In an open type wind tunnel experiment of flow over a bluff body (bluff body is kept in wind tunnel test section), initially without the bluff body the velocity was calculated and this velocity is used for calculating Cd and Cl. Presence of the bluff body obviously increases the blockage and the velocity reduces in the test section. Does it make sense if we calculate the velocity with the presence of the bluff body and this reduced velocity value is used for further calculations?
Yes, the flow velocity downstream of the bluff body is used to calculate Cd and Cl. You can refer to the following paper for a better understanding.
10.1016/j.expthermflusci.2018.12.021
I would highly recommend to measure the actual flow velocity and always use the actual velocity to normalize the forces. In addition, there are methods for wind tunnel correction that depend on the blockage by the wind tunnel model ("Wind Tunnel Wall Corrections AGARD-AG-336").
You can find AGARD-336 here:
https://www.sto.nato.int/publications/AGARD/Forms/AGARD%20Document%20Set/docsethomepage.aspx?ID=8838&FolderCTID=0x0120D5200078F9E87043356C409A0D30823AFA16F60B00B8BCE98BB37EB24A8258823D6B11F157&List=03e8ea21-64e6-4d37-8235-04fb61e122e9&RootFolder=%2Fpublications%2FAGARD%2FAGARD-AG-336
Thank you Stephan Löffler , Most of the wind tunnel corrections are made for Blockage ratio of less than 0.3. My concern is, In case of higher Blockage Ratio (BR>0.5), How to do the blockage corrections.
Dear
For any experiment, first of all you should calibrate your tunnel, i.e. to find any velocity variations along and across test section. Then plot these velocity profiles in empty tunnel. After actual measurements with bluff body inside test section, you may apply all your corrections with help of calibration curves.
Dear prof, Mohsen Jahanmiri , we have done the calibrations. The actual problem is, we have higher blockage ratio bluff body. If we use lower blockage ratio bluff body, the blockage corrections are included in order to nullify the local acceleration of the air. In this higher blockage ratio we have deceleration of air. how to account this deceleration?
Please see the book,
Low Speed Wind Tunnel Testing by Barlow, Rae and Pope.
There all methods of corrections is mentioned.
why don't you check with blockage corrections in automotive wind tunnel? The blockage problem should not be reduced to the jet expansion effect alone. There is a nozzle effect, a collector effect and a static pressure gradient effect of the empty test section as well (see SAE Technical Paper Series).
It is generally recommended to use the velocity measurements taken without the presence of the bluff body as a reference, and use this value to calculate the drag and lift coefficients with the presence of the bluff body. This is because the presence of the bluff body will introduce additional drag and lift forces that are not present in the reference case, and these forces should be accounted for in the calculations of the drag and lift coefficients. Using the reduced velocity with the presence of the bluff body would not properly account for these additional forces, and could lead to inaccurate or misleading results.
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