Electric Car Body Drag Reduction: Rear Taper and Underbody

The problem

For an electric car at highway speeds, aerodynamic drag is the largest single loss, so every reduction in Cd adds range. Rear-end shape and the rough underbody are two of the biggest contributors on a typical hatchback. A structured CFD study on a generic body teaches how designers trade cabin space and styling against drag.

Objectives

• Build a generic, parametric compact car body with controllable rear taper and underbody features.
• Set up an external aerodynamic study with moving ground and a mesh-convergence check.
• Measure Cd for rear taper angles, a flat underbody, a rear diffuser and wheel covers.
• Estimate the highway energy and range effect of the best combination.

What you need

Key concepts

Rear taper (boat-tailing)

Gently tapering the rear of the body lets the flow stay attached longer and reduces the size of the low-pressure wake, which is the main source of pressure drag.

Underbody drag

Suspension parts, exhaust tunnels and rough floors disturb air beneath the car. EVs can have flatter floors because the battery is a flat slab, which helps drag.

Diffuser

An upswept rear underbody section that slows the underbody flow gradually, recovering pressure and reducing wake size if it is not too steep.

Turbulence model

Flow Simulation uses a k-ε model with two-scale wall functions, which is adequate for comparing body variants but less accurate for exact separation points.

Step-by-step guide

1. 1

   Set targets and example vehicle

   Define a generic compact hatchback: example length 4.0 m, width 1.75 m, height 1.55 m and wheelbase 2.55 m. Choose a test speed of 100 km/h. Plan the variants: rear taper angles of 0, 6, 10 and 14 degrees, a flat underbody panel, a rear diffuser and wheel covers. Output Cd, CdA, lift, and drag power at 100 km/h. Note that the body is generic and must not copy a real model.

   Checkpoint: A study plan with dimensions, speed, variants and outputs, labelled as a generic example.

2. 2

   Build the baseline body

   Sketch side, top and front profile curves on the three planes, using global variables for overall dimensions. Build surfaces with Boundary Surface and Lofted Surface, knit them with Knit Surface and use Try to form solid, or thicken. Keep the body smooth; check curvature with Curvature and Zebra Stripes and remove kinks that would cause false separation. Add simple wheel cylinders and wheel arches cut into the body.

   Checkpoint: A single closed solid body, with smooth zebra stripes across main surface boundaries.

3. 3

   Make the rear and underbody variants

   Drive the rear taper by an angle dimension on the roof and side trailing curves, so one value changes the shape, and create a configuration for each angle. Add a rough underbody baseline with blocks representing suspension and the floor tunnel, and a configuration with a flat panel covering them. Add a diffuser configuration with an upsweep angle, and wheel-cover configurations with flat discs. Keep wheelbase and frontal area constant where possible.

   Checkpoint: All configurations rebuild and frontal area stays within a small tolerance across variants.

4. 4

   Set up the external study

   Run the Wizard as External analysis with air at 20 °C, laminar and turbulent flow, and free-stream velocity of 100 km/h along the car's axis. Use a half model with a symmetry boundary on the centre plane, since the generic body is symmetric. Size the domain about three lengths ahead, six to eight behind, and two to three to the side and above. Lower the domain floor to the tyre contact line.

   Checkpoint: The domain and symmetry plane are set, and a short trial run starts without geometry warnings.

5. 5

   Apply moving ground and rotating wheels

   Add a ground plate and apply a Real Wall condition with translational velocity equal to the free stream, so the road moves relative to the car. Optionally give the wheels a rotating wall velocity matching road speed. Note which you used, because both change underbody flow. Insert Global Goals for force in X and Z, and Equation Goals for Cd and CdA using frontal area measured from a silhouette sketch.

   Checkpoint: Cut plots show the road moving at free-stream speed and goals listed in the Goal Plot.

6. 6

   Mesh and convergence

   Use automatic global mesh with a Local Mesh box around the body and a longer one in the wake. Run the baseline at three mesh levels and plot Cd against cell count. Choose the mesh where Cd changes by under 2 per cent to the next finer level. Confirm goals are flat at the end of each run. Record run time so you can plan the number of variants.

   Checkpoint: A mesh-convergence plot for the baseline with less than 2 per cent change at the chosen level.

7. 7

   Run the rear taper sweep

   Run every taper angle on the chosen mesh. Plot Cd against angle and make pressure Surface Plots on the rear and velocity Cut Plots on the centre plane. You should see the wake shrink as taper increases, until the flow separates from the tapered surfaces and the benefit fades. Report the angle where drag is lowest and comment on how it affects rear headroom and boot volume.

   Checkpoint: A Cd-versus-angle plot with a clear trend and flow images explaining it.

8. 8

   Run underbody, diffuser and wheel variants

   Run the flat underbody, diffuser and wheel-cover configurations, each alone and then combined with the best taper. Tabulate Cd and change from baseline. Use Flow Trajectories under the car to show how the flat panel and diffuser change the underbody flow. Check whether the changes add up: interactions often make the combined gain smaller than the sum of single gains.

   Checkpoint: A table of single and combined variants with Cd differences larger than the mesh uncertainty.

9. 9

   Estimate energy and range effect

   Compute drag power at 100 km/h as 0.5·ρ·CdA·v³ for baseline and best variants. Add an example rolling resistance and drivetrain efficiency to get battery power, and estimate highway consumption in Wh/km. For an example 40 kWh usable pack, estimate highway range for each case. State clearly that this is a constant-speed estimate and city driving would show less benefit, because drag power falls with the cube of speed.

   Checkpoint: A table of highway Wh/km and range for baseline and best design.

10. 10

    Report and recommend

    Write up the body definition, CFD set-up, mesh study, variant results, combined effects and energy estimate. Discuss the packaging cost of each change. List limitations: generic body, steady k-ε model, no cooling flow, mirrors or panel gaps. Recommend a validation route such as a scale-model wind-tunnel test or a comparison with published Cd ranges for similar body types, and say which result you trust most.

    Checkpoint: A report that ranks changes by drag saving and packaging cost, with limitations and a validation plan.

Testing & validation

• Mesh convergence on Cd within 2 per cent.
• Domain enlargement changes Cd by under 1 per cent.
• Baseline Cd falls within the typical range for a generic hatchback without detail parts.
• Variant differences exceed the mesh uncertainty before being treated as real.

Expected results

A parametric generic car body, converged Cd results for rear taper, underbody, diffuser and wheel-cover variants, a ranked list of changes with combined effects, and a highway range estimate for the best design.

Take it further

• Add a cooling-air path with a porous-media radiator and measure cooling drag.
• Run a yaw sweep for crosswind sensitivity.
• Rebuild the best body in Autodesk Alias for Class-A styling and re-check Cd.
• 3D print a scale model for a smoke-tunnel visual check.

Viva and review questions

Learn it at DIYguru

These DIYguru courses and programmes teach what this project needs.