Modal Analysis of Volumetric Flow Fields around a Quadcopter from Experimental Shake-The-Box Results

Abstract

Unmanned aerial vehicles experience a rapid rise in various industries as well as in the private sector. This motivates the investigation of their wake in free flight, to allow understanding and development of efficient, safe and silent vehicles. Therefore, a Shake-the-Box experiment of a quadcopter with an overall span of approximately 0.5 m in free flight was conducted at the DLR site in Göttingen, covering a variety of operating conditions, such as flight in ground effect (IGE) and out of ground effect (OGE) [1]. A momentum analysis neglecting the unknown pressure terms was performed on the volumetric, time resolved velocity data from an OGE-case, yielding an estimate of the quadcopter’s thrust. The result exhibits strong fluctuations and a systematic deviation to the expected value derived from the vehicle’s weight (see Figure 1). Large scale flow structures, going along with pressure gradients, might be responsible and can be identified by a modal analysis. This presentation features the use of a Proper Orthogonal Decomposition (POD) code on the STB-data of both IGE and the OGE cases to investigate the modal structure of the flow around a free flying helicopter.