Peculiarities of separated flows in ventilation elbows and their nodes studied numerically

This study examines vortex zones (VZs) that form in single ventilation elbows and their U-shaped and Z-shaped nodes. Reducing local drag in these elements is important for reducing energy consumption in ventilation systems, which can account for up to 80% of the total network drag. A mathematical model based on the discrete vortex method (DVM) was developed to calculate the VZ boundary in a steady state. The model places attached vortices along the flow boundary and free vortices detached from sharp edges, iteratively determining their circulation and position. Convergence is achieved with a sampling step of = = 0.125 m (3522 boundary elements), yielding an average vertical velocity in the control section that differs by only 0.18% from the solution with = 0.0025 m, and a difference in the VZ width of 0.47%. For a single 90° elbow, with an increase in the relative channel width from = 0.5 to 2.0, the maximum VZ width increases from approximately 0.27 to 1.20 The DVM results are in good agreement with the experimental outlines of VZs at a distance of up to two calibers from the sharp edge. Two-dimensional calculations of the Reynolds-averaged Navier-Stokes equations were performed in the Ansys Fluent computational fluid dynamics (CFD) software package using the standard model with extended wall modeling. The determined local drag coefficients (LDCs) for a single elbow differ by less than 1% from the previous numerical data and have the same change trend as the experimental results, with a difference of about 20%. For U-shaped and Z-shaped nodes with distances between individual elbows equal to = 2 and 4, the DVM shows the merging of VZs at a shorter distance, while the CFD method shows separate VZs, demonstrating the influence of the downstream ventilation elbow. The developed approach using DVM and CFD allows for the precise determination of VZ outlines in nodes consisting of ventilation elbows and opens up prospects for the design of shaped energy-efficient ventilation system components.