Effects of Inclination Angle of Ribs on the Flow Behavior in Rectangular Ducts

2004 ◽  
Vol 126 (4) ◽  
pp. 692-699 ◽  
Author(s):  
Xiufang Gao ◽  
Bengt Sunde´n
Keyword(s):  
Velocity Component ◽  
Flow Direction ◽  
Flow Behavior ◽  
Three Dimensional ◽  
Fluid Flows ◽  
Streamwise Velocity ◽  
Flow Structures ◽  
Local Flow ◽  
Flow Motion ◽  
Streamwise Velocity Component

The flow behavior in rib-roughened ducts is influenced by the inclination of ribs and the effect is investigated in the present study by Particle Image Velocimetry (PIV). The local flow structures between two adjacent ribs were measured. The Reynolds number was fixed at 5800. The flow field description was based on the PIV results in planes both parallel and perpendicular to the ribbed walls at various locations. The rib angle to the main flow direction was varied as 30 deg, 45 deg, 60 deg and 90 deg. The ribs induce three dimensional flow fields. The flow separation and reattachment between adjacent ribs are clearly observed. In addition, the inclined ribs are found to alter the spanwise distribution of the streamwise velocity component. The streamwise velocity component has its highest values at the upstream end of the ribs, and decreases continuously to its lowest values at the downstream end. Strong secondary flow motion occurs over the entire duct cross section for the inclined ribs. The flow structures between two consecutive ribs show that the fluid flows along the ribs from one end of the ribs to the other end, and then turns back at the transverse center. Downwash and upwash flows are observed at the upstream end and downstream end of the ribs, respectively.

A resonance mechanism in plane Couette flow

1980 ◽  
Vol 98 (1) ◽  
pp. 149-159 ◽  
Author(s):  
L. HÅKan Gustavsson ◽  
Lennart S. Hultgren
Keyword(s):  
Reynolds Number ◽  
Couette Flow ◽  
Velocity Component ◽  
Three Dimensional ◽  
Phase Speed ◽  
Maximum Amplitude ◽  
Streamwise Velocity ◽  
Plane Couette Flow ◽  
Mean Velocity ◽  
Streamwise Velocity Component

The temporal evolution of small three-dimensional disturbances on viscous flows between parallel walls is studied. The initial-value problem is formally solved by using Fourier–Laplace transform techniques. The streamwise velocity component is obtained as the solution of a forced problem. As a consequence of the three-dimensionality, a resonant response is possible, leading to algebraic growth for small times. It occurs when the eigenvalues of the Orr–Sommerfeld equation coincide with the eigenvalues of the homogeneous operator for the streamwise velocity component. The resonance has been investigated numerically for plane Couette flow. The phase speed of the resonant waves equals the average mean velocity. The wavenumber combination that leads to the largest amplitude corresponds to structures highly elongated in the streamwise direction. The maximum amplitude, and the time to reach this maximum, scale with the Reynolds number. The aspect ratio of the most rapidly growing wave increases with the Reynolds number, with its spanwise wavelength approaching a constant value of about 3 channel heights.

Three-dimensional instabilities in oscillatory flow past elliptic cylinders

2016 ◽  
Vol 798 ◽  
pp. 371-397 ◽  
Author(s):  
José P. Gallardo ◽  
Helge I. Andersson ◽  
Bjørnar Pettersen
Keyword(s):  
Circular Cylinder ◽  
Flow Direction ◽  
Three Dimensional ◽  
Circular Cylinders ◽  
Flow Structures ◽  
Three Dimensional Flow ◽  
Elliptical Cross ◽  
Dimensional Flow ◽  
Aspect Ratios ◽  
Flow Past

We investigate the early development of instabilities in the oscillatory viscous flow past cylinders with elliptic cross-sections using three-dimensional direct numerical simulations. This is a classical hydrodynamic problem for circular cylinders, but other configurations have received only marginal attention. Computed results for some different aspect ratios ${\it\Lambda}$ from 1 : 1 to 1 : 3, all with the major axis of the ellipse aligned in the main flow direction, show good qualitative agreement with Hall’s stability theory (J. Fluid Mech., vol. 146, 1984, pp. 347–367), which predicts a cusp-shaped curve for the onset of the primary instability. The three-dimensional flow structures for aspect ratios larger than 2 : 3 resemble those of a circular cylinder, whereas the elliptical cross-section with the lowest aspect ratio of 1 : 3 exhibits oblate rather than tubular three-dimensional flow structures as well as a pair of counter-rotating spanwise vortices which emerges near the tips of the ellipse. Contrary to a circular cylinder, instabilities for an elliptic cylinder with sufficiently high eccentricity emerge from four rather than two different locations in accordance with the Hall theory.

Numerical Study of the Winter-Kennedy Method—A Sensitivity Analysis

2018 ◽  
Vol 140 (5) ◽  
Author(s):  
Binaya Baidar ◽  
Jonathan Nicolle ◽  
Chirag Trivedi ◽  
Michel J. Cervantes
Keyword(s):  
Boundary Conditions ◽  
Secondary Flow ◽  
Numerical Study ◽  
Flow Behavior ◽  
Three Dimensional ◽  
Flow Distribution ◽  
Local Flow ◽  
Spiral Case ◽  
Computational Fluid Dynamics Cfd ◽  
Flow Changes

The Winter-Kennedy (WK) method is commonly used in relative discharge measurement and to quantify efficiency step-up in hydropower refurbishment projects. The method utilizes the differential pressure between two taps located at a radial section of a spiral case, which is related to the discharge with the help of a coefficient and an exponent. Nearly a century old and widely used, the method has shown some discrepancies when the same coefficient is used after a plant upgrade. The reasons are often attributed to local flow changes. To study the change in flow behavior and its impact on the coefficient, a numerical model of a semi-spiral case (SC) has been developed and the numerical results are compared with experimental results. The simulations of the SC have been performed with different inlet boundary conditions. Comparison between an analytical formulation with the computational fluid dynamics (CFD) results shows that the flow inside an SC is highly three-dimensional (3D). The magnitude of the secondary flow is a function of the inlet boundary conditions. The secondary flow affects the vortex flow distribution and hence the coefficients. For the SC considered in this study, the most stable WK configurations are located toward the bottom from θ=30deg to 45deg after the curve of the SC begins, and on the top between two stay vanes.

Three-Dimensional Simulation of Gas Flow in Microducts

2005 ◽  
Author(s):  
Abhishek Agrawal ◽  
Amit Agrawal
Keyword(s):  
Aspect Ratio ◽  
Knudsen Number ◽  
Gas Flow ◽  
Flow Direction ◽  
Pressure Ratio ◽  
Three Dimensional ◽  
Streamwise Velocity ◽  
Theoretical Development ◽  
Width Ratio ◽  
Two Dimensional

Three-dimensional lattice Boltzmann method based simulations of a microduct have been undertaken in this paper. The objective is to understand the different physical phenomena occurring at these small scales and to investigate when the flow can be treated as two-dimensional. Towards this end, the Knudsen number and aspect ratio (depth to width ratio) are varied for a fixed pressure ratio. The pressure in the microduct is non-linear with the non-linearity in pressure reducing with an increase in Knudsen number. The pressure and velocity behaves somewhat similar to two-dimensional microchannels even when the aspect ratio is unity. The slip velocity at the impenetrable wall has two components: along and perpendicular to the flow. Our results show that the streamwise velocity near the centerline is relatively invariant along the depth for aspect ratio more than three, suggesting that the microduct can be modeled as a two-dimensional microchannel. However, the velocity component along the depth is never identically zero, implying that the flow is not truly two-dimensional. A curious change in vector direction in a plane normal to the flow direction is observed around aspect ratio of four. These first set of three-dimensional results are significant because they will help in theoretical development and flow modeling at micro scales.

scholarly journals Instantaneous pressure measurements on a spherical grain under threshold flow conditions

2014 ◽  
Vol 741 ◽  
pp. 60-97 ◽  
Author(s):  
Ahmet O. Celik ◽  
P. Diplas ◽  
C. L. Dancey
Keyword(s):  
Circular Cylinder ◽  
Channel Flow ◽  
Velocity Component ◽  
Lift Force ◽  
Open Channel ◽  
Streamwise Velocity ◽  
Normal Velocity ◽  
Flow Conditions ◽  
Local Flow ◽  
Dominant Feature

AbstractThe aim of this investigation was to experimentally examine the surface pressures and resulting forces on an individual sediment grain whose size is comparable to the scales of the turbulent channel flow in an effort to discern details of the flow/grain interaction. This was accomplished by measuring the pressure fluctuations on the surface of a coarse, fully exposed, spherical grain resting upon a bed of identical grains in open channel turbulent flow. This spherical particle was instrumented with low-range, high-frequency-response pressure transducers to measure the individual surface pressures simultaneously on its front, back, top and bottom. The local flow velocity was measured synchronously with a laser Doppler velocimeter. The flow and sediment are near threshold conditions for entrainment with the channel and particle Reynolds numbers varying between 31 000–39 000 and 330–440 respectively. The emphasis was on determining the characteristics of the flow field with the potential to dislodge a spherical grain under uniform flow conditions as well as in the wake of a circular cylinder placed spanwise across the flow in otherwise fully developed open channel flow. It is concluded that the streamwise velocity near the bed is most directly related to those force events (and associated individual surface pressure distributions) crucial for particle entrainment. The lift force was observed to momentarily reach values which can be consequential for particle stability, although it is poorly correlated with the fluctuating normal velocity component. Turbulence intensity near the bed, rather than being the causative factor for increased force fluctuations, was shown to be an indicator of changes in the average lift force experienced by the grain during the application of extreme drag forces, at least for this particular flow condition (the upstream, spanwise-mounted circular cylinder). This effect is known to alter the sediment transport rates significantly. The characteristics of the temporal durations of flow events about the local maxima in the stagnation pressure, drag and lift forces, using a conditional sampling method, revealed the prevalence of sweep-type near-bed flow events in generating favourable conditions for particle dislodgement, although the dominant feature is the positive streamwise velocity fluctuation, not the normal velocity component. The duration of such events was the highest in the fourth and first quadrants in the $u,w$ plane, inducing high impulses on the grain.

Flow Structures and Losses in the Exhaust Port of an Internal Combustion Engine

2013 ◽  
Author(s):  
Yue Wang ◽  
Bernhard Semlitsch ◽  
Mihai Mihaescu ◽  
Laszlo Fuchs
Keyword(s):  
Combustion Engine ◽  
Numerical Study ◽  
Flow Direction ◽  
Three Dimensional ◽  
Numerical Data ◽  
Valve Lift ◽  
Flow Structures ◽  
Exhaust Manifold ◽  
Valve Body ◽  
Exhaust Port

A numerical study of the flow in the exhaust port geometry of a Scania heavy-duty Diesel engine is carried out mainly by using the Large Eddy Simulation (LES) approach. Unsteady Reynolds Averaged Navier-Stokes (URANS) simulation results are included for comparison purposes. The calculations are performed with fixed valve and stationary boundary conditions for which experimental data are available. The simulations include a verification study of the solver using different grid resolutions and different valve lift states. The calculated numerical data are compared to existent measured pressure loss data. The results show that even global parameters like total pressure losses are predicted better by LES than by URANS. The complex three-dimensional flow structures generated in the flow field are qualitatively assessed through visualization and analyzed by statistical means. The near valve region is a major source of losses. Due to the presence of the valve, an annular, jet-like flow structure is formed where the high-velocity flow follows the valve stem into the port. Flow separation occurs immediately downstream of the valve seat on the walls of the port and also on the surface of the valve body. Strong longitudinal, non-stationary secondary flow structures (i.e. in the plane normal to the main flow direction) are observed in the exhaust manifold. Such structures can degrade the efficiency of a possible turbine of a turbocharger located downstream on the exhaust manifold.

Performance and Flow Regime of Annular Diffusers With Axial Turbomachine Discharge Inlet Conditions

1976 ◽  
Vol 98 (2) ◽  
pp. 236-242 ◽  
Author(s):  
S. O. Adenubi
Keyword(s):  
Experimental Investigation ◽  
Flow Regime ◽  
Turbulence Intensity ◽  
Velocity Component ◽  
Axial Compressor ◽  
Streamwise Velocity ◽  
Rotor Blades ◽  
Two Dimensional ◽  
Streamwise Velocity Component ◽  
Inlet Conditions

An experimental investigation of the flow regime and the performance of straight-core annular diffusers operating immediately downstream of an axial compressor was carried out. Features of the compressor’s discharge including the size of the blade wakes, the streamwise relative turbulence intensity and the periodic streamwise component of velocity induced by the rotor blades were studied. The periodic streamwise velocity component was found to be quite small and it decayed as the flow proceeded through the diffusers. The relative turbulence intensities were comparatively large and increased along the diffusers. The statistical distribution of the stall periods for the diffuser with transitory stall had characteristics similar to those found for two-dimensional diffusers.

Effect of Jet Shape of Square Array of Multi-Impinging Jets on Heat Transfer

2013 ◽  
Author(s):  
Yoshisaburo Yamane ◽  
Makoto Yamamoto ◽  
Masahiro Motosuke ◽  
Shinji Honami
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Flow Direction ◽  
Flow Behavior ◽  
Impinging Jets ◽  
Intermittent Flow ◽  
Inlet Temperature ◽  
Thermochromic Liquid Crystal ◽  
Local Flow ◽  
Flow Intermittency

It is necessary to increase turbine inlet temperature to improve the performance of the aircraft gas turbine engine. Therefore, effective cooling techniques are still required. The purpose of the present study is to clarify the heat transfer characteristics for the high cooling performance of multiple impinging jets. A focus is placed on the effect of the jet ejection shape, since the shape of jet is expected to enhance flow mixing in accordance with a change in vortex structures at the shear layer of the jet. Experiment was made on the wall jet interaction between adjacent impinging jets by changing the jet ejection shape. Both heat transfer and aerodynamic characteristics in 3×3 square arrays of three types of jet hole shapes, which are circle, cross-shape and oblique cross-shape, are investigated at jet diameter Reynolds number of 4,680. Injection distance is ranged from 2D to 6D, and jet-to-jet spacing is 6D where D is a jet hole diameter. Steady state thermochromic liquid crystal technique is employed to measure local and area averaged Nusselt number. A micro flow sensor, which can detect both flow direction and flow intermittency near the wall, is used to clarify the characteristics of the unsteady flow behavior. It is found that higher local Nusselt number area is spread outward in the concave direction of the cross-shaped jet on the target surface. Characteristic flow behavior due to the flow intermittency and the local flow fluctuation induced by the effect of jet shape are observed in the region surrounded by the adjacent impinging jets in the cases of cross-shaped jet and oblique cross-shaped jet. This intermittent flow phenomenon is considered to contribute to the enhancement of the heat transfer in the intermediate region enclosed by surrounded impinging jets.

Numerical Simulation of Fluid Flow in Microchannels With Superhydrophobic Walls

2005 ◽  
Author(s):  
Todd Salamon ◽  
Wonsuck Lee ◽  
Tom Krupenkin ◽  
Marc Hodes ◽  
Paul Kolodner ◽  
...  
Keyword(s):  
Velocity Field ◽  
Axial Velocity ◽  
Slip Length ◽  
Flow Behavior ◽  
Three Dimensional ◽  
Element Analysis ◽  
Local Flow ◽  
Apparent Slip ◽  
Navier’S Slip ◽  
Flow Enhancement

The three-dimensional flow of a Newtonian fluid in a microchannel with superhydrophobic walls is computed using a finite element analysis. Calculations of the fully-developed laminar flow of water under a pressure gradient of 1 psi/cm in an 80 μm high channel with superhydrophobic upper and lower surfaces containing a 2 μm pitch array of 0.2 μm square posts shows a 40 percent flow enhancement relative to the smooth, non-patterned surface case, and an apparent slip length of 5.4 μm. A sharp gradient is observed in the axial velocity field within 0.5 μm of the post surface and normal to the post center. The calculated axial velocity field away from the superhydrophobic surface agrees well with the analytical solution for two-dimensional channel flow with Navier’s slip condition applying at the channel wall. Mesh refinement studies indicate the important role that adequate resolution of the sharp gradient in the velocity field adjacent to the post surface plays in obtaining accurate flow enhancement predictions. Decreasing the relative contact area of the fluid with the solid portion of the channel surface, either by increasing the post-to-post spacing or decreasing the post size, results in a monotonic increase in the flow enhancement. Wetting of the fluid into the post structure is shown to dramatically decrease the calculated flow enhancement. Calculations of the flow enhancement for fixed surface properties and varying channel heights result in apparent slip lengths that agree to within 1 percent, suggesting that the macroscopic flow behavior is adequately characterized in terms of an apparent slip model, with the magnitude of the slip length a function of the post size, post spacing and wetting behavior that characterize the local flow field.

Analyse du sillage proche d'une sphère par une méthode de reconnaissance de formes

1995 ◽  
Vol 73 (3-4) ◽  
pp. 199-210
Author(s):  
P. Legentilhomme ◽  
J. A. Ferré ◽  
Francesc Giralt
Keyword(s):  
Three Dimensional ◽  
Streamwise Velocity ◽  
Near Wake ◽  
Circular Pattern ◽  
Spatial Periodicity ◽  
Karman Vortex ◽  
Streamwise Velocity Component ◽  
Main Components ◽  
Reconnaissance De Formes ◽  
Longitudinal Position

A pattern recognition technique has been applied to analyse the near-wake of a sphere for a Reynolds (Re) criteria of 12 900 and 25 900. Measurements of the main components of the down-stream turbulent flow of the dynamic field of a sphere allowed to confirm literature data. Fluctuations of the streamwise velocity component were measured simultaneously by eight hot-wire anemometers arranged in a circular pattern with the radius corresponding to the position of the extrema of the Reynolds stress [Formula: see text]. For streamwise locations xld = 18 (d = 4 cm) and 36 (d = 2 cm) (d, diameter of the sphere; x, longitudinal position relative to the centre of the sphere), in the near-wake of the sphere, structures occur periodically. The Strouhal number characterizing this periodic activity is 0.18. These structures, which also exhibit some spatial periodicity, seem to constitute the counterpart of the Kármán vortex path detected in the wake of a cylinder perpendicular to the main flow. In the wake of a sphere, these structures might appear in the three-dimensional form of continuous crenellated rings for xld = 18, or interrupted in one or several points of their circumference for xld = 36.

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