Key Flow Structures Research Articles

Numerical investigation of optical distortions by turbulent wake and shock wave in the transonic flow

Large eddy simulation with the explicit fifth-order weighted compact nonlinear schemes is performed to investigate the aero-optical distortion caused by a transonic flow over a cylinder at Reynolds numbers of Re = 1 × 105, Re = 2 × 105, and Re = 4 × 105. Proper orthogonal decomposition identifies two dominant modes: the antisymmetric “shifting” mode and the symmetric “breathing” mode, both peaking at the non-dimensionalized Strouhal number StD = 0.18. Optical distortion refers to the phenomenon where a beam passes through a non-uniform and fluctuating flow field, resulting in defocus, jitter, and significant energy reduction. Optical calculations for 12 cases demonstrate that local shock waves and turbulent wakes notably exacerbate optical distortion. In terms of temporal results, the peak frequency of the beam aligns with StD = 0.18 when passing through the cylinder shockwave and falls within StD = 0.16–0.42 as it crosses the shear layer. Significant fluctuations are observed in the turbulent wake and local shock wave, with frequency peaks ranging from StD = 0.12–0.72. Additionally, streamwise flow structures are found to primarily impact optical distortion. Comparative analysis across the three Reynolds numbers indicates that optical distortion is insensitive to Reynolds number variations within the same order of magnitude. The employed grid sufficiently resolves key flow structures impacting beam transmission.

Numerical Investigation of Transverse-Jet-Assisted Initiation of Oblique Detonation Waves in a Combustor

Detonation initiation is a prerequisite to normal operations of an oblique detonation engine (ODE), and initiation-assistant measures are imperative in cases of initiation failure that occur in a length-limited combustor under wide-range flight conditions. This study numerically investigates the initiation characteristics of oblique detonation waves (ODWs) in H2-fueled ODE combustors at wide-range flight Mach numbers Maf or flight altitudes Hf. Failures of ODW initiation are observed at both low Maf and high Hf if no measure is taken to assist initiation. Through analyses of the flow fields and theoretical predictions of the ignition induction length Lind, the data reveal that the detonation failure at low Maf is raised by the significant decrease in the post-shock temperature due to insufficient shock compression, leading to a significant increase in Lind. The detonation failure at high Hf is caused by the rapid decrease in the combustor inflow pressure as Hf increases, which also results in an increase in Lind. With further identifications of the key flow structures crucial to detonation initiation, an initiation-assistant concept employing a transverse H2 jet is proposed. The simulation results show that through an interaction between the incident oblique shock wave and the jet shock wave, the transverse jet helps to initiate an ODW in the combustor at a low Maf, and the initiation location is relatively fixed and determined by the jet location. At high Hf, a Mach reflection pattern is formed in the combustor under the effects of the transverse jet, and detonative combustion is achieved by the generated Mach stem and its reflected shock waves. The proposed concept of using transverse jets to assist detonation initiation provides a practical reference for future development of ODEs that are expected to operate under wide-range flight conditions.

Mechanism of hysteresis and uncontrolled deflection in jet vectoring control based on Coanda effect

We investigated the mechanism of hysteresis and uncontrolled deflection in jet vectoring control based on the Coanda effect. Thrust vectoring control based on the Coanda effect is highly applicable in the field of fluid dynamics because they can achieve jet deflection control with a simple geometric structure and low energy consumption. However, the hysteresis and uncontrolled deflection phenomena considerably hinder the practical application of these technologies. We designed a new passive fluidic thrust vectoring control model to analyze the mechanism of these disadvantages. We developed a synchronous pressure, force, and flow field measurement system to investigate the evolutions of flow structures and their effects on wall pressure distribution and vectoring force. The characteristics of the key flow structures, including the separation bubble, the secondary flow, and the backflow, were investigated. The transient features of jet attachment and jet detachment were researched to identify the transition of near-wall flow structures and wall pressure distribution during jet uncontrolled deflection. The mechanism of uncontrolled deflection is that the formation/breakdown of the separation bubble changes the flow state of the backflow, and the near-wall mass flux balance is collapsed. This causes a dramatic change in the jet vectoring angle, which is out of the valve's control. During attachment, the shear layer first touches the wall and forms a separation bubble, and then, the bubble shrinks. During detachment, the bubble first enlarges and then breaks down. Therefore, the formation and breakdown of the separation bubble correspond to different valve positions, which is the mechanism of hysteresis.

Influence of Solidity on Aerodynamic Performance of Axial Supersonic Through-Flow Fan Cascades

To determine the effects of solidity on its aerodynamic performance and key flow structures, this study reports numerical simulations of the cascades of an axial supersonic through-flow fan with an inlet Mach number of 2.36 over a 15 deg range of incidence. The results indicate that, as the solidity increased, the overall loss first dropped and then increased. At the design incidence, loss at optimal solidity was 78% lower than the maximum loss. The overall loss was heavily dependent on the solidity but insensitive to incidence, especially at low solidity. At a critical solidity (2.38), the shock at the leading edge on the pressure side impinged on the suction surface near the trailing edge. At the optimal solidity, the wake loss was minimized and the number of reflected shocks decreased. The wake loss can be reduced by causing the shock to impinge on the suction surface near the throat of the passage. Moreover, this shock loss can be controlled by weakening its intensity. While reducing the local separation induced by the shock/boundary-layer interaction is ineffective for simultaneously reducing the loss of viscosity, reducing the loss of friction dominated by the surface area of the blade per unit of mass flow is effective.

Flow Characteristics at a River Diversion Juncture and Implications for Juvenile Salmon Entrainment

Flow structures at a river diversion juncture are complex and have been studied extensively. Their impact on the juvenile salmon entrainment into the side channel, however, is less investigated, and based mostly on empiricism. In this study, a Eulerian fish tracking model is developed and used in conjunction with a 3D flow solver to quantitatively evaluate the implications of complex flow characteristics at typical junctures on fish entrainment. First, the flow model is validated with the available experimental data, key flow structures are examined using the results, and their implications for fish entrainment are discussed. Next, the numerical fish tracking model is used to show that the cross-sectional fish distribution immediately upstream of a juncture is an important factor that controls fish entrainment efficiency. Fish entrainment efficiency curves are developed for different flow diversion ratios and fish distribution patterns and used to shed light on the reasons behind some field-observed fish entrainment patterns. Further, the model is used to show that the secondary flow in a river bend may have a significant impact on fish entrainment at flow junctures, in agreement with field observations. Finally, a submerged vane is demonstrated to be a potential management option to locally generate secondary flows upstream of a juncture to achieve the desired fish entrainment property.

Parameter-correlation study on shock–shock interaction using a machine learning method

To predict the maximum heating load induced by shock–shock interaction more reliably and accurately, the geometrical scale of the overall wave configuration of shock–shock interaction is very useful. However, it is hard to be solved with traditional shock theory due to its complexity. The results of numerical and experimental studies are case-by-case. Concise formulas correlating the geometrical scales of shock–shock interaction with the given flow parameters are desired but still unavailable. In the present work, a set of correlative formulas for the triple-points' coordinates of type IVa, IV, and III shock–shock interaction are derived by multilevel block building algorithm, a functional machine learning method. The key flow structure of shock–shock interaction, i.e., the supersonic impinging jet, can be determined with the help of shock theories and the formulas. In addition, the transition criteria respectively for the overall wave configuration transitions of type IVa ↔ type IV and type IV ↔ type III shock–shock interaction can be obtained by the machine learning based method.

Three-dimensional flow dynamics and mixing in a gas-centered liquid-swirl coaxial injector at supercritical pressure

Three-dimensional flow dynamics and mixing in a gas-centered liquid-swirl coaxial injector at supercritical pressure is numerically studied using the large-eddy-simulation technique. In this class of injectors, typical of liquid-fueled propulsion engines, high-temperature gaseous oxygen (GOX) is axially delivered into the center tube and kerosene is tangentially injected through discrete orifices into the coaxial annulus. The operating conditions and geometry mimic those of the main injector elements used in staged-combustion propulsion engines. The present work details the full three-dimensional flow evolution over the entire injector configuration, including axial and circumferential dynamics that are essential for small-scale mixing between GOX and kerosene. Various key flow structures and instability mechanisms in the injector, including axial and azimuthal shear-layer instabilities, secondary instabilities (baroclinic torque and volume dilatation), centrifugal instability, flow recirculation, and acoustic motion, are identified. The significance of these instability mechanisms is explored in the context of streamwise and azimuthal vorticity transport. The GOX core is found to exhibit a hexagonal shape, mainly due to the interactions of vortex rings detached from the center post and coaxial annulus. For comparison, a cylindrical sector of the configuration is also simulated. The results of the present study will support the design and development of high-performance injectors for future propulsion applications.

ストリークの存在する境界層において乱流遷移のきっかけとなる流れの構造

Numerical simulations are carried out aimed at finding a key flow structure which leads to a laminar-turbulent transition of a boundary layer with streaky structures. In the preliminary computation, an array of cuboids is used to form streaky structures inside a boundary layer. Then, a disturbance is introduced into the boundary layer by ejecting a short-duration jet from a hole in the wall into a low-speed region in the streaky structures. Although the boundary layer returns to a laminar state when the jet velocity is set to 18% of the uniform flow velocity, it eventually turns into a turbulent state downstream in the 20% case. The differences are investigated in detail in terms of the vortical structures. As a result, only in the stronger jet case, a flat spanwise vortex is generated beside one leg of a hairpin vortex and it merges with the streamwise vortex nearby forming an inclined streamwise vortex. On the other hand, the flat spanwise vortex disappears without being connected to the streamwise vortex in the weaker jet case. The inclined streamwise vortex is stretched by the mean velocity gradient of the boundary layer growing into a strong vortex, and new vortex structures are generated around the inclined one, which leads to turbulence. Therefore, the results suggest that formation of the inclined vortex is the key to transition of the boundary layer.

Petiolate wings: effects on the leading-edge vortex in flapping flight.

The wings of many insect species including crane flies and damselflies are petiolate (on stalks), with the wing planform beginning some distance away from the wing hinge, rather than at the hinge. The aerodynamic impact of flapping petiolate wings is relatively unknown, particularly on the formation of the lift-augmenting leading-edge vortex (LEV): a key flow structure exploited by many insects, birds and bats to enhance their lift coefficient. We investigated the aerodynamic implications of petiolation P using particle image velocimetry flow field measurements on an array of rectangular wings of aspect ratio 3 and petiolation values of P = 1–3. The wings were driven using a mechanical device, the ‘Flapperatus’, to produce highly repeatable insect-like kinematics. The wings maintained a constant Reynolds number of 1400 and dimensionless stroke amplitude Λ* (number of chords traversed by the wingtip) of 6.5 across all test cases. Our results showed that for more petiolate wings the LEV is generally larger, stronger in circulation, and covers a greater area of the wing surface, particularly at the mid-span and inboard locations early in the wing stroke cycle. In each case, the LEV was initially arch-like in form with its outboard end terminating in a focus-sink on the wing surface, before transitioning to become continuous with the tip vortex thereafter. In the second half of the wing stroke, more petiolate wings exhibit a more detached LEV, with detachment initiating at approximately 70% and 50% span for P = 1 and 3, respectively. As a consequence, lift coefficients based on the LEV are higher in the first half of the wing stroke for petiolate wings, but more comparable in the second half. Time-averaged LEV lift coefficients show a general rise with petiolation over the range tested.

Efficiency of an Inertial Particle Separator

An inertial particle separator is one method of reducing the amount of sand and dust ingested by gas turbine engines. This is important because sand and dust often limit the useful life of engines operating in austere environments. To further improve the efficiency of inertial particle separator systems, an in-depth study of the multiphase flow dynamics has been undertaken. An experimental method was chosen to fill the void of available data and provide useful information for model validation. Using a facility at the University of Virginia, experiments have been conducted using three different inertial particle separator geometries. Surface flow visualization of each geometry is conducted using an oil-streak method to identify key flow structures. Separation efficiencies were calculated for an A4 coarse Arizona test dust (International Organization for Standardization 12103-1) over a range of operating conditions for each geometry.

SPATIO-TEMPORAL COMPLEXITY OF THE AORTIC SINUS VORTEX.

The aortic sinus vortex is a classical flow structure of significant importance to aortic valve dynamics and the initiation and progression of calific aortic valve disease. We characterize the spatio-temporal characteristics of aortic sinus voxtex dynamics in relation to the viscosity of blood analog solution as well as heart rate. High resolution time-resolved (2KHz) particle image velocimetry was conducted to capture 2D particle streak videos and 2D instantaneous velocity and streamlines along the sinus midplane using a physiological but rigid aorta model fitted with a porcine bioprosthetic heart valve. Blood analog fluids used include a water-glycerin mixture and saline to elucidate the sensitivity of vortex dynamics to viscosity. Experiments were conducted to record 10 heart beats for each combination of blood analog and heart rate condition. Results show that the topological characteristics of the velocity field vary in time-scales as revealed using time bin averaged vectors and corresponding instantaneous streamlines. There exist small time-scale vortices and a large time-scale main vortex. A key flow structure observed is the counter vortex at the upstream end of the sinus adjacent to the base (lower half) of the leaflet. The spatio-temporal complexity of vortex dynamics is shown to be profoundly influenced by strong leaflet flutter during systole with a peak frequency of 200Hz and peak amplitude of 4 mm observed in the saline case. While fluid viscosity influences the length and time-scales as well as the introduction of leaflet flutter, heart rate influences the formation of counter vortex at the upstream end of the sinus. Higher heart rates are shown to reduce the strength of the counter vortex that can greatly influence the directionality and strength of shear stresses along the base of the leaflet. This study demonstrates the impact of heart rate and blood analog viscosity on aortic sinus hemodynamics.

Analytical and Numerical Investigation of Two-Dimensional Katabatic Flow Resulting from Local Surface Cooling

The analysis of katabatic flows is often complicated by heterogeneity in surface characteristics. This study focuses on an idealized type of katabatic flow driven by a simple form of inhomogeneous surface forcing: a buoyancy or buoyancy flux that varies down the slope as a top-hat profile (cold strip). We consider the two-dimensional Boussinesq system of governing flow equations with the slope angle, Brunt–Vaisala frequency, and coefficients of eddy viscosity and diffusivity treated as constants. The steady-state problem is solved analytically in a linearized boundary-layer framework. Key flow structures are a primary katabatic jet (essentially the classical one-dimensional Prandtl jet), a rotor-like feature straddling the upslope end of the strip, and two nearly horizontal jets: an inward jet of environmental air feeding into the primary jet on the upslope end of the strip and an outward jet resulting from the intrusion of the primary katabatic jet into the environment on the downslope end of the strip. Next, the corresponding nonlinear initial value problem is solved numerically until a steady state is reached at low levels. The main features of the linear solution are seen in the numerical results, but with some notable differences: (i) the primary jet in the numerical simulation requires a longer distance to attain a one-dimensional boundary-layer structure and extends further downslope off the strip before intruding into the environment; (ii) the numerically simulated outward environmental jet is narrower and more intense than the inward jet, and has a pronounced wave-like structure.

Airborne lidar observations in the inflow region of a warm conveyor belt

AbstractWarm conveyor belts (WCBs) are key flow structures associated with extratropical cyclones. They transport moist air from the cyclone's warm sector poleward and upward close to the tropopause level, leading to the formation of elongated cloud bands, intense latent heating and surface precipitation. In this study a comprehensive dataset of airborne lidar observations of moisture and wind from different campaigns has been investigated with a trajectory‐based approach to identify ‘lucky encounters’ with WCBs. On 19 July 2007, an upstream flight over the Iberian Peninsula during the European THORPEX Regional Campaign (ETReC 2007) in Central Europe intersected two WCBs: one in the upper tropospheric outflow region about 3 days after starting the ascent, and the other one in the boundary layer inflow region over Spain just prior to the strong ascent. Comparison of the lidar humidity measurements with analysis fields from the European Centre for Medium‐Range Weather Forecasts (ECMWF) reveals significant positive deviations, equivalent to an overestimation of the modelled humidity, in this low‐tropospheric WCB inflow region (of about 1 g kg−1 (14%) on average and with peak deviations up to 7 g kg−1). It is noteworthy that this substantial bias occurs in a potentially dynamically highly relevant air mass that will be subsequently lifted within a WCB to the upper troposphere. A Lagrangian moisture source diagnostic reveals that these large moisture deviations occur within air masses that, according to the ECMWF analyses, are coherently transported from the western Mediterranean towards Spain and experience intense moisture uptake over the Ebro valley. It is suggested that inaccuracies in surface evapotranspiration, horizontal moisture advection, and turbulent vertical transport of moisture in the atmospheric boundary layer potentially contribute to the erroneous low‐tropospheric humidity in the inflow region of this particular summertime WCB over Spain in the ECMWF analyses. Copyright © 2011 Royal Meteorological Society

Investigation of flow mechanism of a robotic fish swimming by using flow visualization synchronized with hydrodynamic force measurement

When swimming in water by flapping its tail, a fish can overcome the drag from uniform flow and propel its body. The involved flow mechanism concerns 3-D and unsteady effects. This paper presents the investigation of the flow mechanism on the basis of a 3-D robotic fish model which has the typical geometry of body and tail with periodic flapping 2-freedom kinematical motion testing in the case of St = 0.78, Re = 6,600 and phase delay mode (φ = −75°), in which may have a greater or maximum propulsion (without consideration of the optimal efficiency). Using a special technique of dye visualization which can clearly show vortex sheet and vortices in detail and using the inner 3-component force balance and cable supporting system with the phase-lock technique, the 3-D flow structure visualized in the wake of fish and the hydrodynamic force measurement were synchronized and obtained. Under the mentioned flapping parameters, we found the key flow structure and its evolution, a pair of complex 3-D chain-shape vortex (S–H vortex-rings, S1–H1 and S2–H2, and their legs L1 and L2) flow structures, which attach the leading edge and the trailing edge, then shed, move downstream and outwards and distribute two anti-symmetric staggering arrays along with the wake of the fish model in different phase stages during the flapping period. It is different with in the case of St = 0.25–0.35. Its typical flow structure and evolution are described and the results prove that they are different from the viewpoints based on the investigation of 2-D cases. For precision of the dynamic force measurement, in this paper it was provided with the method and techniques by subtracting the inertial forces and the forces induced by buoyancy and gravity effect in water, etc. from original data measured. The evolution of the synchronized measuring forces directly matching with the flow structure was also described in this paper.

Numerical Simulation of Laminar Flow and Heat Transfer Inside a Microchannel With One Dimpled Surface

Steady, laminar flow and heat transfer, inside a rectangular microchannel with a dimpled bottom surface, are numerically studied. The microchannel is 50×10−6m(50μm) deep and 200×10−6m(200μm) wide. The dimples are placed in a single row along the bottom wall with a pitch of 150×10−6m(150μm). The dimple depth is 20×10−6m(20μm), and the dimple footprint diameter is 98×10−6m(98μm). Fully developed periodic velocity and temperature boundary conditions are used at the inlet and outlet of one unit cell of the dimpled microchannel. Key flow structures such as recirculating flow and secondary flow patterns and their development along the flow directions are identified. The impact of these flow structures on the heat transfer is described. Heat transfer augmentations (relative to a channel with smooth walls) are present both on the bottom-dimpled surface, and on the sidewalls of the channel. The pressure drops in the laminar-microscale flow are either equivalent to, or less than, values produced in smooth channels with no dimples. It is concluded that dimples, proven to be an effective passive heat transfer augmentation for macroscale channels, can also be used to enhance heat transfer inside microchannels.

Investigation of Wedge Probe Wall Proximity Effects: Part 2—Numerical and Analytical Modeling

An experimental study of wedge probe wall proximity effects is described in Part 1 of this paper. Actual size and large-scale model probes were tested to understand the mechanisms responsible for this effect, by which free-stream pressure near the outer wall of a turbomachine may be overindicated by up to 20 percent dynamic head. CFD calculations of the flow over two-dimensional wedge shapes and a three-dimensional wedge probe were made in support of the experiments, and are reported in this paper. Key flow structures in the probe wake were identified that control the pressures indicated by the probe in a given environment. It is shown that probe aerodynamic characteristics will change if the wake flow structures are modified, for example by traversing close to the wall, or by calibrating the probe in an open jet rather than in a closed section wind tunnel. A simple analytical model of the probe local flows was derived from the CFD results. It is shown by comparison with experiment that this model captures the dominant flow features.