This section describes how to determine the aerodynamic drag area (CDA) of your vehicle using the coastdown procedure in 40 CFR part 1066 or an alternative method correlated to it.
(a) General. The primary method for measuring the aerodynamic drag area of vehicles is specified in paragraph (b) of this section. You may determine the drag area using an alternate method, consistent with the provisions of this section and good engineering judgment, based on wind tunnel testing, computational fluid dynamic modeling, or constant-speed road load testing. See 40 CFR 1068.5 for provisions describing how we may evaluate your engineering judgment. All drag areas measured using an alternative method (CDAalt) must be adjusted to be equivalent to the corresponding drag areas that would have been measured using the coastdown procedure as follows:
(1) Unless good engineering judgment requires otherwise, assume that coastdown drag areas are proportional to drag areas measured using alternative methods. This means you may apply a single constant adjustment factor (Falt-aero) for a given alternate drag area method using the following equation: CDA = CDAalt x Falt-aero
(2) Determine Falt-aero by performing coastdown testing and applying your alternate method on the same vehicle. Unless we approve another vehicle, the vehicle must be a Class 8, high-roof, sleeper cab with a full aerodynamics package, pulling a standards trailer. Where you have more than one model meeting these criteria, use the model with the highest projected sales. If you do not have such a model you may use your most comparable model with prior approval. If good engineering judgment allows the use of a single, constant value of Falt-aero, calculate it from this coastdown drag area (CDAcoast) divided by alternative drag area (CDAalt): Falt-aero = CDAcoast / CDAalt
(3) Calculate Falt-aero to at least three decimal places. For example, if your coastdown testing results in a drag area of 6.430, but your wind tunnel method results in a drag area of 6.200, Falt-aero would be 1.037.
(b) Recommended method. Perform coastdown testing as described in 40 CFR part 1066, subpart D, subject to the following additional provisions:
(1) The specifications of this paragraph (b)(1) apply when measuring drag areas for tractors. Test high-roof tractors with a standard box trailer. Test low- and mid-roof tractors without a trailer (sometimes referred to as in a ``bobtail configuration''). You may test low- and mid-roof tractors with a trailer to evaluate innovative technologies.
(2) The specifications of this paragraph (b)(2) apply for tractors and standard trailers. Use tires mounted on steel rims in a dual configuration (except for steer tires). The tires must--
(i) Be SmartWay-Verified tires or have a rolling resistance below 5.1 kg/ton.
(ii) Have accumulated at least 2,175 miles of prior use but have no less than 50 percent of their original tread depth (as specified for truck cabs in SAE J1263).
(iii) Not be retreads or have any apparent signs of chunking or uneven wear.
(iv) Be size 295/75R22.5 or 275/80R22.5.
(3) Calculate the drag area (CDA) in m\2\ from the coastdown procedure specified in 40 CFR part 1066.
(c) Approval. You must obtain preliminary approval before using any methods other than coastdown testing to determine drag coefficients. Send your request for approval to the Designated Compliance Officer. Keep records of the information specified in this paragraph (c). Unless we specify otherwise, include this information with your request. You must provide any information we require to evaluate whether you are apply the provisions of this section consistent with good engineering judgment.
(1) Include all of the following for your coastdown results:
(i) The name, location, and description of your test facilities, including background/history, equipment and capability, and track and facility elevation, along with the grade and size/length of the track.
(ii) Test conditions for each test result, including date and time, wind speed and direction, ambient temperature and humidity, vehicle speed, driving distance, manufacturer name, test vehicle/model type, model year, applicable model engine family, tire type and rolling resistance, weight of tractor-trailer (as tested), and driver identifier(s).
(iii) Average drag area result as calculated in 40 CFR 1066, subpart D) and all of the individual run results (including voided or invalid runs).
(2) Identify the name and location of the test facilities for your wind tunnel method (if applicable). Also include the following things to describe the test facility:
(i) Background/history.
(ii) The layout (with diagram), type, and construction (structural and material) of the wind tunnel.
(iii) Wind tunnel design details: corner turning vane type and material, air settling, mesh screen specification, air straightening method, tunnel volume, surface area, average duct area, and circuit length.
(iv) Wind tunnel flow quality: temperature control and uniformity, airflow quality, minimum airflow velocity, flow uniformity, angularity and stability, static pressure variation, turbulence intensity, airflow acceleration and deceleration times, test duration flow quality, and overall airflow quality achievement.
(v) Test/working section information: test section type (e.g., open, closed, adaptive wall) and shape (e.g., circular, square, oval), length, contraction ratio, maximum air velocity, maximum dynamic pressure, nozzle width and height, plenum dimensions and net volume, maximum allowed model scale, maximum model height above road, strut movement rate (if applicable), model support, primary boundary layer slot, boundary layer elimination method, and photos and diagrams of the test section.
(vi) Fan section description: fan type, diameter, power, maximum rotational speed, maximum top speed, support type, mechanical drive, and sectional total weight.
(vii) Data acquisition and control (where applicable): acquisition type, motor control, tunnel control, model balance, model pressure measurement, wheel drag balances, wing/body panel balances, and model exhaust simulation.
(viii) Moving ground plane or rolling road (if applicable): construction and material, yaw table and range, moving ground length and width, belt type, maximum belt speed, belt suction mechanism, platen instrumentation, temperature control, and steering.
(ix) Facility correction factors and purpose.
(3) Include all of the following for your computational fluid dynamics (CFD) method (if applicable):
(i) Official name/title of the software product.
(ii) Date and version number for the software product.
(iii) Manufacturer/company name, address, phone number and Web address for software product.
(iv) Identify if the software code is Navier-Stokes or Lattice-Boltzmann based.
(4) Include all of the following for any other method (if applicable):
(i) Official name/title of the procedure(s).
(ii) Description of the procedure.
(iii) Cited sources for any standardized procedures that the method is based on.
(iv) Modifications/deviations from the standardized procedures for the method and rational for modifications/deviations.
(v) Data comparing this requested procedure to the coastdown reference procedure.
(vi) Information above from the other methods as applicable to this method (e.g., source location/address, background/history).
(d) Wind tunnel methods. (1) You may measure drag areas consistent with the modified SAE procedures described in this paragraph (d) using any wind tunnel recognized by the Subsonic Aerodynamic Testing Association. If your wind tunnel is not capable of testing in accordance with these modified SAE procedures, you may ask us to approve your alternate test procedures if you demonstrate that your procedures produce equivalent data. For purposes of this paragraph (d), data are equivalent if they are the same or better with respect to repeatability and unbiased correlation with coastdown testing. Note that, for wind tunnels not capable of these modified SAE procedures, good engineering judgment may require you to base your alternate method adjustment factor on more than one vehicle. You may not develop your correction factor until we have approved your alternate method. The applicable SAE procedures are SAE J1252, SAE J1594, and SAE J2071 (incorporated by reference in Sec. 1037.810). The following modifications apply for SAE J1252:
(1) You may measure drag areas consistent with the modified SAE procedures described in this paragraph (d) using any wind tunnel recognized by the Subsonic Aerodynamic Testing Association. If your wind tunnel is not capable of testing in accordance with these modified SAE procedures, you may ask us to approve your alternate test procedures if you demonstrate that your procedures produce equivalent data. For purposes of this paragraph (d), data are equivalent if they are the same or better with respect to repeatability and unbiased correlation with coastdown testing. Note that, for wind tunnels not capable of these modified SAE procedures, good engineering judgment may require you to base your alternate method adjustment factor on more than one vehicle. You may not develop your correction factor until we have approved your alternate method. The applicable SAE procedures are SAE J1252, SAE J1594, and SAE J2071 (incorporated by reference in Sec. 1037.810). The following modifications apply for SAE J1252:
(i) The minimum Reynold's number (Remin) is 1.0 x 10\6\ instead of the value specified in section 5.2 of the SAE procedure. Your model frontal area at zero yaw angle may exceed the recommended 5 percent of the active test section area, provided it does not exceed 25 percent.
(ii) For full-scale wind tunnel testing, use good engineering judgment to select a test article (tractor and trailer) that is a reasonable representation of the test article used for the reference method testing. For example, where your wind tunnel is not long enough to test the tractor with a standard 53 foot trailer, it may be appropriate to use shorter box trailer. In such a case, the correlation developed using the shorter trailer would only be valid for testing with the shorter trailer.
(iii) For reduced-scale wind tunnel testing, a one-eighth (1/8th) or larger scale model of a heavy-duty tractor and trailer must be used, and the model must be of sufficient design to simulate airflow through the radiator inlet grill and across an engine geometry representative of those commonly used in your test vehicle.
(2) You must perform wind tunnel testing and the coastdown procedure on the same tractor model and provide the results for both methods. Conduct the wind tunnel tests at a zero yaw angle and, if so equipped, utilizing the moving/rolling floor (i.e., the moving/rolling floor should be on during the test, as opposed to static) for comparison to the coastdown procedure, which corrects to a zero yaw angle for the oncoming wind.
(e) Computational fluid dynamics (CFD). You may determine drag areas using a CFD method, consistent with good engineering judgment and the requirements of this paragraph (e) using commercially available CFD software code. Conduct the analysis assuming zero yaw angle, and ambient conditions consistent with coastdown procedures. For simulating a wind tunnel test, the analysis should accurately model the particular wind tunnel and assume a wind tunnel blockage ratio consistent with SAE J1252 (incorporated by reference in Sec. 1037.810) or one that matches the selected wind tunnel, whichever is lower. For simulation of open road conditions similar to that experienced during coastdown test procedures, the CFD analysis should assume a blockage ratio at or below 0.2 percent.
(1) Take the following steps for CFD code with a Navier-Stokes formula solver:
(i) Perform an unstructured, time-accurate, analysis using a mesh grid size with total volume element count of at least 50 million cells of hexahedral and/or polyhedral mesh cell shape, surface elements representing the geometry consisting of no less than 6 million elements, and a near-wall cell size corresponding to a y+ value of less than 300, with the smallest cell sizes applied to local regions of the tractor and trailer in areas of high flow gradients and smaller geometry features.
(ii) Perform the analysis with a turbulence model and mesh deformation enabled (if applicable) with boundary layer resolution of 95 percent. Once result convergence is achieved, demonstrate the convergence by supplying multiple, successive convergence values for the analysis. The turbulence model may use k-epsilon (k-[epsi]), shear stress transport k-omega (SST k-[omega]), or other commercially accepted methods.
(2) For Lattice-Boltzman based CFD code, perform an unstructured, time-accurate analysis using a mesh grid size with total surface elements of at least 50 million cells using cubic volume elements and triangular and/or quadrilateral surface elements with a near wall cell size of no greater than 6 mm on local regions of the tractor and trailer in areas of high flow gradients and smaller geometry features, with cell sizes in other areas of the mesh grid starting at twelve millimeters and increasing in size from this value as the distance from the tractor-trailer model increases.
(3) All CFD analysis should be conducted using the following conditions:
(i) A tractor-trailer combination using the manufacturer's tractor and the standard trailer, as applicable.
(ii) An environment with a blockage ratio at or below 0.2 percent to simulate open road conditions, a zero degree yaw angle between the oncoming wind and the tractor-trailer combination.
(iii) Ambient conditions consistent with the coastdown test procedures specified in this part.
(iv) Open grill with representative back pressures based on data from the tractor model,
(v) Turbulence model and mesh deformation enabled (if applicable).
(vi) Tires and ground plane in motion consistent with and simulating a vehicle moving in the forward direction of travel.
(vii) The smallest cell size should be applied to local regions on the tractor and trailer in areas of high flow gradients and smaller geometry features (e.g., the a-pillar, mirror, visor, grille and accessories, trailer leading and trailing edges, rear bogey, tires, and tractor-trailer gap).
(viii) Simulate a speed of 55 mph.
(4) You may ask us to allow you to perform CFD analysis using parameters and criteria other than those specified in this paragraph (e), consistent with good engineering judgment, if you can demonstrate that the specified conditions are not feasible (e.g., insufficient computing power to conduct such analysis, inordinate length of time to conduct analysis, equivalent flow characteristics with more feasible criteria/parameters) or improved criteria may yield better results (e.g., different mesh cell shape and size). To support this request, we may require that you supply data demonstrating that your selected parameters/criteria will provide a sufficient level of detail to yield an accurate analysis, including comparison of key characteristics between your criteria/parameters and the criteria specified in paragraphs (e)(1) and (2) of this section (e.g., pressure profiles, drag build-up, and/or turbulent/laminar flow at key points on the front of the tractor and/or over the length of the tractor-trailer combination).
(f) Yaw sweep corrections. You may optionally apply this paragraph (f) for vehicles with aerodynamic features that are more effective at reducing wind-averaged drag than is predicted by zero-yaw drag. You may correct your zero-yaw drag area as follows if the ratio of the zero-yaw drag area divided by yaw sweep drag area for your vehicle is greater than 0.8065 (which represents the ratio expected for a typical aerodynamic Class 8 high-roof sleeper cab tractor):
(1) Determine the zero-yaw drag area and the yaw sweep drag area for your vehicle using the same alternate method as specified in this subpart. Measure drag area for 0[deg], -6[deg], and +6[deg]. Use the arithmetic mean of the -6[deg] and +6[deg] drag areas as the 6[deg] drag area.
(2) Calculate your yaw sweep correction factor (CFys) using the following equation:[GRAPHIC] [TIFF OMITTED] TR15SE11.012
(3) Calculate your corrected drag area for determining the aerodynamic bin by multiplying the measured zero-yaw drag area by CFys. The correction factor may be applied to drag areas measured using other procedures. For example, we would apply CFys to drag areas measured using the recommended coastdown method. If you use an alternative method, you would also need to apply an alternative correction (Falt-aero) and calculate the final drag area using the following equation:
CDA = Falt-aero [middot] CFys [middot] (CDA)zero-alt
(4) You may ask us to apply CFys to similar vehicles incorporating the same design features.
(5) As an alternative, you may choose to calculate the wind-averaged drag area according to SAE J1252 (incorporated by reference in Sec. 1037.810) and substitute this value into the equation in paragraph (f)(2) of this section for the 6[deg] yaw-averaged drag area.