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When stumbling upon this post I went to the source for the post, the 2016 and 2017 DOE Annual Merit Review reports. The subject DOE slides claim “radical drag reductions”, “breakthrough”, “aerodynamic sailing”, “enhanced sailing”, “performance is radically different” , etc… Indicating Nobel Prize type results. However nothing is further from the truth. The claims reflect a limited understanding of aerodynamics as well as the data obtained. When a test is performed and results are presented there are three areas to assess, test process, data and findings. And there are 3 questions to ask; 1) is the test BAD or GOOD, 2) is the data BAD or GOOD and 3) are the findings BAD or GOOD. Where BAD and GOOD relate to the validity of the item in question. You can have a BAD test with GOOD data and the data can be used to present either BAD or GOOD findings. In fact you can have any combination of the three areas. GOOD findings can be presented for a BAD test that produced BAD data. Back to the presented material. It appears to be a BAD test with BAD findings but some of the data may be GOOD and some may be BAD. Neither the post or the DOE review material provide sufficient details to determine the status of the data. The results do not show a “sailing effect” that generates a negative drag force. However we can look to peer reviewed published journal documents and NASA reports to decipher the results. The force labeled drag is actually the axial force (ie body axis system). I know - the body axis is the drag resistive force for a ground vehicle. However, it is well known that a cross wind acting on an aerodynamic forebody will result in a reduction in axial force (see high speed train data) where the axial force can decrease with increasing crossflow. This same effects can be seen for a tapered closed afterbody. These trends are well know and well established by the aerodynamic rail and aircraft communities (not a new revelation). Despite these trends the total drag will not become negative, but will increase with yaw angle, for a constant forward speed. Note; in the current test the representative vehicle forward speed is reduced with increasing yaw angle such that at 90 deg yaw the test would be modeling a stationary vehicle with a cross wind. The aerodynamics for the subject test body are quite complex are not fully corrected by the author. It is clear that data corrections are not accounting for the following errors in the reported drag force value; blockage at yaw, ground boundary layer, dynamic similitude, Reynolds number, wall interference, force transformation errors between the wind tunnel balance axis and the model axis systems,........
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Sep 18, 2017