Navier-Stokes predictions of the individual components of the pitch-damping coefficient sum

1995 ◽  
Author(s):  
Paul Weinacht
Keyword(s):  
Damping Coefficient ◽  
Navier Stokes ◽  
The Individual ◽  
Pitch Damping

Formation of Standing Waves in Radial Tires

1984 ◽  
Vol 12 (1) ◽  
pp. 44-63 ◽  
Author(s):  
Y. D. Kwon ◽  
D. C. Prevorsek
Keyword(s):  
High Speed ◽  
Critical Speed ◽  
Unit Length ◽  
Standing Waves ◽  
Rolling Resistance ◽  
Damping Coefficient ◽  
Circular Membrane ◽  
Critical Speeds ◽  
Steel Cord ◽  
The Individual

Abstract Radial tires for automobiles were subjected to high speed rolling under load on a testing wheel to determine the critical speeds at which standing waves started to form. Tires of different makes had significantly different critical speeds. The damping coefficient and mass per unit length of the tire wall were measured and a correlation between these properties and the observed critical speed of standing wave formation was sought through use of a circular membrane model. As expected from the model, desirably high critical speed calls for a high damping coefficient and a low mass per unit length of the tire wall. The damping coefficient is particularly important. Surprisingly, those tire walls that were reinforced with steel cord had higher damping coefficients than did those reinforced with polymeric cord. Although the individual steel filaments are elastic, the interfilament friction is higher in the steel cords than in the polymeric cords. A steel-reinforced tire wall also has a higher density per unit length. The damping coefficient is directly related to the mechanical loss in cyclic deformation and, hence, to the rolling resistance of a tire. The study shows that, in principle, it is more difficult to design a tire that is both fuel-efficient and free from standing waves when steel cord is used than when polymeric cords are used.

scholarly journals Statistical Uncertainty of DNS in Geometries without Homogeneous Directions

2021 ◽  
Vol 11 (4) ◽  
pp. 1399
Author(s):  
Jure Oder ◽  
Cédric Flageul ◽  
Iztok Tiselj
Keyword(s):  
Channel Flow ◽  
A Priori ◽  
Time Integration ◽  
Navier Stokes ◽  
Statistical Uncertainty ◽  
Time Step ◽  
Order Of Magnitude ◽  
Averaging Time ◽  
The Individual ◽  
Time Required

In this paper, we present uncertainties of statistical quantities of direct numerical simulations (DNS) with small numerical errors. The uncertainties are analysed for channel flow and a flow separation case in a confined backward facing step (BFS) geometry. The infinite channel flow case has two homogeneous directions and this is usually exploited to speed-up the convergence of the results. As we show, such a procedure reduces statistical uncertainties of the results by up to an order of magnitude. This effect is strongest in the near wall regions. In the case of flow over a confined BFS, there are no such directions and thus very long integration times are required. The individual statistical quantities converge with the square root of time integration so, in order to improve the uncertainty by a factor of two, the simulation has to be prolonged by a factor of four. We provide an estimator that can be used to evaluate a priori the DNS relative statistical uncertainties from results obtained with a Reynolds Averaged Navier Stokes simulation. In the DNS, the estimator can be used to predict the averaging time and with it the simulation time required to achieve a certain relative statistical uncertainty of results. For accurate evaluation of averages and their uncertainties, it is not required to use every time step of the DNS. We observe that statistical uncertainty of the results is uninfluenced by reducing the number of samples to the point where the period between two consecutive samples measured in Courant–Friedrichss–Levy (CFL) condition units is below one. Nevertheless, crossing this limit, the estimates of uncertainties start to exhibit significant growth.

A CFD-Based Parametric Study on Modification of Discrete Sister Holes Location for the Film Cooling Flow

2014 ◽  
Author(s):  
Siavash Khajehhasani ◽  
Bassam Jubran
Keyword(s):  
Film Cooling ◽  
Numerical Study ◽  
Three Dimensional ◽  
Navier Stokes ◽  
Base Case ◽  
Spanwise Direction ◽  
Cooling Performance ◽  
Cooling Flow ◽  
Lift Off ◽  
The Individual

A numerical study on the effects of sister holes locations on film cooling performance is presented. This includes the change of the location of the individual discrete sister holes in the streamwise and spanwise directions, where each one of these directions includes 9 different locations, The simulations are performed using three-dimensional Reynolds-Averaged Navier Stokes analysis with the realizable k–ε model combined with the standard wall function. The variation of the sister holes in the streamwise direction provides similar film cooling performance as the base case for both blowing ratios of 0.5 and 1. On the other hand, the spanwise variation of the sister holes’ location has a more prominent effect on the effectiveness. In some cases, as a result of the anti-vortices generated from the sister holes and the repositioning of the sister holes in the spanwise direction, the jet lift-off effect notably decreases and more volume of coolant is distributed in the spanwise direction.

Analytical and Semi-empirical Models of Tornadoes and Downbursts

2021 ◽  
Author(s):  
Djordje Romanic ◽  
Horia Hangan
Keyword(s):  
Numerical Simulations ◽  
Stokes Equations ◽  
Empirical Models ◽  
Navier Stokes ◽  
Mean Flow ◽  
Navier Stokes Equations ◽  
Boundary Layer Equations ◽  
Simplifying Assumptions ◽  
The Individual ◽  
Semi Empirical

Analytical and semi-empirical models are inexpensive to run and can complement experimental and numerical simulations for risk analysis-related applications. Some models are developed by employing simplifying assumptions in the Navier-Stokes equations and searching for exact, but many times inviscid solutions occasionally complemented by boundary layer equations to take surface effects into account. Other use simple superposition of generic, canonical flows for which the individual solutions are known. These solutions are then ensembled together by empirical or semi-empirical fitting procedures. Few models address turbulent or fluctuating flow fields, and all models have a series of constants that are fitted against experiments or numerical simulations. This chapter presents the main models used to provide primarily mean flow solutions for tornadoes and downbursts. The models are organized based on the adopted solution techniques, with an emphasis on their assumptions and validity.

Spray Flames Simulation Using a Two Phase Combustion Model With Single Droplet Combustion Mode

2012 ◽  
Author(s):  
F. Wang ◽  
Y. Huang
Keyword(s):  
Droplet Diameter ◽  
Navier Stokes ◽  
Combustion Model ◽  
Droplet Combustion ◽  
Single Droplet ◽  
Two Phase ◽  
Spray Flame ◽  
Spray Flames ◽  
The Individual ◽  
Individual Droplet

There are three combustion regimes of individual droplet combustion behavior: the fully enveloped flame, the partially enveloped flame, and the wake flame. From PLIF measurement results, single droplet combustion phenomenon happens in spray flame, as well as lean type gas turbine combustion chamber sometimes. The drag coefficient, evaporation rate, and combustion rate are different according to the burning modes. At present, in Reynolds Averaged Navier Stokes (RANS) method and Large Eddy Simulation (LES) method, the droplets are treated as point source because the grid scale is bigger than the droplet diameter. A two phase combustion model with the consideration of the individual droplet burning mode is proposed before. In this paper, this model is tested by spray flames here again. Furthermore, this model was used in a concept lean premixed pre-vaporized (LPP) combustion case too. In spray flame, the predicted results are close to the experimental data.

Thermoacoustic Spectrum of a Swirled Premixed Combustor With Partially Reflecting Boundaries

2019 ◽  
Author(s):  
Kah Joon Yong ◽  
Max Meindl ◽  
Wolfgang Polifke ◽  
Camilo F. Silva
Keyword(s):  
State Space ◽  
Stokes Equations ◽  
Stability Margin ◽  
Navier Stokes ◽  
Model Parameters ◽  
Navier Stokes Equations ◽  
Acoustic Reflection ◽  
Wide Range ◽  
Space Formulation ◽  
The Individual

Abstract This study investigates the effect of partial acoustic reflection at inlet or outlet of a combustor on thermoacoustic stability. Parametric maps of the thermoacoustic spectrum are utilized for this purpose, which represent frequencies and growth rates of eigenmodes for a wide range of model parameters. It is found that a decrease of the acoustic reflection at the boundaries does not always imply an increase in the stability margin of the thermoacoustic system. As a matter of fact, a reduction in the acoustic reflection may sometimes destabilize a thermoacoustic mode. Additionally, we show that perturbed passive thermoacoustic modes may become ITA modes in the fully anechoic case. We briefly discuss the mode definitions ‘acoustic’ and ‘intrinsic’ commonly found in the literature. The computational analysis is based on a state-space formulation of the Linearized Navier-Stokes Equations (LNSE) with discontinuous Galerkin discretization. This approach allows to describe the thermoacoustic system as a linear combination of internal acoustics, flame dynamics and acoustic boundaries. Such a segregation grants a clear analysis of the respective effects of the individual subsystems on the general stability of the system, expressed in terms of adjoint-based eigenvalue sensitivity. The state-space formulation of the LNSE proposed in this paper offers a powerful and flexible framework to carry out thermoacoustic studies of combustors with arbitrary geometry and acoustic boundary conditions.

Roll Damping Simulations of an Offshore Heavy Lift DP3 Installation Vessel Using the CFD Toolbox OpenFOAM

2020 ◽  
Author(s):  
Brecht Devolder ◽  
Florian Stempinski ◽  
Arjan Mol ◽  
Pieter Rauwoens
Keyword(s):  
Boundary Element ◽  
Damping Coefficient ◽  
Navier Stokes ◽  
Roll Motion ◽  
Viscous Effects ◽  
Roll Damping ◽  
Two Phase ◽  
Damping Behavior ◽  
External Moment ◽  
Heavy Lift

Abstract In this work, the roll damping behavior of the offshore heavy lift DP3 installation vessel Orion from the DEME group is studied. Boundary element codes using potential flow theory require a roll damping coefficient to account for viscous effects. In this work, the roll damping coefficient is calculated using the Computational Fluid Dynamics (CFD) toolbox OpenFOAM. The two-phase Navier-Stokes fluid solver is coupled with a motion solver using a partitioned fluid-structure interaction algorithm. The roll damping is assessed by the Harmonic Excited Roll Motion (HERM) technique. An oscillating external moment is applied on the hull and the roll motion is tracked. Various amplitudes and frequencies of the external moment and different forward speeds, are numerically simulated. These high-fidelity full-scale simulations result in better estimations of roll damping coefficients for various conditions in order to enhance the accuracy of efficient boundary element codes for wave-current-structure interactions simulations.

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