Experimental and Numerical Characterization of the Flow Around the Keel of a Yacht

2008 ◽  
Author(s):  
Renata M. B. Chaves ◽  
Atila P. S. Freire ◽  
Alexandre T. P. Alho
Keyword(s):  
Laboratory Data ◽  
Turbulence Models ◽  
Detailed Comparison ◽  
Mean Velocity ◽  
Flow Parameters ◽  
Numerical Characterization ◽  
Sst Model ◽  
Order Of Magnitude ◽  
Measured Flow ◽  
Turbulence Data

The present work carries out a detailed comparison between numerical computations for the flow around the keel and the bulb of a sailboat and some newly obtained laboratory data. Two typical turbulence models are tested: the eddy-viscosity SST model and the second-moment model BSL-RSM-ω. Hot-wire anemometry (HWA) and particle image velocimetry (PIV) are used to characterize the flow around the keel and the bulb of a yacht. The experiments are conducted in a low speed wind tunnel. Measured flow parameters include the mean velocity profiles and second order moments. Both turbulence models are shown to perform well regarding mean velocity and global predictions. Turbulence data predictions, however, are shown to be erroneous by at least one order of magnitude.

scholarly journals Dynamics of Subsiding Shells in Actively Growing Clouds with Vertical Updrafts

2020 ◽  
Vol 77 (4) ◽  
pp. 1353-1369 ◽  
Author(s):  
Vishnu Nair ◽  
Thijs Heus ◽  
Maarten van Reeuwijk
Keyword(s):  
Shell Thickness ◽  
Large Scale ◽  
Cloud Environment ◽  
Initial State ◽  
Cumulus Clouds ◽  
Mean Velocity ◽  
Flow Parameters ◽  
Shallow Cumulus ◽  
Order Of Magnitude ◽  
Self Similar

Abstract The dynamics of a subsiding shell at the edges of actively growing shallow cumulus clouds with updrafts is analyzed using direct numerical simulation. The actively growing clouds have a fixed in-cloud buoyancy and velocity. Turbulent mixing and evaporative cooling at the cloud edges generate a subsiding shell that grows with time. A self-similar regime is observed for first- and second-order moments when normalized with respective maximum values. Internal scales derived from integral properties of the flow problem are identified. A self-similarity analysis using these scales reveals that contrary to classical self-similar flows, the turbulent kinetic energy budget terms and velocity moments scale according to the buoyancy and not with the mean velocity. The shell thickness is observed to increase linearly with time. The buoyancy scale remains time invariant and is set by the initial cloud–environment thermodynamics. The shell accelerates ballistically with a magnitude set by the saturation value of the buoyancy of the cloud–environment mixture. In this regime, the shell is buoyancy driven and independent of the in-cloud velocity. Relations are obtained for predicting the shell thickness and minimum velocities by linking the internal scales with external flow parameters. The values thus calculated are consistent with the thickness and velocities observed in typical shallow cumulus clouds. The entrainment coefficient is a function of the initial state of the cloud and the environment, and is shown to be on the same order of magnitude as fractional entrainment rates calculated for large-scale models.

scholarly journals The turbulent/non-turbulent interface bounding a far wake

2002 ◽  
Vol 451 ◽  
pp. 383-410 ◽  
Author(s):  
DAVID K. BISSET ◽  
JULIAN C. R. HUNT ◽  
MICHAEL M. ROGERS
Keyword(s):  
Large Scale ◽  
Turbulence Models ◽  
Passive Scalar ◽  
Reynolds Stresses ◽  
Mean Velocity ◽  
Velocity Fluctuations ◽  
Eddy Motion ◽  
Order Of Magnitude ◽  
Fixed Reference ◽  
Velocity Variances

The velocity fields of a turbulent wake behind a flat plate obtained from the direct numerical simulations of Moser et al. (1998) are used to study the structure of the flow in the intermittent zone where there are, alternately, regions of fully turbulent flow and non-turbulent velocity fluctuations on either side of a thin randomly moving interface. Comparisons are made with a wake that is ‘forced’ by amplifying initial velocity fluctuations. A temperature field T, with constant values of 1.0 and 0 above and below the wake, is transported across the wake as a passive scalar. The value of the Reynolds number based on the centreplane mean velocity defect and half-width b of the wake is Re ≈ 2000.The thickness of the continuous interface is about 0.07b, whereas the amplitude of fluctuations of the instantaneous interface displacement yI(t) is an order of magnitude larger, being about 0.5b. This explains why the mean statistics of vorticity in the intermittent zone can be calculated in terms of the probability distribution of yI and the instantaneous discontinuity in vorticity across the interface. When plotted as functions of y−yI the conditional mean velocity 〈U〉 and temperature 〈T〉 profiles show sharp jumps at the interface adjacent to a thick zone where 〈U〉 and 〈T〉 vary much more slowly.Statistics for the conditional vorticity and velocity variances, available in such detail only from DNS data, show how streamwise and spanwise components of vorticity are generated by vortex stretching in the bulges of the interface. While mean Reynolds stresses (in the fixed reference frame) decrease gradually in the intermittent zone, conditional stresses are roughly constant and then decrease sharply towards zero at the interface. Flow fields around the interface, analysed in terms of the local streamline pattern, confirm and explain previous results that the advancement of the vortical interface into the irrotational flow is driven by large-scale eddy motion.Terms used in one-point turbulence models are evaluated both conventionally and conditionally in the interface region, and the current practice in statistical models of approximating entrainment by a diffusion process is assessed.

A New Hybrid RANS/LES Modeling Methodology for CFD Applications

2011 ◽  
Author(s):  
Mohammad F. Alam ◽  
D. Keith Walters ◽  
David S. Thompson
Keyword(s):  
Turbulence Models ◽  
Detached Eddy Simulation ◽  
Backward Facing Step ◽  
Mean Velocity ◽  
New Model ◽  
Rans Model ◽  
Eddy Simulation ◽  
Modeling Methodology ◽  
Sst Model ◽  
Les Model

The primary weakness of current hybrid RANS/LES (HRL) models lies in the treatment of the “transition zone,” where the value and the physical interpretation of the eddy viscosity changes from LES to RANS, or vice versa. In order to address this problem, the initial version of a new HRL modeling methodology has been developed that incorporates two separate turbulent stress parameters (one from the LES model and the other from the RANS model). In this paper, the viability of the new model is demonstrated by predictions of the flow over a backward facing step, which is one of the canonical test cases used for the validation of turbulence models. The simulation results of backward facing step flow at ReH = 37,000 provided by Menter’s Shear Stress Transport (SST) model, a new version of Detached Eddy Simulation (DES) i.e. delayed DES model, and the new model are compared with experiments. Mesh sensitivity of the models is also studied employing two different types of mesh, in order to test the wide applicability of the HRL models in various realistic flow simulations. Pressure and skin friction distributions and mean velocity profiles obtained with the new HRL model show improved agreement with the experimental measurements versus DES, and less sensitivity to the mesh details. Turbulent kinetic energy profiles of both the new model and the RANS model show qualitatively good agreement with experiments.

scholarly journals Analysis of Flow Characteristics and Effects of Turbulence Models for the Butterfly Valve

2021 ◽  
Vol 11 (14) ◽  
pp. 6319
Author(s):  
Sung-Woong Choi ◽  
Hyoung-Seock Seo ◽  
Han-Sang Kim
Keyword(s):  
Turbulence Model ◽  
Dissipation Rate ◽  
Flow Characteristics ◽  
Turbulence Models ◽  
Flow Properties ◽  
Nominal Diameter ◽  
Large Pressure ◽  
Sst Model ◽  
Turbulence Effect ◽  
Valve Opening

In the present study, the flow characteristics of butterfly valves with different sizes DN 80 (nominal diameter: 76.2 mm), DN 262 (nominal diameter: 254 mm), DN 400 (nominal diameter: 406 mm) were numerically investigated under different valve opening percentages. Representative two-equation turbulence models of two-equation k-epsilon model of Launder and Sharma, two-equation k-omega model of Wilcox, and two-equation k-omega SST model of Menter were selected. Flow characteristics of butterfly valves were examined to determine turbulence model effects. It was determined that increasing turbulence effect could cause many discrepancies between turbulence models, especially in areas with large pressure drop and velocity increase. In addition, sensitivity analysis of flow properties was conducted to determine the effect of constants used in each turbulence model. It was observed that the most sensitive flow properties were turbulence dissipation rate (Epsilon) for the k-epsilon turbulence model and turbulence specific dissipation rate (Omega) for the k-omega turbulence model.

scholarly journals Using the SWEPT Methodology for a Preliminary Wind Potential Estimation

2014 ◽  
Vol 2014 ◽  
pp. 1-7
Author(s):  
Jean-Luc Menet
Keyword(s):  
Wind Turbine ◽  
Characteristic Curve ◽  
Rayleigh Distribution ◽  
Mean Velocity ◽  
Technical Data ◽  
Wind Potential ◽  
Order Of Magnitude ◽  
General Data ◽  
The Mean ◽  
A Site

The implantation of wind turbines generally follows a wind potential study which is made using specific numerical tools; the generated expenses are only acceptable for great projects. The purpose of the present paper is to propose a simplified methodology for the evaluation of the wind potential, following three successive steps for the determination of (i) the mean velocity, either directly or by the use of the most occurrence velocity (MOV); (ii) the velocity distribution coming from the single knowledge of the mean velocity by the use of a Rayleigh distribution and a Davenport-Harris law; (iii) an appropriate approximation of the characteristic curve of the turbine, coming from only two technical data. These last two steps allow calculating directly the electric delivered energy for the considered wind turbine. This methodology, called the SWEPT approach, can be easily implemented in a single worksheet. The results returned by the SWEPT tool are of the same order of magnitude than those given by the classical commercial tools. Moreover, everybody, even a “neophyte,” can use this methodology to obtain a first estimation of the wind potential of a site considering a given wind turbine, on the basis of very few general data.

Development of an Inverse Plume Model for Mass Eruption Rate in Unsteady Conditions

2021 ◽  
Author(s):  
Stephen A Solovitz
Keyword(s):  
Laboratory Data ◽  
Model Performance ◽  
Volcanic Eruptions ◽  
Reynolds Numbers ◽  
Model Parameters ◽  
Plume Height ◽  
Eruption Rate ◽  
Mass Eruption Rate ◽  
Downstream Effects ◽  
Order Of Magnitude

Abstract Following volcanic eruptions, forecasters need accurate estimates of mass eruption rate (MER) to appropriately predict the downstream effects. Most analyses use simple correlations or models based on large eruptions at steady conditions, even though many volcanoes feature significant unsteadiness. To address this, a superposition model is developed based on a technique used for spray injection applications, which predicts plume height as a function of the time-varying exit velocity. This model can be inverted, providing estimates of MER using field observations of a plume. The model parameters are optimized using laboratory data for plumes with physically-relevant exit profiles and Reynolds numbers, resulting in predictions that agree to within 10% of measured exit velocities. The model performance is examined using a historic eruption from Stromboli with well-documented unsteadiness, again providing MER estimates of the correct order of magnitude. This method can provide a rapid alternative for real-time forecasting of small, unsteady eruptions.

Flow Characteristics of Transitional Boundary Layers on an Airfoil in Wakes

2000 ◽  
Vol 122 (3) ◽  
pp. 522-532 ◽  
Author(s):  
H. Lee ◽  
S.-H. Kang
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Skin Friction ◽  
Velocity Fluctuation ◽  
Plate Boundary ◽  
Flow Characteristics ◽  
Turbulence Models ◽  
Transitional Region ◽  
Mean Velocity ◽  
Measured Velocity

Transition characteristics of a boundary layer on a NACA0012 airfoil are investigated by measuring unsteady velocity using hot wire anemometry. The airfoil is installed in the incoming wake generated by an airfoil aligned in tandem with zero angle of attack. Reynolds number based on the airfoil chord varies from 2.0×105 to 6.0×105; distance between two airfoils varies from 0.25 to 1.0 of the chord length. To measure skin friction coefficient identifying the transition onset and completion, an extended wall law is devised to accommodate transitional flows with pressure gradient and nonuniform inflows. Variations of the skin friction are quite similar to that of the flat plate boundary layer in the uniform turbulent inflow of high intensity. Measured velocity profiles are coincident with families generated by the modified wall law in the range up to y+=40. Turbulence intensity of the incoming wake shifts the onset location of transition upstream. The transitional region becomes longer as the airfoils approach one another and the Reynolds number increases. The mean velocity profile gradually varies from a laminar to logarithmic one during the transition. The maximum values of rms velocity fluctuations are located near y+=15-20. A strong positive skewness of velocity fluctuation is observed at the onset of transition and the overall rms level of velocity fluctuation reaches 3.0–3.5 in wall units. The database obtained will be useful in developing and evaluating turbulence models and computational schemes for transitional boundary layer. [S0098-2202(00)01603-5]

Computational Fluid Dynamics Evaluation of Heat Transfer Correlations for Sodium Flows in a Heat Exchanger

2010 ◽  
Vol 132 (5) ◽  
Author(s):  
Seok-Ki Choi ◽  
Seong-O Kim ◽  
Hoon-Ki Choi
Keyword(s):  
Heat Transfer ◽  
Heat Exchanger ◽  
Turbulence Model ◽  
Numerical Solutions ◽  
Numerical Study ◽  
Cross Flow ◽  
Turbulence Models ◽  
Parallel Flow ◽  
Sst Model ◽  
Heat Transfer Correlations

A numerical study for the evaluation of heat transfer correlations for sodium flows in a heat exchanger of a fast breeder nuclear reactor is performed. Three different types of flows such as parallel flow, cross flow, and two inclined flows are considered. Calculations are performed for these three typical flows in a heat exchanger changing turbulence models. The tested turbulence models are the shear stress transport (SST) model and the SSG-Reynolds stress turbulence model by Speziale, Sarkar, and Gaski (1991, “Modelling the Pressure-Strain Correlation of Turbulence: An Invariant Dynamical System Approach,” J. Fluid Mech., 227, pp. 245–272). The computational model for parallel flow is a flow past tubes inside a circular cylinder and those for the cross flow and inclined flows are flows past the perpendicular and inclined tube banks enclosed by a rectangular duct. The computational results show that the SST model produces the most reliable results that can distinguish the best heat transfer correlation from other correlations for the three different flows. It was also shown that the SSG-RSTM high-Reynolds number turbulence model does not deal with the low-Prandtl number effect properly when the Peclet number is small. According to the present calculations for a parallel flow, all the old correlations do not match with the present numerical solutions and a new correlation is proposed. The correlations by Dwyer (1966, “Recent Developments in Liquid-Metal Heat Transfer,” At. Energy Rev., 4, pp. 3–92) for a cross flow and its modified correlation that takes into account of flow inclination for inclined flows work best and are accurate enough to be used for the design of the heat exchanger.

scholarly journals Impact of Turbulence Models on the Air Flow in a Confined Rectangular Space

2020 ◽  
pp. 46-53
Author(s):  
Jakub Mularski ◽  
Amit Arora ◽  
Muhammad Azam Saeed ◽  
Łukasz Niedźwiecki ◽  
Samrand Saeidi
Keyword(s):  
Experimental Data ◽  
Air Flow ◽  
Turbulence Models ◽  
The Other ◽  
Experimental Measurements ◽  
Wall Region ◽  
Sst Model ◽  
Computational Fluid Dynamics Cfd ◽  
The Impact ◽  
The Given

The paper regards the impact of four different turbulence models on the air flow pattern in a confined rectangular space. The following approaches are analyzed. The Baseline (BSL) Reynolds model, the Speziale-Sarkar-Gatzki (SSG) Reynolds model, the Menter's shear-stress transport (SST) model and the basic k-ε model. Computational fluid dynamics (CFD) results are compared with the experimental measurements in four different planes. The Reynolds number for the given conditions is equal to 5000. The k-ε model yielded the most accurate results with regard to the experimental data but its reliability decreased near the wall region. With respect to the other models, it was also found that the k-ε approach generated the least circulating flow.

Conditional Sampling in a Transitional Boundary Layer Under High Freestream Turbulence Conditions

2003 ◽  
Vol 125 (1) ◽  
pp. 28-37 ◽  
Author(s):  
Ralph J. Volino ◽  
Michael P. Schultz ◽  
Christopher M. Pratt
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Streamwise Velocity ◽  
Turbulent Shear ◽  
Conditional Sampling ◽  
Freestream Turbulence ◽  
Mean Velocity ◽  
Transitional Boundary Layer ◽  
Turbulent Shear Stress ◽  
Order Of Magnitude

Conditional sampling has been performed on data from a transitional boundary layer subject to high (initially 9%) freestream turbulence and strong (K=ν/U∞2dU∞/dx as high as 9×10−6) acceleration. Methods for separating the turbulent and nonturbulent zone data based on the instantaneous streamwise velocity and the turbulent shear stress were tested and found to agree. Mean velocity profiles were clearly different in the turbulent and nonturbulent zones, and skin friction coefficients were as much as 70% higher in the turbulent zone. The streamwise fluctuating velocity, in contrast, was only about 10% higher in the turbulent zone. Turbulent shear stress differed by an order of magnitude, and eddy viscosity was three to four times higher in the turbulent zone. Eddy transport in the nonturbulent zone was still significant, however, and the nonturbulent zone did not behave like a laminar boundary layer. Within each of the two zones there was considerable self-similarity from the beginning to the end of transition. This may prove useful for future modeling efforts.

Sign in / Sign up

Export Citation Format

Share Document