Automotive Aerodynamic Experimental Techniques
Automotive aerodynamic experimental techniques fall into two general categories:
(1) Wind tunnel testing
(2) Road testing.
Wind tunnels vary in size from miniature table-top devices that are almost toys to huge facilities that can hold full-size vehicles that are not models, but the real road vehicle. Historically, wind tunnels have been used to test air foils, propellers and models of all scale.
The model or shape under study is placed in the tunnel and rigged for testing. This rigging may be for determining drag, lift, pressure patterns or flow over the surface.
Advantages of tunnel testing include:
(1) an absence of weather interference;
(2) control of air velocity;
(3) control of air turbulence;
(4) instrumentation for measurement of drag, lift, yaw, surface pressure and flow patterns are all easily set up and may be left in place from day to day;
(5) access to work shops (usually found nearby or incorporated into the facility)
(6) no interference by road traffic; and
(7) no legal constraints on velocity of testing.
Disadvantages of tunnel testing include:
(1) very high capital cost (this varies with size and tends to be exponential ... the 3rd power based on size and/or velocity, at least!);
(2) high cost of operation, ownership, and upkeep (This requires the facility must have a high utilization in order to be economical.);
(3) accuracy of results which stems from problems related to:
(a) less than full scale models;
(b) the “ground effect” (in fact, the absence of the ground effect contributes to some artificiality of tunnel testing for road vehicles);
(c) the relation of the size of the tunnel to the model size;
(d) deviations from actual shape, particularly in small scale models, and intricate detail from the real, full size device.
Variations in Wind Tunnel Types
Since ownership of wind tunnels carries a high cost, there are a number of significant departures from full scale size. If the need for information can be limited, large savings can be obtained.
Categories of tunnels:
(1) Full size tunnel;
(2) Scaled-down tunnel;
(3) Smoke-stream tunnels;
(4) Force only tunnels;
(a) one-force (such as drag or lift) only;
(b) two-force (such as drag and lift);
(c) three-force (where the forces on a shape, model, or vehicle can all be measured);
(5) Vehicle-mounted devices:
(a) tunnel mounted above slip-stream (outside the boundary layer);
(b) test probe or instrument/force stinger positioning shape (model) into the undisturbed air flow above the slip-stream (outside the boundary layer).
Road tests usually involve taking the full-size vehicle and performing various test on it “in the field”; that is, on the road. Public roads offer some constraints but, in many instances, can be overcome. Test tracks offer more technical freedom.
Models or scaled-down versions are also used where the smaller size is more economical or the use or availability of the full-size vehicle is impractical.
The advantages of road testing are real and powerful:
(1) often the full-size vehicle (that is, the actual vehicle, not a model or mock-up) can be used. This avoids the costly construction of a model which sometimes have significant differences from the prototype or production machines;
(2) the results can carry greater accuracy due to the use of the full-size test vehicle and the elimination of ground effect considerations;
(3) there is no problem of the test vehicle size relationship to tunnel size since the test is conducted in open air and the vehicle is in actual operating conditions;
(4) capital investment can be held to a minimum through the use of ingenious techniques;
(5) operating costs and upkeep are nil, especially if quiet, back roads are used;