Different numbers, positions, and shapes of air guide vanes in the elbow were used to reduce the pressure loss and also played a crucial role in improving the uniformity of airflow in the elbow.
Through CFD simulation, the optimized specifications of guide vanes in the elbow on the airflow and local pressure loss coefficients were explored. This study will provide a reference for numerical prediction and engineering application of using guide vane to minimize the local pressure loss of large prefabricated air ducts.
The results show the erosion rate for the elbow of 90 higher than in low angle elbows (30 and 60), also the high erosion rate occurs in the upper half of the elbow 4.

When air enters a turn (elbow) airflow is most disrupted coming out of the turn.To most accurately measure airflow in a pipe that has no airflow vanes to make the airflow more uniform you would average multiple pitot measurements using the following pattern. Not trivial as you can see.
This is "Suspension Material Study" by Andrew Halonen on Vimeo, the home for high quality videos and the people who love them.

Airflow Elbow Support How to Apply. Materials: Elastane, Polyamide. Wash and Care: Hand Wash Cold Only and Dry Flat. Do not use Fabric Softener, Do Not Tumble Dry, Do Not Bleach, Do Not Iron. Store in a cool, dry place away from direct heat and sunlight.
Airflow Enhancement System The Airflow Enhancement Kit improves the cabin airflow in General Aviation aircraft, specifically Cessnas.Q: What material are these Airflow Enhancement System vents and what are the dimensions? Part # 05-04080.