Influence of Turbulent Flow Characteristics and Coherent Vortices on the Pressure Recovery of Annular Diffusers: Part B — Scale-Resolving Simulations

2015 ◽  
Author(s):  
Bastian Drechsel ◽  
Christoph Müller ◽  
Florian Herbst ◽  
Joerg R. Seume
Keyword(s):  
Boundary Layer ◽  
Flow Characteristics ◽  
Boundary Layer Separation ◽  
Numerical Approach ◽  
Pressure Recovery ◽  
Experimental Investigations ◽  
Physical Parameters ◽  
Shear Stresses ◽  
Local Flow ◽  
Coherent Vortices

This paper examines the diffuser flow with consideration to turbine outflow conditions. The setup consists of a low-speed axial diffuser test rig, that represents a 1/10 scaled heavy-duty exhaust diffuser with an annular and a conical diffuser part. In part A of this paper it was shown through experimental investigation that the turbulent kinetic energy as well as the Reynolds shear stresses are the relevant physical parameters that correlate with diffuser pressure recovery. To complement the experimental investigations, unsteady scale-resolving CFD simulations are performed, applying the SST-SAS turbulence model. As a first step, the numerical approach is validated by means of the experimental data with regards to the diffuser’s integral parameters as well as the prediction of local flow characteristics. In a second step, the interaction of coherent vortices generated by the rotor and the diffuser’s boundary layer are analyzed by means of the validated SST-SAS results. These vortices are found to have a major impact on the boundary layer separation in the region immediately downstream of the rotor and at the diffuser inlet.

scholarly journals A Numerically Supported Investigation of the 3D Flow in Centrifugal Impellers: Part I — The Validation Base

1996 ◽  
Author(s):  
Ch. Hirsch ◽  
S. Kang ◽  
G. Pointel
Keyword(s):  
Boundary Layer ◽  
Three Dimensional ◽  
Boundary Layer Separation ◽  
Numerical Approach ◽  
Secondary Flows ◽  
Pump Impeller ◽  
High Loss ◽  
Leakage Flows ◽  
Fine Mesh ◽  
Radial Pump

The three-dimensional flow in centrifugal impellers is investigated on the basis of a detailed analysis of the results of numerical simulations. In order to gain confidence in this process, an in-depth validation is performed, based on computations of Krain’s centrifugal compressor and of a radial pump impeller, both with vaneless diffusers. Detailed comparisons with available experimental data provide high confidence in the numerical tools and results. The appearance of a high loss ‘wake’ region results from the transport of boundary layer material from the blade surfaces to the shroud region and its location depends on the balance between secondary and tip leakage flows and is not necessarily connected to 3D boundary layer separation. Although the low momentum spots near the shroud can interfere with 3D separated regions, the main outcome of the present analysis is that these are two distinct phenomena. Part I of this paper focuses on the validation base of the numerical approach, based on fine mesh simulations, while Part II presents an analysis of the different contributions to the secondary flows and attempts to estimate their effect on the overall flow pattern.

Correlation Between Pressure Recovery of Highly Loaded Annular Diffusers and Integral Stage Design Parameters

2017 ◽  
Author(s):  
Dajan Mimic ◽  
Bastian Drechsel ◽  
Florian Herbst
Keyword(s):  
Boundary Layer ◽  
Dynamic Pressure ◽  
Boundary Layer Separation ◽  
Flow Coefficient ◽  
Tip Vortex ◽  
Pressure Recovery ◽  
Design Parameters ◽  
Steam Turbines ◽  
Pressure Losses ◽  
Induced Velocity

Exhaust diffusers significantly enhance the available power output and efficiency of gas and steam turbines by allowing for lower turbine exit pressures. The residual dynamic pressure of the turbine outflow is converted into static pressure, which is referred to as pressure recovery. Since total pressure losses as well as construction costs increase drastically with diffuser length, it is more than favourable to design shorter diffusers with rather steep opening angles. However, those designs are more susceptible to boundary layer separation. In this paper, the stabilising properties of tip leakage vortices generated in the last rotor row and their effect on the boundary layer characteristics are examined. Based on analytical considerations, for the first time a correlation between the pressure recovery of the diffuser and integral rotor parameters of the last stage, namely the loading coefficient, flow coefficient and reduced frequency, is established. Both, experimental data and scale resolving simulations, carried out with the SST-SAS method, show excellent agreement with the correlation. Blade tip vortex strength predominantly depends on the amount of work performed in the rotor, which in turn is described by the non-dimensional loading coefficient. The flow coefficient influences mainly the orientation of the vortex, which affects the interaction between vortex and boundary layer. The induced velocity field accelerates the boundary layer, essentially reducing the thickness of the separated layer or even locally preventing separation.

Flow Characteristics around Circular Cylinders with a Normal Slit

2017 ◽  
Vol 379 ◽  
pp. 48-57 ◽  
Author(s):  
Cheng Hsiung Kuo ◽  
Hwa Wei Lin ◽  
Chih Tao Chai ◽  
Fred Cheng
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Boundary Layer Flow ◽  
Rear Surface ◽  
Flow Characteristics ◽  
Boundary Layer Separation ◽  
Circular Cylinders ◽  
Flow Structures ◽  
Phase Lock ◽  
Layer Flow

Alterations of boundary layer separation along the upper-rear surface of a baseline and slit cylinder and the formation of a vortex in the near-wake are investigated by particle image velocimetry (PIV) at Reynolds number 1000. The slit ratio (S/D) is 0.3. The phase-lock flow structures are referred to the time-dependent volume flux at the slit exit and are achieved by the modified phase-averaged technique. The alterations and the evolution of boundary-layer flow along the upper-rear surface are demonstrated by the phase-lock flow structures. It is found that the alternate blowing and suction at the slit exit serves as a perturbation to the boundary layer near the shoulder of the slit cylinder leading to a significant delay of flow separation and the flow reattachment of boundary-layer flow along the upper-rear surface of the cylinder. After perturbation, the vortex street behind a slit cylinder is more organized and stronger than that behind a baseline cylinder at Reynolds number 1000.

Effects of Coriolis Force on Turbulence Structure in Rotating Channels With High and Low Aspect Ratios

2019 ◽  
Author(s):  
Hide S. Koyama ◽  
Shoichiro Tatsuta ◽  
Ema Tamura ◽  
Hani H. Nigim
Keyword(s):  
Boundary Layer ◽  
Aspect Ratio ◽  
Coriolis Force ◽  
Experimental Investigations ◽  
Shear Stresses ◽  
Turbulence Structure ◽  
Hot Wire ◽  
Aspect Ratios ◽  
Flow Effects ◽  
Vorticity Transport

Abstract To clarify the stabilization, destabilization and secondary flow effects of the Coriolis force on the turbulence structure in rotating channels with high and low aspect ratios, experimental investigations were undertaken by using a rotating wind tunnel about 2 meters in diameter. A hot-wire anemometer and more than 6 hot-wire probes with fine resistance wire, each inclined at a known angle to the main flow, were used for the measurements of time-mean velocities and 6 components of Reynolds normal and shear stresses. An optical transmission system of electrical signals from a rotating apparatus to the stationary system was used to immunize the electrical noise. The hot-wire probe was traversed with sufficient accuracy. Phenomena of the stratification, Taylor-Görtler type vortices in the destabilized nominal two-dimensional turbulent boundary layer, relaminarization from turbulent to laminar boundary layer in the high aspect ratio channel and a pair of longitudinal flat vortices in the low aspect ratio channel were observed, and discussed in detail by using the low of conservation of angular momentum, the vorticity transport equation and the Reynolds stress transport equations in a rotating frame.

Numerical Investigation of the Wake-Boundary Layer Interaction on a Highly Loaded LP Turbine Cascade Blade

2002 ◽  
Author(s):  
Pasquale Cardamone ◽  
Peter Stadtmu¨ller ◽  
Leonhard Fottner
Keyword(s):  
Boundary Layer ◽  
Separation Bubble ◽  
Transition Process ◽  
Reynolds Numbers ◽  
Numerical Approach ◽  
Experimental Investigations ◽  
Turbine Cascade ◽  
Suction Surface ◽  
Time Resolved ◽  
Highly Loaded

The effects of wake passing on the development of the profile boundary layer of a highly loaded low-pressure turbine cascade are studied using the RANS code TRACE-U. The numerical results are compared with available experimental data to verify the accuracy of the code in predicting the periodic-unsteady transition and separation mechanisms at low Reynolds number conditions. The experimental investigations have been carried out on a turbine cascade called T106D-EIZ subjected to wakes generated by an up-stream moving bar-type generator. The cascade pitch was increased by about 30% with respect to design conditions without modifying the blade geometry in order to obtain a large separation bubble on the suction surface. The extensive database containing time-averaged as well as time-resolved results was presented in a separate paper by Stadtmu¨ller and Fottner (2001) and is discussed only briefly. The time-accurate multistage Navier-Stokes solver TRACE-U developed by the DLR Cologne used for the numerical simulations employs a modified version of the one-equation Spalart-Allmaras turbulence model coupled with a transition correlation based on the work of Abu-Ghannam and Shaw in the formulation of Drela. The objective of this paper is to provide further insight into the aerodynamics of the wake-induced transition process and to rate the application limits of the numerical approach for exit Reynolds numbers as low as 60.000. The CFD predictions for two different flow conditions are compared with the measurements. Plots of wall-shear stress, blade loading, shape factor and loss behaviour are used to verify the reliability of the code. The periodic-unsteady development of the boundary layer as well as the loss behaviour is well reproduced for higher Reynolds numbers. For the case with massive separation, large discrepancies between numerical and experimental results are observed.

Transient Performance of Separated Flows: Characterization and Active Flow Control

2018 ◽  
Author(s):  
J. Saavedra ◽  
G. Paniagua
Keyword(s):  
Boundary Layer ◽  
Flow Control ◽  
Boundary Layer Separation ◽  
Low Pressure ◽  
Separated Flows ◽  
Shear Stresses ◽  
Recirculation Bubble ◽  
Control Approach ◽  
Layer Separation ◽  
Inlet Conditions

The aerothermal performance of the low pressure turbine in UAVs’ is significantly abated at high altitude, due to boundary layer separation. During past years different flow control strategies have been proposed to prevent boundary layer separation, such as dielectric barrier discharges, synthetic jets, vortex generators. However, the optimization of the control approach requires a better characterization of the separated regions at several frequencies. The present investigation analyzes the behavior of separated flows, and specifically reports the inception, reattachment and separation length, that allows the development of more efficient methods to manipulate flow separation under non-tempo-rally uniform inlet conditions. The development of separated flows under sudden flow accelerations or pulsating inlet conditions were investigated with series of numerical simulations including Unsteady Reynolds Average Navier Stokes and Large Eddy Simulations. The present research was performed on a wall mounted hump, which imposes an adverse pressure gradient representative of the suction side of low pressure turbines. The heat transfer and wall shear stresses were fully documented, as well as the flow velocity and temperature profiles at different axial locations to characterize the near wall flow properties and the thermal boundary layer. Through a sudden flow acceleration we looked into the dynamic response of the shear layer detachment as it is modulated by the mean flow evolution. Similarly, we studied the behavior of the recirculation bubble under periodic disturbances imposed by sinusoidal inlet total pressure signals at various frequencies ranging from 10 to 500 Hz. During each period the Reynolds number oscillates between 40000 and 180000 (based on a characteristic length of 0.1 m). Finally, as a first step into the flow control approach we added a slot in our geometry to allow flow injection and ingestion just upstream of the separation inception. Exploring the behavior of the separated region at different slot pressure conditions we defined the envelope for its periodic actuation. Thanks to that analysis, we found that matching the actuator frequency with the frequency response of the separated region the performance of the actuation is boosted.

Studies on Blood Rheology in a Coronary Artery Using CFD Technique With an AE Sensor

2007 ◽  
Author(s):  
Yong Hyun Kim ◽  
Goddy Chungag ◽  
Joon Sang Lee ◽  
Emmanuel Ayorinde ◽  
Xin Wu
Keyword(s):  
Coronary Artery ◽  
Experimental Studies ◽  
Flow Characteristics ◽  
Blood Rheology ◽  
Arterial System ◽  
Experimental Investigations ◽  
Flow Induced Vibration ◽  
Local Flow ◽  
Physiological Conditions ◽  
Rbc Aggregation

There still exists a need for developing more accurate generalized models for multiscale biofluids systems that enable clearer understanding of normal microcirculation and complexities of disease hemorheology. Such work will yield enhanced computational and experimental techniques for a wider class of flows having fluid-solid interactions, complex moving boundaries, and involving red blood cell (RBC) aggregation under physiological conditions. The work reported here has involved the multiphase non-Newtonian fluid simulations of pulsatile flow in an idealized coronary artery model have been performed using numerical and experimental studies. The secondary flow affected a local RBC accumulation on the inside curvature and it changed the local flow characteristics as well. RBC viscosity and wall shear stress (WSS) were changed with a function of local hemotocrit. In practical work involving specialized velocity measurement and acoustic emission monitoring of flow characteristics, flow-induced vibration effects, as well as material and physiological aspects of arterial systems were conducted. Computations of arterial flows were made and experimental investigations using glass microtube simulations of arteries were carried out. This work contributes to an understanding of the mechanics of relationship between the progression of certain inherited diseases and the mechanical deformation characteristics of the arterial system and the RBC.

scholarly journals Characteristics of Helium Gas with High Temperature and High Pressure Flowing through a 90-Degree Elbow

2014 ◽  
Vol 2014 ◽  
pp. 1-6 ◽  
Author(s):  
Beibei Feng ◽  
Yanfei Sun ◽  
Xingtuan Yang ◽  
Shengqiang Li ◽  
Jiyuan Tu ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Static Pressure ◽  
Flow Characteristics ◽  
Boundary Layer Separation ◽  
Separation Process ◽  
Centrifugal Forces ◽  
Helium Flow ◽  
Primary Loop ◽  
Helium Gas ◽  
Simulation Results

There exists a certain 90° elbow structures in the helium circulation of HTGR-10. In terms of energy-saving and design simplification of reactor’s primary loop, 90° elbow can be used to measure the helium flow and the content of water vapor, both of which are significant in an accident. It is necessary to make an in-depth research of the flow characteristics of helium flowing 90° elbow. Simulation results indicate that fluid’s motion in the elbow is under the control of the centrifugal forces. Static pressure near the extrados is higher than that near the intrados. Boundary layer separation occurs at the latter half intrados of the elbow. The vortex emerges during the separation process and increases the energy dissipation. Velocity in the near-intrados region is higher than that in the near-extrados region, which is opposite to the pressure distribution trend. Under the action of the centrifugal forces, the secondary flow emerges in the latter half of the elbow and complicates the flow field by generating two vortexes which rotate in a different direction.

scholarly journals Correlation Between Pressure Recovery of Highly Loaded Annular Diffusers and Integral Stage Design Parameters1

2018 ◽  
Vol 140 (7) ◽  
Author(s):  
Dajan Mimic ◽  
Bastian Drechsel ◽  
Florian Herbst
Keyword(s):  
Boundary Layer ◽  
Dynamic Pressure ◽  
Boundary Layer Separation ◽  
Flow Coefficient ◽  
Tip Vortex ◽  
Pressure Recovery ◽  
Steam Turbines ◽  
Construction Costs ◽  
Pressure Losses ◽  
Induced Velocity

Exhaust diffusers significantly enhance the available power output and efficiency of gas and steam turbines by allowing lower turbine exit pressures. The residual dynamic pressure of the turbine outflow is converted into static pressure, which is referred to as pressure recovery. Since total pressure losses and construction costs increase drastically with diffuser length, it is strongly preferred to design shorter diffusers with steeper opening angles. However, these designs are more susceptible to boundary layer separation. In this paper, the stabilizing properties of tip leakage vortices generated in the last rotor row and their effect on the boundary layer characteristics are examined. Based on analytical considerations, for the first time, a correlation between the pressure recovery of the diffuser and the integral rotor parameters of the last stage, namely, the loading coefficient, flow coefficient, and reduced frequency, is established. Experimental data and scale-resolving simulations, carried out with the shear stress transport scale-adaptive simulation (SST-SAS) method, both show excellent agreement with the correlation. Blade tip vortex strength predominantly depends on the amount of work exchanged between fluid and rotor, which in turn is described by the nondimensional loading coefficient. The flow coefficient influences mainly the orientation of the vortex, which affects the interaction between vortex and boundary layer. The induced velocity field accelerates the boundary layer, essentially reducing the thickness of the separated layer or even preventing separation locally.

Sub boundary-layer vortex generators for the control of shock induced separation

2006 ◽  
Vol 110 (1106) ◽  
pp. 215-226 ◽  
Author(s):  
G. S. Cohen ◽  
F. Motallebi
Keyword(s):  
Boundary Layer ◽  
Total Pressure ◽  
Static Pressure ◽  
Pressure Rise ◽  
Boundary Layer Separation ◽  
Vortex Generators ◽  
Pressure Recovery ◽  
Flow Visualisation ◽  
Pressure Distributions ◽  
Pressure Profiles

Abstract The results of an investigation into the effects that sub-boundary layer vortex generators (SBVGs) have on reducing normal shock-induced turbulent boundary-layer separation are presented. The freestream Mach number and Reynolds number were M = 1·45 and 15·9 × 106/m, respectively. Total pressure profiles, static pressure distributions, surface total pressure distributions, oil flow visualisation and Schlieren photographs were used in the results analysis. The effects of SBVG height, lateral spacing and location upstream of the shock were investigated. A novel curved shape SBVG was also evaluated and comparisons against the conventional flat vane type were made. The results show that in all but two cases, separation was completely eliminated. As expected, the largest SBVGs with height, h = 55%δ, provided the greatest pressure recovery and maximum mixing. However, the shock pressure rise was highest for this case. The experiments showed that the mid height SBVG array with the largest spacing provided similar results to the SBVG array with the largest height. Reducing the distance to shock to 10δ upstream also showed some improvement over the SBVG position of 18δ upstream. It was suggested that total elimination of the separated region may not be required to achieve a balance of improved static pressure recovery whilst minimising the pressure rise through the shock. The effect of curving the SBVGs provided an improved near wall mixing with an improved static and surface total pressure recovery downstream of the separation line. The optimum SBVG for the current flow conditions was found to be the curved vanes of h = 40%δ, with the largest spacing, located at 18δ upstream of the shock. Overall, it was apparent from the results that in comparison to larger vortex generators with a height comparable to δ, for SBVGs the parameters involved become more important in order to obtain the highest degree of mixing from a given SBVG configuration.

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