Oblique Flow Research Articles

Quantitative time-of-flight MR angiography for simultaneous luminal and hemodynamic evaluation of the intracranial arteries.

To report a quantitative time-of-flight (qTOF) MRA technique for simultaneous luminal and hemodynamic evaluation of the intracranial arteries. Implemented using a thin overlapping slab 3D stack-of-stars based 3-echo FLASH readout, qTOF was tested in a flow phantom and for imaging the intracranial arteries of 10 human subjects at 3 Tesla. Display of the intracranial arteries with qTOF was compared to resolution-matched and scan time-matched standard Cartesian 3D time-of-flight (TOF) MRA, whereas quantification of mean blood flow velocity with qTOF, done using a computer vision-based inter-echo image analysis procedure, was compared to 3D phase contrast MRA. Arterial-to-background contrast-to-noise ratio was measured, and intraclass correlation coefficient was used to evaluate agreement of flow velocities. For resolution-matched protocols of similar scan time, qTOF portrayed the intracranial arteries with good morphological correlation with standard Cartesian TOF, and both techniques provided superior contrast-to-noise ratio and arterial delineation compared to phase contrast (20.6 ± 3.0 and 37.8 ± 8.7 vs. 11.5 ± 2.2, P < .001, both comparisons). With respect to phase contrast, qTOF showed excellent agreement for measuring mean flow velocity in the flow phantom (intraclass correlation coefficient = 0.981, P < .001) and good agreement in the intracranial arteries (intraclass correlation coefficient = 0.700, P < .001). Stack-of-stars data sampling used with qTOF eliminated oblique in-plane flow misregistration artifacts that were seen with standard Cartesian TOF. qTOF is a new 3D MRA technique for simultaneous luminal and hemodynamic evaluation of the intracranial arteries that provides significantly greater contrast-to-noise ratio efficiency than phase contrast and eliminates misregistration artifacts from oblique in-plane blood flow that occur with standard 3D TOF.

System Identification for Propellers at High Incidence Angles

Propellers used for electric vertical takeoff and landing (eVTOL) aircraft propulsion systems experience a wide range of aerodynamic conditions, including large incidence angles relative to oncoming airflow. In oblique flow, propellers exhibit deviations in thrust and torque oriented along the propeller axis of rotation, as well as significant off-axis forces and moments. Although important for modeling eVTOL aircraft aerodynamics, sparse experimental data or mathematical models exist for propellers at incidence. This paper describes a propulsion system modeling methodology for the LA-8 tandem tilt-wing, eVTOL aircraft. System identification methods are applied to isolated propeller wind-tunnel data gathered across the vehicle’s flight envelope to develop a mathematical model of the propulsion system, including a static motor model, dynamic motor model, and propeller aerodynamic model. Modeling results validated against data withheld from the modeling process indicate good predictive capability and agree with theoretical expectations. The results are followed by a discussion of model implementation strategies into high-fidelity eVTOL aircraft simulations.

The formation and development of oblique detonation wave with different chemical reaction models

In this paper, we investigate the formation and development characteristics of oblique detonation wave in stoichiometric hydrogen-oxygen mixtures diluted with argon using Euler equations coupled with the 19-step or 34-step chemical reaction model. The novelty of this work lies in the study of differences in the characteristics of the initiation region and the formation processes of oblique detonation by different chemical kinetic processes. The formation processes of radicals in the induction region and heat release region for the oblique detonation flow field are emphatically analyzed, and the differences in the sensitivity on the key chemical reaction channels to radicals are also discussed. The results show that under the action of 19-step mechanism, the oblique detonation forms relatively early, the wave morphology of the flow field is complex, and the type of the initiation region is abrupt transition. While for the 34-step mechanism, the structure of flow field is simple with smooth transition type. The existence of transverse wave in the mainstream region contributes a lot to the formation of a sonic region and the abrupt transition structure. The chemical kinetic process directly affects the flow field structure of oblique detonation, and H2O2 plays an important role on triggering and accelerating the exothermic process. The radicals of H, O and H2O2 have different properties in different chemical reaction models.

Experimental investigation of single blade loads by captive model tests in pure oblique flow. Part II: Propeller in-plane loads and preliminary comparison of single blade loads during transient phases

Off–design and realistic operative conditions of the propulsion system have been progressively considered a frontier topic for the development of novel and successful design procedures. The availability of high accurate data on propeller performance, both experimental or numerical, is pivotal for the development of reliable tools and the enhancement of traditional ones. In this context, this paper continues the investigation described in (Ortolani et al., 2020), that was dedicated to analyze the single blade loads obtained by oblique towing tests and assess their consistency with those measured during the steady turning phase, reproduced by free running model tests. In this paper, the same analysis is broadened to two aspects. At first, the in–plane forces and moments that are obtained by a conversion from the rotating frame of the measure to the fixed frame are discussed. These loads need quantification, because they are the primary cause of damages of the shafting structure, vibratory loads and also contribute to dynamic response of the ship. Then, the study is switched to the comparison of single blade loads during transient motions of the turning maneuvers, at weak and tight rudder angles, performed at the same reference speed as the captive model test. The analysis lays the basis for the enhancement of ship system (comprehensive) mathematical models, already used in ship design, to real–time analysis of vibratory loads and emitted noise.

CFD study of hull wakes in oblique flow at model and full scales

The characteristic of hull wake for a ship operating in oblique flow significantly affects the ship performance at off-design conditions. This paper presents a numerical study on the hull wakes under oblique flow conditions and analyzes the scale effect. RANS method combined with SST k-ω turbulence model is adopted and an international benchmark ship, Japan Bulk Carrier (JBC), is computed. Drift angles ranging from 0° to 20° with an interval of 5° are considered. Results show that the oblique flow considerably changes the hull wake field. Unlike a left-right symmetric pattern at zero drift angle, the nominal wake in oblique flow conditions shows a strong asymmetry since the flow at the windward side is close to the incoming flow whereas the flow at the leeward side is featured by the vortices detached from the hull. It is also revealed that the mean wake fraction decreases as the drift angle increases. Both model and full scales are simulated and the comparison between them shows significant differences in the wake pattern on the leeward side and the magnitude of the mean wake fraction. Compared with the model scale, the full scale has a stronger aft-body vortex and a smaller mean wake fraction.

Fish-like swimming in oblique flows: A numerical investigation

Although being a common situation for fish and underwater vehicles, limited research has been reported for a fish-like body swimming in oblique incoming flows. The present work employs the immersed boundary-lattice Boltzmann method to numerically study the dynamics of this problem. A more comprehensive analysis method, which utilizes the effective force parameters that consolidate the angle of attack (AOA) into the traditional force parameters, is proposed for investigating the dynamics of cases with non-zero AOA. Several distinct features in terms of the dynamic performance, the periodic features, and the flow field characteristics are presented. For the dynamic performance, undulation amplitude plays a more dominant role than the undulation form in thrust generation. The periodic features revealed by the Fourier spectra present increasing complexity with larger AOA. Besides, three flow regimes are identified, one of which observes hierarchical vortex structures with different orders in time and space. The small-scale vortex street generated by undulation is “local” in both time and space, and it is embedded in the large scale vortex street generated by the body and the free-stream.

Mix convection non- boundary layer flow of unsteady MHD oblique stagnation point flow of nanofluid

This article demonstrates the analysis of hydrodynamics in the existence of a magnetic field. Heat transfer is analyzed for the unsteady flow of non-Newtonian fluid. The thermal conductivity is enhanced by introducing nanoparticles in the non-Newtonian fluid. Magnetohydrodynamics is introduced to control the turbulence in the flow. The dimensional equations satisfying the stated assumptions of the fluid flow and the motion of nanoparticles are presented. Fourier law is incorporated in the evaluation of heat flux. The analysis is tested in the fixed frame of reference. The mathematical coupled equations of motion are solved numerically. It is concluded that the Casson parameter leads to enhance velocity profile whereas the temperature profile is declined.

Thermal analysis of oblique stagnation point flow with slippage on second-order fluid

It is well-established fact that thermal resistance models are highly effective passive devices to transfer large quantities of heat for predicting the thermal performance. In the present investigation, we analyzed the thermal analysis of an unsteady oblique stagnation point flow of an incompressible second-grade fluid on a stretching surface with some slip effects. The governing equations of the model under consideration are presented. The governing PDEs are altered into nonlinear ODEs by utilizing non-similar and similar variables and then solved numerically. The analysis further reveals that these solutions sustain in a definite domain of corresponding parameters. Moreover, the variations in temperature and velocity are presented in graphical form to show the influence of controlling parameters. The numerical details of the heat transfer rate for the several thermophysical parameters and skin friction are illustrated in tabular form. The increment in the local second-grade parameter causes the Sherwood number to decrease. The value of the Nusselt number enhances if we decrease the value of the local second-grade parameter.

Characteristics of the oblique detonation flow field induced by a complex wave structure

In this paper, the initiation characteristics of the oblique detonation flow field induced by single- and double-wedge surfaces of finite length in a confined space are investigated. Numerical simulations with a detailed H2/air reaction and theoretical shock polar analyses are combined to study the influence mechanism of a complex wave system structure on the characteristics of the oblique detonation. The effects of expansion waves on the oblique detonation waves (ODWs) and their flow field characteristics for different equivalent ratios and geometric sizes are analyzed in single-wedge and double-wedge structures with the same inflow parameters. The results show that the length of the induced ODW is shorter in the double-wedge structure than in the single-wedge structure. For the single-wedge structure, the strength of the expansion wave increases, the wall temperature drops, and the characteristic length of the induction zone decreases with increasing deflection angle of the second wedge. If the strength of the expansion wave is sufficiently large, the ODW is initiated. For the double-wedge structure, the ODWs interact and form a complex wave system structure, consisting of a Mach stem, two reflected detonation waves and slip lines. The length and the temperature before and after the Mach stem decrease with an increase in the strength of the expansion waves. The effects of the expansion waves on the flow field of the ODW are relatively small at a large equivalent ratio and significantly larger at a small equivalent ratio.

Analysis of Pollutant Entrainment from Localized Sources in a Street Network

The propagation of a pollutant emitted from localized sources both within and above a regular street network is studied by analyzing data from direct numerical simulations of passive scalar dispersion. Two wind directions are considered, corresponding to aligned and oblique flow with respect to the street axes. Particular attention is paid to the role of entrainment of the scalar into the urban canopy from an elevated source and re-entrainment of material originally released further upstream from a ground source. The variation of concentration differences and vertical fluxes between the streets and the air above as a function of distance reveals important differences between the rate of lateral and vertical mixing for the two sources. Detrainment and entrainment need a longer fetch to equilibrate for the elevated source than for the ground source. There are large differences between the advection and detrainment velocities for the aligned and oblique cases, so that a change in wind direction could affect ventilation efficiency considerably. Time scales associated with different dispersion processes are computed and the time of first appearance of the scalar from the onset of release in different streets is mapped. It is shown that re-entrainment can provide a shortcut dispersion pathway for reaching certain parts of the network. This is particularly striking in the case of oblique flow, when material can be transferred by entrainment up to twice as rapidly as by advection. Taken together, these results highlight the overall message that vertical exchange is a two-way process and that entrainment needs to be considered in the context of emergency response as well as urban ventilation.

Simulation of turbulent effective wakes for propellers in off-design conditions by a correction factor approach

This paper presents a procedure for the estimation of propeller effective wakes in oblique flows. It shows how a recently developed method for controlling coupling errors can be applied to analyze propellers operating in off-design conditions. The approach allows the use of fast potential flow methods for the representation of the propeller in the context of viscous flow solvers and works accurately for a wide range of advance numbers and incidence angles with a minimum computational cost. The new method makes it possible to disclose flow phenomena on the effective wake that were hidden in conventional approaches of effective wake simulation. Different application cases are analyzed, such as a propeller-shaft configuration in inclined flow, a pod propulsor in an oblique inflow, and a ship hull advancing at a yaw angle. A dipole-like distortion on the effective wake is unmasked for a uniform flow incident to a propeller mounted on an inclined shaft. The flow component perpendicular to the axis is found to be responsible for the distortion. The effect of the direction of propeller rotation on the effective wake is illustrated for a single-shaft ship moving at a yaw angle. In particular, keel vortices are either attracted to or repelled from the propeller disk depending on the sign of the yaw angle or alternatively on that of the propeller rotation.

Dynamic modeling, simulation, and parameter study of electric quadrotor system of Quad-Plane UAV in wind disturbance environment

Quad-Plane UAV(quadrotor fixed-wing hybrid vertical take-off and landing Unmanned Aerial Vehicle) is vulnerable to wind disturbance in quadrotor mode, and the performance of the electric quadrotor system directly affects its wind disturbance rejection performance. To study and optimize the wind disturbance rejection performance of the electric quadrotor system of Quad-Plane precisely and efficiently, a dynamic electric quadrotor simulation system integrating high-precision submodels of electric quadrotor system components is required. However, there are few papers on this kind of simulation system. This paper proposed a simulation system with both high computational efficiency and precision, and it can be used in optimization and flight simulation of Quad-Plane. The simulation system includes submodels of rotor, brushless direct current (BLDC) motor, electronic speed controller (ESC), and Li-ion Polymer battery. The surrogate-based rotor model is established to calculate the aerodynamic performance of the rotor in oblique flow. The BLDC motor performance calculation model considering inductance is presented and the power loss of the ESC is considered. The discharge characteristics of the battery are modeled considering the rate capacity effect. All submodels are validated and the simulation results show good agreements with test data. A flight test of Quad-Plane in quadrotor mode is conducted to validate the integrated dynamic simulation system. The results show that the simulation system has the advantages of high computational efficiency and precision. Based on the simulation system, the influence of the electric quadrotor system parameters on the performance of the electric quadrotor system, and furthermore on the wind disturbance rejection performance of the Quad-Plane are found, the conclusions are helpful to the selection and optimization of the electric quadrotor system components.

On homogeneous-heterogeneous reactions in oblique stagnation-point flow of Jeffrey fluid involving Cattaneo-Christov heat flux

The existing investigation highlights oblique Jeffrey fluid with mixed convection on a stretched surface. Also chemical reactions with properties of homogeneity and heterogeneity are considered. The leading physical model is converted in a non-linear system of ODE by means of proper similarity alterations. Influence of all relative physical constraints on velocity, temperature as well as on concentration are expressed geometrically. Physical magnitudes of interest like friction measurements, thermal and concentration transport rates of chemical spices at the surface are studied numerically.

Finite difference simulation for oblique stagnation point flow of viscous nanofluid towards a stretching cylinder

This study describes the effects of hydrodynamic, thermal and solutal slips on the flow of viscous nanofluid impinging obliquely on the surface of stretching cylinder. An appropriate coordinate decomposition converts the governing non-linear partial differential equations into system of ordinary differential equations along with boundary conditions. An implicit finite difference scheme is used to approximate the governing system for particular values of the involved parameters. Behaviors of various involved parameters on velocity, temperature and concentration profiles are graphically presented and discussed. The numerical results for both orthogonal and non-orthogonal flow are computed and compared with the existing literature for stretching sheet case as well as for stretching cylinder. It is observed that shear velocity of nanofluid decreases by increasing velocity ratio parameter and increases by increasing curvature of cylinder, also the position of the stagnation point for flat plate and cylinder is rapidly change with increase in slip.

A numerical simulation of mixed convective and arbitrarily oblique radiative stagnation point slip flow of a CNT-water MHD nanofluid

Numerical simulation of a non-linear mathematical model governing an arbitrarily oblique slip flow of a nanofluid, with suspended carbon nanotubes in water, agitated by magnetic field, mixed convection currents and thermal radiation is carried out. The cases for both single-wall carbon nanotubes (SWCNTs) and multi-wall carbon nanotubes (MWCNTs) are taken under consideration to gain a wider perspective. Globally, heat transfer rate for SWCNTs is found to be more effective in comparison to the MWCNTs, as the former offers less resistance in terms of the overall drag or shear. In addition, it is found that the magnetic field increases the skin friction by increasing the obliqueness of the flow. Numerical validation with available studies in the literature shows excellent conformity. The results will have bearing in thermal management of the heat exchange processes occurring within the devices used in polymer industries and polymer technology.

Determining the acceptable turning circle manoeuvring modes to avoid loss of efficiency of the twin propeller system

Nowadays, a large number of newly built inland and navy vessels are equipped with a twin propeller-twin rudder configuration (TPTR). Observations of the modes of work of each unit of the TPTR system when performing manoeuvres at curvilinear trajectory show asymmetrical loading. The nature and parameters of the phenomenon are not sufficiently studied, which in specific manoeuvres could create overloading and loss of effectiveness. The purpose of the task is to study the effectiveness of the ship’s TP system in manoeuvring, taking into account the specifics of working in oblique flow of the “internal” and “external” propellers, respectively to the trajectory of motion. Appropriate manoeuvring experiments with self-running ship model have been carried out by which the propeller thrust and torque have been measured. In the present paper, based on obtained investigation results, analysis of interaction effects in the TP system has been performed. Coefficients for estimating the asymmetric efficiency of the twin-screw system have been developed, and related conclusions were summarized.

Progress in multi-wave structure and stability of oblique detonations

Oblique detonation is an important direction in gaseous detonation physics and has great potential in the application of new-concept aeronautic and astronautic propulsion. As the fast combustion induced by shock, the oblique detonation wave could be simplified into a discontinuity with energy input. However, in oblique detonation flow, there concern several complicated fluid phenomena, such as shock wave and turbulence, which are coupled with the heat release and result in complicated flow and combustion mechanisms. A theoretical investigation is hard to be performed due to the characteristics of multi-scale and nonlinearity. Meanwhile, experimental investigation encounters the difficulties from measuring the flow fields of high temperature, high pressure, and high velocity. In the last two decades, the main progress of oblique detonation is achieved by numerical investigation through comprehensive simulation and analysis. First of all, the multi-wave structure of the initiation region and surface stability are introduced in the ideal inflow conditions. Then, derived from the application in engines, the effects of inflow inhomogeneity and interaction with expansion waves are studied and analyzed. Finally, some suggestions on future work are proposed and discussed.

Numerical simulation on the hydrodynamic performance of an azimuthing pushing podded propulsor in reverse flow and rotation

In this study, the hydrodynamic performance of a reverse podded propulsor under azimuthing conditions is evaluated. The shear stress transfer (SST) k-omega turbulence model and sliding mesh technique are adopted for this evaluation, and the commercially available software, STAR-CCM+ is used for the numerical simulation. An extensive convergence study was performed with respect to time step and grid size to verify the numerical simulation. The hydrodynamic performance is analyzed under different advance velocities as the propulsor sinusoidally azimuths from 130° to 230°. The forces and moments generated by the propulsor in steady oblique flow are compared with those under azimuthing condition. It is found that the forces and axial moments of the propulsor in oblique flow generally varies in between the envelopes of the transient ones under an azimuthing state, while the vertical and horizontal moments are observed too strongly deviate from one another.

Numerical simulation of thermal-solutal Marangoni convection in a shallow rectangular cavity with mutually perpendicular temperature and concentration gradients

A series of three-dimensional numerical simulations have been carried out to examine the characteristics of thermal-solutal Marangoni convection in a rectangular cavity that is subjected to mutually perpendicular temperature and concentration gradients. In the simulations, the thermal Marangoni number MaT is selected as 0, 1, 3, and 7 × 104, but the solutal Marangoni number MaC is varied in order to be able to investigate the complex flow patterns and flow transitions. Results show that the flow is steady at relatively small MaC. Then, at this MaC value, we observe three types of steady flows as MaT increases, namely, a longitudinal surface flow, an oblique stripe flow, and a lateral surface flow. When MaC exceeds a critical value, the stability of the Marangoni flow is destroyed, and a three-dimensional oscillatory flow appears. For the oscillatory flow, the wave patterns of temperature and concentration fluctuations are highly dependent on the coupling of the thermal and solutal Marangoni effect. Two different propagation directions of wave patterns coexist on the free surface when the contributions of thermal and solutal flows are in the same order (i.e., MaC is approximately equal to MaT). In addition, a sudden drop in the wave frequency and a backward transition phenomenon from chaotic to oscillatory are also observed. For all the cases of the thermal Marangoni numbers, thermal-solutal Marangoni convection becomes chaotic at higher MaC values. The present study would provide more physical insights into industrial processes such as painting and drying.

Breaking of Internal Kelvin Waves Shoaling on a Slope

AbstractIn stratified flow, breaking of internal waves over slopes induces resuspension of bottom sediments and transport of mass. When internal waves shoal and break, flow dynamics and mass transport differ significantly according to whether the Coriolis force is included or neglected. Despite its importance, the currents generated by breaking internal Kelvin waves remain uninvestigated. Therefore, this study considers breaking of internal waves over a uniform slope under Coriolis with equivalent upper‐ and lower‐layer depths. Laboratory experiments, using a 6.0‐m rotating tank, were undertaken to visualize currents using particle image velocimetry. Experimental data validated a three‐dimensional fluid dynamics model, in which a phase‐averaged velocity (residual jet) was simulated to occur at the lateral wall (to the right) of the progressive internal Kelvin waves in the breaking zone, with the generation of an oblique downslope return flow (downdraft) under Coriolis. The geostrophic balance drove the residual jet, and the equation for estimating the residual current, due to the jet, was formulated and was discussed by referring a coastal jet in Lake Erie. The results provide insight on mass transport in lakeshore and coastal zones.