Supersonic Nozzles - What happens next will SHOCK you!

00:18:09
https://www.youtube.com/watch?v=I8ntKdaKxV4

Résumé

TLDREn aquest vídeo es discuteix el comportament dels fluxos compressibles en boquilles que convergeixen i divergeixen, així com la diferència contraintuitiva entre els fluxos subsònics i supersònics. Es descriu com les boquilles varien el flux depenent dels gradients de pressió i de les diferències de pressió entre les diferents parts de la boquilla i l'entorn. Aquests conceptes s'expliquen amb animacions que mostren com el canvi en la geometria i les condicions ambientals poden afectar la velocitat del flux a través del principi de conservació de la massa i el moment. També es toquen temes com l'expansió sub/supersònica, xocs normals i oblicus, així com l'ús d'una boquilla per ajustar la velocitat del flux per tal que es correspongui amb les pressions ambientals i les limitacions imposades per la velocitat del so.

A retenir

  • 🚀 Les boquilles convergents acceleren el flux sónic fins a supersònic.
  • 🔄 Els fluxos subsònics deceleren a la gola d'una boquilla.
  • ⚙️ El número de Mach és clau per a definir la compressibilitat del flux.
  • 📉 Les pèrdues d'energia passen amb les ones de xoc normals.
  • ⚠️ La pressió del final de la boquilla ha de coincidir amb la pressió ambiental.
  • 🌐 La transició de subsònic a supersònic inclou xocs normals.
  • 🔊 El so no es pot propagar deu a un flux supersònic.
  • 📏 La geometria de la boquilla determina el comportament del flux supersònic.
  • 💎 Els dissenys de boquilles usos l'expansió i contracció per controlar el flux.
  • 💡 Entendre els gradients de pressió ajuda a predir el comportament del flux.

Chronologie

  • 00:00:00 - 00:05:00

    El vídeo comença parlant sobre un tema confús relacionat amb les toveres convergents i divergents, i el comportament oposat dels fluxos compressibles subsonics i supersonics. L'autor vol mostrar que el flux supersonic no és tan contraintuïtiu com es pensa. Per això, explica que els fluids s'acceleren o desacceleren segons les diferències de pressió. Es menciona que en règim supersonic, hi ha un gradient de pressió a la gola de la tovera que accelera el flux per sobre de Mach 1, mentre que en règim subsonic, el gradient de pressió desaccelera el flux des de Mach 1.

  • 00:05:00 - 00:10:00

    La discussió sobre les toveres continua amb una explicació sobre el Principi de Bernoulli aplicat a fluxos incompressibles i com això varia en fluxos compressibles. S'explicita que, en la gola de la tovera, els gradients de pressió influeixen en si el flux és subsonic o supersonic. A mesura que es disminueix més la pressió atmosfèrica, la velocitat de sortida del gas augmenta per compensar la reducció de pressió estàtica. Això crea un percentatge de velocitat més alt en comparació amb l'ambient, generant fluxos més alts en zones de baixa pressió.

  • 00:10:00 - 00:18:09

    S'introdueixen i es descriuen les ones de xoc normals i obliqües, fent relació a com aquestes afecten el flux en avions supersònics i toveres. Les ones de xoc normals apareixen quan hi ha una transició entre fluxos subsonics i supersonics, i són ineficients perquè es perd energia en el procés. Les ones de xoc obliqües són menys intenses i es fan servir per disminuir gradualment el flux supersonic augmentant la pressió sense una ona de xoc normal forta. També es discuteix el paper de la geometria de la tovera per comunicar el comportament desitjat en els fluxos supersonics.

Carte mentale

Mind Map

Questions fréquemment posées

  • Què és el número de Mach?

    Es la velocitat del flux dividida per la velocitat del so al mateix punt.

  • Com es comporten els fluxos subsònics i supersònics a la gola d'una boquilla?

    Els fluxos se subsònics es deceleren i els supersònics s'acceleren.

  • Què significa 'flux sónic'?

    S'anomena nivell on el flux arriba a la velocitat del so.

  • Per què es produeixen pèrdues d'energia amb les ones de xoc normals?

    Les pèrdues d'energia es produeixen perquè les ones de xoc no són un procés de compressió eficient.

  • Quan es considera que una boquilla està perfectament expandida?

    Una boquilla està perfectament expandida quan no hi ha xocs i el flux supersonic es fa sense entrebancs.

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  • 00:00:00
    g'day space cadets
  • 00:00:02
    in this video i want to talk about a
  • 00:00:04
    topic which really confused me for a
  • 00:00:06
    long time it's something that often gets
  • 00:00:08
    dismissed as a counter-intuitive topic
  • 00:00:11
    where you just have to trust the maths
  • 00:00:13
    i'm talking of course about converging
  • 00:00:15
    diverging nozzles and the opposite
  • 00:00:17
    behavior of subsonic and supersonic
  • 00:00:19
    compressible flows
  • 00:00:21
    there are already some really good
  • 00:00:23
    videos out there which cover the
  • 00:00:24
    mathematical description of what's going
  • 00:00:26
    on
  • 00:00:27
    i've linked a good video from josh the
  • 00:00:29
    engineer in the description which will
  • 00:00:31
    help you get an engineering level of
  • 00:00:32
    understanding
  • 00:00:34
    i don't want to repeat too much of that
  • 00:00:36
    content here instead i want to try and
  • 00:00:38
    convince you that there's nothing
  • 00:00:39
    counter-intuitive about supersonic flow
  • 00:00:42
    i'm not saying that it's simple to
  • 00:00:44
    understand
  • 00:00:45
    but if we take some time to think about
  • 00:00:47
    the situation in the right way and focus
  • 00:00:49
    on the fundamental physics i think we
  • 00:00:52
    can build an intuitive picture
  • 00:00:54
    i'm going to start with a couple of
  • 00:00:56
    points that may not make total sense to
  • 00:00:58
    you right now but keep them in mind as
  • 00:01:00
    we build up our understanding of the
  • 00:01:02
    problem
  • 00:01:03
    fluid flows speed up or slow down
  • 00:01:06
    depending on pressure differences
  • 00:01:08
    if a fluid flowing in a pipe has lower
  • 00:01:10
    pressure downstream than upstream this
  • 00:01:13
    pressure difference will cause the fluid
  • 00:01:15
    to accelerate
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    if the downstream pressure is higher the
  • 00:01:19
    fluid will decelerate
  • 00:01:21
    this is also true at the throat of a
  • 00:01:23
    nozzle
  • 00:01:24
    when the nozzle is operating in the
  • 00:01:26
    supersonic regime there is a pressure
  • 00:01:28
    gradient at the throat which speeds up
  • 00:01:30
    the flow above mach 1.
  • 00:01:32
    when the nozzle is operating in the
  • 00:01:34
    subsonic regime there is a pressure
  • 00:01:36
    gradient at the throat which slows the
  • 00:01:38
    flow back down from mach 1.
  • 00:01:40
    understanding these gradients and how
  • 00:01:42
    they form is what we will talk about for
  • 00:01:44
    most of this video
  • 00:01:46
    secondly the fluid pressure at the
  • 00:01:48
    nozzle exit must match the environment
  • 00:01:51
    pressure
  • 00:01:52
    this is a fundamental truth like
  • 00:01:54
    conservation of mass and conservation of
  • 00:01:56
    energy
  • 00:01:57
    the pressure in the nozzle can be lower
  • 00:01:59
    or higher than the atmospheric pressure
  • 00:02:01
    at the exit but then there will be shock
  • 00:02:03
    waves or expansion fans to enforce
  • 00:02:06
    pressure or quality
  • 00:02:08
    here we have a typical venturi with a
  • 00:02:11
    gas flowing through it the flow comes in
  • 00:02:13
    from the left and moves to the right
  • 00:02:15
    as the cross-section area gets smaller
  • 00:02:18
    the fluid must move faster to conserve
  • 00:02:20
    energy and momentum in the system
  • 00:02:22
    i'm guessing most of you will be
  • 00:02:24
    familiar with bernoulli's principle for
  • 00:02:26
    incompressible flows where the total
  • 00:02:28
    energy of the fluid must be conserved
  • 00:02:30
    this means that as the flow speeds up
  • 00:02:33
    its pressure drops and vice versa the
  • 00:02:36
    same general idea applies here but when
  • 00:02:38
    we talk about compressible flows instead
  • 00:02:41
    of total energy we use properties called
  • 00:02:43
    the total pressure and total temperature
  • 00:02:46
    these values are the pressure and
  • 00:02:47
    temperature the flow would have if we
  • 00:02:49
    slowed it to a speed of zero with no
  • 00:02:52
    losses
  • 00:02:53
    if we plot the total pressure through
  • 00:02:55
    the venturi we can see that it is
  • 00:02:57
    constant everywhere as we have perfect
  • 00:02:59
    subsonic flow with no friction and no
  • 00:03:01
    other losses
  • 00:03:03
    if we also plot the static pressure
  • 00:03:06
    which is what we would measure with a
  • 00:03:07
    pressure sensor
  • 00:03:08
    we can see that it is close to the total
  • 00:03:10
    pressure everywhere
  • 00:03:12
    this is telling us that the fluid is
  • 00:03:14
    moving slowly and that we can quite
  • 00:03:16
    accurately treat this flow as
  • 00:03:18
    incompressible
  • 00:03:20
    for this video
  • 00:03:21
    we're not concerned with the total
  • 00:03:23
    temperature so we'll just skip straight
  • 00:03:24
    to the mach number
  • 00:03:26
    the mach number is the ratio of the
  • 00:03:28
    fluid speed at a particular location to
  • 00:03:31
    its speed of sound at that same location
  • 00:03:34
    let's plot the mach number now
  • 00:03:36
    as we expected from the pressure plot we
  • 00:03:39
    can see that the mach number is close to
  • 00:03:40
    zero everywhere and gets just a little
  • 00:03:42
    bit higher at the throat
  • 00:03:44
    below mach numbers of about 0.3 we can
  • 00:03:48
    treat fluids as incompressible without
  • 00:03:50
    sacrificing much accuracy
  • 00:03:52
    we can't really see anything interesting
  • 00:03:54
    with this slow flow so let's speed it up
  • 00:03:58
    as we increase the speed we see changes
  • 00:04:00
    in both the mach number curve and the
  • 00:04:02
    static pressure curve
  • 00:04:03
    the static pressure drops a little bit
  • 00:04:05
    everywhere as the increased velocity
  • 00:04:07
    converts energy which had previously
  • 00:04:09
    been stored as pressure into kinetic
  • 00:04:11
    energy
  • 00:04:12
    the pressure drop is particularly
  • 00:04:14
    noticeable at the throat as this is the
  • 00:04:16
    point of highest velocity
  • 00:04:20
    eventually we reach mach 1 at the throat
  • 00:04:24
    now let's pause and recall the points we
  • 00:04:26
    introduced earlier the fluid speeds up
  • 00:04:29
    or slows down due to pressure gradients
  • 00:04:32
    if we consider two points in the
  • 00:04:34
    converging section of the nozzle we can
  • 00:04:36
    see that the pressure is higher at point
  • 00:04:38
    one than it is at point two
  • 00:04:40
    now remember that pressure is really
  • 00:04:42
    just molecules whizzing around and
  • 00:04:44
    colliding with each other the speed of
  • 00:04:46
    the particles and also the speed of
  • 00:04:48
    sound in the gas are determined
  • 00:04:50
    primarily by the gas temperature
  • 00:04:53
    a higher pressure means more particles
  • 00:04:55
    in a given volume and therefore more
  • 00:04:57
    collisions
  • 00:04:59
    the slice of gas between points one and
  • 00:05:01
    two will be experiencing more collisions
  • 00:05:04
    on the left hand side than on the right
  • 00:05:06
    hand side
  • 00:05:07
    these extra collisions from the left
  • 00:05:09
    hand side create a force which
  • 00:05:10
    accelerate the slice of gas to the right
  • 00:05:14
    now if we look at the downstream side we
  • 00:05:17
    can see the reverse effect happening
  • 00:05:19
    the pressure at point 4 is higher than
  • 00:05:22
    at 0.3 creating a net force which is
  • 00:05:24
    slowing down the slice of gas
  • 00:05:27
    this pressure gradient is why subsonic
  • 00:05:30
    nozzles are subsonic
  • 00:05:32
    you might be thinking wait
  • 00:05:34
    we have sonic flow in the throat and an
  • 00:05:36
    expanding nozzle doesn't that mean the
  • 00:05:38
    flow should be supersonic
  • 00:05:40
    let's recall point two
  • 00:05:43
    the nozzle exit pressure must match the
  • 00:05:45
    atmospheric pressure
  • 00:05:47
    the short story here is that the exit
  • 00:05:49
    pressure is too high to achieve
  • 00:05:51
    supersonic flow
  • 00:05:53
    in other words with a high atmospheric
  • 00:05:55
    pressure we don't have a large enough
  • 00:05:57
    pressure difference to achieve
  • 00:05:59
    supersonic flow
  • 00:06:00
    we've primarily used an area contraction
  • 00:06:03
    to achieve sonic flow
  • 00:06:05
    but we also reduce the atmospheric
  • 00:06:08
    pressure slightly to encourage higher
  • 00:06:10
    flow velocities in the subsonic regions
  • 00:06:13
    using our knowledge of pressure and
  • 00:06:15
    pressure gradients
  • 00:06:16
    we know that if we lowered the
  • 00:06:18
    environment pressure even further we
  • 00:06:20
    would expect to see higher velocities
  • 00:06:23
    let's do that now
  • 00:06:25
    we're going to see some interesting
  • 00:06:26
    stuff in this animation so i'll just
  • 00:06:28
    shut up and let you watch and we'll chat
  • 00:06:30
    about what we saw later
  • 00:07:17
    as we reduce the atmospheric pressure
  • 00:07:19
    further the exit gas velocity increased
  • 00:07:21
    to compensate for the reduced static
  • 00:07:23
    pressure
  • 00:07:24
    this pressure reduction propagated up
  • 00:07:26
    the subsonic flow region until the
  • 00:07:28
    choked flow of the throat
  • 00:07:30
    we know that in sonic and supersonic
  • 00:07:32
    flows pressure information can't travel
  • 00:07:34
    upstream
  • 00:07:36
    the effect of a downstream pressure
  • 00:07:38
    reduction when the throat is already
  • 00:07:39
    choked is to remove the particle
  • 00:07:42
    collisions from the right hand side
  • 00:07:44
    no particle collisions means no
  • 00:07:46
    communication method
  • 00:07:48
    the mach 1 flow essentially fills a
  • 00:07:50
    vacuum to its right
  • 00:07:52
    however
  • 00:07:53
    the gas particles can still easily
  • 00:07:55
    interact with those particles above and
  • 00:07:57
    below them as they're all traveling at
  • 00:07:59
    roughly the same speed
  • 00:08:01
    we can exploit these vertical particle
  • 00:08:03
    interactions in order to tell the flow
  • 00:08:05
    what to do
  • 00:08:06
    instead of communicating via pressure
  • 00:08:08
    gradients and particle collisions we
  • 00:08:11
    instead communicate with the flow via
  • 00:08:13
    the nozzle shape
  • 00:08:14
    as the supersonic flow moves down the
  • 00:08:17
    nozzle the area gets larger and the
  • 00:08:19
    particles communicate this to each other
  • 00:08:21
    vertically
  • 00:08:22
    due to conservation of mass the
  • 00:08:24
    particles expand to fill the larger
  • 00:08:26
    cross-section and the pressure drops
  • 00:08:29
    from conservation of momentum a
  • 00:08:31
    reduction in pressure means an increase
  • 00:08:33
    in velocity
  • 00:08:34
    the flow will keep accelerating as we
  • 00:08:36
    keep increasing the area
  • 00:08:39
    the flow can communicate vertically at
  • 00:08:40
    roughly the speed of sound but it's
  • 00:08:43
    traveling through the nozzle at multiple
  • 00:08:44
    times the speed of sound so this
  • 00:08:46
    vertical communication is really
  • 00:08:48
    diagonal
  • 00:08:50
    this idea is the basis of the method of
  • 00:08:52
    characteristics which we may discuss in
  • 00:08:54
    a later video
  • 00:08:56
    this image here was produced via the
  • 00:08:57
    method of characteristics and
  • 00:08:59
    illustrates these diagonal communication
  • 00:09:02
    lines in a typical rocket nozzle
  • 00:09:04
    let's briefly return to the subsonic
  • 00:09:07
    supersonic transition animation
  • 00:09:09
    initially a small region of supersonic
  • 00:09:12
    flow develops but then something strange
  • 00:09:14
    happens there's a large increase in
  • 00:09:16
    pressure and a large drop in mach number
  • 00:09:18
    and this happens almost instantaneously
  • 00:09:21
    at a certain point in the nozzle
  • 00:09:23
    as the pressure gets lower this point
  • 00:09:25
    gets closer towards the end of the
  • 00:09:27
    nozzle
  • 00:09:28
    this strange effect is a normal shock
  • 00:09:31
    neither the supersonic or the subsonic
  • 00:09:34
    expansion alone can match the nozzle
  • 00:09:36
    exit pressure with the atmospheric
  • 00:09:37
    pressure and so a small amount of both
  • 00:09:40
    is needed
  • 00:09:41
    a normal shock is nature's way of
  • 00:09:43
    transitioning between the two flow
  • 00:09:45
    regimes
  • 00:09:47
    let's detour for a minute and have a
  • 00:09:48
    quick chat about normal shocks
  • 00:09:51
    i think everyone watching this video is
  • 00:09:53
    probably already familiar with the idea
  • 00:09:55
    of shock waves forming around a
  • 00:09:56
    supersonic aircraft
  • 00:09:58
    the airplane is moving faster than the
  • 00:10:00
    speed of sound so the air can't get out
  • 00:10:03
    of the way and piles up creating a shock
  • 00:10:06
    wave and a region of high pressure
  • 00:10:08
    behind the shock wave
  • 00:10:10
    we're interested in the part of the
  • 00:10:11
    shock wave that forms at the very tip of
  • 00:10:14
    an aircraft's nose or wing leading edge
  • 00:10:17
    here the shock wave is perpendicular to
  • 00:10:19
    the direction of motion of the aircraft
  • 00:10:21
    this kind of shock wave can also occur
  • 00:10:23
    in tubes and nozzles as we saw before
  • 00:10:26
    the flow behind a normal shock wave is
  • 00:10:28
    always subsonic
  • 00:10:30
    the faster the plane goes the more air
  • 00:10:33
    piles up and the higher the pressure
  • 00:10:35
    behind the shock becomes
  • 00:10:37
    a higher mach number means a bigger
  • 00:10:39
    pressure difference
  • 00:10:41
    this is easy to understand from the
  • 00:10:42
    point of view of the aircraft where the
  • 00:10:44
    shock wave appears stationary
  • 00:10:46
    but
  • 00:10:47
    consider what this situation would look
  • 00:10:49
    like to someone standing on the ground
  • 00:10:51
    to them the airplane and its shock wave
  • 00:10:54
    are moving
  • 00:10:55
    to the observer a higher pressure
  • 00:10:57
    difference across the shock means a
  • 00:10:59
    louder boom and a shock that travels
  • 00:11:02
    past them faster
  • 00:11:04
    higher pressure differences create
  • 00:11:06
    shocks that travel faster
  • 00:11:08
    this is an important point for our
  • 00:11:10
    nozzle example
  • 00:11:12
    one final note on shocks they are an
  • 00:11:14
    inefficient compression process and we
  • 00:11:16
    lose a lot of energy across a normal
  • 00:11:18
    shock if you were paying close attention
  • 00:11:21
    to our transition example before you
  • 00:11:23
    would have noticed that the total
  • 00:11:24
    pressure in the nozzle dropped sharply
  • 00:11:27
    when there was a normal shock present
  • 00:11:29
    and the higher the mach number the
  • 00:11:31
    greater this total pressure loss
  • 00:11:36
    in this intermediate expansion situation
  • 00:11:38
    we only have a small region of
  • 00:11:40
    supersonic flow if we continued the
  • 00:11:43
    supersonic expansion all the way to the
  • 00:11:45
    nozzle end we would have about a mach 4
  • 00:11:47
    flow
  • 00:11:48
    but the nozzle pressure would be much
  • 00:11:51
    lower than the atmospheric pressure
  • 00:11:52
    which is still quite high
  • 00:11:54
    in fact this large pressure difference
  • 00:11:57
    would correspond to something like a
  • 00:11:59
    mach 6 shock
  • 00:12:00
    a mach 6 shock in a mach 4 flow would
  • 00:12:03
    travel up the nozzle at about mach 2.
  • 00:12:07
    while pressure information can't
  • 00:12:08
    transmit upper supersonic flow via
  • 00:12:11
    traditional particle collisions
  • 00:12:13
    normal shocks can propagate up
  • 00:12:15
    supersonic flows
  • 00:12:17
    this mach 6 shock would move up the
  • 00:12:19
    nozzle and the pressure immediately
  • 00:12:21
    before the shock would increase as the
  • 00:12:24
    amount of supersonic expansion prior to
  • 00:12:26
    the shock has decreased
  • 00:12:28
    the pressure difference across the shock
  • 00:12:30
    is now smaller
  • 00:12:31
    making it a weaker shock which would
  • 00:12:34
    travel slower
  • 00:12:36
    as it continued propagating up the
  • 00:12:38
    nozzle
  • 00:12:38
    it would travel slower and slower until
  • 00:12:41
    it reached an equilibrium point where it
  • 00:12:43
    stops moving
  • 00:12:45
    at this equilibrium point the supersonic
  • 00:12:48
    pressure drop
  • 00:12:50
    shock pressure increase
  • 00:12:52
    and subsonic pressure increase combined
  • 00:12:55
    would match the nozzle end pressure with
  • 00:12:57
    the atmospheric pressure
  • 00:12:59
    it's important to note that the shock
  • 00:13:01
    doesn't affect the flow properties in
  • 00:13:03
    the supersonic flow region
  • 00:13:05
    only the shape of the nozzle can do this
  • 00:13:08
    the shock position simply determines
  • 00:13:10
    where the supersonic flow region ends
  • 00:13:14
    as the exit pressure is reduced
  • 00:13:17
    this lower pressure information can
  • 00:13:19
    propagate up the subsonic flow section
  • 00:13:21
    until it reaches the shock
  • 00:13:24
    with a lower downstream pressure the
  • 00:13:26
    pressure differential across the shock
  • 00:13:28
    has been decreased
  • 00:13:29
    making it a weaker and slower shock and
  • 00:13:32
    so it will move down the nozzle until it
  • 00:13:34
    finds a new equilibrium point
  • 00:13:37
    eventually the shock will reach the
  • 00:13:39
    nozzle exit
  • 00:13:40
    if we reduce the pressure further the
  • 00:13:42
    pressure difference becomes too small
  • 00:13:44
    for a normal shock to exist and we will
  • 00:13:46
    transition into two-dimensional oblique
  • 00:13:48
    shocks and fancy shock structures like
  • 00:13:50
    muck diamonds
  • 00:13:52
    i haven't animated these here for
  • 00:13:54
    simplicity but the end result is that
  • 00:13:56
    the flow doesn't slow down as much as it
  • 00:13:58
    would have for a normal shock
  • 00:14:01
    eventually we reach a perfectly expanded
  • 00:14:03
    nozzle with no shocks anywhere
  • 00:14:07
    finally i want to address one last
  • 00:14:10
    confusing point
  • 00:14:11
    why can't we increase the mach number
  • 00:14:13
    above 1 by contracting the throat area
  • 00:14:16
    further
  • 00:14:17
    we'll work through a couple of examples
  • 00:14:19
    to understand this
  • 00:14:20
    first let's consider a choked converging
  • 00:14:23
    nozzle
  • 00:14:24
    if we suddenly removed part of the
  • 00:14:26
    converging section
  • 00:14:27
    we would create a large pressure
  • 00:14:29
    difference between the subsonic flow at
  • 00:14:31
    the new throat location and the
  • 00:14:33
    atmospheric pressure
  • 00:14:34
    this pressure difference would quickly
  • 00:14:36
    accelerate throat flow
  • 00:14:39
    as the throat flow is initially subsonic
  • 00:14:41
    this low pressure would propagate back
  • 00:14:43
    up the flow as a low pressure pulse and
  • 00:14:46
    increase the flow speed everywhere
  • 00:14:49
    if the total pressure is high enough we
  • 00:14:51
    will still achieve choke flow at the
  • 00:14:53
    throat but now with a larger area and
  • 00:14:55
    higher mass flow
  • 00:14:56
    rate now let's consider what would
  • 00:14:59
    happen if instead of increasing the
  • 00:15:01
    throat diameter we decreased it
  • 00:15:03
    the accelerated flow would be supersonic
  • 00:15:06
    and have reduced pressure at the exit
  • 00:15:08
    the environment pressure would need to
  • 00:15:10
    be modified to prevent shocks from
  • 00:15:12
    forming at the exit
  • 00:15:13
    however the real problem here is that we
  • 00:15:16
    can't have converging supersonic flows
  • 00:15:18
    without shocks forming
  • 00:15:20
    whenever supersonic flow streamlines are
  • 00:15:23
    turned into themselves a shock will
  • 00:15:25
    occur
  • 00:15:26
    and these shocks are known as oblique
  • 00:15:28
    shocks
  • 00:15:30
    remember that in supersonic flows the
  • 00:15:32
    particles can't communicate back up the
  • 00:15:34
    stream and warn the incoming particles
  • 00:15:36
    that they will need to change direction
  • 00:15:38
    soon
  • 00:15:39
    instead the particles continue straight
  • 00:15:41
    until they collide with the deflected
  • 00:15:43
    downstream flow
  • 00:15:45
    the particles pile up this time creating
  • 00:15:47
    an oblique shock
  • 00:15:49
    oblique shocks are weaker than normal
  • 00:15:51
    shocks as the flow is simply being
  • 00:15:53
    deflected a little bit rather than
  • 00:15:55
    almost completely stopped
  • 00:15:57
    a supersonic contraction and the
  • 00:15:59
    resulting oblique shock waves are
  • 00:16:01
    exploited in supersonic wind tunnel
  • 00:16:03
    diffusers to gradually slow a supersonic
  • 00:16:05
    flow and increase its pressure back to
  • 00:16:08
    ambient without needing a strong normal
  • 00:16:10
    shock
  • 00:16:11
    we can clearly see in our image that the
  • 00:16:13
    streamlines are converging
  • 00:16:15
    if these flows were supersonic this
  • 00:16:18
    convergence would cause shock waves to
  • 00:16:20
    form
  • 00:16:20
    however our flow is only sonic and so an
  • 00:16:24
    infinitely weak shock wave would exist
  • 00:16:26
    temporarily
  • 00:16:28
    in reality it would not get to the point
  • 00:16:30
    where supersonic flow and shocks existed
  • 00:16:33
    as a high pressure pulse would propagate
  • 00:16:35
    up the nozzle to slow the flow
  • 00:16:38
    we would achieve choke flow again at the
  • 00:16:40
    throat but with a reduced mass flow due
  • 00:16:42
    to the smaller area
  • 00:16:45
    i would like to conclude this video by
  • 00:16:47
    reiterating a few key points
  • 00:16:50
    subsonic nozzles have a pressure
  • 00:16:52
    gradient of the throat which decelerates
  • 00:16:54
    the flow
  • 00:16:55
    while supersonic nozzles have a pressure
  • 00:16:57
    gradient at the throat which accelerates
  • 00:16:59
    the flow
  • 00:17:01
    a high total pressure and an area
  • 00:17:03
    contraction are required to generate
  • 00:17:05
    sonic mach 1 flow at the throat
  • 00:17:09
    we can't accelerate flows above mach 1
  • 00:17:12
    with an area contraction alone
  • 00:17:14
    a low atmospheric pressure and an area
  • 00:17:17
    expansion after the throat are required
  • 00:17:19
    to generate supersonic flow
  • 00:17:22
    we can communicate down a supersonic
  • 00:17:24
    flow with particle interactions but not
  • 00:17:27
    up the flow
  • 00:17:29
    instead we can communicate the desired
  • 00:17:31
    behavior of a supersonic flow
  • 00:17:33
    via the nozzle geometry
  • 00:17:36
    normal shocks can travel up a supersonic
  • 00:17:38
    flow but they can't influence the
  • 00:17:41
    properties in a supersonic flow region a
  • 00:17:44
    normal shock can only terminate a
  • 00:17:46
    supersonic flow region
  • 00:17:48
    the subsonic nozzle can be thought of as
  • 00:17:51
    a special case of supersonic flow where
  • 00:17:53
    the normal shock has progressed all the
  • 00:17:55
    way to the throat and then disappeared
  • 00:17:57
    leaving subsonic flow everywhere
  • 00:18:01
    that's all for this video please leave
  • 00:18:03
    any comments or questions you have in
  • 00:18:04
    the comments section and like and
  • 00:18:06
    subscribe if you enjoyed this video
Tags
  • fluxos compressibles
  • boquilles
  • subsònic
  • supersònic
  • gradients de pressió
  • ona de xoc
  • número de Mach
  • expansió de gasos
  • acústica
  • conservació de massa