Instantaneous Flow Fields Research Articles

Direct simulation of sound generation by a two-dimensional flow past a wedge

Direct simulations of sound generation due to two-dimensional, unsteady, laminar flow past a wedge at several angles of incidence have been performed using a highly accurate, physical dispersion relation preserving scheme. We have considered a uniform flow past a wedge at a Mach number of M = 0.2 and a Reynolds number of Re = 100, at thirteen different angles of incidence 0° ≤ α ≤ 60°. Results show that the vortex shedding phenomena which in turn strongly depend on the angle of incidence are responsible for triggering negative and positive pressure pulses. We have in particular focused our attention on a special case α = 30° where the mean drag attains a lowest value among all angle of incidence cases and also reports a highest root mean square value for the lift coefficient. We have closely related the effects of the fluctuations in flow field parameters on the frequency and amplitudes of generated sound waves. The generated sound field displays dipolar nature. The lift dipole contributes more to the sound field as compared to the drag dipole. The dominating nature of the lift dipole has been confirmed by the proper orthogonal decomposition of the disturbance pressure field. Using Doak’s decomposition technique, the instantaneous flow field for different cases is decomposed into acoustic, entropic, and hydrodynamic modes. Doak’s decomposition further confirms that the amplitude and the frequency associated with lift and drag coefficient fluctuations characterize sound field generation and its propagation. It has been found that the generated sound field is greatly enhanced as α varies from 30° to 60°.

Detection algorithm for turbulent interfaces and large-scale structures in intermittent flows

A robust algorithm is introduced for detection of large-scale coherent structures in transitional and intermittent flows that feature turbulent/non-turbulent (T/NT) interfaces. The algorithm is applicable the instantaneous flow fields of wall-bounded and free shear flows, and can effectively identify coherent events in the velocity or vorticity fields, or sweep/ejection motions. A database from direct numerical simulation (DNS) of transitional boundary layer is used to develop and demonstrate the capabilities of the algorithm which consists of three steps. The first is identification of the T/NT interface by comparing the normalized vorticity magnitude to a threshold value that is independent of the Reynolds number. The vorticity normalization is specifically designed to be applicable in transitional flows, where regions of the flow can host juxtaposed regions of laminar and turbulent flow. With the definition of the T/NT interface, conditional statistics are computed and perturbation quantities are defined relative to their respective conditional means. Second, the influence of the small-scale turbulence is excluded by applying an anisotropic Gaussian filter. The filter size is determined from the spatial characteristics of the small-scale vortical motions. In the third step, one-dimensional cores and two-dimensional surfaces within the flow structures of interest are identified from local extrema in the fields, and are tracked as Lagrangian objects. Using the algorithm, the population trends and advection speeds of large-scale sweep/ejection events are computed in the transitional boundary layer. Two additional flow configurations are also considered: turbulent jet flow emerging from a circular nozzle and the turbulent flow in a channel with a wavy surface.

Numerical Study of Iced Airfoils with Horn Features Using Large-Eddy Simulation

Separated flowfields around two iced airfoils with horn features near stall are numerically studied using wall-modeled large-eddy simulation. The iced airfoils are the GLC305 airfoil with rime ice 212 and the NLF0414 airfoil with glaze ice 623. A grid-quality-based hybrid central/upwind scheme is applied for numerical flux computation. The close agreement between the computed lift, drag, and surface pressure distribution and the experimental results illustrates the capability of the present method. In particular, the current method can accurately predict the Kelvin–Helmholtz instability of a free shear layer. The statistical results, instantaneous flowfields, pressure fluctuations, and characteristic frequencies are investigated. Both flowfields are dominated by separation bubbles. The bubble length increases with the angle of attack according to a quadratic relation. A strong-fluctuation region in the flowfield grows rapidly at high angles of attack, especially near stall. High- and low-frequency peaks are found in the initial and further downstream regions of the free shear layer, respectively. The high frequency is caused by the Kelvin–Helmholtz instability and vortex shedding. The low frequency might be caused by the vortex merging in the further downstream region of the free shear layer.

Aeromechanics Analysis of a Hummingbird-Like Flapping Wing in Hover

This paper presents an in-depth investigation into the instantaneous forces, flowfield, and wing deflections of a flapping wing used on a hover-capable robotic hummingbird. The goal was to understand the relationship between instantaneous lift and wing shape. An experimental rig was constructed, which flapped the wing at 20 Hz and measured instantaneous loads normal to the stroke plane. To separate inertial and aerodynamic loads, a novel approach was developed in which the wing spatial and temporal displacement was obtained using digital image correlation. From this, the instantaneous inertial force normal to the stroke plane was calculated based on mass distribution. The flowfield on the wing was resolved using particle image velocimetry, which revealed attached flow at steep angles of attack and several strong vortices. It was found that lift was sensitive to both unsteady aerodynamics and deflection. This study marks the first time that these quantities have been measured for a flapping wing used on a hover-capable vehicle, using a procedure developed for extracting pure aerodynamic force on a flexible structure. This effort has resulted in experimental data useful for validating high-fidelity flapping wing aeroelastic computational models.

Filamentary anemometry using femtosecond laser-extended electric discharge - FALED.

We demonstrate a non-contact spatiotemporally resolved comprehensive method for gas flow velocity field measurement: Filamentary Anemometry using femtosecond Laser-extended Electric Discharge (FALED). A faint thin plasma channel was generated in ambient air by focusing an 800-nm laser beam of 45 fs, which was used to ignite a pulsed electric discharge between two electrodes separated over 10 mm. The power supplier provided a maximum voltage up to 5 kV and was operated at a burst mode with a current duration of less than 20 ns and a pulse-to-pulse separation of 40 μs. The laser-guided thin filamentary discharge plasma column was blowing up perpendicularly by an air jet placed beneath in-between the two electrodes. Although the discharge pulse was short, the conductivity of the plasma channel was observed to sustain much longer, so that a sequence of discharge filaments was generated as the plasma channel being blown up by the jet flow. The sequential bright thin discharge filaments can be photographed using a household camera to calculate the flow velocity distribution of the jet flow. For a direct comparison, a flow field measurement using FLEET [Appl. Opt. 50, 5158 (2011)] was also performed. The results indicate that the FALED technique can provide instantaneous nonintrusive flow field velocity measurement with good accuracy.

Marginally turbulent Couette flow in a spanwise confined passage of square cross section

The results of direct numerical simulations to determine the critical conditions for self-sustained turbulence in wall-driven (Couette) square duct flow and its characteristics at relatively low turbulent Reynolds numbers are presented. We focus on the case in which a pair of opposite counter-moving walls translating with the same speed drives the flow. Stabilisation by the side walls is found to play a crucial role in the transition to turbulence, the minimum Reynolds number for maintaining a turbulent state (Rec ≈ 875) being much greater than that in a plane channel. At Reynolds numbers close to the critical, an alternation of the flow field, in time, between two states characterised by a four-vortex secondary flow pattern is observed, one being a mirror reflection of the other, and the flow remains approximately symmetrical about the common bisector of the moving walls. Due to the intermittency, large velocity fluctuations about the long-term mean are observed at different locations in the duct. These findings are consistent with results of previous studies on turbulent pressure-driven (Poiseuille) square duct flow at low Reynolds numbers; hence, the phenomenon is not unique to Poiseuille flows. Instantaneous flow field visualisations reveal the existence of coherent structures which are persistent over the length of the duct, thus indicating that the states are very stable in the streamwise direction. Quadrant analysis of the Reynolds shear stress shows that the secondary motions are closely related to the near-wall ejection and sweeping events.

Large-Eddy Simulations of Inclined Jets in Crossflow with Different Holes

Film cooling is a key technology to improve the thermal performance of high-pressure turbines. The mixing of the cooling air and the main flow is inherently unsteady, but the unsteady flow physics of different cooling holes are seldom investigated. In the current study, the large-eddy simulation method is used to investigate inclined jets in crossflow via a cylindrical hole and a fan-shaped hole. The angle between the hole and the main flow is 35 deg, and the blowing ratio is 0.5, which are representative for film cooling. First, the results are analyzed in a traditional way using counter-rotational vortices based on the time-averaged results. Then, the instantaneous flowfield is presented. The mixing of the injected flow with the main flow is highly related to the unsteady coherent vortices. The results showed that the instantaneous flowfield can provide a better explanation of the distribution of film-cooling effectiveness than the time-averaged flowfield. The effects of instantaneous vortices on the temperature distribution for different cooling hole geometry are discussed in detail.

Evaluating the hydrodynamics of a round jet in a vegetated crossflow through large eddy simulation

Vegetation plays an important role on the turbulence structures of the effluent spreading in an open channel, which are insufficiently studied. This paper employs a large eddy simulation model to investigate the hydrodynamic processes of a round jet in a vegetated crossflow. The time-averaged simulated results are consistent with the experimental data. Analyses of mean flow characteristics including velocity distributions in different planes are showed that the array of rigid vegetation affects the averaged flow field and the jet structure, diminishing the velocity, with a significant increase of the jet penetration height. Moreover, this simulation successfully reproduces the coherent structures of classical and well-documented types based on the mean and instantaneous flow fields, including shear-layer vortices, wake vortices, counter-rotating vortex pair and windward vortex pair. The momentum transport mechanism is quantitatively elaborated by the quadrant analysis. Spectral analysis is adopted to obtain the dominant frequencies of vortex shedding and investigated the characteristic length of the large-scale vortex at different probes in the flow field.

LBE simulation of impinging stream inside a T-junction mixer

Lattice Boltzmann Equation (LBE) method is utilized to simulate impinging stream (IS) in a T-junction mixer using a TD2G9 model. It aims to investigate the influence of Reynolds number (Re), aspect ratio of outlet diameter to inlet diameter, ratio of opposite inlet velocities, and the thermal boundary conditions on flow, mixing and heat transfer characteristics. In particular, the vortex evolution, velocity distribution, mixing index and Nusselt number (Nu) distribution in the T-junction mixer are explored in details. Four types of vortices and flow regimes are observed. The instantaneous and time-averaged flow and thermal fields, including vortex structure, transition of flow regimes, streamline and the Nusselt number distribution are discussed. Distinct quantitative transitions, even for dramatic change, are observed near the critical Re. At a low or moderate aspect ratio, the symmetric coherent structure is observed in an unstable flow regime. At a larger aspect ratio, the flow in the T-mixer becomes turbulent and asymmetric. The unequal injections velocities of the nozzles impose significant influence on the flow structure, mixing and heat transfer in vertical tube. Using larger difference between the two inlet velocities can result in more obvious change in flow characteristics. Moreover, mixing index is found to be valid in evaluating the mixing degree under a sinusoidal inlet velocity.

Development of a three-dimensional phase-field lattice Boltzmann method for the study of immiscible fluids at high density ratios

Based on the recent work by Fakhari et al. (2017b), a three-dimensional phase-field lattice Boltzmann method was developed to investigate the rise of a Taylor bubble in a duct. The proposed approach couples the conservative phase-field equation with a velocity-based lattice Boltzmann scheme equipped with a weighted multiple-relaxation-time collision operator to enhance numerical stability. This makes the model ideal for numerical simulation of immiscible fluids at high density ratios and relatively high Reynolds numbers. Several benchmark problems, including the deformation of a droplet in a shear flow, the Rayleigh–Taylor instability, and the rise of a Taylor bubble through a quiescent fluid, were considered to asses the accuracy of the proposed solver. The Rayleigh–Taylor instability simulations were conducted for a configuration mimicking an air-water system, which has received little attention in the literature. After detailed verification and validation, the presented formulation was applied to study the flow field surrounding a Taylor bubble, for which numerical results were compared with the experimental work of Bugg and Saad (2002). The findings highlighted that the experimental bubble rise velocity, instantaneous flow field, and interface profile can be accurately captured by the presented model. In particular, the rise velocity of the present model indicated an improvement in accuracy when compared to the reference numerical solutions. The agreement between various numerical schemes, in some instances, indicated potential experimental difficulties in measuring the local flow field. Future application of the present model will facilitate detailed investigation of the pressure and flow profile surrounding Taylor bubbles evolving in co-current and counter-current flows.

Numerical study of turbulent flow fields around of a row of trees and an isolated building by using modified k-ε model and LES model

Turbulent flow fields over two typical urban elements, a row of trees with low packing density and an isolated building with high packing density, are investigated by a modified k−ε model and a LES model. The applicability of these two models is evaluated by the validation metrics. Instantaneous flow fields are visualized by vortex cores and examined by quadrant analysis. In the wake region of the row of trees, predicted mean wind speed by the modified k−ε model shows favourable agreement with the measured data, but turbulent kinetic energy is underestimated since the modified k−ε model is not capable of simulating organized motions. In the wake region of the isolated building, both predicted mean wind speed and turbulent kinetic energy by the modified k−ε model are slightly underestimated due to lack of the vortex shedding in the simulation. On the other hand, LES model well predicts both mean wind speed and turbulent kinetic energy since all large vortices are directly resolved by LES model.

Propeller wake evolution mechanisms in oblique flow conditions

In the present study the wake flow past an isolated propeller operating in oblique flow conditions is investigated experimentally. In particular, the investigation concerns a systematic topological comparison of the wake behaviour in axisymmetric and in oblique inflow conditions, for three inclination angles, and is focused on an analysis of the underlying mechanisms of wake evolution and instability. To this end, the experiment has been designed to investigate the dynamics of propeller vortical structures over a wide spatial extent covering the wake region from the propeller disk up to 4.5 diameters in the streamwise direction. Detailed flow measurements have been undertaken by particle image velocimetry (PIV), using a multicamera configuration with three cameras arranged side by side. This allowed simultaneous acquisition of a large flow extent at a spatial resolution adequate to resolve the smallest vortical structures involved in the process of propeller wake instability. The analysis has been based on both phase-locked averaged and instantaneous flow fields. The study extends the knowledge on the subject of propeller wake dynamics, highlighting the major hydrodynamic effects that non-axisymmetric propeller operating conditions exert on the mechanisms of wake evolution, instability and breakdown, such as asymmetric destabilization of the tip vortices on the leeward and windward sides of the wake, and the interference between the tip and the junction vortices, as well as the cause–effect relation between the breakdown of the blade trailing wake and the instability of the tip and hub vortices.

Across-wind excitation mechanism for interference of twin tall buildings in staggered arrangement

This paper explores the excitation mechanism for across-wind force interference of two tall buildings in staggered arrangement through detailed measurement of turbulent flow fields past the two buildings and concurrent pressure measurement on the downstream principal building. Analysis of the instantaneous flow fields and wall pressure distributions reveals a synchronization process between the sideway oscillation of the upstream building wake with vortex development and shedding from the downstream principal building. Further analysis using the phase averaging technique confirms that the synchronization of five quasi-periodic aerodynamic phenomena is responsible for the magnification of across-wind force fluctuations in a region of building arrangements centered at a longitudinal building separation of 5 building breaths (D) and a lateral separation of 2.5D. In addition to this “wake interference region I”, three other flow regions of staggered arrangement are classified, namely, “wake interference region II” in which wake development on the principal building is affected by impingement of the meandering upstream building wake in a close lateral separation, “proximity interference region” which is characterized by channeled flow through a narrow gap between the two buildings, and “weak interference region”.

LES and Wind Tunnel Test of Flow around Two Tall Buildings in Staggered Arrangement

Wind flow structures and their consequent wind loads on two high-rise buildings in staggered arrangement are investigated by Large Eddy Simulation (LES). Synchronized pressure and flow field measurements by particle image velocimetry (PIV) are conducted in a boundary layer wind tunnel to validate the numerical simulations. The instantaneous and time-averaged flow fields are analyzed and discussed in detail. The coherent flow structures in the building gap are clearly observed and the upstream building wake is found to oscillate sideways and meander down to the downstream building in a coherent manner. The disruptive effect on the downstream building wake induced by the upstream building is also observed. Furthermore, the connection between the upstream building wake and the wind loads on the downstream building is explored by the simultaneous data of wind pressures and wind flow fields.

Aeroelasticity Validation Study for a Three-Dimensional Membrane Wing

The objective of this effort is to study the aeroelastic characteristics of a flexible membrane wing at different Reynolds numbers using direct numerical simulation and to investigate techniques to reduce the adverse effects of flow separation and unsteady aerodynamics. For the aeroelastic predictions, a first-principles-based approach is undertaken, where the flow solver is fully coupled with the structural dynamics solver. This approach is able to accurately and efficiently predict the unsteady aerodynamics, boundary-layer separation, lift and drag, and the stress and structural deformation of the flexible membrane wing. The focus of this paper is on the systematic validation of this coupled solver for a specific membrane configuration, where extensive experimental measurements are available. Comparisons between the present simulations and experiments are made for the following quantities: 1) mean membrane shape; 2) dominant membrane oscillating frequencies under various conditions; 3) flowfield visualizations; 4) instantaneous velocity field; 5) membrane displacement response to instantaneous flowfield; 6) use of rigid and flexible membranes; 7) distribution of mean velocity field; and 8) distribution of turbulence intensity. Comparisons of these quantities show the favorable agreement of the present prediction against measurement, and hence build a sound validation study for the coupled fluid–structure solver.

Turbulent channel flow perturbed by triangular ripples

This paper presents an experimental investigation of the perturbation of a turbulent closed-conduit flow by two-dimensional triangular ripples. Two ripple configurations were employed: one single asymmetric triangular ripple, and two consecutive asymmetric triangular ripples, all of them with the same geometry. Different water flows were imposed over either one or two ripples fixed to the bottom wall of a closed conduit, and the flow field was measured by PIV (particle image velocimetry). Reynolds numbers based on the channel height were moderate, varying between 27500 and 34700. The regime was hydraulically smooth, and the blockage ratio was significant. Experimental data for this specific case remain scarce, and the physics involved has yet to be fully elucidated. Using the instantaneous flow fields, the mean velocities and fluctuations were computed, and the shear stresses over the ripples were determined. The velocities and stresses obtained in this way for the single ripple and for the pair of ripples are compared, and the surface shear stress is discussed in terms of bed stability. Our results show that for the single and upstream ripples the shear stress increases at the leading edge and decreases toward the crest, while for the downstream ripple it decreases monotonically between the reattachment point and the crest. The stress distribution over the downstream ripple, which is different from both the upstream and single ripples, is shown to sustain existing ripples over loose beds.

Effect of Inflow Turbulence on an Airfoil Flow with Laminar Separation Bubble: An LES Study

The present paper is concerned with numerical investigations on the effect of inflow turbulence on the flow around a SD7003 airfoil. At a Reynolds number Rec = 60,000, an angle of attack α = 4∘ and a low or zero turbulence intensity of the oncoming flow, the flow past the airfoil is known to be dominated by early separation, subsequent transition and reattachment leading to a laminar separation bubble with a distinctive pressure plateau. The objective of the study is to investigate the effect of inflow turbulence on the flow behavior. For this purpose, a numerical methodology relying on a wall-resolved large-eddy simulation, a synthetic turbulence inflow generator and a specific source term concept for introducing the turbulence fluctuations within the computational domain is used. The numerical technique applied allows the variation of the free-stream turbulence intensity (TI) in a wide range. In order to analyze the influence of TI on the arising instantaneous and time-averaged flow field past the airfoil, the present study evaluates the range 0% ≤ TI ≤ 11.2%, which covers typical values found in atmospheric boundary layers. In accordance with experimental studies it is shown that the laminar separation bubble first shrinks and finally completely vanishes for increasing inflow turbulence. Consequently, the aerodynamic performance in terms of the lift-to-drag ratio increases. Furthermore, the effect of the time and length scales of the isotropic inflow turbulence on the development of the flow field around the airfoil is analyzed and a perceptible influence is found. Within the range of inflow scales studied decreasing scales augment the receptivity of the boundary layer promoting an earlier transition.

Continuous control of asymmetric forebody vortices in a bi-stable state

Aiming at the problem of continuous control of asymmetric forebody vortices at a high angle of attack in a bi-stable regime, a dual synthetic jet actuator embedded in an ogive forebody was designed. Alternating unsteady disturbance with varying degree asymmetrical flow fields near the nozzles is generated by adjusting the duty cycle of the drive signal of the actuator, specifically embodying the asymmetric time-averaged pattern of jet velocity, vorticity, and turbulent kinetic energy. Experimental results show that within the range of relatively high angles of attack, including the angle-of-attack region in a bi-stable state, the lateral force of the ogive forebody is continuously controlled by adjusting the duty cycle of the drive signal; the position of the forebody vortices in space, the vorticity magnitude, the total pressure coefficient near the vortex core, and the vortex breakdown location are continuously changed with the duty cycle increased observed from the time-averaged flow field. Instantaneous flow field results indicate that although the forebody vortices are in an unsteady oscillation state, a continuous change in the forebody vortices’ oscillation balance position as the duty cycle increases leads to a continuous change in the model’s surface pressure distribution and time-averaged lateral force. Different from the traditional control principle, in this study, other different degree asymmetrical states of the forebody vortices except the bi-stable state are obtained using the dual synthetic jet control technology.

Experimental investigation on flow past nine cylinders in a square configuration

An experimental investigation on flow past nine cylinders in a square configuration was carried out using the particle image velocimetry technique and load cell in a water channel. The center-to-center spacing ratio L/D was in the range of 1.5–3.0 and the Reynolds number Re was varied from 1500 to 5000. The effects of spacing ratio and Reynolds number on the instantaneous time-averaged flow fields and force coefficients are investigated. The results show that three distinct flow regimes are categorized with variation of the spacing ratios and Reynolds numbers, namely, shielding flow regime, transition flow regime and vortex shedding flow regime. Depending on the interferences of shear layers around the nine cylinders, each flow regime is further divided into two types of flow patterns. An interesting feature of bistable flow pattern with different flow modes is observed at small spacing ratio L/D = 1.5. The non-dimensional vortex shedding frequencies appear to be more associated with the individual shear layers rather than the multiple cylinders. Moreover, force analysis, streamline topologies and Reynolds stress contours are presented to elucidate the effects of spacing ratio and Reynolds number on the complex wake interference among the nine cylinders. The flow characteristics and force coefficients are found to be more sensitive to L/D rather than Re.

Beating motion of a circular cylinder in vortex-induced vibrations

In this paper, beating phenomenon of a circular cylinder in vortex-induced vibration is studied by numerical simulations in a systematic manner. The cylinder mass coefficients of 2 and 10 are considered, and the Reynolds number is 150. Two distinctive frequencies, namely cylinder oscillation and vortex shedding frequencies, are obtained from the harmonic analysis of the cylinder displacement. The result is consistent with that observed in laboratory experiments. It is found that the cylinder oscillation frequency changes with the natural frequency of the cylinder while the reduced velocity is varied. The added-mass coefficient of the cylinder in beating motion is therefore estimated. Meanwhile, the vortex shedding frequency does not change dramatically in the beating situations. In fact, it is very close to 0.2. Accordingly, the lift force coefficient has two main components associated with these two frequencies. Besides, higher harmonics of the cylinder oscillation frequency appear in the spectrum of the lift coefficient. Moreover, the vortex shedding timing is studied in the beating motion by examining the instantaneous flow fields in the wake, and two scenarios of the vortex formation are observed.