Tag: fluid mechanics

Engineering Riddle – No. 5 (Fluid Mechanics) “Dimensionless”

I’m a fraction made of forces, comparing different strength
I have no final units, only characteristic length
I sample speed and weight and size, these are the things I measure
My subject as it ebbs and flows, changes shape forever
On top I show inertia, the power to keep things steady
Below I measure thickness, stickiness and eddy
In pipe or duct or channel, o’er wings be smooth, or rough
I’ll tell you dominant motive, whether sheering’s weak or tough
 
What am I?

Tunnel Vision Part III

The terminal and smells of hydraulics and motors are now well behind us and the status console indicates NORMAL operating conditions. It is no longer possible to look at the tube structural support rings as they pass at a rate faster than the flapping of hummingbird wings.

As the tube slowly turns to go around and through mountains, the pod rotates like a banking roller coaster keeping our sense of gravity directed down through our seats.

The on-board stereo is softly playing the soundtrack from a popular science fiction space trek show and my mind links up with a scene from an episode where the space ship is propelled to a distant part of the galaxy by mental control of thought, time, and energy. I am awakened from this brief daydream by a yellow WARNING sign above each seat and a soft professional automated voice instructing passengers to place their heads against the head rest for the upcoming increase in acceleration.

Just moments ahead is the greatest design challenge of the whole system: analysis of the geometric, thermal, and dynamic parameters of the wind tunnel throat such that this high volume of air will indeed transition from subsonic to supersonic speeds as the tunnel re-expands. While reflecting on the many details studied during the design phase, my mind wanders for an instant to the many potential applications to use this mode of high speed transportation. A ski lift in the Wasatch mountains, underwater transoceanic links from San Francisco to China, satellite launchers, postal payload and letter delivery networks, and large scale, 100 plus passenger models, like a commercial aircraft or train fuselage with no wings, tail, or landing gear. My reflection is interrupted by a subtle vibration caused by a shock wave forming on the leading edges of the pod…

Tunnel Vision Part II

Just before stepping through the hatch of our five foot diameter bullet, I look behind the craft at the 165 foot diameter fan stretching above and all around me and I realize more than I had ever before, that I would be sharing space with this huge mass of turbulent energy as we both accelerate into the common 5 foot diameter tunnel just ahead. Although invisible, I could feel the airstream converging in on itself, and me, creating a pressure difference across the pod.

We take our seats, which are highly reclined to minimize the pod and tunnel diameter, and fasten our seat belts for the non-stop trip. This is no ordinary craft. There are no wheels, no bearings, no engine, and no brakes. This pod is a pressure vessel with forward and aft regulators to balance the inside environment at a comfortable pressure, temperature and air circulation. Forward and aft circumferential hydrodynamic seals turn the cylinder into a piston, a very fast piston, driven at cruse by pneumatic pressure.

As the hatch is closed, I sit uneasy. I look out through the mostly transparent tubular walls at those who we leave behind and it occurs to me that in a moment, they will be many, many miles away. My eyes scan from the right, over head and to my left, taking in the exciting moment, and then after a brief wink at each member of my family, I look straight ahead and become acutely aware of the small  hole which is to be our immediate destination. The ALL CLEAR indicator illuminates on the front console, a small bump sounds from below, and we move out at a brisk 1/2 the acceleration of gravity, 15 ft/sec2.

v = a*t

x =  ½ a*t^2

(constant acceleration equations with zero initial velocity and position)

At this rate we reach 100 miles per hour in 10 seconds (not exactly a drag race car), having traveled a quarter mile. After 20 seconds, we pass 200 miles per hour and just over one mile. This constant acceleration continues as the air behind us converges and we approach the throat, minimum tunnel diameter, and 640 miles per hour…

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