Millions saw the apple fall, But Newton asked why.
Rockets 101 – How to turn during flight ?
To be able to control is what distinguishes a toy rocket from a real one. And it is of quintessence to be able to channel the rocket’s direction. To be able to fly is cool, but you what is ever more cool, to be able to pinpoint the destination and its trajectory.
In most modern rockets, this is accomplished by a system known as Gimbaled Thrust.
In a gimbaled thrust system, the exhaust nozzle of the rocket can be swiveled from side to side. As the nozzle is moved, the direction of the thrust is changed relative to the center of gravity of the rocket and a torque is generated. As a result, the rocket changes direction. After necessary corrections are made, the exhaust nozzle is brought back to its initial state.
The angle by which the rocket’s nozzle swivels is known as the Gimbaled Angle.
To decelerate the aircraft, the forward thrust of the aircraft which propels the aircraft forward is reversed by a Thrust Reverser. This is accomplished by redirecting all the air ( in small engines ) or partially ( in larger engines ) to the front.
Due to the flow of air to the front end of the aircraft, a reaction force acts in the opposite direction to the motion of the aircraft. This aids in the deceleration of the aircraft.
Perpetual motion machines… Wow! I haven’t had any productive design ideas for PMM’s at all. But I wish to share with you one of my favorites. It is highly efficient, but it eventually does succumb to nature.
On November 7 the Tacoma Narrows was seen dancing happily to the song of the winds. But unfortunately, the bridge collapsed only after an hour of swaying.
The reason for collapse of the Tacoma Narrows bridge is often attributed by many physics textbooks as Resonance. But it is incorrect. Atleast, the way in which it is explained in textbooks.
This real reason for it’s collapse is what engineer’s call as Flutter.
The collapse Mechanism.
I’ll start explaining this by highlighting some key points in the bridges design.
The bridge itself had a span of 4944 ft (1506.9 m) and connected the city of Tacoma to the Kitsap Peninsula. The bridge consisted of two pillars which suspended the central span which itself was 2800 (852.4 m) feet long and 39 ft (11.9 m) wide. During the construction of the bridge itself, it was reported that some transverse (vertical) oscillations occurred across all three segments of the bridge with the two pillars and the connection to the shore acting as nodes (areas of no oscillation).
To counteract this, the left and right sections of the bridge were reinforced by diagonal ties and hydraulic buffers that damped the oscillations but the center of the bridge was still free to vertically oscillate.
During the short commercial lifetime of the bridge (between it’s completion on 1st July, 1940, until 7th November 1940), it earned the affectionate name of “Gallopin’ Gertie” as it frequently oscillated with a range of 0 – 8 nodes between the two pillars. The maximum amplitude before the gale that caused the collapse of the bridge was recorded to be 5 ft (1.52 m) from crest to trough at a frequency of 0.13 Hz and well within the range of maximum stress the bridge was designed to withstand.
Several days before the 7th November, it is believed that that K-bracing under the deck and diagonal ties at the support pillars had been weakened during a storm; with one witness reporting to have seen the bridge behaving differently (this is later interpreted to mean that the bridge had been displaying larger than normal transverse oscillations).
On the morning of 7th November, the Tacoma Narrow bridge was buffeted by wind velocity of 42 miles per hour. The high winds, suspected structural damage and the loosening of diagonal ties combined to cause a fatal combination of transverse and torsional (twisting) oscillations.
But what caused the eventual collapse of the bridge?
Most A level text books will attribute the collapse of the bridge to resonance caused by the frequency of the driving force (supplied by the wind) being similar to the natural frequency of the bridge causing a drastic increase in the amplitude of oscillation (see resonance curve below) and the eventual collapse of the bridge due to the stress exerted by the increased amplitude.
This has several fatal assumptions, the most obvious of which is that the gusts of wind would occur with any defined or regimented period (which is clearly ridiculous).
A more credible explanation is that that collapse was triggered by a phenomenon called Vortex Shedding.
A vortex is a region in fluid medium that flows around an axis. Vortex shedding is where a fluid flows past a buff (non streamlined) object and results in an oscillating flow of vortexes that detach periodically from either side of the body. It is believed that these periodic vortexes exerted a periodic force alternately on the top and bottom causing the torsional oscillation around the central line of the bridge.
It can also be inferred that the frequency at which vortexes were produced must have been similar to the natural frequency of the bridge. This would have caused resonance and thus would explain why the amplitude of the torsional oscillation was large enough (and able to overcome frictionaltension forces that would have reduced the amplitude) to exert enough strain on the bridge to cause it to collapse.
However, there is more to this argument than originally appears.
For the mathematically inclined, the frequency of shredding vortexes is defined by,
Where St is a constant called the Strouhal number (for the Tacoma Bridge this constant is 0.11), f is the frequency of the detachment of vortexes, D is frontal dimension (in this case, the depth of the bridge platform was 8 ft (2.44 m)) and V is the Velocity of the wind.
From this we can calculate the the frequency of the vortex shredding is close to 1 Hz (1 vortex being produced per second). The observed frequency of the bridges oscillations was close to 0.2 Hz.
This means that the Shredding Vortexes cannot have caused resonance and there must have been another phenomenon in action.
Whilst the shredding vortexes may have caused the initial torsional oscillation, the increasing amplitude was a self induced.
When an object changes direction in a fluid stream, it causes new vortexes to form behind it. This is known as a Flutter Wake. This is caused when the air flow is disconnected from the surface and vortexes flow into the newly formed low pressure region. This can be seen with a schematic of a plane wing changing direction.
Similar to the shredding vortexes, the flutter wake exerted a force on the bridge increasing the amplitude of oscillations. As the amplitude of the oscillations increased, so did the difference in air pressure between the surface of the bridge and the undisturbed air flow causing more vorticity. This in turn increased the force exerted by the flutter wake and as a result increased the amplitude until the bridge reached breaking point.
The mean net force experienced by the bridge can be described as negative damping, rather than the amplitude decreasing over time, the amplitude increases until breaking point.
This is a perhaps similar to “Which came first, the chicken or the egg?”.
The (shredding) vortexes causes motion, and the motion causes more (flutter) vortexes.
The wind supplies the power, and the motion supplies the power tapping mechanism.
A big shout out to Sophie Meredith who painstakingly drafted the collapse mechanism. This post wouldn’t have been possible if not for her. Thanks a lot !
Exploring the Realms of water – The Leidenfrost effect ( #1 )
The skittering of water droplets of a hot surface that you just witnessed is known as the Leidenfrost Effect.
The effect can be seen as drops of water are sprinkled onto a pan at various times as it heats up. There is no specific temperature, beyond which the Leidenfrost effect kicks in.
Unveiling the mystery.
The bottom part of the water droplet vaporizes immediately on contact with the hot plate.
The resulting gas suspends the rest of the water droplet just above it, preventing any further direct contact between the liquid water and the hot plate.
As steam has much poorer thermal conductivity, further heat transfer between the pan and the droplet is slowed down dramatically. This also results in the drop being able to skid around the pan on the layer of gas just under it.
It is safe to presume from the increasing number of Car racing games and the profound popularity of the Fast and Furious Franchise that you know what nitrous does. (It gives a significant power boost to the engine)
Nitrous Oxide or popularly known as Nitrous in the racing world is N2O. When you heat nitrous oxide to about 570 degrees F (~300 C), it splits into oxygen and nitrogen.
When Nitrous is supplied into the engine, you are increasing the amount of oxygen inside the engine.
And due to the presence of that excess oxygen you can inject and burn more fuel. As a result, you get a significant power boost!
Have a great weekend!
Why can’t you use pressurized Oxygen ?
Well.. You need controlled combustion! Nitrous Oxide splits into oxygen and nitrogen only when you heat it to 300 Celsius… Whereas the entire oxygen tank would explode almost immediately when administered into the engine! Boom!