M.Sc Thesis
M.Sc Student Kominer Sivan A New Actuator Disk Model for Rotor at Steady Flight Conditions Department of Aerospace Engineering Professor Emeritus Aviv Rosen

Abstract

Actuator disk models were amongst the earliest tools to analyze rotary wings. Even though other more advanced and accurate models have been developed throughout the years, the use of actuator disk models is still very common, especially models that are based on the momentum theory. Momentum models are popular because of their simplicity, and relatively good accuracy. However, the momentum theory has also significant disadvantages: According to the theory the integral representing the conservation of axial momentum, is replaced by its differential form. This step has no theoretical justification, and it expresses a weakness of the model. In addition, the momentum theory has been developed for the case of axial flow. Using it for inclined flight is based on various simplifying assumptions. Thus it is not surprising that substantial deviations appear in many cases when the results of the momentum model are compared with experimental data. Improved actuator disk models have been developed in order to overcome the above mentioned problems. These models are usually based on solving the flow equations over the entire flow field. Therefore they are much more complicated and lose the main advantages of actuator disk models that are based on momentum theory - simplicity and efficiency.

The new model concentrates on the flow field near the plane of the disk. This model is relatively simple, efficient, and accurate. It enables the analysis of a wide spectrum of steady flight conditions, especially inclined flight.

The derivation of the model starts by presenting an approach of combining blade element theory with the actuator disk principle. It includes a method of calculating the induced velocities. This is done by distributing sinks over the disk's plane, which induce velocities over the entire flow field. The sinks simulate the sucking action of rotors and propellers, which accelerate the flow that passes through them.

The above described new actuator disk model is used to analyze various rotary wing problems, that have been reported previously in the literature. First, results of the new model are compared with theoretical results of two other theoretical models that include a solution of the entire flow field. In addition, a comparison with wind tunnel test results of a four bladed propeller is also presented. In most of the cases good agreements are shown. In certain cases differences appear between the results of the present model and test results, which are discussed and explained in the text.