Oblique Flow Conditions Research Articles

A Simplified Model for Propeller Thrust in Oblique Flow

New aircraft architectures are being proposed for unmanned aerial vehicles and air taxis, which include tilt-able motor and propellers. These propulsive units operate with a propeller axis at an angle oblique to the flight direction, and thus it is important to understand and model how thrust is produced by a propeller operating under these conditions. Propellers in oblique flow have been modeled using Blade Element Momentum Theory coupled to an inflow model, and the Vortex Lattice Method. In the present work, we develop a much simpler approach that neglects the crossflow component of the incoming air velocity. An advance ratio is developed based on the parallel inflow component, and is coupled to existing propeller data collected in axial flow conditions. The proposed model is evaluated using existing experimental data collected under oblique flow conditions, and is predicts thrust to within \(5\,\%\) of experimental values for most conditions. The greatest discrepancy between the model and experiments occurs in the pure crossflow case, which is of lesser importance in the application to unmanned aerial vehicles and air taxis.

Research on the hydrodynamic performance of propellers under oblique flow conditions

The phenomenon of oblique water inflow is widespread in the operation of thrusters, which will cause adverse effects on the hydrodynamic performance of thrusters. In order to investigate the hydrodynamic performance and stress variation of the propeller in oblique flow, based on computational fluid dynamics theory, this paper takes the DTMB4119 propeller as the research object and conducts numerical simulation research on the propeller in oblique flow by solving the RANS equation. By calculating the open water performance curve and surface pressure distribution of the propeller, the rationality of the numerical method and the meshing are verified. Considering the flow field information such as velocity, oblique flow angles, flow line distribution, and pressure distribution, the changes in hydrodynamic characteristics of the propeller are simulated and analyzed. The results show that the uneven distribution of pressure on the propeller blade surface increases with a decrease in the advance coefficient. The force pulsation amplitude of a single blade increases with an increase in oblique flow angles. With the increase in oblique flow angles, the thrust, torque, and efficiency of the propeller show different increasing trends. With the increase in propeller advance coefficient, propeller thrust and torque decrease gradually, and propeller efficiency increases gradually. By using mature propellers, this paper has more reference value for studying the flow field around the ship hull and the hydrodynamic performance of propellers in the process of ship maneuvering.

Effects of a nozzle on the propeller wake in an oblique flow using modal analysis

The effect of a nozzle on the wake dynamics of a four-bladed propeller operating in an oblique flow is investigated via modal decomposition and flow visualization of the results obtained from numerical simulations using delayed detached eddy simulations. The wake characteristics and destabilization mechanisms of a non-ducted propeller (NP) and ducted propeller (DP) in axisymmetric and oblique flow conditions are systematically analysed. The wake characteristics on the windward side are very different from those on the leeward side in an oblique flow, and the nozzle has a crucial role in mitigating the asymmetry and weakening the wake deflection. More destabilization mechanisms are present in an oblique flow than in an axisymmetric flow, including the asymmetric evolution and destabilization of the helixes on the windward and leeward sides of the NP wake, the interaction between the vortex shedding and the helixes in the DP leeward region, and the generation of a tube-shaped wake envelope around the nozzle and its rolling-up. Moreover, the effect of the nozzle on wake meandering is discussed based on modal analysis.

Numerical simulation of the wake dynamics of the pumpjet propulsor in oblique inflow

A numerical analysis based on detached eddy simulations is conducted to investigate vortex dynamics of a pre-swirl pumpjet propulsor (PJP) in oblique inflow. In this paper, the working conditions of PJP operating in axisymmetric flow and drift with two angles (10° and 20°) are considered. The effects of incidence α and propeller loading on the wake dynamics of PJP as well as the mechanism leading to its destabilization are discussed. The results show that high hydrodynamic efficiency loss is found for PJP operating in drift. In addition, a different “secondary vortex structure” caused by the duct is found for PJP in both axisymmetric and oblique flow conditions. The instability mechanism of tip vortices shows obvious asymmetry. On the leeward side, it is dominated by the interaction caused by the duct-induced vortex, while it is dominated by the secondary vortices on the windward side. Furthermore, the fluctuation frequency of tip vortex for PJP is characterized by the rotor blade-passing frequency and the stator blade-passing frequency. In addition, the hub rotation frequency is important in oblique flow conditions.

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.

Application of multi-axis force/torque sensor system for experimental study of small unmanned aerial vehicles propulsion systems – Preliminary results

The experimental evaluation of the performance of small propellers plays important role in the process of multicopter UAV design perfection. This paper presents a small UAV propeller test stand, based on a multi-axis Force/Torque sensor system. The system makes possible to measure the thrust, torque and the pitching moment or lateral force (when in oblique flow) of the tested propeller. The data processing of the experimental results is discussed. Results for static, axial flow and oblique flow conditions are presented. Some conclusions about the measurement methodology are drawn. The results verify that the test bench has good accuracy and is easy for operation.

An Annular Wing VTOL UAV: Flight Dynamics and Control

A vertical takeoff and landing, unmanned aerial vehicle is presented that features a quadrotor design for propulsion and attitude stabilization, and an annular wing that provides lift in forward flight. The annular wing enhances human safety by enshrouding the propeller blades. Both the annular wing and the propulsion units are fully characterized in forward flight via wind tunnel experiments. An autonomous control system is synthesized that is based on model inversion, and accounts for the aerodynamics of the wing. It also accounts for the dominant aerodynamics of the propellers in forward flight, specifically the thrust and rotor torques when subject to oblique flow conditions. The attitude controller employed is tilt-prioritized, as the aerodynamics are invariant to the twist angle of the vehicle. Outdoor experiments are performed, resulting in accurate tracking of the reference position trajectories at high speeds.

CFD analysis of the lateral loads of a propeller in oblique flow

Large propeller lateral loads may be generated under oblique flow condition, which is critical to the safety of stern tube bearing of a ship. In the present study, the effect of oblique flow on propeller lateral loads is numerically investigated based on URANS method, coupled with SST k-ω turbulent model and sliding mesh method. An 82, 000 DWT bulk carrier with a single right-handed screw is computed. Both open water and behind-hull conditions are simulated for a range of drift angles from −20 to 20°. The lateral loads in open water increase with the drift angle increasing in a slightly non-linear trend. However, the behind-hull results are quite different from that in open water. At behind-hull conditions, the horizontal loads monotonously, non-linearly increase as the drift angle increases, while the vertical loads peak at the −5 deg drift angle (corresponding to the transverse flow coming from the portside). In large drift angles, the behind-hull lateral loads reach a larger percentage of the axial loads compared with the open-water results. Scale effect is also investigated by simulating the behind-hull flows with three scales. It is revealed that the increase in scales causes a slight decrease on the horizontal loads independently of drift angles, while it causes a remarkable increase and decrease on the vertical components for positive and negative drift angles, respectively.

Propeller Hydrodynamic Characteristics in Oblique Flow by Unsteady Ranse Solver

Abstract Propellers may encounter oblique flow during operation in off-design conditions. Study of this issue is important from the design and ship performance points of view. On the other hand, a propeller operating in oblique flow may sometimes result in a better propulsion efficiency. The main goal of the present study is to provide an insight on the propeller characteristics in the oblique flow condition. In this research, the performance of the DTMB 4419 propeller is studied by the numerical method based on solving Reynolds Averaged Navier–Stokes (RANS) equations in several inflow angles. The sliding mesh approach is used to model the rotary motion of the propeller. Initially, the numerical method is verified by grid and time step dependency analysis at various inflow angles. Additionally, computed results at zero inflow angle are compared with the available experimental data and good agreement is achieved. Finally, the forces and moments acting on the propeller are obtained for 0° to 30° inflow angles. It is concluded that the inflow angle up to 10° has no significant influence on the thrust and torque coefficients as well as the propeller efficiency. However, at high angles up to 30°, the thrust and torque coefficients increase as the inflow angle increases, which may result in a significant improvement of propeller efficiency.

Computationally Efficient Force and Moment Models for Propellers in UAV Forward Flight Applications

Two low-order, parametric models are developed for the forces and moments that a rotating propeller undergoes in forward flight. The models are derived using a first-principles-based approach, and are computationally efficient in the sense of being represented by explicit expressions. The parameters for the models can be identified either using supervised learning/grey-box fitting from labelled data, or can be predicted using only the static load coefficients (i.e., the hover thrust and torque coefficients). The second model is a multinomial model that is derived by means of a Taylor series expansion of the first model, and can be viewed as a lower-order lumped parameter model. The models and parameter generation methods are experimentally tested against 19 propellers tested in a wind tunnel under oblique flow conditions, for which the data is made available. The models are tested against 181 additional propellers from existing datasets.

Hydrodynamic loads and wake dynamics of ducted propeller in oblique flow conditions

ABSTRACT The hydrodynamic loads and wake dynamics of the ducted propeller in oblique flow are investigated by detached eddy simulation (DES), with particular emphasis on the characteristics of wake fields and kinetic energy (KE) spectra. The influence of the duct, together with the strengthened induction of the propeller, cause the non-uniform velocity distributions in the inflow plane, resulting in unstable and periodic hydrodynamic loads on the blades. Shedding vortices from the leading edge of the duct dominate the leeward wake, and strengthen the self- and mutual induction of tip vortices. Large deformation of the tip vortices is observed due to the complex interference of the shear-layer vortices, shedding vortices and secondary vortices. The trajectories of the tip vortices are found distorted but still recognisable on the windward wake. The spiral-to-spiral distances of tip vortices increase and the re-alignment effect of the main flow enhances in the downstream wake.

Performance of counter-rotating tandem propellers at oblique flow conditions

Tidal currents are a potential source of reliable and renewable energy. Various power generation systems have been proposed. The authors developed a counter-rotating type tidal stream power unit in which the tandem runners make the inner and the outer armatures counter-rotate. The unit is composed of tandem propellers with 3 front and 5 rear blades and has a promising advantage that the rotational torque between the front runner and the rear propellers, namely the inner and the outer armatures, is balanced in the unit. That is, this tidal stream power unit can be moored with only one cable and then its nose and tail will naturally line up in response to the tidal stream by yawing. In this paper, the performance of the propellers in oblique flow conditions were investigated experimentally and numerically. When the coming flow obliquely to the propeller by 30 degrees, the performance deteriorates by 20%.

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.

Numerical Analysis of Azimuth Propulsor Performance in Seaways: Influence of Oblique Inflow and Free Surface

In the present work, a generic ducted azimuth propulsor, which are frequently installed on a wide range of vessels, is subject to numerical investigation with the primary focus on performance deterioration and dynamic loads arising from the influence of oblique inflow and the presence of free surface. An unsteady Reynolds-Averaged Navier-Stokes (RANS) method with the interface Sliding Mesh technique is employed to resolve interaction between the propulsor components. The VOF formulation is used to resolve the presence of free surface. Numerical simulations are performed, separately, in single-phase fluid to address the influence of oblique inflow on the characteristics of a propulsor operating in free-sailing, trawling and bollard conditions, and in multi-phase flow to address the influence of propulsor submergence. Detailed comparisons with experimental data are presented for the case of a propulsor in oblique flow conditions, including integral propulsor characteristics, loads on propulsor components and single blade loads. The results of the study illustrate the differences in propulsor performance at positive and negative heading angles, reveal the frequencies of dynamic load peaks, and provide quantification of thrust losses due to the effect of a free surface without waves. The mechanisms of ventilation inception found at different propulsor loading conditions are discussed.

Modeling Techniques of Puller Podded Propulsor in Extreme Conditions

This article presents the outcome of a numerical simulation-based research to evaluate the propulsive characteristics of a puller podded propulsor in extreme azimuthing and at multiple initial loading conditions. A Reynolds-Averaged Navier-Stokes (RANS)-based computational fluid dynamics (CFD) code is used to model and predict the propulsive characteristics of the podded propulsor at extreme oblique flow conditions in which highly unsteady, fully 3D, and separated flows exist. The forces and moments of the podded propulsor in the three coordinate directions as well as the thrust and torque of the propeller in static azimuthing conditions in the range of -180°–180° at advance coefficients of 0.20 and 0.8 are predicted and compared with a corresponding dataset obtained in a previous model testing campaign. The predictions of the performance characteristics of the pod unit were within 1–5% of the corresponding measurements, which also captured the nonlinear trends for all the loading conditions, and azimuthing angles. In addition, the velocity and pressure distributions around the podded propulsor as derived from the RANS predictions reveal the highly separated and 3D flow and complex interaction between the propeller and pod-strut bodies, which was more evident in the extreme azimuthing and moderate to light propeller loading conditions. This study demonstrates that the RANS-based CFD code, with proper meshing arrangement, boundary conditions, and setup techniques, can predict the performance characteristics of the podded propulsor in extreme azimuthing and in various loaded conditions with the same level of accuracy of the measurements. 1. Introduction Podded propulsors have the potential to become a popular main propulsion system because they allow more flexibility in the design of the internal arrangement of a ship, potentially reduce noise and vibration, and increase maneuverability (Pakaste et al. 1999; Carlton 2002). The propeller, pod, and strut are the three main components of a podded propulsor. There are multiple propeller-pod-strut configurations that are available commercially, namely the puller type, pusher type, duel-end type, ducted type, and contrarotating type configurations. One of the popular configurations of the podded systems is the puller type, in which the propeller is located upstream of the pod and strut (Islam 2009).

Analysis of the performances of a marine propeller operating in oblique flow

The present work is aimed to assess the capability of a numerical code based on the solution of the Reynolds averaged Navier–Stokes equations for the study of propeller functioning in off design conditions; this aspect is becoming of central interest in naval hydrodynamics research because of its crucial implications on design aspects and performance analysis of the vessel during its operational life. A marine propeller working in oblique flow conditions is numerically simulated by the unsteady Reynolds averaged Navier–Stokes equations (uRaNSe) and a dynamically overlapping grid approach. The test case considered is the CNR-INSEAN E779A propeller model. Two different loading conditions have been analyzed at different incidence angles (10–30°) in order to characterize the propeller performance during idealized off-design conditions, similar to those experienced during a tight manoeuvre. The main focus is on hydrodynamic loads (forces and moments) that act on a single blade, on the hub and on the complete propeller; peculiar characteristics of pressure distribution on the blade and downstream wake will be presented as well. Verification of the numerical computations have been assessed by grid convergence analysis.

Experimental and Theoretical Analysis of Propeller Shaft Loads in Oblique Inflow

A series of model tests on an azimuth thruster model in oblique inflow conditions were performed for different heading angles and at different advance coefficients in pushing and pulling modes. Tests were performed in ventilating and non-ventilating conditions. A novel shaft dynamometer was used to measure all six component forces and moments on the propeller shaft. It was found that the propeller shaft lateral force and bending moment were quite large, and thus, the load at the shaft-bearing positions was about three times larger than when only the propeller weight was considered. The results also showed that oblique inflow due to steering gives higher bending loads than when the propeller is subject to ventilation in the straight-ahead condition. A basic blade element momentum method (BEMT) was used to predict the forces and moments on the propeller shaft in oblique flow conditions. Fairly good agreement was found between the BEMT results and the experimental results.

Flow dynamics over a foredune at Prince Edward Island, Canada

Time-averaged windspeed profiles at eight locations over a relatively high (8 m), vegetated, topographically simple foredune were measured using cup and ultrasonic anemometry during an onshore wind event in May 2002. The experiment was part of a larger study on the sedimentary dynamics of a beach–dune complex in the Greenwich Dunes, Prince Edward Island (PEI), Canada. The foredune is vegetated with Ammophila breviligulata, and vegetation density ranged from 0% on the foredune ramp to upward of 70% at the dune crest. Topographic forcing and resulting flow acceleration was observed distinctly in windspeed profiles, and linear speedup occurred for all elevations above the vegetation canopy. A distinct inflection point is evident in normalized windspeed profiles over the dune indicating a net momentum sink below the vegetation canopy top, and systematic speed-down within the vegetation was observed up the foredune stoss slope. Wind speed profile structure was consistent for the range of incident wind speeds measured. A comparison of normal and slightly oblique incident winds indicated that for similar approach winds, wind speeds are consistently higher during oblique flow conditions. The implications for foredune morphodynamics and sedimentation are discussed.

An empirical model of aeolian dune lee‐face airflow

ABSTRACTAirflow data, gathered over dunes ranging from 60‐m tall complex‐crescentic dunes to 2‐m tall simplecrescentic dunes, were used to develop an empirical model of dune lee‐face airflow for straight‐crested dunes. The nature of lee‐face flow varies and was found to be controlled by the interaction of at least three factors (dune shape, the incidence angle between the primary wind direction and the dune brinkline and atmospheric thermal stability). Three types of lee‐face flow (separated, attached and deflected along slope, or attached and undeflected) were found to occur. Separated flows, characterized by a zone of low‐speed (0–3O% of crestal speed) back‐eddy flow, typically occur leeward of steep‐sided dunes in transverse flow conditions. Unstable atmospheric thermal stability also favours flow separation. Attached flows, characterized by higher flow speeds (up to 84% of crestal speed) that are a cosine function of the incidence angle, typically occur leeward of dunes that have a lower average lee slope and are subject to oblique flow conditions. Depending on the slope of the lee face, attached flow may be either deflected along slope (lee slopes greater than about 20°), or have the same direction as the primary flow (lee slopes less than about 20°). Neutral atmospheric thermal stability also favours flow attachment.As each of the three types of lee‐face flow is defined by a range of wind speeds and directions, the nature of lee‐face flow is intimately tied to the type of aeolian depositional process (i.e. wind ripple or superimposed dune migration, grainflow, or grainfall) that occurs on the lee slope and the resulting pattern of dune deposits. Therefore, the model presented in this paper can be used to enhance the interpretation of palaeowind regime and dune type from aeolian cross‐strata.

Investigation into the Propeller Cavitation in Oblique Flow

In order to serve the improvement of performance prediction of high-speed vessels, theoretical and experimental investigations into the propeller cavitation in oblique flow condition have been carried out.A series of supercavitating propellers were tested in cavitating oblique flow condition. The test results show that in non-cavitating condition the variation of propeller characteristics due to the shaft inclination is related closely with pitch ratio but little with expanded area ratio, while in cavitating condition the variation decreases with cavitation number and the effect of geometrical particulars of propeller is not so clear as in non-cavitating condition.In view of the analysis and the application of the test results, a quasi-steady method was developed to calculate the propeller characteristics in oblique flow condition from those in axial flow condition. The comparision of the experimental and the calculated results showed pretty good coincidence.