Vibration, synchronization and localization of three-bladed rotor: theoretical and experimental studies

Numerical and experimental methods in free and forced vibrations of the rotating structure consisting of the rigid hub and three flexible beams are considered. Firstly, the system of four mutually coupled dimensionless differential governing equations is presented and then forced response of the system as well as synchronization phenomenon are investigated. Next, the finite elements method is used to design the rotating structure and analyse complex dynamic response. During the numerical calculations symmetric, as well as de-tuned rotor are analyzed. Finally, results obtained from ordinary differential equations and numerical simulations are compared with experimental tests.

Alternative Blade Profile based on Savonius Concept for Effective Wind Energy Harvesting

The drag-based vertical-axis Savonius wind rotor is a potential candidate for harvesting renewable energy. It is very simple in design and can be deployed as an off-the-grid electricity system in remote locations having no access to electricity. The present work aims to develop a novel blade profile for the Savonius rotor in order to improve its performance. In that connection, an arc-elliptical-blade profile has been developed and rotor performance has been assessed through wind tunnel testing at three different Reynolds numbers (Re = 87039, 107348, and 131066). Further, its performance is compared to that of a conventional semicircular-bladed rotor under identical test conditions. The experiments revealed the maximum power coefficient (CPmax) of 0.11, 0.162, 0.213 at Re = 87039, 107348, and 131066, respectively for the arc-elliptical-bladed rotor. To complement the experimental findings and to examine the flow behaviour around the rotor blades, the computational fluid dynamics (CFD) simulations have also been performed using ANSYS FLUENT software. The local torque is found to be greater around the advancing arc-elliptical blade than around the advancing semicircular blade. It has been also noticed that the pressure distributions over concave sides are similar regardless of the blade shape.

Validation of a Modal Work Approach for Forced Response Analysis of Bladed Disks

The paper describes a numerical method based on a modal work approach to evaluate the forced response of bladed disks and its validation against numerical results obtained by a commercial FEM code. Forcing functions caused by rotor–stator interactions are extracted from CFD unsteady solutions properly decomposed in time and space to separate the spinning perturbation acting on the bladed disk in a cyclic environment. The method was firstly applied on a dummy test case with cyclic symmetry where the forcing function distributions were arbitrarily selected: comparisons for resonance and out of resonance conditions revealed an excellent agreement between the two numerical methods. Finally, the validation was extended to a more realistic test case representative of a low-pressure turbine bladed rotor subjected to the wakes of two upstream rows: an IGV with low blade count and a stator row. The results show a good agreement and suggest computing the forced response problem on the finer CFD blade surface grid to achieve a better accuracy. The successful validation of the method, closely linked to the CFD environment, creates the opportunity to include the tool in an integrated multi-objective procedure able to account for aeromechanical aspects.

Numerical Investigations on Application of Cantilever Stator on Aerodynamic Performance of Tandem Bladed Axial-Flow Compressor

Adoption of a tandem bladed rotor configuration brings special flow features at the exit compared to the conventional rotor. For tandem bladed rotor, there is the presence of strong dual-tip leakage flow, atypical exit flow angle distributions, corner blade separations leading to thicker dual wakes at the exit of the rotor to name a few. This makes the aerodynamic design of downstream stator more challenging in terms of overall performance as well as operational stability. The modern compressor requisite of being lighter and cost-efficient needs to be taken care of both aerodynamic and mechanical requirements. To overcome all these challenges, the cantilever type stator (without hub rotation) has been chosen and been analyzed for the present study. The effects of different hub gap sizes of the cantilever stator in combination with the tandem bladed axial compressor stage are investigated in order to explore passive flow control mechanism near the hub. The goal of the work is to get further insights into the aerodynamic aspects of flow using a detailed flow field analysis. The numerical study was performed using ANSYS TurboGrid® for mesh generation and the commercial package ANSYS CFX® 18.0 was used as solver for steady-state simulation. Stationary hub boundary conditions have been employed for the stator in all 3 cases [baseline, 1% and 2% (of span) part clearance]. For no clearance case, the regions of momentum deficit were observed in the vicinity of the hub endwall and suction surface of the stator. The region keeps growing along both streamwise and spanwise direction as a low momentum bubble is formed near trailing edge. This low momentum bubble seems to be transported along the span and moved more towards the suction surface. The solution strategy explored to mitigate the effect of hub corner separation by adapting hub clearance. The role played by secondary flow in feeding the low momentum flow along the span is seen to be moderated by the high momentum leakage flow from the pressure side. The hub leakage flow from the blade pressure side reenergized the low momentum fluid on the suction side refraining it to travel along the span and mitigate its effect by suppressing the separation tendency near end wall region. The formation of large size bubble gets reduced in overall size both in the circumferential and span-wise direction. This phenomenon compels the low momentum flow to pass along the low span region. Numerically obtained results provide an insightful mechanism of the interaction of secondary flow structures and the influence of hub clearance flow. Hub corner stall, which is the consequence of low momentum fluid sweeping across the blade passage near the end wall got wiped out in the presence of hub clearance. This phenomenon diminishes the extent and overall effect of the hub corner stall. The interaction of hub leakage vortex and passage vortex leads to mitigation of overall secondary flow adverse effects. As a result, performance improvement at design flow conditions have been elucidated by implementation of cantilever stator. The peak pressure operation is dominated by mid-span flow complexities and as a result cantilevered stator doesn’t show much improvements. Nevertheless, the improvements in design point operating conditions do justify the study for gaining physical insights.

Integrally Bladed Rotor Mistuning Identification and Model Updating Using Geometric Mistuning Models

Non-uniform manufacturing variations and uneven usage wear and damage, referred to as mistuning, can drastically alter the dynamic response of Integrally Bladed Rotors (IBRs). Optical scanners, combined with Finite Element Model mesh metamorphosis algorithms, have provided capabilities to create analytical models that reduce the effect of geometrical uncertainties in numerical predictions. However, deviations in material properties cannot be obtained via optical scanning, so additional approaches are needed. A geometric mistuning Reduced-Order Model (ROM) is developed and modified to solve for unknown IBR sector eigenvalues that are linearly proportional to Elastic modulus. The developed approach accounts for both proportional and non-proportional mistuning and allows updating of the Elastic modulus for each sector in the ROM. Different tuned and mistuned modal reduction procedures are employed to understand the implications of each for identifying mistuning. Simulated test data with known inputs indicate the efficiency and accuracy of the method and improvements over using a traditional, tuned mode approach. The developed methods are then extended to bench-level traveling wave excitation data to discern how sector frequencies vary due to geometry and modulus mistuning.

Influence of the Solidity Ratio on the Small Wind Turbine Aerodynamics

Increasing popularity of individualised electricity generation from wind by prosumers creates a strong demand for profitable and highly efficient small wind turbines. This paper investigates the influence of rotor blade solidity parameter on device efficiency in hope of determining its optimal value as a part of the development process of the GUST small wind turbine. The study involved experimental analysis in the wind tunnel and numerical simulations performed in QBlade software. Different solidities of the rotor were achieved by alteration of (1) number of blades and (2) chord distribution along the blade span. The increase of rotor solidity resulted in augmentation of the aerodynamic efficiency in both approaches. The elongation of the chord by 33% in a 3-bladed rotor resulted in a bigger power coefficient increment than addition of a 4th blade with original chord distribution. Even though the solidity was the same, the 3-bladed rotor performed better, possibly due to lower form drag. The results emphasize the importance of the rotor solidity optimization during the small wind turbine rotor development and may significantly influence overall power output.

Boundary Layer Transition Measured by DIT on the PSP Rotor in Forward Flight

A well-defined reference set of data for computational fluid dynamics and comprehensive code validation for a scaled helicopter main rotor with boundary layer transition in forward flight is presented. The boundary layer transition was measured using differential infrared thermography (DIT) on the top (suction) side of the NASA/Army “PSP rotor” in the NASA Langley 14-by-22-Foot Subsonic Tunnel. The tests used a FLIR X8500 SLS long-wave infrared camera to observe the three-bladed rotor. The boundary layer transition was detected for forward flight at an advance ratio of 0.3 (115 kt). The measured boundary layer transition positions are consistent with previous measurements and predicted boundary layer transition locations. A method for the analysis of DIT images for a rotor in forward flight is shown and validated based on computational analysis of a pitching airfoil with varying inflow, showing both qualitative and quantitative similarity to experimental data.

Blade Tip-Vortices of a Four-Bladed Rotor with Axial Inflow

The vortex system of four rotating and pitching DSA-9A blades was examined numerically and experimentally. Numerical computations were performed using German Aerospace Center (DLR)'s finite-volume solver TAU and were validated against experimental data gathered using particle image velocimetry carried out at the rotor test facility (RTG) in Göttingen. Algorithms deriving the vortex position, swirl velocity, circulation, and core radius were implemented. Hover-like conditions with a fixed blade pitch were analyzed giving further physical insights of the static vortex system. These results are used to understand the vortex development for the unsteady pitching conditions, which can be described as a superpositioning of static vortex states. The use of a zonal detached-eddy simulations approach improved physical modeling of the vortex development by resolving finer scales than URANS. Trimmed cases agree well with differences less than 0.5% in the circulation and swirl velocity.