scholarly journals High-frequency forcing to mitigate unsteady separation from a bursting separation bubble

2018 ◽  
Vol 3 (1) ◽  
Author(s):  
Stuart I. Benton ◽  
Miguel R. Visbal
Keyword(s):  
High Frequency ◽  
Separation Bubble ◽  
Unsteady Separation

Behavior of Underexpanded Jet Impinging on Sphere

2007 ◽  
Author(s):  
Masaki Endo ◽  
Yoko Sakakibara ◽  
Junjiro Iwamoto
Keyword(s):  
Flow Field ◽  
High Frequency ◽  
Laser Cutting ◽  
Separation Bubble ◽  
Jet Impingement ◽  
Screech Tone ◽  
Underexpanded Jet ◽  
Underexpanded Jets ◽  
Shock Region ◽  
Jet Structure

An underexpanded jet is utilized in industries as well as aviation field, e.g. to cool a body by the jet impingement, to remove molten metal in laser cutting, etc. One of the biggest problems is noise radiating from the jet which has high frequency, or screech tone. It is pointed out that the noise is closely related to the structure of the jet. In this paper, the underexpanded jets on a plate and hemispheres of different radii are visualized using the shadowgraph and Schlieren methods so as to analyze the jet structure, especially the flow field above the object, or the shock region. As a result, the radius of the hemisphere is found to have an effect not on the shape greatly, but on the location of the plate shock, and furthermore on the formation of separation bubble on the surface.

Full-Annular Numerical Investigation of the Rim Seal Cavity Flows Using GPU’s

2014 ◽  
Author(s):  
A. M. Basol ◽  
A. Raheem ◽  
M. Huber ◽  
R. S. Abhari
Keyword(s):  
High Frequency ◽  
Separation Bubble ◽  
Pressure Fluctuations ◽  
Injection Rate ◽  
Main Flow ◽  
Cavity Flows ◽  
Hot Gas ◽  
Purge Flow ◽  
Unsteady Interaction ◽  
Complete Revolution

In high pressure turbines to prevent the ingestion of the hot gas into the disk cavity cold air is purged from the cavities in-between the blade rows into the main flow. This numerical paper investigates this unsteady interaction between the main flow and the purge flow on a full-annular computational model of the LEC’s in-house axial turbine facility “LISA” at ETH Zurich. The simulations have been conducted on the GPU accelerated in-house solver “MULTI3”. One complete revolution of the 360° model could be completed in 48h using 18 GPU’s. Two injection rates have been considered. For the 0.9% injection rate a separation bubble has been captured at the cavity neck which triggered high frequency pressure fluctuations. The 20° model also predicted the high frequency fluctuations; however their amplitude was underestimated due to the forcing of the flow field to periodicity. On the other hand, at the low injection rate of 0.4% no considerable separation bubble was formed inside the cavity. The unsteady simulations have demonstrated the inherent unsteadiness in the purge flow - main flow interaction.

Experimental Flow Visualization Study of Flow Separation Control With High-Frequency Translational Surface Actuation

2020 ◽  
Author(s):  
Kenechukwu Okoye ◽  
Wing Lai ◽  
Taiho Yeom
Keyword(s):  
Flow Separation ◽  
High Frequency ◽  
High Speed ◽  
Free Stream ◽  
Separation Bubble ◽  
Angle Of Attack ◽  
Small Displacement ◽  
Peak Displacement ◽  
Flow Separation Control ◽  
Translational Surface

Abstract Flow separation causes aircraft to experience an increase in drag degrading their aviation performance. The current study aims to delay flow separation on an airfoil by embedding a high-frequency translational piezoelectric actuator along the surface of the airfoil. The actuators with two actuation surfaces were embedded on the suction surface of an Eppler 862 airfoil model and placed in a low-speed wind tunnel. Consecutive pictures of the flow fields with dry ice fogs around the airfoil were taken using a high speed camera in order to observe the flow separation phenomenon before and after turning on the high-frequency translational surface actuation. The effects of the actuation on the flow separation were observed at various actuation displacements, angles of attack, and free stream velocities. The operating frequency of the surface actuation was 565 Hz. The measured actuation mean-to-peak displacement ranged up to 0.12 mm at the maximum applied voltage of 150 V. The angle of attack of the airfoil varied from 6° to 24°. The chord Reynolds number was increased up to around 262,000. It was confirmed that the actuation had a very strong influence on the flow separation even at a very small displacement of 0.024 mm remaining significantly reduced separation bubble compared to the one before activating the actuators at 4.3 m/s of velocity and 14° of angle of attack. The flow separation was completely suppressed when the actuation displacement reached around 0.082 mm under the same conditions of flow velocity and angle of attack. This implied that the actuation should generate a strong enough momentum relative to the free stream in order to completely suppress the flow separation. In summary, the study confirmed that the employed high-frequency translational surface actuation had the obvious control authority on delaying or suppressing the flow separation over the airfoil depending on the parameters changed.

Numerical Study of a Cascade Unsteady Separation Flow

2004 ◽  
Author(s):  
Zongjun Hu ◽  
GeCheng Zha ◽  
J. Lepicovsky
Keyword(s):  
Mach Number ◽  
Stokes Equations ◽  
Numerical Study ◽  
Separation Bubble ◽  
Leading Edge ◽  
Pressure Oscillation ◽  
Separation Flow ◽  
Inlet Flow ◽  
Unsteady Separation ◽  
Unsteady Simulation

A CFD solver is developed to solve a 3D, unsteady, compressible Navier-Stokes equations with the Baldwin-Lomax turbulence model to study the unsteady separation flow in a high incidence cascade. The second order accuracy is obtained with the dual time stepping technique. The code is first validated for its unsteady simulation capability by calculating a 2D transonic inlet diffuser flow. Then a 3D steady state calculation is carried out for the cascade at an incidence of 10°. The surface pressure distributions compare reasonably well with the experiment measurement. Finally, the 3D unsteady simulation is carried out with 3 inlet Mach numbers at the incidence of 10°. The separation bubble oscillation and the static pressure oscillation on the leading edge of the blade suction surface exhibit clear periodicity. The details of the leading edge vortex shedding is captured. The inlet Mach number is shown to be an important factor to determine the pattern of the separation flow. In the subsonic inlet flow region, increasing the inlet Mach number enlarges the separation region and the pressure oscillation intensity. The separation flow is weakened when the inlet flow becomes supersonic.

Alternatives to Kelvin-Helmholtz Instabilities to Control Separation Bubbles

2006 ◽  
Author(s):  
Mark P. Simens ◽  
Javier Jime´nez
Keyword(s):  
Pressure Gradient ◽  
Separation Bubble ◽  
Low Frequency ◽  
Adverse Pressure Gradient ◽  
Reynolds Numbers ◽  
Two Dimensional ◽  
Laminar Separation ◽  
Unsteady Separation ◽  
Separation Bubbles ◽  
Frequency Regime

We study the control of two-dimensional laminar separation bubbles on a flat plate at low Reynolds numbers, using two-dimensional DNS. A range of steady separation bubbles is obtained varying the pressure gradient. They are forced by a zero-mass flow, oscillatory wall blowing with different perturbation amplitudes and frequencies. The reduction in bubble length as a function of frequency has two minima for sufficient high amplitudes. One of them is related to the Kelvin-Helmholtz instability of the separated boundary layer, while the other, most effective one, is here denoted as the low-frequency regime. In this regime large vortices are created which are not a consequence of an instability of the original bubble. On the contrary the forcing creates an unsteady separation bubble which evolves into a large vortex. These vortices have large radii and attach to the wall due to their self-induced pressure field while convecting across the adverse pressure gradient zone. Scaling relations for the effect of the forcing are proposed and tested.

The Microstructure of Steatite (Protoenstatite)

1985 ◽  
Vol 43 ◽  
pp. 236-237
Author(s):  
W. E. Lee ◽  
A. H. Heuer
Keyword(s):  
High Frequency ◽  
Glass Ceramics ◽  
Magnesium Silicate ◽  
Beam Current ◽  
Glassy Matrix ◽  
Angle Of Incidence ◽  
X Ray ◽  
Mechanical And Electrical Properties ◽  
Argon Ions ◽  
High Temperature Polymorph

IntroductionTraditional steatite ceramics, made by firing (vitrifying) hydrous magnesium silicate, have long been used as insulators for high frequency applications due to their excellent mechanical and electrical properties. Early x-ray and optical analysis of steatites showed that they were composed largely of protoenstatite (MgSiO3) in a glassy matrix. Recent studies of enstatite-containing glass ceramics have revived interest in the polymorphism of enstatite. Three polymorphs exist, two with orthorhombic and one with monoclinic symmetry (ortho, proto and clino enstatite, respectively). Steatite ceramics are of particular interest a they contain the normally unstable high-temperature polymorph, protoenstatite.Experimental3mm diameter discs cut from steatite rods (∼10” long and 0.5” dia.) were ground, polished, dimpled, and ion-thinned to electron transparency using 6KV Argon ions at a beam current of 1 x 10-3 A and a 12° angle of incidence. The discs were coated with carbon prior to TEM examination to minimize charging effects.

Simulations of high-resolution images of amorphous silicon films

1986 ◽  
Vol 44 ◽  
pp. 544-545
Author(s):  
G. Y. Fan ◽  
J. M. Cowley
Keyword(s):  
Electron Microscope ◽  
High Frequency ◽  
Fourier Transformation ◽  
Amorphous Materials ◽  
Low Frequency ◽  
Chromatic Aberration ◽  
Structure Information ◽  
Frequency Components ◽  
High Resolution Images ◽  
Spatial Frequency Spectrum

It is well known that the structure information on the specimen is not always faithfully transferred through the electron microscope. Firstly, the spatial frequency spectrum is modulated by the transfer function (TF) at the focal plane. Secondly, the spectrum suffers high frequency cut-off by the aperture (or effectively damping terms such as chromatic aberration). While these do not have essential effect on imaging crystal periodicity as long as the low order Bragg spots are inside the aperture, although the contrast may be reversed, they may change the appearance of images of amorphous materials completely. Because the spectrum of amorphous materials is continuous, modulation of it emphasizes some components while weakening others. Especially the cut-off of high frequency components, which contribute to amorphous image just as strongly as low frequency components can have a fundamental effect. This can be illustrated through computer simulation. Imaging of a whitenoise object with an electron microscope without TF limitation gives Fig. 1a, which is obtained by Fourier transformation of a constant amplitude combined with random phases generated by computer.

SEM image sharpness analysis

1996 ◽  
Vol 54 ◽  
pp. 142-143
Author(s):  
M. T. Postek ◽  
A. E. Vladar
Keyword(s):  
High Frequency ◽  
Low Frequency ◽  
Quantitative Information ◽  
Primary Electron ◽  
Image Sharpness ◽  
Critical Dimensions ◽  
Sem Image ◽  
Frequency Changes ◽  
Electron Microscopes ◽  
Scanning Electron

Fully automated or semi-automated scanning electron microscopes (SEM) are now commonly used in semiconductor production and other forms of manufacturing. The industry requires that an automated instrument must be routinely capable of 5 nm resolution (or better) at 1.0 kV accelerating voltage for the measurement of nominal 0.25-0.35 micrometer semiconductor critical dimensions. Testing and proving that the instrument is performing at this level on a day-by-day basis is an industry need and concern which has been the object of a study at NIST and the fundamentals and results are discussed in this paper.In scanning electron microscopy, two of the most important instrument parameters are the size and shape of the primary electron beam and any image taken in a scanning electron microscope is the result of the sample and electron probe interaction. The low frequency changes in the video signal, collected from the sample, contains information about the larger features and the high frequency changes carry information of finer details. The sharper the image, the larger the number of high frequency components making up that image. Fast Fourier Transform (FFT) analysis of an SEM image can be employed to provide qualitiative and ultimately quantitative information regarding the SEM image quality.

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