of special tests one can 'see' what is going on by visualizing the flow on as different as inaccessible areas (brake disc for example), which 'is inconceivable on the road; some well equipped wind tunnels may simulate various climate conditions, while the testing installation permits to absorb the engine power in different running configurations. . The computer-aided sorting of the data ijs very tast and allows the utilization of a pieasuring method otherwise unusable. Among the several problems examined and solved through the tunnel experimentation of full-s ize prototypes, worth noting are those regarding windshield wipers, rear window, air entries and exits, aerodynamic noise, exhaust pipes, ventilation and conditioning circuits.
(From an article in Style auto 31 1972)

Citroën SM windtunnel