Three-dimensional Viscous Flow Field Research Articles

A Novel Vortex Identification Technique Applied to the 3D Flow Field of a High-Pressure Turbine

Abstract The efficiency of modern axial turbomachinery is strongly driven by the secondary flows within the vane or blade passages. The secondary flows are characterized by a complex pattern of vortical structures that origin, interact, and dissipate along the turbine gas path. The endwall flows are responsible for the generation of a significant part of the overall turbine loss because of the dissipation of secondary kinetic energy and mixing out of nonuniform momentum flows. The understanding and analysis of secondary flows requires a reliable vortex identification technique to predict and analyze the impact of specific turbine designs on the turbine performance. However, the literature shows a remarkable lack of general methods to detect vortices and to determine the location of their cores and to quantify their strength. This paper presents a novel technique for the identification of vortical structures in a general 3D flow field. The method operates on the local flow field, and it is based on a triple decomposition of motion proposed by Kolář. In contrast to a decomposition of velocity gradient into the strain and vorticity tensors, this method considers a third, pure shear component. The subtraction of the pure shear tensor from the velocity gradient remedies the inherent flaw of vorticity-based techniques, which cannot distinguish between rigid rotation and shear. The triple decomposition of motion serves to obtain a 3D field of residual vorticity whose magnitude is used to define vortex regions. The present method allows to locate automatically the core of each vortex, to quantify its strength, and to determine the vortex bounding surface. The output may be used to visualize the turbine vortical structures for the purpose of interpreting the complex three-dimensional viscous flow field and to highlight any case-to-case variations by quantifying the vortex strength and location. The vortex identification method is applied to a high-pressure turbine with three optimized blade tip geometries. The 3D flow field is obtained by computational fluid dynamics (CFD) computations performed with Numeca FINE/Open. The computational model uses steady-state Reynolds-averaged Navier–Stokes (RANS) equations closed by the Spalart-Allmaras turbulence model. Although developed for turbomachinery applications, the vortex identification method proposed in this work is of general applicability to any three-dimensional flow field.

Influence of Polymeric Sand Porous Material on the Aerodynamic Effect when a High-Speed Train is Entering into a Tunnel

In order to effectively reduce the compression wave in tunnel and the micro pressure wave at the exit of tunnel, improve the comfort of passengers, this study put forward laying polymeric sand porous material in tunnel by using the sound absorption properties of porous media, and give the numerical simulation calculation method of the aerodynamic effect of three-dimensional compressible, viscous and unsteady flow field when a high-speed train entering into a tunnel, furthermore, the influence of sound absorption board on the aerodynamic effect in tunnel was studied. Theoretical analysis and numerical results show the compression wave in tunnel and the micro pressure wave at the exit of tunnel can be decreased significantly when pasting porous materials in tunnel. The reason is mainly due to there are lots of micro-pores in the material and it could absorb the energy from compression wave, then the peak value of it and pressure gradient are decreased. Secondly, the method of laying polymeric sand sound absorption material has influence on the change of the pressure in the tunnel and the micro pressure wave at the exit of tunnel. When the length and height of sound absorption materials are same, calculation results show that the best pressure reduction effect is achieved when laying the material at the entrance of tunnel. However, when laying the material at different circumferential positions of tunnel, the pressure reduction effect at haunch is better than vault and spandrel. Based on that, considering the pressure noise reduction problem, material cost and height of construction in practical engineering, when laying the material at haunch, the relationship between the reduction effect of pressure gradient and the length of laying is summarized with 3 different speeds, which have certain reference value for the design of the method of laying sound absorption material in tunnel.

Blade design loads on the flow exciting force in centrifugal pump

The three-dimensional viscous flow field of two centrifugal pumps, which have the same volute, design head, design flow rate and rotational speed but the blade design load, are analyzed based on large eddy simulation. The comparisons are implemented including the hydraulic efficiencies, flow field characteristics, pressure pulsations and unsteady forces applied on the impellers to investigate the effect of the design blade load on hydraulic performance and flow exciting force. The numerical results show that the efficiency of the pump, the impeller blade of which has larger design load, is improved by 1.1%~2.9% compared to the centrifugal pump with lower blade design load. The pressure fluctuation of the pump with high design load is more remarkable. Its maximum amplitude of coefficient of static pressure is higher by 43% than the latter. At the same time the amplitude of unsteady radial force is increased by 11.6% in the time domain. The results also imply that the blade design load is an important factor on the excitation force in centrifugal pumps.

New Hybrid Method for Predicting the Flowfields of Helicopter Rotors

Based on a Navier-Stokes/full potential/free wake solver, a new hybrid method was developed for efficient predictions of the three-dimensional viscous flowfield of a helicopter rotor under both hover and forward flight conditions. The developed flow solver was composed of three modules: 1) a compressible Navier-Stokes analysis to model the viscous flow and near wake about the blade, 2) a compressible potential flow analysis to model the inviscid isentropic potential flow region far away from the rotor, and 3) a free wake model to account for tip vortex effects once the tip vortex leaves the viscous flow region and enters the potential flow region. In this hybrid method, a moving embedded grid methodology was adopted that accounts for rigid blade motions in rotation, flapping, and pitching. A dual-time method was employed to fulfill the calculation of the unsteady flowfields of helicopter rotors, and a third-order upwind scheme (MUSCL) and flux-difference splitting scheme without introducing artificial viscosity were used to calculate the flux. To search suitable donor elements in embedded grids to pass information between the viscous flow and potential flow zones, a new searching scheme was implemented. The sectional pressure distributions of a UH-60A helicopter rotor and an AH-1G model rotor in hover and forward flight, with and without the wake model, were calculated, and the developed hybrid model was validated by comparing with available experimental data. The simulated steady and unsteady lifting results of a model rotor with three different blade-tip planforms demonstrate the benefits of the curvilinear swept tip and constant swept tip in suppressing supercritical flows.

천음속 회전익에서의 누설유동

It is known that tip clearance flows reduce the pressure rise, flow range and efficiency of the turbomachinery. So, the clear understanding about flow fields in the tip region is needed to efficiently design the turbomachinery. The Navier-Stokes code with the proper treatment of the boundary conditions has been developed to analyze the three-dimensional steady viscous flow fields in the transonic rotating blades and a numerical study has been conducted to investigate the detail flow physics in the tip region of transonic rotor, NASA Rotor 67. The computational results in the tip region of transonic rotors show the leakage vortices, leakage flow from pressure side to suction side and their interaction with a shock. Depen ding on the operating conditions, toad distributions and the position of shock-wave on the blade surface are very different close to the blade tip of the transonic compressor rotor. The load distribution and the shock-wave position close to the blade tip had the close relationship with the starting position of leakage vortex and the direction of leakage flow.

Influence of Tip Clearance on the Flow Field and Aerodynamic Performance of the Centrifugal Impeller

AbstractThis paper studies numerically the influence of the tip clearance on the three-dimensional viscous flowfield and performance of the NASA Low-Speed Centrifugal Compressor (LSCC) impeller with a vaneless diffuser. A three-dimensional viscous code developed by the authors is applied with several acceleration methods: local time-step, multigrid and residual smoothing. The computations were performed under several operating conditions with four different tip clearance sizes (0.0%, 50%, 100% and 200% design tip-clearance). The numerical results show that: (1) the throughflow wake is considerably influenced by the tip-clearance. The low-kinetic-energy fluid accumulated in the shroud/pressure side of the unshrouded impeller, but in the shroud/suction region for 0.0 tip clearance; (2) there possibly exists an optimum size of the tip-clearance which is not the zero tip clearance to make the flow loss minimized.

Unstructured Mesh Navier-Stokes Calculations of the Flow Field of a Helicopter Rotor in Hover

Three-dimensional viscous flow field around a lifting helicopter rotor in hover is calculated by using an unstructured mesh methodology. The flow solver utilizes a cell-centered finite-volume scheme that is based on the Roe’s flux-difference splitting and an implicit Jacobi/Gauss-Seidel time integration. The effect of turbulence is estimated by the Spalart-Allmaras one-equation model coupled with a wall function boundary condition. A solution-adaptive mesh refinement technique is used for efficient capturing of the tip vortex. Calculations are performed at two operating conditions with varying tip Mach number and collective pitch setting. Formation of the tip vortex is well captured through a series of adaptive mesh refinement procedure starting from a coarse initial mesh. Good agreements are obtained between the numerical result and the experiment for both the blade loading and the tip vortex behavior. It is demonstrated that the rotor performance and the flow field are significantly affected by viscosity. The process of tip vortex formation around the blade tip is also qualitatively investigated.

Three-Dimensional Blade Boundary Layer and Endwall Flow Development in the Nozzle Passage of a Single Stage Turbine

The three-dimensional viscous flow field development in the nozzle passage of an axial flow turbine stage was measured using a “x” hot-wire probe. The measurements were carried out at two axial stations on the endwall and vane surfaces and at several spanwise and pitchwise locations. Static pressure measurements and flow visualization, using a fluorescent oil technique, were also performed to obtain the location of transition and the endwall limiting streamlines. The boundary layers on the vane surface were found to be very thin and mostly laminar, except on the suction surface downstream of 70 percent axial chord. Strong radial pressure gradient, especially close to the suction surface, induces strong radial flow velocities in the trailing edge regions of the blade. On the endwalls, the boundary layers were much thicker, especially near the suction corner of the casing surface, caused by the secondary flow. The secondary flow region near the suction surface-casing corner indicated the presence of the passage vortex detached from the vane surface. The boundary layer code accurately predicts the three-dimensional boundary layers on both vane surfaces and endwall in the regions where the influence of the secondary flow is small.

Topology method for analyses of 3-D viscous flow structure in transonic turbomachinery

A topology method is presented in this paper to reveal flow structure occurring inside turbomachinery, in which near wall flow structure is revealed by using wall limiting streamlines and space flow feature is revealed by using space streamlines and cross-section streamlines. As an example, a computational three-dimensional viscous flow field inside a transonic turbine cascade is studied. Through the analysis, the form and evolution of vortex system and the whole process of separation occurring within this cascade are revealed. The application of topology method for analyze flow structure inside turbomachinery is very important for understanding flow features and mechanism of flow loss even for improving the design of turbomachinery and increasing its efficiency.

An application of topological method to analysing the three-dimensional flow in cascades (II)—Topological analysis on the vector field patterns of skin-frictions and section streamlines

With an application of topological analysis, in this paper the skin-friction line patterns on compressor and turbine cascade surfaces are depicted and the streamline patterns of the secondary flow fields in the cross-section of a curved pipe and a turbine cascade are drawn under given conditions. In addition the structures of vortices within three-dimensional viscous flow fields in cascades are analysed.

Secondary Flows and Vortex Motion in a High-Efficiency Backswept Impeller at Design and Off-Design Conditions

The three-dimensional viscous flow field of a 4.7:1 pressure ratio backswept impeller was studied experimentally and numerically by using laser velocimetry and an advanced three-dimensional viscous code. The impeller was designed by a CAD method, and a maximum rotor efficiency of 94 percent was achieved. Both the experimental and the theoretical approach revealed comparatively smooth impeller discharge velocity profiles at all three operating conditions (design, choke, and near surge) differing widely from the well-known jet/wake-type flow pattern. The three-dimensional viscous code was used for detailed flowfield studies, i.e., secondary flows; vortex motion and tip-clearance effects were analyzed at design and off-design conditions. The comparison of experimental and numerical results indicates that the tip-clearance effect should be properly modeled to predict the impeller flow pattern properly and that optimum shape of rotor exit flow pattern can be obtained by controlling the swirling vortex motion.

Numerical analysis of a three-dimensional cascade flow by solving Navier-Stokes Equations.

Three-dimensional viscous flowfields in two different types of cascades, such as a turbine nozzle and fan blade, were numerically simulated by solving the Navier-Stokes Equations. For turbine nozzle flows, computed flow patterns on a vane in stationary and rotating frames were compared with those on a differently stacked vane. In a fundamental case, the computed results showed good agreement with experimental data. Three-dimensional flow separations as well as secondary flows caused by passage vortices could be predicted and effectively visualized using surface oil flow and streakline techniques, which were useful methods of three-dimensional flow representation.

Calculation of forces and moments in vortex methods

An alternative formulation for the calculation of forces and moments acting on a body in a three-dimensional unsteady viscous incompressible flow field is derived. The formulation is especially useful when the Navier-Stokes equations are solved in vorticity formulation.

Three-Dimensional Viscous Flowfield Computations in a Streamline Coordinate System

A method of predicting the three-dimensional laminar hypersonic viscous flowfield over a body at very high angles of attack has been developed by using the parabolized Navier-Stokes equations in a streamline coordinate system. At a very high angle of attack, the primary flow direction deviates substantially from the marching coordinate direction of the conventional body-generator coordinate direction. Hence, in the present approach the viscous flow equations are parabolized along the surface streamline direction instead of the body-generator coordinate direction, which reduces the restriction on the angle of attack of a flight vehicle. The streamline traces on the body surface are evaluated from a three-dimensio nal inviscid flowfield solution. Computational results for a test case have been obtained by both the present streamline approach (SLPNS) and the conventional body-generator coordinate method (PNS) for the purpose of comparison. The SLPNS results for the test case at moderate angle of attack compare well with conventional PNS results.