Numerical simulation for finding the effect of blockage and tunnel interference in wind tunnel testing of a bluff body

Abstract

Flows past bluff bodies are ubiquitous in various engineering applications, such as flows past tall buildings, bridge piers, undersea pipelines, and launch vehicles. These flows exhibit complex phenomena, prompting extensive experimental and numerical investigations to unravel their intricacies. Wind tunnel testing is a pivotal experimental technique employed to analyse flow patterns around objects, notably used to determine drag coefficients for aircraft. However, the accuracy of wind tunnel tests is often compromised by blockage effects, where the rigid tunnel walls impede the free lateral displacement of the fluid stream around the body, leading to "tunnel interference". This interference causes the streamlines to converge near the body, resulting in higher velocities than those in an unbounded stream. This study addresses the necessity to quantify the impact of blockage ratio on flow characteristics around a bluff body. The blockage ratio, defined as the ratio of the model's projected area to the wind tunnel's cross-sectional area, significantly influences experimental outcomes. Additionally, supporting systems used to mount models in wind tunnels introduce "supporting system interference", where the interaction between the system and the body's wake alters drag measurements. These systems affect the wake width and velocity gradients near the body, thereby increasing skin friction drag and total drag. A comprehensive review of the literature on blockage effects and supporting system interference has been conducted. The study provides a detailed examination of how blockage ratio and supporting systems impact drag measurements. In this investigation, numerical simulations have been performed to study the axial flow of air past conical bluff bodies in both confined and unconfined flow domains. The drag coefficient has been computed numerically for each scenario, and the difference in drag coefficients between confined and unconfined flows has been plotted against the Reynolds number, illustrating the tunnel interference effect. Furthermore, to assess support interference, each conical bluff body has been mounted on a supporting system within a confined domain, and the resulting drag coefficient has been compared to that of an unsupported body. The variations in drag coefficient due to the supporting system have also been plotted against the Reynolds number. The findings reveal significant insights into the effects of tunnel interference and support interference on drag measurements. Quantifying these effects is crucial for improving the accuracy of wind tunnel tests, thereby enhancing the reliability of experimental data used in designing and optimizing engineering structures subjected to bluff body flows. This study underscores the importance of considering both blockage and support effects in experimental setups and provides a foundation for future research to mitigate these interferences in wind tunnel testing.