Session OL: Turbulence and Instability Control III

Characterization of Combustion Powered Actuators for Flow Control

The performance of a high-power small-scale combustion-based fluidic actuator for flow control applications is characterized with specific focus on comparisons between premixed and nonpremixed operating modes for the device. Momentary (pulsed) actuation jets are produced by the ignition of a mixture of gaseous fuel and oxidizer within a small (cubic centimeter scale) combustion chamber. The combustion process yields a high pressure burst (1 to 3 ms in duration in the typical configurations) and the ejection of a high-speed exhaust jet. The actuation frequency can be continuously varied by independently controlling the flow rate of the fuel/oxidizer and the spark ignition frequency up to a maximum determined by the operating characteristics of the actuator. The actuator performance is characterized by both its peak thrust and net total impulse, with increases in peak jet momentum often two to three orders of magnitude above the baseline steady jet. Results for operation of the device in both premixed and nonpremixed modes are presented and analyzed, with nonpremixed operation typically yielding higher pressures and greater frequency ranges in the present configurations. Operating frequencies up to 500 Hz are demonstrated for nonpremixed operation.   

Optimal Control of Flows in Moving Domains

This investigation concerns adjoint--based optimization of viscous incompressible flows (the Navier-Stokes problem) coupled with heat conduction involving change of phase (the Stefan problem) and occurring in domains with moving boundaries such as the free and solidification surfaces. This problem is motivated by optimization of advanced welding techniques used in automotive manufacturing. We characterize the sensitivity of a suitable cost functional defined for the system with respect to control (the heat input) using adjoint equations. Given that the shape of the domain is also a dependent variable, characterizing sensitivities necessitates the introduction of ``non-cylindrical'' calculus required to differentiate a cost functional defined on a variable domain. As a result, unlike the forward problem, the adjoint system is defined on a domain with a predetermined evolution in time and also involves ordinary differential equations defined on the domain boundary (``the adjoint transverse system''). We will discuss certain computational issues related to numerical solution of such adjoint problems.   

Flight experiments on laminar flow control in swept-wing boundary layers

Data are presented on boundary-layer transition to turbulence in low-disturbance environments. It uses a combination of hotfilm anemometry and infra-red thermography to study a variety of roughness related issues in flight. The hotfilm measurements give the important passband and spanwise scales while the thermography gives transition location. A swept-wing model is mounted on the wing of a Cessna O-2 aircraft. An Euler code is used calculate the aircraft flowfield while parabolized stability equations correlate the stability measurements and transition locations. The laminarization scheme of spanwise-periodic distributed roughness elements is investigated at chord Reynolds numbers of 7.5 million. In the past year, a number of flight tests have been conducted. Measurements were made to determine the pressure distribution on the model and the transition locations for clean configurations, and transition locations for enhanced surface roughness that simulates an operational surface finish. For clean configurations, natural laminar flow was achieved over 80{\%} of the surface of a 30$^{\circ}$ swept-wing model at chord Reynolds numbers of 7.55 million. The corresponding amplification factors were at N = 14.   

Distributed dynamic phasors in empirical Galerkin models for separated flows over an airfoil

A low order Galerkin model is proposed for the flow over an airfoil. The model includes modes representing the shedding and actuation frequencies and base flow dynamics. The method addresses the difficulty of POD techniques to capture distinct harmonics in dedicated modes during transients, diminishing the link to flow physics and the efficiency of a minimal order model. Dynamic phasors representations capture transient behavior of periodic characteristics in periodically dominated systems. In a distributed variant, they are suggested as natural tools to identify and extract distinct periodic contributions to dominant modes, and are used in conjunction with subsequent POD analysis.   

Simulation and Experiment of Plasma Flow Control over a Hump Model

This work is focused on the development of active control of a turbulent boundary layer separation over a hump model used in a NASA Langley Workshop on CFD Validation of Synthetic Jets and Turbulent Separation Control. In this study, Fluent with various turbulence models was used to predict the flow over a wall-mounted hump at Reynolds number of $9.74 \times 10^5$ based on the hump chord length. For the base flow, three forms of $k-\varepsilon$ models showed favorable agreement with the experiment data over the whole hump. Separation control using steady and unsteady SDBD plasma actuators was also simulated in this study. The plasma actuator was positioned upstream of the flow separation locations. A body force model for the plasma actuator was used in the simulations. Different arrangements of the actuator were used: one to produce periodic spanwise vortices at an optimum frequency for reattaching the flow, and the other that was designed to produce streamwise vortices. The results show that the plasma actuator was effective in turbulent separation control, and that the simulations with the plasma actuator agreed well with the experiments.   

Generalized mean-field model of oscillatory flow using continuous mode interpolation

A `least order' Galerkin model is proposed for the transient and post-transient cylinder wake targeting model-based flow control. The underlying Galerkin approximation comprises two mean-flow dependent modes representing the developing von K\'arm\'an vortex street and the shift mode resolving mean-flow changes. A continuous mode interpolation between global stability eigenmodes and POD modes enables a uniformly accurate flow prediction during the whole transient from steady to periodic state. Additionally, the shift mode and the interpolated oscillatory modes are derived from first principles employing a generalized mean-field theory.   

Lumped-element Circuit Model for Single-Dielectric Barrier Discharge Plasma Actuator

This work presents an extension of our previous studies on a single-dielectric barrier discharge (SDBD) plasma actuators. The space-time lumped-element circuit model that had been developed for the actuator is intended to model the details of the ionization process to provide predictions of the body force for a range of parameters without the need of experimental calibration. In this model, the domain over the covered electrode is divided into several parallel equivalent circuit networks, each consisting of resistive and capacitive elements and zenor diodes. The results of this numerical model show very good agreement with the space-time resolved experimental observations of the plasma illumination over the dielectric surface for a range of applied voltage amplitudes and frequencies. These characteristics include plasma sweep-out velocity and spatial extent, and spatial intensity decay. The model provides the boundary conditions on the electric potential over the dielectric that is needed in solving for the actuator space-time body force. The plasma body force is then used in a Navier-Stokes flow solver to study the effects of the plasma actuator. Examples of simulations with the plasma actuator on a flat surface and the leading edge of an airfoil are presented. The simulations show good agreement with comparable experiments. Supported under a USAF SBIR Phase II Contract FA8650-04-C-3405.   

Scaling law of efficient drag reduction due to Lorenz actuators

Drag reduction due to Lorenz pulsing in seawater turbulent boundary layers is considered focusing simultaneously on efficiency which is generally low in this technique. Experimental and numerical data from two- and three-dimensional pulsing actuators are examined in the context of vorticity reorientation hypothesis of Stokes' drag reduction (Bandyopadhyay, Jou. App. Mech., V73, 483-489, 2006). Drag reduction is compared between actuators based on their efficiencies rather than the level of their drag reduction alone. This leads to an effective interaction parameter of inertia and Lorenz forces based on the effective value of Lorenz force rather than on the applied levels. Viscous sublayer thickness is taken as the length scale that is appropriate for drag reduction due to both two- and three-dimensional actuators rather than any large scale boundary layer integral parameter or actuator geometric scale. A universal relationship of drag reduction with effective interaction parameter is obtained.