In electric machines, there are two common causes of eddy currents: (1) time-varying currents in coils, and (2) motion of conductors relative to sources of magnetic field. In this post, we show how to estimate the current density arising from a time-varying magnetic field passing through a plate.
Continue reading Eddy Currents Induced by a Time-Varying Magnetic Field
Low Reynolds number flow can be a very interesting topic. Low Reynolds number flow (Re 
) is also called Stokes flow. At very low Reynolds number, the Navier-Stokes equations can be greatly simplified. Fluid mechanics at human length scales, such as swimming, is generally not very low Reynolds number. Developments in microfluidics, nanotechnology, and biomimicry has increased the frequency with which engineers encounter low Reynolds flows problems. Because humans often encounter fluids at moderate of high Reynolds numbers, our intuition can deceive us. Two of the most basic results of low Reynolds flow is that it is fully reversible and independent of time.
As a refresher, the Reynolds number is the ratio of inertial to viscous forces and is given by
![\[ \mbox{Re}=\frac{\rho v d}{\mu} \]](https://i0.wp.com/mechanicsandmachines.com/wp-content/ql-cache/quicklatex.com-8490d2369c334339b5b0f400f232dfc3_l3.png?resize=70%2C37&ssl=1 "Rendered by QuickLaTeX.com")
where 
is the fluid density, 
is the velocity, 
is a characteristic length such as a diameter, and 
is the viscosity. The Reynolds number can also be thought of as the ratio of the momentum diffusion rate to the viscous diffusion rate. At Reynolds numbers less than approximately 2000, the flow is laminar. For Reynolds numbers greater than approximately 4000, the flow is turbulent. Continue reading Low Reynolds Number Flow
A great set of video lectures by Ascher Shapiro of MIT on Fluid Mechanics is available on youtube. The videos are old, but fluid mechanics hasn’t changed. Each video is presented by a world renowned fluids expert such as Ascher Shapiro (wikipedia link) or G.I. Taylor (wikipedia link). An accompanying set of notes is available at http://web.mit.edu/hml/notes.html. He uses experiments to explain the topics in a way which helps develop one’s intuition for understanding and solving fluids problems.
Mechanical supports for mirrors and other optical components and substrates to maintain their initial undeformed shape is a common engineering problem. Ideally a mirror or similar substrate can be supported on three points if the mirror or substrate is stiff enough. However in many cases, the deflections are too large and more support is required. One of the earliest areas where this problem arose was for the mirrors in early telescopes. Irishman Howard Grubb came up with a novel solution by supporting the mirror on a set of levers known as a whiffletree. For a historical bio of Howard Grubb see Biographical Encyclopedia of Astronomers or the Museum Victoria (Australia) bio or a history of the Armagh Observatory and Grubb’s telescope.
High vacuum systems are becoming more common and a number of semiconductor processes already operate in high vacuum. The following references are ones that I have found useful in performing vacuum system calculations.
Pfeiffer Vacuum, which manufactures vacuum pumps and other components, has a nice PDF handbook which is free to download. It covers the basics of vacuum as well as operating principles of various pumps and as well as a number of practical issues.
https://www.pfeiffer-vacuum.com/filepool/File/Vacuum-Technology-Book/Vacuum-Technology-Book-II-Part-2.pdf?referer=1456&request_locale=en_US
The Handbook of Vacuum Technology edited by Karl Jousten is a thorough reference with detailed calculations for wide variety of problems in vacuum systems.
amazon.com link
Continue reading Good References for Vacuum Flow Calculations
In this article, we compare the performance of a tuned-mass damper mounted at the end of a cantilever beam to the Lanchester damper which was shown in the previous article. The classic single-degree-of-freedom (SDOF) tuned-mass damper is sketched in the figure below. The design approach is to find the equivalent SDOF system for the cantilever beam’s mode of interest and then use the design formulas for an optimal SDOF TMD to determine the stiffness and damping of the absorber.
In this article, we show the robust and broadband performance of a Lanchester damper applied to a cantilever beam and how it achieves good performance without tuning and good performance over a number of modes, not just the primary mode.
This post shows how to set exact view orientation, zoom, pan, and lighting in CFD post. This is especially useful for making consistent figures for reports or presentations.
This picture below is a stream-ribbon visualization of a swirling flow, viewed using the built in “Isometric View (Z up).” The view is centered too low and the lighting angle is not favorable.
Continue reading Setting Views and Lighting in ANSYS CFD Post
We describe how to obtain the constraint equations for a two point pivot and three point pivot. Designing a mechanism which can obtain a desired set of constraints is often an important step in kinematic or exact constraint machine design.
We begin with the simple lever mechanism shown in the figure below constraining the motion of two points A and C using the pivot at O.
Continue reading Writing Constraint Equations for Two-Point and Three-Point Mounts
Beams are often used in precision engineering applications. One common question is “what are the optimal support locations for a beam?” The answer depends on the desired objective. Below we describe some of the most common support locations: Airy points, Bessel points, minimum deflection, and nodal points. It turns out that these points are relatively close to each other for the uniform beam. The basic problem is sketched in the figure below. A uniform beam is supported on two points and the objective is the determine the placement of the supports in the presence of gravity.
