scholarly journals Modelling spanwise heterogeneous roughness through a parametric forcing approach

2021 ◽  
Vol 930 ◽  
Author(s):  
K. Schäfer ◽  
A. Stroh ◽  
P. Forooghi ◽  
B. Frohnapfel
Keyword(s):  
Secondary Flow ◽  
Physical Mechanism ◽  
Mean Value ◽  
Channel Height ◽  
Smooth Wall ◽  
Flow Topology ◽  
Topological Features ◽  
Roughness Model ◽  
Flow Formation ◽  
Parametric Forcing

Inhomogeneous rough surfaces in which strips of roughness alternate with smooth-wall strips are known to generate large-scale secondary motions. Those secondary motions are strongest if the strip width is of the order of the half-channel height and they generate a spatial wall shear stress distribution whose mean value can significantly exceed the area-averaged mean value of a homogeneously smooth and rough surface. In the present paper it is shown that a parametric forcing approach (Busse & Sandham, J. Fluid Mech., vol. 712, 2012, pp. 169–202; Forooghi et al., Intl J. Heat Fluid Flow, vol. 71, 2018, pp. 200–209), calibrated with data from turbulent channel flows over homogeneous roughness, can capture the topological features of the secondary motion over protruding and recessed roughness strips (Stroh et al., J. Fluid Mech., vol. 885, 2020, R5). However, the results suggest that the parametric forcing approach roughness model induces a slightly larger wall offset when applied to the present heterogeneous rough-wall conditions. Contrary to roughness-resolving simulations, where a significantly higher resolution is required to capture roughness geometry, the parametric forcing approach can be applied with usual smooth-wall direct numerical simulation resolution resulting in less computationally expensive simulations for the study of localized roughness effects. Such roughness model simulations are employed to systematically investigate the effect of the relative roughness protrusion on the physical mechanism of secondary flow formation and the related drag increase. It is found that strong secondary motions present over spanwise heterogeneous roughness with geometrical height difference generally lead to a drag increase. However, the physical mechanism guiding the secondary flow formation, and the resulting secondary flow topology, is different for protruding roughness strips and recessed roughness strips separated by protruding smooth surface strips.

scholarly journals Prandtl’s Secondary Flows of the Second Kind. Problems of Description, Prediction, and Simulation

2021 ◽  
Vol 56 (4) ◽  
pp. 513-538
Author(s):  
N. V. Nikitin ◽  
N. V. Popelenskaya ◽  
A. Stroh
Keyword(s):  
Secondary Flow ◽  
Turbulent Flows ◽  
Longitudinal Component ◽  
Secondary Flows ◽  
Physical Mechanisms ◽  
Numerical Studies ◽  
Flow Formation ◽  
Active Research ◽  
Prediction And Simulation ◽  
Turbulent Pulsations

Abstract— The occurrence of turbulent pulsations in straight pipes of noncircular cross-section leads to the situation, when the average velocity field includes not only the longitudinal component but also transverse components that form a secondary flow. This hydrodynamic phenomenon discovered at the twenties of the last century (J. Nikuradse, L. Prandtl) has been the object of active research to the present day. The intensity of the turbulent secondary flows is not high; usually, it is not greater than 2–3% of the characteristic flow velocity. Nevertheless, their contribution to the processes of transverse transfer of momentum and heat is comparable to that of turbulent pulsations. In this paper, a review of experimental, theoretical, and numerical studies of secondary flows in straight pipes and channels is given. Emphasis is placed on the issues of revealing the physical mechanisms of secondary flow formation and developing the models of the apriori assessment of their forms. The specific features of the secondary flow development in open channels and channels with inhomogeneously rough walls are touched upon. The approaches of semiempirical simulation of turbulent flows in the presence of secondary flows are discussed.

Investigations on Aerodynamic Loading Limits of Subsonic Compressor Tandem Cascades: End Wall Flow

2014 ◽  
Author(s):  
Charlotte Hertel ◽  
Christoph Bode ◽  
Dragan Kožulović ◽  
Tim Schneider
Keyword(s):  
Secondary Flow ◽  
Pressure Rise ◽  
Compressor Cascade ◽  
Flow Topology ◽  
Plane Flow ◽  
Aerodynamic Loading ◽  
Oil Flow ◽  
Contour Plots ◽  
End Wall ◽  
Tandem Cascades

An optimized subsonic compressor tandem cascade was investigated experimentally and numerically. Since the design aims at applications under incompressible flow conditions, a low inlet Mach number of 0.175 was used. The experiments were carried out at the low speed cascade wind tunnel at the Technische Universität Braunschweig. For the numerical simulations, the CFD-solver TRACE of DLR Cologne was used, together with a curvature corrected k-ω turbulence model and the γ-Reθ transition model. The aerodynamic loading was varied by incidence variation. Results are presented and discussed for different inlet angles: spanwise loss coefficient, turning, pressure rise coefficient and AVR together with contour plots of the wake plane, flow visualization and oil flow pictures. Experimental and numerical results were compared and found to be in good agreement. The secondary flow topology of the front blade is considerably altered by the aerodynamic loading variation, whereas the topology of the rear blade surface is almost unchanged. The effect of the nozzle between the tandem blades, was observable up to the end wall for all investigated incidences. In addition, a comparison is made to published results of previous experimental and numerical investigations of a transonic tandem compressor cascade [1] and its reference single compressor cascade [2]. The comparison of the tandem cascades revealed that the general structures of the secondary flow seem to be similar for similar loading.

Particle Image Velocimetry Measurements in a Two-Pass 90 Degree Ribbed-Wall Parallelogram Channel

2015 ◽  
Vol 137 (4) ◽  
Author(s):  
Tong-Miin Liou ◽  
Shyy-Woei Chang ◽  
Shu-Po Chan ◽  
Yu-Shuai Liu
Keyword(s):  
Particle Image Velocimetry ◽  
Secondary Flow ◽  
Particle Image ◽  
Separation Bubble ◽  
Channel Height ◽  
Mean Velocity ◽  
Cross Sectional ◽  
Local Flow ◽  
Image Velocimetry ◽  
Sharp Turn

A parallelogram channel has drawn very little or no attention in the open literature although it appears as a cross-sectional configuration of some gas turbine rotor blades. Particle image velocimetry (PIV) is presented of local flow structure in a two-pass 90 deg ribbed-wall parallelogram channel with a 180 deg sharp turn. The channel has a cross-sectional equal length, 45.5 mm, of adjacent sides and two pairs of opposite angles are 45 deg and 135 deg. The rib height to channel height ratio is 0.1. All the measurements were performed at a fixed Reynolds number, characterized by channel hydraulic diameter of 32.17 mm and cross-sectional bulk mean velocity, of 10,000 and a null rotating number. Results are discussed in terms of the distributions of streamwise and secondary-flow mean velocity vector, turbulent intensity, Reynolds stress, and turbulent kinetic energy of the cooling air. It is found that the flow is not periodically fully developed in pitchwise direction through the inline 90 deg ribbed straight inlet and outlet leg. Pitchwise variation of reattachment length is revealed, and comparison with reported values in square channels is made. Whether the 180 deg sharp turn induced separation bubble exists in the ribbed parallelogram channel is also documented. Moreover, the measured secondary flow results inside the turn are successively used to explain previous heat transfer trends.

Flow processes and sedimentation in a straight submarine channel on the Qiongdongnan margin, northwestern South China Sea

2020 ◽  
Vol 90 (10) ◽  
pp. 1372-1388
Author(s):  
Chenglin Gong ◽  
Dongwei Li ◽  
Kun Qi ◽  
Hongxiang Xu
Keyword(s):  
Flow Velocity ◽  
Secondary Flow ◽  
Cross Sections ◽  
Vertical Channel ◽  
Mean Value ◽  
Flow Dynamics ◽  
Straight Channel ◽  
Channel Depth ◽  
Water Entrainment ◽  
Stacking Patterns

ABSTRACT Straight channels are ubiquitous in deep-water settings, yet flow dynamics and sedimentation in them are far from being well understood. Stratigraphy and flow dynamics of a middle to late Miocene straight channel in Qiongdongnan Basin were quantified, in terms of angle of channel-complex-growth trajectories (Tc), stratigraphic mobility number (M), Froude number (Fr), layer-averaged flow velocity (U), flow thickness (h), and water entrainment coefficient (Ew). The documented channels are composed of three channel complexes (CC1 to CC3) all of which are all characterized by symmetrical channel cross sections without levees and by organized vertical channel-stacking patterns (represented by high mean value of Tc = 37.4° and low mean value of M = 0.038). Turbidity currents in them were estimated to have U of 1.6 to 2.0 m/s (averaging 1.8 m/s), h of 63 to 89 m (averaging 78), Fr of 0.849 to 0.999 (averaging 0.912), and Ew of 0.0003 to 0.0005. They were, in most case, subcritical over most of the channel length, and had a low degree of water entrainment and low flow height scaled to the channel depth (i.e., 0.786 to 0.81 of the channel depth), most likely inhibiting the gradual loss of sediment to form levees. With reference to modeling results of secondary flow velocity vectors of numerical straight channels with the same sinuosity, two parallel gullies seen on both sides of the interpreted channel beds are interpreted to be induced by high-velocity downward backflows produced by the negative buoyancy. Such symmetrical secondary flow structures most likely promoted symmetrical intrachannel deposition (i.e., less deposition along both channel margins but more deposition near the channel center), and thus forced individual channel complexes to progressively aggrade in a synchronous manner, forming straight-channel complexes with symmetrical channel cross sections and organized vertical channel-stacking patterns.

Generalized expressions for secondary vorticity using intrinsic co-ordinates

1973 ◽  
Vol 59 (1) ◽  
pp. 97-115 ◽  
Author(s):  
B. Lakshminarayana ◽  
J. H. Horlock
Keyword(s):  
Viscous Flow ◽  
Secondary Flow ◽  
Physical Mechanism ◽  
Vector Equation ◽  
Coupled Equations ◽  
Rotating Systems

Equations for the development of streamwise (or secondary) vorticity, for stationary and rotating systems, are derived directly from the vector equation for vorticity, using intrinsic co-ordinates. This approach places emphasis on the coupled equations for the vorticity components parallel and normal to the streamline. The equations derived are more general than those hitherto available, being valid for compressible, stratified and viscous flow. Some interpretation is given of the physical mechanism causing secondary flow.

On the Efficiency of Secondary Flow Suction in a Compressor Cascade

2010 ◽  
Author(s):  
Karsten Liesner ◽  
Robert Meyer ◽  
Matthias Lemke ◽  
Christoph Gmelin ◽  
Frank Thiele
Keyword(s):  
Mach Number ◽  
Secondary Flow ◽  
Total Pressure ◽  
Main Flow ◽  
Loss Coefficient ◽  
Total Loss ◽  
Compressor Cascade ◽  
Flow Topology ◽  
Different Types ◽  
Suction Rate

An experimemtal investigation in a high speed compressor cascade has been carried out to show the effect of different types of secondary flow suction. In order to get deeper insight into the separated three dimensional flow topology and to determine appropriate suction positions, numerical simulations are performed additionally for the baseline cascade. To obtain the flow solution, an implicit, pressure based solver, elaN3D (by ISTA TU Berlin), is employed in steady RANS mode, whereby the Menter SST-k model is used for turbulence treatment. Both investigations are conducted at Mach number Ma = 0.67 and Reynolds number Re = 560.000. The aerodynamic design condition is used. The examined cascade consists of NACA65-K48 type vanes. The experiments include measurements with four different types of suction geometries plus reference measurements. Total pressure and flow angle measurements in the wake show the flow deflection, total pressure loss and the rise of the static pressure of the cascade. The best suction geometry follows the design of R.E. Peacock, designed for low Mach number cascades, with small changes. Using a maximum suction rate of 2% of the main flow the total loss coefficient was reduced by 23%. In this case the stage efficiency — calculated with a reference rotor — is increased by almost 1%. The vacuum pump energy consumption has been taken into account for this calculation. In another case the suction geometry has been chosen in a way that the suction slot is placed along the sidewall from suction side to pressure side following the wall streamlines. With an increased suction rate of 5% of the main flow, the vortex system in the passage is eliminated and the total loss coefficient is decreased to 0.055, which equals to a decrease of 37%. Taking into account that compressors in aero-engines provide bleed air for the plane’s air system, enormous efficiency increase is possible. For this the air bleed valves need to be redesigned.

scholarly journals Tomographic PIV measurement of a re-entrant jet in a cavitating venturi

2021 ◽  
Vol 1 (1) ◽  
Author(s):  
Udhav Ulhas Gawandalkar ◽  
Christian Poelma
Keyword(s):  
Physical Mechanism ◽  
Thin Liquid Film ◽  
Shear Layers ◽  
Flow Topology ◽  
Tomographic Piv ◽  
Partial Cavitation ◽  
Piv Measurement ◽  
Order Of Magnitude ◽  
Insight Into

Partial cavitation occurs when low-pressure regions caused by separated shear layers are filled with vapours. Partial cavitation is inherently unsteady and leads to periodic cloud shedding. The periodically generated re-entrant jet travelling beneath the vapour cavity is considered as one of the mechanisms responsible for the periodic cloud shedding (Callenaere et al. (2001)). However, the exact physical mechanism that drives the shedding remains unclear. The re-entrant flow exists as a thin liquid film wedged between the wall and the vapour cavity. The flow in this thin film is generally assumed to move with the same order of magnitude as the bulk flow, yet in the opposite direction. There have been several attempts to measure the velocity of the re-entrant flow to get insight into the physics of re-entrant flow and its contribution to cloud shedding. However, the flow topology of the re-entrant jet poses a major challenge to experimentally study it. The unsteady nature of the flow and the opacity of the cavitation cloud adds to the further complexity. In this work, we show that tomographic PIV (Elsinga et al. (2006)) can be extended to exploit the flow topology to accurately measure the velocity and thickness of the re-entrant flow. This in turn provides better insight into the role of re-entrant flow in periodic cloud shedding.

Concerning secondary flow in straight pipes

1938 ◽  
Vol 34 (3) ◽  
pp. 335-344 ◽  
Author(s):  
L. Howarth
Keyword(s):  
Turbulent Flow ◽  
Secondary Flow ◽  
Mean Value ◽  
Central Plane ◽  
Curved Pipe ◽  
Transfer Theory ◽  
Two Dimensional Flow ◽  
Flow Through ◽  
The Mean ◽  
Linear Differential

In this paper the condition for the existence of a secondary flow in a straight non-circular pipe is determined according to the modified vorticity transfer theory, with Goldstein's assumed form for the tensorIt is shown that a secondary motion arises if the mixture length is not constant on the curves along which | gradu| is constant,ubeing the velocity parallel to the pipe axis.In problems of turbulent flow treated by means of the modified vorticity transfer theory, the quantitywhereis the mean value of the square of the velocity fluctuation andpthe mean pressure, plays a part analogous to the pressure in laminar flow. In two-dimensional flow through a channel the theory shows the existence of a gradient ofacross the channel from the central plane to each wall. Qualitative arguments such as are used to explain the existence of a secondary flow for laminar motion in a curved pipe are applied here to show that a secondary flow is to be expected near the short sides in the turbulent flow through a straight rectangular pipe of large length/breadth ratio.The equations to determine the secondary flow through an almost circular elliptic pipe are discussed, the mixture length being assumed constant on ellipses similar to and concentric with the pipe section. For a first approximation the problem is reduced to the numerical solution of three simultaneous ordinary linear differential equations.

Super-Hydrophobic Effect in Single Phase Fluids

2012 ◽  
Author(s):  
Jerzy M. Floryan ◽  
A. Mohammadi
Keyword(s):  
Short Wavelength ◽  
Single Phase ◽  
Solid Wall ◽  
Hydrophobic Effect ◽  
Gas Bubbles ◽  
Smooth Wall ◽  
Flow Topology ◽  
Pressure Drag ◽  
Transverse Grooves ◽  
Two Phases

Super-hydrophobic effect involves capture of gas bubbles in pores of a solid wall, which separates liquid from the solid, resulting in the reduction of the shear drag experienced by the liquid. This effect occurs in the presence of two phases. A similar effect might be produced by creating separation bubbles made of the same fluid through proper shaping of the surface. Use of transverse grooves with a sufficiently short wavelength creates the required flow topology. The shear drag decreases by up to 50% compared with the smooth wall but the interaction pressure drag increases, resulting in only minor reduction in the overall drag. Proper shaping of the grooves may reduce the interaction pressure drag.

Fan Root Aerodynamics for Large Bypass Gas Turbine Engines: Influence on the Engine Performance and 3D Design

2009 ◽  
Author(s):  
Giulio Zamboni ◽  
Liping Xu
Keyword(s):  
Secondary Flow ◽  
Engine Performance ◽  
Flow Path ◽  
Numerical Code ◽  
Flow Topology ◽  
3D Design ◽  
Turbofan Engines ◽  
The Core ◽  
Inlet Conditions ◽  
The Impact

The exit flow field of the fan root of large turbofan engines defines the inlet conditions to the core compressor. This in turn could have significant impact to the performance of the core compressor. This study is aimed to resolve two related issues concerning the impact of the fan root flow on the core compressor performance: to establish the effect of an increased loss at the inlet on the engine specific fuel consumption (SFC) and to assess the effect of the radial distribution of the fan root flow on the engine performance. With understanding of these issues, the geometric parameters and design details which can produce a more uniform core flow at the exit of the fan stage module can be identified. The fan root flow is analysed with methods of different complexity and fidelity. A simple cycle analysis is used to assess the impact on engine SFC of a stagnation pressure deficit at the fan root; a throughflow code is used for the preliminary study of the curvature effect of the root flow path and 3D RANS CFD calculations are then used to simulate the flow path from the inlet of the fan to the first stage of the core compressor. The adequacy of the application of the numerical code in this case has been assessed and confirmed by the comparison with the experimental data for two rig configurations. The results of this study show that the flow at the fan hub region is very complex and dominated by 3D effects. The interaction of the secondary flow with real geometries, such as leakage flows, is found to have a strong detrimental effect on the core performance. The curvature of the hub end-wall is a key parameter controlling the fan root flow topology; it influences the strength of the secondary flow, the spanwise distribution of the flow and its sensitivity to leakage flow. With this understanding it is possible to redesign of the fan hub flow path to reduce the loss generation by a significant amount.

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