مقاله مکانیک ()
مقاله مکانیک 6 - خصوصیات دینامیک جامدات - فصل دوم
by: امید ترجمان
فصل دوم کتاب " خصوصیات دینامیک جامدات" به مبحث "کاربردهای دینامیک شبکه پتانسیل های موثر متغیر در فرمول بندی انتگرال مسیر مکانیک آماری" می پردازد. این فصل به قلم ای راجر کراولی و جورج کی هورتون نوشته شده است. و مباحث فرعی همچون " فرمول بندی انتگرال مسیر و تابع تقسیم بندی" ، " روند متغیر و پتانسیل موثر" ، " پتانسیل متغیر درجه دوم" ، " پارامتر گینزبرگ" ، "تقریب جفت شدگی پایین" و بخش های فرعی دیگر را در بر می گیرد. در اینجا متن انگلیسی بخش اول این فصل برای شما دوستان درج می شود و در صورت تمایل به ترجمه می توانید از لینک های دوستان ما که در بخش پیوندهای وبلاگ موجود است اقدام به سفارش ترجمه این فصل کتاب کنید
While there exist several formalisms to calculate the lattice dynamical
properties of crystals, no one approach is universally superior. The original
harmonic, or quasi-harmonic, formalism is valid only if the amplitudes of
the atomic vibrations are small. For most materials this restricts the method
to low temperatures. For quantum crystals, the harmonic approximation is
never satisfactory because even the zero-point motion of the atoms is too
large. Anharmonicity can be included through a perturbation expansion and
this extends the temperature range over which the theory can be applied.
However, the successive terms in the perturbation expansion become more
and more complicated, and in some cases the expansion appears to be only
asymptotically convergent. Self-consistent phonon theories were originally
devised to deal with the quantum crystals. The harmonic force constants
are replaced by their averages calculated self-consistently over a trial set of
harmonic oscillator functions. The theory can be shown to be equivalent
to a summation of an infinite subset of the perturbation theory terms. The
great triumph of the first order self-consistent phonon theory (SC1) is that it
provides a physically reasonable starting point for the description of quan-
tum crystals such as helium, but when careful, quantitative, comparisons
are made, the first order theory is usually found to be inadequate. Prob-
ably the best lattice dynamical formalism is the improved self-consistent
theory (ISC), which adds on to the first order theory a non-self-consistent
calculation of the first missing perturbation theory term. It has been found
to give good agreement with experiment in many applications but is clearly
not a complete theory. At some temperature, which is not easily predicted,
the omitted terms must become significant.
This leaves us with the simulation techniques which have been widely
used in recent years. Molecular dynamics and the classical Monte Carlo
method each yield values for a wide range of average quantities which are
in principal exact if quantum mechanical effects are neglected. As more
powerful computers have become widely available, the statistical uncertain-
ties in the averages have been greatly reduced. These theories seem to us to
provide the most satisfactory description of the thermal properties of solids
at high enough temperatures. When quantum effects must be included, at
مقاله مکانیک 5 - فصل اول خصوصیت دینامیک جامدات
by: امید ترجمان
فصل اول از کتاب " خصوصیات دینامیک جامدات" به مبحث " تحقیقات مونت کارلو کوانتوم انتگرال مسیر متعلق به خصوصیات ارتعاشی کریستال ها می پردازد. این فصل نوشته آرتور آر مورگان از دپارتمان فیزیکی دانشگاه میشیگان غربی است. این فصل مباحتی همچون " روش های مونت کالرلو کلاسیک: جامدات گاز نجیب" ، " روش های مونت کارلو کوانتومی" ف مونت کارلو کوانتومی وابسته به زمان را دنبال می کند.در این پست بخشی از این مقاله در این کتاب مکانیک جامدات معتبر را می خوانید. برای ترجمه این فصل می توانید از لینک دوستان ما در بخش پیوندهای وبلاگ استفاده کنید.
In this chapter I will look at some of the recent developments in Monte
Carlo simulations for the static and dynamic thermodynamic properties of
lattice vibrations in quantum mechanical crystaline solids (i.e., the develop-
ment of quantum Monte Carlo methods as they apply to phonons in crys-
tals). This is a relatively new topic in solid state physics even though the
study of the thermodynamics of quantum vibrations in crystaline materials
is one of the first branches of modern physics to be developed (Born and
Huang 1954; Maradudin 1969; Paskiewicz 1987; Xia et al. 1990). The basis
of quantum Monte Carlo simulation techniques as applied to lattice vibra-
tions is the reformulation of the quantum mechanical partition function in
terms of a path integral expressed solely in classical (commuting) variables
(Suzuki 1976a, b, 1987; Suzuki et al. 1977; de Raedt and Lagendijk 1985;
Gubernatis 1986; Negele and Orland 1988; Doll and Gubernatis 1990; Ru-
binstein 1981; Binder 1984, 1986). In the course of this reformulation, one
finds that a d-dimensional quantum partition function is reexpressed as a
(d 1)-dimensional path integral so that the complications of evaluating
a partition function formed of non-commuting operators is carried over to
the evaluation of a higher dimensional classical problem. Our goal in this
chapter will be to present this path-integral reformulation and then to dis-
cuss the evaluation of the thermodynamics of the quantum system in terms
of the evaluation by classical Monte Carlo techniques of averages formed
in the corresponding path-integral formulation. As a final point, contact
will also be made with recent efforts to obtain approximate evaluations of
the path-integral formulation using variational techniques (Feynman 1988;
Samathiyakanit and Glyde 1973; Giachetti and Tognetti 1985-1987; Feyn-
man and Kleinett 1986; Giachetti et al. 1988a, b; Cuccoli et al. 1990,
1992a, b, 1993a; Liu et al. 1991, 1993).
lمقاله مکانیک 4 - جریان لزج تراکم ناپذیر
by: امید ترجمان
این پست مقدمه ای بر جریان لزج تراکم ناپذیر از کتاب " مقدمه ای بر مکانیک سیالات" را بیان می کند و به توزیع سرعت در این قبیل سیالات می پردازد. برای ترجمه مقاله از لینک های دوستان در سایدبار وبلاگ استفاده کنید.
In Chapter 3 the effects of pressure in a ﬂuid were isolated (because the ﬂuid was not
moving), and in Chapter 4 the inertia terms were added. The inclusion of viscosity,
its effects, and the resulting velocity distribution are discussed here. For example,
the velocity distribution for the laminar ﬂow inside a pipe is formulated and the
average velocity is calculated. This provides the relation between the simple 1D
average velocity model (of Chapter 4) and the more complex (and realistic) 2D or
The solutions presented early in this chapter are often called exact solutions.
This means that, for a few limited cases, a set of logical assumptions leads to simpliﬁ-
cation of the ﬂuid dynamic equations, which allows their solution (for laminar ﬂow)!
Also, the cases presented in this chapter (e.g., the ﬂow in pipes) is often termed as
internal ﬂows. The discussion on external ﬂows is delayed to the following three
The second part of this chapter demonstrates the approach that evolved during
the past 200 years for solving ﬂuid dynamic problems (because there is no closed-
form analytic solution to the complete ﬂuid dynamic equations). According to this
approach, to develop a practical engineering solution, we must start with a simple
but exact solution that determines the major parameters and the basic trends of the
problem (e.g., the pressure drop in a circular pipe versus the Reynolds number).
Based on these parameters, an empirical database can be developed for treating
a wider range of engineering problems. As an example, the viscous laminar ﬂow
model in circular pipes is extended into the high-Reynolds-number range and the
effects of turbulent ﬂow are discussed.
مقاله مکانیک 3 -بررسی آزمایشی انتشار موج در صفحه های ایزوتروپیک
by: امید ترجمان
این مقاله به بررسی واکنش انتقالی یک صفحه تخت با شعاع بزرگ به اثر متقطع محلی می پردازد
برای ترجمه این بخش از کتاب یا ترجمه مقاله های تخصصی خود می توانید به سایت ترجمه ای که در بخش پیوندهای وبلاگ درج شده اند مراجعه کنید
EXPERIMENTAL INVESTIGATION OF WAVE
PROPA GA TION IN ISOTROPIC ELA STIC PLA TES
Medick, M. A., and Steele, J. R.
Avco Corp., Wilmington, Mass.
This paper is concerned with the transient response
of a flat plate of large radius to a localized transverse
impact. Impulsive loadings were generated by ex-
plosive sources and high-speed projectiles. Capacitance-
type transducers and wire strain gages were used to
record transverse displacement and inplane strain-
time histories, respectively.
The basic characteristics of the response were
determined, as well as its variation with details of loading
(e.g., force-time history, total impulse and impact
mechanism), material, plate thickness and distance
from impact area. The ability of classical plate theory
to predict the flexural displacement response was studied
through comparisons of theory with experiments.
مقاله مکانیک - اثر حمل و نقل ریلی بر گسترش مهندسی مکانیک
by: امید ترجمان
این فصل از کتاب " شاخص های مهندسی مکانیک" به بررسی اثر گسترش حمل و نقل ریلی بر روی توسعه دانش مهندسی مکانیک می پردازد.
برای ترجمه این بخش از کتاب یا ترجمه مقاله های تخصصی خود می توانید به سایت ترجمه مقاله ایمان مراجعه کنید
Mechanical engineers have been stimulated by the challenges of railroading from its earliest days. In many ways railroads and engineering have grown up together. The
need to travel and transport materials overland goes back to ancient times. No one knows who first moved objects by rolling them on logs, thus making more efficient
use of animal and human power, and no one has identified that inspired individual who first conceived the wheel, axle, and bearing combination that made rolling
vehicles truly practical. The challenge then became, and has remained, how to carry more with greater comfort, speed, efficiency, and safety.
The concept of a railroad was born in England around 1630 when flanged rails were first used to guide coal wagons. In the early part of the nineteenth century, the
revision of this concept into one using flanged wheels on unflanged rails and the concurrent development of the steam locomotive set the stage for the development of
modern railroads. That blend of art and science we call mechanical engineering has played a major part in every step of this development.
The Baltimore & Ohio "Old Main Line" and the St. Charles Avenue streetcar line in New Orleans were two early efforts at practical railroads in the United States,
the former an intercity route powered first by horses and later by steam and diesel locomotives, and the latter a local carrier that experimented with several power
sources before settling on electricity. Both lines remain in service.
The continuing need for power to move heavier trains at faster speeds with greater efficiency has been the genus for several landmark locomotives. These include
Texas & Pacific No. 610, an early "Super Power" locomotive that revolutionized modern steam locomotive design, and Southern Pacific No. 4294 and Norfolk &
Western No. 611, two later applications of these same concepts to meet two vastly different needs. The New Haven's AC electrification of its New York
New Haven main line in 1907 pioneered mainline electrification in America. Almost thirty years later, Pennsylvania No. 4800, the prototype for a fleet of 139 electric
locomotives that were arguably the best every built, began operation. The early dieselelectric locomotives are represented here, too. The Pioneer Zephyr combined
a lightweight diesel engine with a train built with new materials and techniques to usher in the "streamline age." ElectroMotive FT freight diesel No. 103 has aptly been
called "the diesel that did it," for this was the locomotive that showed how diesels could outperform steam in freight as well as passenger service.
مقاله مکانیک -یاتاقان فشاری کینگزبری
by: امید ترجمان
این فصل از کتاب «شاخص های مهندسی مکانیک» به بررسی یاتاقان فشاری کینگزبری می پردازد و برای شما عزیزان درج شده است.
به علت مشغله کاری و کمبود فرصت امکان ترجمه کل این بخش وجود ندارد. اما طبق هماهنگی های به عمل آمده می توانید برای سفارش ترجمه کتاب یا این مقاله می توانید به سایت ترجمه مقاله ایمان رجوع کنید و از تخفیف مناسب دانشجوئی برخوردار شوید
The first Kingsbury thrust bearing was put into service on June 22, 1912, under the 10,000kilowatt Unit 5 at the Holtwood hydroelectric station of the Pennsylvania
Water & power Company. The 48inch (1,219mm ) diameter bearing has been at work ever since, effortlessly carrying 410,000 pounds (186,000 kg) at a speed
of 94 rpm.
All rotating machinery must use bearings to maintain the correct location between stationary and revolving parts, and to maintain the correct relative position of the
shaft and its supporting structure. Specifically, a thrust bearing maintains the relative axial location of a shaft and its supporting structure. Helicopter rotors, for
example, and boat and airplane propellers need thrust bearings on their shafts. So do water and steam turbines, which must operate continuously for long periods of
time—usually several years—with no maintenance.
The Kingsbury thrust bearing was the brainchild of Pittsburgh mechanical engineer Albert Kingsbury (1863–1943). Kingsbury's idea was deceptively simple: instead
of roller bearings, a series of adjustable bearing surfaces would carry the weight, gliding, as they did so, over a continuous film of oil.
Patented in 1910 (No. 947,242), the Kingsbury thrust bearing consists of a stationary castiron ring (called a "runner"), a cupshaped frame or collar (to
Albert Kingsbury (right) and Frederick A. Allner, who later became
a vice president of the Pennsylvania Water and Power
Company, inspect the Unit 5 thrust bearing in 1937.
contain the lubricant), the shaft, and a segmental ringbearing member comprised of several wedgeshaped bearing shoes (usually six, as in the case of Holtwood Unit
5) that are identical in size. Each shoe is loosely bolted through a tapped hole at its midpoint so that it can rock a bit. As the shaft rotates, a film of oil is forced
between the stationary ring and the shoes, where the pressure is highest. The oil actually supports the weight—there is no physical contact between the runner and the
shoes—resulting in extremely low friction and almost no mechanical wear.
Until the advent of the Kingsbury thrust bearing, units like
Holtwood represented the upper limit of hydroelectric turbine size; even then, roller thrust bearings commonly used in such installations wore out quickly and had to be
repaired or replaced with annoying (and expensive) frequency. Kingsbury bearings could support one hundred times the load of roller bearings with negligible wear
and were rapidly adopted for hydraulic and steamturbine use. Eventually, Pennsylvania Water & Power put them on all ten Holtwood units.
When Holtwood Unit 5 was rebuilt for sixtycycle service in 1950, the original Kingsbury bearing was found still to be in perfect condition. The bearing was inspected
again in 1969 with the same result. "Not a single part has ever been replaced", reads a plaque attached to the unit in recognition of Albert Kingsbury's singular
mechanical achievement, which made possible the design of much larger hydroelectric units, including those of the Tennessee Valley and Bonneville power authorities.
Kingsbury thrust bearings have also found wide application on the propeller shafts of ocean liners.