Vehicle aerodynamics: Drag reduction through surface dimples

by Dr. Sharul Sham Dol

The successful application of dimples on golf balls have inspired vehicle aerodynamics engineers for a long time. A golf ball with a dimpled surface can travel higher and further than a smooth surfaced golf ball when subjected to identical force.

Dimples on golf balls induce turbulence at lower Reynolds number (a non-dimensionless number that determines whether the flow is laminar or turbulent), providing extra momentum or energy to the boundary layer and causing delay in flow separation (Figure 1). This phenomenon causes smaller wake areas or swirling flow regions behind the ball, thus reducing the total drag.

Drag force can be divided into two different types, that is, pressure drag and friction drag. Pressure drag forms due to the eddying motions that are set up in the fluid by the passage of the body and is related to the formation of a wake. Friction drag comes from friction between the fluid and the contacted surface and is related to the development of boundary layer and the Reynolds number.

When drag force of a body is dominated by friction drag, the body is defined as a streamline body, for example, a fish, bird or airfoil that is in small or zero angles of attack. On the other hand, the drag force of a bluff body, for example, a brick, golf ball or an airfoil that is in large angle of attack, is dominated by pressure drag.

In the popular science entertainment television programme Mythbusters, Jamie and Adam created dimples on a car surface with clay (Figure 2). They found out that there was an increase in fuel consumption efficiency by 11%. Although it was not an official experiment or research, it is possible to apply dimples on car, train or aircraft surfaces to reduce fuel consumption.

Past studies (experimental and numerical) observed that dimples on an airfoil created extra turbulence to delay the boundary layer separation. The effect was improved when the airfoil was at angle of attack. It was suggested that a smart dimple matrix over the airfoil will sense boundary layer separation and arrange the dimples in the least drag and high lift configuration (Figure 3).

German researchers proved an effect which could revolutionise the design of ships, vehicles and aircrafts. Experiments in a wind tunnel showed dimples on the surface of a body massively reduced the flow resistance and that trains with dimples have 16% less frictional resistance depending on the speed.

Adding a dimple on a streamlined body might help delay the flow separation and reduce the size of the wake but it might also increase the friction drag as a trade-off.

Curtin Sarawak final-year mechanical engineering project student Chear Chie Khan has further investigated the energy-saving effects of dimples on the flow of car bodies.

A parameter called dimple ratio (DR) was introduced. DR is the ratio between the depth of a half dimple over the print diameter of a dimple (Figure 4). In his work, a car model (Figure 5) was simulated with a DR of 0.05 – 0.5.

His Ahmed body car model is a simplified car model for accurate flow simulation, retaining its standard car features such as curved fore body, straight centre section and angled rear end. It is a typical bluff body commonly used for simulation to study the flow past of a car (Figure 4). Flow was simulated using k-ε turbulence model in ANSYS Fluent software with tetrahedral meshing (Figure 6).

For the model without dimple application, there is insignificant turbulent kinetic energy on the car surface (Figure 7). When compared to the model without a dimple, kinetic turbulent energy is generated within the dimple and at the vicinity of the dimple edge. These results suggest that the flows manage to go further before flow separation takes place. The coefficient of drag, CD, is reduced by 1.9% for the model with DR = 0.4.

The results are encouraging since the simulation is only based on one dimple. Different parameters like dimple position, number of dimples and dimple orientation will be tested in order to fully understand the performance of the dimple application on vehicle aerodynamics.

Comprehensive data from the study were presented and discussed at the 8th Curtin University Technology, Science and Engineering (CUTSE) International Conference held at Curtin Sarawak on 2 to 3 December 2013.

Figure 1: Dimple pattern on golf ball.	Figure 2: The Mythbusters crew experimented with dimples on this Ford Taurus.
	Figure 3: Dimples matrix as suggested by Figure 2.

Figure 4: Dimple ration.	Figure 5: Isometric view of the Ahmed body car model.
Figure 6: Tetrahedral mesh around dimples.

Figure 7: Turbulence kinetic energy for model without dimple and for model with DR = 0.4.

Dr. Sharul Sham Dol is the Head of Department of Mechanical Engineering, School of Engineering and Science at Curtin Sarawak. Besides teaching, he supervises a number of postgraduate students and is involved in research and consulting projects in the area of fluid dynamics, turbulent flows, oil and gas hydrodynamics, vortex dynamics and renewable energy. He can be contacted at 085-443 823 or by e-mail tosharulsham@curtin.edu.my