Influence of different contraction nozzle designs of an open-loop wind tunnel on aerodynamic parameters

Purpose The objective of this study is to investigate the optimal geometry configuration of the contraction section. To achieve the lowest turbulence intensity, a uniform velocity distribution within the test section, a reduction in the metal and the design cost of the contraction section are required. Design/methodology/approach A design for the contraction section based on a polynomial profile is presented. Computational fluid dynamics simulations using the K–ω SST transition model for axisymmetric were performed to study five different wind tunnel contraction profiles at different contraction ratios (CR = 3, 5, 7, 9 and 11) with various contraction lengths from 0.5 to 1.1 m. The simulation results were verified using the experimental data of the National Institute of Standards (NIS) for CR = 7 and a contraction length of 0.9 m. Most previous studies focused on designing the contraction section in terms of the contraction ratio within the recommended range. Findings It was found that the best result, producing the most uniform velocity profile at the inlet to the working section and preventing separation of the boundary layer within the contraction section, was obtained when CR = 5 was lower than the recommended values but with a contraction length of 1.1 m. Therefore, when using a CR lower than the recommended value, a contraction length of at least 2 CR should be used to enhance the accuracy of the aerodynamic parameter measurements inside the test section. Originality/value The novelty of this study is determining the direct impact of the contraction ratio and length on the flow uniformity, turbulence intensity, and adverse pressure across the contraction section at a CR below the recommended range. It was found that the best result, producing the most uniform velocity profile at the inlet to the working section and preventing separation of the boundary layer within the contraction section, was obtained when CR = 5 was lower than the recommended values but with a contraction length of 1.1 m. Graphical abstract A flowchart for optimizing contraction section design via C F D simulation steps. The vertical flowchart with the rectangles explains the process for optimizing a contraction section in fluid dynamics. It begins from the top left box labeled “Input variable dimension C R, and contraction length”. An arrow from this box leads right to “Re-Design the contraction section”, which leads downwards to ”C F D simulation” and ”Selection optimum design”. A decision arrow between ”C F D simulation” and ”Selection optimum design” moves to the top left input box to repeat the process. ”Selection optimum design” leads to a rectangle “C R equals 5, and Contraction length equals 1.1 meters” at the bottom center. “C R lower than recommended value,” from the middle left leading below to “Contraction length equals 2 C R approximately.” connects to the main branch “C R equals 5, and Contraction length equals 1.1 meters”. The main branch produces three outputs on the bottom right, represented by three vertically stacked boxes labeled “Velocity, more Uniformity”, “Turbulence intensity less than 0.39 percent”, and “Lower adverse pressure gradient at the exit”.