Induced Velocity Field Research Articles

Dynamic impact of transiting weather systems on coastal currents in the northern Gulf of Mexico

Moving weather systems determine the history of wind variations with patterns as the systems transit through the ocean. These weather systems are integrated entities that can provide a system level perspective. A vessel-based survey repeatedly occupying a 30-km transect 12 times in 36 h provided non-aliased measurements of velocity field that showed how the along-shelf transport varied by more than three-fold in response to a transiting high-pressure weather system following an atmospheric cold front. To further illustrate the impact of different weather systems, we analyzed time series data from moored ADCPs, which showed influence on the velocity field from infrequent summertime cold fronts and remote hurricanes moving through the region, one on the west and the other on the east of the study site. Rotary spectrum analysis showed that the flow field rotated mostly clockwise with a smaller but non-negligible counterclockwise component, consistent with near inertial oscillations mixed with weak tidal currents. A theoretical model is presented by a Laplace transform and a general relationship between the velocity field and forcing functions is obtained, which shows that the contributions to the rotary velocity field from various forcing functions are through mathematical convolutions between the forcing functions and the complex frictional-rotary inertial function (CFRIF). These convolutions include an integrated effect of history of the forcing. CFRIF is effectively a frictional rotary filter that favors inertial oscillations. The wind-stress induced velocity field over a few days is computed and it shows significant variations after the passage of a cold front, with a magnitude consistent with observations. The wind-stress induced velocity is a few times greater than the density driven flow during the ship-based observations. The weather systems passing through the region can impact coastal currents causing a great variability over short time scales.

VAST as a generic, modular aeromechanics code for wind turbine simulation

In order to leverage the full potential wind energy, more research is needed both in the field and numerically. At DLR, the need for simulation competencies is expected to be met by the in-house developed generic and highly flexible multi-physics code VAST. While initially aimed at low- to medium-fidelity simulations of helicopter aeromechanics, the code is applicable to other problems like wind turbine computations. The goal of this paper is to present the general concept of VAST and briefly introduce the available physics models. Next to examples of successful application to helicopter simulation problems, first results in the field of wind energy are shown. The well-known IEA 15 MW reference wind turbine is used as a basis to compare VAST — both with a BEMT and a vortex theory inflow model — to other tools with similar and higher-fidelity physics modeling. For the rigid turbine, blade loads are compared with results of other tools using engineering models as well as CFD, showing good overall agreement. A more detailed evaluation of aerodynamic quantities like the angle of attack and the induced velocity field is presented for VAST and the CFD results. An outlook on the ongoing VAST developments is given.

Wake characteristics and vortex structure evolution of floating offshore wind turbine under surge motion

Offshore wind energy is booming and floating offshore wind has been proved to be a promising clean energy. Wake effect is one of the biggest challenges in large-scale floating wind farm which results in power loss and increased fatigue load. In this paper, actuator line model (ALM) coupled with unsteady Reynolds-averaged Navier-Stokes (URANS) is adopted to investigate the wake characteristics and evolution of floating wind turbine under motion. Besides, modal analysis is adopted to extract typical features of turbine wake. Velocity fluctuation is the prominent characteristic of floating turbine wake. The increase of surge frequency results in the decrease of velocity fluctuating region and large surge amplitude leads to bigger velocity deficit fluctuations. In addition, the surge motion brings a faster recovery rate than fixed wind turbine. The induced velocity field generated by vortex ring structure is the key to understanding complex evolution of turbine wake and the vortex ring structure is analyzed in this paper. With the deep understanding of FOWT wake, an innovative vortex ring structure model is proposed in this paper. This work is the basic work of developing the engineering wake model of floating wind turbine, to cope with the wake effect challenge in the large-scale floating wind farm.

Theory of the momentum source method for synthetic turbulence

The interaction between turbulence and blade leading edges is known to have a significant impact on the aerodynamic and aeroacoustic performance of propellers. In addition to directly simulating turbulence, synthetic turbulence, such as the momentum source method, has been developed as a popular method for studying this interaction process in computational fluid dynamics and computational aeroacoustics. However, it is found that for non-periodic disturbances, although the induced velocity field is divergence-free, spurious noise may be generated in the source region and contaminate simulation results. To address this issue, the present work proposes adding a correction term so that the divergence-free condition is satisfied globally and the unwanted acoustic waves are suppressed, as an extension to our previous work for time-periodic gusts [H. Jiang, Phys. Fluids 35, 096115 (2023)]. The strength of the proposed approach lies in its simplicity, flexibility, and generality. First, it derives explicit source terms, which are straightforward for numerical implementations, to generate unsteady flow fluctuations. Second, the sources can be added inside the computational domain, saving computational costs for turbulence convection and being compatible with most existing boundary conditions. Third, the proposed method can obtain analytical expressions for the needed momentum source of the Navier–Stokes equation subject to any desired isotropic or anisotropic divergence-free turbulence fields. The method has been verified by examples of synthesizing harmonic gusts, Gaussian eddies, and random turbulence. The synthetic velocity results characterized by different spectral components are directly compared to target velocity fields, verifying the proposed approach and showing its capability. Parameters that influence the distribution of added sources are systematically investigated to identify an optimal combination for different scenarios. Finally, the model is employed to evaluate the aerodynamic interaction between an incoming turbulence and a thin airfoil. The obtained results exhibit good correspondence with analytical solutions.

Understanding the Influence of the Buoyancy Sign on Buoyancy-Driven Particle Clouds

A numerical model was developed to investigate the behavior of round buoyancy-driven particle clouds in a quiescent ambient. The model was validated by comparing model simulations with prior experimental and numerical results and then applied the model to examine the difference between releases of positively and negatively buoyant particles. The particle cloud model used the entrainment assumption while approximating the flow field induced by the cloud as a Hill’s spherical vortex. The motion of individual particles was resolved using a particle tracking equation that considered the forces acting on them and the induced velocity field. The simulation results showed that clouds with the same initial buoyancy magnitude and particle Reynolds number behaved differently depending on whether the particles were more dense or less dense than the ambient fluid. This was found even for very low initial buoyancy releases, suggesting that the sign of the buoyancy is always important and that, therefore, the Boussinesq assumption is never fully appropriate for such flows.

The Full Non-linear Vortex Tube-Vorton Method: the pre-stall condition

The present hybrid vortex tube-vorton method is based entirely on the Full Multi-wake Vortex Lattice Method (FMVLM) concepts, which means detaching vorticity with precise vortex strength and orientation along all separation lines between each discretized element of a shell-body, including all external edges. Since the classic Vortex Particle Method (VPM) is unstable by itself because it does not conserve the total amount of circulation as time evolves (Kelvin’s circulation theorem), an isolated Vortex (regularized) Filament Method (VFM) approach is implemented to obtain advection of vorticity, while the induced velocity field is obtained through its corresponding full vorton cloud. Further, a novel vortex squeezing/stretching scheme for such a vortex cylinder-sphere approach is proposed based on variation in time for vortex volumes in order to precisely (zero residual) conserve both circulation and vorticity at each time step (for each detached vortex element), while the viscous effect can be accounted for via the Core Spreading Method (CSM).

Numerical study on the forces and moments on a bottom-mounted cylinder by internal solitary wave

The forces and moments on a bottom-mounted cylinder by internal solitary wave are investigated with numerical simulation. The solution of Dubreil–Jacotin–Long equation is used to initialize internal solitary wave in the computational domain, which overcomes the deficiencies of internal solitary wave generated by gravity collapse method. The continuous density stratification is taken into account in this study and numerical results show that the shape of pycnocline could influence the forces and moments exerted on cylinder by internal solitary wave. The forces and moments on different cylinder sections are calculated separately, the derived forces exhibit similar patterns observed in previous laboratory measurements. The results show that the loads by internal solitary wave on cylinder are quite complex as the wave induced velocity field is unsteady and the directions are opposite between upper and lower layers. The performance of Morison equation is examined and three coefficient determination methods are tested, force based global coefficients, moment based global coefficients and local coefficients. The analysis shows that the global coefficients are sensitive to the data adopted, which explains why universal coefficients are not available from previous studies.

Characteristics of a tip-vortex generated by a single rotor used in agricultural spraying drone

A better understanding of downwash flow is required to increase the efficiency of unmanned aerial vehicle sprayers. The tip vortex rolled up by the rotor blade tip rotating at high speed produces a complex induced velocity field, which has an important impact on the downwash flow in the wake of the rotor. Usually, a symmetrical experimental setup with a central rotor support perpendicular to the rotation plane is used in studies on rotating blade wake and vortices, potentially leading to differences in observed wake and vortices in real situations. whereas in reality, the rotor is typically supported by a parallel arm extending below the rotor in the rotation plane. It is crucial to consider these differences for accurate results in analyzing blade rotation. The behavior of the tip vortices, dynamic characteristics, and velocity field of the rotor wake in agricultural drone single-rotor downwash flow was experimentally investigated using particle image velocimetry (PIV) combined with high-speed camera imaging. The dynamics of blade tip vortices at different wake ages and rotational speeds of the rotor, 1000–3000 rpm, were measured and analyzed. The results indicate that in the early stages of a vortex's life, tip vortices generated by rotors do not have an axisymmetric structure, and the swirl velocity in the core decreases logarithmically as the wake age increases. Vortex models were validated by examining the swirl velocity structure of the tip of the vortex. Vortices within the vortex core exhibit a comparable self-similar velocity profile to the Iverson model's laminar profile, despite the transition flow outside the vortex core. With these measurements, the behavior of vortices in the rotor’s wake could be described with generalized semi-empirical models.

Particle orbiting constrained by elastic filament as a model cilium for fluid pumping

Many microorganisms use cilia to propel themselves in low Reynolds number ( $Re$ ) environments. In this work, we study the dynamics of a composite cilium consisting of an elastic filament and a spherical particle attached at the filament tip driven by an external time-periodic force acting on the particle. The elastic filament is modelled numerically using a slender body theory with hydrodynamic interactions. When tilted at a large angle from the normal direction of the wall, the filament buckles, and the induced velocity field by the cilium shows a large net flux. By varying the tilt angle or the force amplitude, the particle trajectory and the net flux display abrupt changes along with a reversal of the buckling direction. We further demonstrate through a segmental model that the abrupt changes arise from the deviation of the cilium orientation at the start of the recovery stroke from the natural orientation. Our results suggest a simple approach to engineering particle motions and designing artificial cilia for fluid pumping in low $Re$ environments.

The formation process of annular starting jets

The formation and evolution process of the outer vortex ring (denoted as OV0) in annular starting jets has been studied numerically at stroke ratio $3\leqslant L_m/D\leqslant 8$ , $\beta \leqslant 0.8$ and Reynolds number $400\leqslant Re_{\triangle R}\leqslant 1400$ , where $D$ is the outer diameter of orifice, $L_m$ is the length of the fluid column, $\beta$ and $\triangle R$ are the ratio and difference between inner and outer radii of the annular orifice, respectively. The initial formation process of OV0 is found to be similar to that of the leading vortex ring in a circular starting jet, but the circulation strength is relatively lower and decreases gradually with increasing $\beta$ . The pinch-off process will also appear after OV0 grows to a certain size but is different from that of the circular starting jet. On one hand, OV0 is stretched and deformed by the strain field generated by the trailing jet's instability, and vorticity would be shed. On the other hand, OV0 accelerates the irrotational fluids around it under viscous action so as to form the induced velocity field, which would consume energy causing it to shed vorticity further under the limit of the Kelvin–Benjamin variational principle. Furthermore, OV0 would eventually become a stable vortex ring whose non-dimensional circulation ( $\varGamma /(U_{0max}D$ )) is found to be between 0.59–0.62 and it would not change with different $L_m/D$ at $\beta =0.5$ , which is much less than the 2–2.5 of circular starting jets.

Spatial–temporal evolution of the pressure field generated by a plasma actuator in quiescent air

Dielectric barrier discharge plasma actuators, capable of generating quasi-steady wall jets, are well suited for flow control on various problems. Compared with the detail related to the induced velocity field, there are limited results available for the pressure field created by the plasma actuator. However, the profound merits of understanding the evolution of the pressure field are to reveal the controlling mechanism. Here, the time-averaged and the phase-averaged pressure field distributions are obtained by using a pressure reconstruction method based on the velocity field from particle image velocimetry experiments. According to the discharge regimes, the formation mechanism of the pressure field is discussed. During the streamer discharge stage, the pressure close to the upper electrode is decreased under the influence of the induced heating caused by the high-frequency and high-amplitude pulsed current, leading to the air above the plasma actuator being drawn toward the wall surface. During the glow discharge stage, under the effect of suction generated by the streamer discharge, the pressure near the wall is increased and the plasma actuator generates a favorable pressure gradient, which provides advantageous conditions for the airflow acceleration. During the discharge quenching stage, the effect of the plasma actuator vanishes and the influence of viscous force is strengthened. Therefore, the adverse pressure gradient is gradually formed and the velocity of the wall jet is decreased compared to that of the glow discharge stage. The change of pressure field in a period can be summarized into three processes: pressurization, pressure release, and pressure recovery.

A Fast Analysis of Pesticide Spray Dispersion by an Agricultural Aircraft Very near the Ground

This study provides a fast analysis of pesticide spray trails and dispersion influenced by crosswind, headwind, velocity field and the wake of an agricultural aircraft approaching the ground, to improve operational efficiency and reduce environmental impact. The lifting line-wingtip vortices mixture model is proposed to calculate induced velocity field around a monoplane far to 190 wingspans downstream, and N-vortex system based on point vortex dynamics is applied to simulate vortex rebound and vortex motion on considering extreme ground effect. The droplet trajectories governed by wake vortices and their induced velocity field are therefore determined under the Lagrangian framework. According to the ground deposition of typical droplets, the Gaussian mixture model is employed to predict droplet drift or dispersal for the whole spectrum of droplets in the spanwise direction. The fast analysis is compared to AGDISP and computational fluid dynamics (CFD) simulation for Thrush 510G aircraft, which runs on a common personal computer (CPU 2 GHz, memory 2 GB) within 3.2 s, faster than AGDISP and CFD, and does not rely on swath width input needed by AGDISP. The results suggest crosswind speed and droplet size are two leading factors affecting the drift and ground deposition. To increase droplet size or reduce the portion of fine droplets, the pesticide drift can be suppressed in some degrees.

Heat transfer characteristics of plasma actuation in different boundary-layer flows

The coupling characteristics of the aerodynamic and thermal effects of a surface dielectric barrier discharge plasma actuator and its transfer characteristics in different boundary-layer flows are studied experimentally. The actuator is attached to the surface of a flat-plate airfoil and driving by an alternative-current signal. Different boundary-layer flows are achieved in the wind tunnel by adjusting the airfoil's angle of attack with a Reynolds number of 2.02 × 105. The spatial temperature-rise distributions and velocity fields induced by plasma actuation in quiescent air show that the influence range of temperature is consistent with that of the induced velocity field. The aerodynamic and thermal effects induced by plasma actuation have strong coupling characteristics. The heat around the actuator is limited within the boundary-layer flows with a 15 m/s incoming flow. The temperature rise outside the boundary layer is close to zero. In the turbulent boundary-layer flow, the temperature is lower than that in the laminar boundary-layer flow as a whole. The maximum temperature-rise difference exceeds 10 °C. In the leading-edge separation-bubble flow, most heat generated by the plasma actuation is restricted inside the separation bubble. The results provide references for the mechanism detection of related plasma icing-control and flow-control research.

Prediction of steady hydrodynamic performance of pump jet propulsor considering vessel body influence

The actual working environment of a pump jet propulsor is formed under the joint action of the vessel and thruster. The disturbance created by the pump jet propulsor changes the flow field of the vessel body. And this changing flow field environment in turn affects the hydrodynamic performance of the pump jet propulsor. Therefore, a steady-state substitution relationship is formed between two systems. In this study, the vessel was regarded as a three-dimensional non-lifting body in an ideal fluid. A numerical model of the induced velocity field of the vessel by an arbitrary surface panel method was established. The resulting velocity field was combined with a numerical model for pump jet propulsor performance prediction, which accounts for the interaction between the vessel and the pump jet propulsor according to induced velocity. A method for the prediction of hydrodynamic performance of pump jet propulsor is thereby realised considering vessel body influence.

Analytical solutions for nonionic and ionic diffusio-osmotic transport at soft and porous interfaces

In this paper, we develop one of the first models for closed-form fully analytical solutions for describing the nonionic and ionic diffusio-osmotic (DOS) transport at interfaces grafted with a soft and porous polymeric film in the presence of a neutral solute concentration gradient (for nonionic diffusio-osmosis) and a salt concentration gradient (for ionic diffusio-osmosis). The nonionic DOS velocity depends on this solute concentration gradient and the drag force from the polymeric film. The ionic DOS transport is characterized by the diffusio-osmotically induced electric field and the diffusio-osmotically induced velocity field. This induced electric field is primarily dictated by the conduction of the mobile ion imbalance present within the electric double layer, induced at the charged solid, in the presence of the applied salt concentration gradient. The DOS velocity, on the other hand, is driven by a combination of the induced pressure gradient and an induced electro-osmotic body force (triggered by this induced electric field) and is opposed by the drag from the polymer layer. The result is a velocity field whose magnitude increases rapidly at near wall locations, decreases away from the wall, and depending on the salt concentration, may or may not increase outside the polymeric layer. This unique velocity profile ensures the presence of significant hydrodynamic shear stress across a wide zone extending from the wall in a non-confined fluidic system: This will ensure that finite-sized species (e.g., biological cells) can be conveniently made to access locations of large hydrodynamic stresses for a myriad of engineering and biological applications.

Comparative Study of Coaxial Main Rotor Aerodynamics at Forward Flight Based on Free Wake Model and Unsteady ReynoldsAveraged Navier–Stokes Method

This paper is dedicated to the numerical modeling of the aerodynamic characteristics of the full-scale coaxial main rotor. The modes of forward flight in the range of values V = 30-60 m∙s –1 (108-216 km∙h –1) have been under consideration. The simulation has been performed with the usage of two computational fluid dynamics (CFD) approaches: the free wake model (FWM) developed by the authors and the unsteady Reynolds-averaged Navier –Stokes (URANS) equations method based on the Ansys Fluent software. The coefficients of rotor thrust and torque, vortex wake shapes and induced velocity fields have been obtained and analyzed. The FWM and the URANS modeling data match satisfactorily. The FWM demonstrates a significant advantage in computing time and resources costs with sufficient accuracy in resulting rotor’s basic aerodynamic characteristics. That’s why it seems appropriate to use the FWM for preliminary simulations, taking into account before using the high time and resource intensive the URANS method for comprehensive investigation of coaxial rotor aerodynamics at forward flight modes.

Characterization of flame front wrinkling in a highly pressure-charged spark ignition engine

Particle Image Velocimetry (PIV) technique has been employed to investigate turbulent flame propagation in a relatively quiescent, optically accessed, boosted, two-stroke spark ignition engine. Turbulence scales and flame structure have been characterized based on Fourier analysis of an independent stationary coordinate at elevated pressures. Analysis shows that turbulence, with an initial rms value of u', is modified by flames, and the strong induced velocity field ahead of it. As the flame grows, the persistence of larger scale structures as well as smaller scale wrinkling is apparent. Power spectral density (PSD) of flame wrinkling exhibit the same general trend under different operating conditions. These are in a good agreement with the previous data measured from other rigs (i.e. burners and bombs), which featured lower pressures. These functions have been normalized by terms of wrinkling parameters and other turbulence parameters. General PSD functions are developed. These can predict the flame wrinkling behavior at low and high pressures, in different experimental apparatuses.

Numerical Study of the Main Rotor Wake Structures and Induced Velocity Fields at the Tail Rotor Location When Flying Near the Ground

This paper considers the Mi-8 helicopter main rotor aerodynamics when flying near the infinite ground surface. The research is based on the free wake model developed by authors at Moscow Aviation Institute. The distance from the rotor's hub to the ground surface in the range of H = 6-16 m and the values of free stream (flight) velocity in the range of V = 0-15 m/s are considered. The results of the visualization for both rotor wake shapes and streamlines are obtained. The influence of the ground proximity on the rotor wake shape, including the formation of "supervortex" and "ground vortex" structures are analyzed. The induced velocity fields of the main rotor in the area of the tail rotor location for various azimuth positions relative to the main rotor axis are studied. The conclusion is made about the significant influence of the ground effect on the rotor wake structure and induced velocities field, including the area of the tail rotor location. Particularly, at slip flight with speed V = 10 m/s, when the distance to the ground surface H increases from 6 to 12 m, the value of the average induced velocity at the tail rotor plane is growing up to four times. The obtained data have allowed taking into account such effects of aerodynamic interference in the simplified mathematical model of the research flight simulator of JSC Helicopters Mil and Kamov.

Structure and stability of Joukowski's rotor wake model

Abstract

Comparative Study of Coaxial Main Rotor Aerodynamics in the Hover with the Usage of Two Methods of Computational Fluid Dynamics

The paper is focused on numerical modeling of the aerodynamic characteristics of a full-scale coaxial main rotor in hover. The simulation was performed using two approaches of computational fluid dynamics (CFD): the original free wake model (FWM) developed by the authors and the unsteady Reynolds-averaged Navier–Stokes (URANS) equations method based on the Ansys Fluent software. The structure of the rotor vortex wake, flow images, vorticity and induced velocity fields, total and distributed aerodynamic characteristics of the rotor, including the rotor performance and figure of merit diagrams, have been analyzed. A comparison between FWM/URANS based calculations and calculated/experimental data by other authors has been performed. A satisfactory match of these data confirms reliability of used methods when modeling the aerodynamic characteristics of coaxial rotors. The FMW demonstrates a significant advantage in speed and resources intensity when calculating the total aerodynamic characteristics of a coaxial rotor. The URANS method makes it possible to model with significant accuracy the effects associated with the blade vortex interactions and pressure distribution over the blade surface. Finally, conclusions about most effectiveness of joint application of considered methods in solving complex problems of coaxial rotor aerodynamics has been made.