Оscillatory regime in liquid jet veil separating gas areas with different pressure
In G. V. Logvinovich's monograph "Hydrodynamics of currents with free borders" (in Russian) the general properties of flows of liquid with free boundaries are considered. To them treat as a current with formation of cavities on streamline bodies, and jet flows with the boundaries divi...
Збережено в:
| Дата: | 2013 |
|---|---|
| Автори: | , , |
| Формат: | Стаття |
| Мова: | English |
| Опубліковано: |
Інститут гідромеханіки НАН України
2013
|
| Назва видання: | Прикладна гідромеханіка |
| Теми: | |
| Онлайн доступ: | http://dspace.nbuv.gov.ua/handle/123456789/116413 |
| Теги: |
Додати тег
Немає тегів, Будьте першим, хто поставить тег для цього запису!
|
| Назва журналу: | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| Цитувати: | Оscillatory regime in liquid jet veil separating gas areas with different pressure / I.I. Kozlov, S.A. Ocheretyany, V.V. Prokof'ev // Прикладна гідромеханіка. — 2013. — Т. 15, № 1. — С. 45-52. — Бібліогр.: 4 назв. — англ. |
Репозитарії
Digital Library of Periodicals of National Academy of Sciences of Ukraine| id |
irk-123456789-116413 |
|---|---|
| record_format |
dspace |
| spelling |
irk-123456789-1164132017-04-26T03:02:45Z Оscillatory regime in liquid jet veil separating gas areas with different pressure Kozlov, I.I. Ocheretyany, S.A. Prokof'ev, V.V. Науковi статтi In G. V. Logvinovich's monograph "Hydrodynamics of currents with free borders" (in Russian) the general properties of flows of liquid with free boundaries are considered. To them treat as a current with formation of cavities on streamline bodies, and jet flows with the boundaries dividing liquid and gas. Questions of behavior of the non-stationary free borders, raised and studied in the monograph, are actual still and inspire many authors on new researches. Below one of such researches - a problem of creation of the air cushion by means of a jet veil is presented. Presented results of experimental studies for self-oscillating modes of fluid jet discharging in a plane channel with an air cushion. Investigated effect on the flow from the air cavity volume and thickness, width of the channel in a wide range of magnitude of jet rate and amount of gas discharge. Oscillatory flow regimes were realized under constant water pressure in the pressure tank and a constant mass flow rate of the blower to the air cushion. It was found that the previously studied low-frequency mode exists in a certain range of values of gas flow rate to the cavity, with the range depending on the cavity volume. It is shown that in some cases this mode is replaced by a high-frequency oscillatory regime with low amplitude, and in transitional range of air flow-rate both modes are simultaneously presented (there is an intermittency). Video recording of high-frequency regime has shown that unlike in the low-frequency regime there is no direct interaction between the outflowing jet with the channel wall. We found that in both modes the oscillation characteristics of the flow (frequency, amplitude) are independent of the thickness of the cushion (the channel width). However, the regime change event significantly depends on the thickness of the cushion. В монографии Г.В.Логвиновича "Гидродинамика течений со свободными границами" рассмотрены общие свойства течений жидкости со свободными границами, к которым относятся как течения с образованием каверн на обтекаемых телах, так и струйные течения с границами, разделяющими жидкость и газ. Вопросы поведения нестационарных свободных границ, поставленные и изученные в монографии, актуальны до сих пор и вдохновляют многих авторов на новые исследования. Ниже представлено одно из таких исследований -- задача создания воздушной подушки с помощью струйной завесы. Задача исследовалась экспериментально на плоской струйной установке, где струи жидкости истекали в плоский канал, с поддувом воздуха в заглушенную его часть. Исследовалось возникновение нестационарных - автоколебательных режимов, их зависимость от параметров течения. Обнаружено, что в некотором диапазоне величин поддува газа в каверну имеет место низкочастотный режим. С ростом поддува этот режим течения сменяется другим, с более высокой частотой и более низкой амплитудой, причем имеется область поддувов, где одновременно существуют оба режима (имеет место перемежаемость). Скоростная видеосъемка показала, что в отличие от низкочастотного режима при высокочастотном нет непосредственного взаимодействия истекающей струи со стенкой канала. Обнаружено, что в обоих режимах характеристики колебаний не зависят от толщины подушки (от ширины канала). Однако граница смены режимов существенно зависит от толщины подушки. В монографiї Г.В.Логвиновича "Гидродинамика течений со свободными границами" розглянутi загальнi властивостi течiї рiдини з вiльними границями, до яких можна вiднести як течiї з утворенням порожнини (каверни) на тiлi, що обтiкається, так i струйнi течiї з границями, якi роздiляють рiдину та газ. Питання з поведiнкою i росповсюдженням нестацiонарних вiльних границь, що поставленi й розглянутi в монографії, актуальнi на сьогоднi, спонукають i дають наснагу багатьом авторам до нових дослiджень. В розглянутому нижче маємо одне з таких дослiджень, а саме -- задачу створення повiтряної подушки завдяки струйнiй завiсi. Задача дослiджувалась експериментально на плоскiй установцi, де струї рiдини витiкають у плоский канал з наявнiстю пiддува повiтря в заглушену його частину. Дослiджувались виникнення нестацiонарних автоколивальних режимiв, їхня залежнiсть вiд параметрiв течiї. Виявлено, що в деякому дiапазонi кiлькостi пiддува газу в каверну маємо низькочастотний режим коливать. Iз збiльшенням пiддуву такий стан течiї змiнюється на iнший з бiльш високою частотою та бiльш низькою амплiтудою, причому наявнi в перехiдному дiапазонi пiддува i одночасно спiвiснують обидва режима течiї. Швидкiсне вiдео демонструє, що на вiдмiну вiд низькочастотного режиму при високочастотному нема беспосереднього дотика витiкаючої струї з стiнкою каналу. Виявлено, що в обох режимах автоколивань характеристики коливань (частота, амплiтуда) не залежать вiд товщини подушки (ширини каналу). Але момент змiни режимiв течiї суттєво залежить вiд товщини подушки. 2013 Article Оscillatory regime in liquid jet veil separating gas areas with different pressure / I.I. Kozlov, S.A. Ocheretyany, V.V. Prokof'ev // Прикладна гідромеханіка. — 2013. — Т. 15, № 1. — С. 45-52. — Бібліогр.: 4 назв. — англ. 1561-9087 http://dspace.nbuv.gov.ua/handle/123456789/116413 532.528 en Прикладна гідромеханіка Інститут гідромеханіки НАН України |
| institution |
Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| collection |
DSpace DC |
| language |
English |
| topic |
Науковi статтi Науковi статтi |
| spellingShingle |
Науковi статтi Науковi статтi Kozlov, I.I. Ocheretyany, S.A. Prokof'ev, V.V. Оscillatory regime in liquid jet veil separating gas areas with different pressure Прикладна гідромеханіка |
| description |
In G. V. Logvinovich's monograph "Hydrodynamics of currents with free borders" (in Russian) the general properties of flows of liquid with free boundaries are considered. To them treat as a current with formation of cavities on streamline bodies, and jet flows with the boundaries dividing liquid and gas. Questions of behavior of the non-stationary free borders, raised and studied in the monograph, are actual still and inspire many authors on new researches. Below one of such researches - a problem of creation of the air cushion by means of a jet veil is presented. Presented results of experimental studies for self-oscillating modes of fluid jet discharging in a plane channel with an air cushion. Investigated effect on the flow from the air cavity volume and thickness, width of the channel in a wide range of magnitude of jet rate and amount of gas discharge. Oscillatory flow regimes were realized under constant water pressure in the pressure tank and a constant mass flow rate of the blower to the air cushion. It was found that the previously studied low-frequency mode exists in a certain range of values of gas flow rate to the cavity, with the range depending on the cavity volume. It is shown that in some cases this mode is replaced by a high-frequency oscillatory regime with low amplitude, and in transitional range of air flow-rate both modes are simultaneously presented (there is an intermittency). Video recording of high-frequency regime has shown that unlike in the low-frequency regime there is no direct interaction between the outflowing jet with the channel wall. We found that in both modes the oscillation characteristics of the flow (frequency, amplitude) are independent of the thickness of the cushion (the channel width). However, the regime change event significantly depends on the thickness of the cushion. |
| format |
Article |
| author |
Kozlov, I.I. Ocheretyany, S.A. Prokof'ev, V.V. |
| author_facet |
Kozlov, I.I. Ocheretyany, S.A. Prokof'ev, V.V. |
| author_sort |
Kozlov, I.I. |
| title |
Оscillatory regime in liquid jet veil separating gas areas with different pressure |
| title_short |
Оscillatory regime in liquid jet veil separating gas areas with different pressure |
| title_full |
Оscillatory regime in liquid jet veil separating gas areas with different pressure |
| title_fullStr |
Оscillatory regime in liquid jet veil separating gas areas with different pressure |
| title_full_unstemmed |
Оscillatory regime in liquid jet veil separating gas areas with different pressure |
| title_sort |
оscillatory regime in liquid jet veil separating gas areas with different pressure |
| publisher |
Інститут гідромеханіки НАН України |
| publishDate |
2013 |
| topic_facet |
Науковi статтi |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/116413 |
| citation_txt |
Оscillatory regime in liquid jet veil separating gas areas with different pressure / I.I. Kozlov, S.A. Ocheretyany, V.V. Prokof'ev // Прикладна гідромеханіка. — 2013. — Т. 15, № 1. — С. 45-52. — Бібліогр.: 4 назв. — англ. |
| series |
Прикладна гідромеханіка |
| work_keys_str_mv |
AT kozlovii oscillatoryregimeinliquidjetveilseparatinggasareaswithdifferentpressure AT ocheretyanysa oscillatoryregimeinliquidjetveilseparatinggasareaswithdifferentpressure AT prokofevvv oscillatoryregimeinliquidjetveilseparatinggasareaswithdifferentpressure |
| first_indexed |
2025-07-08T10:20:50Z |
| last_indexed |
2025-07-08T10:20:50Z |
| _version_ |
1837073775093350400 |
| fulltext |
ISSN 1561 -9087 Прикладна гiдромеханiка. 2013. Том 15, N 1. С. 45 – 52
УДК 532.528
OSCILLATORY REGIME IN LIQUID JET VEIL SEPARATING
GAS AREAS WITH DIFFERENT PRESSURE
I. I. K OZ L OV, S. A. OC H E R ET YAN Y, V. V. PR OK O F’E V
Institute of mechanics of the Moscow State University, Rusia,
119899 Moscow, prosp. of Michurin, № 1,
prokof@imec.msu.ru
Received 11.07.2012
In G. V. Logvinovich’s monograph “Hydrodynamics of currents with free borders” [1] (in russian) the general properties
of flows of liquid with free boundaries are considered. To them treat as a current with formation of cavities on streamline
bodies, and jet flows with the boundaries dividing liquid and gas. Questions of behavior of the non-stationary free borders,
raised and studied in [1], are actual still and inspire many authors on new researches. Below one of such researches –
a problem of creation of the air cushion by means of a jet veil is presented. Presented results of experimental studies
for self-oscillating modes of fluid jet discharging in a plane channel with an air cushion. Investigated effect on the flow
from the air cavity volume and thickness, width of the channel in a wide range of magnitude of jet rate and amount of
gas discharge. Oscillatory flow regimes were realized under constant water pressure in the pressure tank and a constant
mass flow rate of the blower to the air cushion. It was found that the previously studied low-frequency mode exists in a
certain range of values of gas flow rate to the cavity, with the range depending on the cavity volume. It is shown that in
some cases this mode is replaced by a high-frequency oscillatory regime with low amplitude, and in transitional range
of air flow-rate both modes are simultaneously presented (there is an intermittency). Video recording of high-frequency
regime has shown that unlike in the low-frequency regime there is no direct interaction between the outflowing jet with
the channel wall. We found that in both modes the oscillation characteristics of the flow (frequency, amplitude) are
independent of the thickness of the cushion (the channel width). However, the regime change event significantly depends
on the thickness of the cushion.
KEY WORDS: cavitation, jet, self-oscillations, Rayleigh-Taylor’s instability, high-speed video
В монографiї Г. В. Логвiновича “Гидродинамика течений со свободными границами” [1] розглянутi загальнi влас-
тивостi течiї рiдини з вiльними границями, до яких можна вiднести як течiї з утворенням порожнини (каверни)
на тiлi, що обтiкається, так i струйнi течiї з границями, якi роздiляють рiдину та газ. Питання з поведiнкою
i росповсюдженням нестацiонарних вiльних границь, що поставленi й розглянутi в [1], актуальнi на сьогоднi,
спонукають i дають наснагу багатьом авторам до нових дослiджень. В розглянутому нижче маємо одне з таких
дослiджень, а саме – задачу створення повiтряної подушки завдяки струйнiй завiсi. Задача дослiджувалась
експериментально на плоскiй установцi, де струї рiдини витiкають у плоский канал з наявнiстю пiддува повiтря в
заглушену його частину. Дослiджувались виникнення нестацiонарних автоколивальних режимiв, їхня залежнiсть
вiд параметрiв течiї. Виявлено, що в деякому дiапазонi кiлькостi пiддува газу в каверну маємо низькочастотний
режим коливать. Iз збiльшенням пiддуву такий стан течiї змiнюється на iнший з бiльш високою частотою та
бiльш низькою амплiтудою, причому наявнi в перехiдному дiапазонi пiддува i одночасно спiвiснують обидва
режима течiї. Швидкiсне вiдео демонструє, що на вiдмiну вiд низькочастотного режиму при високочастотному
нема беспосереднього дотика витiкаючої струї з стiнкою каналу. Виявлено, що в обох режимах автоколивань
характеристики коливань (частота, амплiтуда) не залежать вiд товщини подушки (ширини каналу). Але момент
змiни режимiв течiї суттєво залежить вiд товщини подушки.
КЛЮЧОВI СЛОВА: кавiтацiя, струмiнь, автоколивання, нестiйкiсть Релея-Тейлора, швидкiсна вiдеозйомка
В монографии Г. В. Логвиновича “Гидродинамика течений со свободными границами” [1] рассмотрены общие
свойства течений жидкости со свободными границами, к которым относятся как течения с образованием каверн
на обтекаемых телах, так и струйные течения с границами, разделяющими жидкость и газ. Вопросы поведения
нестационарных свободных границ, поставленные и изученные в [1], актуальны до сих пор и вдохновляют многих
авторов на новые исследования. Ниже представлено одно из таких исследований – задача создания воздушной
подушки с помощью струйной завесы. Задача исследовалась экспериментально на плоской струйной установке, где
струи жидкости истекали в плоский канал, с поддувом воздуха в заглушенную его часть. Исследовалось возникно-
вение нестационарных – автоколебательных режимов, их зависимость от параметров течения. Обнаружено, что в
некотором диапазоне величин поддува газа в каверну имеет место низкочастотный режим. С ростом поддува этот
режим течения сменяется другим, с более высокой частотой и более низкой амплитудой, причем имеется область
поддувов, где одновременно существуют оба режима (имеет место перемежаемость). Скоростная видеосъемка
показала, что в отличие от низкочастотного режима при высокочастотном нет непосредственного взаимодействия
истекающей струи со стенкой канала. Обнаружено, что в обоих режимах характеристики колебаний не зависят
от толщины подушки (от ширины канала). Однако граница смены режимов существенно зависит от толщины
подушки.
КЛЮЧЕВЫЕ СЛОВА: кавитация, струя, автоколебания, неустойчивость Рэлея-Тейлора, скоростная видеосъемка
INTRODUCTION The Institute of Mechanics, MSU conducted
experimental studies of transverse fluid jet dischargi-
ng in a plane channel with ventilated cavity at
c© I. I. Kozlov, S. A. Ocheretyany, V. V. Prokof’ev, 2013 45
ISSN 1561 -9087 Прикладна гiдромеханiка. 2013. Том 15, N 1. С. 45 – 52
pressure higher than external. In this setup, we model
a flow with formation of artificial cavity with a
negative cavitation number, which is characterized
by the presence of a concave, unstable in Rayleigh-
Taylor sense, boundary. We have previously noted
[2] that in addition to the supercritical jet unsteadi-
ness associated with the development of the Rayleigh-
Taylor waves [3], the flow may develop substantially
unsteady self-oscillating form. For oscillatory modes
plane (2D) experimental facility is a good way to
model the problem of a high pressure chamber (air
bag) bounded by jet curtain as shown in FIG. 1.
1. EXPERIMENTAL SETUP
In this experiment, half of FIG. 1 flow was investi-
gated taking advantage of the symmetry. The experi-
mental setup has 2 transparent side walls (with a gap
of 5 or 9 mm). FIG. 2 shows a general view of the
working area of the machine, and on FIG. 3 we show
a photo of the flow obtained through the transparent
side wall. Unlike scheme FIG. 1 on experiment (FIG.
3) the stream is directed up. It has no strong impact
on a current as far as acceleration of particles of li-
quid is much more than gravity acceleration g (that
is Froude’s numbers are great - for example, at small
difference of pressure P0=0. 02 MPa and maximum
channel width in expermental H= 70 mm centrifugal
acceleration is equal to 51).
Fig. 1. Scheme to create an air cushion
bounded by liquid jet curtains
The stream of water flows out of the nozzle strai-
ght up, on the right there is a cavity with high air
pressure, on the left there is an outflow of liquid and
gas into the atmosphere. On FIG. 3 we show a near-
critical flow regime, when the oscillations are absent.
The solid lines represent the theoretical boundary
of the jet. On jet’s right boundary waves (Rayleigh-
Taylor structures) are formed, which are responsible
for discharge of air from the cavity [3].
Fig. 2. General view of the working area of the setup
Fig. 3. Flow photo through the transparent side wall
2. TERMINOLOGY
Below is list of variables we will be using for the
rest of the article:
Pk = pk − pa, P0 = p0 − pa(pk, p0, pa – average
pressure in the cavity, the pressure of the water flow
and atmospheric pressure, respectively),
A − Average range of pressure fluctuations in the
cavity for the measurement period
D, H − the width of the nozzle and the channel
width,
f − the frequency of oscillation,
h − the gap between the plates (9 mm),
V∞ =
√
2Po/ρ characteristic velocity of the jet,
Qg − volumetric flow rate of gas in the cavity of
the blower,
Ql − the average flow rate of water,
Cd = Pk/P0 − the coefficient of the pressure in
the cavity (factor of base pressure),
Cq = Qg/Ql − factor of carry-over of gas (or
blowed air),
St = fD/V∞ − the Strouhal number,
Ωk − the volume of the cavity,
Ck = Ωk/DHh − the relative volume of the cavi-
ty,
Kp = Ql/(DhV∞)− flow rate coefficient.
46 I. I. Kozlov, S. A. Ocheretyany, V. V. Prokof’ev
ISSN 1561 -9087 Прикладна гiдромеханiка. 2013. Том 15, N 1. С. 45 – 52
Fig. 5. Oscillograms of dimensionless pressure in the cavity (thick line) and in the settling chamber at Cq =14
for the coefficients of the cavity Ck = 36.6, 60.5, 74.8, 83.6 (a, b, c, d, respectively)
Fig. 4. Dependance of pressure pulsations intensity
on the relative volume of the cavity
3. RESULTS AND DISCUSSION
For FIG. 3 flow configuration (we consider only
liquid jet discharging perpendicular to the screen),
the coefficient of gas discharge at the critical mode
approximately equals to 1 (see [4]). A further increase
in pressure in the cavity (increase Cd) leads to rapid
growth of air injection (Cq). In the theoretical soluti-
on at the supercritical Cq the jet ceases to interact
with the screen. Practically, one more mechanism of
entrainment of gas emerges: gas flow along the screen.
With the increase of blowing of air at some point
the flow becomes unsteady – self-oscillatory. It was
found that the threshold at which oscillations start
to develop, is heavily dependent on the volume of the
cavity. Therefore, in the experiment it was important
to eliminate the influence of the air supply system
volume, this was achieved by means of a tap installed
at the entrance to the working area with the flow at
the tap in critical mode (locked tap). This ensured
the constancy of air mass flow to the cavity. In the
oscillatory regime the pressure fluctuations occur not
only in the cavity, but also in the water line feed. The
water pressure of the feed was stabilized by an air
cushion in a special container, which was connected
to the working area by 50 mm reinforced 1m long
tube. Thus, for all experiments the water supply zone
(tube, pre-chamber and nozzle on FIG. 2), in whi-
ch the unsteady fluid motion takes place, was fixed.
The flow rate of air and water were measured in the
stationary conditions. As an indicator of the water
jet momentum the time-averaged pressure p0 in the
I. I. Kozlov, S. A. Ocheretyany, V. V. Prokof’ev 47
ISSN 1561 -9087 Прикладна гiдромеханiка. 2013. Том 15, N 1. С. 45 – 52
pre-chamber was used. Also time-averaged pressure
in the cavity pk was used to characterize the flow.
We also introduce a volume factor of the cavity Ck
– i.e. the ratio of the volume of the cavity Ωk to the
characteristic volume DHh associated with the out-
flowing jet. FIG. 4 shows the intensity A/P0 of the
pressure fluctuations in the cavity depending on the
coefficient of the cavity volume Ck at constant blowi-
ng ratio Cq. We see quite a strong decrease in the
intensity fluctuations with increase in volume of the
cavity, and the slope of the curve becomes smaller wi-
th growth of Cq. The threshold value of Ck depends
strongly on the gas intake rate. Or, conversely, the
threshold of Cq increases with Ck. For example, when
Ck= 20, auto-oscillatory motion occurs at Cq > 6; for
Ck= 40, critical Cq= 10, and for Ck = 80 oscillati-
ons occur at Cq > 23. FIG. 5 shows the variation
of dimensionless pressure (p − pa)/P0 (in the cavi-
ty and the pre-chamber) by the dimensionless time
T = tV∞/D with the growth of the cavity at same
Cq ≈ 14 and average P0= 0.02 MPa. It is seen that
the pressure oscillations in the cavity (thick lines) are
accompanied by no-less intense pressure oscillations
in the settling chamber. With growing volume of the
cavity is not only the pressure oscillations in the cavi-
ty drop, but also the phase shift between the pulsati-
ons in the cavern and the settling chamber increases.
In the case of FIG. 5, d. pressure fluctuations in
the cavity are ahead by about two units of pressure
pulsation in the settling chamber (with the pulsati-
on period approximately equal to 10 dimensionless
units).
Fig. 6. Graph of pressure coefficient of blowing air
Below we present results of experimental studies
for a series of experiments conducted in a relatively
small volume of the cavity (Ck ≈ 5, in this case the
transition to self-oscillating mode occurs at Cq ≈2)
for different values of the thickness of the cushion Н.
FIG. 6 and 7 show dependence of the cavity
pressure and the water flow rate on rate of air blowi-
Fig. 7. Graph of flow rate of the blowing air
ng into the cavity at four channel widths (thickness
of the air cushion). Average excess head P0 pressure
varies from 0.02 to 0.07 MPa – some scatter of points
is associated with the existing scale effect. It can be
seen that the coefficient of blowing rather strongly
depends on the thickness of the air cushion. Without
blowing air (Cq= 0) in a water-filled cavity has hi-
gh pressure, which increases with decreasing H . If
with the constant parameters we replace water wi-
th air in the jets then the cushion air pressure will
remain the same – but required air flow rate is 1.000
times greater than water. Large black circles on the
graph show the theoretical values of the critical values
C∗
d . In [4] it was shown that in the test configuration
(turning the water jet at 900) the coefficient of blowi-
ng Cq ≈ 1 and pressure ratio increases approximately
linearly from zero to a critical blowing. FIG. 6 graphs
for presented range of flow rate (supercritical blowi-
ng) are well approximated by polynomials (for H =
28 mm – 4-th order polynomials, for other values –
third). For all the values of cushion thickness when
the critical value C∗
d is reached, the pressure ratio is
increased by the same amount of about 0.1 [4], while
in supercritical conditions, the greater the thickness
of the cushion the more efficient is the supercritical
blowing. So at H =70 mm pressure can be increased
by 90%, and at H= 28 mm, only by 27%. Moreover,
the maximum pressure in the cushion for all cases is
achieved with Cq ≈ 20, and then pressure ratio is
slightly reduced (in FIG. 6 not shown).
FIG. 7 shows that actual flow rate of air blowing to
the cavity is not growing as much as the value of Cq
due to the drop of water flow as the pressure in the
cavity increases. FIG. 7 points represent same Cq as
in FIG 6. It is seen that, in spite of a slight decrease
in the average pressure in the cavity at Cq > 20, the
48 I. I. Kozlov, S. A. Ocheretyany, V. V. Prokof’ev
ISSN 1561 -9087 Прикладна гiдромеханiка. 2013. Том 15, N 1. С. 45 – 52
Fig. 8. Oscillograms and spectrum of the cavity pressure (thick line) and pressure
in the settling chamber at P0 = 0.01 MPa, Cq =15.6
water flow rate at fixed average head pressure conti-
nues to decrease with increasing blowing. This can
not be explained in terms of the concept of stationary
jet flow.
Let’s turn to the study of pulsation characteristics
for oscillatory flow regimes. FIG. 5 shows oscillograms
for oscillatory flow regime (observed oscillations of
the jet near the steady state - see illustrations in [2]).
In the problem under consideration there is a
strong influence of the size effect on the fluctuati-
ng characteristics. FIG. 8, a. shows similar to FIG.
5 waveforms with similar coefficient of blowing, but
at smaller rate of discharge, here there is another -
surge (intermittent [4]) regime of flow.
Fig. 9. Strouhal number dependence on the coefficient
of gas flow rate into the cavity
at P0= 0.02 MPa and various H
FIG. 8, b. shows a spectrum of the waveforms.
Here F is the frequency. Following of oscillograms to
evolution of the flow was studied in details in [2]. The
spectrum of pressure fluctuations in the cavity (solid
line) is very close to spectrum of the ramp - so, despi-
te the presence of harmonics, it is a single-frequency
regime. Pressure oscillations in the pre-chamber in
surge mode are quite different from oscillations in
the cavity. In the stage of "purging"of the channel
and the discharge of the jet into "Empty"channel
(the pressure in the cushion close to the atmospheric
pressure) a second hump of pressure is observed in
the pre-chamber. As a result, the first harmonic of
the signal spectrum decreases and (in some cases it
is even less than the second).
Fig. 10. Dependence of relative magnitude of pressure
fluctuations in the cavity on the coefficient of flow rate
of gas blown into the cavity
at P0 = 0.02 MPa and various H
FIG. 9 and FIG. 10 shows Strouhal number (St =
fD/V∞) and relative magnitude of pressure fluctuati-
ons in the cavity (A/P0) of the blowing ratio in the
cavity with the same medium head (P0= 0.02 MPa)
and different thicknesses of the air cushion.
FIG. 9 shows that with increasing blowing for
some of its rate there is an abrupt change in the
frequency of pressure fluctuations in the cavity (note
that Strouhal number is determined by the leading
frequency in the spectrum). The transition to this hi-
I. I. Kozlov, S. A. Ocheretyany, V. V. Prokof’ev 49
ISSN 1561 -9087 Прикладна гiдромеханiка. 2013. Том 15, N 1. С. 45 – 52
Fig. 11. Waveforms of pressure fluctuations in the cavity and their spectrum show the process
of regime change in the oscillations at P0= 0.02 MPa, H = 28 mm
Fig. 12. Snapshots of surges mode (left, Cq =20.1) and high-frequency mode
(right, Cq =41.8 for P0= 0.01 MPa, H = 25.6 mm, D = 25 mm, pitch 0.003 s frames)
gh frequency is different from the transition from the
"sine wave"to the surge mode where frequency and
amplitude are continuous. We can see that in both
modes (low and high frequency), Strouhal number
is independent of the cushion thickness and approxi-
mately linearly increases with increasing of blowing
ratio. The thickness of the cushion depends signifi-
cantly on the transition from low to high frequency
regime.
This is well illustrated by the data on FIG.
50 I. I. Kozlov, S. A. Ocheretyany, V. V. Prokof’ev
ISSN 1561 -9087 Прикладна гiдромеханiка. 2013. Том 15, N 1. С. 45 – 52
10. It shows that average peak-time drops signifi-
cantly during transition to the high-frequency mode.
Intensity of the vibration increases significantly wi-
th the increase in low-frequency mode and does not
depend on the thickness of pads and practically does
not change depending on the blowing in the high
mode, the range of variation in this case is approxi-
mately equal. Thus variations (in the intensity and
frequency) in the studied range do not depend on the
thickness of the cushion with the exception of the
transition modes.
It is seen that the transition is not abrupt but
it takes a whole range of change in blowing ratios
(about 10 to 20 – as seen in FIG. 10). The spectrum
shows that in this area two modes co-exist (there is
an intermittency). In addition, it appears that as the
blowing increases the sinusoidal mode is not always
progressing to surge. For example, the data of FIG.
11 show that with increasing pressure the sinusoi-
dal mode transitions to a different regime, bypassing
surge mode.
FIG. 12 shows a transition to a different mode
from the surge mode (under lower water pressure –
P0= 0.01 MPa). The left sequence of frames shows
only part of the period of low-frequency pulsations –
flow from the nozzle, the interaction with the screen,
and early discharge gas liquid plug. Right plot shows
sequence of frames flow at the same pressure of the
jet, but with a much larger blowing factor. Typi-
cally discharging jet ceases to interact directly wi-
th the screen – the jet only interacts with the gas
flow. As shown in FIG. 10, changes of the frequency
and intensity of the pressure fluctuations dramati-
cally decreases in the gas chamber, in this regard,
there is no periodic separation of the flow of liquid
from the nozzle edge which is common for surge regi-
me.
FIG. 13 and 14 show effect of the discharge
rate (scale effect) on the characteristics of pressure
fluctuations in the cavity at constant geometry of
the boundaries. Thus the scale effect on the relati-
ve intensity of fluctuations is particularly high.
The lower the velocity of discharge (or the average
pressure P0), the higher is the relative amplitude. At
P0 = 0.01 MPa value A/P0 reaches 3.7, and P0 = 0.07
MPa only 1.1. Additionally, the more is the pressure
P0 the earlier (in terms of Cq) the transition to high-
frequency oscillation mode takes place. But in high-
frequency mode, the scale effect is not presented, as
in FIG. 10 swings in pressure variations in this mode
is approximately equalP0 .
If the transition to surge, intermittent regime is
possible at sufficiently high relative amplitude of the
pressure oscillations, then with increasing pressure
conditions for this transition disappear.
Fig. 13. Dependence of Strouhal number
on the coefficient of blowing at H = 39 mm
and different speeds of the liquid flow
So, it was found that, at other fixed parameters
with increase of the relative cavity volume the
"threshold"for the transition to self-oscillating Cq1
mode increases. On the other hand, for a fixed volume
of the cavity there is a quantity Cq2 (depending also
on P0), when coefficients of blowing above that there
would be a reorganization of vibrations and dischargi-
ng jet would not interact with the screen.
Fig. 14. Dependence of coefficient of blowing at
H = 39 mm and different speeds of the liquid flow
Thus, low-frequency oscillations occur in the range
Cq1 < Cq < Cq2, for example if with increase in
Ck Cq1 = Cq2condition is achieved, this Cklimits
at the top the range of existence for the oscillations.
FIG. 15 shows experimental data for the
dependence of the flow rate of water through the
nozzle Kpon the pressure coefficient of the cavity
I. I. Kozlov, S. A. Ocheretyany, V. V. Prokof’ev 51
ISSN 1561 -9087 Прикладна гiдромеханiка. 2013. Том 15, N 1. С. 45 – 52
Fig. 15. Dependence of jet flow rate factor on pressure
factor in the cavity at Ck ≈ 5
Cd at different cushion thicknesses Hand average
pressuresP0. Solid line is theoretical curve for the
steady plane flow of an ideal fluid. At Cd = 0 flow
coefficient is less than 1, because the water nozzle has
a constriction as covered in the theoretical calculati-
ons. Large gray circle marks the limit - the critical
point for the flow of the jet with the accession to
the screen for all four values of cushion thickness.
Points corresponding to three different rates of di-
scharge of FIG. 15 are shown in symbols of different
shapes. Transparent and opaque symbols alternate
for successive values of H . We see that at supercritical
flow the discharge coefficient is less than calculated
by steady-state theory (as presented in FIG. 15, up
to 2 times). In [4] it was shown that in the subcritical
flow regime the average flow characteristics are well
described by the ideal liquid steady theory. Hence,
there is a strong influence of significantly unsteadi-
ness of the flow on average parameters. At fixed Cd
the rate of fluid flow from the nozzle depends on the
thickness of the cushion and the pressure of water.
CONCLUSION
When creating a jet veil with high-pressure cavity
(air cushion) in supercritical air flow rate the auto-
oscillatory modes of jet flow are observed. In this regi-
me the liquid jet directly interacts with the screen
(note, the theoretical steady stream in supercriti-
cal mode does not interact with the screen). It was
found that the range of blowing coefficients where
this regime exists is strongly dependent on volume
of the cavity. With a slight overpressure in the cavi-
ty (about 0.01 MPa, these are the values common for
the hovercrafts) with increasing air blowing the auto-
oscillating mode turns into surge (intermittent) flow
regime. Despite the fact that average pressure in the
air cushion depends on its thickness, the characteri-
stics of vibration (frequency and intensity) in the
studied range do not depend on the cushion thi-
ckness. There is only dependence of transition poi-
nt to high-frequency regime on this parameter. The
relative intensity of pressure pulses in the cavity noti-
ceably depends on velocity of the jet (scale effect),
and in the region high-frequency mode the scale effect
is not observed and amplitude of the pressure pulsati-
on in the cavity is approximately equal P0.
1. Г.В. Логвинович Гидродинамика течений со сво-
бодными границами.– Киев: Наук. думка, 1969.–
215 с.
2. Kozlov I.I., Ocheretyany S.A., Prokof’ev V.V. Experi-
mental Investigation of Liquid Jet Outflow into a
Plane Ventilated Channel in Self-Oscillatory Regi-
mes // Fluid Dynamics.– 2011.– 46 (4).– P. 548.
3. I.I. Kozlov, V.V. Prokof’ev, and A.A. Puchkov
High-Speed Videocamera Investigation of the Wave
Structure Development on an Unstable Cavity
Boundary // Fluid Dynamics.– 2008.– 43 (2).–
P. 287.
4. I.I. Kozlov and V.V. Prokof’ev Gas Entrainment
from a Ventilated Cavity with a Negative Cavitation
Number // Fluid Dynamics.– 2001.– 36 (5).– P. 751.
52 I. I. Kozlov, S. A. Ocheretyany, V. V. Prokof’ev
|