Aerodynamic Styling Iteration Loop between Alias and CFD

The problem

Aerodynamic drag strongly affects EV range at higher speeds. Designers and engineers often iterate slowly because styling and CFD models are separate and inconsistent. A disciplined loop with consistent geometry, mesh and boundary conditions lets students explore how styling changes affect drag and range, with honest uncertainty.

Objectives

• Create a baseline concept body in Alias and a consistent CFD-ready export.
• Set up an external flow study in SOLIDWORKS Flow Simulation with goals for drag force and coefficient.
• Run a mesh study and establish uncertainty.
• Iterate styling changes and track drag change against design intent.
• Estimate range impact for an example vehicle on a drive cycle.

What you need

Key concepts

Drag coefficient

Cd = 2F/(ρv²A), drag force normalised by dynamic pressure and frontal area. The product CdA is what affects energy use.

Wake and rear taper

Much of the drag comes from the wake. A tapered rear and sharp separation edges can shrink it.

Ground effect and wheels

Underbody and rotating wheels change the flow around a car. A moving ground and rotating wheels are needed for accurate results; a fixed ground overestimates some effects.

Mesh study and uncertainty

Drag can change with mesh. Comparing results at several mesh levels gives an uncertainty band to compare variants against.

Step-by-step guide

1. 1

   Set targets and example vehicle

   Define an example compact EV with 2.2 m² frontal area, 1300 kg mass and a 40 kWh battery, and a styling goal of reducing Cd without losing design intent. Pick a test speed, for example 100 km/h, and a range estimation method based on a road-load model. List the styling parameters you will vary, such as rear taper angle, roof drop, boat-tail length and underbody covers.

   Checkpoint: A target and parameter table with design-intent constraints stated.

2. 2

   Build a CFD-ready baseline

   Create a closed baseline body in Alias with simplified wheels, closed lamp recesses and a smooth underbody, removing small details that do not matter at concept stage. Keep the parameters you plan to vary on clean, editable curves. Export as STEP, import into SOLIDWORKS, knit the surfaces into a solid with Knit Surface and confirm it is watertight before any flow study.

   Checkpoint: A watertight solid body in SOLIDWORKS with no import errors.

3. 3

   Set up the Flow study

   Create an external flow project with air at the test speed. Define a computational domain several body lengths around the car, place the ground as a wall moving at vehicle speed where possible, and use a symmetry plane to halve the domain and the cell count. Add a global goal for drag force and an equation goal for Cd using the frontal area and dynamic pressure.

   Checkpoint: The study converges with steady drag goal values over the last iterations.

4. 4

   Run a mesh study

   Run the baseline at three mesh levels with local mesh refinement around the body and in the wake region behind it. Plot drag against cell count and estimate uncertainty from the change between the two finest meshes. Choose a working mesh that balances solve time and accuracy, and use exactly the same mesh settings for every variant so comparisons stay fair.

   Checkpoint: Drag changes by less than about 3 per cent between the two finest meshes, and an uncertainty band is stated.

5. 5

   Check plausibility

   Compare the baseline Cd with typical ranges for similar body types from textbooks or papers, rather than with claims for specific products. Use cut plots of pressure and velocity and flow trajectories to see where flow separates and how large the wake is. Confirm the solver goals have converged, and investigate if the result falls well outside the expected range.

   Checkpoint: Baseline Cd in a plausible range for the body type and a wake picture that matches expectations.

6. 6

   Iterate rear taper

   In Alias, create variants with different rear taper angles and roof drop by editing the controlling curves and rebuilding surfaces, keeping everything else unchanged. Export each variant, run it with the same mesh settings and record Cd. Have a reviewer rate design intent for each variant on a simple scale so aerodynamic gain is always seen next to the cost to the design.

   Checkpoint: A table of Cd and design-intent rating for each rear variant.

7. 7

   Iterate underbody and wheels

   Test underbody covers, wheel covers, small rear diffuser shapes and spoilers as separate variants. Rotating wheels can be represented in Flow Simulation with a rotating region or a wall with a rotation condition; state which simplification you used and why. Record the Cd change for each variant and compare every change with the uncertainty band from the mesh study. Keep one file per variant.

   Checkpoint: A table of Cd changes for underbody and wheel variants, each compared with the uncertainty band.

8. 8

   Combine and confirm

   Combine the most effective changes into a final variant and run it, together with the baseline, at the finest mesh level. Check that the improvement is larger than the uncertainty band, since effects of individual changes do not always add up. Visualise the wake of the baseline and final variant side by side with the same cut plot settings to explain the difference.

   Checkpoint: Final variant Cd reduction greater than the uncertainty band, with wake comparison plots.

9. 9

   Estimate range impact

   Use a road-load model in Excel or MATLAB to estimate Wh/km at steady speeds of 50, 80 and 100 km/h and on a drive cycle for the baseline and final variant, keeping mass, rolling resistance and efficiency fixed. Convert to range for the example battery. State assumptions clearly and show that the range gain is largest at high speed, where drag dominates.

   Checkpoint: A table of energy use and range for both variants at steady speeds and on a drive cycle.

10. 10

    Report

    Report the method, mesh study, variants, results with uncertainty, design-intent ratings, range estimate and limitations, including simplified wheels and ground, steady-state solution and missing cooling flow. Show renders of the baseline and final variant next to each other. Recommend wind tunnel testing of a scale model or higher-fidelity CFD to validate the trend before it informs a real design. Archive the Flow project files.

    Checkpoint: A report with tables, plots and renders of baseline and final variant.

Testing & validation

• Mesh study meets the convergence criterion with a stated uncertainty.
• Baseline Cd is plausible for the body type.
• Reported improvements exceed the uncertainty band.
• Range estimates use a stated and reproducible road-load model.

Expected results

A styling-to-CFD iteration loop with consistent geometry and mesh, a set of drag-reduced variants with design-intent ratings, and an estimate of range impact.

Take it further

• Add cooling flow through the front intake and compare open and closed shutters.
• Run transient analysis to look at wake unsteadiness.
• Compare with a scale model in a small wind tunnel if available.
• Automate variant generation with Alias scripting or parametric sweeps.

Viva and review questions

Learn it at DIYguru

These DIYguru courses and programmes teach what this project needs.