CFD
COMPUTATIONAL FLUID DYNAMICS

CFD is a branch of fluid mechanics that uses numerical analysis and data structures to analyse and solve problems that involve fluid flows. Computers are used to perform the calculations required to simulate the free-stream flow of the fluid, and the interaction of the fluid (liquids and gases) with surfaces defined by boundary conditions.

VALIDATION TF1 MODEL VS CERVELO P5
CFD model used to create the baseline CFD case

This document presents the CFD model that TotalSim will use to create the baseline CFD case. The intention of this document is to Illustrate the supplied geometry and Check for any missing or misplaced components.

Cervelo P5 considered the industry standard aero bench mark for Time Trial derived Triathlon bikes.

Cervelo is the largest brand by participants at the Kona Ironman Triathlon World Championships and has been for a number of years.

In a recent independent wind tunnel study* of the current state of the art bikes used in triathlon the P5 was only just beaten by the non UCI legal P5x from the same company.

Frame and wheel geometry was provided by Aperta BV. Rider geometry was scanned by TotalSim and assembled. For the P5 the fork and brake fairing was scanned as well.

The different colours depict different force groups. Each of these force groups will be able to be isolated individually to monitor the drag on that specific area.

CERVELO P5

TF1 MODEL

GEOMETRY OVERVIEW HANDLEBARS

Crank, armrest cups, extensions, contact patch and ground plane

CRANK
HANDLE BARS
HANDLE BARS
CONTACT PATCH AND GROUND PLANE

CONCLUSIONS

TOTALSIM SUMMARY

The baseline TF1 configuration has been tested at three yaw angles. The results have been compare to the P5 frame, with the same rider. analysis has been
presented, with future development opportunities included in order to improve the performance of the model.

The TF1 configuration shows decreased rider drag and increased bike drag in comparison to the P5 configuration. This results in a small overall increase total drag.

The baseline Aperta frame has not been optimised, therefore the increased bike drag is to be expected at this stage.

Overall, the Aperta frame shows potential for improved aerodynamic performance.

CFD

FORCES | TOTALS AND SUMMARY

To: Richard McAinsh
Company: KÚ Cycle
Author: GO
Reviewer: MC
Revision: 1

SUMMARY:

This document presents a brief analysis of the forces and flow of the TF1. This documents also outlines some areas of the frame that have been suggested for potential improvement in terms of their aerodynamic performance.

GEOMETRY OVERVIEW TF1 MODEL

Geometry origin

Frame and wheel geometry was provided by KÚ Cycle.
Rider geometry was scanned by TotalSim and assembled.

GEOMETRY OVERVIEW CERVELO P5

Geometry origin

Frame and wheel geometry was provided by KÚ Cycle.
Rider geometry was scanned by TotalSim and assembled. For the P5, the fork and brake fairing was scanned as well.

GEOMETRY OVERVIEW CERVELO P5 | TF1 MODEL

Differences between the TF1 MODEL and CERVELO P5

Delta plots highlight the changes between the TF1 frame and the Cervelo P5 frame. Green indicates geometry added (I.e. updated parts of the TF1 frame).
Red indicates geometry removed (I.e. removed parts of the P5 frame).

GEOMETRY OVERVIEW FORCE GROUPS - RIDER & TF1 MODEL

Different colours depict different groups

The different colours depict different force groups of the bike.
This is used to extract the forces acting on specific parts of the bike; such as forks or the down tube.

The different colours depict different force groups of the rider.
This is used to extract the forces acting on specific parts of the rider; such as the rider’s head.

GEOMETRY OVERVIEW CASE SETUP

Key aerodynamic terms

This page defines a number of key aerodynamic terms used in our CFD analysis.

Cp: Pressure coefficient – Non-dimensionalised static pressure. Red/orange indicates positive static pressure and blue/green indicates negative static pressure.

CpX: Pressure coefficient resolved in the x-direction.

CpT: Total pressure coefficient – A measure of the energy in the flow. Red/orange indicates high energy flow and blue/green indicates low energy flow (i.e. separation and wake).

Uw: Near wall velocity – Velocity of flow near the surface of the car, which can be used to identify regions of slow or separated flow.

GEOMETRY OVERVIEW CASE SETUP

Wind tunnel Style testing

Runs are carried out using DES unsteady methodology, without transition. The flow is assumed fully turbulent.

The cases are modelled in ‘clean’ flow. I.e. idealised wind tunnel style testing.

All cases are tested with a rolling road, i.e. the ground plane is moving and the wheels are spinning.

The wheels are considered as moving walls with a given angular velocity.

The model is run without spokes as these add significant complexity to the model but typically have very minimal effect on the forces and flow structures.

Three wind conditions are tested: 2, 8 and 14 degrees yaw (cross wind). However, no weighting has been applied to these values at this stage.

In cross-wind condition, the X component of the wind velocity is held constant (12.5m/s). Hence the magnitude of the wind velocity will increase as the cross wind yaw angle is increased. This is analogous to the situation where a rider experiences a side wind whilst moving along a straight road.

The ground velocity is fixed at 12.5 m/s for all cases and is aligned to the X axis (bike longitudinal axis).

MESH RECIPE CFD

35 Millons cells

The geometry has been volume meshed based on TotalSim’s current best practice for time trial bikes. Each case contains around 35 million cells.

CPX SURFACE PILOTS TF1 MODEL

At 2 degrees yaw key aerodynamic terms

CpX plots show the x-component of static pressure acting on the surface of the bike and rider. Red/orange indicates positive static pressure (i.e. drag). Blue/green indicates negative static pressure (i.e. thrust).

NEAR WALL VELOCITY SURFACE PLOTS TF1 MODEL

At 8 degrees yaw key aerodynamic terms

Flow largely separated from the rear of the rider. Flow separates towards the rear of the forks.
Good attachment of flow to upper region of seat tube.

FLOW VISUALISATION HANDLEBARS AND HEAD TUBE TF1 MODEL

At 8 degrees yaw key aerodynamic terms

At yaw, flow to the leeawrds side of the fork crown becomes tired. Tis, combined with the high satgnation pressures ahead of the left leg (at this rider position) cause flow to recirculate between the crown and the rider’s left leg.

TF1 Z-CpT Slice through handlebars
TF1 static pressure
TF1 Near wall velocity

FLOW VISUALISATION FORKS TF1 MODEL

At 2 degrees yaw key aerodynamic terms

The position of the forks could be optimised to lower stagnation pressures acting on the legs.
The images below show slices through the forks at 2 and 8 degrees yaw.

TF1 Z-CpT Slice at 2 degrees yaw. Wake from forks.
TF1 Z-Cp Slice at 2 degrees yaw.
TF1 Z-Cpt Slice at 8 degrees yaw. Wake from fork interacts with left leg, reducing stagnation pressures.
TF1 Z-Cp Slice at 8 degrees yaw. Wake from fork interacts with left leg, reducing stagnation pressures.

FLOW VISUALISATION FORK CROWN TF1 MODEL

At 2 degrees yaw key aerodynamic terms

At yaw, the high stagnation pressures ahead of the left leg (at this rider position) cause flow to recirculate between the crown and the rider’s left leg.

TF1 Z-CpT Slice through fork crown profile
TF1 Y-CpT Slice through fork crown profile
TF1 Z-Cp Slice through fork crown profile
TF1 Y-Cp Slice through centreline

FLOW VISUALISATION DOWN-TUBE PROFILES CERVELO P5 | TF1 MODEL

At 8 degrees yaw key aerodynamic terms

The images below show slices through the down tube at 8 degrees yaw.

P5: Z-CpT Slice
P5: Z-Cp Slice
Aperta: Z-CpT Slice
perta: Z-Cp Slice

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