Development Of Shear Layer Research Articles

Cold Flow Measurements of Supersonic Low Aspect Ratio Jet-Surface Interactions

In streamlined airplane configurations, additional noise sources can be created from interactions between the jet flow and surfaces on an aircraft’s body. During takeoff and landing procedures the ground is close enough to already cause jet-surface interference. To assess this interaction, the presence of a flat plate impinging on a supersonic jet of a low aspect ratio (2:1) rectangular nozzle of equivalent exit diameter, De = 20.65 mm, is studied from the minor and major axis orientation. The impact of the plate, 30De in length to resemble an aircraft carrier deck, is studied at supersonic nozzle pressure ratios (NPRs) of 2.5–4.5 for a low jet temperature ratio of TR = 1.1. Streamwise particle image velocimetry (PIV) data was taken to extract average velocity and turbulence kinetic energy (TKE) of the flow. Plate offset (h) distances of h/De = 0, 1, 2, and 3 from the nozzle lip are studied to assess trends related to shock cell spacing, potential core length, and shear layer development relative to a free jet configuration. The surface offset from the nozzle is shown to vary flow properties and even increase screech tones particularly at h/De = 1 and 3, while the in between distance of h/De = 2 is shown to reduce them. These off-design conditions causing screech are of particular interest in this study.

Effects of eddy shocklets on the segregation and evaporation of droplets in highly compressible shear layers

Numerical studies of droplet-laden spatially developing shear layers are conducted with a high convective Mach number (Mc = 1.0), in which a high-order hybrid weighted essentially nonoscillatory scheme is used for the turbulence as well as shock capturing. The evaporating droplets are tracked as point mass in the Lagrangian manner, and the two-way coupling between the flow and droplets is considered by adding the source terms to the governing equations of the gas-phase. This research focuses on the preferential concentration of droplets and the interactions between droplets and eddy shocklets in the shear layers with high flow compressibility and analyzes the underlying mechanisms of momentum and thermal response behaviors of droplets to eddy shocklets. The segregation of droplets exhibits the strongest spatial preference in the highly compressible shearing vortices, and droplets tend to accumulate as stripes behind the shocklets, associated with the coherent structures. The high flow compressibility results in the strong spatiotemporal variations of pressure and temperature, and the distributions of the expansion zones with low temperature and the compression zones with high temperature occur alternately in the shear layer. The relaxation response behaviors of the droplets to the change of momentum and thermal features of the surrounding carrier gas result in the delay of droplet evaporation in the high temperature region and then the enhancement of droplet-vapors in the low temperature region. The present observations can be ascribed to the interactions between the dispersed droplets and the distributed eddy shocklets in the shear flows with high compressibility.

Control of Upswept Afterbody Vortices Using Continuous and Pulsed Blowing

An experimental study was performed in a water tunnel to evaluate the effects of continuous and pulsed blowing jets on the counter-rotating vortices generated by the afterbody of a slanted base cylinder. Drag reductions from continuous blowing through circular jets were found to vary significantly with direction and location, and approached 7% when blowing outboard from upstream locations on the upswept face. However, for all circular jets tested, the external power required was larger than the power saved due to the drag reduction. Jet vortices restricted shear layer development, leading to smaller afterbody vortex cores further from the surface. A high-aspect-ratio jet flap, ejecting nearly parallel to the freestream, achieved drag reductions close to 9%, equating to the net energy savings of almost 3% for the best case. Jet vortices shortened the shear layer, resulting in vortices with reduced circulation, which were displaced away from the upswept face. Pulsing the jet flap resulted in improved drag reductions and energy savings (up to around 6%) compared with the equivalent continuous blowing case at the same time-averaged jet momentum coefficient. Pulsed blowing caused an increase in vortex separation and meandering, whereas the circulation was reduced by up to 10% of that for continuous blowing.

Modeling the interplay between the shear layer and leading edge suction during dynamic stall

The dynamic stall development on a pitching airfoil at Re = 106 was investigated by time-resolved surface pressure and velocity field measurements. Two stages were identified in the dynamic stall development based on the shear layer evolution. In the first stage, the flow detaches from the trailing edge and the separation point moves gradually upstream. The second stage is characterized by the roll up of the shear layer into a large scale dynamic stall vortex. The two-stage dynamic stall development was independently confirmed by global velocity field and local surface pressure measurements around the leading edge. The leading edge pressure signals were combined into a single leading edge suction parameter. We developed an improved model of the leading edge suction parameter based on thin airfoil theory that links the evolution of the leading edge suction and the shear layer growth during stall development. The shear layer development leads to a change in the effective camber and the effective angle of attack. By taking into account this twofold influence, the model accurately predicts the value and timing of the maximum leading edge suction on a pitching airfoil. The evolution of the experimentally obtained leading edge suction was further analyzed for various sinusoidal motions revealing an increase in the critical value of the leading edge suction parameter with increasing pitch unsteadiness. The characteristic dynamic stall delay decreases with increasing unsteadiness, and the dynamic stall onset is best assessed by critical values of the circulation and the shear layer height which are motion independent.

Numerical simulation of the flow past six-circular cylinders in rectangular configurations

Two-dimensional numerical simulations of the flow past six cylinders in the 2 × 3 matrix configuration and 3 × 2 matrix configuration are performed at a low Reynolds number (Re = 100) to provide a comprehensive understanding of the complicated physics for flow past multiple cylinder arrays. The spacing ratio L/D is varied from 1.25 to 7.0 (where L is the distance between the adjacent cylinder centers and D is the diameter of the cylinder). The effects of spacing ratio on the flow pattern, force coefficient, pressure distribution, and Strouhal number are investigated. The underlying flow physics for flow past six-cylinder arrays are interpreted from the view of shear layer development and force characteristics. Depending on the spacing ratio, four wake flow regimes are identified: a single bluff body flow regime (L/D = 1.25), shielding flow regime (1.5 ≤ L/D ≤ 2.5), reattachment flow regime (3.0 ≤ L/D ≤ 4.0), and co-shedding flow regime (L/D ≥ 4.5). For both configurations, regardless of L/D, the mean force coefficients on the downstream cylinders are always smaller than those of upstream cylinders, and an abrupt change of the standard deviation of the force coefficients is observed during the transition of different flow regimes. The Strouhal numbers of the six cylinders generally increase with L/D, and approach to that of a single cylinder in the co-shedding flow regime, a broadband FFT spectrum with multiple peaks is observed in the shielding and reattachment flow regimes.

Turbulent flow and noise sources on a circular cylinder in the critical regime

The flow around a circular cylinder with long spanwise length has been investigated in the critical regime using a compressible wall-resolved Large Eddy Simulation (LES) for the first time. The flow at such a critical Reynolds number combines complex features: large favorable and adverse pressure gradient, separation and turbulence transition and flow reattachment. The results of the present simulation agree well with previous experimental and incompressible LES data, for the distribution of the mean wall-pressure coefficient that dominates the drag coefficient, and the skin-friction coefficient that illustrates the flow separation and reattachment behaviors. A weak reattachment is observed from the quasi-zero skin-friction in the reattachment region. A detailed study of the boundary-layer and shear-layer development around the cylinder with profiles of mean velocity and turbulence intensities confirms the transition, separation and reattachment behaviors shown by the skin-friction coefficient. The maximum tangential velocity and its location above the wall have also proven to be adequate measures of the edge velocity and associated boundary layer thickness. The Kelvin-Helmholtz instabilities have been observed in the shear layer and the ratio of the frequency of these instabilities and the fundamental vortex shedding frequency matches well with the existing scaling based on experimental data. The far-field noise obtained by both direct computation and acoustic analogy shows a dominant vortex shedding tone, but with additional broadband sources in the cylinder wake. These sound sources are evaluated from maps of filtered pressure signals and cross-correlation analysis of the pressure fluctuations around the cylinder and in the far-field.

Large Eddy Simulation Requirements for the Flow over Periodic Hills

Large eddy simulations are carried out for flows in a channel with streamwise-periodic constrictions, a well-documented benchmark case to study turbulent flow separation from a curved surface. Resolution criteria such as wall units are restricted to attached flows and enhanced criteria, such as energy spectra or two-point correlations, are used to evaluate the effective scale separation in the present large eddy simulations. A detailed analysis of the separation above the hill crest and of the early shear layer development shows that the delicate flow details in this region may be hardly resolved on coarse grids already at Re = 10595, possibly leading to a non monotonic convergence with mesh refinement. The intricate coupling between numerical and modeling errors is studied by means of various discretization schemes and subgrid models. It is shown that numerical schemes maximizing the resolution capabilities are a key ingredient for obtaining high-quality solutions while using a reduced number of grid points. On this respect, the introduction of a sharp enough filter is an essential condition for separating accurately the resolved scales from the subfilter scales and for removing ill-resolved structures. The high-resolution approach is seen to provide solutions in very good overall agreement with the available experimental data for a range of Reynolds numbers (up to 37000) without need for significant grid refinement.

Wake structures and acoustic resonance excitation of a single finned cylinder in cross-flow

This paper presents an experimental and numerical investigation to clarify the effect of circular fins on the wake structures, the dynamic loading, as well as the flow–sound interaction mechanism downstream of a single finned cylinder in cross-flow. It is revealed that adding circular fins to the cylinder enhances the flow coherence along the cylinder’s span, strengthens the intensity of the vortex shedding process downstream of the cylinder, reduces the vortex formation length, and increases the dynamic lift force acting on the cylinder. Thus, adding circular fins to the cylinder makes the flow more susceptible to acoustic resonance excitation, compared to an equivalent bare cylinder. Three-dimensional numerical simulations of the flow field around the cylinder show that the flow is entrained between the fins which results in the generation of an accelerated jet-like flow in the developing shear layer region. The jet-like flow emanating from the fins leads to a closer and stronger shear layer roll-up, and hence a shorter vortex formation region. This, in turn, increases the fluctuating lift force on the cylinder, as the strong oscillating shear layers occur closer to base of the cylinder. Additionally, with the reduction of the vortex formation length, the base pressure coefficient decreases and therefore the drag force on the cylinder increases. These intrinsic flow features explain the global effect of circular fins on the vortex shedding process and there by on the excitation mechanism of acoustic resonance.

A hybrid RANS/LES approach with scale-adaptive capabilities for highly separated flows

ABSTRACTA new hybrid RANS/LES approach with scale-adaptive capabilities is developed. The blending function in the SST model is adopted to prevent the invasion of the von Karman length scale to the RANS region, and the compressibility correction proposed by Wilcox is incorporated to produce a realistic shear layer development in compressible flows. The new model is validated for a subcritical flow past a circular cylinder and a supersonic base flow. Time-averaged turbulent statistics predicted by the new model show fairly good agreement with the experimental data, slight improvements over DES simulations, and are much better than SAS results. The main advantage of the new model over the DES method is that the distribution of the blending function reflects local vortex structures instead of grid spacing in the turbulent wake. The sequence of the effect intensity of the compressibility correction from strong to weak is SAS, the new model and DES.

Stereoscopic TR-PIV measurements of mixed convection flow in a vertical channel with an open cavity with discrete heating

An experimental investigation of opposing mixed convection is carried out using stereoscopic time-resolved particle image velocimetry (TR-PIV) to assess the thermal effects on the flow structure over an open cubical cavity that lies at the midsection of a vertical water channel. The ratio between the channel and cavity depth is of 1, the wall facing the opening is isothermal and all other walls are adiabatic. The complex interaction between the induced incompressible forced flow at ambient temperature and buoyancy-induced flow from the heated wall of the cavity are studied experimentally for fixed Prandtl number of Pr=7, buoyancy strength or Richardson number in the range 0 ≤ Ri ≤ 20, and two Reynolds numbers defined with the channel hydraulic diameter and the mean velocity in the upstream channel of Re=1500 and 4500 that correspond to laminar and turbulent upstream boundary layers, respectively. The 3D flow features are presented in the form of mean statistics and instantaneous flow properties in planes normal to the cavity floor. It is found that the shear layer development is highly 3D due to the confinement effect induced by the lateral walls. We also show the modulation effect of opposing buoyancy on the flow distributions and the time-evolution of the shear layer. Also, the complex dynamics of the coherent structures in the region surrounding the cavity show that the entrainment rate is modified with increasing buoyancy. Root-mean-square contours of the fluctuating laminar velocity fields have been obtained, and Reynolds stress distribution and proper orthogonal decomposition analysis for the turbulent case show that an increase in the fractional energy of the characteristic POD modes takes place for increasing Ri numbers.

Validation of steady RANS modelling of isothermal plane turbulent impinging jets at moderate Reynolds numbers

Abstract The numerical modelling of impinging jet flows is not straightforward as it should not only solve the shear layer development in the free jet region, but also the near-wall behaviour (streamline curvature) and the resulting wall jets after impingement. This study presents a validation study of steady Reynolds-averaged Navier–Stokes turbulence models for predicting isothermal plane turbulent impinging jets at two different slot Reynolds numbers, i.e. Re = 8,000 (case I) and Re = 13,000 (case II), based on 2D particle image velocimetry measurements. In addition, an in-depth analysis of the results provided by the five different turbulence models: standard k − e (SKE), realisable k − e (RKE), RNG k − e , SST k − ω , and a Reynolds stress model (RSM), is performed. The results show that: (1) for both Reynolds numbers the best agreement with measured velocities and turbulent kinetic energy in the region near the jet nozzle is achieved with SST; (2) the best predictions of potential core length are provided by RNG (case I) and RKE (case II); (3) centreline distributions of velocities and turbulent kinetic energy are most accurately predicted by RNG and RKE for case I, while for case II the best agreement with experimental data is obtained by SKE and RNG; (4) the best overall performance for both cases in predicting velocities is provided by RKE, and by RKE and RNG when considering turbulent kinetic energy; (5) all models more accurately predict the jet spreading rate in the intermediate region than in the potential core region; (6) for both Reynolds numbers SKE provides the most accurate estimation of jet decay rate.

Flow and acoustics of an isothermal supersonic 2:1 rectangular jet with an adjacent surface

In integrated airframe/propulsion system configurations, additional noise sources can be created from the interactions between the jet flow and the adjacent airframe surfaces. Another situation of surfaces of close proximity occur during takeoff and landing when the ground is close enough to cause the jet-surface interference. The impact of a plate oriented parallel to the axis of a 2:1 rectangular supersonic jet (De = 20.65mm) at the minor axis side is studied. Plate offset (h) distances of h = 0, 1, 2, and 3 equivalent diameters from the nozzle lip to the surface are included. The impact of the plate is studied at nozzle pressure ratios (NPRs) of 2.5–4.5 when the design Mach number is 3.67, at a temperature ratio of TR = 1.1. Mean and turbulent flow field data are acquired using streamwise particle image velocimetry (PIV). Trends of shock cell spacing, potential core length, and shear layer development are analyzed. Near and far field data are taken and correlated with the flow field details. The offset from the nozzle is shown to vary flow and acoustic properties and impact screech tones.

Large-eddy simulation of sonic coaxial jets with different total pressure ratios of the inner to outer nozzle

Large-eddy simulation of sonic coaxial jets issuing into a quiescent environment was carried for two total pressure ratios of the inner to outer nozzle, i.e. 5/3 and 3/5. The effects of the total pressure ratio on the flow characteristics were mainly investigated. Various fundamental mechanisms dictating the complex flow characteristics, including jet shear layer evolution, shock system formation, shock/shear-layer interaction, turbulence behavior, and mixing property, have been studied. It is found that the total pressure ratio has an important influence on the flow evolution of coaxial jets as well as turbulence behavior and jet mixing property. The fluid-dynamic shearing and compressing processes are analyzed based on the Lamb vector curl and divergence. The multi-layer structures of the shear layers and shock waves are reasonably captured and the relevant fluid-dynamic processes are clearly clarified. It is also identified that the turbulence behavior and mixing property of the coaxial jets are mainly associated with the shearing effect in the outer shear layer region and the shearing and compressing coupled effect in the jet core region.

Critical evolution of leading edge suction during dynamic stall

Dynamic stall dominates the aerodynamic performance, the robustness, and the wake dynamics of vertical axis wind turbines. To better assess the dynamic stall onset and its associated unsteady effects, this paper analyzes experimentally the evolution of the leading edge suction vector on a sinusoidally pitching airfoil based on time-resolved surface pressure measurements and particle image velocimetry. During the dynamic stall stage, we linked the shear layer evolution with the evolution of the leading edge suction. The dynamic stall development prior to dynamic stall onset consists of two stages: a primary instability stage and a vortex formation stage. The transition between the stages is marked by a maximum in the leading edge suction. During the primary instability stage, the leading edge suction increases linearly while the shear layer height with respect to the airfoil’s surface also increases linearly. During the vortex formations stage, the leading edge suction decreases linearly while the shear layer rolls up and creates a dynamic stall vortex. The maximum leading edge suction increases nearly linearly with the normalized effective unsteadiness. The leading edge suction at dynamic stall onset appears to be independent of unsteadiness of the pitching motion.

Stress-Blended Eddy Simulation of Coherent Unsteadiness in Pressure Side Film Cooling Applied to a First Stage Turbine Vane

Within the framework of scale resolving simulation techniques, this paper considers the application of the stress-blended eddy simulation (SBES) model to pressure side (PS) film cooling in a high-pressure turbine nozzle guide vane. The cooling geometry exhibits two rows of film cooling holes and a trailing edge cutback, fed by the same plenum chamber. The blowing conditions investigated were in the range of coolant-to-mainstream mass flow ratio (MFR) from 1% to 2%. The flow regime resembles that in a real engine (exit isentropic Mach number of Ma2is = 0.6), but also low speed conditions (Ma2is = 0.2) were considered for comparison purposes. The predicted results were validated with measurements of surface adiabatic effectiveness and instantaneous off-wall visualizations of the flow field downstream of cooling holes and cutback slot. The focus is on SBES ability of developing shear layer structures, because of their strong influence on velocity field, entrainment mechanisms and, thus, vane surface temperature. Special attention has been paid to the development and dynamics of coherent unsteadiness, since measured values of shedding frequency were also available for validation. SBES provided significant improvement in capturing the unsteady physics of cooling jet-mainstream interaction. The effects of changes in flow regime and blowing conditions on vortex structures were well predicted along the cutback surface. As regards the cooling holes, the high speed condition made it difficult to match the experimental Kelvin–Helmholtz breakdown in the shear layer, in the case of high velocity jets.

Turbulent shear layers in confining channels

ABSTRACTWe present a simple model for the development of shear layers between parallel flows in confining channels. Such flows are important across a wide range of topics from diffusers, nozzles and ducts to urban air flow and geophysical fluid dynamics. The model approximates the flow in the shear layer as a linear profile separating uniform-velocity streams. Both the channel geometry and wall drag affect the development of the flow. The model shows good agreement with both particle image velocimetry experiments and computational turbulence modelling. The simplicity and low computational cost of the model allows it to be used for benchmark predictions and design purposes, which we demonstrate by investigating optimal pressure recovery in diffusers with non-uniform inflow.

Numerical Study of Controlling Jet Flow and Noise using Pores on Nozzle Inner Wall

In this paper, the feasibility of controlling the subsonic jet flow and its noise using pores of blind holes added on the nozzle inner wall is explored numerically. These pores are intended to introduce disturbances to the shear layer so as to change the flow mixing. This passive strategy has not been attempted so far. A convergent nozzle with a cylindrical extension is selected as the baseline case. Three nozzles with pores on the inner wall are set up. Validations of the numerical settings are carried out, then the compressible turbulent jets at the exit Mach number M j = 0.6 in the four nozzles are calculated by large eddy simulations (LES), while the radiated sounds are predicted by the FW-H acoustic analogy. The results show that the blind holes have produced some effects on weakening the turbulence intensity in the shear layer. Comparison reveals that both temporal and spatial correlations of the turbulent fluctuations in the modified cases are suppressed to some extent. Meanwhile, the porous nozzles are shown to suppress the pairing of vortices and enhance the flow mixing, and therefore, the development of shear layer and the fragmentation of large scale vortices are accelerated.

Shear Layer Development, Separation, and Stability Over a Low-Reynolds Number Airfoil

The shear layer development for a NACA 0025 airfoil at a low Reynolds number was investigated experimentally and numerically using large eddy simulation (LES). Two angles of attack (AOAs) were considered: 5 deg and 12 deg. Experiments and numerics confirm that two flow regimes are present. The first regime, present for an angle-of-attack of 5 deg, exhibits boundary layer reattachment with formation of a laminar separation bubble. The second regime consists of boundary layer separation without reattachment. Linear stability analysis (LSA) of mean velocity profiles is shown to provide adequate agreement between measured and computed growth rates. The stability equations exhibit significant sensitivity to variations in the base flow. This highlights that caution must be applied when experimental or computational uncertainties are present, particularly when performing comparisons. LSA suggests that the first regime is characterized by high frequency instabilities with low spatial growth, whereas the second regime experiences low frequency instabilities with more rapid growth. Spectral analysis confirms the dominance of a central frequency in the laminar separation region of the shear layer, and the importance of nonlinear interactions with harmonics in the transition process.

A physical study of air–water flow in planar plunging water jet with large inflow distance

A plunging jet is commonly encountered in nature. It is also widely used in industry for its capacity to enhance fluid mixing and entrain gases into the liquid fluid when the impact velocity exceeds a critical value. This paper presents a physical study of vertical supported planar water plunging jets, in a relatively large-size facility. Air-water flow properties were measured in the falling jet and in the plunging pool using an intrusive phase-detection probe, with jet impact velocities between 2.5 m/s and 7.4 m/s and a fixed jet length. The falling jet was characterised by large disturbance and substantial pre-aeration. Intense air-water mixing was observed downstream of the impingement point. The development of air diffusion layer and turbulent shear layer was characterised by the streamwise evolution of void fraction, bubble count rate, bubble chord length and interfacial velocity profiles. The results compared favourably with the literature, albeit some difference was observed associated with different inflow jet turbulence levels as well as instrumentation development and signal processing refinement, including instrumental size, scanning rate and duration. The clustering properties were derived using the near-wake criterion. Results were comparable to those in horizontal hydraulic jumps. The air-entrainment rate was derived, highlighting the significant contribution of jet pre-aeration.

The Influence of Carrier Air Preheating on Autoignition of Inline-Injected Hydrogen–Nitrogen Mixtures in Vitiated Air of High Temperature

The use of highly reactive hydrogen-rich fuels in lean premixed combustion systems strongly affects the operability of stationary gas turbines (GT) resulting in higher autoignition and flashback risks. The present study investigates the autoignition behavior and ignition kernel evolution of hydrogen–nitrogen fuel mixtures in an inline co-flow injector configuration at relevant reheat combustor operating conditions. High-speed luminosity and particle image velocimetry (PIV) measurements in an optically accessible reheat combustor are employed. Autoignition and flame stabilization limits strongly depend on temperatures of vitiated air and carrier preheating. Higher hydrogen content significantly promotes the formation and development of different types of autoignition kernels: More autoignition kernels evolve with higher hydrogen content showing the promoting effect of equivalence ratio on local ignition events. Autoignition kernels develop downstream a certain distance from the injector, indicating the influence of ignition delay on kernel development. The development of autoignition kernels is linked to the shear layer development derived from global experimental conditions.