Tom McLaughlin

FEEDBACK CONTROL OF A CIRCULAR CYLINDER WAKE IN EXPERIMENT AND SIMULATION (INVITED)

The effect of feedback flow control on the wake of a circular cylinder at a Reynolds number of 100 is investigated in both water tunnel experiment and direct numerical simulation. Our control approach uses a low dimensional model based on proper orthogonal decomposition (POD). The controller applies linear proportional and differential feedback to the estimate of the first POD mode. The range of validity of the POD model is explored in detail. Actuation is implemented as displacement of the cylinder normal to the flow. We demonstrate that the threshold peak amplitude below which the control actuation ceases to be effective is in the order of 5% of the cylinder diameter. The closed loop feedback simulations explore the effect of both fixed phase and variable phase feedback on the wake. While fixed phase feedback is effective in reducing drag and unsteady lift, it fails to stabilize this state once the low drag state has been reached. Variable phase feedback, however, achieves the same drag and unsteady lift reductions while being able to stabilize the flow in the low drag state. In the low drag state, the near wake is entirely steady, while the far wake exhibits vortex shedding at a reduced intensity. We achieved a drag reduction of close to 90% of the vortex-induced drag, and lowered the unsteady lift force by the same amount.

Feedback Flow Control of a Shear Layer for Aero-Optic Applications

A Reduced Order Model (ROM) is formulated for the shear layer behind a backward facing step for the purpose of developing feedback control strategies. The ultimate goal of controlling the shear layer is the mitigation of optical aberrations caused by large scale density variations in the flow field. The results of simulations of the unforced and openloop forced shear layer, based on the compressible Navier-Stokes equations, were used to compile a Proper Orthogonal Decomposition (POD) database of the flow states. A Reduced Order Model for the POD time coefficients was then built using Wavenets (WN). This WN model was utilized to simulate closed loop dynamics and ultimately develop feedback control algorithms. Results obtained from applying direct adaptive control to the WN model indicate that a 35% reduction in the optical aberrations is possible.

Reduced Order Model of Cylinder Wake with Direct Adaptive Feedback Control

This paper demonstrates a very systematic approach for feedback flow controller design. Open loop forcing and unforced CFD simulation training data is used to build a reduced order model via the Double Proper Orthogonal Decomposition (DPOD) process. Nonlinear system identification is then used realize a very simple, low dimensional plant. The model has the ability to simulate interior points on the forcing envelope and also predict closed loop dynamics if the model is developed correctly. For simplicity, the model and controller development strategy is shown for a relatively well know flow field, the two dimensional, circular cylinder wake. This paper shows the validity of neural network (ANNARX) models to recreate closed loop simulations while only trained from open loop forcing cases. Direct adaptive feedback control is then applied to the ANN-ARX model. Once satisfying results are seen on the model, the feedback controller is scaled up to a CFD simulation. This control design technique is not limited to laminar two-dimensional flows, but also has capability to model and control more turbulent, non-linear three dimensional flows.

Plasma Actuator for Wake Flow Control of High Camber Blades During Part Load Operation

Plasma actuators, composed of two electrodes with a constant or time-varying voltage difference applied between them are known to impart a directed momentum on the gas in the vicinity of one of the electrodes. This work focuses on a plasma actuator installed on one blade of a gas turbine blade cascade. These high-camber angle blades are used for transportation and stationary applications, and at partial load (i.e. low flow speeds) they exhibit flow separation on the suction side. A plasma actuator, optimized in terms of insulation thickness and applied voltage waveform, is placed on the suction-side, near the trailing edge of the blade, and airfoil plasma-off performance compared to plasma-on. Separation is detected via surface pressure measurements, and loss of stagnation pressure via measurements of total pressure with Pitot tubes. Flow directions are measured in a few cases as well. The actuator is found to decrease the stagnation pressure loss at most experimental conditions, and to increase the flow turning angle. Conclusions as to the plasma actuator effectiveness are derived from blade loss coefficients. The plasma actuator can reduce stagnation pressure losses by 50% with the most effective actuator of those investigated thus far in this cascade. An approximate ratio of electrical to dynamic forces is defined and calculated as a means of characterizing the relative magnitude of the plasma force required to avert separation. Since the loss is measured in the wake of the blade, the term “wake filling configuration” seems an appropriate description of this specific actuator location.Copyright

Turbine Cascade Flow Control Using a “Wake Filling” Pulsed Plasma Actuator

Separation bubbles in high-camber blades under part-load conditions have been addressed via continuous and pulsed jets, and also via plasma actuators. Numerous passive techniques have been employed as well. In this type of blades, the laminar boundary layer cannot overcome the adverse pressure gradient arising along the suction side, resulting on a separation bubble. When separation is abated, a common explanation is that kinetic energy added to the laminar boundary layer speeds up its transition to turbulent. In the present study, a plasma actuator installed in the trailing edge (i.e. “wake filling configuration”) of a cascade blade is used to excite the flow in pulsed and continuous ways. The pulsed excitation can be directed to the frequencies of the large coherent structures (LCS) of the flow, as obtained via a hot-film anemometer, or to much higher frequencies present in the suction-side boundary layer, as given in the literature. It is found that pulsed frequencies much higher than that of LCS reduce losses and improve turning angles further than frequencies close to those of LCS. With the plasma actuator 50% on time, good loss abatement is obtained. Larger “on time” values yield improvements, but with decreasing returns. Continuous high-frequency activation results in the largest loss reduction, at increased power cost. The effectiveness of high frequencies may be due to separation abatement via boundary layer excitation into transition, or may simply be due to the creation of a favorable pressure gradient that averts separation as the actuator ejects fluid downstream. Both possibilities are discussed in light of the experimental evidence.© 2005 ASME

CANCELLATION OF NON-HARMONIC WAVES USING A CYCLOIDAL TURBINE

We computationally investigate the ability of a cycloidal turbine to cancel two-dimensional non-harmonic waves in deep water. A cycloidal turbine employs the same geometry as the well established Cycloidal or Voith-Schneider Propeller. It consists of a shaft and one or more hydrofoils that are attached eccentrically to the main shaft and can be independently adjusted in pitch angle as the cycloidal turbine rotates. We simulate the cycloidal turbine interaction with incoming waves by viewing the turbine as a wave generator superimposed with the incoming flow. The generated waves ideally are 180° out of phase and cancel the incoming wave downstream of the turbine. The upstream wave is very small as generation of single-sided waves is a characteristic of the cycloidal turbine as has been shown in prior work. The superposition of the incoming wave and generated wave is investigated in the far-field and we model the hydrofoil as a point vortex. This model has previously been used to successfully terminate regular deep water waves as well as intermediate depth water waves. We explore the ability of this model to cancel non-harmonic waves. Near complete cancellation is possible for a non-harmonic wave with components designed to match those generated by the cycloidal turbine for specified parameters. Cancellation of a specific wave component of a multi-component system is also shown. Also, step changes in the device operating parameters of circulation strength, rotation rate, and submergence depth are explored to give insight to the cycloidal turbine response characteristics and adaptability to changes in incoming waves. Based on these studies a linear, time-invarient (LTI) model is developed which captures the steady state wave frequency response. Such a model can be used for control development in future efforts to efficiently cancel more complex incoming waves.Copyright

A Methodology Based on Experimental Investigation of a DBD-Plasma Actuated Cylinder Wake for Flow Control

The main purpose of flow control is to improve the mission performance of air vehicles. Flow control can either be passive or active and active flow control is further characterized by open-loop or closed-loop techniques. Gad-el-Hak (1996) provides an insight into the advances in the field of flow control. Research of closed-loop flow control methods has increased over the past two decades. Cattafesta et al (2003) provide a useful classification of active flow control.

Structure identification in turbulent flows for feedback flow control

Feedback flow control strategies have traditionally been developed by focusing on prototype laminar flows such as the wake behind a cylinder or a free shear layer. Many strategies rely on some form of proper orthogonal decomposition (POD) of the velocity field to identify the flow structures that need to be mitigated. However, in turbulent flows, POD does not readily identify the large, coherent motion targeted by flow control. Ongoing research at the US Air Force Academy is focused on developing feedback flow control strategies for the turbulent shear layer behind a backward facing step. POD of the velocity and density fields showed that the density is a better marker of the coherent structures in the shear layer and nearly completely eliminates the contribution of the small-scale turbulent motion to the POD spatial modes, allowing for a focused development of strategies to control the coherent motion in the shear layer.

Reduced order modeling for feedback flow control of a shear layer

Computations using the compressible Navier-Stokes equations were used to investigate feedback flow control for a shear layer behind a backward facing step. The controller was built based on a reduced order model (ROM), designed using Wavelet Neural Nets (WNN) for the Proper Orthogonal Decomposition (POD) mode amplitudes of unforced and open-loop forced data. Results of the model simulations indicate that a 35% reduction in the optical aberrations is possible when feedback flow control is applied. While the CFD results show a similar OPD reduction, modifications to the controller parameters based on the CFD results improved the performance and revealed a more effective mechanism to reduce the optical aberrations.