Mean Flow Properties Research Articles

TURBULENT STRUCTURE IN OPEN-CHANNEL FLOW OVER FLAT BED WITH BED-LOAD TRANSPORT

In open channel fows with bed-load transport, mean flow properties, turbulence intensities and Reynolds stress profiles are affected by the bed-load motions. In this study, LDA measurements over flat movable sand bed were conducted with high accuracy. The sediment transport was also measured simultaneously. It was found that the mean velocity gradient near the bed decreases with an increase of the bedload transport rate. Both the horizontal and vertical turbulence intensities, also increase in the inner layer as compared with those of rough fixed-bed open channel flows. Reynolds stress profiles near the bed deviate systematically from the triangular distribution which is theoretically valid in a two dimensional uniform open channel flow.

Turbulent flow field measurements in a model gas turbine combustion chamber

The results of the investigation of the reactive flow in a scaled-down (≅1:3) model of a gas turbine combustion chamber are presented. The lean-premix combustion chamber model was tested at atmospheric pressure; measurements in the reactive flow were taken by using a two-channel fibre-optic laser Doppler anemometer with Al 2O 3 seeding on the air flow. Measurements cover a wide region of the axial-symmetric combustion chamber at points characterised by different seeding conditions and data rates, including the flame development and recirculating zones. This work presents a detailed analysis of the fluctuating components of flow velocity; mean flow properties are presented as a prerequisite. Frequency and time domain characteristics of turbulence including power density spectra, autocorrelation coefficients and time integral scales are reported and discussed, and a physical description of the reacting turbulent flow field is attempted.

A Loosely Coupled Unsteady Flow Simulation of a Single Stage Compressor

A steady/unsteady, three-dimensional Navier- Stokes solver that utilizes a semi-implicit, pressure-based solution procedure is developed to simulate the three-dimensional, incompressible flow through a single stage compressor. The present numerical scheme features the implementation of a second-order plus fourth-order artificial dissipation formulation to prevent the numerical oscillation due to central differencing schemes. A low-Reynolds-number form of the two-equation turbulence model is used to account for the turbulence effects. For unsteady flow computations, the coupling between the mean flow properties and the turbulence is enhanced by an inner-iteration procedure during each time step. The steady flow field in the rotor passage is computed first. This is used as input for the computation of the unsteady flow in the subsequent stator. The predicted unsteady pressure on the stator blades and unsteady velocities at several locations inside the passage are compared with the experimental data. The unsteady pressures on the stator blade surfaces are in good agreement with the experimental data. The predicted unsteady velocity components at various locations inside the stator blade rows are generally smaller than the measured values in the endwall regions. The phase, angle variations of the unsteady velocity are in good agreement with the measured values. The effects of the rotor wake, secondary and tip clearance flows on the unsteady flow through the subsequent stator are studied. An attempt is also made to quantify the contributions of incoming tip leakage flows and the endwall boundary layers on the unsteady flow through the downstream stator. It was found that the endwall boundary layers and tip leakage flows have a much stronger influence on the unsteady flow development than the wake.

On Eddy Characteristics, Eddy Transports, and Mean Flow Properties

Abstract Estimates of eddy energy and eddy scales obtained previously from TOPEX/POSEIDON (T/P) altimeter data are interpreted in the context of a baroclinically unstable flow field. From the observations an integral timescale Talt can be defined that—combined with estimates of the eddy kinetic energy—sets a mixing length scale. Results are compared with theories of a baroclinically unstable flow field. For such conditions, the Eady theory predicts a timescale Tbc = Ri/f from the mean-flow Richardson number Ri, which shows some qualitative agreement with T/P results in terms of a geographical distribution. A factor of 2 difference between the timescales can be explained in terms of a systematic difference between the specific definitions of scale estimates. Although transfer length and velocity scales emerging out of scaling arguments lack resemblance with observations, a transfer length scale based on Tbc and the observed eddy kinetic energy is strikingly consistent with observed eddy scales. Primarily i...

Further studies onDatura metel Linn powder I: effect of surfactants on the granule and compact properties

A study has been made of the effect of the presence of surfactant in binder mucilage on the granules and compacts of D. metel powder (DMP). The mean size, friability, sinking times and flow property of the granules have been measured. Bigger and less friable granules were produced with the presence of surfactants due to increased wettability of the hydrophobic D. metel powder (DMP). The flow rate increased while the angle of repose decreased with the presence of surfactants and increase in surfactant concentration. Inclusion of surfactant resulted in the production of weaker compacts with a reduction in disintegration times. The improved wetting of the powder resulted in the decrease in disintegration times. © 1998 John Wiley & Sons, Ltd.

Contribution towards the second-moment closure modelling of separating turbulent flows

Six cases of turbulent flows over backward-facing steps and in sudden plane and axisymmetric expansions at a range of Reynolds numbers were analyzed computationally, using two variants of high- Re number second-moment closures and a new model which accounts separately for low- Re number, wall blockage and pressure reflection effects, thus allowing integration up to the wall. Attention was focused on a back-step flow at a step-height Reynolds number of 5000, for which detailed data were recently supplied both by experiments (Jović, S. and Driver, D. M., Backward-facing step measurements at low Reynolds number, Re H =5000, TM 108807, NASA, 1994, 1993) and by direct numerical simulation (Le, H., Moin, P. and Kim, J., Direct numerical simulation of turbulent flow over a backward-facing step. J. Fluid Mech. 1997, 330, 349. The new model (shown earlier to reproduce well several classes of equilibrium and nonequilibrium wall-parallel flows) improved the predictions of most mean and turbulent flow properties, and particularly the turbulent stress budget in all three characteristic flow subregions: separating, reattachment and recovery zones. Anomalous performances of some popular Reynolds-stress models are also analyzed and possible causes of the models' deficiencies and their elimination are discussed. It is demonstrated that the introduction of a new term in the invariant form into the ε-equation to suppress the excessive growth of the turbulence scale obviates the anomaly and better reproduces the streamline pattern.

EFFECT OF BED LOAD ON TURBULENT STRUCTURE IN AN OPEN CHANNEL FLOW

In open channel flows with bed-load transport, mean flow properties and turbulent characteristics are affected by the bed load motions. In this study, FLDA measurements over flat movable bed were conducted in an open channel flow. It was found that the mean velocity profile can be expressed by the log-law in the inner layer, but near the bed, the velocity profile is uniform, and its tendency become stronger as the volume of bed-load transport increases. Turbulence-intensity distributions also change in the inner layer; it curves upward from the exponential relation.

Measurements in Rollup Region of the Tip Vortex from a Rectangular Wing

The evolving three-dimensional flowfield of the tip vortex in the near wake of a rectangular wing at incidence was studied in detail, using three-component laser Doppler velocimetry. The flow quantities measured were the three components of the instantaneous velocity. These data were used to obtain the distributions of velocity, vorticity, and circulation across the vortex at several axial locations in the flow for several angles of incidence. The data have been used to understand the process of rollup of the shear layer into the vortex in the near wake, as well as its kinematic structure. The data indicate that the rollup takes place quite quickly and the inner part of the three-dimensional vortex becomes nearly axisymmetric within a distance of about two chord lengths downstream of the trailing edge. Even though the vortex behavior in the near wake is, in general, strongly dependent on the initial conditions, the vortex trajectory appears to be described reasonably well by the overall wing lift and the freestream velocity. Also, even in the near wake, circumferentially averaged mean flow properties in the inner part of the nearly axisymmetric vortex begin to exhibit a universal structure characteristic of conceptual asymptotic trailing vortices

NOISE FROM SUPERSONIC COAXIAL JETS, PART 2: NORMAL VELOCITY PROFILE

Instability waves have been established as noise generators in supersonic jets. Recent analysis of these slowly diverging jets has shown that these instability waves radiate noise to the far field when the waves have components with phase velocities that are supersonic relative to the ambient speed of sound. This instability wave noise generation model has been applied to supersonic jets with a single shear layer and is now applied to supersonic coaxial jets with two initial shear layers. In this paper the case of coaxial jets with normal velocity profiles is considered, where the inner jet stream velocity is higher than the outer jet stream velocity. To provide mean flow profiles at all axial locations, a numerical scheme is used to calculate the mean flow properties. Calculations are made for the stability characteristics in the coaxial jet shear layers and the noise radiated from the instability waves for different operating conditions with the same total thrust, mass flow and exit area as a single reference jet. The effects of changes in the velocity ratio, the density ratios and the area ratio are each considered independently.

NOISE FROM SUPERSONIC COAXIAL JETS, PART 1: MEAN FLOW PREDICTIONS

Recent theories for supersonic jet noise have used an instability wave noise generation model to predict radiated noise. This model requires a known mean flow that has typically been described by simple analytic functions for single jet mean flows. The mean flow of supersonic coaxial jets is not described easily in terms of analytic functions. To provide these profiles at all axial locations, a numerical scheme is developed to calculate the mean flow properties of a coaxial jet. The Reynolds-averaged, compressible, parabolic boundary layer equations are solved using a mixing length turbulence model. Empirical correlations are developed to account for the effects of velocity and temperature ratios and Mach number on the shear layer spreading. Both normal velocity profile and inverted velocity profile coaxial jets are considered. The mixing length model is modified in each case to obtain reasonable results when the two stream jet merges into a single fully developed jet. The mean flow calculations show both good qualitative and quantitative agreement with measurements in single and coaxial jet flows.

Effect of Bed-Load on Turbulent Structure in Open Channel Flows

Mean flow properties and turbulent characteristics of open channel flow with bed-load transport were experimentally studied in the 550×25.8×20cm flume with erodible gravel bed. The sand with the diameters of 0.59mm and 1.19mm were used and the rate of bed-load transport reach to 13g/cm·sec. The velocity were measured by ADV, and the characteristics of the turbulent intensity and the spectra of turbulent velocity were analyzed. The experimental results show that: 1) the mean velocity profile of open channel flow with bed-load can be expressed by a log-law; 2) the turbulent intensity near bed decreases with the increase of bed-load transport rate; 3) the spectrum of turbulent velocity also changes as the increase in bed-load transport.

Combined laser Doppler velocimetry and cross-wire anemometry analysis for supersonic turbulent flow

A synergistic laser Doppler velocimetry and multiple overheat cross-film anemometry analysis was developed to allow for direct measurements of the mean and turbulent flow properties in a supersonic turbulent flow. The present technique facilitated, for thin-layer, nonreacting flows, the measurement of the Reynolds (time) and Favre (mass-weighted time) averaged turbulent shear stress, heat flux, and apparent mass flux, without the usual ad hoc assumption of negligible static pressure fluctuations. The mean velocity, mass flux, and density were also acquired. The directly measured mean flow and turbulence results, in a Mach 2.8 wall boundary layer (Re θ = 1.12 x 10 4 ), were found to compare very well with numerical predictions based on the k-w two-equation (for the Favre approach) and the compressible apparent mass mixing length extension algebraic (for the Reynolds approach) turbulence models.

Hydraulic Jump in Sloping Channels

Experiments have been carried out to determine the streamwise variations of the mean flow properties of the hydraulic jump formed in sloping channels. Measurements of velocities have been made with hot-film anemometer for various Froude numbers of the flow. Strip integral method, with an assumption that the jump is a wall jet, is applied to determine the mean velocity field and the variation of depth along the jump. An empirical correction is made for the deviation of static pressures in the jump from hydrostatic pressures. Satisfactory agreement is found between the predicted and experimental results of the mean flow field. A body of data pertaining to the roller length collected by various investigators has been presented, and an explanation is provided for their scatter.

Flow Analyses in a Single-Stage Propulsion Pump

Steady-state analyses of the incompressible flow past a single-stage stator/rotor propulsion pump are presented and compared to experimental data. The purpose of the current study is to validate a numerical method for the design application of a typical propulsion pump and for the acoustic analysis based on predicted flowfields. A steady multiple-blade-row approach is used to calculate the flowfields of the stator and the rotor. The numerical method is based on a fully conservative control-volume technique. The Reynolds-averaged Navier–Stokes equations are solved along with the standard two-equation k–ε turbulence model. Numerical results for both mean flow and acoustic properties compare well with measurements in the wake of each blade row. The rotor blade has a thick boundary layer in the last quarter of the chord and the flow separates near the trailing edge. These features invalidate many Euler prediction results. Due to the dramatic reduction of the turbulent eddy viscosity in the thick boundary layer, the standard k–ε model cannot predict the correct local flow characteristics near the rotor trailing edge and in its near wake. Thus, a modification of the turbulence length scale in the turbulence model is applied in the thick boundary layer in response to the reduction of the turbulent eddy viscosity.

Spanwise Mean Flow Properties of Turbulent Boundary Layer through Gaps between Roughness Elements.

The off-centerline development of a turbulent boundary layer flow through a gap in an isolated wall-mounted roughness element has been studied experimentally in order to examine the effect of gap size on the spanwise mean flow properties. All measurements were made when the ratio of roughness height to boundary layer thickness is 0.135 at a Reynolds number of 3300, based on obstacle height and free stream velocity. From the off-centerline features in the horizontal plane at y+=125, the present flow field can be divided into two regions : the outer layer region and internal mixing region. The internal mixing region, furthermore, can be divided into four small groups : center region, interaction region, weak interaction region and mixing region. The interaction is more intense for a large gap than for a small one. The classification of gap size as mentioned in our previous paper is clearly related to the division of present flow field.

Predictions of the Structure of Turbulent, Highly Underexpanded Jets

A mathematical model capable of predicting the shock and flow structure of turbulent, underexpanded jets is described. The model is based on solutions of the fluid flow equations obtained using a second-order accurate, finite-volume integration scheme together with an adaptive grid algorithm. Closure of these equations is achieved using a k-ε turbulence model coupled to the compressible dissipation rate correction proposed by Sarkar et al. (1991a). Extending earlier work which demonstrated the ability of this model to predict the structure of moderately underexpanded jets, the present paper compares model predictions and experimental data, reported in the literature, on a number of highly underexpanded releases. The results obtained demonstrate that the model yields reliable predictions of shock structure in the near field, inviscid region of such jets, while in the far field results derived using the compressibility corrected turbulence model are adequate for predicting mean flow properties, and are superior to those obtained using a standard k-ε approach.

Control of a Jet Impinging on a Wedge having a Circular Cylinder at the Vertex. Mean Flow and Turbulence Properties.

An experimental study has been performed on the mean flow and turbulence properties of a two-dimensional jet impinging on a wedge having a circular cylinder with various diameters at the wedge vertex. The nondimensional profiles of the mean velocity in the fully developed flow region of the wall jet approach those of the conventional two-dimensional wall jet when a cylinder is attached to the wedge vertex and if the cylinder diameter is increased. Also both the slopes of the growth of the half-width and of the decay of the maximum velocity approach these of the two-dimensional case. The non-dimensional profiles of the turbulence intensities and of Rexnolds stress in the fully developed flow region of the wall jet behave in the same manner, as mentioned above. In the variations along the wall of the integral value of the total production term of Reynolds stress, there exists the negative value region of it near the wedge vertex, and it extends as the cylinder diameter increases.

Experiments on the Injection of a Circular Jet into a Wake of an Airfoil. Mean Flow Properties.

An experimental study has been performed on the three-dimensional mixing process in a circular jet which was injected into each wake of a two-dimensional symmetrical airfoil that was fitted with attack angles of 0° and 5° in a uniform flow. The results were compared with the that in the case of injection into the wake of the circular cylinder which has been examined by us previously. The profile of mean velocity relative to the wakes in the cases of the airfoil with the attack angles of 0° and 5° is axisymmetric, which agrees almost with the Gaussian profile. The decay of the relative maximum velocity takes place with the difference owing to the variation of the surrounding wake field. In all cases, the half width which was determined from the relative velocity distribution obeys the power law. In the case of the attack angle of 0°, the length scale shows symmetry, but in the case of the attack angle of 5°, the length scale shows asymmetry. It confirmed that this phenomenon is produced by the secondary flow.

Uniform flow in open channels with movable gravel bed

Mean flow properties and turbulent characteristics of uniform open-channel flow were experimentally studied in a reasonably steep channel with an erodible gravel bed. A recently developed Acoustic Doppler Velocity Profiler (ADVP) was used to obtain instantaneously the information on the flow profiles. From these measurements, the mean velocities, the turbulence-intensities as well as the Reynolds-stress profiles were obtained. The experimental results show that: i) the mean velocity profile can be expressed by the log-law with Br ≃ 8.42 in the inner region; Coles' law can be used to explain the velocity profiles in the entire region with Π ≃ 0.08; ii) the flow resistance coefficient,f, is explained with a theoretical relation; iii) the horizontal turbulence- intensity distribution agrees well with the exponential relation; iv) the vertical turbulence-intensity distribution agrees with the exponential relation only in the outer region; in the inner region, it curves backwards; v) the Reynolds stress are linearly distributed over the depth; vi) the friction velocities calculated with three different methods agree reasonably well with each other, this is considered as a proof that the flow is nearly two-dimensional; vii) the measured bed-load transport rates compare favorable with a bed-load relation; viii) all the above conclusions are in general valid for flows without and with bed-load transport; only small differences are noticed.

Fully developed turbulent pipe flow: a comparison between direct numerical simulation and experiment

Direct numerical simulations (DNS) and experiments are carried out to study fully developed turbulent pipe flow at Reynolds numberRec≈ 7000 based on centreline velocity and pipe diameter. The agreement between numerical and experimental results is excellent for the lower-order statistics (mean flow and turbulence intensities) and reasonably good for the higher-order statistics (skewness and flatness factors). To investigate the differences between fully developed turbulent flow in an axisymmetric pipe and a plane channel geometry, the present DNS results are compared to those obtained from a channel flow simulation. Beside the mean flow properties and turbulence statistics up to fourth order, the energy budgets of the Reynolds-stress components are computed and compared. The present results show that the mean velocity profile in the pipe fails to conform to the accepted law of the wall, in contrast to the channel flow. This confirms earlier observations reported in the literature. The statistics on fluctuating velocities, including the energy budgets of the Reynolds stresses, appear to be less affected by the axisymmetric pipe geometry. Only the skewness factor of the normal-to-the-wall velocity fluctuations differs in the pipe flow compared to the channel flow. The energy budgets illustrate that the normal-to-the-wall velocity fluctuations in the pipe are altered owing to a different ‘impingement’ or ‘splatting’ mechanism close to the curved wall.