Demetri P. Telionis

Acoustics of Aircraft Engine-Duct Systems

Noise generated in aircraft engines is usually suppressed by acoustically treating the engine ducts. The optimization of this treatment requires an understanding of the transmission and attenuation of the acoustic waves. A critical review is presented of the state of the art regarding methods of determining the transmission and attenuation parameters and the effect on these parameters of (1) acoustic properties of liners, (2) the mean velocity, including uniform and shear profiles and nonparallel flow, (3) axial and transverse temperature gradients, (4) slowly and abruptly varying cross sections, and (5) finite-amplitude waves and nonlinear duct liners.

VORTEX SHEDDING AND LOCK-ON OF A CIRCULAR CYLINDER IN OSCILLATORY FLOW

An experimental study has been made of a circular cylinder in steady and oscillatory flow with non-zero mean velocity up to a Reynolds number of 40000. The results for the stationary cylinder are in close agreement with previously published data. Skin-friction measurements revealed the amplitude of fluctuation of the boundary layer for different angular locations. It has been universally accepted that bluff bodies shed vortices at their natural frequency of shedding (Strouhal frequency), or, when synchronized with an external unsteadiness, at the frequency of the disturbance or half of it, depending of the direction of the unsteadiness. Our findings, instead, indicate that the shedding frequency may vary smoothly with the driving frequency before locking on its subharmonic. Moreover, the present results indicate that, at the lowest frequency limit of lock-on, vortices are shed simultaneously on both sides of the model. A more traditional alternate pattern of vortex shedding is then recovered at higher driving frequencies.

Unsteady Laminar Separation--An Experimental Study.

Abstract : The design of most aerodynamic surfaces, as for example the helicopter rotor, is based essentially on quasi-steady theories. However the dynamics of a rotating blade introduce unexpected fluctuations and overshoots of properties like lift, drag, etc. The phenomenon of unsteady stall is intimately connected with the development of an oscillating boundary layer and separation. Experimental investigation of such flows was undertaken by a method of visualization developed especially for the study of laminar or turbulent boundary layers and separation. The method captures the instantaneous 2-D flow field, including regions of separated flow and provides accurate quantitative information. Laser doppler anemometer measurements complement the optically received data. Results reveal that separation responds with time-lag to external disturbances, in agreement with unsteady stall data. Oscillating outer flows result in displacement of the point of separation and under certain conditions, the Despard and Miller criterion was found to hold. Earlier theoretical models of separation are confirmed qualitatively and for the early stages of the transient phenomena. The findings provide physical insight and quantitative data that may help understand the phenomenon of unsteady stall and unsteady separation. (Author)

Handbook of Fluid Dynamics and Fluid Machinery

This definitive reference contains contributions from renowned global experts who discuss not only the new generation of fluid dynamics and machinery but classical topics as well. Volume One covers the basics; Volume Two describes advanced aspects such as computational fluid dynamics and fluid machinery; and Volume Three covers the applications of fluid dynamics. This set is illustrated with over 1,000 line drawings and tables.

Blade-vortex interaction

A conformal transformation and discrete vortex dynamics are used to calculate the interaction of a blade with vortices drifting with the free stream. An unsteady Kutta condition is employed to dictate the strength of vortices shed at the trailing edge. Instantaneous pressure distributions are calculated and compared with earlier experimental and analytical data.

Time-Resolved DPIV Analysis of Vortex Dynamics in a Left Ventricular Model Through Bileaflet Mechanical and Porcine Heart Valve Prostheses

The performance of the heart after a mitral valve replacement operation greatly depends on the flow character downstream of the valve. The design and implanting orientation of valves may considerably affect the flow development. A study of the hemodynamics of two orientations, anatomical and anti-anatomical, of the St. Jude Medical (SJM) bileaflet valve are presented and compared with those of the SJM Biocor porcine valve, which served also to represent the natural valve. We document the velocity field in a flexible, transparent (LV) using time-resolved digital particle image velocimetry (TRDPIV). Vortex formation and vortex interaction are two important physical phenomena that dominate the filling and emptying of the ventricle. For the three configurations, the following effects were examined: mitral valve inlet jet asymmetry, survival of vortical structures upstream of the aortic valve, vortex-induced velocities and redirection of theflow in abidance of the Biot-Savart law, domain segmentation, resonant times of vortical structures, and regions of stagnantflow. The presence of three distinct flow patterns, for the three configurations, was identified by the location of vortical structures and level of coherence corresponding to a significant variation in the turbulence level distribution inside the LV. The adverse effect of these observations could potentially compromise the efficiency of the LV and result in flow patterns that deviate from those in the natural heart.

Flow Control of a Sharp-Edged Airfoil

An experimental study of active control of fully separated flow over a symmetrical circular-arc airfoil at high angles of attack was performed. The experiments were carried out in a low-speed, open-circuit wind tunnel with the airfoil at angles of attack from 10 to 40 deg. Low-power input, unsteady excitation was applied to the leading or trailing edge. The actuation was provided by the periodic oscillation of a 4% chord flap placed on the suction side of the airfoil and facing the sharp edge

Periodic vortex shedding over delta wings

again the edges were nearly parallel, and except for the formation of cells, the wakes organized themselves in a nearly twodimensional fashion. In the problem under discussion here, the edges form an angle of 30 deg, and moreover they are inclined with respect to the flow. Preliminary findings on this problem were published by the present authors in a short Note.18 The variation of the Strouhal number vs the angle of attack and the Reynolds number was partially investigated. The main thrust in the present effort is to explore the flowfield for organized natural oscillations and to identify their character. The existence of two different shedding modes (simultaneous and alternate) is now documented in terms of pairs of hot-wire signals. The angle-of-attack ranges in which each models present are investigated for the entire domain of shedding angles of attack (35 < a < 90 deg). The orientation of the shed vortices with respect to the wing and their evolution in time are studied. Finally, flow visualization techniques are used to support the earlier findings.

Unsteady laminar separation over impulsively moved cylinders

Abstract A numerical method is employed to integrate the unsteady laminar boundary layers about circular and elliptic cylinders started impulsively from rest. An upwind differencing scheme permits the integration through thin layers of reversed flow that are embedded in the boundary layer. The results are in agreement with numerical solutions of the full Navier-Stokes equations and with small time inner-outer expansion solutions. At later times, a singularity of the Goldstein type appears; Sears and Telionis defined this singularity as unsteady boundary-layer separation. For the circular cylinder, this singularity travels upstream and for large times, arrives asymptotically at the point of steady-state separation.

Dynamic pitch-up of a delta wing

The transient flowfield over a delta wing during pitch-up motions to very large angles of attack was investigated. Emphasis was directed at the growth and the eventual breakdown of the leading-edge vortices. Delta wing models were tested in a wind tunnel at Reynolds numbers of order 10 5 . The flowfield along the trailing edge was mapped out via a seven-hole probe designed, constructed, and calibrated to general time-resolved information. Instantaneous surface pressure measurements were also obtained. Earlier qualitative evidence of hysteresis in the development of the flow was confirmed. Moreover, the present data indicate significant differences of vorticity content between the steady and the steady motions, for reduced frequencies as low as 0.01