Subharmonic excitation of the shear layer between two ribs - Vortex interaction and pressure field

AIAA Journal ◽  
1991 ◽  
Vol 29 (9) ◽  
pp. 1390-1399 ◽  
Author(s):  
S. Acharya ◽  
T. A. Myrum ◽  
S. Inamdar
Keyword(s):  
Shear Layer ◽  
Pressure Field ◽  
Vortex Interaction ◽  
Subharmonic Excitation

Direct Numerical Simulations of Transitional Separation–Bubble Development in Swept–Blade Flow Conditions

2012 ◽  
Author(s):  
Joshua R. Brinkerhoff ◽  
Metin I. Yaras
Keyword(s):  
Boundary Layer ◽  
Numerical Simulations ◽  
Shear Layer ◽  
Pressure Field ◽  
Separation Bubble ◽  
Three Dimensional ◽  
Direct Numerical Simulations ◽  
Small Scale ◽  
Flow Conditions ◽  
Crossflow Instability

This paper describes numerical simulations of the instability mechanisms in a separation bubble subjected to a three-dimensional freestream pressure distribution. Two direct numerical simulations are performed of a separation bubble with laminar separation and turbulent reattachment under low freestream turbulence at flow Reynolds numbers and streamwise pressure distributions that approximate the conditions encountered on the suction side of typical low-pressure gas-turbine blades with blade sweep angles of 0° and 45°. The three-dimensional pressure field in the swept configuration produces a crossflow-velocity component in the laminar boundary layer upstream of the separation point that is unstable to a crossflow instability mode. The simulation results show that crossflow instability does not play a role in the development of the boundary layer upstream of separation. An increase in the amplification rate and most amplified disturbance frequency is observed in the separated-flow region of the swept configuration, and is attributed to boundary-layer conditions at the point of separation that are modified by the spanwise pressure gradient. This results in a slight upstream movement of the location where the shear layer breaks down to small-scale turbulence and modifies the turbulent mixing of the separated shear layer to yield a downstream shift in the time-averaged reattachment location. The results demonstrate that although crossflow instability does not appear to have a noticeable effect on the development of the transitional separation bubble, the 3D pressure field does indirectly alter the separation-bubble development by modifying the flow conditions at separation.

scholarly journals Compressible starting jet: pinch-off and vortex ring–trailing jet interaction

2017 ◽  
Vol 817 ◽  
pp. 560-589 ◽  
Author(s):  
Juan José Peña Fernández ◽  
Jörn Sesterhenn
Keyword(s):  
Reynolds Number ◽  
Shear Layer ◽  
Vortex Ring ◽  
Pressure Ratio ◽  
Reynolds Numbers ◽  
Chamber Pressure ◽  
Vortex Interaction ◽  
Dominant Feature ◽  
Chamber Temperature ◽  
Starting Jet

The dominant feature of the compressible starting jet is the interaction between the emerging vortex ring and the trailing jet. There are two types of interaction: the shock–shear layer–vortex interaction and the shear layer–vortex interaction. The former is clearly not present in the incompressible case, since there are no shocks. The shear layer–vortex interaction has been reported in the literature in the incompressible case and it was found that compressibility reduces the critical Reynolds number for the interaction. Four governing parameters describe the compressible starting jet: the non-dimensional mass supply, the Reynolds number, the reservoir to unbounded chamber temperature ratio and the reservoir to unbounded chamber pressure ratio. The latter parameter does not exist in the incompressible case. For large Reynolds numbers, the vortex pinch-off takes place in a multiple way. We studied the compressible starting jet numerically and found that the interaction strongly links the vortex ring and the trailing jet. The shear layer–vortex interaction leads to a rapid breakdown of the head vortex ring when the flow impacted by the Kelvin–Helmholtz instabilities is ingested into the head vortex ring. The shock–shear layer–vortex interaction is similar to the noise generation mechanism of broadband shock noise in a continuously blowing jet and results in similar sound pressure amplitudes in the far field.

On the Mechanisms Affecting Fluidic Vectoring Using Suction

2006 ◽  
Vol 129 (1) ◽  
pp. 91-99 ◽  
Author(s):  
R. D. Gillgrist ◽  
D. J. Forliti ◽  
P. J. Strykowski
Keyword(s):  
Shear Layer ◽  
Secondary Flow ◽  
Mass Flow ◽  
Pressure Field ◽  
Countercurrent Flow ◽  
Primary Flow ◽  
Countercurrent Shear ◽  
Low Pressures ◽  
Direction Opposite ◽  
Subsonic Jet

Suction was applied asymmetrically to the exhaust of a rectangular subsonic jet creating a pressure field capable of vectoring the primary flow at angles up to 15deg. The suction simultaneously creates low pressures near the jet exhaust and conditions capable of drawing a secondary flow along the jet shear layer in the direction opposite to the primary jet. This countercurrent shear layer is affected both by the magnitude of the suction source as well as the proximity of an adjacent surface onto which the pressure forces act to achieve vectoring. This confined countercurrent flow gives rise to elevated turbulence levels in the jet shear layer as well as considerable increases in the gradients of the turbulent stresses. The turbulent stresses are responsible for producing a pressure field conducive for vectoring the jet at considerably reduced levels of secondary mass flow than would be possible in their absence.

Shear-layer pressure fluctuations and superdirective acoustic sources

1990 ◽  
Vol 220 ◽  
pp. 355-368 ◽  
Author(s):  
D. G. Crighton ◽  
P. Huerre
Keyword(s):  
Wave Equation ◽  
Shear Layer ◽  
Large Scale ◽  
Pressure Field ◽  
Pressure Fluctuations ◽  
Turbulent Shear ◽  
Acoustic Fields ◽  
Algebraic Decay ◽  
Boundary Source ◽  
Acoustic Sources

We consider a sequence of boundary-value problems for the acoustic wave equation, with the pressure specified on the boundary as a function of space and time, and simulating features of the pressure field measured just outside a turbulent shear layer supporting large-scale coherent structures. The boundary pressure field has the form of a travelling subsonic plane wave, modulated by a large-scale envelope function. Three models for the envelope distribution are studied in detail, and the particular features which they exhibit are shown to be representative of large classes of amplitude functions.We start by looking at the hydrodynamic near field of the boundary pressure fluctuations, over spatial regions throughout which the motion can be taken as incompressible. Very close to the boundary, the pressure fluctuations decay exponentially with transverse distance, while at sufficiently large distances from the whole wave packet on the boundary, the pressure fluctuations have a dipole algebraic decay. We investigate the transition from exponential to algebraic decay, and find that it is effected through quite a complicated multilayer structure which depends crucially on the detailed form of the envelope.Acoustic fields are then determined both from exact solutions to the wave equation, and from matching arguments. In some cases, where the boundary source is compact, the distant acoustic fields have a simple compressible dipole type of behaviour. In other cases, however, when the boundary source is non-compact, the acoustic field has a superdirective character, the angular variation being described by exponentials of cosines of the angle with the streamwise direction. It is shown how the superdirective acoustic sources are completely compatible with the features of the inner incompressible field, and a criterion for the occurrence of the superdirective acoustic fields will be given. Superdirective fields of this kind have been observed in measurements by Laufer & Yen (1983) on a low-speed round jet of Mach number 0.1, and the general relation of our results to those experiments is explained.

Vortex-corner interactions in a cavity shear layer elucidated by time-resolved measurements of the pressure field

2013 ◽  
Vol 728 ◽  
pp. 417-457 ◽  
Author(s):  
Xiaofeng Liu ◽  
Joseph Katz
Keyword(s):  
Boundary Layer ◽  
Shear Layer ◽  
Pressure Field ◽  
Vertical Wall ◽  
Feedback Mechanism ◽  
Pressure Gradients ◽  
Direct Role ◽  
Time Resolved ◽  
Number Range ◽  
Cavitation Inception

AbstractThe flow structure and turbulence in an open cavity shear layer has been investigated experimentally at a Reynolds number of$4. 0\times 1{0}^{4} $, with an emphasis on interactions of the unsteady pressure field with the cavity corners. A large database of time-resolved two-dimensional PIV measurements has been used to obtain the velocity distributions and calculate the pressure by spatially integrating the material acceleration at a series of sample areas covering the entire shear layer and the flow surrounding the corners. Conditional sampling, low-pass filtering and time correlations among variables enable us to elucidate several processes, which have distinctly different frequency ranges, that dominate the shear layer interactions with the corners. Kelvin–Helmholtz shear layer eddies have the expected Strouhal number range of 0.5–3.2. Their interactions with the trailing corner introduce two sources of vorticity fluctuations above the corner. The first is caused by the expected advection of remnants of the shear layer eddies. The second source involves fluctuations in local viscous vorticity flux away from the wall caused by periodic variations in the streamwise pressure gradients. This local production peaks when the shear layer vortices are located away from the corner, creating a lingering region with peak vorticity just above the corner. The associated periodic pressure minima there are lower than any other point in the entire flow field, making the region above the corner most prone to cavitation inception. Flapping of the shear layer and boundary layer upstream of the leading corner occurs at very low Strouhal numbers of ∼0.05, affecting all the flow and turbulence quantities around both corners. Time-dependent correlations of the shear layer elevation show that the flapping starts in the boundary layer upstream of the leading corner and propagates downstream at the free stream speed. Near the trailing corner, when the shear layer elevation is low, the stagnation pressure in front of the wall, the downward jetting flow along this wall, the fraction of shear layer vorticity entrained back into the cavity, and the magnitude of the pressure minimum above the corner are higher than those measured when the shear layer is high. However, the variations in downward jetting decay rapidly with increasing distance from the trailing corner, indicating that it does not play a direct role in a feedback mechanism that sustains the flapping. There is also low correlation between the boundary/shear layer elevation and the returning flow along the upstream vertical wall, providing little evidence that this returning flow affects the flapping directly. However, the characteristic period of flapping, ∼0.6 s, is consistent with recirculation time of the fluid within the cavity away from boundaries. The high negative correlations of shear/boundary layer elevation with the streamwise pressure gradient above the leading corner introduce a plausible mechanism that sustains the flapping: when the shear layer is low, the boundary layer is subjected to high adverse streamwise pressure gradients that force it to widen, and when the shear layer is high, the pressure gradients decrease, allowing the boundary layer to thin down. Flow mechanisms that would cause the flapping-induced pressure changes, and their relations to the flow within the cavity are discussed.

On the hydrodynamics of three in-line square cylinders in a uniform flow

2019 ◽  
Vol 33 (06) ◽  
pp. 1950066 ◽  
Author(s):  
Qiao-Gao Huang ◽  
Guang Pan
Keyword(s):  
Fluid Dynamics ◽  
Reynolds Number ◽  
Shear Layer ◽  
Uniform Flow ◽  
Bluff Bodies ◽  
Vortex Interaction ◽  
Square Cylinder ◽  
Hydrodynamic Problem ◽  
Square Cylinders ◽  
Number Formula

In this paper, is the hydrodynamics of three in-line square cylinders in a uniform flow, where the gap between two neighboring square cylinders is equivalent is under investigation. The fluid dynamics around those multiple bluff bodies, including time-mean drag coefficients, time-mean lift coefficients and Strouhal numbers, are considered at the Reynolds number [Formula: see text]. Through numerically solving the nonlinear hydrodynamic problem, we show that the drag force acting on the first square cylinder is always larger than that acting on the remaining two square cylinders. From the perspective of wake structures, with keeping the increase of the gap between two neighboring square cylinders, the wake structures become much more complicated, including attachment of shear layer, interaction between shear layer and vortex, interaction between vortex and vortex, etc. Moreover, the Strouhal number of three square cylinders are approximately equaled when the dimensionless gap between two neighboring square cylinders is less than 2.

Turbulent structure of high-amplitude pressure peaks within the turbulent boundary layer

2013 ◽  
Vol 735 ◽  
pp. 381-426 ◽  
Author(s):  
S. Ghaemi ◽  
F. Scarano
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Shear Layer ◽  
Pressure Field ◽  
Three Dimensional ◽  
High Amplitude ◽  
Shear Layers ◽  
Turbulent Structure ◽  
Streamwise Vortices ◽  
Ejection Event

AbstractThe positive and negative high-amplitude pressure peaks (HAPP) are investigated in a turbulent boundary layer at $R{e}_{\theta } = $ 1900 in order to identify their turbulent structure. The three-dimensional velocity field is measured within the inner layer of the turbulent boundary layer using tomographic particle image velocimetry (tomo-PIV). The measurements are performed at an acquisition frequency of 10 000 Hz and over a volume of $418\times 149\times 621$ wall units in the streamwise, wall-normal and spanwise directions, respectively. The time-resolved velocity fields are applied to obtain the material derivative using the Lagrangian method followed by integration of the Poisson pressure equation to obtain the three-dimensional unsteady pressure field. The simultaneous volumetric velocity, acceleration, and pressure data are conditionally sampled based on local maxima and minima of wall pressure to analyse the three-dimensional turbulent structure of the HAPPs. Analysis has associated the positive HAPPs to the shear layer structures formed by an upstream sweep of high-speed flow opposing a downstream ejection event. The sweep event is initiated in the outer layer while the ejection of near-wall fluid is formed by the hairpin category of vortices. The shear layers were observed to be asymmetric in the instantaneous visualizations of the velocity and acceleration fields. The asymmetric pattern originates from the spanwise component of temporal acceleration of the ejection event downstream of the shear layer. The analysis also demonstrated a significant contribution of the pressure transport term to the budget of the turbulent kinetic energy in the shear layers. Investigation of the conditional averages and the orientation of the vortices showed that the negative HAPPs are linked to both the spanwise and quasi-streamwise vortices of the turbulent boundary layer. The quasi-streamwise vortices can be associated with the hairpin category of vortices or the isolated quasi-streamwise vortices of the inner layer. A bi-directional analysis of the link between the HAPPs and the hairpin paradigm is also conducted by conditionally averaging the pressure field based on the detection of hairpin vortices using strong ejection events. The results demonstrated positive pressure in the shear layer region of the hairpin model and negative pressure overlapping with the vortex core.

Boundary Layer Transition on the High Lift T106A Low-Pressure Turbine Blade With an Oscillating Downstream Pressure Field

2008 ◽  
Vol 130 (2) ◽  
Author(s):  
Maciej M. Opoka ◽  
Richard L. Thomas ◽  
Howard P. Hodson
Keyword(s):  
Boundary Layer ◽  
Shear Layer ◽  
Surface Pressure ◽  
Pressure Field ◽  
Turbine Blades ◽  
Periodic Variation ◽  
Surface Boundary ◽  
Surface Shear Stress ◽  
Suction Surface ◽  
Surface Shear

This paper presents the results of an experimental study of the interaction between the suction surface boundary layer of a cascade of low-pressure (LP) turbine blades and a fluctuating downstream potential field. A linear cascade equipped with a set of T106 LP turbine blades was subjected to a periodic variation of the downstream pressure field by means of a moving bar system at low-speed conditions. Measurements were taken in the suction surface boundary layer using 2D laser Doppler anemometry, flush-mounted unsteady pressure transducers and surface shear stress sensors. The Reynolds number, based on the chord and exit conditions, was 1.6×105. The measurements revealed that the magnitudes of the suction surface pressure variations induced by the oscillating downstream pressure field, just downstream of the suction peak, were approximately equal to those measured in earlier studies involving upstream wakes. These pressure field oscillations induced a periodic variation of the transition onset location in the boundary layer. Two turbulence levels were investigated. At a low level of inlet freestream turbulence of 0.5%, a separation bubble formed on the rear part of the suction surface. Unsteady measurements of the surface pressure revealed the presence of high-frequency oscillations occurring near the start of the pressure recovery region. The amplitude of these fluctuations was of the order of 7–8% of exit dynamic pressure, and inspection of the velocity field revealed the presence of Kelvin-Helmholtz-type shear layer vortices in the separated free shear layer. The frequency of these shear layer vortices was approximately one order-of-magnitude greater than the frequency of the downstream passing bars. At a higher inlet freestream turbulence level of 4.0%, which is more representative of real engine environments, separation was prevented by an earlier onset of transition. Oscillations were still observed in suction surface shear stress measurements at a frequency matching the period of the downstream bar, indicating a continued influence on the boundary layer from the oscillating pressure field. However, the shear layer vortices seen in the lower turbulence intensity case were not so clearly observed, and the maximum amplitude of suction surface pressure fluctuations was reduced.

Boundary Layer Transition on the High Lift T106A LP Turbine Blade With an Oscillating Downstream Pressure Field

2006 ◽  
Author(s):  
Maciej M. Opoka ◽  
Richard L. Thomas ◽  
Howard P. Hodson
Keyword(s):  
Boundary Layer ◽  
Shear Layer ◽  
Surface Pressure ◽  
Pressure Field ◽  
Turbine Blades ◽  
Surface Boundary ◽  
Surface Shear Stress ◽  
Suction Surface ◽  
Surface Shear ◽  
Lp Turbine

This paper presents the results of an experimental study of the interaction between the suction surface boundary layer of a cascade of LP turbine blades and a fluctuating downstream potential field. A linear cascade equipped with a set of T106 LP turbine blades was subjected to a periodic variation of the downstream pressure field by means of a moving bar system at low-speed conditions. Measurements were taken in the suction surface boundary layer using 2D Laser Doppler Anemometry, flush mounted unsteady pressure transducers and surface shear stress sensors. The Reynolds number, based on the chord and exit conditions, was 1.6×105. The measurements revealed that the magnitudes of the suction surface pressure variations induced by the oscillating downstream pressure field, just downstream of the suction peak, were approximately equal to those measured in earlier studies involving upstream wakes. These pressure field oscillations induced a periodic variation of the transition onset location in the boundary layer. Two turbulence levels were investigated. At a low level of inlet freestream turbulence of 0.5%, a separation bubble formed on the rear part of the suction surface. Unsteady measurements of the surface pressure revealed the presence of high frequency oscillations occurring near the start of the pressure recovery region. The amplitude of these fluctuations was of the order of 7–8% of exit dynamic pressure, and inspection of the velocity field revealed the presence of Kelvin-Helmholtz type shear layer vortices in the separated free shear layer. The frequency of these shear layer vortices was approximately one order of magnitude greater than the frequency of the downstream passing bars. At a higher inlet freestream turbulence level of 4.0%, which is more representative of real engine environments, separation was prevented by an earlier onset of transition. Oscillations were still observed in suction surface shear stress measurements at a frequency matching the period of the downstream bar, indicating a continued influence on the boundary layer from the oscillating pressure field. However, the shear layer vortices seen in the lower turbulence intensity case were not so clearly observed and the maximum amplitude of suction surface pressure fluctuations was reduced.

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