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Plot of the month: External truck aerodynamics

Contributed by Dr. Ilhan Bayraktar
Old Dominion University, Norfolk, VA

External truck aerodynamics animation

Models such as this will lead to modifications that increase fuel efficiency by minimizing drag force. Reducing drag force will also lessen environmental pollution and improve stability and vehicle control.

Truck Animation

Dr. Ilhan Bayraktar is a Ph.D. Candidate at Old Dominion University’s Aerospace Engineering Department. His primary research interest is numerical simulation of complex, large-scale flow problems involving heat transfer and structural interactions in participating media (in this case flow-structure interaction). The applications that he is currently working on include compressible and incompressible flows, detonation in aerodynamics and heat transfer problems.

Outlined results are from Ilhan’s dissertation, which has been conducted at Old Dominion University and Langley Full Scale Wind Tunnel. Old Dominion University has assembled a research group on Ground Vehicle Aerodynamics. This particular research project, directed by Dr. Oktay Baysal, focuses on analyzing heavy ground vehicle aerodynamics and understanding complex wake flow behind vehicle bodies. (Further research areas include under-body and under-the-hood aerodynamics, internal aerodynamics and heat transfer problems.)

The animation above shows pressure contours on a truck surface. Maximum pressure occurs in the front region of the model (red regions). The side of the trailer has relatively low pressure and no separation exists (there is no wake or reattachment in the flow). The figure below shows a circulation region on the back of the truck – a very important region for aerodynamic drag. Although the highest pressure takes place on the front surfaces, most of the drag occurs at the back due to the separation of the flow.

A circulation region on the back of the truck can be easily seen in this image. Most of the drag force takes place due to the separation of the flow at the back.

There are two types of drag forces on bluff bodies; friction drag and pressure drag. Computational studies show that about 80% of total drag is from pressure drag, and the rest is from friction. The maximum pressure difference is observed at the back surface of the truck, where complex flow phenomena, such as separation, reattachment and vortices are found.

Maximum pressure contours can be seen on the front of the truck.

The computational part of this work was conducted on Sun 10000 Supercomputer using up to 32 processors. Full-scale domain was created with 12.5 million mesh elements. Approximately 10 GBs of memory was allocated by implicit Reynolds averaged Navier-Stokes Solver and roughly one week runtime was spent for converged result.

Visualization is a key component in transforming the raw data into something useful for engineering and scientific analysis. Using Tecplot, Ilhan can quickly explore the experimental and computational data to better understand the aerodynamics of the flow and generate plots to communicate the results. In the images above, 3-D stream tubes help identify separation and recirculation regions.

A truck inside the Langley Full Scale Wind Tunnel

Langley Full Scale Wind Tunnel (LFST), the largest university-operated wind tunnel in the world, plays a very important role in the research group. LFST tests full-scale commercial vehicles like trucks, cars and aircraft. Since the research group works on full-scale aerodynamics of ground vehicles, they need a wind tunnel to test full-scale ground vehicles. A purely computational study is not complete without experimental support or vice versa. Because there are just a few places, which can test full-scale ground vehicle models, in the world – LFST is the perfect place for comparison and validation studies for such studies.

This study will be a benchmark case resulting in a database containing both experimental and computational results. These results will lead to modification devices that minimize drag force and increase fuel efficiency. Reducing drag force will also lessen environmental pollution and improve stability and vehicle control.

Pressure contours on various back surface configurations. (Bluff bodies represent simplified ground vehicles.)