Three-dimensional Navier-Stokes Computations Research Articles

Performance analysis of a transonic high-pressure turbine

Efficiency is a crucial design parameter in any turbine development. This research presents a detailed investigation on the efficiency of a modern transonic high-pressure turbine. The work focuses on the study of the efficiency loss at different stage loading factors (ranging from Δ H/U2 = 1.3 to 2.7) and various flow factors ( Vax/ U = 0.41 to 0.90). In particular, the present research is of utmost importance to understand the effect of the vane trailing edge shocks in the turbine stage efficiency. The experimental work was carried out in a compression tube facility that allows testing of the turbine at temperature ratios, Re and Mach numbers, encountered in real engines. The efficiency is measured by the mechanical method. Experiments were performed with two different vanes (cooled and uncooled) at two stagger angles, four rotational speeds (4570–6500 r/min) and three pressure ratios ( p01/ Ps3 ranging from 2.42 to 5.12). The effect of the change of reaction and rotor incidence is correlated with the performance. Three-dimensional Navier-Stokes calculations aid the interpretation of the results.

Computational Investigation and Design Optimization of a Lateral-Jet-Controlled Missile

To investigate the behavior of normal force and pitching moment characteristics for different angles of attack and spouting jet angles, a parametric study of a lateral-jet-controlled missile has been performed. For this purpose, a three-dimensional Navier-Stokes computer code, AADL3D, has been developed and validated. Based on results of the parametric study, pitching moment and normal force coefficients have been selected as the objective and constraint functions, respectively. The flight Mach number, the angle of attack, and the spouting lateral jet angle are the design variables. With implementation of a genetic algorithm for the global optimum and the response surface method, the design optimization of the lateral-jet-controlled missile has been performed to evaluate the most effective flight conditions for the missile control.

Numerical investigation of the shock interaction effect on the lateral jet controlled missile

A computational study on the supersonic flow around the lateral jet controlled missile has been performed. A three-dimensional Navier–Stokes computer code (AADL 3D) has been developed and case studies have been performed by comparing the normal force coefficient and the moment coefficient of a missile body. Different jet flow conditions including jet pressures and jet Mach numbers, and the circumferential jet positions have been incorporated into the case studies. The missile surface is divided into four regions with respect to the center of gravity, and the normal force and moment distribution at each region are compared. The results show conspicuously different normal force and moment variations according to each parameter variation. From the detailed flow field analyses, it has been verified that most of the normal force loss and the pitching moment generation are taking place at the low-pressure region behind the jet nozzle. Furthermore, it is shown that the pitching moment can be efficiently reduced by the lateral thrust obtained through higher jet Mach number rather than high jet pressure. Thus, an angle of yaw is more effective for missile control by side jet than an angle of attack.

再使用宇宙輸送機フライバックブースタ翼空力形状の設計探査

The wing shape of flyback booster for a Two-Stage-To-Orbit reusable launch vehicle has been optimized considering four objectives. The objectives are to minimize the shift of aerodynamic center between supersonic and transonic conditions, transonic pitching moment and transonic drag coefficient, as well as to maximize subsonic lift coefficient. The three-dimensional Reynolds-averaged Navier-Stokes computation using the modified Spalart-Allmaras one-equation model is used in aerodynamic evaluation accounting for possible flow separations. Adaptive range multi-objective genetic algorithm is used for the present study because tradeoff can be obtained using a smaller number of individuals than conventional multi-objective genetic algorithms. Consequently, four-objective optimization has produced 102 non-dominated solutions, which represent tradeoff information among four objective functions. Moreover, Self-Organizing Maps have been used to analyze the present non-dominated solutions and to visualize tradeoffs and influence of design variables to the four objectives. Self-Organizing Maps contoured by the four objective functions and design variables are found to visualize tradeoffs and effects of each design variable.

New Insight Into the Flow Around a Wind Turbine Airfoil Section1

The objective of this paper is an improved understanding of the physics of the aeroelastic motion of wind turbine blades in order to improve the numerical models used for their design. Two- and three-dimensional Navier–Stokes calculations of the flow around a wind turbine airfoil using the k−ω SST and Detached Eddy Simulation (DES) turbulence models, as well as an engineering semiempirical dynamic stall model, are conducted. The computational results are compared to the experimental results that are available for both the static airfoil and the pitching airfoil. It is shown that the Navier–Stokes simulations can reproduce the main characteristic features of the flow. The DES model seems to be able to reproduce most of the details of the unsteady aerodynamics. Aerodynamic work computations indicate that a plunging motion of the airfoil can become unstable.

Effect of the Hub Endwall Cavity Flow on the Flow-Field of a Transonic High-Pressure Turbine

In high-pressure turbines, a small amount of cold flow is ejected at the hub from the cavity that exists between the stator and the rotor disk. This prevents the ingestion of hot gases into the wheel-space cavity, thus avoiding possible damage. This paper analyzes the interaction between the hub-endwall cavity flow and the mainstream in a high-pressure transonic turbine stage. Several cooling flow ratios are investigated under engine representative conditions. Both time-averaged and time-resolved data are presented. The experimental data is successfully compared with the results of a three-dimensional steady Navier-Stokes computation. Despite the small amount of gas ejected, the hub-endwall cavity flow has a significant influence on the mainstream flow. The Navier-Stokes predictions show how the ejected cold flow is entrained by the rotor hub vortex. The time-resolved static pressure field around the rotor is greatly affected when traversing the non-uniform vane exit flow field. When the cavity flow rate is increased, the unsteady forces on the rotor airfoil are reduced. This is linked to the decrease of vane exit Mach number caused by the blockage of the ejected flow.

Symmetry breaking in free-surface cylinder flows

The flow in a stationary open cylinder driven by the constant rotation of the bottom endwall is unstable to three-dimensional perturbations for sufficiently large rotation rates. The bifurcated state takes the form of a rotating wave. Two distinct physical mechanisms responsible for the symmetry breaking are identified, which depend on whether the fluid depth is sufficiently greater or less than the cylinder radius. For deep systems, the rotating wave results from the instability of the near-wall jet that forms as the boundary layer on the rotating bottom endwall is turned into the interior. In this case the three-dimensional perturbations vanish at the air/water interface. On the other hand, for shallow systems, the fluid at radii less than about half the cylinder radius is in solid-body rotation whereas the fluid at larger radii has a strong meridional circulation. The interface between these two regions of flow is unstable to azimuthal disturbances and the resulting rotating wave persists all the way to the air/water interface. The flow dynamics are explored using three-dimensional Navier–Stokes computations and experimental results obtained via digital particle image velocimetry. The use of a flat stress-free model for the air/water interface reproduces the experimental results in the deep system but fails to capture the primary instability in the shallow system, even though the experimental imperfections, i.e. departures from a perfectly flat and clean air/water interface, are about the same for the deep and the shallow systems. The flat stress-free model boundary conditions impose a parity condition on the numerical solutions, and the consideration of an extended problem which reveals this hidden symmetry provides insight into the symmetry-breaking instabilities.

Significance of blade element theory in performance prediction of marine propellers

This paper illustrates the implementation of a combined momentum-blade element theory for light and moderately loaded marine propellers and highlights its relevance, when compared to other more complex procedures, for their design and analysis. For this purpose, the results obtained using the theoretical model are first validated against experimental data concerning four Wageningen B-series propellers, and then these results are compared to those found using a fully three-dimensional Navier–Stokes calculation. The reasons of the differences in the results are analyzed and discussed using theoretical arguments.

Three-dimensional structures of flames over liquid fuel pools

Three-dimensional unsteady Navier-Stokes computations have been performed for the ignition and flame propagation above a liquid fuel (propanol) pool in an airflow duct. The article extends the computations of Cai et al. ( Combustion Science and Technology , Vol. 174, pp. 5-34, 2002) to include a parameter survey that determines the regimes of pulsating flame spread and uniform flame spread in the initial temperature/opposed air velocity, pool depth, pool width, and oxygen concentration planes. The two flame spread regimes are defined for both earth gravity and microgravity conditions. Pulsating flame spread behavior occurs if the initial liquid temperature and/or the oxygen mass fraction in the incoming airflow is sufficiently low. The roles of surface-tension-driven flow, gas buoyancy, liquid buoyancy, and hot gas expansion are analyzed. The effects of oxygen mass fraction, pool depth, and pool width are evaluated. For narrow pools (2 cm), the three-dimensional effects are focused near the poolside edges where the flame front wraps along the poolside edge but remains nearly two-dimensional over most of the pool. For a wider pool (8 cm), three-dimensional variations in flame shape occur over much of the pool. The behavior of the flame along the poolside edges differs significantly for earth gravity and microgravity conditions. The supply of oxygen due to buoyancy at the edges is an important factor. Contrary to previous speculation, the existence of a "valley" in the surface temperature profile is not necessary to explain the behavior of pulsating spread.

Flowfield Around Spike-Tipped Bodies for High Attack Angles at Mach 4.5

The requirements for the design of a new short-range high-velocity missile are both the drag reduction and the correct information acquisition for the optoelectronic sensors embedded in the hemispherical nose. High anglesof attack must be studied to fulfill the maneuverability requirements of present and future missiles. A supersonic missile generates a bow shock around its blunt nose, which causes rather high surface pressure and temperature and, as a result, the development of high drag and damage of embedded sensors. The pressure and the temperature on the hemispherical nose surface can be substantially reduced if an oblique shock is generated by a forward-facing spike. Both the experiments and the computations are carried out to study the flowfield around three-dimensional blunt bodies equipped with forward-facing spikes for a large range of attack angles at a Mach number of 4.5. A blunt body, a classical disk-tip spike, a sphere-tip spike, and a biconical-tip spike are studied. The experiments involve high-pressure shock tunnel investigations using a shock tube facility. The differential interferometry technique is applied to visualize the flowfield around the different missile spike geometries. The differential interferogram pictures as well as surface pressure measurements are compared with numerical results. Numerical simulations based on steady-state three-dimensional Navier-Stokes computations are performed to predict the drag, the lift, and the pitching moment for the blunt body and for each spike-tipped missile. The computations allow one to bring out the advantages of each spike geometry in comparison to the blunt body.

Flow induced patterning at the air–water interface

Patterns on the air–water interface of a swirling cylinder flow are produced via hydrodynamic symmetry-breaking instability of the bulk flow. The patterns are rotating waves breaking the axisymmetry of the system and are longitudinal at the free surface (i.e., not surface deforming). Qualitative observations and quantitative measurements of velocity and vorticity are provided. Three-dimensional Navier–Stokes computations identify the symmetry-breaking mode responsible for the waves. These waves are then used to pattern Langmuir monolayers at concentrations sufficiently below saturation.

Effect of forward sweep in a transonic compressor rotor

This paper presents the design and testing of a transonic compressor rotor with forward sweep. The rotor was used to investigate the influence of forward sweep on the performance and stability of a single-stage transonic compressor compared with a baseline design with radially stacked blade sections. The comparison was done numerically with the three-dimensional Navier—Stokes code TRACE-S and experimentally in the Darmstadt transonic compressor test rig. It was found that the new rotor with forward sweep has an increased efficiency and also a much better stall margin (much more in the rig test than predicted by the three-dimensional Navier—Stokes calculation). Particularly close to stall, the forward sweep diverts the flow towards the blade tip region which helps to stabilise this region. For that reason it is possible to throttle the forward-swept rotor much further than the radially stacked rotor, although the forward-swept rotor does already suffer from separated flow in the hub.

Precessing vortex breakdown mode in an enclosed cylinder flow

The flow in a cylinder driven by the rotation of one endwall for height to radius ratios around three is examined. Previous experimental observations suggest that the first mode of instability is a precession of the central vortex core, whereas a recent linear stability analysis to general three-dimensional perturbations suggests a Hopf bifurcation to a rotating wave at lower rotation rates than those where the precession mode was first detected. Here, this apparent discrepancy is resolved with the aid of fully nonlinear three-dimensional Navier–Stokes computations.

Multistage three-dimensional Navier-Stokes computation of off-design operation of a four-stage turbine

In this paper a three-dimensional computational methodology for multistage turbomachinery flows is developed, validated and applied in analysing the reduced mass flowrate operation of a four-stage turbine. The flow is modelled by the three-dimensional Favre-Reynolds-averaged Navier-Stokes equations using the Launder-Sharma near-wall k-ɛ turbulence closure, which are integrated using an implicit third-order upwind solver. The computation models the eight blade rows, including the tip clearance gaps of the rotors, using a 6 × 106 points grid, and computes the time-averaged flow using mixing planes between rows. Computational results are compared with measurements for various operating points. After this validation the method is used in a first attempt to analyse the three-dimensional flow at reduced-mass-flow operation.

Impact of three-dimensional phenomena on the design of rotodynamic pumps

Three-dimensional Navier—Stokes calculations are expected to be increasingly applied in the future for performance improvement of rotodynamic pumps. Frequently such an optimization process involves a preliminary design—based on one-dimensional methods and empirical data—which is subsequently optimized by computational fluid dynamics (CFD). Employing an empirical database is not only necessary in order to provide a good starting point for the CFD analysis but also to ensure that the design has a good chance of fulfilling part load requirements, since recirculating flows at the impeller inlet and outlet are not easily handled by CFD programs. CFD calculations provide the specific work input to the fluid and information on losses and reveal the complex three-dimensional flow patterns. The designer is faced with the task of interpreting such data and drawing conclusions for the optimization of the impeller. It is the purpose of the present contribution to analyse and describe the impact of various geometric parameters and flow features on the velocity distribution in the impeller and their influence on performance and part load characteristics. Criteria are also provided to select the parameters for the preliminary design. Hydraulic impeller losses calculated by CFD programs may often be misleading if the non-uniformity of the flow distribution at the impeller outlet is ignored. Procedures to quantify such mixing losses in the diffuser or volute downstream of the impeller are discussed.

Modelling of spanwise mixing in compressor through-flow computations

Although three-dimensional Navier-Stokes computations are coming into use more and more, streamline curvature through-flow computations are still needed, especially for multistage compressors, and where codes which run in minutes rather than hours are preferred. These methods have been made more realistic by taking account of end-wall effects and spanwise mixing by four aerodynamic mechanisms: turbulent diffusion, turbulent convection by secondary flow, spanwise migration of aerofoil boundary layer fluid and spanwise convection of fluid in blade wakes. This paper describes the models adopted in the DRA streamline curvature method for axial compressor design and analysis. Previous papers are summarized briefly before describing the new part of the model—that accounting for aerofoil boundary layers and wakes. Other changes to the previously published annulus wall boundary layer model have been made to enable it to cater for separations and end bends. The resulting code is evaluated against a range of experimental and computational results.

Comparison of Computation with Experiment for a Geometrically Simplified Powered-Lift Model

The performance of a representative short takeoff and vertical landing model during transition flight is investigated by comparison of experimental and numerical simulations. The model consists of a 60-deg cropped delta wing planform; a simple fuselage shape blended to the wing; and tandem, circular, highpressure-air lift-jets that exit perpendicularly to the flat lower surface. The configuration minimizes the geometric complexity while retaining the important flow physics of the lift-jet/aerodynamic surface interaction. Three-dimensional, laminar, and turbulent Navier-Stokes computations are made using a multiple, overset grid scheme. Results are presented for jet-off and powered-lift cases and compared with the measured forces and pressures for the model at a freestream Mach number of 0.146, a 10-deg angle of attack, and sonic lift-jets. Computational flow visualization illustrates the presence of primary and secondary wing leading-edge vortices and the deflection of the lift-jets by the freestream. Significantly, both computational fluid dynamics and experiment predict a jet-induced lift loss that is mostly a result of a reduction in the suction pressure at the wing leading edge.

Three-Dimensional Navier-Stokes Computations of Transonic Fan flow Using An Implicit Solver

Article Three-Dimensional Navier-Stokes Computations of Transonic Fan flow Using An Implicit Solver was published on June 1, 1997 in the journal International Journal of Turbo & Jet-Engines (volume 14, issue 2).

MODERN HELICOPTER AERODYNAMICS

▪ Abstract Modern helicopter aerodynamics is challenging because the flow field generated by a helicopter is extremely complicated and difficult to measure, model, and predict; moreover, experiments are expensive and difficult to conduct. In this article we discuss the basic principles of modern helicopter aerodynamics. Many sophisticated experimental and computational techniques have been employed in an effort to predict performance parameters. Of particular interest is the structure of the rotor wake, which is highly three-dimensional and unsteady, and the rotor-blade pressure distribution, which is significantly affected by the strength and position of the wake. We describe the various modern methods of computation and experiment which span the range from vortex techniques to full three-dimensional Navier-Stokes computations, and from classical probe methods to laser velocimetry techniques. Typical results for the structure of the wake and the blade pressure distribution in both hover and forward flight are presented Despite the complexity of the helicopter flow, significant progress has been made within the last ten years and the future will likely bring marked advances.

Experimental and Numerical Investigation of a Shock Wave Within a Swept Shrouded Propfan Rotor

Laser velocity measurements are conducted in a swept propfan rotor in order to investigate the local transonic flow region on the suction surface at high subsonic inlet Mach numbers. The velocity measurements are accompanied by a three-dimensional Navier–Stokes calculation for a selected operating point. The good agreement between computed and measured flow field gives some confidence to study the local three-dimensional passage shock on the suction surface by using the numerical procedure. Apart from the computed three-dimensional shock, structure is investigated in detail. By considering streamlines, it is concluded whether the shock wave is normal or oblique. The results are compared with the one-dimensional shock conditions.