Experimental investigation on the structures and induced drag of wingtip vortices for different wingtip configurations

As an effective method to reduce induced drag and the risk of wake encounter, the winglet has been an essential device and developed into diverse configurations. However, the structures and induced drag, as well as wandering features of the wingtip vortices (WTVs) generated by these diverse winglet configurations are not well understood. Thus, theWTVs generated by four typical wingtip configurations, namely the rectangular wing with blended/raked/split winglet and without winglet (ModelBL/RA/SP/NOfor short), are investigated in this paper using particle image velocimetry technology. Comparing with an isolated primary wingtip vortex generated by ModelNO, multiple vortices are twisted coherently after installing the winglets. In addition, the circulation evolution ofWTVs demonstrates that the circulation for ModelSPis the largest, while ModelRAis the smallest. By tracking the instantaneous vortex center, the vortex wandering behavior is observed. The growth rate of wandering amplitude along with the streamwise location from the quickest to the slowest corresponds to ModelSP, ModelNO, ModelBL, ModelRAin sequence, implying that theWTVs generated by modelSPexhibit the quickest mitigation. Considering that the induced drag scales as the lift to power 2, the induced drag and lift are estimated based on the wake integration method, and then the form factor lambda, defined by lambda=C L2/CDi, is calculated to evaluate the aerodynamic performance. Comparing with the result of ModelNO, the form factor decreases by 7.99%, 4.80%, and 2.60% for ModelRA, ModelBL, ModelSP, respectively. In sum, ModelRAandBLhave a smaller induced drag coefficient but decay slower; while Model SP has a larger induced drag coefficient but decays quicker. An important implication of these results is that reducing the strength ofWTVs and increasing the growth rate of vortex wandering amplitude can be the mutual requirements for designing new winglets.