Adaptive Frequency Responsive Control for Wind Farm Considering Wake Interaction

With the increasing share of wind power,it is expected that wind turbines would provide frequency regulation ancillary service.However,the complex wake effect intensifies the difficulty in controlling wind turbines and evaluating the frequency regulation potential from the wind farm.We propose a novel frequency control scheme for doubly-fed induction generator(DFIG)-based wind turbines,in which the wake effect is considered.The proposed control scheme is developed by incorporating the virtual inertia control and primary frequency control in a holistic way.To facilitate frequency regulation in timevarying operation status,the control gains are adaptively adjusted according to wind turbine operation status in the proposed controller.Besides,different kinds of power reserve control approaches are explicitly investigated.Finally,extensive case studies are conducted and simulation results verify that the frequency behavior is significantly improved via the proposed control scheme.

An Experimental Study on Aerodynamic Sound Generated from Wake Interaction of Circular Cylinder and Airfoil with Attack Angle in Tandem

In our previous study, the effects of the interval between the cylinder and the airfoil on the aerodynamic sound were investigated and compared with the cases of single circular and single airfoil. In this study, the effects of the attack angle of the airfoil located downstream on the characteristics of aerodynamic sound and the wake structure are investigated at a given interval between the cylinder and the airfoil. It is found that the sound pressure level of DFN and the peak frequency decrease with increasing attack angle of airfoil because of the diffusive wake structure due to the increased back pressure of circular cylinder, which is caused by the blocking effect of airfoil. It is shown that the sound sources are corresponded to the attack points of shedding vortex form the upstream circular cylinder to the downstream airfoil. We conclude that the pressure fluctuation at the airfoil surface effects on the sound pressure level, from the flow visualizations and the exploration test of sound source.

The influence of the wake of a flapping wing on the production of aerodynamic forces

The effect of the wake of previous strokes on the aerodynamic forces of a flapping model insect wing is studied using the method of computational fluid dynamics. The wake effect is isolated by comparing the forces and flows of the starting stroke （when the wake has not developed） with those of a later stroke （when the wake has developed）. The following has been shown. （1） The wake effect may increase or decrease the lift and drag at the beginning of a half-stroke （downstroke or upstroke）, depending on the wing kinematics at stroke reversal. The reason for this is that at the beginning of the half-stroke, the wing “impinges” on the spanwise vorticity generated by the wing during stroke reversal and the distribution of the vorticity is sensitive to the wing kinematics at stroke reversal. （2） The wake effect decreases the lift and increases the drag in the rest part of the half-stroke. This is because the wing moves in a downwash field induced by previous half-stroke＇s starting vortex, tip vortices and attached leading edge vortex （these vortices form a downwash producing vortex ring）. （3） The wake effect decreases the mean lift by 6%-18% （depending on wing kinematics at stroke reversal） and slightly increases the mean drag. Therefore, it is detrimental to the aerodynamic performance of the flapping wing.

Experimental Study of Stator Clocking Effects in an Axial Compressor

This paper is focused on the experimental study of the effects of stator clocking on the performance of a low-speed repeating stage axial compressor with compound-lean stators as well as the one with conventional stators （the baseline） for comparison. The experimental results show that as the clocking positions vary, the upstream stator wake enters the following passage at different circumferential positions, and then mixes with the local fluid in the following passage. This is the main reason for the variation of the compressor performance resulted from the stator clocking effects. The variation of the compressor performance due to the clocking effect is less pronounced for the compressor with compound-lean stators than with the baseline. At a certain clocking position, the efficiency of the compressor with compound-lean stators is increased in comparison with that of the baseline, especially on small mass flow rate conditions, e.g., 0.7% at design condition and 3.5% at near-surge condition in this case. The maximum 1.22% and the minimum 0.07% increases in efficiency on design condition are obtained through the combined effects of the stator compound-lean and the stator clocking in this case.

Unsteady Flow Variability Driven by Rotor-stator Interaction at Rotor Exit

Numerical investigation of the unsteady flow variability driven by rotorstator interaction in a transonic axial compressor is performed. Two models with close and far axial gap between rotor and stator rows are studied in the simulation. Particular attention is attached to the analysis of mechanisms involved in driving rotor wake oscillation, rotor wake skewing and flow angle fluctuation at rotor exit. The results show that smaller axial gap is favorable to enhance the interaction in the region between two adjacent rows, and the fluctuation of the static pressure difference between two sides of rotor wake is improved by potential field from down stator, which is the driving force for rotor wake oscillation. The interaction between rotor and stator is weakened by increasing axial distance, rotor wake shifts to suction side of rotor blade with 5%-10% of rotor pitch, the absolute value of flow angle at rotor exit is less than that in the case of close interspace for every time step, and the fluctuation amplitude is also decreased.

Strip distortion generator for simulating inlet flow distortion in gas turbine engine ground test facilities

A methodology has been developed to generate a non-uniform/distoited inlet flow field to test a gas turbine engine in ground test facilities.The distorted flow field is generated by positioning radial and circumferential strips of varying widths upstream of the Aerodynamic Interface Plane.The interacting wakes from these strips are used to generate a given target flow field.The approximate superposition of these wakes is investigated and used to construct the strip arrangement which is subsequently validated by computing the flow field by solving the Navier-Stokes equations.The strip geometry designed using the present methodology is able to produce the target Mach number distribution with a root-mean-square error of 5.06%.

High-speed unsteady flows past two-body configurations

This paper presents a detailed investigation of unsteady supersonic flows around a typical two-body configuration, which consists of a capsule and a canopy. The cases with different trailing distances between the capsule and canopy are simulated. The objective of this study is to examine the detailed effects of trailing distance on the flow fields and analyze the flow physics of the different flow modes around the parachute-like two-body model. The computational results show unsteady pulsating flow fields in the small trailing distance cases and are in reasonable agree- ment with the experimental data. As the trailing distance increases, this unsteady flow mode takes different forms along with the wake/shock and shock/shock interactions, and then gradually fades away and transits to oscillate mode, which is very different from the former. As the trailing distance keeps increasing, only the capsule wake/canopy shock interaction is present in the flow field around the two-body model, which reveals that the unsteady capsule shock/canopy shock interaction is a key mechanism for the pulsation mode.

L2F Anemometry Measurements of the Inter-Row FlowField Within a Transonic Axial Compressor Providedwith Curved Windows

When measurements are performed in high speed, small-scale compressors, the use of curved glasswindows is required in order to avoid any mismatch between the measurement window and the casing. However, the glass curvature leads to optical distortions, which hinder acceptable measurementsand can even prevent the acquisition of any data. Thus, an original optical assembly, which consistsin inserting a simple and inexpensive corrective window between the frontal lens of the anemometerand the shroud window, is proposed. The way of determining the geometric characteristics and theposition of this corrective window, which restores very acceptable fool, is presented in the paper. Thereliability of this corrective optical assembly is highlighted by comparative measurements in a test case.Using such an optical setting, L2F measurements were realized along a section, downstream of the inletguide vane (IGV) of a transonic compressor stage. The spatial resolution leads to a good descriptionof the interaction of the wake with the oblique shock emanating from the leading edge of the rotor.A phenomenological study of the wake/shock interaction with a change of frame is realized using thestreamwise equation of the transport of vorticity.