DUCTED FAN INLET/EXIT AND ROTOR TIP FLOW IMPROVEMENTS FOR VERTICAL LIFT SYSTEMS

Open Access
Author:
Akturk, Ali
Graduate Program:
Aerospace Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 12, 2010
Committee Members:
  • Cengiz Camci, Dissertation Advisor
  • Cengiz Camci, Committee Chair
  • Savas Yavuzkurt, Committee Member
  • Dennis K Mc Laughlin, Committee Member
  • Edward C Smith, Committee Member
  • Kenneth Steven Brentner, Committee Member
  • Michael H Krane, Committee Member
Keywords:
  • Ducted fan
  • Inlet flow distortion
  • Tip leakage
  • Tip leakage mitigation
  • Double ducted fan
  • Pressure side platform
  • Fan rotor tip loss
  • Tip treatment
Abstract:
Ducted fan based vertical lift systems are excellent candidates to be in the group of the next generation vertical lift vehicles, with many potential applications in general aviation and military missions for both manned and unmanned systems. Ducted fans provide a higher static thrust/power ratio, improved safety and lower noise levels for a given diameter when compared to open rotors. Although ducted fans provide high performance in many “Vertical Take-Off and Landing” (VTOL) applications, there are still unresolved problems associated with these unique machines. The main problems are distortion of inlet flow due to forward flight and tip leakage related problems. Distorted inlet flow results in lip separation on the inner side of the lip section severely limiting the lift generation and controllability of vertical lift systems. Inlet flow distortions passing through a typical ducted fan rotor become increasingly detrimental with increasing forward flight velocity. Fan rotor tip leakage flow is another source of aerodynamic loss for ducted fan systems. Tip leakage related problems adversely affect the general performance of VTOL “Uninhabited Aerial Vehicles” (UAV) systems. The current research utilized experimental and computational techniques in 5" and 22" diameter ducted fan test systems that have been custom designed and manufactured. Qualitative investigation of flow around the ducted fan was also performed using smoke flow visualizations. Quantitative measurements consisted of 2D and 3D velocity measurements using planar and Stereoscopic Particle Image Velocimetry (PIV and SPIV), high resolution total pressure measurements using Kiel total pressure probes and real time six-component force and torque measurements. The computational techniques used in this thesis included a recently developed radial equilibrium based rotor model (REBRM) and a three dimensional Reynolds-Averaged Navier Stokes (RANS) based CFD model. A radial equilibrium based rotor model (REBRM) developed by the author was effectively integrated into a three-dimensional RANS based computational system. The PIV measurements and computational flow predictions using (REBRM) near the fan inlet plane were in a good agreement at hover and forward flight conditions. The aerodynamic modifications resulting from the fan inlet flow distortions in forward flight regime were clearly captured in 2D PIV results. High resolution total pressure measurements at the downstream of the fan rotor showed that tip leakage, rotor hub separation, and passage flow related total pressure losses were dominant in hover condition. However, the losses were dramatically increased in forward flight because of inlet lip separation and distortion. A novel ducted fan inlet flow conditioning concept named “Double Ducted Fan” (DDF) was developed. The (DDF) concept has a potential to significantly improve the performance and controllability of VTOL UAVs and many other ducted fan based vertical lift systems. The new concept that will significantly reduce the inlet lip separation related performance penalties used a secondary stationary duct system to control “inlet lip separation” occurring especially at elevated forward flight velocities. The (DDF) is self-adjusting in a wide forward flight velocity range. DDFs corrective aerodynamic influence becomes more pronounced with increasing flight velocity due to its inherent design properties. RANS simulations of the flow around rotor blades and duct geometry in the rotating frame of reference provided a comprehensive description of the tip leakage and passage flow in the flow environment of the two ducted fan research facilities developed throughout this thesis. The aerodynamic measurements and results of the RANS simulation showed good agreement especially near the tip region. A number of novel tip treatments based on custom designed pressure side extensions were introduced. Various tip leakage mitigation schemes were introduced by varying the chordwise location and the width of the extension in the circumferential direction. The current study showed that a proper selection of the pressure side bump location and width were the two critical parameters influencing the success of the tip leakage mitigation approach. Significant gains in axial mean velocity component were observed when a proper pressure side tip extension was used. It is also observed that an effective tip leakage mitigation scheme significantly reduced the tangential velocity component near the tip of the axial fan blade. Reduced tip clearance related flow interactions were essential in improving the energy efficiency and range of ducted fan based vehicle. Full and inclined pressure side tip squealers were designed. Squealer tips were effective in changing the overall trajectory of the tip vortex to a higher path in radial direction. The interaction of rotor blades and tip vortex was effectively reduced and aerodynamic performance of the rotor blades was improved. The overall aerodynamic gain was a measurable reduction in leakage mass flow rate. This leakage reduction increased thrust significantly. Full and inclined pressure side tip squealers increased thrust obtained in hover condition by 9.1 % and 9.6 % respectively. A reduction or elimination of the momentum deficit in tip vortices is essential to reduce the adverse performance effects originating from the unsteady and highly turbulent tip leakage flows rotating against a stationary casing. The novel tip treatments developed throughout this thesis research are highly effective in reducing the adverse performance effects of ducted fan systems developed for VTOL vehicles.