I need to identify what is the car drag force in function of the velocity. We can find many equations but I want to know what is the best procedure and the most used model or equation to estimate the drag force and coefficient.
Thanks
Dear Enrique, There are several possible origins for aerodynamic drag. The most important sources of drag for cars are :
1) Parasitic (or profile) drag, Do: Two possible contributions/physical phenomena:
1.1) Friction drag, Df: due to friction between the vehicle's surface (aircraft or car skin) and the air moving around it. This drag is due to the viscosity of the air.
1.2) Pressure (or wake) drag, Dp: associated to the wake formed downwind the vehicle and, roughly speaking, associated to the frontal area of the vehicle, and whether it is blunt or streamlined. For aircraft, which are streamlined and with high aspect ratios, the wake drag is small while the wet surface (surface in contact with air, where friction is acting) is very large. Therefore, the friction drag is dominant in aircraft. Conversely, for cars, trucks, etc. the wake drag is dominant. In speed/race cars, both contributions to drag are approximately of the same order of magnitude
The drag force is generally modeled as: D = 0.5•rho•V^2•S•Cd; where tho is the air density, V is basically the speed of the car, S is the frontal area of the car if we assume that the wake drag is dominant, and Cd is the parasitic drag coefficient. To accurately estimate the latter, you need to use either wind tunnel testing of your car model, or simulations with Computational Fluid Dynamics (CFD) software, etc. But for preliminary computations, it should range from around 0.25 to 1.5 (I know it's a broad range and a factor 6 in the final drag value, but it's the best I can tell you without knowing the exact geometry of the car).
Hope this information is useful,
Jose
Dear Enrique, There are several possible origins for aerodynamic drag. The most important sources of drag for cars are :
1) Parasitic (or profile) drag, Do: Two possible contributions/physical phenomena:
1.1) Friction drag, Df: due to friction between the vehicle's surface (aircraft or car skin) and the air moving around it. This drag is due to the viscosity of the air.
1.2) Pressure (or wake) drag, Dp: associated to the wake formed downwind the vehicle and, roughly speaking, associated to the frontal area of the vehicle, and whether it is blunt or streamlined. For aircraft, which are streamlined and with high aspect ratios, the wake drag is small while the wet surface (surface in contact with air, where friction is acting) is very large. Therefore, the friction drag is dominant in aircraft. Conversely, for cars, trucks, etc. the wake drag is dominant. In speed/race cars, both contributions to drag are approximately of the same order of magnitude
The drag force is generally modeled as: D = 0.5•rho•V^2•S•Cd; where tho is the air density, V is basically the speed of the car, S is the frontal area of the car if we assume that the wake drag is dominant, and Cd is the parasitic drag coefficient. To accurately estimate the latter, you need to use either wind tunnel testing of your car model, or simulations with Computational Fluid Dynamics (CFD) software, etc. But for preliminary computations, it should range from around 0.25 to 1.5 (I know it's a broad range and a factor 6 in the final drag value, but it's the best I can tell you without knowing the exact geometry of the car).
Hope this information is useful,
Jose
In addition to the answers above: Please see my publication "Introduction into longitudinal dynamics". Here you will find the general equations and some exemplary value ranges of the different coefficients.
The drag characteristics can also be determined by a rolling test on a horizontal road (no wind is recommended). Measure the speed of the car, disconnect the clutch and measure the time / distance until stand still (or a certain lower velocity). After that, you will know the deceleration of the vehicle, which is induced by the sum of drag forces. In this way, you can calculate (estimate) the total drag force.
Kind regards,
Mario
CFD Model,
Wind Tunnel Scale model, then full size.
When you have an actual vehicle you can do a 2 ways coast down parasitic loss one a straight away: Estimate Chassis parasitic losses, what is left must be aerodynamic. This cannot be done with a model as part of the flow is affected by under-hood flow and rotating wheels.
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