Stanislav Gordeyev

Coherent Structure in the Turbulent Planar Jet. Part 1. Extraction of Proper Orthogonal Decomposition Eigenmodes and Their Self-Similarity

In this paper the coherent structure in the similarity region of the turbulent planar jet is examined experimentally by application of the proper orthogonal decomposition (POD). In particular, twin cross-stream rakes of X-wire probes are used to take cross-spectral measurements with different spanwise separations between the rakes and at several locations throughout the similarity region. The resulting POD spatial eigenfunctions for each of the three velocity components depend on cross-stream spatial coordinate, Strouhal number, and spanwise wavenumber. Corresponding eigenvalue distributions are obtained in Strouhal number–spanwise wavenumber space. Eigenvalue convergence is demonstrated to be rapid. When properly scaled the eigenfunctions and eigenvalues are shown to exhibit self-similarity though the streamwise location at which this commences depends on the particular velocity component. The results suggest that the flow supports a planar structure aligned in the spanwise direction as well as an essentially three-dimensional structure with asymmetrical shape in the cross-stream direction and pseudo-periodically distributed in the spanwise direction. Comparison of the single- and dual-rake implementations of the POD presented in this paper demonstrate that measurements confined to a single plane are incapable of properly extracting the planar modes. Rather, the single-rake implementation results in modes that appear to be a weighted sum of modes corresponding to different spanwise wavenumbers.

Aero-Optical and Flow Measurements Over a Flat-Windowed Turret

DOI: 10.2514/1.24468An extensive investigation of the aberrating character of flow over a hemisphere-on-cylinder turret with a flatwindow was performed. Optical distortions over the window were measured using a two-dimensional wave frontsensor and a Malley probe. The Malley probe measurements were complemented with simultaneous hot-wiremeasurementsofthevelocityfieldnormaltothewindowatseveralpointsacrossitsdiameter.ThetestswererunforafixedelevationforseveralazimuthalanglesoverarangeofMachnumbers.Theresultsprovidethelevelsofunsteadyopticalaberration acrossthewindow,aswell asthelocalthickness,intensity,andconvectivespeedoftheseparatedflowoverthewindow.ResultsrevealthattheopticaldistortionsgrowapproximatelywiththesquareoftheincomingMach number multiplied by the freestream density, OPD

AERO-OPTICAL CHARACTERISTICS OF COMPRESSIBLE, SUBSONIC TURBULENT BOUNDARY LAYERS

An extensive experimental study of optical aberrations due to propagation through fully-developed turbulent boundary layers at high subsonic Mach numbers was performed. Time-resolved, high- bandwidth, direct optical measurements of the dynamic aberrations were made using a Malley probe. The probe was used to obtain the convective speeds of the optically-significant turbulence structures and to measure the optical path differences. Measurements were made over a range of boundary layer thicknesses and Mach numbers. Optical distortions were found to scale linearly with boundary layer thickness and freestream density, and to go as the square of the freestream Mach number.

Aero-optical environment around a conformal-window turret

This paper presents the aero-optical environment around a generic conformal-window turret formed from a hemisphere on a short cylindrical base. A suite of optical instruments consisting of a Malley probe, a conventional two-dimensional Shack-Hartmann wave-front sensor, and a new high-bandwidth, lower-resolution Hartmann wave-front sensor were used to measure the aberrations on the wave front of a laser beam emanating from the turret at various angles in both the forward and aft direction in the turrets zenith plane. The measurements were made over a range of Mach numbers from 0.35 to 0.45. Complementary steady- and unsteady-pressure measurements over a slightly larger range of Mach numbers were also made, along with a surface-flow-visualization study of the complex flowfield over and around the turret. The use of the suite of sensors allowed for the recognition and separation of the aberrating optical environment into components associated with stationary disturbances and convecting disturbances at the frequency of the turrets separated wake and at order-of-magnitude-higher frequencies associated with structures that form in the separated shear layers, respectively. The optical data separated in this way are valuable because of the implications for adaptive optics.

The Optical Environment of a Cylindrical Turret with a Flat Window and the Impact of Passive Control Devices

Optical aberrations over a cylindrical turret with a flat window were measured using a 2-D wavefront sensor and a Malley probe as a function of laser beam elevation angle. It was found that depending on the window back-facing angle the flow either had a weak separation bubble followed by a reattached boundary layer or a strong separation with a large recirculation region behind the cylindrical turret. It was found that optical aberrations were high in the case of a weak separation bubble and at large look-back elevation angles. Different passive devices placed upstream from the turret were studied for their effectiveness in improving the optical-propagation environment. The results are presented and discussed.

Coherent structure in the turbulent planar jet. Part 2. Structural topology via POD eigenmode projection

The topology of the large-scale structure in the similarity region of a turbulent planar jet is investigated experimentally. The large-scale structure is reconstructed in physical space by projection of measured proper orthogonal decomposition eigenmodes onto instantaneous flow-eld realizations. The instantaneous flow-eld realizations are obtained by a spanwise aligned triple X-wire rake arrangement which is used in conjunction with the linear stochastic estimation technique. Instantaneous realizations are also acquired via a second triple rake arrangement which provides an assessment of the eect of spatial aliasing on the resulting structural topology. Results indicate that the self-similar large-scale structure in the planar jet consists of a dominant planar component consisting of two lines of large-scale spanwise vortices arranged approximately asymmetrically with respect to the jet centreline. This planar component of the structure resembles the classic K arm an vortex street. There is a strong interaction between structures on opposite sides of the jet in the form of nearly two-dimensional lateral streaming motions that extend well across the flow. In addition, results indicate that the eect of the nonplanar spanwise modes is to both tilt and bend the primary spanwise vortex tubes and thereby redistribute large-scale vorticity. The bending occurs primarily in the streamwise direction. The degree to which the spanwise vortices are distorted varies greatly; in some cases they are nearly streamwise oriented and in others only slight distortion of a spanwise vortex is noted. Based upon the experimental results, prospects for low-order modelling of the jet large-scale structure are discussed.

Modal Analysis of Fluid Flows: An Overview

Simple aerodynamic configurations under even modest conditions can exhibit complex flows with a wide range of temporal and spatial features. It has become common practice in the analysis of these flows to look for and extract physically important features, or modes, as a first step in the analysis. This step typically starts with a modal decomposition of an experimental or numerical dataset of the flow field, or of an operator relevant to the system. We describe herein some of the dominant techniques for accomplishing these modal decompositions and analyses that have seen a surge of activity in recent decades. For a non-expert, keeping track of recent developments can be daunting, and the intent of this document is to provide an introduction to modal analysis that is accessible to the larger fluid dynamics community. In particular, we present a brief overview of several of the well-established techniques and clearly lay the framework of these methods using familiar linear algebra. The modal analysis techniques covered in this paper include the proper orthogonal decomposition (POD), balanced proper orthogonal decomposition (Balanced POD), dynamic mode decomposition (DMD), Koopman analysis, global linear stability analysis, and resolvent analysis.

In-flight aero-optics of turrets

Abstract. Recent in-flight aero-optical measurements from the Airborne Aero-Optics Laboratory are provided, along with instrumentation and experimental set-up. Results of an extensive survey of the aero-optical environment at different viewing angles, for both flat-window and conformal-window turrets at different subsonic and low transonic speeds below M=0.65, are presented, compared and extensively discussed. A comparison between two turret geometries, hemisphere-on-cylinder and hemisphere only, plus the statistical analysis of wavefronts at different viewing angles, are also presented and discussed. Additionally, dynamics of a local shock appearing on the conformal-window turret at transonic Mach number are discussed.

Active Control and Optical Diagnostics of the Flow over a Hemispherical Turret

The effects of flow control actuation on the aerodynamic characteristics and aero optical distortions in the near wake of a hemispherical turret model were investigated in a series of wind tunnel experiments at M = 0.3 – 0.64. Flow control was applied using a spanwise array of high-frequency synthetic jet actuators oriented such that the long sides of their rectangular orifices were aligned in the streamwise direction. The effects of actuation on the flow dynamics and aero-optical distortions were characterized using Malley probe measurements, along with distributions of static surface pressure and hot-wire anemometry. One of the main findings of the present work was that the dissipative, small-scale motions that are induced by the actuation (StD > 1) resulted in suppression of turbulent fluctuations within the separated flow over the hemisphere, and concomitantly in significant reduction in the levels of optical distortion (in excess of 40% at M = 0.4). The effect of the momentum limited actuation diminishes at M > 0.45.

Airborne Aero-Optics Laboratory

Abstract. We provide a background into aero-optics, which is the effect that turbulent flow over and around an aircraft has on a laser projected or received by an optical system. We also discuss the magnitude of detrimental effects which aero-optics has on optical system performance, and the need to measure these effects in flight. The Airborne Aero-Optics Laboratory (AAOL), fulfills this need by providing an airborne laboratory that can capture wavefronts imposed on a laser beam from a morphable optical turret; the aircraft has a Mach number range up to low transonic speeds. We present the AAOL concept as well as a description of its optical components and sensing capabilities and uses.