Complex Turbulent Flow Field Research Articles

Large-scale dynamics in the flow around a finite cylinder with a ground plate

To date, physically meaningful representations of the nonstationarity in complex 3D flows with converged turbulent statistics are scarce and shed little light on the nonlinear processes in turbulent motion. This study attempts to address part of this deficit by concentrating on the kinematics of larger scales of motion. Two methods are utilized to describe the kinematics of large-scale unsteady motion in the flow around a wall-mounted finite circular cylinder at Reynolds number ReD = 200 000. The first, Proper Orthogonal Decomposition (POD), is a global method resulting in spatial modes defined over the whole domain and their corresponding temporal coefficients. The second, Coherent Structure Tracking (CST), belongs to a class of local methods that extracts connected domains in the flow data. Modes specific for distinct harmonics are extracted by temporal harmonic filtering. Based on time coefficients of the dominant mode pairs provided by POD or harmonic filtering, phase-averaging has been performed. A scalar-field version of CST is proposed, yielding an intuitively more accessible description of the flow. The extent to which POD and CST are complementary is discussed, as well as the extent to which they partially overlap. The combination of POD, filtering, phase-averaging and CST allowed for identification and quantification of important flow patterns in a complex turbulent flow field.

Estimation of the vortex shedding frequency of a 2-D building using correlation and the POD methods

A method was developed to estimate the frequency of vortex shedding in turbulent flow. In this method, a Particle Image Velocimetry (PIV) system was used to acquire the original complex turbulent flow field. Proper orthogonal decomposition (POD) analysis was applied to the PIV data in order to extract the main structure of the overall physical characteristics from the measured wind velocity field. A bivariate correlation was introduced to the POD results to determine the correlation of the vortex structures. Fast Fourier Transform (FFT) was employed to perform a power spectra analysis of these correlation coefficients to obtain the frequency of vortex shedding from the power peak. A PIV experiment was carried out within a wind tunnel simulated atmospheric surface layer, which involved a two-dimensional building problem with wind from two directions. The developed method was applied to the PIV experimental data and the frequency of vortex shedding was successfully extracted. The results show that the frequency of vortex shedding increases with increase in incoming flow velocity for both wind directions.

TURBINE BLADE FILM COOLING USING PSP TECHNIQUE

Film cooling is widely used to protect modern gas turbine blades and vanes from the ever increasing inlet temperatures. Film cooling involves a very complex turbulent flow-field, the characterization of which is necessary for reliable and economical design. Several experimental studies have focused on gas turbine blade, vane and end-wall film cooling over the past few decades. Measurements of heat transfer coefficients, film cooling effectiveness values and heat flux ratios using several different experimental methods have been reported. The emphasis of this current review is on the Pressure Sensitive Paint (PSP) mass transfer analogy to determine the film cooling effectiveness. The theoretical basis of the method is presented in detail. Important results in the open literature obtained using the PSP method are presented, discussing parametric effects of blowing ratio, momentum ratio, density ratio, hole shape, surface geometry, free-stream turbulence on flat plates, turbine blades, vanes and end-walls. The PSP method provides very high resolution contours of film cooling effectiveness, without being subject to the conduction error in high thermal gradient regions near the hole.

On the generation and maintenance of waves and turbulence in simulations of free-surface turbulence

One of the challenges in numerical simulation of wave–turbulence interaction is the precise setup and maintenance of wave and turbulence fields. In this paper, we investigate techniques for the generation and suppression of specific surface wave modes, the generation of turbulence in an inhomogeneous physical domain with a wavy boundary-fitted grid, and the generation and maintenance of waves and turbulence during the complex wave–turbulence interaction process. We apply surface pressure to generate and suppress waves. Based on the solution of linearized Cauchy–Poisson problem, we derive three pressure expressions, which lead to a δ -function method, a time-segment method, and a gradual method. Numerical experiments show that these methods generate waves as specified and eliminate spurious waves effectively. The nonlinear wave effect is accounted for with a time-relaxation method. For turbulence generation, we extend the linear forcing method to an inhomogeneous physical domain with a curvilinear computational grid. Effects of force distribution and computational grid distortion are examined. For wave–turbulence interaction, we develop an algorithm to instantaneously identify specific progressive and standing waves. To precisely control the wave amplitude in a complex turbulent flow field, we further develop an energy controlling method. Finally, a simulation example of wave–turbulence interaction is presented. Results show that turbulence has unique features in the presence of waves. Velocity fluctuations are found to be strongly dependent on the wave phase; variations of these fluctuations are explained by the pressure–strain correlation associated with the wave-induced strain field.

High resolution and high order accurate solution of complex turbulent flow field in jet elements

In view of the flow field of jet elements had the complex shock wave and the vortex characteristic. One kind of strongly compact—weighted ENO scheme is used for solving this kind of complex flow field. This form is applied to the Reynolds averaged Navier—Stokes equations, its convection term uses the third and fourth order, turbulence model uses Realizable κ–e model, which has calculated 7 kinds of operating conditions. Numerical results indicated that using this scheme may succeed in capturing to Coanda effect, namely if the total pressure under the limiting value, departure of flow will occur when there is no control flow. The circumfluence and the separation of boundary can be seen distinctly, and the high order accurate complex shock wave and the vortex can be caught from the calculating results. Moreover, the numerical results are in good agreement with the experimental ones, which demonstrates that this scheme has feasibility and validity. This is essential to guide the design of jet elements.

Management of a Stronger Wall Jet by a Streamwise Vortex with Periodic Perturbation (Effect of Periodic Perturbation on Reynolds Stress Distributions)

To investigate effect of periodic perturbation in management of a stronger wall jet by introducing a streamwise vortex, six components of the Reynolds stress tensor were measured in the three-dimensional complex turbulent flow field. Peak distributions in Reynolds stress contour maps should be understood by considering both contributions from production and convection terms. Especially, negative spanwise velocity component enlarged by periodic perturbation plays an important role on difference in the Reynolds stress contours. Assuming meander of the introduced streamwise vortex can reasonably explain periodic perturbation effect in (vw)^^- distribution and large discrepancies in the stress-strain relation. The periodic perturbation modifies Reynolds stress field and then development process of the streamwise vortex. Interaction between the streamwise vortex and the shear layer in stronger wall jet enlarges both length and velocity scales of the large-scale eddy in the outer layer.

Measurement of Turbulent Flow Fields in a Agitated Vessel with Four Baffles by Laser-Doppler Velocimetry. Mean Velocity Fields and Flow Pattern.

The three dimensional complex turbulent flow fields induced by a four flat blade paddle impeller in agitated vessel were measured by laser doppler velocimetry. Mixing vessel used was a closed cylindrical tank of 490 mm diameter with a flat bottom and four vertical buffles, giving water volumes of about 100 1. The impellers were at the midheight of the water level in the tank. A height of liquid (water) was equal to the vessel diameter. Three components of mean velocity were measured at three vertical sections θ=7.5°, 45°and 85°, in several horizontal planes. Mixing Reynolds number NRe was 1.2×105. It can be found from the results that circumferential mean velocity profiles show the symmetrical shape in the upper and lower sides of impeller. Secondary velocity components, such as axial and radial velocities, however, were not in symmetry. For this reason, the ratio of circulation flow volume which enter in upper and lower sides of impeller was roughly 7/3. In both the middle and buffle regions, mean flow velocities (flow patterns) were different, dependent of three vertical planes with different circumferential angle measured from buffle.

NUMERICAL STUDY OF TURBULENT FORCED CONVECTION IN A PERIODICALLY RIBBED CHANNEL WITH OSCILLATORY THROUGHFLOW

Engineering Systems International/Uihon ESI K.K., 2-47-18 Uehara, Shibuya. Tobo 151, Japan SUMMARY Numerical computations are performed on the fully developed flow and heat transfer in a periodically ribbed channel with oscillatory throughflow. A uniform heat flux is imposed at the lower plate of the channel. An externally sustained pressure gradient vanes sinusoidally in time. A low-turbulent-Reynolds-number version of the k+ two-equation model of turbulence is invoked, together with a preferential dissipation modification, to predict the complex turbulent flow field. Computed results indicate that much heat transfer enhancement is expected by increasing the Womersley number, which measures the relative strength of the oscillatory motion to the viscous effects. Turbulent heat transfer in a channel with rib-roughened walls is of much engineering relevance. It is known to be an effective means of heat transfer enhancement in compact heat exchangers, cooling of electronic devices and other types of thermal engineering equipment. A higher heat transfer rate may be expected by imposing on the channel an externally sustained oscillatory pressure gradient, by which better mixing of the medium can be accomplished. It is thought to be a promising technique for cooling high-density electronic circuit boards, for which an excessively high flow rate of the cooling agent is not desirable from a mechanical design point of view. Despite its potential for innovative engineering applications, the available information on turbulent heat transfer in oscillatory flow is limited. Ohmi et d.' measured the velocity profiles of oscillatory througMow in a rectangular duct using hot-wire anemometry. Instantaneous velocity profiles for turbulent oscillatory flow were found to be similar to those of non-oscillatory flow. Consequently, the profiles contained little phase shift, contrasting with their laminar flow counterparts. The same authors have also constructed a flow regime diagram, thereby identifying the role of the mechanically imposed frequency (characterized by the Womersley number) in the flow transition. Their experiments were conducted under an isothermal condition. Features of the flow field and mass transfer rate were investigated for a wavy walled channel by Nishimura et aL2 With a smoothly contoured wall, flow separation and subsequent growth of a vortex, which eventually occupied the entire flow passage, were clearly observable in the acceleration stage. In the deceleration stage accompanying the reversal in the flow direction, these vortices were

Direct numerical simulation of turbulent flow in a square duct

A direct numerical simulation of a fully developed, low-Reynolds-number turbulent flow in a square duct is presented. The numerical scheme employs a time-splitting method to integrate the three-dimensional, incompressible Navier-Stokes equations using spectral/high-order finite-difference discretization on a staggered mesh; the nonlinear terms are represented by fifth-order upwind-biased finite differences. The unsteady flow field was simulated at a Reynolds number of 600 based on the mean friction velocity and the duct width, using 96 × 101 × 101 grid points. Turbulence statistics from the fully developed turbulent field are compared with existing experimental and numerical square duct data, providing good qualitative agreement. Results from the present study furnish the details of the corner effects and near-wall effects in this complex turbulent flow field; also included is a detailed description of the terms in the Reynolds-averaged streamwise momentum and vorticity equations. Mechanisms responsible for the generation of the stress-driven secondary flow are studied by quadrant analysis and by analysing the instantaneous turbulence structures. It is demonstrated that the mean secondary flow pattern, the distorted isotachs and the anisotropic Reynolds stress distribution can be explained by the preferred location of an ejection structure near the corner and the interaction between bursts from the two intersecting walls. Corner effects are also manifested in the behaviour of the pressure-strain and velocity-pressure gradient correlations.

A computational study of annular cascade flows using aq-ω turbulence model with a wall function

A computational code has been developed for steady viscous flows in three dimensional annular cascades. This code solves a special form of the thin-layer Navier-Stokes equations with a two-equationq-ω turbulence model in curvilinear coordinates using a time asymptotic method for steady state solutions. It employs a scalar implicit approximate factorization in time and a finite volume formulation with second-order upwind-differencing in space. A wall function treatment is implemented at solid boundaries for turbulence equations instead of integration to the wall to relieve gridding requirements. In order to validate the effectiveness of this code, computational studies have been made to access modeling capability for complex turbulent flow fields in three dimensional annular cascade geometries which typically include laminar-turbulent boundary layer transition. The results have been compared with both the computational studies with integration to the wall and the experimental studies. The wall function treatment was found to be reliable by predicting secondary flows and loss contours reasonably well.

Mean velocity and turbulence fields inside a β=0.50 orifice flowmeter

AbstractThe flow field inside an orifice flowmeter with a beta ratio of 0.50 operating at a Reynolds number of 91,100 has been studied using a three‐color, 3‐D laser Doppler anemometer system. Mean velocity measurements show large radial velocities leading into the orifice, the fluid separating from the orifice plate at the throat, the presence of a vena contracta farther downstream, flow reattachment to the pipe wall 5.3 pipe radii (R) downstream, the presence of a small upstream recirculation zone, and both a primary and secondary recirculation zone downstream of the orifice plate. The static wall pressure distribution attains a minimum pressure at 1.5 R, which does not coincide with the location of the vena contracta (0.75 R). Distributions of the entire Reynolds stress tensor are presented along with calculated values of turbulence kinetic energy, turbulence kinetic energy production, vorticity, and turbulence induced accelerations. These data are analyzed to interpret the complex turbulent flow field.

Labyrinth Seal Rotordynamic Forces Using a Three-Dimensional Navier-Stokes Code

A finite difference method for determining rotordynamic forces on an eccentric whirling labyrinth cavity has been developed. A coordinate-transformation was applied to the Reynolds time-averaged Navier-Stokes equations in order to use the modified bipolar coordinate system. The SIMPLER algorithm with QUICK differencing and the high Reynolds number k–ε turbulence model are used to compute the complex turbulent flowfield. A circular whirl orbit about the geometric center of the housing was specified for simplicity. The new model was tested against the rotordynamic force measurements, and close agreement was found. For the cases considered, the radial and tangential force components become rotor dynamically less desirable with increasing inlet swirl. Also, circumferential pressure variations are included for enhanced insight into the flowfield.

Acoustic vortical interaction in a complex turbulent flow

This investigation is concerned with the interaction between vortical structures and an externally applied acoustic field in a complex turbulent flow field. The flow field is produced by a flow of air over a backward-facing step in a rectangular section duct. The purposes of this investigation are to (a) demonstrate the existence of somewhat organized vortical structures in the shear layer, (b) demonstrate that the vortical structures are influenced (organized) by an applied acoustic field, and (c) determine if the vortical structures in turn affect the applied acoustic field. By anemometry and acoustic pressure measurement, together with aeroacoustic theory, the first demonstrations are made. However, in the flow field used, without strong downstream constrictions, there is no discernible feedback into the acoustic field.

Efficiency of mixing from data on fast reactions in multi‐jet reactors and stirred tanks

AbstractSome fluid mechanical information concerning mixing generation in complex flow fields can be obtained from conversion data of fast chemical reactions. Computation of such information and averages required in the procedure are presented here. Complex turbulent flow fields present deterministic‐like behavior when analyzed in average sense with regard to mixing generation.