Memahami Perbedaan Antara Total, Static dan Tekanan Dinamis

Memahami Perbedaan Antara Total, Static dan Tekanan Dinamis

Ketika mendiskusikan sistem perpipaan, istilah "tekanan" sering digunakan untuk menggambarkan sifat fluida kunci yang memainkan peran penting dalam pengoperasian peralatan seperti pompa, katup kontrol, tank dan kapal dan perangkat lainnya. Namun, seperti banyak istilah yang digunakan dalam rekayasa, ada nuansa makna yang harus diperhitungkan untuk menghindari miskomunikasi, kebingungan dan kesalahan Fatal. Cukup sering, kualifikasi kunci yang membedakan antara tekanan total, tekanan statis dan tekanan dinamis tidak digunakan. Kadang-kadang perbedaan penting, seperti perbedaan antara laju aliran massa dan laju alir volumetrik harus dilakukan untuk menjadi ringkas ketika membahas laju aliran.

Definisi dan Unit
Definisi klasik dari tekanan adalah jumlah gaya yang bekerja pada permukaan per satuan luas, seperti yang ditunjukkan pada Gambar 1.

Tekanan (P) = Gaya (F)/Luas Penampang (A)

F


Hal ini memberikan tekanan unit pound per inci persegi (lb / in2 atau psi) atau newton(N) per meter persegi (Pascal). unit lain yang biasa digunakan untuk tekanan termasuk suasana, bar, kilopascal, torr, inci (atau mm) dari merkuri dan inci (atau mm) air.

Gambar 1. Tekanan adalah gaya per satuan luas.

Total Tekanan
Tekanan total adalah gaya per satuan luas yang dirasakan saat cairan mengalir dibawa untuk beristirahat dan biasanya diukur dengan alat pitot jenis tabung, yang ditunjukkan pada Gambar 2. tekanan total adalah jumlah dari tekanan statis dan tekanan dinamis .

Ptotal=Pstatic+Pdynamic

Tekanan total sering disebut sebagai tekanan stagnasi.

Tekanan statis
tekanan statis dirasakan ketika cairan adalah saat istirahat atau saat pengukuran diambil ketika bepergian bersama dengan aliran fluida. Ini adalah gaya yang diberikan pada partikel cairan dari segala arah, dan biasanya diukur dengan alat pengukur dan pemancar yang melekat pada sisi pipa atau tangki dinding. Karena tekanan statis adalah apa yang kebanyakan tekanan pengukur ukuran, tekanan statis biasanya apa yang tersirat ketika istilah "tekanan" yang digunakan dalam diskusi.

Tekanan dinamis
Perbedaan antara tekanan total dan statis tekanan dinamis, yang merupakan energi kinetik fluida yang mengalir. Tekanan dinamis adalah fungsi dari kecepatan fluida dan kepadatan dan dapat dihitung dari:

Pdynamic=ρv22g





Gambar 2. Mengukur Total, Static, dan Tekanan Dinamis.

Ketika Membuat Perbedaan tersebut
Tergantung pada aplikasi, perbedaan antara tekanan total dan statis mungkin dapat diabaikan, tetapi untuk orang lain, mengabaikan perbedaan dapat mengakibatkan kesalahan mahal.

Untuk banyak aplikasi cair, jaringan pipa yang berukuran untuk memastikan kecepatan fluida rendah untuk mengurangi kehilangan kepala dan penurunan tekanan untuk laju alir yang diberikan, sehingga nilai kecil tekanan dinamis. Juga, karena akurasi dan skala instrumen yang digunakan untuk mengukur tekanan, perbedaan antara tekanan total dan statis dapat diabaikan.

Pada Gambar 3, ukuran pipa berubah untuk menghasilkan kecepatan cairan yang berbeda untuk 700 gpm aliran air, mengakibatkan jumlah yang berbeda dari tekanan dinamis dan statis untuk tekanan total inlet dari 100 psig. Untuk kasus kecepatan rendah dengan ukuran pipa 6 inci, 700 hasil gpm dalam kecepatan sekitar 7,8 ft / sec. Dari total tekanan 100 psig, 99,59 psig tekanan statis dan 0,41 psi tekanan dinamis. Jika tekanan diukur pada pengukur tekanan 0-150 psig, perbedaan antara tekanan total dan statis akan kemungkinan besar tidak akan dilihat.

Dalam kasus kecepatan moderat dengan pipa 4 inci, 700 hasil gpm dalam kecepatan fluida dari 17,6 ft / detik, tekanan dinamis 2,1 psi dan tekanan statis 97,9 psig. Dalam kasus kecepatan tinggi dengan pipa 3 inci, fluida mengalir di sekitar 30 ft / sec. Tekanan dinamis sekitar 6,2 psi, sehingga dari total tekanan 100 psig, 93,8 psig tekanan statis. Dalam pipa 2,5 inci untuk skenario kecepatan yang sangat tinggi, 47 ft / sec hasil kecepatan di 15 psi tekanan dinamis dan 85 psig tekanan statis.





Gambar 3. Perbedaan antara Total, Static, dan Tekanan Dinamis untuk berbagai kecepatan fluida dalam aplikasi cair.

Untuk aplikasi gas ditunjukkan pada Gambar 4, perbedaan antara tekanan total dan statis lagi lagi akan tergantung pada jumlah tekanan dinamis, tetapi karena kepadatan gas jauh lebih rendah dari cairan, kecepatan yang jauh lebih tinggi diperlukan sebelum perbedaan antara tekanan total dan statis perlu dibuat. Perhatikan berbagai ukuran pipa, kecepatan cairan dan tekanan statis untuk tekanan total inlet dari 100 psig dan laju aliran massa 7500 lb / h dari 350 ° F uap dengan kepadatan 0,248 lb / ft3.




Gambar 4. Perbedaan antara Total, Static, dan Tekanan Dinamis untuk berbagai kecepatan fluida dalam aplikasi gas.

Gunakan ringkas Terminologi
Ketika mengevaluasi parameter operasi dari sistem perpipaan, perbedaan antara total dan statis sifat fluida mungkin atau mungkin tidak penting. Untuk aplikasi yang paling likuid, kecepatan fluida sengaja tetap rendah untuk meminimalkan jumlah kerugian kepala dan konsumsi daya sistem. Hal ini menyebabkan sejumlah kecil energi fluida dinamis, membuat perbedaan antara total dan statis tekanan dpt dibedakan pada kebanyakan alat pengukur tekanan industri.



literatur : https://eng-software.com/about-us/press/2016/4/understanding-the-distinction-between-total-static-and-dynamic-pressure/

Categories of Waves

Waves come in many shapes and forms. While all waves share some basic characteristic properties and behaviors, some waves can be distinguished from others based on some observable (and some non-observable) characteristics. It is common to categorize waves based on these distinguishing characteristics.

Longitudinal versus Transverse Waves versus Surface Waves
One way to categorize waves is on the basis of the direction of movement of the individual particles of the medium relative to the direction that the waves travel. Categorizing waves on this basis leads to three notable categories: transverse waves, longitudinal waves, and surface waves.
A transverse wave is a wave in which particles of the medium move in a direction perpendicular to the direction that the wave moves. Suppose that a slinky is stretched out in a horizontal direction across the classroom and that a pulse is introduced into the slinky on the left end by vibrating the first coil up and down. Energy will begin to be transported through the slinky from left to right. As the energy is transported from left to right, the individual coils of the medium will be displaced upwards and downwards. In this case, the particles of the medium move perpendicular to the direction that the pulse moves. This type of wave is a transverse wave. Transverse waves are always characterized by particle motion being perpendicular to wave motion.

A longitudinal wave is a wave in which particles of the medium move in a direction parallel to the direction that the wave moves. Suppose that a slinky is stretched out in a horizontal direction across the classroom and that a pulse is introduced into the slinky on the left end by vibrating the first coil left and right. Energy will begin to be transported through the slinky from left to right. As the energy is transported from left to right, the individual coils of the medium will be displaced leftwards and rightwards. In this case, the particles of the medium move parallel to the direction that the pulse moves. This type of wave is a longitudinal wave. Longitudinal waves are always characterized by particle motion being parallel to wave motion.
A sound wave traveling through air is a classic example of a longitudinal wave. As a sound wave moves from the lips of a speaker to the ear of a listener, particles of air vibrate back and forth in the same direction and the opposite direction of energy transport. Each individual particle pushes on its neighboring particle so as to push it forward. The collision of particle #1 with its neighbor serves to restore particle #1 to its original position and displace particle #2 in a forward direction. This back and forth motion of particles in the direction of energy transport creates regions within the medium where the particles are pressed together and other regions where the particles are spread apart. Longitudinal waves can always be quickly identified by the presence of such regions. This process continues along the chain of particles until the sound wave reaches the ear of the listener. A detailed discussion of sound is presented in another unit of The Physics Classroom Tutorial.
Waves traveling through a solid medium can be either transverse waves or longitudinal waves. Yet waves traveling through the bulk of a fluid (such as a liquid or a gas) are always longitudinal waves. Transverse waves require a relatively rigid medium in order to transmit their energy. As one particle begins to move it must be able to exert a pull on its nearest neighbor. If the medium is not rigid as is the case with fluids, the particles will slide past each other. This sliding action that is characteristic of liquids and gases prevents one particle from displacing its neighbor in a direction perpendicular to the energy transport. It is for this reason that only longitudinal waves are observed moving through the bulk of liquids such as our oceans. Earthquakes are capable of producing both transverse and longitudinal waves that travel through the solid structures of the Earth. When seismologists began to study earthquake waves they noticed that only longitudinal waves were capable of traveling through the core of the Earth. For this reason, geologists believe that the Earth's core consists of a liquid - most likely molten iron.
While waves that travel within the depths of the ocean are longitudinal waves, the waves that travel along the surface of the oceans are referred to as surface waves. A surface wave is a wave in which particles of the medium undergo a circular motion. Surface waves are neither longitudinal nor transverse. In longitudinal and transverse waves, all the particles in the entire bulk of the medium move in a parallel and a perpendicular direction (respectively) relative to the direction of energy transport. In a surface wave, it is only the particles at the surface of the medium that undergo the circular motion. The motion of particles tends to decrease as one proceeds further from the surface.
Any wave moving through a medium has a source. Somewhere along the medium, there was an initial displacement of one of the particles. For a slinky wave, it is usually the first coil that becomes displaced by the hand of a person. For a sound wave, it is usually the vibration of the vocal chords or a guitar string that sets the first particle of air in vibrational motion. At the location where the wave is introduced into the medium, the particles that are displaced from their equilibrium position always moves in the same direction as the source of the vibration. So if you wish to create a transverse wave in a slinky, then the first coil of the slinky must be displaced in a direction perpendicular to the entire slinky. Similarly, if you wish to create a longitudinal wave in a slinky, then the first coil of the slinky must be displaced in a direction parallel to the entire slinky.

 

Electromagnetic versus Mechanical Waves
Another way to categorize waves is on the basis of their ability or inability to transmit energy through a vacuum (i.e., empty space). Categorizing waves on this basis leads to two notable categories: electromagnetic waves and mechanical waves.
An electromagnetic wave is a wave that is capable of transmitting its energy through a vacuum (i.e., empty space). Electromagnetic waves are produced by the vibration of charged particles. Electromagnetic waves that are produced on the sun subsequently travel to Earth through the vacuum of outer space. Were it not for the ability of electromagnetic waves to travel to through a vacuum, there would undoubtedly be no life on Earth. All light waves are examples of electromagnetic waves. Light waves are the topic of another unit at The Physics Classroom Tutorial. While the basic properties and behaviors of light will be discussed, the detailed nature of an electromagnetic wave is quite complicated and beyond the scope of The Physics Classroom Tutorial.
A mechanical wave is a wave that is not capable of transmitting its energy through a vacuum. Mechanical waves require a medium in order to transport their energy from one location to another. A sound wave is an example of a mechanical wave. Sound waves are incapable of traveling through a vacuum. Slinky waves, water waves, stadium waves, and jump rope waves are other examples of mechanical waves; each requires some medium in order to exist. A slinky wave requires the coils of the slinky; a water wave requires water; a stadium wave requires fans in a stadium; and a jump rope wave requires a jump rope.

The above categories represent just a few of the ways in which physicists categorize waves in order to compare and contrast their behaviors and characteristic properties. This listing of categories is not exhaustive; there are other categories as well. The five categories of waves listed here will be used periodically throughout this unit on waves as well as the units on sound and light.

Waves

Longitudinal Waves

In a longitudinal wave the particle displacement is parallel to the direction of wave propagation. The animation at right shows a one-dimensional longitudinal plane wave propagating down a tube. The particles do not move down the tube with the wave; they simply oscillate back and forth about their individual equilibrium positions. Pick a single particle and watch its motion. The wave is seen as the motion of the compressed region (ie, it is a pressure wave), which moves from left to right.
The second animation at right shows the difference between the oscillatory motion of individual particles and the propagation of the wave through the medium. The animation also identifies the regions of compression and rarefaction.

The P waves (Primary waves) in an earthquake are examples of Longitudinal waves. The P waves travel with the fastest velocity and are the first to arrive.
To see a animations of spherical longitudinal waves check out:

• Sound Radiation from Simple Sources
• Radiation from Cylindrical Sources

 

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Transverse Waves

In a transverse wave the particle displacement is perpendicular to the direction of wave propagation. The animation below shows a one-dimensional transverse plane wave propagating from left to right. The particles do not move along with the wave; they simply oscillate up and down about their individual equilibrium positions as the wave passes by. Pick a single particle and watch its motion. The S waves (Secondary waves) in an earthquake are examples of Transverse waves. S waves propagate with a velocity slower than P waves, arriving several seconds later.

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Water Waves (updated 2016)

Water waves are an example of waves that involve a combination of both longitudinal and transverse motions. As a wave travels through the waver, the particles travel in clockwise circles. The radius of the circles decreases as the depth into the water increases. The animation at right shows a water wave travelling from left to right in a region where the depth of the water is greater than the wavelength of the waves. I have identified two particles in orange to show that each particle indeed travels in a clockwise circle as the wave passes.

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Rayleigh surface waves (Updated 2016)

Another example of waves with both longitudinal and transverse motion may be found in solids as Rayleigh surface waves (named after John W. Strutt, 3rd Baron Rayleigh who first studied them in 1885). The particles in a solid, through which a Rayleigh surface wave passes, move in elliptical paths, with the major axis of the ellipse perpendicular to the surface of the solid. As the depth into the solid increases the "width" of the elliptical path decreases.
Rayleigh waves in an elastic solid are different from surface waves in water in a very important way. In a water wave all particles travel in clockwise circles. However, in a Rayleigh surface wave, particles at the surface trace out a counter-clockwise ellipse, while particles at a depth of more than 1/5th of a wavelength trace out clockwise ellispes. This motion is often referred to as being "retrograde" since at the surface, the horizontal component of the particle motion is in the opposite direction as the wave propagation direction. I have identified two particles in orange in this animation to illustrate the retrograde elliptical path at the surface and the reversal in the direction of motion as a function of depth.
The Rayleigh surface waves are the waves that cause the most damage during an earthquake. They travel with velocities slower than S waves, and arrive later, but with much greater amplitudes. These are also the waves that are most easily felt during an earthquake and involve both up-down and side-to-side motion