Integral form of the skin friction coefficient suitable for experimental data

An integral method to evaluate skin friction coefficient for turbulent boundary layer flow is presented. The method replaces streamwise gradients with total stress gradients in the wall-normal direction and is therefore useful in cases when measurements at multiple streamwise locations are not available or feasible. It is also shown to be especially useful for experimental data with typical noisy shear stress profiles such as rough-wall boundary layer flows for which there are limited ways by which skin friction can be determined.     

Design and test of a friction damper to reduce engine vibrations on a space launcher

Space launchers are submitted to complex vibration environments and this can impact the payload it is carrying. Ensuring the protection of the payload therefore requires the addition of a secondary system. In this paper, a rapid design method for the dimensioning of a friction damper is developed, based on the equivalent energy dissipation with that of a viscous damper. A friction damper is designed and a prototype is built. The friction damper is first characterised alone and it is then mounted inside a scale model of a launcher last stage. The friction damper is adequately modelled by a spring in series with a friction element. The damper prototype proves to efficiently damp the rocket engine vibrations, and the design method used for dimensioning the friction damper gives a good approximation for the optimal sliding force of the damper.     

Application of the dynamic mode decomposition to experimental data

The dynamic mode decomposition (DMD) is a data-decomposition technique that allows the extraction of dynamically relevant flow features from time-resolved experimental (or numerical) data. It is based on a sequence of snapshots from measurements that are subsequently processed by an iterative Krylov technique. The eigenvalues and eigenvectors of a low-dimensional representation of an approximate inter-snapshot map then produce flow information that describes the dynamic processes contained in the data sequence. This decomposition technique applies equally to particle-image velocimetry data and image-based flow visualizations and is demonstrated on data from a numerical simulation of a flame based on a variable-density jet and on experimental data from a laminar axisymmetric water jet. In both cases, the dominant frequencies are detected and the associated spatial structures are identified.     

Equations of motion of ball lightning in a rocket airstream

Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 1, pp. 174–177, January–February, 1992.     

The non-linear chaotic model reconstruction for the experimental data obtained from different dynamic system

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Experimental determination of the friction factor coefficient in microchannels

Experimental data used to determine the friction factor in microchannels are presented. Glass microchannels with a circular cross section, diameters of 34.5, 33.6, and 24.5 μm, and different lengths are studied in the experiments. Pure deionized degassed water is used as a working fluid. The range of the Reynolds numbers is 13 ⩽ Re ⩽ 330. A method of two channels is used to calculate the friction factor. The results obtained are in good agreement with theoretical predictions for the case of a developed laminar flow in circular channels, but the use of the method of two channels has some constraints.     

Stability of motion of a two-mass system with nonlinear friction

This paper concerns the motion of different elastically coupled masses. One of the masses is subjected to a motive force, while the other mass is acted upon by friction. The motive force decreases linearly, while friction changes nonlinearly. The differential equations of motion are derived and are reduced to the standard form (after Bogolyubov). The averaging method is used to find steady-state solutions, one of which agrees with the exact steady-state solution of the initial system of equations. It is found that the actual conditions of stability of the steady-state solution are differ greatly from the conditions calculated on the basis of avaraged equations. These differences are due to the difference in the degrees of the characteristic Rouse-Hurwitz polynomials calculated on the basis of the initial and averaged equations. The analytical results are illustrated by modeling on a microcomputer. S. P. Timoshenko Institute of Mechanics, National Academy of Sciences of Ukraine, Kiev; Regional Scientific Research and Experimental Design Institute, Kiev, Ukraine. Translated from Prikladnaya Mekhanika, Vol. 35, No. 8, pp. 94–100, August, 1999.     

Multi-degree-of-freedom systems with a Coulomb friction contact: analytical boundaries of motion regimes

Nonlinear Dynamics - This paper proposes an approach for the determination of the analytical boundaries of continuous, stick-slip and no motion regimes for the steady-state response of a...     

摩擦力对两个恢复系数公式等价性条件的影响

文章讨论两个作一般运动的刚体在考虑摩擦力的情况下碰撞(不受外力作用)．首先由欧勒动力学方程和质心运动定理导出在碰撞的压缩阶段和恢复阶段二碰撞点沿公法线的相对速度的变化量, 然后给出两个恢复系数公式的等价性条件与刚体之间摩擦力的关系．     

The frequency response of dynamic friction: Model comparisons

Any linearised theory of the initiation of friction-excited vibration via instability of the state of steady sliding requires information about the dynamic friction force in the form of a frequency response function for sliding friction. Recent measurements of this function for an interface consisting of a nylon pin against a glass disc are used to probe the underlying constitutive law. Results are compared to linearised predictions from the simplest rate-state model of friction, and a rate-temperature model. In both cases the observed variation with frequency is not compatible with the model predictions, although there are some significant points of similarity. The most striking result relates to variation of the normal load: any theory embodying the Coulomb relation F∝N would predict behaviour entirely at variance with the measurements, even though the steady friction force obtained during the same measurements does follow the Coulomb law.     

Effect of friction on motion of a piston driven by combustion products

In recent years the two-stage light gas ballistic apparatus with deformable plastic pistons has become widely used in experimental aerodynamics. The existing methods of calculating such devices either completely neglect friction of the piston on the channel wall [1–3] or use a schematization of the frictional forces [4–6], which does not have a satisfactory physical basis. In a number of studies [7, 8] the friction force was considered constant, and its value was specified not from physical considerations, but to produce the best agreement between calculated and experimental values of object velocity or driving gas pressure. Since friction is such a significant factor, its proper consideration in calculating piston motion parameters requires special study. In this connection, it is useful to consider the operation of only the first stage of the ballistic apparatus, which sets the piston in motion. Below we will consider the problem of the internal ballistics of a one-stage powder-driven apparatus in the column channel of which a piston made of polymer material moves, experiencing friction. The friction model is constructed on the basis of a series of experiments on the slow forcing of polymer specimens compressed in the longitudinal direction through a steel channel. An experimental study was made of the relationship between the gunpowder gas pressure and time within a constant volume chamber, allowing establishment of the true powder-burning law, and its deviation from the geometric law of [9]. Calculated and experimental values of maximum gas pressure and muzzle velocity of a polyethylene piston are compared.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 5, pp. 73–80, September–October, 1978.The authors express their gratitude to S. S. Grigoryan for his interest in the study and valuable advice, and to G. N. Suntsov for aid in performing the experimental studies.