Skewed Flow Research Articles

A closure model with plumes

Oscillations of stellar p modes, excited by turbulent convection, are investigated. We take into account the asymmetry of the up and downflows created by turbulent plumes through an adapted closure model. In a companion paper, we apply it to the formalism of excitation of solar p modes developed by Samadi & Goupil 2001. Using results from 3D numerical simulations of the upper most part of the solar convection zone, we show that the two-scale-mass-flux model (TFM) is valid only for quasi-laminar or highly skewed flows (Gryanik & Hartmann 2002). We build a generalized-Two-scale-Mass-Flux Model (GTFM) model which takes into account both the skew introduced by the presence of two flows and the effects of turbulence in each flow. In order to apply the GTFM to the solar case, we introduce the plume dynamics as modelled by Rieutord & Zahn (1995) and construct a Closure Model with Plumes (CMP). When comparing with 3D simulation results, the CMP improves the agreement for the fourth order moments, by approximatively a factor of two compared with the use of the quasi-normal approximation or a skewness computed with the classical TFM. The asymmetry of turbulent convection in the solar case has an important impact on the vertical-velocity fourth-order moment which has to be accounted for by models. The CMP is a significant improvement and is expected to improve the modelling of solar p-mode excitation.

Wind Tunnel Hotwire Measurements, Flow Visualization and Thrust Measurement of a VAWT in Skew

The results of experimental research on the wake and induced flow around a vertical axis wind turbine (VAWT) in skew are presented. The previous research on VAWTs in skew is limited because this operation mode has only recently been found to be significant in the operation of VAWTs in the built environment. These results contain hotwire measurements of the incoming flow and wake of a VAWT in nonskewed and skewed flow. The high sampling rate of the hotwire data allows the effects of blade passing to be identified. Flow visualization of the tip vortices is also presented. Thrust measurements of the rotor were performed to understand the effect of skew on thrust variation and to compare with analytical predictions.

Pattern formation of underwater sand ripples with a skewed drive

In this paper we present an experimental study of the dynamics of underwater sand ripples when a regular pattern of ripples is subjected to a skewed oscillatory flow, i.e., one not perpendicular to the direction of the ripple crests. Striking patterns with new, superposed ripples on top of the original ones occur very quickly with a characteristic angle, which is, in general, not perpendicular to the flow. A slower, more complex transition then follows, leading to the final state where the ripples are again perpendicular to the flow. We investigate the variation of the superposed pattern as a function of the direction, amplitude, and frequency of the drive, and as a function of the viscosity (by changing the temperature). We quantify the dynamics of the entire transition process and finally study the grain motion around idealized (solid) skewed ripples. This leads to a characteristic mean path of a single particle. The path has a shape close to a parallelogram, with no apparent connection to the pattern of real, superposed ripples. On the other hand, a thin layer of sand sprinkled on the solid ripples leads to qualitatively similar patterns.

Performance of an H-Darrieus in the Skewed Flow on a Roof1

Application of wind turbines on roofs of higher buildings is a subject of increasing interest. However, the wind conditions at the roof are complex and suitable wind turbines for this application are not yet developed. This paper addresses both issues: the wind conditions on the roof and the behavior of a roof-located wind turbine with respect to optimized energy yield. Vertical Axis Wind Turbines (VAWTs) are to be preferred for operation in a complex wind environment as is found on top of a roof. Since the wind vector at a roof is not horizontal, wind turbines on a roof operate in skewed flow. Thus the behavior of an H-Darrieus (VAWT) is studied in skewed flow condition. Measurements showed that the H-Darrieus produces an increased power output in skewed flow. The measurements are compared with a model based on Blade Element Momentum theory that also shows this increased power output. This in contradiction to a HAWT in skewed flow which suffers from a power decrease. The paper thus concludes that due to this property an H-Darrieus is preferred above the HAWT for operation on a flat roof of higher buildings.

Numerical Study of Coolant Mixing Caused by the Flow Deflectors in a Nuclear Fuel Bundle

A numerical study was conducted to investigate the nuclear fuel assembly coolant flow mixing that is promoted by the flow deflectors on the grid spacer. Four typical flow deflectors (split vane, side-supported vane, swirl vane, and twisted vane) were chosen for this study. A single subchannel of one grid span is modeled using the flow symmetry. The predicted axial and lateral mean flow velocities, and the turbulent kinetic energy in the subchannel for the split-vane design, are in good agreement with the experimental results.The split vane and the twisted vane generate a large cross flow between the subchannels and a skewed elliptic swirling flow in the subchannel near the grid spacer. The cross flow rapidly decreases and the swirling flow becomes dominant downstream of the spacer. The side-supported vane induces a horizontally elongated elliptic swirl in the subchannel and a secondary flow in the near downstream of the spacer. The swirl vane produces a circular swirling flow in the subchannel and a negligible cross flow. For the twisted-vane and side-supported vane designs, the change in direction of the cross flow was predicted. The average turbulent kinetic energy in the subchannel sharply increases near the spacer and rapidly decreases to a fully developed level. In summary, the numerical results showed a somewhat large difference from the experimental results near the spacer but represented the overall characteristics of coolant mixing well in a nuclear fuel bundle with the flow deflectors on the grid spacer.

A Study on the Choice of Dependent Variables of Momentum Equations in the General Curvilinear Coordinate

This paper represents the importance of dependent variables in non-orthogonal curvilinear coordinates just as the importance of those variables of convective scheme and turbulence model in computational fluid dynamics. Each of Cartesian, physical covariant and physical contravariant velocity components was tested as the dependent variables of momentum equations in the staggered grid system. In the flow past a circular cylinder, the results were computed to use each of three variables and compared to experimental data. In the skewed driven cavity flow, the results were computed to check the grid dependency of the variables. The results used in Cartesian and physical contravariant components of velocity in cylinder flow show the nearly same accuracy. In the case of Cartesian and contravariant component, the same number of vortex was predicted in the skewed driven cavity flow. Vortex strength of Cartesian component case has about 30% lower value than that of the other two cases.

Simulation and modelling of a skewed turbulent channel flow

A time-dependent three-dimensionally skewed flow is investigated using direct numerical simulations of the incompressible Navier-Stokes equations. The effect on the instantaneous and mean turbulent field is investigated. Instantaneous flowfields reveal that the skewing has the effect of initially reducing the strength and height of quasi-streamwise vortices of both signs of rotation with respect to the skewing. A mechanism for this process is put forward. The mean flowfields show drops in turbulence quantities such as turbulence kinetic energy. In addition to this, two-equation turbulence modelling of the flow is carried out. This highlights a deficiency, in that the standard turbulence models are unable to capture the drop in turbulence intensity due to the skewing. A modification based on the exact dissipation equation is found to significantly improve the model behaviour for this flow.

Local heat transfer measurements of an orthogonally rotating square duct with angled rib turbulators

Abstract The effect of angled rib turbulators on the heat transfer of an orthogonally rotating square duct were experimentally investigated. The Reynolds and rotation numbers were varied in 10,000–20,000 and 0–0.08, respectively. The leading and trailing walls were rib-roughened with angles of 90° and 60°. The results showed that the rotation induced a peripheral variation of the heat transfer and the higher heat transfer was seen using the 60° angled rib; judging from the local heat transfer variation, the skewed secondary flow induced by the angled rib seemed to result in additional heat transfer enhancement.

Simulation of skewed turbulent flow past a surface mounted cube

The flow past a cubic ground-mounted obstacle placed in a turbulent wind environment is studied using the large eddy simulation technique. The wind environment is taken from a pre-computed database containing the time-dependent inflow boundary conditions and representing a typical full-scale urban wind environment (Jenson number J=60). The Reynolds number R=10 000 is high enough for viscous scaling effects to be ignored, the turbulence intensity is about 15% at the cube height, and the integral length Lux is about 1.1 times the cube height h. The cube is aligned with one corner pointing upstream so that a pair of conical roof vortices are created. The computational grid used is effectively 362×226×98 in the streamwise, spanwise, and vertical directions, i.e. about 3×107 degrees of freedom, and uses 32 grid points along the sides of the cube. Two simulations are performed: (a) the flow with the cube absent so that the reference wind environment can be assessed; and (b) the flow past the cube for that wind environment. We present the flow topology as given by the mean streamlines, the roof pressures, the mean and fluctuating velocity and pressure field, and flow visualisation of the unsteady vortex shedding. A new shedding mechanism is identified which explains the turbulence statistics found in the wake.

Wall Turbulent Flow with Mean Skewed Velocity. 1st Report. Mean Velocity and Statistic Properties of Turbulence.

The skewed wall turbulent flow, driven by sudden span wise wall motion superimposed on plane Poiseulle flow, has been simulated by Direct Numerical Simulation (DNS). The time evolution of the mean velocities and the Reynolds stresses have been studied. Just after the spanwise wall motion starts the flow skewness is confined to the narrow wall region (y+<30) and the flow soon becomes another collateral flow again. During the evolution stage, turbulent kinetic energy first decreases and then increases. The time when q^-2 takes its minimum depends on the Reynolds number of the flow. The difference between shear driven and pressure driven skewed flow is also shown. The production and pressure strain terms in the Reynolds stress transport equation are shown to play an important role in leading to non parallel vectors of velocity gradient and Reynolds shear stresses.

Development of a parallel code to simulate skewed flow over a bluff body

This paper outlines a new complex geometry large-eddy simulation code using finite differences and a multigrid Poisson solver written for a parallel computer. The flow domain may be constructed from an arbitrary arrangement of rectangular blocks thus permitting flow in regions with complicated shapes, and may be mapped to any number of processors up to the number of blocks. The code is used to simulate turbulent flow at a Reynolds number of 3000 past a cube placed at ground level in rough terrain with its sides set 45° to mean flow direction. Preliminary results are presented which reproduce the conical vortices on the top of the cube.

The three-dimensional turbulent boundary layer flow around bridge piers

The three-dimensional skewed flow around circular cylinders placed on smooth, rough rigid and rough mobile (with scour hole) beds was analyzed in terms of the theories of the three-dimensional turbulent boundary layer (3DTBL). Performance of various cross-flow and near-wall similarity models was evaluated and the effects of roughness and scour hole were discerned. Perry and Joubert's model gave best results among cross-flow models. All near-wall models gave nearly identical results because of the small cross-flow but none gave good results. However, some indications of the near-wall similarity were found. The polar plots were found to be helpful in interpreting the cross-flow behavior, especially the effects of roughness and scour hole on it.

Binormal Model of Ensemble Partial Cloudiness

Abstract A new formulation of partial cloudiness parameterization has been introduced that agrees with that for a random model in one limit and approaches the simple updraft/downdraft model of larger-scale models in the limit of very highly skewed flow. Each of the conserved variables, liquid potential temperature and total humidity, along with the vertical velocity, are assumed to have probability distributions that may be parameterized as combinations of two multivariate normal distributions. This allows the skewness of the variables to be controlled by the bias between the means of the two normals and their relative fractions. It also provides a smooth transition between the normal distribution and the two limiting delta function distribution of the updraft/downdraft model. Comparisons with large-eddy-simulation data show this new model to be valid over a much wider range of conditions than the single normal distribution. When a simple cloud-top entrainment instability (CTEI) analysis is made using the...

Two-dimensional shear dispersion for skewed flows in narrow gaps between moving surfaces

Flows transporting material between nearby moving surfaces are ubiquitous in machinery of all scales and with a variety of geometries. Here a general derivation is given of the effective two-dimensional mixing process in a narrow gap for a solute or miscible fluids. Explicit formulae are given for the shear dispersion tensor in laminar and turbulent (logarithmic velocity profile) flows. It is shown that if improved mixing is required, then the optimum direction for additional boundary motion or stress is at right angles to the primary flow direction.

SERC flood channel facility: skewed flow experiments

This article describes experiments carried out on a skewed compound channel in the SERC Flood Channel Facility operated at Hydraulics Research Ltd, Wallingford, England. The project is one of a co-ordinated group of four being run simultaneously by U.K. University research groups into the hydraulics of overbank river flow. The other three projects in the group deal with parallel flow in a prismatic compound channel but this one examines the interactive cross-flows that occur between floodplain and main channel when the two are not parallel. The project was designed to prepare the way for a later examination of overbank flow in meandering channels. Results show that the velocity distribution both in the main channel and over the two floodplains are distorted by this cross-flow, as is also the boundary shear stress distribution. The momentum equation is used to analyse the forces acting on the flow both for the whole cross-section and also for the sub-units comprising the floodplains alone. The apparent she...

Velocity and Turbulence Fields in Pipe Entrance Regions in the Presence of Cross Flows

This paper reports on an experimental investigation into the velocity and turbulence fields in the entrance region of a circular pipe with cross flow at the entry. The pipe entry is a sharp square-edged type with air as the working fluid and measurements are made over the region 0&lt;X/D&lt;21. Observations are presented for a range of cross flows with a maximum Reynolds number, based on pipe diameter, of 64,000. The hot-wire anemometer measurements show a separated nonaxisymmetric flow at entry followed by a skewed flow which progressively smooths out with distance along the pipe.

In-Tube Heat Transfer for Skewed Inlet Flow Caused by Competition Among Tubes Fed by the Same Plenum

Measurements were made of the axial and circumferential distributions of the heat transfer coefficient in a tube in which the entering airflow is highly skewed. The skewness was caused by competition between the test section tube and a parallel tube which draws air from the same plenum chamber. For each of several fixed Reynolds numbers in the test section tube, the flow imbalance between the competing tubes was varied parametrically (up to a factor of eighteen), as was the center-to-center separation distance between the tubes (separation = 1.5, 3, and 4.5 times the tube diameter). Measurements were also made of the pressure drop, and a visualization technique was employed to examine the pattern of fluid flow. Practically significant effects of the flow imbalance on the axial distribution of the heat transfer coefficient were encountered only at the smallest of the investigated intertube spacings. Even for that case, the effects were moderate; for example, the imbalance-related changes for an imbalance ratio of two did not exceed 7 percent. The experiments involved naphthalene sublimation, and a new technique was developed for coating the inside surface of a tube with naphthalene.

Experimental Study on Diffusers for Mixed-Flow Machines

As a result of examining a relationship between inlet and inside flow patterns in the pure conical mixed-flow diffuser, it was disclosed that a comparatively uniform flow in the diffuser would be obtained resulting in a good pressure recovery. Also to find out an optimum configuration that would enable a stable and effective flow, tests on some model diffusers were made and the following was found: the skewed flow in the pure conical diffuser was rectifiable by means of fitting some guide fences on the face of the diffuser inner wall; though a little twist was observed in the main flow between the inner and outer walls, a curved diffuser gave a slight improvement on the recovered pressure over the case of pure conical one.

Generation of skewed annual flows using fast fractional Gaussian noise generator

The fast fractional Gaussian noise process is modified to generate skewed streamflows and those having negative lag 1 autocorrelation. The modified procedures are applied to two Australian streams, and the results are compared with the corresponding historical values.

The Decay of Highly Skewed Flows in Ducts

The development of a very nonuniform or skewed velocity profile in passing through a diffuser has been examined from the point of view of velocity components induced by line vortices. Account is taken of the effect of dissipation and straining on the strengths of the vortices and the intensities of the induced velocities. Changes with the integrated momentum of the perturbation velocities contribute to the local pressure in the duct, or diffuser, along with friction forces and area changes. An expression is derived for the pressure coefficient of a highly skewed profile flowing through a duct of arbitrary section. The results of experiments with an ejector apparatus are presented and found to support the analytical conclusions. In contrast with slightly skewed flows, diffusion will not inhibit the decay of highly skewed profiles.