Shear Layer Behavior Research Articles

Noise reduction of an airfoil model covered by bio-inspired herringbone riblets

It is curious that whether the typical herringbone pattern of bird flight feathers plays a role in attenuating aeroacoustic noise. Being motivated by this, experiments are conducted to investigate noise reduction of the NACA0012 (National Advisory Committee for Aeronautics) airfoil-based model with one side surface covered by bio-inspired herringbone patterns of riblets. The herringbone-ribbed surface is defined by the divergent angle β (= 60°) of the riblets pattern, the spanwise wavelength λ (= 0.2c and 0.4c) of the pattern, and the riblet height h (= 0.6%c and 1.2%c), where c is the chord length of the airfoil. While far-field sound pressure fluctuations are measured via microphones in an anechoic wind tunnel, flow fields around the model are captured using particle image velocimetry (PIV) in a water tunnel. The effective angle of attack of the test models ranges from αeff = −11.1° to 11.1° and Reynolds number considered is from Re = 2.3 × 105 to 7.8 × 105. Compared with the baseline smooth models, the models with the riblet pattern on the pressure side are able to substantially suppress the tonal noise, associated with considerable reduction in the overall noise. The reduction in the overall sound pressure levels of the tonal noise and the overall noise are up to 21.3 dB and 20.5 dB, respectively, at αeff = −2.2° and Re = 3.6 × 105. The noise reduction is attributed to the transition of laminar to turbulent boundary layers over the herringbone-ribbed surface, particularly in the saddle location where the spanwise-repeated herringbone pattern converges. Behavior of shear layers separating from the trailing edge of the model is examined, corroborating the proposition that the acoustic feedback loop is impaired by the herringbone-ribbed surface.

Quasi-steady aerodynamic characteristics of Terminal Sensitive Bullets with short cylindrical portion

The aerodynamic characteristics are vital for short cylindrical Terminal Sensitive Bullets (TSB) with low aspect ratio, especially in terminal trajectory. Currently, there is little research in terms of the TSB and short cylinder with two free ends, and particularly in this trajectory, where the scanning angle β and roll angle α vary over a broad range between 0° and 180°. In this work, wind tunnel experiments are first conducted to learn the effects of Reynolds number and scanning angle on aerodynamic parameters for short cylinder with aspect ratio L/D=1. Similar to infinite cylinder, for the short cylinder with two free ends, the drag crisis phenomenon still exists in the critical regime 1.7×105≤Re≤6.8×105. Then 3D simulations are performed to demonstrate the aerodynamic characteristics of short cylinder and TSB over a broad range of Re,L/D, α and β. The sensitivity analysis of time step and grid are presented as well. When β=0°, for short cylinder, the drag crisis phenomenon was also observed in the simulation, but not as obvious as in the wind tunnel test. In some attitudes, there is an obvious Kármán vortex in the wake of short cylinder and TSB. The correlation between time-averaged aerodynamic coefficients and L/D, Re, α&β is discussed. The vortex shedding frequency and shear layer behavior are obtained for quasi-steady and unsteady flow. Finally, the effect of end’s shape on drag reduction and vortex shedding frequency is analyzed.

Energetic scales in a bluff body shear layer

A detailed experimental campaign into separated shear layers stemming from rectangular sections (having aspect ratios of 5 : 1, 3 : 1 and 1 : 1) was carried out at Reynolds numbers range between $1.34\times 10^{4}$ and $1.18\times 10^{5}$ based on the body thickness. Particle image velocimetry was used to locate the highest concentration of fluctuations in the velocity field and subsequent hot-wire measurements at those locations provided adequate spectral resolution to follow the evolution of various instabilities that are active within the separated shear layer. Similar to recent findings by this same group, the shear layer behaviour is observed to contain a combination of Reynolds invariant characteristics, including its time-averaged position, while other properties demonstrate clear Reynolds number dependency, including the spatial amplification of turbulent kinetic energy. Additional results here show that the ratio of side lengths of the body is a key parameter in revealing these effects. One reason for this is the level of coupling between modes of instability, which is evaluated using two-point correlation methods. These findings indicate that the separated shear layer on a bluff body is highly nonlinear. A specific set of scales responsible for these unique behaviours is identified and discussed, along with their relationship to other scales in the flow.

Statistical behavior of shear layers of reactive oxygen/kerosene spray

Reactive gaseous oxygen/kerosene spray was visualized by a shadowgraph imaging technique in a lab-scale rocket engine combustor. Dynamic behaviors of inner and outer shear layer were observed morphologically and analyzed quantitatively by image processing. Correlation methods were used to statistically analyze dynamic behaviors of each shear layer and interaction between two shear layers. Relationship between shear layer dynamics and combustion flow-field was investigated by analyzing periodicity of jet-core length, lump-detachment, and luminous flame intensity. The results of correlation and frequency analysis show that dominant frequency of behaviors and interaction of shear layers is 270 Hz and the degree of correlation becomes stronger along the flow direction. The dominant frequency of core length variation and luminous flame formation is around 270 Hz and, which is the same as that of behaviors and interaction of shear layers. This proposes that the surface behaviors and interaction of the inner and outer jets in the upstream affect the downstream combustion flow-field.

Numerical characterization of three-dimensional bluff body shear layer behaviour

Three-dimensional bluff body aerodynamics are pertinent across a broad range of engineering disciplines. In three-dimensional bluff body flows, shear layer behaviour has a primary influence on the surface pressure distributions and, therefore, the integrated forces and moments. There currently exists a significant gap in understanding of the flow around canonical three-dimensional bluff bodies such as rectangular prisms and short circular cylinders. High-fidelity numerical experiments using a hybrid turbulence closure that resolves large eddies in separated wakes close this gap and provide new insights into the unsteady behaviour of these bodies. A time-averaging technique that captures the mean shear layer behaviours in these unsteady turbulent flows is developed, and empirical characterizations are developed for important quantities, including the shear layer reattachment distance, the separation bubble pressure, the maximum reattachment pressure, and the stagnation point location. Many of these quantities are found to exhibit a universal behaviour that varies only with the incidence angle and face shape (flat or curved) when an appropriate normalization is applied.

The effect of Reynolds numbers for unequal gap spacings on flow past three square cylinders arranged in-line

Numerical investigations are carried out to study the effect of Reynolds numbers (Re) for unequal gap spacing on the flow past three square cylinders arranged in-line. Three different cases of unequal gap spacing between the cylinders are chosen in this work: the first case is related to the subcritical spacing range (g 1, g 2) = (1, 1.5) and (g 1, g 2) = (1.5, 1), the second one is related to the post-critical spacing range (g 1, g 2) = (4.5, 5) and (g 1, g 2) = (5, 4.5) and the third one is related to critical gap spacing (g 1, g 2) = (3, 4) and (g 1, g 2) = (4, 3). The Re values are varied from 75 to 175. Effect of equal gap spacing at Re = 150 is also discussed. A two-dimensional single-relaxation time lattice Boltzmann code was employed to simulate the important flow characteristics. In this study, three major cases concerning the behavior of shear layers, emerging from first cylinder, are observed: (a) without reattachment and without roll up, (b) without reattachment and with roll up and (c) with reattachment and with roll up. For equal spacing case, it is found that the critical spacing lies between 2–4. In the subcritical unequal spacing range, the single slender body flow is observed while the reattachment and binary vortex streets are observed in the post-critical unequal spacing range. The mean drag coefficient, Strouhal number, root-mean-square values of drag and lift coefficients are also calculated and compared with the experimental and numerical data in existing literature. It is found that the value of critical Re is 75 where maximum changes in the flow characteristics are observed.

Examination of thermo-acoustic instability in a low swirl burner

Low-swirl burners are of interest in industrial applications due to their low NOx emissions. In the present work, a 3.81 cm diameter low-swirl burner is acoustically forced with different fuel mixtures. The measured results are compared to 2.54 cm diameter low-swirl burner data to infer scaling properties. The experiments and analysis show that three coupling modes were present in the 3.81 cm burner: base mode coupling, shear layer generated coupling, and transitional coupling. The 3.81 cm burner was observed to have a critical acoustic driving pressure amplitude, similar to the 2.54 cm burner; however, the 3.81 cm burner had higher frequency natural acoustic modes. This counter-intuitive result is shown to arise because the modes are tied to shear layer behavior rather than burner size. It was also observed that adding hydrogen to the fuel stream resulted in less coherence. This is likely the result of increased flame speed and decreased ignition limit, allowing the flame to interact less with both large and small vortices. Small scale interaction leads to more small scale wrinkling while strong large scale vortices produce more flame role up and displacement of heat release zones. The ability of hydrogen addition to negate these effects suggests selective hydrogen addition as a possible method of inhibiting combustion instability.

A numerical study on shear layer behaviour in flow over a square unit of four cylinders at Reynolds number of 200 using the LB method

The major purpose of the present research is developing the Lattice Boltzmann (LB) method as an extremely efficient computational technique in the pragmatic field of fluid-structure interactions. Hence, the LB method is employed in simulating the incompressible flow past a square unit of four equal diameter circular cylinders. The simulation is conducted at Re of 200 and the spacing ratio (L/D) increases from 1.5 to 5 in 0.5 steps to cover all closely, moderately and widely spaced cases. Accordingly, the effects of the spacing ratio on the shear layer’s behaviour and vortex shedding pattern are studied in detail. Furthermore, its effects on the main force characteristics, i.e., lift and drag coefficients and also the Strouhal number are fully investigated. The observations and results obtained by the LB method employed in the current work are in strict accordance with experimental evidence of other researchers and other existent numerical data in the literature.

Shear layer behavior resulting from shock wave diffraction

All previous studies on shock wave diffraction in shock tubes have spatial and temporal limitations due to the size of the test sections. These limitations result from either the reflection of the expansion wave, generated at the corner, from the top wall and/or of the reflection of the incident diffracted shock from the bottom wall of the test section passing back through the region of interest. This has limited the study of the evolution of the shear layer and its associated vortex, which forms a relatively small region of the flow behind the shock with an extent of only a few centimeters, and yet is a region of significant interest. A special shock tube is used in the current tests which allow evolution of the flow to be examined at a scale about an order of magnitude larger than in previously published results, with shear layer lengths of up to 250 mm being achieved without interference from adjacent walls. Tests are presented for incident shock wave Mach numbers of nominally 1.3–1.5. Studies have been undertaken with wall angles of 10, 20, 30 and 90°. Significant changes are noted as the spatial and temporal scale of the experiment increases. For a given wall angle, the flow behind the incident shock is not self-similar as is usually assumed. Both shear layer instability and the development of turbulent patches become evident, neither of which have been noted in previous tests.

Vortex Shedding of an Airfoil at Low Reynolds Numbers

Characteristics of airfoil wake vortices in low Reynolds number flows are investigated. The focus of this investigation is the effect of Reynolds number and airfoil thickness on the shedding frequency of these coherent structures. Experiments were conducted on a NACA 0018 airfoil at an angle of attack of 10° for a range of chord Reynolds numbers from 30,000 to 200,000. The experimental conditions were selected to investigate both laminar boundary-layer separation without reattachment and separation-bubble formation as well as to enable a comparison with data from previous studies. The results show that vortex shedding occurs in the airfoil wake for both flow regimes investigated. The dimensionless wake vortex shedding frequency is shown to vary linearly with Reynolds number; however, the slope of the linear fit depends on the shear-layer behavior. When an increase in Reynolds number results in shear-layer reattachment on the upper surface of the airfoil, the wake shedding frequency increases significantly, and the vortical structures become less coherent. The present results are compared with those available for NACA 0012 and NACA 0025 profiles as well as selected bluff bodies. The comparison demonstrates a similar dependence of the dimensionless frequency on the Reynolds number. In the absence of shear-layer reattachment on the upper surface, airfoil wake vortex shedding resembles that of a bluff body; however, the results show a more pronounced effect of the Reynolds number on the Strouhal number for airfoils. Using a frequency scaling based on a characteristic wake length, the data for two different airfoil profiles are shown to collapse on to a universal Strouhal number of approximately 0.18.

Effect of blockage ratio on wake transition for flow past square cylinder

Two-dimensional numerical investigations are carried out to study the behavior of wake transition for flow past square cylinder confined between two parallel walls. It is known from past literature that nature of wall confinement affects the behavior of vortex shedding behind the obstacle. The present work reports dependence of the critical Reynolds number ( Re cr ) for onset of planar vortex shedding on blockage ratio. The effect of wall confinement on characteristic features of the wake, such as wake bubble size and shear layer behavior has been studied. The characteristic relationship between Strouhal number and flow Reynolds number has been presented for different values of blockage ratios. The temporal characteristics of the wake are presented.

障壁まわりの乱流場・拡散場のＬＥＳ解析

Large-eddy simulation(LES) is applied to the problem of atmospheric dispersion around a normal plate for oncoming turbulence, which is time-sequentially generated by unsteady numerical simulation of spatially-developing boundary layer in the driver unit as another computational domain. The present numerical model is validated in comparison with the previous wind tunnel experimental data. Also, based on the computed data, we investigate turbulence and dispersion fields strongly influenced by complex dynamic behavior of separated shear layers and large eddies in the wake, and clarify physical mechanism of plume entrainment into the wake region of a normal plate.

LES analysis on aeroelastic instability of prisms in turbulent flow

This paper discusses the aeroelastic instability of prismatic structures on the basis of the numerical results by large eddy simulation for turbulent flows around a square and a rectangular cylinders. This type of aeroelastic instability is a classical problem continuing for over 30 years , so there have been a number of studies to understand the physical meanings by theoretical or experimental approach. From the wind engineering point of view, the turbulence effect on unstable oscillations should be clarified. Turbulence of the oncoming flows changes sensitively the behavior of separated shear layers and the vortex formation around a prism, so we need to investigate details of the flow structures under unstable oscillations, as well as the aeroelastic characteristics of a prism. Over 30 years have passed since the start, current Computational Fluid Dynamics technique makes it possible to analyze the above matters and give a solution. First, we examine the inflow homogeneous turbulence generated by the statistical method, which is utilized for study of aerodynamics of prisms in turbulent flows. Second, we investigate the turbulence effects on aerodynamic characteristics of stationary prisms to validate the numerical model. Finally, we clarify the aeroelastic instability of prisms in smooth and turbulent flows and its physical mechanism is discussed by the visualized flow structure around an oscillating prism.

An experimental study of turbulent flow over a low‐angle dune

Many large, sand bed alluvial channels are dominated by dunes that possess low‐angle lee sides, often <10°, which play a critical role in the transportation of sediment and generation of significant bed form roughness. Despite the fact that these low‐angle dunes are very common in such channels many current models of dune flow dynamics are based on bed forms with an angle of repose slip face that generates a zone of permanent separated flow in the dune lee. Study of flow associated with low‐angle dunes in the field is inherently difficult since it is usually both hard to measure very near the bed and hard to quantify the nature of turbulence over these bed forms. Results from a detailed scale model experimental study of flow over a low‐angle dune, which is based on a prototype dune from the Fraser River, Canada, present a necessary link between flume and field studies and document the origins of macroturbulence associated with these bed forms. Two‐dimensional laser Doppler anemometer measurements over a low‐angle dune (maximum lower lee side slope = 14°) show that dune morphology exerts a dominant control on the turbulent flow, causing flow deceleration in the lower lee and development of an intermittent layer of shear at the interface with the higher velocity flow above. The scale model confirms that permanent flow separation does not occur over low‐angle dunes but, instead, is replaced by a small region (here ∼7% of the dune wavelength in length) of intermittent flow reversal, which may be present for up to 4% of the time. Shear layers generated along this small zone of decelerated and/or separated flow in the lower lee have a much smaller velocity differential than is characteristic of shear layers generated by flow separation in the lee of angle of repose dunes. Turbulence production associated with low‐angle dunes is dominated by eddies generated along this shear layer, which produce highly variable horizontal and vertical velocities and large Reynolds stresses in this region. These results show that macroturbulence associated with low‐angle dunes is generated by intermittent separation or shear layer generation due to velocity gradients established in the zone of lee side flow expansion. Velocity profiles and maps of turbulence structure from the scale model are in reasonable agreement with field measurements from low‐angle dunes in natural sand bed rivers. These results highlight the need to consider the temporal evolution and intermittency of shear layer behavior, often very near the bed, when interpreting the generation of macroturbulence and dispersal of sediment associated with low‐angle dunes.

PREDICTION OF LOW-FREQUENCY TONES PRODUCED BY FLOW THROUGH A DUCT WITH A GAP

A method is presented for predicting the frequencies of tones produced by high Reynolds number, subsonic flow through a duct with a gap. An inviscid, linear model of the fluid motion is developed in which the shear layer delimiting the jet formed between the inlet and exit of the duct gap is modelled as a vortex sheet. The linear analysis is used to calculate the Rayleigh conductivity parameter which is then analyzed to find the frequency dependence of the shear layer behavior. The calculations for the case when an external load is applied across the shear layer have been carried out for Strouhal numbers from 0 to 10, Mach numbers from 0·001 to 0·5, and a gap length-to-diameter ratio of 0·5 to 2·0. The resonant frequencies have been calculated for Mach numbers ranging from 0·001 to 0·8 at a length-to-diameter ratio of 1·0. The results for the forced motion in the low Mach number limit compare well with previous incompressible predictions for flow past wall apertures. The resonant frequency predictions have been validated at the Mach number of 0·52 through comparison with experimental data.

Velocity measurements and turbulence statistics of a confined isothermal swirling flow

A two-component laser Doppler velocimeter was employed to measure the flow properties of a confined, isothermal, swirling flowfield in an axisymmetric sudden expansion research combustor. A free vortex swirler was used to stir the flow at the inlet of the combustor. Measurements of mean velocities, Reynolds normal and shear stresses, and triple products were carried out at axial distances ranging from 0.38 H (step height) to 18 H downstream of the swirler. Detailed experimental data are provided to help in the understanding of the behavior of swirling, recirculating, axisymmetric, and turbulent flows inside dump combustors. A balance of the turbulence energy equation has been performed in order to get a detailed insight into the turbulent shear layer behavior. The turbulent kinetic energy terms: production, diffusion, and convection terms were computed directly from the experimental data using central differences. The viscous dissipation term was obtained from the balance of the kinetic energy equation. The analysis of the data was successful in identifying the various areas of interest in the flowfield where different turbulent transport mechanisms dominate. In addition, the data from this study will be available for upgrading advanced numerical codes. The swirling flow streamlines data are compared with the simple dump flow and it is shown that swirl reduces the size of the corner recirculation region in addition to creating a central toroidial recirculation region. In summary, swirling enhances the production and distribution of turbulence energy in the combustor which, in turn, indicates thorough flow mixing and earlier flow recovery.

Subharmonic transition to turbulence in a flat-plate boundary layer at Mach number 4.5

The subharmonic transition process of a flat-plate boundary layer at a free-stream Mach number of M∞ = 4.5 and a Reynolds number of 10000 based on free-stream velocity and initial displacement thickness is investigated by direct numerical simulation up to the beginning of turbulence. A second-mode instability superimposed with random noise of low amplitude is forced initially. The secondary subharmonic instability evolves from the noise in accordance with theory and leads to a staggered Λ-vortex pattern. Finite-amplitude Λ-vortices initiate the build-up of detached high-shear layers below and above the critical layer. The detached shear-layer generation and break-up are confined to the relative-subsonic part of the boundary layer. The breakdown to turbulence can be separated into two phases, the first being the break-up of the lower shear layer and the second being the break-up of the upper shear layer. Four levels of subsequent roll-up of the lower, Y-shaped shear layer have been observed, leading to new vortical structures which are unknown from transition at low Mach numbers. The upper shear layer behaviour is similar to that of the well-known high-shear layer in incompressible boundary-layer transition. It is concluded that, as in incompressible flow, turbulence is generated via a cascade of vortices and detached shear layers with successively smaller scales. The different phases of shear-layer break-up are also reflected in the evolution of averaged quantities. A strong decrease of the shape factor, as well as an increase of the skin friction coefficient, and a gradual loss of spanwise symmetry indicate the final breakdown to turbulence, where the mean velocity and temperature profiles approach those measured in fully turbulent flow.

Shear layer behavior under a streamwise vibrating gate and its significance to gate vibration.

A flow visualization study of the shear layer movement beneath a submerged, vertical, flat-bottomed gate model with underflow and which undergoes forced streamwise vibration of fixed amplitude and frequency is presented. Computer simulation based on the flow visualization photos, coupled with measured discharge flow velocities and hydrodynamic pressure distributions on the gate, permits the estimation of the fluctuating discharge coefficient and the phase lag between the fluctuating discharge and the gate motion. The motion of the downstream portion of the shear layer is shown to correlate Well with the observed discharge fluctuations. A mechanism of energy transfer for self-excited vibrations of vertical gates positioned above a horizontal bed is identified. The excitations due to the pressure distributions along the upstream and downstream faces of the gate are shown to be approximately equal.

Closure to “Discussion of ‘Free Shear Layer Behavior in Rotating Systems’” (1979, ASME J. Fluids Eng., 101, p. 120)

Closure to “Discussion of ‘Free Shear Layer Behavior in Rotating Systems’” (1979, ASME J. Fluids Eng., 101, p. 120)

Free Shear Layer Behavior in Rotating Systems

Experiments are reported concerning turbulent separated flow downstream of a backward-facing step in a two-dimensional channel that was rotated at a steady rate about a spanwise axis. Reattachment distance is reported as a function of Reynolds number, rotation direction and number and passage aspect ratio. Extensive flow visualization films have been produced. It is demonstrated that turbulent motions in a free shear layer may be suppressed or enhanced by system rotation according to the sense of the rotation. Two-dimensional, spanwise vortices which have been observed in the free shear layer are found to be relatively insensitive to system rotation in the stabilizing direction. These vortices are believed to be important contributors to the high rates of free shear layer entrainment, even in stationary systems at moderate Reynolds numbers.